Fly ashes are finely divided residue resulting from the combustion of ground or powdered coal. They are generally finer than cement and consist mainly of glassy-spherical particles as well as residues of hematite and magnetite, char, and some crystalline phases formed during cooling. Use of fly ash in concrete started in the United States in the early 1930's. The first comprehensive study was that described in 1937, by R. E. Davis at the University of California (Kobubu, 1968; Davis et al., 1937). The major breakthrough in using fly ash in concrete was the construction of Hungry Horse Dam in 1948, utilizing 120,000 metric tons of fly ash. This decision by the U.S. Bureau of Reclamation paved the way for using fly ash in concrete constructions.
In addition to economic and ecological benefits, the use of fly ash in concrete improves its workability, reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-aggregate reaction, and enhances sulfate resistance. Even though the use of fly ash in concrete has increased in the last 20 years, less than 20% of the fly ash collected was used in the cement and concrete industries (Helmuth 1987).
One of the most important fields of application for fly ash is PCC pavement, where a large quantity of concrete is used and economy is an important factor in concrete pavement construction. FHWA has been encouraging the use of fly ash in concrete. When the price of fly ash concrete is equal to, or less than, the price of mixes with only portland cement, fly ash concretes are given preference if technically appropriate under FHWA guidelines (Adams 1988).
Two major classes of fly ash are specified in ASTM C 618 on the basis of their chemical composition resulting from the type of coal burned; these are designated Class F and Class C. Class F is fly ash normally produced from burning anthracite or bituminous coal, and Class C is normally produced from the burning of subbituminous coal and lignite (as are found in some of the western states of the United States) (Halstead 1986). Class C fly ash usually has cementitious properties in addition to pozzolanic properties due to free lime, whereas Class F is rarely cementitious when mixed with water alone. All fly ashes used in the United States before 1975 were Class F (Halstead 1986: ACI Comm. 226 1987c).
Fly ash which is produced at base loaded electric generating plants is usually very uniform. Base loaded plants are those plants which operate continuously. The only exception to uniformity is in the start-up and the shut-down of these plants. Contamination may occur from using other fuels to start the plant, and inconsistencies in carbon content occur until the plant reaches full operating efficiency. The ash produced from the start-up and shut-down must be separated from what is produced when the plant is running efficiently. In addition, when sources of coal are changed, it is necessary to separate the two types of fly ashes. Peak load plants are subjected to many start-up and shut-down cycles. Because of this, these plants may not produce much uniform fly ash.
The most-often-used specifications for fly ash are ASTM C 618 and AASHTO M 295. While some differences exist, these two specifications are essentially equivalent. Some state transportation agencies have specifications that differ from the standards (Admixtures and Ground Slag 1990). The general classification of fly ash by the type of coal burned does not adequately define the type of behavior to be expected when the materials are used in concrete.
There are also wide differences in characteristics within each class. Despite the reference in ASTM C 618 to the classes of coal from which Class F and Class C fly ashes are derived, there was no requirement that a given class of fly ash must come from a specific type of coal. For example, Class F ash can be produced from coals that are not bituminous. and bituminous coals can produce ash that is not Class F (Halstead 1986). It should be noted that current standards contain numerous physical and chemical requirements that do not serve a useful purpose. Whereas some requirements are needed for ensuring batch-to-batch uniformity, many are unnecessary (RILEM 1988).
The substitution rate of fly ash for portland cement will vary depending upon the chemical composition of both the fly ash and the portland cement. The rate of substitution typically specified is a minimum of 1 to 1 ½ pounds of fly ash to 1 pound of cement. It should be noted that the amount of fine aggregate will have to be reduced to accommodate the additional volume of fly ash. This is due to fly ash being lighter than the cement.
The amount of substitution is also dependent on the chemical composition of the fly ash and the portland cement. Currently, States allow a maximum substitution in the range of 15 to 25 percent.
Effects of fly ash, especially Class F, on fresh and hardened concrete properties has been extensively studied by many researchers in different laboratories, including the U.S. Army Corps of Engineers, PCA, and the Tennessee Valley Authority. The two properties of fly ash that are of most concern are the carbon content and the fineness. Both of these properties will affect the air content and water demand of the concrete.
The finer the material the higher the water demand due to the increase in surface area. The finer material requires more air-entraining agent to five the mix the desired air content. The important thing to remember is uniformity. If fly ash is uniform in size, the mix design can be adjusted to give a good uniform mix.
The carbon content, which is indicated by the loss of ignition, also affects the air entraining agents and reduces the entrained air for a given amount of air-entraining agent. An additional amount of air-entraining agent will need to be added to get the desired air content. The carbon content will also affect water demand since the carbon will absorb water. Again uniformity is important since the differences from non-fly ash concrete can be adjusted in the mix design.
Fresh Concrete Workability. Use of fly ash increases the absolute volume of cementitious materials (cement plus fly ash) compared to non-fly-ash concrete; therefore, the paste volume is increased, leading to a reduction in aggregate particle interference and enhancement in concrete workability. The spherical particle shape of fly ash also participates in improving workability of fly ash concrete because of the so-called "ball bearing" effect (Admixtures and Ground Slag for Concrete 1990; ACI Comm. 226 1987c). It has been found that both classes of fly ash improve concrete workability.
Bleeding. Using fly ash in air-entrained and non-air-entrained concrete mixtures usually reduces bleeding by providing greater fines volume and lower water content for a given workability (ACI Comm. 226, 1987c; Idorn and Henrisken, 1984). Although increased fineness usually increases the water demand, the spherical particle shape of the fly ash lowers particle friction and offsets such effects. Concrete with relatively high fly ash content will require less water than non-fly-ash concrete of equal slump (Admixtures and ground slag for concrete, 1990).
Time of Setting. All Class F and most Class C fly ashes increase the time of setting of concrete (Admixtures and ground slag 1990; ACI Comm. 226, 1987c). Time of setting of fly ash concrete is influenced by the characteristics and amounts of fly ash used in concrete. For highway construction, changes in time of setting of fly ash concrete from non-fly-ash concrete using similar materials will not usually introduce a need for changes in construction techniques; the delays that occur may be considered advantageous (Halstead 1986).
Strength and Rate of Strength of Hardened Concrete. Strength of fly ash concrete is influenced by type of cement, quality of fly ash, and curing temperature compared to that of non-fly-ash concrete proportioned for equivalent 28-day compressive strength. Concrete containing typical Class F fly ash may develop lower strength at 3 or 7 days of age when tested at room temperature (Admixtures and ground slag for concrete, 1990; ACI Comm. 226 1987c). However, fly ash concretes usually have higher ultimate strengths when properly cured. The slow gain of strength is the result of the relatively slow pozzolanic reaction of fly ash. In cold weather, the strength gain in fly ash concretes can be more adversely affected than the strength gain in non-fly-ash concrete. Therefore, precautions must be taken when fly ash is used in cold weather (Admixtures and ground slag 1990).
Freeze-thaw Durability of Hardened Concrete. On the basis of a comparative experimental study of freeze-thaw durability of conventional and fly ash concrete (Soroushian 1990; Virtanen 1983; Lane and Best 1982), it has been observed that the addition of fly ash has no major effect on the freeze-thaw resistance of concrete if the strength and air content are kept constant. The addition of fly ash may have a negative effect on the freeze-thaw resistance of concrete when a major part of the cement is replaced by it. The use of fly ash in air-entrained concrete will generally require an increase in the dosage rate of the air-entraining admixture to maintain constant air. Air-entraining admixture dosage depends on carbon content, loss of ignition, fineness, and amount of organic material in the fly ash (ACI Comm. 226, 1987c).
Carbon content of fly ash, which is related to the coal burned by the producing utility of the type and condition of furnaces in the production process of fly ash, influences the behavior of admixtures in concrete. It has been found that high-carbon-content fly ash reduces the effectiveness of admixtures such as air-entraining agents (Joshi, Langan, and Ward 1987: Hines 1985).
Alkali-silica Reaction of Hardened Concrete. One of the important reasons for using fly ash in highway construction is to inhibit the expansion resulting from ASR. It has been found that 1) the alkalies released by the cement preferentially combine with the reactive silica in the fly ash rather than in the aggregate, and 2) the alkalies are tied up in nonexpansive calcium-alkali-silica gel. Thus hydroxyl ions remaining in the solution are insufficient to react with the material in the interior of the larger reactive aggregate particles and disruptive osmotic forces are not generated (Halstead 1986; Olek, Tikalsky, and Carrasquillo 1986; Farbiarz and Carrasquillo 1986).
In a paper presented at the 8th International Conference on alkali-aggregate reactivity held in Japan in 1989, Swamy and Al-Asali indicated that ASR expansion is generally not proportional to the percentage of cement replacement by fly ash. The rate of reactivity, the replacement level, the method of replacement, and the environment all have a profound influence on the protection against ASR afforded by fly ash. Several investigators (Mehta, 1980; Diamond, 1981; Hobbs, 1982) have stated that ASR expansions correlated better with water-soluble alkali-silica contents than with total alkali content. The addition of some high-calcium fly ash containing large amounts of soluble alkali sulfate might increase rather than decrease the alkali-aggregate reactivity (Mehta, 1983). The effectiveness of different fly ashes in reducing long-term expansion varied widely; for each fly ash, this may be dependent upon its alkali content or fineness (Soroushian, 1990).
The following will discuss on the Type "IP", "P" and "I(PM)" cements. The specifications for these cements are in AASHTO M-240 and ASTM C-595. Blended cements can be manufactured by either intimate blending of portland cement and pozzolan or intergrinding of the pozzolan with the cement clinker in the kiln. Type "I(PM)" (pozzolan modified cement) allows up to 15 percent replacement of cement with fly ash. The Type "IP" and Type"P" are pozzolan-modified portland cements which allow 15-40 percent replacement with pozzolans. The differences in the two types of cements is in the ultimate strength and the rate of strength gain of the concretes. Most States specify limits on the pozzolanic content on Type "IP" cement. These limits are between 15 and 25 percent.
It is well known now that both classes of fly ash improve the properties of concrete, but several factors and cautions should be considered when using fly ashes especially in highway construction, where fly ash is heavily used. In a report prepared by the Virginia Highway and Transportation Research Council (VHTRC) and summarized by Halstead (1986), several restraints relating to the use of fly ash concrete for construction of highways and other highway structures were discussed. These restraints include the following: 1) special precautions may be necessary to ensure that the proper amount of entrained air is present; 2) not all fly ashes have sufficient pozzolanic activity to provide good results in concrete; 3) suitable fly ashes are not always available near the construction site, and transportation costs may nullify any cost advantage; and 4) mix proportions might have to be modified for any chance in the fly ash composition.
Since the cement-fly ash reaction is influenced by the properties of the cement, it is important for a transportation agency not only to test and approve each fly ash source but also to investigate the properties of the specific fly ash-cement combination to be used for each project (Halstead 1986).
The State highway agencies should develop certification programs similar to those in existence for portland cement. This program should include testing by the supplier with check tests on grab samples taken by the agency. The plan should also require that the supplier's laboratory participate in the Cement and Concrete Reference Laboratory (CCRL) program which includes inspection of facilities and testing of comparative samples.
Until the certification programs are in place, it is suggested that the States test the fly ash and use sealed silos and transports. Five tests per silo should be run to insure uniformity of the fly ash. Once uniformity of a source is established, sampling could be reduced to one per 400 tons as specified in ASTM C-311. It is recommended that 10,000 tons of fly ash be tested before reducing the testing frequency.
The air content of each load of concrete should be monitored at least in the beginning of production. This would indirectly monitor the uniformity of the fly ash.
Specifications should contain strength requirements with minimum substitution ratio and maximum replacement. This would allow maximum substitution without sacrificing strength. The water cement ratio should be based on the total cementious materials, i.e., the portland cement plus the fly ash substituted.
Substitution ratios on a minimum of 1 to 1 on a mass basis with a maximum substitution should be specified. A substitution rate of 15 to 25 percent maximum is currently being specified for typical concrete production. These values should be established based on the actual fly ashes and portland cements that are available.
Mix designs should be performed by the State on each combination of materials, or by the contractor with the requirement to provide the test data to the State for verification with trial batches.
Since the chemical composition of fly ashes and portland cements vary considerably, substantial problems could result if fixed rates and percentages of substitutions are used for all combinations of fly ashes and cements.
The EPA guideline on the substitution of fly ash requires the State highway agency to document the reasons for not allowing the substitution of fly ash for cement if it feels that it is technically inappropriate. The following two cases will not require documentation.
Fly ash should not be substituted for a portion of Type "IP", Type "I" (PM) or Type "P".
Substitution should not be specified for high early strength concrete. In this case, concrete that contains fly ash gains strength slower so it would not be capable of having high early strength.
Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.
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