Chapter 1 - Background and History of We Energies Coal Combustion Products (CCPs)
Figure 1-1: Fly ash "flying away" from We Energies’ Lakeside Power Plant prior to the advent of collection in electrostatic precipitators and bag houses.
In the early days of the power generation industry, coal com-bustion products (CCPs) were considered to be a waste material. The properties of these materials were not studied or evaluated seriously and nearly all of the coal combustion products were landfilled. In the course of time, the cementitious and pozzolanic properties of fly ash were recognized and studied by several individuals and institutions. The products were tested to understand their physical properties, chemical
properties and suitability as a construction material. During the last few decades these "waste" materials have seen a transformation to the status of “by-products” and more recently “products” that are sought for construction and other applications.
During the past several decades, generation of electricity through various coal combustion processes has grown to accommodate increased population and associated industrial and commercial development in the United States and other parts of the world. These coal combustion processes leave behind residues that are referred to as CCPs.
The initial CCPs were called cinders and were formed from burning lump coal on grates in stoker furnaces. These cinders were sometimes used as road gravel and as a lightweight aggregate in manufacturing masonry “cinder” blocks.
Figure 1-2: Bottom ash "cinders" from We Energies’ Wells Street Power Plant destined for road surfacing and other applications.
In the 1920's, more effective methods of firing power plant boilers were invented. These new processes involved burning pulverized coal instead of lump coal. While the process was a more efficient method of firing, the process generated an increased stream of fine combustion products and lower quantities of cinders. This fine combustion product is called fly ash, and the cinders that are relatively coarser are called bottom ash. As environmental awareness and landfilling costs have grown, CCP generators and government regulators have encouraged the beneficial use of industrial by-products, including coal ash.
According to the American Coal Ash Association (ACAA), combustion of coal in the United States alone generated approximately 130 million tons of coal combustion products in 2010, including approximately 68 million tons of fly ash, 18 million tons of bottom ash, 32 million tons of flue gas desulfurization (FGD) materials, and 2 million tons of boiler slag. Of the fly ash produced, approximately 13 million tons were used in cement, concrete, and grout applications; and another 13 million tons were used in various other applications (1).
In some parts of the world, CCP utilization rates are much higher than that of the United States with a utilization rate of 42.5% in 2010, per ACAA. For example, in the European Union (EU15) the CCP utilization rate was 89% in 20071. CCP utilization in Japan was 97% in 20062, and was 58% in China in 2000. According to ECOBA, EU15 generated 61.2 thousand metric tons of coal combustion products (including 41.8 tons of fly ash, 5.7 tons of bottom ash, 10.8 tons of FGD and 1.5 tons of boiler slag) in 2007.
1 European Coal Ash Association (ECOBA), “Production and Utilization of CCPs in 2007 in Europe (EU 15)”
2 Japan Coal Energy Center, “Status of coal ash production”, 2005
The United States is the world's second largest producer of fly ash with 68 million tons (second only to China with 70 million tons)4. Opportunities exist to make use of these valuable mineral resources (2) with approximately 43% of coal combustion products used in the United States in 2010. The ACAA survey reported the usage included a number of applications, with construction industries and civil engineering at 32.0%, followed by mining applications with 9.9% and other applications with 1.1%. These percentages are expected to increase, as a result of the development of new uses for CCPs, increased awareness of proven technologies, and global focus on sustainable development for the remaining 57% of the total CCPs produced in the USA that are being stockpiled or disposed in landfills.
Figure 1-3: We Energies CCP Production and Utilization
3 Wang,F., &WU,Z, ”A Handbook For Fly Ash Utilization (2ed.)”, Beijing: China Power Press, 2004.
4 Fu, J., “Challenges To Increased Use of Coal Combustion Products in China”, Spring 2010
Coal fired power generation has gone through several process modifications to improve efficiency, control the quality of air emissions, and to improve the quality of CCPs. The variety of coal that is burned influences the chemistry of CCPs significantly. The introduction of low sulfur coal has improved the quality of air emissions and also generally improved the quality of fly ash.
The provisions of the Clean Air Act Amendments (CAAA) have also affected nitrogen oxide (NOx) emissions and its controls for the electric utility industry. Further reductions are possible if the Cross State Air Pollution Rule (CSAPR) is implemented.
The process for reducing NOX emissions through combustion control technologies has generally increased the amount of unburned carbon content and the relative coarseness of fly ash at many locations. In particular, post-combustion control technologies for NOX emissions such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) both utilize ammonia injection into the boiler exhaust gas stream to reduce NOX emissions. As a result, the potential for ammonia impacts of the fly ash due to excessive ammonia slip from SCR/SNCR operation is an additional concern. An SCR installed at We Energies Pleasant Prairie Power Plant (P4) began operation in 2003 and at the Oak Creek Site Units in 2010-2012. Ammonia impacts can occur especially near the end of an SCR catalysts life, and daily fly ash testing is in place to ensure that ammonia levels are acceptable for the intended use of the fly ash. We Energies has also developed and patented a fly ash beneficiation process to remove and reuse ammonia if needed in the future.
Regulations to reduce sulfur dioxide emissions have resulted in the introduction of either dry or wet scrubber flue gas desulfurization (FGD) systems which can produce calcium sulfite or calcium sulfate (gypsum) as a by-product, respectively. The scrubbers capture more than 97% of the sulfur dioxide (SO 2) from combustion exhaust gas. According to the U.S EPA, in 2005, the overall annual SO2 emissions from power plants were 9% lower than the year 2000 and 41% lower than 1980. In 2010, the total SO2 emissions were reduced by over 10 million tons since 1990 (67%). The Clean Air Interstate Rule (CAIR) was issued by U.S. EPA in 2005. The U.S Clean Air Act Amendments of 1990 established the Acid Rain Program (ARP) . The former NOx Budget Trading Program (NBP) was promulgated by U.S. EPA in 1998. From the CAIR, ARP, and Former NBP 2010 progress report, the electric utility companies nationwide emitted about 5.2 million tons of SO2 (well below the statutory annual cap of 8.95 million tons). Many western coals and some eastern coals are naturally low in sulfur and have been used to help meet SO2 compliance requirements. Blending coals of different sulfur contents to achieve a mix that is in compliance with applicable regulations is also common. Nearly more than 200 coal-fired power plants in more than 35 states use compliance coals such as low sulfur Powder River Basin coal to achieve the SO2 emission level currently mandated5. Wet FGD systems are currently installed on about 25% of the coal-fired utility generating capacity in the United States (3). Currently, there are wet FGD systems operating on We Energies new supercritical Oak Creek Units 1-2, Oak Creek Units 5-8 and Pleasant Prairie Power Plants.
Figure 1-4: This 170-acre coal ash landfill is located in Oak Creek, Wisconsin, where over 3,700,000 cubic yards of coal ash are stored.
In the 1990 Clean Air Act Amendments, mercury is also identified to be an air toxic metal, and this element is emitted in three forms from the coal-fueled power plants. About 60% of mercury is typically in the elemental form (Hg⁰), 40% in the oxidized (Hg2+ or HgCl2) form, and the remainder is condensed mercury on ash particles (Hgp). Since the oxidized mercury is water -soluble, small amounts end up in waste water treatment residuals. Under the right conditions mercury can form a toxic organic form called Methylmercury (which can be taken in by fish). The U.S. EPA conducted as analysis on mercury emissions from coal-fueled power plants and regional deposition patterns in U.S waters. A case study was conducted for Wisconsin in 2002 as part of the state rule-making process, and has concluded that all of the state’s coal-fueled power plants combined contribute approximately 1-4 % of the mercury being deposited in Wisconsin’s lakes and rivers. A significant reduction of mercury emissions was achieved through existing pollution controls such as fabric filters (for particulate matter), scrubbers (for SO2) and SCRs (for NOx).
5 Ward Jr.,K., “Powder River Basin not a ‘coal producing region’?”, Coal Business in Legal Actions, February 11, 2011.
The Presque Isle Power Plant installed the TOXECON process that uses a fabric filter in conjunction with sorbent (activated carbon) injection to remove mercury and other emissions downstream of the plant’s existing particulate control devices. Results have shown that TOXECON has been able to capture 90% of the mercury in the flue gas. One of the disadvantages of injecting activated carbon is its impact on the salability or reuse of ash. Tests have shown that the activated carbon interferes with admixtures used in concrete. However, if a TOXECON baghouse is placed downstream of an Electrostatic Precipitator (ESP) to capture the spent sorbent, the fly ash quality is then preserved for subsequent use. We Energies Oak Creek (Units 5-8), Pleasant Prairie (Units 1 & 2) and Presque Isle Power Plants (Units 7-9) use sub -bituminous coal and these power plants increase the capture process of mercury by using calcium bromide (CaBr 2) as an additive to the coals. CaBr2 is a cost effective method to oxidize mercury for facilitating its absorption in the wet FGD slurry. The adsorbed mercury is then primarily captured within the FGD waste water treatment system solids.
The Oak Creek Expansion, Units 1 - 2 burn Eastern bituminous coal with the use of advanced air quality control equipment including selective catalytic reduction to remove nitrogen oxides, baghouse filters to remove particulate matter (ash in the exhaust gas), scrubbers to remove sulfur dioxide, and wet electrostatic precipitators (WESP) to remove sulfuric acid mist, aerosols and ultrafine particulates from the flue gas. The WESP consists of a series of electrically charged collecting plates located in the casings of the WESP where discharge electrodes between the plates create the electrical field which in turn repels the sulfuric acid mist, aerosols and ultrafine particulates toward the collecting plates. The plates are continuously (or intermittently, depending on the gas condition) washed with spray water to remove the collected material. This wash water is collected and returned either to the WESP spray wash system or added to the FGD system for neutralization. The WESP captures more than 94% of the sulfuric acid mist, aerosols and ultrafine particulates on collection plates from the flue gas (4).
It is important to distinguish fly ash, bottom ash, and other CCPs from incinerator ash. CCPs result from the burning of coal under controlled conditions. The U.S EPA (RCRA orientation manual, 2008) has conclusively determined CCPs being non-hazardous after studying the coal-fired utility wastes in 1993 that excluded large volume of coal fired utility wastes (inclusive of fly ash, bottom ash, boiler slag and flue gas desulfurization materials) from the definition of hazardous waste. In December of 2008, an impoundment dike failed at the Kingston Plant in Tennessee that has resulted in EPA proposing both hazardous and non-hazardous rules for comment. The outcome will likely establish federal standards for disposal of CCPs. Even though trace elements of mercury are retained in the coal-combustion residue, it is unlikely to be leached at levels of environmental concern (U.S. EPA, January 2006).
The other constituents of coal ash are commonly found in everyday products and natural materials, including soil (ACAA Educational Foundation, March 2009). Incinerator ash is the ash obtained as a result of burning combinations of municipal wastes, medical waste, paper, wood, etc. and sometimes will test as hazardous waste. The mineralogical composition of coal ash and incinerator ash consequently are very different. The composition of ash from a single coal source is typically very consistent and uniform, unlike the composition of incinerator ash, which varies tremendously because of the wide variety of waste materials burned.
The disposal cost of CCPs has escalated significantly during the last couple of decades due to significant changes in landfill design regulations. Utilization of CCPs helps preserve existing licensed landfill capacity and thus reduces the demand for additional landfill sites. Due to continued research and marketing efforts, We Energies was able to utilize 110% of coal combustion products in 2010 compared to only 5% in 1980. Increased commercial use of CCPs translates to additional revenues and reduced disposal costs for We Energies, which in turn translates to lower electric bills for electric customers. The use of CCPs in construction reduces the need for quarried raw materials, manufactured aggregates and Portland cement. Replacement of these virgin and manufactured materials with CCPs helps to conserve energy and reduce emissions associated with manufacturing and processing. When fly ash and bottom ash are used beneficially as engineered backfill material, these materials are replacing sand or gravel that would otherwise have been quarried and transported from various locations. The use of CCPs helps preserve mineral materials from sand and gravel pits and quarries as well as provides construction cost savings associated with operation. It is also important to keep in mind that every time Portland cement is replaced or displaced with fly ash, CO2 and other emissions to the atmosphere from cement production are reduced by decreasing the need for limestone calcination as well as the fossil fuel that is consumed for production. Beginning in 2006, We Energies began production of flue gas desulfurization (FGD) gypsum at Pleasant Prairie Power Plant. The FGD gypsum produced has all been used in place of natural mined gypsum in the manufacture of wallboard products and in agricultural applications.
The Wisconsin Department of Natural Resources (WDNR) has been monitoring the progress of beneficial utilization of industrial by-products, including CCPs. In 1998, the WDNR introduced a new chapter to the Wisconsin Administrative Code - Chapter NR 538 “Beneficial Use of Industrial Byproducts”, to encourage the environmentally responsible use of industrial by-products. According to the WDNR, the purpose of Chapter NR 538 is “to allow and encourage to the maximum extent possible, consistent with the protection of public health and the environment and good engineering practices, the beneficial use of industrial by-products in a nuisance-free manner.
The department encourages the beneficial use of industrial by-products in order to preserve resources, conserve energy, and reduce or eliminate the need to dispose of industrial by-products in landfills.”
Figure 1-5: Landfilling of fly ash can seem overwhelming.
We Energies has made significant progress in finding uses for its coal ash, and it is interesting to look
back at this quote from Path of a Pioneer page 210 (5):
Solving one problem in the air created another on the ground: what to do with millions of tons of fly ash. Recycling had provided an early solution to some of the company’s waste problems. In the late 1920’s, cinders from the Commerce and East Wells plants had been mixed in a building material called Cincrete, which was used in the Allen-Bradley plant, the Tripoli Shrine, and other Milwaukee landmarks. Cinders were in short supply after the system converted to pulverized coal, but fly ash found some acceptance as a concrete additive after World War II. Hard, heat-resistant, and convincingly cheap, it was used in everything from oil well casings to airport runways. Demand, however, never threatened to outstrip supply; most of WEPCO’s “used smoke” ended up in landfills.
We Energies, doing business as Wisconsin Electric, and its past affiliate Minergy Corporation also produced several light weight aggregate products such as structural-grade light weight aggregate suitable for use in a broad range of concrete products and geotechnical applications, light weight concrete masonry with higher fire rating and higher R-values, and light weight soils for roof top gardens and parks. However, Minergy Corporation was closed in 2000.
Concrete continues to be the leading utilization application today; however many new and promising technologies have also been introduced and proven which are discussed in the balance of this handbook.
Chapter 2 - CCPs and Electric Power Generation
Coal is one of the most commonly used energy sources for the generation of electricity. In the process of generating power from coal, large quantities of CCPs are produced. CCPs are the solid residues that remain after the combustion of coal within a furnace, and are collected in emission control processes.
In the early years of power generation at coal-fueled generating plants, coal was fired in a furnace with stoker grates. Today most coal-fueled power plants are fired with pulverized coal.
Electric Power Generation
In the most simplified form, a coal-fired power plant process can be described as follows. Coal is first passed through a pulverizer where it is milled to the consistency of flour. The powdered coal is mixed with a steady supply of air and is blown to the furnace where it burns like a gas flame. Pulverized coal firing is more efficient than stoker firing. With stoker firing, there is always a bed of coal on the grate, which contains a considerable amount of heat that is lost when it is removed. With pulverized coal, the coal burns instantly, and in this way the heat is released quickly and the efficiency of the process is higher. If the coal supply is cut off, combustion ceases immediately (6).
The heat generated by burning pulverized coal in the furnace in the presence of air is used to generate steam in a boiler. In its simplest form, the boiler consists of steel tubes arranged in a furnace. The hot gases pass through the banks of tubes, heating the tubes. The boiler is supplied with a steady flow of water, which is turned to steam in the tubes. The steam is collected in the upper drum of the boiler and is directed to pipes leading to a turbine (6).
The turbine can be compared to a windmill. The steam generated in the boiler is directed to the fan blades in the turbine and causes the rotor assembly to turn. The blades are arranged in groups or stages and the steam is forced to flow through the different stages. In doing so, the steam loses some of its energy at each stage, and the turbine utilizes the steam energy efficiently to spin the rotor shaft.
The turbine rotor shaft is coupled to an electric generator. When the steam from the boiler pushes against the blades fitted to the turbine rotor, it spins together with the generator rotor. The generator rotor is simply a large electromagnet. The electromagnet rotates inside a coil of wire. The magnetic field issuing from the rotating electromagnet travels across the turns of wire in the stationary coil and generates electric current in the wire.
Depending on the number of turns in the coil, the magnitude of the current in the coil will increase or decrease. The electric voltage and current generated in the generator can be increased or decreased using a power transformer for transmission to consumers. Figure 2-1 is a basic flow diagram of a typical coal-fired power plant. The above description of the turbine/generator is very simple, but in a real power plant, the system is more complex with multiple stages and additional equipment to increase efficiency and protect the environment.
In addition to the above pulverized coal technology, an alternate power generation technology is Integrated Gasification Combined Cycle (IGCC). The IGCC process is designed to break down coal into its basic constituents and obtain a synthetic gas (syngas) that is burned in combustion turbines. The gas conditioning process enables the separation of any contaminants from the syngas prior to its use as fuel. Excess heat is also utilized to produce steam for steam turbine use. The IGCC system consists of coal gasifiers, air separation units, gas conditioning systems, steam turbine generators, and sulfur recovery systems, etc. Figure 2-2 shows a basic diagram of an IGCC plant process. One of the most significant advantages of IGCC is that the technology can easily capture CO2 and also achieve greater emissions reductions. An IGCC unit was proposed as part of the company’s Power the Future plan, but was not approved due to the immaturity of the processes at the time. As of 2012, IGCC generation units have not been added to the We Energies fleet of power generation units.
CCPs Generation
The description in the past few paragraphs summarizes the primary operations taking place in a coal-fueled power plant for the generation of electricity. In the coal combustion process, CCPs are also generated in direct proportion to the variety, quantity and ash content of coal consumed. The pulverized coal is burned in the furnace to generate heat, and the hot gases then pass around the bank of tubes in the boiler and are eventually cleaned and discharged through the plant chimney. In large power plants that consume large quantities of coal, substantial quantities of coal ash are produced. The ash that is collected in electrostatic precipitators or baghouses is called fly ash.
In electrostatic precipitators the flue gas is passed between electrically charged plates where the fly ash particles are then attracted to the plates. Baghouses can also be used to collect ash with bags that filter the fly ash out of the flue gas stream. The fly ash particles are periodically knocked off the plates or bags and fall into the hoppers located at the bottom of the electrostatic precipitators or baghouses. The fly ash is then pneumatically transported to storage silos. The storage silos are equipped with dry unloaders for loading dry bulk semi tankers or rail cars, and wet unloaders for conditioned ash or disposal applications.
Bottom ash is formed when ash particles soften or melt and adhere to the furnace walls and boiler tubes. These larger particles agglomerate and fall to hoppers located at the base of the furnace where they are collected and normally ground to a predominantly sand size gradation. Some bottom ash is transported to storage dry, but most is transported wet from the furnace bottom to dewatering bins where water is removed prior to unloading and transport to construction sites or storage stockpiles. Figure 2-3 shows the typical ash generation process in a coal-fueled power plant.
The ash collected from pulverized-coal-fired furnaces is fly ash and bottom ash. For such furnaces, fly ash constitutes a major component (80 to 90%) and the bottom ash component is in the range of 10 to 20%. Boiler slag is formed when a wet-bottom furnace is used. The non-combustible minerals are kept in a molten state and tapped off as a liquid. The ash hopper furnace contains quenching water. When the molten slag contacts quenching water, it fractures, crystallizes, and forms pellets, resulting in the coarse, black, angular, and glassy boiler slag. The boiler slag constitutes the major component of cyclone boiler by-products (70 to 85%). The remaining combustion products exit along with the flue gases. Currently, We Energies power plants do not produce boiler slag.
Flue gas desulfurization (FGD) material is the solid material resulting from the removal of sulfur dioxide gas from the utility boiler stack gases in the FGD process. The material is produced in the flue gas scrubbers by reacting slurried limestone or lime with the gaseous sulfur dioxide to produce calcium sulfite. At We Energies, “wet” FGD systems are installed where the sulfur dioxide removal takes place downstream of the fly ash removal device. Then the calcium sulfite is further oxidized to calcium sulfate (synthetic gypsum) which has the same chemical composition as natural gypsum. The dewatering system removes water from the calcium sulfate leaving the FGD absorber modules into hydrocyclone centrifuges and onto belt filter presses. Vacuum pumps beneath the belt, siphon the water out of the material, leaving it with about a 10 percent moisture content. A belt conveyor system transports the dewatered materials from the dewatering building to an adjacent storage shed.
In the FGD process, a small fraction of the calcium sulfate slurry is regularly removed to a water treatment system for dewatering to remove chlorides and fines from the process. The solids from the water treatment system are captured and removed in a filter press. This material is typically referred to as waste water system filter cake (a second by-product) and consists of fine gypsum particles, unreacted limestone fines, calcium sulfite particles and a minor amount of fly ash. It is a brown clay-like chunky material with a high (107% ±) water content. Due to the high content of water, chlorides, sulfites and trace metals, filter cake cannot be used in pavements or other applications without stabilization.
The CCPs described above are produced in pulverized coal-fueled plants. In IGCC facilities, the sulfur-containing gases from the acid gas removal system are converted to elemental sulfur or sulfuric acid. Sulfur dioxide combines with oxygen and water to form sulfuric acid; the reaction of hydrogen sulfide and sulfur dioxide forms water and elemental sulfur. Elemental sulfur or sulfuric acid in sufficiently pure forms can be suitable for sale to other industries for various uses. If elemental sulfur is produced, a storage tank is provided to hold molten sulfur until it can be transferred to railcars for shipment off-site. Sulfur can be used in bituminous mixtures, sulfur-concrete, and in the manufacture of fertilizer, paper, etc. If sulfuric acid is produced, above ground storage tanks are constructed to temporarily hold the acid until it is transported off site by specially designed rail cars or trucks for commercial use, such as wastewater treatment or in the production of phosphate fertilizers.
Figure 2-1: Diagram of Pulverized Coal-Fueled Power
Figure 2-2: Diagram of an IGCC Plant Process (7)
Figure 2-3: Typical Ash Generation Process in a Coal Fired Power Plant
Properties of Fly Ash
Fly ash is a fine powder that is collected from the combustion gases of coal-fueled power plants with electrostatic precipitators and/or baghouses. Fly ash particles are very fine, mostly spherical and vary in diameter. Under a microscope they look like tiny solidified bubbles or spheres of various sizes. The average particle size is about 10µm but can vary from <1µm to over 150µm (8).
The properties of fly ash vary with the mineral make-up of coal used, grinding equipment, the furnace and the combustion process itself. ASTM C618 (American Society for Testing and Materials) “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete”, classifies fly ash into two categories – Class F and Class C fly ash. Combustion of bituminous or anthracite coal normally produces Class F (low calcium) fly ash and combustion of lignite or sub-bituminous coal normally produces Class C (high calcium) fly ash. Table 2-1 shows the normal range of the chemical composition for fly ash produced from different coal types.
Table 2-1: Normal Range of Chemical Composition for
Although ASTM does not differentiate fly ash by CaO content, Class C fly ash generally contains more than 15% CaO, and Class F fly ash normally contains less than 5% CaO. In addition to Class F and Class C fly ash, ASTM C618 defines a third class of mineral admixture - Class N. Class N mineral admixtures are raw or natural pozzolans such as diatomaceous earths, opaline cherts and shales, volcanic ashes or pumicites, calcined or uncalcined, and various other materials that require calcination to induce pozzolanic or cementitious properties, such as some shales and clays (9).
Table 2-2 gives the typical composition of Class F fly ash, Class C fly ash and Portland cement.
Table 2-2: Typical Chemical Composition of Fly Ash
Determining Fly Ash Quality (99)
The loss on ignition (LOI) is a very important factor for determining the quality of fly ash for use in concrete. The LOI values primarily represent residual carbonaceous material that may negatively impact fly ash use in air-entrained concrete. A low and consistent LOI value is desirable in minimizing the quantity of chemical admixtures used and producing consistent durable concrete. Activated carbon powder is sometimes now being used in power plant air quality control systems to remove mercury from combustion gases. Ordinary activated carbons that are commingled with fly ash can present two issues when used as a cementitious material in concrete. First, conventional activated carbon has a high affinity for air entraining admixtures, making predictable air content in concrete very difficult. This phenomenon may also be true for other chemical admixtures as well. Secondly, carbon particles can present aesthetic issues for architectural concrete in terms of a darker color or black surface speckles.
Another important fly ash parameter with respect to affecting concrete quality is fineness, which is a measure of the percent of material retained on the no. 325 sieve. The condition and the type of coal crusher can affect the particle size of the coal itself. A coarser ground coal may leave a higher percentage of unburned residues. Also, a coarser resulting fly ash gradation means there is less particle surface area of contact, which leads to a less reactive ash.
Uniformity of fly ash is important in most applications. The characteristics of the fly ash can change when a new coal source is introduced in the power plant. Each generating station's fly ash is different and it is important to determine its chemical and physical properties before it is used in commercial applications.
Based on the Unified Soil Classifications System, fly ash particles are primarily in the silt size range with the low end falling in the clay category and top end in the sand range. For geotechnical applications, fly ash is sometimes classified as a sandy silt or silty sand, having a group symbol of ML or SM (10).
The specific gravity of fly ash is generally lower than that of Portland cement, (SG = 3.15). We Energies fly ash sources typically range from a specific gravity of 2.05 to 2.68. Table 2-3 shows some typical geotechnical engineering properties of fly ash. These properties are useful when fly ash is designed for use in applications such as backfilling for retaining walls or constructing embankments.
Table 2-3: Typical Geotechnical Properties of Fly Ash
Major Fly Ash Uses
Class C fly ash has been widely used for soil stabilization. It can be incor-porated into the soil by disking or mixing (12). Fly ash can increase the subgrade support capacity for pavements and increase the shear strength of soils in embankment sections when proportioned, disked and compacted properly.
One of the ways that fly ash stabilizes soil is by acting as a drying agent. Soil with high moisture content can be difficult to compact during Spring and Fall. Adding fly ash to the soil and mixing will quickly reduce the moisture content of the soil to levels suitable for compaction. Fly ash has been widely used to reduce the shrink-swell potential of clay soils. The cementitious products formed by the hydration of fly ash bond with the clay particles. The swell potential is substantially reduced to levels comparable to lime treatment.
When fly ash is used to stabilize subgrades for pavements, or to stabilize backfill to reduce lateral earth pressure or to stabilize embankments to improve slope stability, better control of moisture content and compaction is required. The construction equipment needed for proper placement and compacting fly ash includes a bulldozer for spreading the material, a compactor (vibrating or pneumatic tired roller), a water truck to provide water for compaction (if needed) and to control dusting, and a motor grader, where final grade control is critical.
Class C and F fly ashes are pozzolanic and Class C fly ash is also cementitious. It reacts with calcium hydroxide produced by the hydration of cement in the presence of water to form additional cementitious compounds. This property of fly ash gives it wide acceptance in the concrete industry.
Class C fly ash has been successfully used in reconstructing and/or upgrading existing pavements. In this process, commonly known as cold-in-place recycling (CIR) or full depth reclamation (FDR), existing asphalt pavement is pulverized with its base, and the pulverized mixture is stabilized by the addition of fly ash and water. The cementitious and pozzolanic properties of fly ash enhance the stability of the section. Fly ash recycled pavement sections have structural capacities substantially higher than crushed stone aggregate base. A new asphaltic concrete or other wearing surface is then installed above the stabilized section.
Fly ash is a by-product pozzolan. The pozzolanic property of volcanic ash was known to the Romans almost 2000 years ago. Pozzolans are the vitamins that provide specific benefits to a particular mixture (13). The word “pozzolan” comes from the village of Pozzuoli, near Vesuvius, where volcanic ash was commonly used. The Romans used a mixture of lime and volcanic ash or burnt clay tiles in finely ground form as a cementing agent. The active silica and alumina in the ash combined with the lime and was used to produce early pozzolanic cement. Some of the old Roman structures like the Coliseum and the Pont du Gard are good examples of structures built with early volcanic ash cements (14).
Extensive research has been conducted in utilizing fly ash in concrete, masonry products, precast concrete, controlled low strength materials (CLSM), asphalt and other applications. These applications are discussed in the following chapters.
Properties of Bottom Ash
Bottom ash particles are much coarser than fly ash. The grain size typically ranges from fine sand to gravel in size. The chemical composition of bottom ash is similar to that of fly ash but typically contains greater quantities of carbon. Bottom ash tends to be relatively more inert because the particles are larger and more fused than fly ash. Since these particles are highly fused, they tend to show less pozzolanic activity and are less suited as a binder constituent in cement or concrete products. However, bottom ash can be used as a concrete aggregate or for several other civil engineering applications where sand, gravel and crushed stone are used. Table 2-4 shows the typical chemical composition of bottom ash obtained by burning bituminous coal and sub-bituminous coal.
Table 2-4: Chemical Composition of Bottom Ash
Table 2-5 shows the gradation of bottom ash from two We Energies power plants. The gradation of bottom ash can vary widely based on the coal pulverization and burning processes in the power plant, the variety of coal burned, and the bottom ash handling equipment. Table 2- 6 gives typical geotechnical properties of bottom ash produced from the combustion of bituminous coal. These values are based on research conducted in Australia (10). Table 2-7 shows some geotechnical properties of bottom ash from two We Energies power plants, based on studies performed by Gestra Engineering, Inc. in the USA.
Table 2-5: Gradation of Bottom Ash*
Table 2-6: Geotechnical Properties of Bottom Ash (10)
Table 2-7: Geotechnical Properties of Bottom Ash
Properties of Boiler Slag
Boiler slags are predominantly single-sized and within a range of 5.0 to 0.5 mm. Ordinarily, boiler slag particles have a smooth texture, but if gases are trapped in the slag as it is tapped from the furnace, the quenched slag will become somewhat vesicular or porous. Boiler slag from the burning of lignite or subbituminous coal tends to be more porous than that of the bituminous coals. The gradation of typical boiler slag is shown in Table 2-8. Compared to natural granular materials, the maximum dry density values of boiler slag are from 10 to 25% lower; while the optimum moisture content values are higher.
Table 2-8: Gradation of Boiler Slag (11)
Table 2-9 shows the chemical composition of boiler slag. The chemical composition of boiler slag is similar to that of bottom ash, as shown in Table 2-4, though the production process of boiler slag and bottom ash is relatively different.
Table 2-10 gives the typical geotechnical properties of the boiler slag. The friction angle of boiler slag is within the same range as those for sand and other conventional fine aggregates. Boiler slag exhibits high CBR value, comparable to those of high-quality base materials. Compared to bottom ash, boiler slag exhibits less abrasion and soundness loss because of its glassy surface texture and lower porosity (11).
Table 2-9: Chemical Composition of Selected Boiler Slag (11)
Table 2-10: Geotechnical Properties of Boiler Slag (11)
Boiler slag has been frequently used in hot mix asphalt because of its hard durable particles and resistance to surface wear. It can also be used in asphalt wearing surface mixtures because of its affinity for asphalt and its dust-free surface, thus increasing the asphalt adhesion and anti-stripping characteristics. Since boiler slag has a uniform particle size, it is usually mixed with other size aggregates to achieve the target gradation used in hot mix asphalt. Boiler slag has also been used very successfully as a seal coat aggregate for bituminous surface treatments to enhance skid resistance.
Properties of FGD Gypsum
FGD scrubber material is initially generated as calcium sulfite; but We Energies’ plants use wet FGD systems that utilize calcium-based sorbents and forced oxidation that converts calcium sulfite (CaSO3) to calcium sulfate (CaSO4). Since this process is carried out in the aqueous phase, FGD gypsum is produced. Calcium sulfite FGD scrubber material can be expansive and needs to be fixated or stabilized prior to most construction uses. FGD gypsum is frequently used for wallboard, in agriculture, and as a cement additive. Table 2-11 shows the typical physical properties (particle size and specific gravity) of calcium sulfite and calcium sulfate (gypsum), indicating gypsum is typically coarser than calcium sulfite (11). The purity of FGD gypsum typically ranges from 96%-99%, depending on the sorbent used for desulfurization. Table 2-12 presents the typical chemical composition of FGD gypsum (15) and Table 2-13 shows the typical geotechnical properties (16).
Table 2-11: Typical Particle Size (%) Properties of FGD
Compared to mined rock gypsum, the handling of fine grained FGD gypsum is more difficult because FGD gypsum is abrasive, sticky, compressive, and considerably finer (<0.2 mm). The adhesiveness of FGD gypsum decreases with the increase in particle size and the decrease of free water content. Temperature has little effect on the adhesiveness of FGD gypsum in storage. High temperatures, however, can cause a significant amount of degradation of FGD gypsum particles (15). The bulk physical properties of FGD gypsum are similar to silty sand and can be handled similarly. FGD gypsum is primarily crystalline in its morphology. The typical moisture content of FGD gypsum is in the range of about 5-15%. FGD gypsum can be transported by rail, truck, or barge and is easily transferred using mechanical conveyors.
Table 2-12: Typical Chemical Composition of
Table 2-13: Typical Geotechnical Properties of
The quantity of gypsum produced is directly proportional to the sulfur content of the fuel being used. Quality FGD gypsum material produced from wet scrubbers is currently being used for wallboard manufacture and for agricultural applications. Gypsum has reportedly been also utilized for road base or structural fill construction by blending with quicklime and pozzolanic fly ash, cement, or self-cementitious fly ash. Approximately 5% gypsum is used in the manufacturing of Portland cement to control the time of set. FGD gypsum in wet form can benefit the cement grinding process by introducing the inherent moisture into the ball mill, thus providing additional cooling.
Current We Energies CCP Sources
Fly ash, bottom ash and FGD gypsum are the predominant CCPs produced at We Energies’ six coal-fueled power plants. These power plants generate electricity for use by residential, industrial and commercial customers and also generate fly ash, bottom ash and gypsum as end products. We Energies together with regulators, universities, consultants and research institutions are committed to developing alternative environmentally protective beneficial use applications for fly ash, bottom ash and gypsum materials.
During the past three decades, several construction products have been developed and marketed. The beneficial utilization of these materials in agriculture, concrete and other construction products can preserve virgin resources, lower energy costs and yield high-performance materials. We Energies has conducted extensive testing of these products to evaluate their properties. The product test information is given in the following chapters to help potential users better understand the materials and potential applications.
Annual fly ash and bottom ash production at We Energies typically totals approximately 625,000 tons of which nearly 491,000 tons of fly ash and 100,000 tons of bottom ash was beneficially used in 2010 (18). In the same year, FGD Gypsum production at We Energies’ two power plants (PPPP and OCXP) totaled approximately 166,000 tons of which nearly 102,000 tons of gypsum were utilized in 2010. The breakdown by power plant is shown in Table 2-14. The primary uses of We Energies bottom ash include pavement and foundation sub-base materials and landfill drainage layer construction. For We Energies fly ash, the primary uses include cementitious material for concrete and concrete products, feedstock for Portland cement manufacture, and subsidence prevention in underground mines. Uses for We Energies FGD gypsum presently include agriculture and wallboard manufacturing.
Table 2-14: Annual Coal Combustion Products Production*
The following coal-fueled power plants are owned and operated by We Energies:
- Milwaukee County Power Plant (MCPP)
- Oak Creek Power Plant (OCPP)
- Oak Creek Expansion Units (OCXP)
- Pleasant Prairie Power Plant (PPPP)
- Valley Power Plant (VAPP)
- Presque Isle Power Plant (PIPP)
Of the above power plants, the first five are located in southeastern Wisconsin and the last, Presque Isle Power Plant, is located in upper Michigan.
Milwaukee County Power Plant (MCPP)
9250 Watertown Plank Road, Wauwatosa, Wisconsin 53226
Oak Creek Power Plant Units 5-8 (OCPP)
11060 S. Chicago Road, Oak Creek, Wisconsin 53154
Oak Creek Power Plant also has a 20,000 ton fly ash storage facility for winter production.
Oak Creek Expansion Units 1 and 2 (OCXP)
10800 S. Chicago Road, Oak Creek, Wisconsin 53154
The fly ash is removed by a baghouse and can be used in various construction activities (replacement for Portland cement in concrete, an ingredient in controlled low strength materials, and as a raw feed material for manufactured products). The bottom ash is removed from the bottom of the boiler and is used primarily as base material in place of aggregates beneath pavement and foundations. The FGD gypsum is used in wallboard manufacturing and agriculture.
The OCXP has installed Air Quality Control Systems (AQCS) on the new units to reduce nitrogen oxides by more than 85 percent, capture more than 99 percent of particulate matter, 97 percent of sulfur dioxide, and more than 90 percent of mercury. The AQCS consists of baghouses, Selective Catalytic Reduction (SCR), Wet Flue Gas Desulfurization (WFGD), and wet precipitator emission control components.
All bottom ash (from both OCPP and OCXP) is used by the company's designated bottom ash marketer, A.W. Oakes & Son. An on-site stock pile allows for beneficial use activities that require larger quantities of materials.
Pleasant Prairie Power Plant (PPPP)
8000 95th Street, Kenosha, Wisconsin 53142
PPPP was the first power plant in Wisconsin to get an advanced combustion technology, Air Quality Control System (AQCS) installed to reduce nitrogen oxide (NOx), sulfur dioxide (SO2) and mercury emissions. The AQCS consists of Selective Catalytic Reduction (SCR) and Wet Flue Gas Desulfurization (WFGD) emission control components.
The flue gas desulfurization (FGD) gypsum is produced in the wet scrubbing process for SO2 removal from coal combustion gases. It is used in wallboard manufacturing and agriculture. The FGD gypsum and the bottom ash are shipped to users or stored on a compacted high recycled content concrete “Eco-Pad” at this site.
Fly ash that is not immediately transported offsite by the Company’s designated fly ash marketer, Lafarge, can be stored on site in a company-owned 12,000 ton capacity storage building. All bottom ash is removed as necessary by the company's designated bottom ash marketer, A.W. Oakes & Son, who manages a stockpile for this product on site. The stockpile allows for beneficial use activities that require larger quantities of material.
Port Washington Generating Station (PWGS) replaced Port Washington Power Plant (PWPP) – Retired in 2004
146 South Wisconsin Street, Port Washington, Wisconsin 53074
Valley Power Plant (VAPP)
1035 West Canal Street, Milwaukee, Wisconsin 53233
Presque Isle Power Plant (PIPP)
2701 Lake Shore Boulevard, Marquette, MI 49855
In 2004, a TOXECON unit was installed on the combined flue gas stream of Units 7, 8, and 9. “TOXECON is an integrated emission control system that achieves high levels of mercury removal, increases the collection efficiency of particulate matter (PM) and determines the viability of sorbent injection for SO2 and NOx control, while maximizing the use of coal combustion by-products” (17). The PIPP TOXECON unit uses activated carbon as a sorbent, and the by-product is about a 50/50 blend of ultrafine Class C fly ash and spent activated carbon sorbent. About 400 tons of this material is presently being landfilled each year.
We Energies is committed to developing and implementing full utilization of its CCPs. The company is working with several research groups, universities, regulators, consultants, and trade associations to develop environmentally friendly “green” products and applications for its CCPs. We Energies gas and electric utility service area is shown on Figure
Figure 2-4: Wisconsin Energy Corporation Service Territories and Generation Facilities
Chapter 3 - Properties of We Energies Coal Combustion Products
Fly ash, bottom ash, and flue gas desulfurization (FGD) produced at the coal-fueled power plants that are owned and operated by We Energies have been subjected to extensive tests for physical and chemical properties. The type of coal, percentage of incombustible matter in the coal, sulfur content, the pulverization process, furnace types and the efficiency of the combustion process determine the chemical composition of the coal combustion products (CCP).
Another factor affecting the quality of CCPs is whether the power plant is base loaded or frequently being brought in and out of service. A base loaded plant operates at consistent temperatures and combustion rates. Plants that are frequently changing load or coming in and out of service tend to produce more variability in coal ash characteristics. The use of low NOx burners at power plants has sometimes resulted in an increase in loss on ignition and carbon content in the fly ash. Other NOx reduction technologies such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) have sometimes added ammonia to fly ash with associated odors. Depending on the configuration of other air quality control systems for SOx and Hg removal, the potential exists to also effect fly ash quality characteristics. We Energies has taken measures in early system design planning to minimize or eliminate the effects by applying these controls after the fly ash is collected.
Figure 3-1: Fly ash particles are spherical and average about 10 microns in diameter
We Energies purchases coal from several mines. Various factors affect the selection of coal sources, but quality and cost of coal are two very important considerations. The consistency of fly ash does not change significantly if the coal used in the plant is from a single geological formation or from a consistent blend of coals. But when coal sources change, the chemical and physical properties of the fly ash may change significantly if the type or chemistry of coal is changed. At times, coal from different sources may be blended to improve air emissions, to reduce generation costs, to increase the efficiency of combustion and/or to improve the quality of fly ash generated.
Physical, Chemical and Mechanical Properties of Fly Ash
Table 3-1 gives the chemical composition of fly ash from various We Energies power plants. The results shown are based on tests performed at We Energies state-certified lab and other outside certified testing facilities. We Energies fly ash marketers have on-site labs that test the fly ash generated from the power plant daily and more often if warranted. The quality and chemical composition of fly ash do not change very often because coal is usually purchased on long-term contracts. Fly ash from We Energies plants has actually been more consistent than many Portland cement sources.
Figures 3-2 and 3-3 show the fineness consistency and loss on ignition for Pleasant Prairie's fly ash. A customer may request samples for independent testing on a particular fly ash to independently determine its properties. As can be seen from Table 3-1, the chemical composition of fly ash differs from plant to plant and sometimes from unit to unit within a power plant.
Table 3-1: Chemical Composition of We Energies Fly Ash*
Fly ash is classified as Class F or Class C by ASTM C-618 based on its chemical and physical composition. We Energies contracts with marketers that distribute and test fly ash to ensure that customer supply, quality and consistency requirements are met.
The chemical composition of We Energies fly ash generated by burning sub-bituminous coal is different from that generated by burning bituminous coal. For example, burning 100% Wyoming Powder River Basin (PRB) sub-bituminous coal produces fly ash with calcium oxide content, typically in the range of 16 to 28%. However, burning 100% bituminous coal generates a fly ash with calcium oxide content in the range of 1 to 4%.
Figure 3-2: Fineness Consistency of PPPP Fly Ash
According to ASTM C-618, when the sum of SiO2, Al2O3 and Fe2O3 is greater than 70%, the fly ash can be classified as Class F and when the sum is greater than 50% it can be classified as Class C fly ash. The fly ash must also meet the ASTM C-618 limits for SO3, loss on ignition, fineness and other requirements.
Presque Isle Power Plant generated both Class C and Class F fly ash and had separate silos for each variety (see Table 3-1). By reviewing the chemical composition of fly ash from each plant, it is easy to determine if the fly ash is Class C or Class F and to select an ash that best meets end use requirements. In November of 2011, PIPP Units 5-6 switched from bituminous coal to subbituminous coal, and the fly ash now meets the ASTM C-618, Class C criteria.
By graphing individual parameter test results, it is possible to identify any significant changes. This is helpful in order to determine if a specific fly ash is suitable for a particular application or whether a blend of one or more materials is needed.
Figure 3-3: Loss on Ignition Consistency for PPPP Fly Ash
Table 3-2 shows the physical properties of fly ash at various We Energies power plants, along with the ASTM standard requirements.
Table 3-2: Fly Ash Physical Properties
Physical, Chemical and Mechanical Properties of Bottom Ash
The coal combustion process also generates bottom ash, which is second in volume to the fly ash. Bottom ash is a dark gray, black, or brown granular, porous, predominantly sand size material. The characteristics of the bottom ash depend on the type of furnace used to burn the coal, the variety of coal, the transport system (wet or dry), and whether the bottom ash is ground prior to transport and storage. We Energies generates over 106,000 tons of bottom ash each year at its coal-fired power plants.
The coal combustion process also generates bottom ash, which is second in volume to the fly ash. Bottom ash is a dark gray, black, or brown granular, porous, predominantly sand size material. The characteristics of the bottom ash depend on the type of furnace used to burn the coal, the variety of coal, the transport system (wet or dry), and whether the bottom ash is ground prior to transport and storage. We Energies generates over 106,000 tons of bottom ash each year at its coal-fired power plants.
Figure 3-4: Bottom ash
It is important that the physical, chemical and mechanical properties of bottom ash be studied before it can be beneficially utilized. The primary chemical constituents of We Energies bottom ash are shown in Table 3- 3. The chemical characteristics of bottom ash are generally not as critical as for fly ash, which is often used in concrete, where cementitious properties and pozzolanic properties are important.
Table 3-3: Chemical Composition of We Energies
In the case of bottom ash, physical and mechanical properties are critical. We Energies has been studying the properties of bottom ash that are important in construction applications for comparison to virgin materials currently dominating the market.
An additional consideration for bottom ash is its staining potential if used as an aggregate in concrete masonry products. Staining can occur if certain iron compounds such as pyrite are present. Pyrites can also present a potential for corrosion of buried metals. For these applications, it is important to identify if pyrites exist in sufficient quantity to present a problem (> 3.0 %).
Moisture-Density Relationship (ASTM D1557)
Bottom ash samples were tested to determine maximum dry density and optimum moisture content per the ASTM D-1557 test method. The test results are shown in Table 3-4.
Table 3-4: Physical Properties of Bottom Ash*
We Energies bottom ashes are generally angular particles with a rough surface texture. The dry density of bottom ash is lower than sand or other granular materials typically used in backfilling.
The grain size distribution is shown in Table 3-5; Figures 3-5 through 3-10 show the grain size distribution curves for the various We Energies bottom ashes tested during 2011 following the U.S standards.
Engineering Properties of We Energies Bottom Ash
Unlike fly ash, the primary application of bottom ash is as an alternative for aggregates in applications such as sub-base and base courses under rigid and flexible pavements. It has also been used as a coarse aggregate for hot mix asphalt (HMA) and as an aggregate in masonry products. In these applications, the chemical properties are generally not a critical factor in utilizing bottom ash.
However, some engineering properties of the material are important and may need to be evaluated. These properties influence the performance of the material when exposed to freezing and thawing conditions and associated stress cycles.
Table 3-5: Bottom Ash - Grain Size Distribution (ASTM D-422)
The major test procedures and standards established by AASHTO and followed by many Transportation and highway departments, including the Wisconsin Department of Transportation (WisDOT) and Michigan Department of Transportation (MODOT), are listed in Table 3-6.
Table 3-6: AASHTO Test Procedures
Figure 3-5: PPPP Bottom Ash Grain Size Distribution Curve (2011)
Figure 3-6: MCPP Bottom Ash Grain Size Distribution Curve (2011)
Figure 3-7: OCPP Bottom Ash Grain Size Distribution Curve (2011)
Figure 3-8: OCXP Bottom Ash Grain Size Distribution Curve (2011)
Figure 3-9: VAPP Bottom Ash Grain Size Distribution Curve (2011)
Figure 3-10: PIPP Bottom Ash Grain Size Distribution Curve (2011)
Results of Testing Bottom Ash to AASHTO Standards
In early 1994, 2004 and 2011, testing was performed on We Energies bottom ash to evaluate its use as a base course material, as granular fill for subbase and as a coarse aggregate for hot mix asphalt (HMA), following the procedures in the AASHTO Standards. The test results were then compared with the requirements in the WisDOT’s standard specifications (19) and the MDOT’s standard specifications for construction (20). The test results are tabulated in Tables 3-7 and 3-8.
Atterberg Limit tests were performed on Pleasant Prairie, Oak Creek Expansion and Presque Isle bottom ashes. The results show that all three materials tested are non-liquid and non-plastic. Section 301.2.3.5 of WisDOT Standard Specifications require that the base course aggregate not have a liquid limit of more than 25 and not have a plastic index of more than 6. WisDOT standard specifications do not identify a maximum liquid limit for hot mix asphalt coarse aggregate. Therefore, the bottom ash materials meet the WisDOT standard specification requirements for Atterberg Limits.
The Los Angeles Abrasion test results showed that the bottom ash samples tested were not as sound or durable as natural aggregate. However, the test results fall within the WisDOT limits of maximum 50% loss by abrasion for Mixtures E-0.3 and E-1.
WisDOT standard specifications require a minimum 58% fracture face for dense base course aggregate. The bottom ash also meets these specifications.
MDOT specifications limit a maximum loss of 50% for dense graded aggregates. Other grades of aggregates have a lower limit on abrasion loss. Hence, the samples tested meet only MDOT specifications for dense graded aggregates.
Pleasant Prairie and Oak Creek Expansion bottom ash meet the requirements of WisDOT section 460.2.2.3 of the Standard specifications for coarse aggregate for the HMA, Presque Isle bottom ash did not meet this requirement. However, Pleasant Prairie, Oak Creek Expansion and Presque Isle bottom ash did not meet the gradation requirements of WisDOT section 305.2.2.1 of the Standard Specifications for base course aggregate. The material requires blending with other aggregates and/or screening to meet requirements of WisDOT sections 305.2.2.1 and 460.2.2.3.
Pleasant Prairie, Oak Creek Expansion and Presque Isle bottom ash met the gradation requirements for Grade 2 granular fill specified by WisDOT although these materials need to be blended, washed or screened to meet the WisDOT specification for Grade 1 granular fill.
Table 3-7: Summary Of We Energies Bottom Ash Test Data and Comparison to WisDOT Specifications (19)
Table 3-8: Summary of We Energies Bottom Ash Test Data and Comparison to Michigan DOT Specifications (20)
Soundness test results for all three samples are well within the allowable limits per section 301.2.3.5 and section 460.2.7 of the WisDOT standard specifications with maximum % loss of 18% and 12%, respectively. MDOT specifies a maximum percent material loss by washing through the No. 200 sieve in lieu of the soundness test. Since MDOT relies on results of freeze-thaw durability for soundness requirements, the AASHTO T-103 limits to 20% for freeze-thaw durability.
Physical and Chemical Properties of We Energies Flue Gas Desulfurization (FGD) Gypsum
FGD Gypsum
As part of We Energies environmental commitment to reduce emissions and minimize landfilling of coal combustion products, the company has installed FGD systems that produce a high purity gypsum by-product. The FGD gypsum is composed of tetrahedron crystals, ranging on average from 40-50 µm in particle size, appears light brown in color, with soil-like consistency, no odor, and low moisture content. It is chemically known as calcium sulfate dihydrate (CaSO4.2H2O). The typical characteristics and the chemical composition are shown on Table 3-9. We Energies generates over 166,000 tons of FGD gypsum each year cumulatively at Pleasant Prairie and the Oak Creek site power plants. The gypsum is used for wallboard manufacturing and agriculture.
Figure 3-11: We Energies FGD Gypsum
Table 3-9: Typical Characteristics of We Energies FGD Gypsum*
One important application of FGD gypsum is in agriculture. Due to local production, Wisconsin farmers have benefited economically by using FGD gypsum over mined natural gypsum. It provides soil and plant nutrients and also improves the soil’s physical and chemical properties. It increases the soil permeability and water infiltration reducing erosion and lowering silt loadings in field runoff. The fine particle size of synthetic gypsum makes it soluble, releasing calcium (Ca2+) and sulfate (SO42-) ions. The Ca2+ provides structural support and enzyme signal activation, perception and transduction as an addition to the plant nutrients (21). By spreading gypsum to the soil, it doesn’t alter the pH but rather neutralizes some acidity on a short-term basis. The neutralization occurs as the SO42- displaces OH- from the iron and aluminum hydrated oxide on soil surfaces.
The purity of FGD gypsum (> 95%) is an advantage over most natural rock gypsums (purity range of 80% to 96%) when used for wallboard for the purpose of lowering the weight of gypsum board. Table 3-10 presents the geotechnical properties and Figure 3-12 shows the grain size distribution curve for FGD gypsum produced at the Pleasant Prairie power plant.
Table 3-10: Geotechnical Properties of PPPP Gypsum*
Figure 3-12: PPPP Gypsum Grain Size Distribution Curve (2009)
Overview of the Chemical Reaction from a Wet-Limestone Scrubber (22)
Flue-gas scrubbing is a stepped chemistry process (Figure 3-13), where the overall reaction is a classic example of aqueous acid-base chemistry applied on an industrial scale. The limestone slurry (composed primarily of calcium carbonate, CaCO3) reacts with acidic sulfur dioxide, as represented in Equation 1.
CaCO3 + 2H⁺ + SO3⁻² → Ca⁺² + SO3⁻² + H2O + CO2↑ [1]
In the absence of any other reactants, calcium and sulfite ions will precipitate as a hemihydrate, where water is actually included in the crystal lattice of the scrubber byproduct.
Ca⁺² + SO3⁻² + ½H2O → CaSO3 · ½H2O↓ [2]
Many wet-limestone scrubbers operate at a solution pH of around 5.6 to 5.8. A very acidic scrubbing solution inhibits SO2 transfer from gas to liquid; while excessive basic slurry (pH > 6.0) indicates an overfeed of limestone.
The oxygen in the flue gas greatly influences chemistry. Aqueous bisulfite and sulfite ions react with oxygen to produce sulfate ions (SO4-2).
2SO3⁻² + O2 → 2SO4⁻² [3]
Approximately the first 15 mole percent of sulfate ions co-precipitates with sulfite to form calcium sulfite-sulfate hemihydrate [(0.85CaSO3·0.15CaSO4) ·½H2O]. Any sulfate above the 15 percent mole ratio precipitates with calcium as gypsum.
Ca⁺² + SO4⁻² + 2H2O → CaSO4·2H2O↓ [4]
In summation, for every part of SO2 removed from the flue gas, one part of calcium carbonate from the limestone must react with it. Hence, for every part of SO2 removed, one part of gypsum by-product is generated.
SO2 + CaCO3 → CaSO3 · ½H2O + CO2 + O2 → CaSO4·2H2O (gypsum)
FGD Filter Cake
We Energies power plants also produce filtered solids out of waste water treatment during the process of removing sulfur dioxide using wet FGD systems, known as the FGD Filter Cake. The process is shown in Figure 3-14. The filter cake is a brown clay-like chunk with about 107% water content. The 2011 production of FGD Filter Cake was 1624 tons from PPPP and 3477 tons from OCXP compared to 2010, with an estimated production of 1540 tons from PPPP and 2204 tons from OCXP. Presently, the FGD filter cake is being stored for use as an internal landfill leveling layer.
In 2008, FGD filter cake from PPPP was tested from a stockpile and its geotechnical properties are shown on Table 3-11. Table 3-12 shows the chemical composition of FGD filter cake from samples collected in 2011 at OCXP and NR538 PPPP FGD Filter Cake leachate test results are summarized in chapter 9 and was found to contain chloride, sulfate, boron, selenium, strontium and arsenic amongst other expected compounds. Thus, the filter cake material needs to be stabilized before it can be used for construction.
In the summer of 2009, a landfill access road (stretching approximately 425 feet long) was constructed to support heavy loaded multi-axle truck traffic at the Pleasant Prairie Power Plant. For a research based demonstration, the FGD filter cake was stabilized with the addition to PPPP Class C Fly ash for use in stabilizing a road base. The 12-inch base consisted of 80% recycled concrete, 20% FGD filter cake and 120 lbs/yd2 of Class C fly ash. During the construction, rain complicated completion of the project because the handling and the compaction of the material became difficult due to an affinity for water. However construction was successfully completed and 2009 Falling Weight Deflectometer test results indicated that stabilization of the recycled crushed concrete with fly ash and filter cake likely increased the base course layer strength significantly. The road continues to provide good service and performance. Perhaps one day in the future, the minerals contained in the FGD filter cake can be evaluated further for additional applications.
Table 3-11: Geotechnical Properties of PPPP FGD Filter Cake
Table 3-12: Chemical Composition of We Energies
Figure 3-13: Schematic Diagram of a typical We Energies Wet FGD system.
Figure 3-14: Schematic diagram of the production of FGD filter cake from the wet scrubber waste water treatment process.
Chapter 4 - Concrete and Concrete Masonry
Introduction
Coal combustion products have been used in the construction industry since the 1930’s (8). Although the utilization of these products was limited to small-scale applications in the early days, the use of coal combustion products has gained increasing acceptance in the construction industry in the last few decades. The interest in coal combustion products significantly increased during the 1970’s because of the rapid increase in energy costs and the corresponding increase in cement costs.
We Energies has been conducting extensive research to beneficially utilize fly ash, bottom ash and FGD gypsum generated at company-owned coal-fueled power plants for construction and agricultural applications. Many of these research efforts have been conducted in conjunction with universities, research centers and consultants, resulting in the development of cost effective and environmentally friendly products.
Today, We Energies fly ash, bottom ash and FGD gypsum are being widely used in the construction industry. Applications range from utilizing fly ash in the manufacture of concrete, concrete products, controlled low strength material (CLSM), liquid waste stabilization, roller-compacted no fines concrete, high-volume fly ash concrete, cold-in -place recycling of asphalt, lightweight aggregate, and soil stabilization. Of all these applications, the use of fly ash as an important ingredient in the production of concrete is by far the largest application.
Background on Hydration Reaction, Cementitious, And Pozzolanic Activity
To understand the behavior of fly ash in contact with water or in a concrete mixture, it is important to understand the reaction that takes place in freshly mixed concrete and the process by which it gains strength. The setting and hardening process of concrete, which occurs after the four basic components consisting of coarse aggregate, fine aggregate, cement and water are mixed together, is largely due to the reaction between the components cement and water. The other two components, coarse aggregate and fine aggregate, are more or less inert as far as setting and hardening is concerned.
The major components of cement that react with water to produce hydration reaction products are tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF). The reactions can be summarized as shown below:
In the absence of sulfate, C3A may form the following reaction products (8):
The major reaction that takes place is between the reactive silica of the pozzolan and calcium hydroxide producing calcium silicate hydrate. The alumina in the pozzolan may also react with calcium hydroxide and other components in the mixture to form similar products.
High-calcium fly ash is both cementitious and pozzolanic and has self-hardening properties in the presence of moisture. The reaction products include ettringite, monosulphoaluminate and C-S-H. These products are also formed when cement reacts with water and causes hardening in the cement-water mixture.
The rate of formation of C-S-H in the fly ash-water mixture is normally slower than that in a cement-water mixture. Because of this, at ages greater than 90 days, fly ash-cement-water continues to gain strength; while the cement-water pastes do not show as significant a gain in strength. However, this hydration behavior of C3A and C2S in fly ash is the same as that in cement. Low calcium fly ash has very little or no cementing properties alone, but will hydrate when alkalis and Ca(OH)2 are added.
Concrete Containing We Energies Fly Ash
For centuries, concrete has been widely used for a variety of applications ranging from sidewalk slabs to bridges and tall buildings. Concrete used in the early days had low strength and the applications were limited, partly due to the strength of the concrete and partly due to the lack of understanding of design principles.
With the evolution of more sophisticated materials and engineering designs, many problems associated with strength were solved and high -strength concrete designs were developed. Today, engineers can select a concrete mixture with a specified strength for a particular application. In most cases, strength of concrete is not a limiting factor in project design.
Durability of concrete has been a challenge since the early days of concrete production. With applications increasing, the demand to find concrete that “performs” is increasing. Most durability problems associated with concrete get worse in adverse weather conditions. For example, in cold weather regions, concrete that is subjected to freezing and thawing tends to disintegrate faster if it is porous. Porosity is generally considered the most significant factor affecting the long-term performance of concrete.
Portland cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water. The properties of concrete prepared by mixing these four components depends on the physical and chemical properties and the proportions in which they are mixed. The properties of concrete produced can be enhanced for specific applications by adding admixtures and/or additives.
The use of a particular admixture or additive has a definite purpose. For a particular application, it is important that the properties of the concrete be tailored to meet performance requirements.
Fly ash added in concrete as a supplementary cementing material achieves one or more of the following benefits:
- Reduces the cement content.
- Reduces heat of hydration.
- Improves workability of concrete.
- Attains higher levels of strength in concrete especially in the long term.
- Improves durability of concrete.
- Increases the “green” recycled material content of concrete.
- Attains a higher density.
- Lowers porosity and permeability.
The properties of fly ash, whether ASTM C-618, Class C or Class F, and the percentages in which they are used greatly affect the properties of concrete. Mixture proportioning and trial batches are critical to obtaining concrete with the desired fresh and hardened properties. Fly ash may be introduced in concrete as a blended cement containing fly ash or introduced as a separate component at the mixing stage.
Most of the We Energies fly ash is being used in concrete as a separate component at the concrete batching and mixing stage. This allows the flexibility of tailoring mixture proportions to obtain the required concrete properties for the particular application. Ready-mixed concrete producers have greater control with respect to the class and amount of fly ash in the concrete mixture to meet the specified performance requirements.
Fly ash has several other properties, in addition to its cementitious and pozzolanic properties, that are beneficial to the concrete industry (24). Low-calcium fly ash (ASTM C-618 Class F) has been used as a replacement for Portland cement in concrete used for the construction of mass gravity dams. The primary reason for this application has been the reduced heat of hydration of Class F fly ash concrete compared to Portland cement concrete. ASTM C-618 Class C fly ash concrete may also have a slightly lower heat of hydration when compared to Portland cement concrete. However, low calcium Class F fly ash concrete generates still lower heat of hydration, a desirable property in massive concrete construction, such as dams and large foundations.
Studies have also revealed that certain pozzolans increase the life expectancy of concrete structures. Dunstan reported that as the calcium oxide content of ash increases above a lower limit of 5% or as the ferric oxide content decreases, sulfate resistance decreases (25).
Dunstan proposed the use of a resistance factor (R), calculated as follows:
R = (C-5)/F
Where C = percentage of CaO
Where F = percentage of Fe2O3
Dunstan summarized his work in terms of the selection of fly ash for sulfate-resistant concrete as follows (25):
Alkali-aggregate reactions (AAR) also cause expansion and damage in concretes produced with reactive aggregates and available alkalis from the paste. However, a variety of natural and artificial pozzolans and mineral admixtures, including fly ash, can be effective in reducing the damage caused by AAR. Researchers have reported that the effectiveness of fly ash in reducing expansion due to AAR is limited to reactions involving siliceous aggregate. The reactive silica in power plant fly ash combines with the cement alkalis more readily than the silica in aggregate. The resulting calcium-alkali-silica “gel” is nonexpansive, unlike the water- absorbing expansive gels produced by alkali-aggregate reactions. In addition, adding fly ash to concrete increases ASR resistance and improves the concrete’s ultimate strength and durability while lowering costs.
The following factors are important in determining the effectiveness of using fly ash to control AAR.
- The concentration of soluble alkali in the system.
- The amount of reactive silica in the aggregate.
- The quantity of fly ash used.
- The class of fly ash.
According to Electric Power Research Institute (EPRI) studies (26), both Class C and Class F fly ash can be effective at mitigating ASR in concrete when used as substitutes for Portland cement. The major difference between the two ash types is that a greater portion of cement must be replaced with Class C ash to provide the same effect as using Class F ash in a mix design with a smaller ash-to-cement ratio. According to EPRI studies, replacing Portland cement with Class C ash at volumetric rates of 30-50% is effective in controlling ASR. The greater the proportion of Class C fly ash used in a mix, the greater the ASR control benefit.
The concentration of soluble (available) alkali and not the total alkali content is critical for the reaction. Studies have shown that if the acid soluble alkali-content is in excess of 5.73 lb/cu yd, then it can cause cracking, provided that reactive aggregates are present. (This is approximately equivalent to 4.21 lb/cu yd as water- soluble alkali.) For high-calcium Class C fly ash, the amount of alkali in the ash affects the effectiveness of expansion reduction. Another study by EPRI (27) indicated that for high-calcium (22.5% CaO) moderate-alkali (2.30% Na2Oeq) fly ash, the amount of fly ash required to control expansion due to ASR varies significantly from one aggregate to another. In the case of the extremely reactive aggregate, between 50%-60% of fly ash would be required to reduce expansion under the 0.10% level. For less reactive aggregate, a lower fly ash replacement level is required. Even high-calcium (21.0% CaO) high-alkali (5.83% Na2Oeq) fly ash contributed in reducing ASR expansion; however, an expansion higher than 0.10% level occurred. Therefore, it is necessary to test the amount of alkali in the fly ash prior to incorporating it in the concrete to control ASR.
The following aggregates and their mineralogical constituents are known to react with alkalis:
- Silica materials - opal, chalcedony, tridymite and cristobalite
- Zeolites, especially heulandite
- Glassy to cryptocrystalline rhyolites, dacites, andesites and their tuffs
- Certain phyllites
Low-calcium (ASTM C-618, Class F) fly ash is most effective in reducing expansion caused by alkali-silica reactions where the fly ash is used at a replacement level of approximately 20% to 30%. Once the replacement threshold has been reached, the reduction in expansive reaction for a given cement alkali level is dramatic. Additionally, the greater the proportion of cement replaced with Class F fly ash, the greater the ASR reduction. In some cases where silica fume, a very fine material that is high in reactive SiO2, is used in concrete for high strength, adding Class F or Class C fly ash to create a “ternary blend” can significantly reduce ASR susceptibility without diminishing concrete performance. The actual reaction mechanism for the alkali-aggregate reaction and the effect of fly ash is not fully understood today and will require more research to find a satisfactory explanation.
Soundness of aggregates or the freedom from expansive cracking is one of the most important factors affecting the durability of concrete. At early ages, unloaded concrete cracks because of two reasons: thermal contraction and drying shrinkage. When concrete hardens under ambient temperature and humidity, it experiences both thermal and drying shrinkage strains.
The level of shrinkage strain depends on several factors, including temperature, humidity, mixture proportions, type of aggregates, etc. Shrinkage strain in hardened concrete induces elastic tensile stress. Cracks appear in concrete when the induced tensile stress exceeds the tensile strength of the concrete. Creep may reduce the induced tensile stress to a certain extent, but the resultant stress can be large enough for cracking concrete.
Using sufficient steel reinforcement has traditionally controlled cracking. However, using reinforcement does not solve this problem completely. By using reinforcement, fewer large cracks may be reduced to numerous invisible and immeasurable micro-cracks (28). Transverse cracks seen in bridge decks are typical examples. Cracking in concrete is the first step to deterioration, as it results in the migration of harmful ions into the interior of concrete and to the reinforcement.
Several preventive and mitigating measures can be used to minimize the degradation of concrete due to corrosion of reinforcing steel. The use of fly ash as a partial replacement for cement is a cost-effective solution (inclusion of fly ash in a mixture provides the same workability at a lower water content and lower cement content both of which reduces the concrete shrinkage). In several states across the country, the Department of Transportation (DOT) has made it mandatory to include fly ash as an ingredient. The heat of hydration is substantially reduced when fly ash is used in concrete as a partial replacement to cement.
Durability of concrete is very critical in most DOT applications, especially in regions subject to cold weather conditions. In such cases, the incorporation of fly ash in concrete is advantageous, even though the setting and hardening process may be slightly slower than ordinary Portland cement concrete.
Fly ash has been used in concrete for several decades. Research work on short-term and long-term behavior of concrete containing fly ash has been conducted by several research groups. However, the properties of fly ash vary with the specific coal burned as well as the process of coal preparation, firing and collection.
Hence, We Energies has conducted research on the actual fly ash generated at its coal-fueled plants. This research has been conducted with the aid of universities and research institutions in conjunction with concrete producers to develop mix designs that can be readily used for construction. Several parameters, both short- term and long- term, have been studied, and their performances evaluated to identify the suitability of the particular mixture design for a specific field application. One important point is the spherical shape of fly ash with its lubricating effect for pumping and providing the same workability with a lower water to cementitious materials ratio. Also, fly ash is usually finer than Portland cement and thus produces a denser concrete with lower permeability.
Compressive Strength of Concrete Containing We Energies ASTM C-618, Class C Fly Ash (Phase I Study)
Concrete is used in several applications requiring different levels of strength. Most applications require concrete with a compressive strength in the range of 3,000 to 5,000 psi. Based on the type of application, engineers select a mixture design with a specified 28-day compressive strength. Other durability factors such as porosity or freeze-thaw resistance also influence the selection of a concrete mixture.
With the introduction of fly ash concrete, the long-term (56 day or 1 year) properties of concrete have shown dramatic improvement. Since long-term properties of concrete are vital, most construction professionals are interested in understanding the performance of fly ash and the resulting concrete made using fly ash.
The influence of We Energies fly ash on the quality of concrete has been studied for several years. Fly ash is used as a partial replacement for cement at various replacement levels. In order to understand the properties of We Energies fly ash and the short-term and long-term performance of concrete containing We Energies fly ash, a great amount of research work has been conducted.
The following data is from a research project conducted at the Center for By-Products Utilization at the University of Wisconsin-Milwaukee for We Energies (29). This work was done with the objective of developing mixture proportions for structural grade concrete containing large volumes of fly ash. ASTM C-618, Class C fly ash from We Energies Pleasant Prairie Power Plant was used in this research project.
Preliminary mixture proportions were developed for producing concrete on a 1.25 to 1 fly ash to cement weight basis replacement ratio. The replacement levels varied from 0% to 60% in 10% increments. Water to cementitious materials ratios (w/c) of 0.45, 0.55 and 0.65 were used in this project to develop concrete with strength levels of 3,000 psi; 4,000 psi and 5,000 psi. It is interesting to observe that at fly ash utilization levels rising above 50%, Portland cement becomes the admixture or supplementary cementitious material.
Actual concrete production was performed at two local ready mixed concrete plants utilizing different cement and aggregate sources. Three quarter inch maximum size aggregates were used in the mixtures and the slump was maintained at 4”± 1”. Entrained air was maintained in the range of 5-6% ± 1%. The concrete mixtures were prepared at ready mixed concrete plants using accepted industry practices. Six-inch diameter by 12” long cylinder specimens were prepared for compressive strength tests. The compressive strength tests were performed at various ages in accordance with standard ASTM test methods. The chemical and physical properties of PPPP fly ash used in these tests are shown in Table 4-1.
Tables 4-2 to 4-4 show the mixtures designed for concrete in the various strength levels and various percentages of cement replacement with fly ash. The compressive strength results are shown in Tables 4-5 to 4-7.
Table 4-1: Chemical and Physical Test Data Pleasant Prairie Power Plant (PPPP) Fly Ash
Discussion of Test Results - 3,000 psi Concrete
Compressive strength test results for the six different 3000 psi concrete mixtures are shown in Table 4-2. The specified strength for these mixtures is 3,000 psi. These test results show that with an increase in cement replacement levels with fly ash, the early age compressive strength decreases.
The decrease is not significant for concrete with 20% and 30% replacement levels. At the 7-day age, cement replacement with up to a 40% replacement level produces concrete with compressive strength comparable to that of the control mix. At the 28-day age, all mixtures showed strength levels higher than the design compressive strength of 3,000 psi. However, concrete containing 40% replacement of cement with fly ash had the highest strength.
Table 4-2: PPPP Class C Fly Ash Concrete Mix and Test Data - 3000 psi (21 MPa) Specified Strength
Table 4-3: PPPP Fly Ash Concrete Mix and Test Data 4000 psi (28 MPa) Specified Strength
As the age of concrete increased, the compressive strength of all concrete mixtures containing fly ash increased at a level higher than that of the control mix. Concrete with 40% replacement of cement with fly ash continued to show the highest strength level, but all fly ash concrete mixtures showed strength levels higher than that of the control mix at the 56- and 91-day ages.
Discussion of Test Results - 4,000 psi Concrete
Mixes P4-7 through P4-12 were designed for a compressive strength of 4,000 psi. At an age of 3 days, 20% fly ash concrete showed the highest strength.
At the 7-day age, concrete with up to 50% cement replacement showed compressive strength levels comparable to that of the control mix P4-7. Mixes P4-8 and P4-9 with 20% and 30% replacements showed strengths higher than the control mixture at the 7-day age.
At the 28-day age, all mixtures showed strengths higher than the design strength of 4,000 psi. Also, all mixtures containing fly ash showed higher levels of strength compared to the control mix. Mix P4-10 with 40% replacement of cement showed the maximum strength.
This trend continued at later ages with P4-11, the 50% replacement of cement with fly ash, showing the highest strength of 7,387 psi at the 91-day age.
Table 4-4: PPPP Class C Fly Ash Concrete Mix and Test Data 5000 psi (34 MPa) Specified Strength
Discussion of Test Results: 5,000 psi Concrete
Mixes P4-13 to P4-18 were designed with a 28-day compressive strength of 5,000 psi. At the 3-day age, concrete with 20% cement replacement showed compressive strength higher than that of the control mix P4-13.
However, concrete with up to 40% cement replacement showed compressive strength in the acceptable range. At the 7-day age, concrete with up to 40% cement replacement showed strength comparable to the control mix. At the 28-day age, all mixes showed strengths higher than the design strength of 5,000 psi. Also, all fly ash concrete mixes showed strengths higher than the control mix, with the 40% cement replacement concrete showing the highest strength.
At the 56- and 91-day ages, the trend continued with the 50% cement replacement concrete showing the highest strength. Even the 60% replacement concrete showed 38% higher strength compared to the control mix at the 91-day age.
Conclusions: 3000 psi; 4000 psi and 5000 psi Concrete
In conclusion, these tests establish that good quality structural concrete can be made with high cement replacements by fly ash. Even 50% and 60% replacements showed higher strengths than the control mixture at 56- and 91-day ages. But this level of cement replacement with fly ash generally will not be made for structural grade concrete for flexural members, such as beams where rapid form stripping is required.
However, these higher replacements may be used for mass concrete where temperature control is needed and early age strength levels are not needed. At the 40% cement replacement level, the strength levels at early ages are within acceptable limits and can be used for structural grade concrete.
Therefore, it can be concluded that fly ash from Pleasant Prairie Power Plant can be used in the manufacture of structural grade concrete with cement replacement levels of up to 40%, on a 1.25 to 1 fly ash to cement weight basis replacement ratio.
The following figures and tables show strength versus age and give the test data.
Figure 4–1: Compressive Strength vs. Age Comparison – Mix Nos. P4-1 through P4-6
Other important observations from this study are the following:
- Replacement of cement with fly ash in concrete increases workability of the mixture.
- The water demand decreases with the increase in fly ash content. For a given workability, the water to cementitious materials ratio decreases with increases in fly ash content.
- Pleasant Prairie Power Plant fly ash can be used for the manufacture of structural grade concrete.
Table 4-5: PPPP Class C Fly Ash Concrete Strength Test
Figure 4–2: Compressive Strength vs. Age Comparison – Mix Nos. P4-7 through P4-12
Table 4-6: PPPP Class C Fly Ash Concrete Strength Test Data - 4000 psi (28 MPa) Specified Strength
Figure 4-3: Compressive Strength vs. Age Comparison – Mix Nos. P4-13 through P4-18
Table 4-7: PPPP Class C Fly Ash Concrete Strength Test Data - 5000 psi (34 MPa) Specified Strength
Water Demand
Figures 4-4, 4-5 and 4-6 show the relationship between the amount of water and the percentage of fly ash replacement for the same workability corresponding to 3,000 psi, 4,000 psi and 5,000 psi nominal compressive strength concrete mixtures shown in Tables 4-2 through 4-4. For a given workability (slump 4” ± 1”), it can be seen that as the percentage of fly ash increases in the mixture, the water demand decreases (30).
Figure 4-4: Relationship Between Water Demand and Cement Replacement by Fly Ash (3000 psi Concrete with the Same Workability)
Figure 4-5: Relationship Between Water Demand and Cement Replacement by Fly Ash (4000 psi Concrete with the Same Workability)
Figure 4-6: Relationship Between Water Demand and Cement Replacement by Fly Ash (5000 psi Concrete with the Same Workability)
Figure 4-7 shows the relation between the water to cementitious material ratio and the percentage of cement replacement by fly ash for 3,000 psi; 4,000 psi and 5,000 psi concrete. The figure shows that as the percentage of cement replacement with fly ash increases the water to cementitious material ratio decreases. These results confirm that fly ash concrete requires less water when compared to a similar concrete mix without fly ash for a given slump. Less water equates to denser, less permeable concrete with higher durability.
Figure 4–7: Relationship Between Water to Cementitious Ratio and Cement Replacement by Fly Ash (3000, 4000 and 5000 psi Concrete with the same Workability)
Workability
Slump is one measure of workability. Throughout the project, slump was measured and noted. Earlier researchers have reported that workability increases with the increase in fly ash content. This research confirms this same observation. Though the water to cementitious material ratio was reduced as the fly ash content increased, the same workability was obtained.
Time of Set, Modulus of Elasticity, Drying Shrinkage and Poisson’s Ratio for We Energies ASTM C-618 Class C Fly Ash Concrete (Phase II Study)
As an extension of the project to determine the compressive strength of ASTM C-618, Class C fly ash concrete, it was decided to study the effects of Class C fly ash on time of set, modulus of elasticity, drying shrinkage and Poisson’s ratio. Mixture proportions were developed for producing concrete on a 1.25 to 1 fly ash replacement for cement basis. The replacements were in the amounts of 35, 45 and 55%, on a weight basis. Basic w/c ratios of 0.45, 0.55 and 0.65 were proportioned for no fly ash concrete. Table 4-8 shows the mixture proportions with the actual w/c ratios for these fly ash concrete mixtures.
Time of Set
In order to determine the time of set, another set of mixtures were prepared. Table 4-8 shows the mixture proportions. P4-43, P4-24 and P4-25 are mixture designs with a 28-day compressive strength of 3,000 psi. Mixtures P4-44, P4-26 and P4-27 are designed for a 28-day compressive strength of 4,000 psi, and P4-45, P4-28 and P4-29 are designed for a 28-day compressive strength of 5,000 psi. Table 4- 9 shows the initial and final setting time for fly ash concrete with cement replacement levels up to 55%. For 3,000 psi concrete, the initial set time increased about an hour for every 10% increase in fly ash.
However, the actual initial setting time of 8 hours ± one hour is essentially the same for the 35, 45 and 55% cement replacement levels. The final set time is seen to increase about 90 minutes for every 10% increase in fly ash content, when compared to the 35% fly ash mix. But the actual final setting time of 8½ to 11½ hours would not have any serious effect on a typical construction project.
Table 4-8: PPPP ASTM C-618 Class C Fly Ash Concrete Mix Data
Table 4-9: Time of Setting*
The final setting time for 4000 psi and 5000 psi concrete showed a much less increase in time with increase in the fly ash content. The 5000 psi concrete with 55% fly ash content actually showed a decrease by 10 minutes for final setting time compared to 5000 psi concrete with 45% fly ash content.
The initial and final setting time for air-entrained concrete is also shown on Table 4-9. It can be seen from the results that the initial and final setting time for air-entrained fly ash concrete is not significantly different as the fly ash replacement is increased to levels of 55% for the 3,000; 4,000; and 5,000 psi concrete.
The final setting time for 5000 psi air-entrained concrete is actually less than that of 3000 psi and 4000 psi air- entrained concrete. The 3000 psi air -entrained concrete showed the maximum increase in setting time, when fly ash content is increased from 35% to 45%. But for the same strength concrete with 55% fly ash content, the setting time was lower than that of the mixture containing 45% fly ash. Hence, it is reasonable to believe that initial and final setting time is not significantly different for normal strength concrete with up to 55% replacement of cement with this source of Class C fly ash.
Modulus of Elasticity, Poisson’s Ratio and Compressive Strength
Static modulus of elasticity, Poisson’s ratio and compressive strength were determined for six different types of concrete. All six of the mixtures contained 45% replacement of cement with fly ash on a 1 to 1.25 ratio by weight. Mixtures P4-24, P4-26, and P4-28 were non-air-entrained concrete and mixes P4-47, P4-39, and P4-41 were air-entrained concrete mixtures. P4- 24, P4-26, and P4-28 were designed for 3,000 psi; 4,000 psi; and 5,000 psi compressive strength, respectively. Also, P4-47, P4 -39 and P4-41 were designed for 3,000 psi; 4,000 psi; and 5,000 psi compressive strengths respectively.
Table 4-10: ASTM C-469 Test Results at 28 Days * (Non-Air-Entrained Concrete)
Table 4-11: ASTM C-469 Test Results at 28 Days * (Air-Entrained Concrete)
As can be seen from Tables 4-10 and 4-11, the compressive strengths obtained were much higher than the design strength. In accordance with the ACI 318 Building Code, the static modulus of elasticity is equal to 57,000 √f’c. The values of modulus of elasticity shown in Table 4-10 for non-air-entrained and Table 4-11 for air-entrained fly ash concrete follow nearly the same well-established relationship between compressive strength and the static modulus of elasticity. A detailed discussion of the results can be obtained in reference 31.
The static Poisson’s ratios obtained for these mixtures (both non-air-entrained and air-entrained) fall within the accepted limits for concrete of 0.15 to 0.20, with higher strength concrete showing a higher value.
Length Change: Drying Shrinkage in Air and Expansion in Water
The test results for both air-entrained and non-air-entrained concrete with 45% replacement of cement with fly ash are shown on Table 4-12. The data from all of these mixtures fell between 0.014 and 0.046 for non-air-entrained mixtures and between 0.02 and 0.044 for the air-entrained mixtures.
The test results for expansion in water fell between 0.002 and 0.01 for non-air-entrained concrete and between 0.003 and 0.015 for air-entrained concrete.
Table 4-12: Length Change*
Freezing and Thawing Durability
Freezing and thawing tests were performed on two 4,000 psi, 28- day compressive strength concrete mixtures with 45% fly ash replacement for cement. Mix P4-26 was non-air-entrained, and mix P4-39 was air-entrained. Tables 4-13 and 4-14 give the freeze-thaw test results for non-air-entrained concrete and air-entrained concrete, respectively. ASTM Test Designation C666-84, Procedure A, was followed. Non-air-entrained concrete failed after a low number of cycles of rapid freezing and thawing as expected. However, air-entrained concrete didn’t indicate failure even after 300 cycles of freezing and thawing.
These test results demonstrate that properly air-entrained fly ash concrete with 45% of cement replacement with this source of Class C fly ash exhibits a high durability against freezing and thawing.
Table 4-13: Freeze-Thaw Tests* -
Table 4-14: Freeze-Thaw Tests* (Air-Entrained Concrete)
Phase II Test Result Conclusions
The following are the major results of this study:
- For both air-entrained and non-air-entrained concrete, the initial and final setting time is not significantly different for normal strength concrete with up to 55% replacement of cement with fly ash.
- For non-air-entrained and air-entrained fly ash concrete, with fly ash replacement of up to 45% and compressive strength in the range of 3,000 to 5,000 psi, the static modulus of elasticity is in conformance with established relationships to compressive strength.
- Poisson’s ratio of these fly ash concretes is within the accepted limits for concrete.
- Properly air-entrained high-volume fly ash concrete exhibits good resistance to freezing and thawing.
Abrasion Resistance of Concrete Containing We Energies ASTM C-618, Class C Fly Ash
Abrasion is a common form of wear observed in pavements due to friction forces applied by moving vehicles. Abrasion wear can also occur due to rubbing, scraping, skidding or sliding of other objects on the pavement/concrete surface.
Resistance of concrete surfaces to abrasion is influenced by several factors including concrete strength, aggregate properties, surface finishing and type of toppings. Previous studies have reported that the abrasion resistance of a concrete surface is primarily dependent on the compressive strength of concrete.
Typically, higher compressive strength concrete has better resistance to abrasion provided that the concrete has properly cured hard surface material consisting of aggregate and paste having low porosity and high strength which all contribute to the abrasive resistance of concrete.
Abrasion Test Sample Preparation
ASTM C-618, Class C fly ash from Pleasant Prairie Power Plant of We Energies was used in this study. Fine and coarse aggregate used in this project met ASTM C-33 gradation requirements.
The Portland cement was Lafarge Type 1, meeting requirements of ASTM C-150. Commercially available Catexol AE 260, air-entraining agent and a DaracemTM 100 superplastisizer were also used.
Mixture proportions are shown on Table 4-15. Of the 11 mixtures produced, three were control mixtures and the other eight mixtures contained ASTM C-618, Class C fly ash. Mixture proportions containing fly ash replacement for cement on a 1.25 to 1 basis in the range of 15% to 75% by weight were established. The water to cementitious materials ratio was maintained at 0.35 ± 0.02 and air content was kept at 6% ± 1% for the primary mixtures. The mixtures that didn’t meet the above requirements were classified as secondary mixtures and these were not used for detailed analysis of test results.
Slab specimens for abrasion resistance were prepared according to ASTM C-31 procedures. Fresh concrete properties are reported in Table 4-15. Compressive strength test results are shown in Table 4-16.
Abrasion resistance tests were performed at 28 and 91 days after moist curing of the slab specimens. Abrasion tests were conducted on the specimens using ASTM C-944 test methods. The ASTM C-944 test produced a depth of abrasion of about one mm (0.04”) after about 60 minutes of exposure to the abrasive force. This method was too slow. An accelerated method was developed as an alternative. Details of the method can be obtained from reference 32.
Table 4-15: Mixture Proportions Using Pleasant Prairie Power Plant - Class C Fly Ash, 6000 PSI (41.8 MPa) Specified Strength*
Abrasion Test Results and Discussion
The compressive strengths were measured at ages 1, 3, 7, 28 and 91 days, and are shown in Table 4 -16. At early ages, fly ash concrete exhibited lower compressive strength compared to the control mix. At the 28-day age, 30% fly ash concrete showed peak compressive strength.
Beyond 30% cement replacement, the compressive strength decreased with an increase in fly ash content. The compressive strength of concrete also decreased with increasing air content. This is expected and has been reported by earlier researchers.
Abrasion tests were performed at ages of 28 and 91 days. Abrasion measurement using the modified method is a relative indicator of abrasion and is reported in Tables 4-17 and 4-18. Abrasion wear decreased with an increase in specimen age and resulting increased strength.
Concrete mixtures of up to 30% cement replacement by fly ash had abrasion resistance similar to that for concrete produced without fly ash. Beyond 30% cement replacement, abrasion resistance decreased. It can also be said that with the decrease in compressive strength, abrasion resistance decreased (abrasion wear increased).
The above work leads to the following key conclusions:
- Concrete containing up to 30% cement replacement by fly ash exhibited similar or better compressive strength when compared to concrete produced without fly ash, at ages of three days and beyond (See Figure 4-8).
- Compressive strength is the key factor affecting abrasion resistance. Stronger concrete mixtures exhibited higher resistance to abrasion (See Figure 4-9).
- Effect of air content on abrasion resistance of concrete was insignificant within the tested range.
Table 4-16: Compressive Strength Test Data
Table 4-17: Abrasion Resistance Test Results at 28-Day Age
Table 4-18: Abrasion Resistance Test Results at 91-Day Age
Figure 4–8: Abrasion Resistance vs. Cement Replacement with Class C Fly Ash
Figure 4–9: Abrasion Resistance vs. Compressive Strength of Concretes Containing Different Percentages of Fly Ash
Chloride Ion Permeability of High Strength We Energies Fly Ash Concrete Containing Low Cement Factor
Permeability of concrete is a very important factor affecting its durability. A decrease in permeability of concrete increases the resistance to the ingress of aggressive agents, which in turn, would lead to improved concrete durability.
The following discussion is based on a study conducted at the Center for By- Products Utilization at the University of Wisconsin in Milwaukee for We Energies. Several concrete mixes were designed with and without fly ash. The control mixture was designed for a 28-day compressive strength of 5800 psi without any fly ash. However, other mixtures were designed with various percentages of fly ash as a partial replacement of cement. ASTM C-618, Class C fly ash from Pleasant Prairie Power Plant was used in these tests.
Table 4-19 shows the mixture proportions for the various mixtures, including fresh concrete properties. For this study, the water-to-cementitious materials ratio and air content for the primary mixtures were maintained at about 0.35 ± 0.02 and 6% ± 1%, respectively. The mixtures that did not meet these target parameters were called secondary mixes. The primary mixtures were used to make major conclusions, while the secondary mixes were used to study the effect of air content on concrete strength and permeability (33).
The concrete mixing procedure was performed according to ASTM C-192 procedures, and specimens were also cast in accordance with ASTM C-192 “Making and Curing Concrete Test Specimens in the Laboratory” procedures.
Compressive Strength Test Results
Compressive strength tests were measured per ASTM C-39 “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens” procedures. Air and water permeability was measured in accordance with the Figg Method. Chloride ion permeability was measured according to ASTM C-1202 “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Permeability”.
Compressive strength results are shown in Table 4-20 and on Figures 4-10 and 4-11. Fly ash with up to 35% cement replacement and replaced on a 1.25 fly ash per 1.00 cement weight ratio, showed results similar to the reference concrete at a 3-day age. Beyond 30% cement replacement, the mixtures exhibited lower compressive strength when compared to the reference mixture. At the 28-day age the concrete showed strength levels comparable to the control mixture.
Table 4-19: Mixture Proportions Using ASTM Class C Fly Ash 5800 PSI Specified Strength
Table 4-20: Compressive Strength Test Results
Figure 4–10: Compressive Strength of Concrete made with and without Fly Ash for Primary Mixtures
Figure 4–11: Compressive Strength of Concrete made with and without Fly Ash having Different Percentage of Air Content
Permeability Test Results
Concrete air and water permeabilities were measured at an age of 14, 28 and 91 days. Also, the chloride ion permeability was determined at 2 months, 3 months and 1 year. Air, water and chloride permeability values decreased with age, as expected, due to the improvement in concrete microstructure.
Air permeability test results are given in Table 4-21 and shown on Figures 4-12 and 4-13. At the 14-day age, concrete without fly ash and 18% fly ash concrete were rated “good” and mixtures with higher fly ash contents were rated “fair.” At the 28-day age, the reference mixture and mixtures with up to 45% fly ash were rated “good.” At the 91 day age, 55% fly ash mixtures showed the maximum resistance to air permeability. Figure 4-13 shows the effect of air content on the concrete’s resistance to air permeability. No specific relationship is seen between air permeability and air content for concretes with and without fly ash.
Figure 4–12: Air Permeability (Time) vs. Fly Ash Content for Primary Mixtures
Table 4-21: Air Permeability Test Results
Figure 4–13: Air Permeability of Concrete with and without Fly Ash having Different Percentages of Air Content
Water permeability decreased as the age of concrete specimens increased, as shown on Figures 4- 14 and 4-15 and on Table 4-22. At the 14- day age, concrete resistance to water permeability was improved for mixes with up to 35% fly ash when compared to the reference mixture without fly ash. The 18% to 45% fly ash mixtures were rated as “good.”
At 91 days, concrete mixtures with fly ash to total cementitious materials ratio of 35% to 55% were rated as “excellent.” All other mixtures were only rated “good.” In these mixtures, due to pozzolanic action, the grain structure showed substantial improvement. Water permeability showed no major variations when compared to variations in air content for all concrete with and without fly ash.
Figure 4–14: Water Permeability (Time) vs. Fly Ash Content for Primary Mixtures
Figure 4–15: Water Permeability of Concrete with and without Fly Ash having Different Percentages of Air Content
Table 4-22: Water Permeability Test Results
The chloride ion permeability of the concrete mixtures is shown in Table 4-23 and Figures 4-16 and 4-17. At the age of 2 months, the high-volume fly ash mixtures showed lower chloride ion permeability when compared to the reference mixture without fly ash, except for the 74% fly ash to total cementitious materials ratio concrete. The permeability in this case was in the range of 2,000 to 4,000 coulombs (rated “moderate”) per ASTM C-1202 criteria. With additional time, the resistance to chloride ion permeability of these mixtures showed substantial improvement.
At the age of one year, all the fly ash concrete mixtures attained a “very low” (100 to 1,000 coulombs) level of chloride ion permeability in accordance with ASTM C-1202 criteria where the reference mixtures exhibited a “low” (1,000 to 2,000 coulombs) level of chloride permeability.
Figure 4–16: Chloride Permeability vs. Fly Ash Content for Primary Mixtures
Figure 4-17: Chloride Ion Permeability of Concrete with and without Fly Ash having Different Percentages of Air Content
Table 4-23: Chloride Ion Permeability Test Results
The chloride ion permeability showed no major variation with change in air content. It can be concluded from this work that:
- The optimum ASTM C-618, Class C fly ash from We Energies PPPP content is in the range of 35% to 55% with respect to compressive strength, air permeability, water permeability and chloride permeability.
- Air-entrained high strength concretes can be produced with up to a 35% fly ash to total cementitious material ratio with good resistance to air, water and chloride ion permeability.
- Concrete mixtures with up to 55% fly ash to total cementitious material ratio showed “good” resistance to air permeability.
- Concrete mixtures with 35% to 55% fly ash to total cementitious material ratio exhibited excellent resistance to water permeability at 91-day age.
- The resistance to chloride ion permeability increased as the concrete aged. At the age of one year, all the fly ash mixtures showed very low chloride ion permeability.
- Air content had little effect on air, water and chloride ion permeability of concrete, within the test limits.
High-Volume Fly Ash Concrete - Pilot Project
Figure 4–18: Sussex Corporate Center boulevard entrance paved with high-volume fly ash concrete
Several pilot projects were completed as part of the research work to demonstrate and better understand the actual performance of We Energies coal combustion products. All the pilot projects were very successful, and have been in service for several years. The following are examples of such projects.
Sussex Corporate Center Pilot
Pavements at the Sussex Corporate Center, Village of Sussex, Wisconsin, were constructed using high-volume fly ash concrete in 1995. Concrete pavements do not require major maintenance for 30 to 50 years, while asphalt pavements typically last only 10-15 years, after which they are generally milled and surfaced or replaced.
Tax Incremental Financing (TIF) was used as a means of encouraging investment on this project. If asphalt pavement is constructed using TIF and it needs replacement in 10 or 15 years, that work will not be funded by most TIF districts. Since the decision to construct concrete pavement using TIF funds was made, there was no reason to worry about finding alternate financing for future pavement maintenance (34).
The Sussex Corporate Center is a 221-acre business park development for small light-industrial business offices and includes approximately 20 commercial parcels. High-volume fly ash concrete was used for paving approximately 4,220 linear feet of dual 28-foot lane divided concrete boulevard and 4,210 linear feet of 36-foot wide concrete pavements placed for the corporate center roadways. 9-inch thick concrete pavements were placed over a 6-inch crushed limestone base course.
Concrete Pavement Mixture
The concrete mixture was designed for a minimum of 4,000 psi compressive strength at 28 days. ASTM C-618, Class C fly ash from Pleasant Prairie Power Plant was used on the project. Table 4-24 gives the mixture design for the concrete pavement.
Table 4-24: Sussex Corporate Center Concrete
The fly ash used met the standards of ASTM C-618 and the cement met ASTM C-150 Type 1 standards. Table 4-25 is a comparison between the Wisconsin Department of Transportation pavement specification and this paving mixture containing 40% fly ash.
Table 4-25: Cement vs. Cement Plus Fly Ash
Figure 4-19 : Aerial view of the Village of Sussex Corporate Center that was paved with high-volume fly ash concrete.
The Sussex Corporate Center saved $34,000 on this project, which was approximately 5.5% of the pavement cost by using high-volume fly ash concrete. Since the success of this initial project, the village of Sussex has paved additional roads and sidewalks with this same mixture.
Figure 4-20 : Maple Avenue roadway and sidewalk located in the village of Sussex and paved with high-volume fly ash concrete.
Pavement Construction with High-Volume We Energies Class C and Class F Fly Ash Concrete
An existing crushed stone road providing access to an ash landfill was paved using fly ash concrete. Five different concrete mixtures, 20% and 50% ASTM C-618, Class C fly ash, and 40, 50, and 60% off-spec ASTM C-618, Class F fly ash were used to pave a 6,600 foot (2,012 m) long roadway carrying heavy truck traffic. A 20-foot wide, 8-inch thick concrete pavement with ¼-inch per-foot slope from the centerline to the edge of the roadway was placed over the existing crushed stone base. The pavement was designed to comply with the State of Wisconsin Standard Specification for Road and Bridge construction with the exception of using four experimental high-volume fly ash concrete mixtures. A concrete mix with a minimum 28-day compressive strength of 3,500 psi was specified. The air content of fresh concrete was specified to be 5 to 7% by volume (35). The road was opened to traffic within 10 days of paving completion. It has been providing good service after several Wisconsin winters.
Figure 4-21: Another view of Maple Avenue located in the village of Sussex paved with high-volume fly ash concrete.
Figure 4-22: Finishing touch to We Energies’ high-volume fly ash concrete demonstration project at Pleasant Prairie Power Plant.
Figure 4-23: High-volume fly ash demonstration road paving at Pleasant Prairie Power Plant. Note the difference between the darker slate colored class F fly ash concrete and lighter tan colored high-volume class C fly ash concrete.
The following observations were made by the contractor during the construction.
- Air entrainment and slump were more difficult to control for the off-spec ASTM C-618 Class F fly ash concrete than ASTM C-618 Class C fly ash concrete.
- ASTM C-618 Class F fly ash concrete was more “sticky” and took a longer time to reach strength at which saw cuts could be made.
- Twenty percent and 50% Class C fly ash concrete showed two shades of tan, earth-tone colors, and 40% Class F concrete had a medium gray slate-tone color when wet.
Off-spec ASTM C-618 Class F fly ash obtained from Oak Creek Power Plant and ASTM C-618 Class C fly ash obtained from Pleasant Prairie Power Plant were used on this project. ASTM C-150, Type I Portland cement was also used. The mixture proportions are shown on Tables 4-26 to 4-27.
Concrete specimens were also made for the following tests:
- Compressive strength
- Splitting tensile strength
- Flexural strength
- Freezing and thawing resistance
- Drying shrinkage
- Deicing salt scaling resistance
- Chloride ion permeability
- Abrasion resistance
Table 4-26: Concrete Mixture and Site Test Data for 3500 psi Specified Design Strength Concrete at 28-Day Age
Table 4-27: Concrete Mixture and Site Test Data for 3500 psi Specified Design Strength Concrete at 28-Day Age
Tables 4-28 to 4-40 show the results of the above tests. It can be concluded from this paving project that:
- Paving grade air-entrained concrete can be produced with 40% of Portland cement replaced with off-spec ASTM C-618, Class F fly ash plus a superplasticizer, when the water-to-cementitious materials ratio is maintained around or below 0.36.
- The 50% ASTM C-618, Class C fly ash concrete mixture is suitable for pavement construction.
- All concrete mixtures gained strength with age. Cores taken from the pavement showed higher compressive strengths than lab-cured concrete cylinders.
- High-volume fly ash concrete mixtures showed higher freezing and thawing resistance than the WDOT reference mix with 20% ASTM C-618, Class C fly ash.
- High-volume fly ash concrete exhibited lower drying shrinkage when compared to the reference mixture.
- The high-volume Class C fly ash mixture (50% replacement) showed lower resistance to de-icing salt scaling when compared to the other two mixtures in the laboratory. This has not been observed in the field.
- All mixtures showed good resistance to chloride ion penetration. High-volume off-spec ASTM C-618 Class F fly ash concrete performed better than the other two mixtures, for resistance to chloride ion penetration.
- The 20% ASTM C-618 Class C fly ash mixture showed better resistance to abrasion than the other two mixes.
Table 4-28: Average Compressive Strength Test Results from the Construction Site - Prepared Concrete Cylinders for Specified Design Strength 3500 psi at 28-Day Age
Table 4-29: Average Compressive Strength Test Results From Ready Mix Plant Cylinders for Specified Design Strength 3500 psi at 28-Day Age
Table 4-30: Core Strength Test Data ASTM C-42 (Compressive Strength)
Table 4-31: Average Tensile Strength Test Results (psi)
Table 4-32: Average Flexural Strength Test Results (psi)
Table 4-33: Summary of Test Results on Concrete Prisms after Repeated Cycles of Freezing and Thawing*
Table 4-34: Changes in Fundamental Longitudinal Resonant Frequency of Test Prisms During Freeze-Thaw Cycling per ASTM C666 Procedure A
Table 4-35: Changes in Ultrasonic Pulse Velocity of Test Prisms During Freeze-Thaw Cycling Per ASTM C666 Procedure A
Table 4-36: Flexural Strength of Reference Moist Cured and Freeze-Thaw Test Specimens
Table 4-37: Shrinkage-Expansion and Moisture Change up to 112 Days for Drying Shrinkage Prisms and Prisms Stored in Water
Table 4-38: Results of De-Icing Salt Scaling Tests on High-Volume Fly Ash Concrete Specimens
Table 4-39: Results for Chloride Ion Permeability from Cores
Table 4-40: Abrasion Resistance of High-Volume Fly Ash Concrete Specimens
Long Term Performance of High Volume Fly Ash Concrete Pavement
To evaluate the long-term strength properties and durability of HVFA concrete systems, a study was conducted by the University of Wisconsin – Milwaukee, Center for By-Products Utilization (36). All concrete mixtures developed in this investigation were used in construction of various pavement sections from 1984 to 1991. Core specimens and beams were extracted from in-place pavements for measurement of compressive strength (ASTM C-39), resistance to chloride-ion penetration (ASTM C-1202), and hardened concrete density (ASTM C-642).
Density of Concrete Mixtures
The fresh density values of the concrete mixtures varied within a narrow range for all mixtures. The fresh concrete values were a similar order of magnitude as that of hardened concrete density values for the mixtures. Thus, both the fresh and hardened density values were not significantly influenced by the variations in fly ash content, type, or age within the tested range.
Compressive Strength
The compressive strength of the concrete mixtures increased with age. The rate of increase depended upon the level of cement replacement, class of fly ash, and age. In general concrete strength decreased with increasing fly ash concentration at the very early ages for both classes of fly ash. Generally the early-age strength of Class F fly ash concrete mixtures were lower compared to Class C fly ash concrete mixtures. However, the long-term strength gain by the high volume Class F fly ash concrete system was better than comparable Class C fly ash concrete, as shown in Figure 4-24. This is probably due to the fact that Class F fly ash made a greater contribution of pozzolanic C-S-H compared to Class C fly ash. This in turn resulted in a greater improvement in the microstructure of the concrete made with Class F fly ash compared to Class C fly ash, especially in the transition zone. Therefore, the use of this Class F fly ash is the most desirable from the long-term perspective for the manufacture of high-performance concrete (HPC) because HPCs are required to possess both long-term high-strength properties and durability. However, Class C fly ash also continued to gain strength over time and is also expected to perform well.
Resistance to Chloride-Ion Penetration
All concrete mixtures tested in this investigation showed excellent resistance to chloride-ion penetration. The general performance trend with respect to resistance to chloride-ion penetration followed a similar trend as indicated by the compressive strength. The highest resistance to chloride-ion penetration for the mixtures containing high volumes of Class F fly ash was due to the same reasons as described for the compressive strength data (i.e., improved microstructure of concrete).
Summary
Based on the data recorded in this investigation, the following general conclusions may be drawn:
- Concrete density was not greatly influenced by either the class or the amount of fly ash or the age within the tested range.
- The rate of early-age strength gain of the Class C fly ash concrete mixtures was higher compared to the Class F fly ash concrete mixtures. This was primarily attributed to greater reactivity of Class C fly ash compared to Class F fly ash.
- Long-term pozzolanic strength contribution of Class F fly ash was greater compared to Class C fly ash. Consequently, long-term compressive strengths of Class F fly ash concrete mixtures were higher than that for Class C fly ash concrete mixtures.
- Concrete containing Class F fly ash exhibited higher long-term resistance to chloride-ion penetration compared to Class C fly ash concrete. The best long-term performance was recorded for both the 50% and 60% Class F fly ash concrete mixtures as they were found to be relatively impermeable to chloride-ions in accordance with ASTM C-1202. All fly ash concrete mixtures irrespective of the type and amount of fly ash, showed excellent performance with respect to chloride-ion penetration resistance.
- Based on the results obtained in this investigation, it is desirable to use significant amounts of Class F fly ash in the manufacture of low-cost HPC concrete systems for improved long-term performance. However, Class C fly ash also continued to gain significant strength over time as well.
Figure 4–24: Compressive Strength vs. Age
Roller Compacted No-Fines Concrete Containing We Energies Fly Ash for Road Base Course
Many problems associated with pavement failure are due to the pressure of water under rigid surface pavements. When high pressure from heavy traffic is applied on pavements in the presence of water, pumping occurs. Pumping causes erosion of the pavement base, as fines along with water are pumped out. The continued effect of pumping is a loss of support, leading to pavement failure. An open-graded permeable base is used to avoid such problems. The open-graded permeable base pavement system consists of a permeable base, separator layer and edge drainage. Permeable bases can be treated or untreated with cementitious binders.
A demonstration project was designed to use an off-spec ASTM C-618, Class F fly ash in the open-graded concrete base course and an ASTM C-618 Class C fly ash in the concrete pavement for an internal road at the Port Washington Power Plant located in Port Washington, Wisconsin.
The roadway cross section (see Figures 4-25 and 4-26) consisted of an initial layer of filter fabric installed to prevent fines from the subgrade moving up and blocking drainage in the base course, topped by a 6” thick layer of open-graded concrete base course and a 10 in. thick, high-volume fly ash concrete pavement. This pavement was designed in compliance with Wisconsin DOT standards, with the exception of using high-volume fly ash in the open-graded base, and concrete pavement. Underdrains, manholes and storm sewer piping were also installed as part of this project, to ensure proper functioning of the pavement system (37).
The properties of fly ash and cement used in this project are shown on Table 4-41. The ASTM C-618, Class F fly ash used on the project is off-specification with a very high LOI.
The mixture proportions for the open-graded base course were composed of 160 lb/cu yd cement, 125 lb/cu yd fly ash, 81 lb/cu yd water, 2600 lb/cu yd ¾ in. coarse aggregate and no fine aggregate.
The mixture proportions for high-volume fly ash concrete pavement included 300 lb/cu yd cement, 300 lb/cu yd Class C fly ash, 221 lb/cu yd water, 1200 lb/cu yd sand, 966 lb/cu yd ¾” aggregate and 966 lb/cu yd 1-1/2” coarse aggregate. The water to cementitious materials ratio was maintained at about 0.37.
Figure 4-25: Pavement Cross Section
Notes:
- Pavement slope varies to maintain drainage. Typical slope 20.8 mm per meter.
- Expansion joints with dowel bars provided at intersection with existing pavement
- Transverse joints at approximately 6 meter intervals
- Transverse joints were saw cut to a minimum depth of 762 mm.
Table 4-41: Properties of Cement and Fly Ashes Used
Field testing was performed during the placement of base course and the concrete pavement. Slump measurements were taken on both the base course mixture and concrete mixture. Also, air content (ASTM C-231) and temperature (ASTM C-1064) measurements were recorded for the concrete mixture.
Compressive strength was also measured on cylinders made from selected batches of base course and paving slab concrete mixtures, in accordance with ASTM procedures.
Figure 4-26: Open-graded cementitious base course material being placed over filter fabric at Port Washington Power Plant's high-volume fly ash concrete pavement demonstration project.
Results and Discussion
- Base Course Material: The compressive strength data is shown in Table 4-42. The permeable base was designed to have a compressive strength in the range of 490 to 990 psi. However, the mixture provided 670 psi at 28-day age and 810 psi at 56-day age.
- Fly Ash Concrete Pavement: Since there already was significant data on high-volume fly ash concrete, only compressive strength of the pavement concrete mixtures was measured. Based on earlier work, it was assumed that a mixture meeting air content and strength requirements would satisfy other durability requirements.
Table 4-43 gives the compressive strength results for the pavement concrete mixtures. The mixture showed a compressive strength of 4870 psi at the 28-day age, which was 20% higher than the design strength of 4000 psi. The pavement was inspected visually to determine its performance over the past several years. No obvious pavement distress was seen during the inspections.
Table 4-42: Open-Graded Base Course Test Results
Table 4-43: High-Volume Fly Ash Concrete Test Results
Sample specifications are included in Appendix 12.1 for current We Energies cast-in-place concrete.
Bricks, Blocks, and Paving Stones Produced with We Energies Fly Ash
Coal combustion product applications have shown a substantial increase in the past decade. However, only a limited amount of fly ash and bottom ash are actually used in the production of masonry units, such as bricks, blocks, and paving stones. Since only limited research was done on room- cured and steam-cured ash bricks and blocks, We Energies funded research on a project to investigate the properties of bricks and blocks containing We Energies fly ash at the Center for By-Products Utilization of the University of Wisconsin-Milwaukee.
Testing Program
The testing program consisted of the following stages:
- Developing mixture proportions for room temperature cured bricks and blocks utilizing ASTM C-618 Class C fly ash.
- Extended testing using different types of (ASTM Class C and Class F) fly ash from different sources, and using bottom ash as a replacement for natural aggregates.
- Studying the effect of different curing systems.
- Producing small size blocks using selected mix recipes and testing their properties.
Stage 1 Testing
Fly ash from power plants other than We Energies was also used in this work. However, the data presented here is only information relevant to We Energies products. In the first stage testing, only ASTM C-618, Class C fly ash from Dairyland Power Corporation was used. The intent of this work was to develop a suitable and economic brick and block mixture utilizing coal ash.
From the Stage 1 studies, it was concluded that:
- The dry-cast vibration method is better for obtaining higher compressive strength masonry units.
- Sufficient strength develops (greater than 2000 psi) when the specimens are cured in a fog room for 28 days. No firing or steam curing is required for this.
- Most masonry products require only a compressive strength of 2000 psi to 3000 psi. Hence, it is appropriate to raise the aggregate to cementitious ratio and introduce the bottom ash as partial replacement of aggregates in the mixtures.
Stage 2 Testing
Two types of fly ash from We Energies were used in this testing, ASTM C-618 Class C (F-2) and an off-spec ASTM C-618 Class F (F -4) fly ash. The chemical properties of fly ash used in this project are given in Table 4-44.
Table 4-44: Chemical Properties of We Energies Fly Ash
Specimens were made by making semi-dry and wet mixtures and casting them directly into the steel mold for vibrating on a vibration table (38). The molded specimens were cured for one day in the fog room, then removed from molds and placed back in the fog room until the time of test.
Nine 2 inch cubes were made for compressive strength and bulk density tests for each mixture. Three cubes were tested at each test age. Compressive strength tests were performed in accordance with ASTM C-192 “Standard Practice for Making and Curing Specimens in the Laboratory”. Bulk density tests were performed in accordance with the ASTM C-642 “Standard Test Method for Density, Absorption, and Voids in Hardened Concrete” procedures. Mixture proportions are shown in Table 4-45.
Table 4-45: Mix Proportions for Concrete Masonry Units
The aggregate used throughout this work was ⅜” size natural pea gravel as coarse aggregate and natural sand as fine aggregate. The aggregate in the mixture consisted of 50% fine and 50% coarse aggregate.
Test Results
Table 4-46 shows the compressive strength and bulk density test results. The specimens made with ASTM C-618 Class C fly ash gave higher compressive strengths than those with ASTM Class F fly ash for the same fly ash content.
ASTM C-618, Class C fly ash generally has a slightly higher specific gravity than Class F fly ash. Hence, Class C fly ash mixtures show a slightly higher bulk density.
Table 4-46: Compressive Strength and Bulk Density
Stage 3 Testing
After reviewing the work done in Stages 1 and 2, and evaluating the commercial block manufacturing process, modifications were made to the mixture design. Commercial manufacturers use a higher aggregate-to-cement ratio in the mixture than used in the laboratory.
Six blocks measuring approximately 4 x 2.5 x 1.8125 inches with two rectangular 1.25 x 1.25 inch open cells were manufactured. The blocks have a gross area of 10 sq. inches and a net area of 6.25 sq. inches (62.5% of gross area). This size is a proportionately reduced size of block manufactured in the local area for testing purposes.
The mixture design is shown in Table 4-47. Dry material components were first blended with water and then the mixture was tamped into a block mold in three layers. Each layer was compacted by a vibrating pressed bar, then removed from the mold, and stored in the curing tank for steam curing or stored in a fog room.
The blocks were tested for compressive strength and bulk density, water absorption and dimensional stability. All tests were carried out in accordance with ASTM C-140. Table 4-48 shows the compressive strength and bulk density test results and water absorption test results.
Table 4-47: Mix Design for Blocks
Table 4-48: Compressive Strength, Bulk Density, and
The compressive strength values were somewhat lower than expected even for the no fly ash mixture. The reason is believed to be the size effect. Local block manufacturing companies have also documented such reduction in strength when small blocks are tested. However, mix no. 3 with Class C fly ash showed compressive strength comparable to the control mix.
The bulk density measurements showed that the blocks containing fly ash are slightly lighter. The lower bulk density translates to better insulating properties, improved resistance to freezing and thawing, lower heat losses, and lower dead load in structures.
The water absorption for all the mixes are within the limits of ASTM C-55. Dimensional stability tests did not show any change. These tests should also be performed on full-size blocks to verify the results.
CalStar Green Bricks and Pavers Using We Energies’ Fly Ash
CalStar opened its first fly ash brick manufacturing plant in Racine, Wisconsin in January 2010. The plant makes the architectural bricks and pavers using Class C fly ash sourced from We Energies OCPP. CalStar green bricks and pavers are non-fired and do not use clay. They are made from ASTM C-618 Class C fly ash, a self-cementing byproduct of coal combustion. Fly ash, aggregate, mineral oxide pigments, and proprietary ingredients are mixed with water, vibro-compacted, and cured into a stable solid.
CalStar brick is used to build in the time-honored tradition of masonry construction. Masonry’s inherently sustainable qualities include its acoustic performance, high thermal mass and exceptional durability. Its thermal mass can stabilize indoor temperatures, saving energy and improving thermal comfort. CalStar pavers are used to construct walkways, plazas, patios and driveways, on flexible or pervious bases providing a beautiful durable surface. The end product can reduce runoff, stormwater impacts, and erosion when laid in open configurations. Light colors can reduce the heat island effect, helping keep cities and developments cooler. CalStar bricks and pavers therefore, add great environmental value to projects because they are manufactured with fly ash and convert it to a strong, beautiful building material instead of mining virgin clay and firing it. Structures constructed using CalStar bricks and pavers have long service lives because of proven durability. Manufactured by CalStar Products, Inc. in Caledonia, Wisconsin using fly ash, both CalStar bricks and pavers save production energy, preserve natural materials, conserve landfill space, reduce carbon emissions, and provide a market for byproduct materials.
Figure 4-27: Environmentally green CalStar bricks (a), and CalStar pavers (b) made from We Energies’ OCPP Class C fly ash.
CalStar bricks offer a green material choice with performance properties and dimensions that meet or exceed requirements of ASTM C-216 for SW (Severe Weathering) and ASTM C-216 Type FBX (the most precise dimensional tolerance criteria) respectively, making it suitable for use as a face brick in severe and freeze-thaw conditions. CalStar pavers equally offer a green material choice with performance which meet or exceed performance requirements of ASTM C-902 for Class SX clay pavers and ASTM C-936 for interlocking concrete paving units, suitable for pedestrian and light vehicular traffic, in severe climates and freeze-thaw conditions.
Why CalStar bricks and CalStar pavers are environmentally green materials
One increasingly useful way to measure a product’s environmental impact is to audit its ‘embodied energy’ and ‘carbon footprint’—the amount of energy consumed and CO2 released in the extraction, processing and transportation of raw materials and manufacture of the finished product. One focus of materials research and development is finding ways to reduce the environmental footprint without sacrificing other product benefits.
Clay brick is high in both embodied energy and carbon footprint. Clay brick manufacturing is energy-intensive because clay requires firing for up to three days to become hard and durable. Brick firing kilns operate at about 2,000 oF, and are generally kept hot even when not in use. The heat for most kilns is generated by burning natural gas, while some brick producers use fuels such as coal and petroleum coke. All of these fuel sources emit CO2 during combustion.
The National Institute of Standards and Technology (NIST) Building for Environmental and Economic Sustainability (BEES) database lists the average embodied energy for a common fired clay brick at 8,800 BTUs. The Brick Industry Association (BIA) notes that a clay brick plant operating at optimal efficiency might reduce this figure to 5,000 BTUs. For purposes of these calculations, a middle ground of 6,000 BTUs of embodied energy was selected. CO2 emissions are often a by-product of energy consumption; each clay brick fired with fossil fuel conservatively releases 0.9 lbs of CO2 into the atmosphere.
Producing bricks from recycled fly ash consumes less energy and emits less CO2 because it does not require firing to harden the masonry units, and does not use cement with its CO2 emissions. Since CalStar bricks and CalStar pavers are not fired, all the energy which could have been used for this purpose is saved, and CO2 emissions are reduced. This makes CalStar bricks and pavers green products.
For more information visit the manufacturer’s website at www.brickstoneinc.com.
Fly Ash Concrete for Precast/Prestressed Products
We Energies’ fly ash has also been used to produce precast/prestressed concrete products. We Energies initiated a study to develop mixture proportioning information for the production of high early strength concrete with high fly ash content for precast/prestressed concrete products (39).
Materials
The ASTM C-618 fly ash used in this project was produced by We Energies at the Pleasant Prairie Power Plant. A Type I cement was used and the replacement quantities with Class C fly ash were 0, 10, 15, 20, and 30%. Twelve different mixture proportions were developed based upon a nominal 5000 psi control mixture that contained no fly ash. Table 4-49 shows the first six mixture proportions.
Concrete Mixing and Specimen Preparation
Concrete was produced at two different precast/prestressed concrete plants. Standard batching and mixing procedures for ready mixed concrete were followed, in accordance with ASTM C-94. Fresh concrete tests included slump and air content. Cylinders were cured following the actual practice of the individual precast/prestressing plant.
Compressive Strength
The test results indicated that the compressive strength of the concrete mixtures increased with the increase of replacement percentage of cement with Class C fly ash after 3 days (5060 psi) and 28 days (8435 psi) of curing as shown in Table 4-50. The maximum compressive strength was obtained for a 25% Class C fly ash replacement. Therefore, Class C fly ash usage increased the early strength of concrete. The strength results also indicated that cement replacement with up to 30% of Class C fly ash increased the early strength relative to the mixture without fly ash.
Workability
Workability was observed and noted throughout the project. All the concrete produced was homogeneous and cohesive. The fly ash replacement did not affect these properties. Slump measurements show variations because of the use of a superplasticizer. Even though the water to cementitious ratio decreased as the fly ash was increased, clearly acceptable workability was maintained.
There are several advantages of using Class C fly ash in concrete precast/prestressed products:
- Improved economics are possible as a result of reduced raw material costs resulting in the use of more competitive products over a wider geographical region.
- Class C fly ash usage in concrete provides higher quality products which include higher density with reduced permeability, increased strength and other properties.
- Fly ash concrete mixes are handled more easily because of improved workability. Faster release of prestressing tendons is also possible because of increased early age strength with use of Class C fly ash.
Table 4-49: Concrete Mixture Proportions
Table 4-50: Concrete Strength Using Prestressed
Conductive Concrete Containing We Energies High Carbon Fly Ash (US Patent 6,461,424 B1) (40)
Materials
Materials utilized in this project consisted of one source of fly ash, cement, clean concrete sand, crushed quartzite limestone aggregates, steel fibers, and taconite pellets. Materials were characterized for chemical and physical properties in accordance with the appropriate ASTM standards. Table 4-51 shows the mixture proportions.
Type I cement (Lafarge Cement Co.) was used throughout this investigation. Its physical and chemical properties were determined in accordance with applicable ASTM test methods.
One source of fly ash was used for this project (We Energies, Port Washington Power Plant, Units 2 and 3). The ash selected for this project was non-standard (not meeting all requirements of ASTM C-618). This selection was made to develop and encourage additional uses for under-utilized sources of fly ash in Wisconsin.
In one concrete mixture, steel fibers were used to enhance electrical resistance. The steel fibers measured about 2” in length by ¼” wide.
All concrete ingredients were manually weighed and loaded in a laboratory rotating-drum concrete mixer for mixing following the procedures of ASTM C-192. The resulting mixture was then discharged into a pan where the concrete was further tested and test specimens were cast.
Table 4-51: Concrete Mixture Proportions
Fresh concrete properties were also measured for the mixtures. Properties measured included: air content (ASTM C-231), slump (ASTM C-143), unit weight (ASTM C-138), and temperature (ASTM C-1064). Air temperature was also measured and recorded. Cylindrical test specimens 6 inches dia. x 12 inches in length were prepared from each mixture for compressive strength (ASTM C-39) and density tests. All test specimens were cast in accordance with ASTM C-192. Concrete specimens were typically cured for one day at about 70 ± 5°F. These specimens were then demolded and placed in a standard moist-curing room maintained at 100% relative humidity and 73 ± 3ºF temperature until the time of test (ASTM D-4832).
Electrical Resistance Measurements
In order to test the effect of the moisture on the electrical resistance of the material and the reliability of the measurements, six identical cylinders were made from each concrete mixture. Three specimens were left to air dry after demolding and three were placed in water to remain in a saturated condition for testing. Both the air-dried and saturated specimens were tested at the same ages for electrical properties. Resistance measurements were taken using a Leader LCR-475-01 multimeter at one pre-determined location on all six cylinders for each mixture across its length (Fig. 4-28).
Reactance Measurement and Calculation of Permeability
Reactance of the test cylinder was measured by placing the cylinder in a copper wire coil and measuring the reactance of the coil with air as the core (L1) and with the test cylinder as the core (L2). The reactance, L1 and L2, were determined using a Leader LCR-475-01 multi-meter. The resistance measurements were converted into resistivity values (ohm-cm). The measured reactance values were then used to calculate the permeability values from the relationship:
Figure 4–28: Electrical Resistance Measurements
Concrete Compressive Strength
The compressive strength of the three concrete mixtures is shown in Table 4-52. The compressive strength of the mixtures was 2340 psi to 2535 psi at the age of 28 days. A typical concrete used for foundations and walls construction has a minimum specified 28-day compressive strength of 3000 psi to 4000 psi. The concrete strengths achieved for the mixtures developed as part of this project are below this usual strength level. The primary focus of this project was to determine the effect of various materials on the electrical properties of the concrete. Therefore, the compressive strength of the mixtures was considered secondary at this stage of the study. Mixtures can be revised in future phases to produce a higher strength material. The compressive strength of the concrete may be increased by increasing the cementitious materials and/or reducing the amount of water in the mixture (reducing the water to cementitious materials ratio). This may also be achieved by using chemical admixtures such as a mid-range or high-range water reducing admixtures (superplasticizer). The strength at various ages for these three mixtures is quite similar due to the fact that the cementitious materials and water to cementitious materials ratios are essentially the same.
Table 4-52: Compressive Strength of Concrete Mixtures
Electrical Properties of Concrete Mixtures
The electrical properties of the concrete mixtures are shown in Tables 4-53 and Figure 4-29. The electrical resistivity of the air dried concrete is in the range of 1 to 128 x 10³ ohm-cm. The air dried conventional concrete typically has a resistivity of the order of 106 ohm-cm, with oven dried conventional concrete having a resistivity of the order of 10¹¹ ohm-cm. Therefore, it is apparent that the electrical resistivity of concrete is less than the electrical resistivity of conventional concrete. In other words, by incorporating high carbon fly ash into a concrete mixture, a more electrically conductive concrete is produced. The permeability of a concrete prepared with high carbon fly ash exceeds that of air, indicating a greater capability to carry an electrical current. The use of fly ash having greater levels of carbon would further decrease the resistivity of the resulting concrete. In addition, the increased concentration of high carbon fly ash in the composition will result in increased conductivity.
Table 4-53: Electrical Properties of Concrete Mixtures
Conductive Concrete Containing We Energies High Carbon Fly Ash and Carbon Fibers (US Patent 6,821,336) (41)
Testing of concrete using carbon fibers was conducted for concrete mixtures. The goal of this testing work was to determine the feasibility of incorporating high carbon fly ash and carbon fibers in concrete to lower electrical resistance of these construction materials. The lowered electrical resistance of concrete mixtures will reduce the required length of, or entirely replace, the grounding electrodes currently in use for protection of electrical equipment from lightning strikes. Other uses can potentially include grounding, heating bridges, sidewalks or airport runways, sensors, and various other applications.
Figure 4–29: Relative Electrical Permeability of Concrete Mixtures
Materials
Materials utilized consisted of one source of fly ash, cement, clean concrete sand, crushed quartzite limestone aggregates, and carbon fibers. One source of clean concrete sand was utilized in this investigation as fine aggregate for concrete mixtures. The aggregate used was a crushed quartzite limestone with a maximum size of ¾” meeting ASTM C-33 requirements. Type I cement (Lafarge Cement Co.) was used throughout this investigation. One source of fly ash was used for this project (We Energies, Presque Isle Power Plant) . This selection was made to represent a typical high-carbon fly ash available from We Energies.
The fibers used for this project were Panex 33 chopped carbon fibers manufactured by the Zoltek Corporation, St. Louis, MO. The carbon fibers were pan-type fibers ½” long and approximately 0.283 mils (7.2 microns) in diameter. The density of the fibers reported by the manufacturer was 0.065 lb/in3.
All concrete ingredients were manually weighed and loaded in a laboratory rotating-drum concrete mixer following the procedures of ASTM C-192. The test concrete was also manufactured. A high-range water reducing admixture was used for the concrete mixture to achieve the desired slump.
The amount of carbon fibers incorporated into the concrete mixture was determined by We Energies. Mixture CON-C contained approximately 40% fly ash by weight of total cementitious materials, a high-range water reducing admixture, and the addition of 14 lb/yd3 of carbon fibers. Table 4-54 shows the mixture components.
Table 4-54: Concrete Mixtures
Mechanical Properties
Compressive strength of the concrete was measured using standard cylinders, 6" diameter x 12" long, following the method of ASTM C -39. The compressive strength of concrete Mixture CON-C is shown in Table 4-55. The compressive strength of the mixture was very low at the early age and could not be measured until the age of 16 days. At the age of 16 days, the compressive strength was only 60 psi. The compressive strength increased at the age of 28 days to 135 psi, and then significantly increased at the 42-day age to 1345 psi. This indicates that the setting time of the concrete mixture was significantly delayed, and reflects the pozzolanic effect of 40% fly ash content contributing to this increase in strength. The delay in setting was attributed to the amount of high-range water reducing admixture (HRWRA) required to be added to the mixture. The amount of HRWRA exceeded the maximum amount recommended by the manufacturer (136 oz/yd³ versus 170 oz/yd³ actually used in the laboratory mixture). Another possibility investigated was to determine if the water-soluble chemical coating on the carbon fibers had any effect on the setting time of the mixtures. The water-soluble “sizing” coating is applied to prevent the agglomeration of the fibers but yet sustain electrical contact of the fibers in the concrete mixture. The sizing (coating) that was used on the carbon fibers was provided by the manufacturer, Zoltec.
A test was conducted on cement mortar cubes per ASTM C-109 using water that was obtained from soaking the carbon fibers for 24 hours. The compressive strength of the cement mortar cubes at the age of seven days was 5070 psi. This indicates that the water-soluble sizing probably did not have any time of setting delay effect on the compressive strength of cement mortar. The concrete compressive strength achieved for the Mixture CON-C tested for this project is below its normally expected strength level. The primary focus of this project was to determine the effect of carbon fibers on the electrical properties of the concrete. Therefore, the compressive strength of the mixtures was considered secondary at this stage of the study. The amount of fibers can be revised in the future phases to produce a good-quality structural-grade concrete. The amount of carbon fibers may be reduced and optimized for electrical properties. Compressive strength of the concrete may be increased by increasing the cementitious materials and/or reducing the amount of water in the mixture.
Table 4-55: Compressive Strength of Concrete Mixture
Electrical Properties
The electrical resistivity obtained for the concrete Mixture CON-C are given in Table 4-56 and Figure 4-30. Overall, resistivity of both air-dried and saturated specimens were comparable with approximately 40 to 50 ohms-cm at the age of 16 days and 60 to 70 ohms-cm at the age of 42 days. Although the compressive strengths were much lower for the Mixture CON-C than a typical concrete used for many construction applications, the lower resistivity values achieved through the incorporation of high-carbon fly ash and carbon fibers are very promising for potential grounding applications. Further refinement of the carbon fiber content to optimize the resistivity and strength properties of the concrete is needed as a part of future laboratory studies. The permeability values show only a slight increase between 16 and 28 days. The relative electrical permeability of air-dried and saturated specimens were very close to each other as shown on Figure 4-29.
For CON-C, the air-dried specimens also had a higher electrical resistivity at the age of 42 days, but the difference between saturated and air-dried specimens was less. Typically the difference between air-dried and saturated specimens was 10 ohm-cm or less. This may be attributed to the conductivity of the carbon fibers used in the mixtures.
Table 4-56: Electrical Resistivity of High Carbon Concrete
Figure 4–30: Electrical Permeability of High Carbon Fly Ash Concrete Contained Carbon Fibers, Mixture CON-C
We Energies Fly Ash and Spent Carbon Sorbent (US Patent 7,578,881) (42)
Testing of concrete using spent carbon sorbent (having small amounts of mercury absorbed/entrapped by the sorbent) was conducted for concrete mixtures. The goal of this work was to determine if the carbon and mercury in the spent carbon sorbent would lower electrical resistance of these construction materials when incorporated in concrete. The lowered electrical resistance of concrete mixtures has the potential to reduce the required length, or entirely replace the grounding electrodes currently in use for protection of electrical equipment from lightning strikes. Other uses can potentially include lowering the impedance conduction path to earth for electric system protection, and stabilization.
Materials
Materials utilized consisted of one source of fly ash, cement, clean concrete sand, gravel aggregates, and a particulate material including fly ash and a spent activated carbon sorbent having small amounts of adsorbed mercury. One source of clean concrete sand was utilized in this investigation as fine aggregate for the concrete mixture, meeting the ASTM C-33 requirements. The coarse aggregate used was natural river gravel with a maximum nominal size of ⅜ inch. Type I Portland cement was used throughout this investigation. A cementitious fly ash was also used for this project (We Energies, Pleasant Prairie Power Plant) meeting the requirements of ASTM C-618, Class C fly ash.
The spent activated carbon particulate material included some Class C fly ash that had passed through the electrostatic precipitator and was captured with the carbon sorbent in the baghouse.
All concrete ingredients were manually weighed and mixed by hand in a mixing bowl. For fresh concrete, an estimate was made of the unit weight for determination of approximate mixture proportions and a general visual observation of the workability was made. Table 4-57 shows the mixture components.
Table 4-57: Concrete Mixture Proportions
Table 4-58: Concrete Mixture Test Results
Source: We Energies
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