FLY ASH IN CONCRETE

APPLICATION OF FLY ASH IN CONCRETE

Over the past several decades, the use of fly ash in concrete has had a successful track record. The performance benefits fly ash provides to mechanical and durability properties of concrete have been thoroughly researched and documented in actual structures. Cement and Concrete Industry accounts for 50% fly ash utilization, the total utilization of which at present stands at 30 million tons (28%). The other areas of application are low lying area fill (17%), roads and embankments (15%), dyke raising (4%), brick manufacturing (2%) etc. The life cycle cost of fly ash based building materials/constructions is much lower taking into account the environmental benefits and durability aspects.

Fly ash concrete, as the two words suggest, is just an alternate version of concrete within which a predefined content of cement has been salvaged by instilling fly ash. The percentage of cement being replaced can vary, depending upon the use and priorities. Normally it gets in between 10-40%, whereas it may go up to 60% in High Volume Fly Ash Concrete. It is generally found that fly ash content in the cementitious material varies from 30-80% for low strength (20MPa) to high strength (100MPa). The amount of substitution is also dependent on the chemical composition of the fly ash and the Portland cement. The two properties of fly ash that are of most concern are the carbon content and fineness. Both of these properties will affect the air content and water demand of concrete.

MODIFICATION IN CONCRETE CHARACTERISTICS

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. Finer fly ash increases the water demand due to increase in surface area. The finer material requires more air-entraining agents to give the mix the desired air content. If fly ash is uniform in size, the mix design can be adjusted to give a good uniform mix.

Use of fly ash increases the absolute volume of cementitious 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. It has been found that both classes of fly ash concrete improve concrete workability.

Using fly ash in air-entrained and non-air entrained concrete usually reduces bleeding by providing greater fines volume and lower water content for a given workability. Concrete with relatively high fly ash content will require less water than non-fly ash concrete of equal slump.

All class F and most class C fly ashes increase the time of setting of 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. However, fly ash concrete usually has higher ultimate strengths when properly cured.

On the basis of a comparative experimental study of freeze-thaw durability of conventional and fly ash concrete, it has been observed that addition of fly ash has no major effect on the freeze-thaw resistance of concrete if the strength and air content are kept constant.

One of the most important reasons for using fly ash in highway construction is to inhibit the expansion resulting from Alkali-Silica Reaction. It has been found that the alkalies released by the cement preferentially combine with the reactive silica in the fly ash rather than in the aggregate and the alkalies are tied up in non-expansive calcium-alkali-silica gel.

FLY ASH CONCRETE – PROS AND CONS

The advantages of using fly ash far outweigh the disadvantages. The most important benefit is reduced permeability to water and aggressive chemicals. Properly cured concrete made with fly ash creates a denser product because the size of the pores is reduced. This increases strength and reduces permeability.

ADVANTAGES

Since fly ash particles are spherical and in the same size range as Portland cement, a reduction in the amount of water needed for mixing and placing concrete can be obtained. In pre-cast concrete, this can be translated into better workability, resulting in sharp and distinctive corner and edges with a better surface appearance. This also makes it easier to fill intricate shapes and patterns. Fly ash also benefits pre-cast concrete by reducing permeability, which is the leading cause of premature failure. The use of fly ash can result in better workability, permeability, cohesiveness, finish, ultimate strength and durability. The fine particles in fly ash help to reduce bleeding and segregation and improve pumpability and finishing, especially in lean mixes.

DISADVANTAGES

Poor quality fly ash can have a negative impact on concrete. It can actually increase the permeability of concrete. Some concrete will set slowly when fly ash is used. Freeze-thaw durability may not be acceptable with the use of fly ash in concrete. The amount of air entrained in the concrete depends on the amount of fly ash used and it controls the freeze-thaw durability. High carbon content in certain fly ash products absorbs some air entraining agents, reducing the amount of air produced in the concrete, making the concrete susceptible to frost damage. High carbon fly ash materials tend to use more water and darken the concrete as well. Most of the time, High-fineness and Low-carbon fly ash will result in high early strength. Some class C fly ash will not protect against the alkali-aggregate reaction. Lastly, it is important for the precast concrete producer to test the mix design continually, because fly ash is a group of materials that comes from burning coal.

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