Fly ash mix design

Mix Design
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).