Coarse Aggregate and its effects in Mix Design of Concrete

April 21, 2020

Coarse aggregates are one of the main components of mix design and it has a large volume in mix design of concrete. That is why it greatly affects the concrete mix design. Its property like strength, size, shape, water absorption capacity influence the demand of water, cement and fine aggregates in mix design.

High maximum coarse aggregate size will provide lower demand of water because this aggregate has lower surface area compare to small aggregate size. Rounded shaped aggregates provide economical mix design for normal concrete. Angular coarse aggregates are used in high strength concrete. Higher aggregate strength would produce higher concrete strength.

Maximum size of coarse aggregate will give the idea about water demand and quantity of fine aggregate in mix design. It will improve workability. Higher maximum size of aggregate lower the surface area and lower aggregate provides large surface area. If surface area is more water demand is higher and surface area is small will demand less water. Maximum size of aggregate is often restricted by clear cover and minimum distance between the reinforcement bars allow aggregates to pass through the reinforcement in congested areas, to produce dense and homogenous concrete.

Now, grading of coarse aggregates is equally important. Grading means the determination of the particle size distribution for aggregates. It affects the amount of cement and water requirements, workability, pumpability, and durability of concrete.

Proper grading of coarse aggregate can minimize the possibility of segregation, especially for higher workability and improves the compatibility of concrete. There is well, poorly and gap graded coarse aggregates.

Coarse aggregates may be round, angular, or irregular shape. Flaky and elongated coarse aggregate particles not only increase the water demand but also increase the tendency of segregation.

Material strength of coarse aggregate is indicated by crushing strength of rock, aggregate crushing value, aggregate impact value, aggregate abrasion value.

For applying a correction factor for aggregates in dry condition and determining water demand for concrete in saturated surface dry condition aggregate absorption is used.

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