Effects of Fly Ash on Properties of Hardened Concrete

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There are various effects of fly ash on properties of hardened concrete such as strength and durability, creep and volume changes of concrete. This article discusses the concrete properties that are affected by use of fly ash in concrete.

Effects of Fly Ash on Properties of Hardened Concrete

Following are the effects of using fly ash on hardened concrete properties:

1. Effects of Fly Ash on Development of Concrete Strength

Amount of cement utilized in concrete and water to cement ratio are considered major factors specify or control the strength of concrete. These significant components of concrete are computed in practice based on cost, hardened concrete durability and strength, and fresh concrete workability which is required for conveying, placing, and costing the mixture of concrete. Fly ash concrete strength development is mainly affected by the degree and way fly as influence the workability. Moreover, it is demonstrated that using fly ash in concrete decrease concrete water requirements and it neither cause issues in the selection of mixture proportion nor prevent development of concrete strength at any rate. There are different factors which influence fly ash-concrete strength development such as particle size, chemical composition, fly ash properties, reactivity, temperature and other condition of curing.

2. Effects of fly ash type on concrete strength

There are various types of fly ashes which their main difference is in amount of calcium content that is an indicator to whether it is cementitious not. ASTM Class C (high calcium) fly ash, which manufactured commonly from burning lignitic sub-bituminous coal in power plants, can replace cement on equal volume or weight basis and its affect on strength development of fly-ash concrete is proven to be similar to cement controlled concrete. ASTM Class F or low calcium fly ash is another type that is the first category to be used as cement replacement fly ash. Fly ashes that were produced from old power plants contained large grain size and unburned coal particles that led fly ash to be less reactive and strength development rate were slow. Long term concrete strength is not influenced by fly and it is shown that Class F fly ash requires longer moist curing time compare with high calcium fly ash.

3. Effects of Temperature and Curing Regime on Strength Development in Fly-Ash Concretes

At standard temperature of 20C^o, early strength development of both cement controlled and fly ash is not affected and lower strength is achieved by fly ash concrete. At high temperature curing, early strength development of both cement controlled and fly ash concrete is increased significantly because of increasing rate of hydration reaction. However, high strength obtained at early ages is decrease at later for both concrete and cement controlled concrete severely affected and its strength decreased substantially by higher temperature. This might be due to uneven distribution of hydrated products that increased pore sizes which in return decrease strength.

4. Effects of Fly-Ash on Elastic Properties of Concrete

It is estimated that fly ash is slightly impact on concrete elastic properties. There is small increase in modulus of elasticity of fly ash concrete compare with cement controlled concrete provided the concrete have the same strength with and without fly ash. Moreover, elastic modulus of fly ash concrete is small at early ages but it is high at later ages.

5. Effects of Fly Ash on Creep Properties of Concrete

Creep is defined as internal strain which is related to the continuous applied stress. It is demonstrated that the percentage of fly ash replacement influence the affect of fly ash on hardened concrete for example, concrete with up to 25 percent fly ash do not show considerable changes in fly ash concrete compare with conventional concrete. However, replacing cement of more than 25 percent will lead to increase the total creep noticeably. It is thought that creep characteristic of concrete is changed when percentage of fly ash replacing cement affect concrete obtaining strength. More importantly, there is a small reduction in basic creep of fly ash concrete when loads are applied at early ages.

6. Effects of Fly Ash on Volume Changes of Concrete

It is claimed that, for the case where the concrete has normal sections such walls and frame in buildings, the drying shrinkage at exposed surfaces of fly ash concrete up to one year of age is slightly lesser or like conventional concrete. However, at a short distance from the exposed surface the drying shrinkage up to the age of one year is considerably less for cement controlled concretes. For extremely thin sections, drying shrinkage of concrete, which contained finely ground high-early-strength cements with normal fineness, could be decreased by employing fly ash.