How Coarse Aggregate Affects Mix Design of Concrete?
Coarse aggregate is one of the essential components of concrete and occupies the largest volume in the mix. That is why it greatly affects the concrete mix design. Its properties such as strength, maximum size, shape, and water absorption influence water demand, the quantity of cement and fine aggregate in concrete mixture.
It is reported that, high maximum coarse aggregate size leads to lower water demand in the mixture since such aggregate has lower surface area compare with small coarse aggregate size. As far as shape is concerned, rounded shape aggregate provides economical mix design for normal strength concrete.
However, angular coarse aggregate is desired in the case of high strength concrete. This possibility of segregation is minimized if coarser aggregate is properly graded. This explains how important is good grading for concrete mix design.
As far as strength is concerned, higher aggregate strength would produce higher concrete strength provided that other controlling factors have been dealt with properly.
1. Maximum Aggregate Size
The maximum size of coarse aggregate is one of the factors that controls water demand to achieve certain work-ability. It also dictates quantity of fine aggregate content needed for producing cohesive mix.
For a given weight, higher the maximum size of aggregate, lower is the surface area of coarse aggregates and vice versa. As the maximum size of coarse aggregate reduces, surface area of coarse aggregate increases. Higher the surface area, greater is the water demand to coat the particles and generate workability.
Smaller maximum size of coarse aggregate would require greater fine aggregate content to coat particles and maintain cohesiveness of concrete mix. So, for the same workability, 40mm down aggregate would have lower water/cement ratio, thus higher strength when compared to 20mm down aggregate. Because of its lower water demand, advantage of higher maximum size of coarse aggregate can be taken to lower the cement consumption.
Maximum size of aggregate is often restricted by clear cover and minimum distance between the reinforcement bars. Maximum size of coarse aggregate less than clear cover or minimum distance between the reinforcement bars. This allow aggregates to pass through the reinforcement in congested areas, to produce dense and homogeneous concrete.
2. Grading of Coarse Aggregate
Grading is the determination of the particle-size distribution for aggregate. It affects the amount of cement and water requirements, workability, pumpability, and durability of concrete. The grading of coarse aggregate is important to get cohesive and dense concrete. The voids left by larger coarse aggregate particles are filled by smaller coarse aggregate particles.
Proper grading of coarse aggregate can minimize the possibility of segregation, especially for higher workability and improves the compatibility of concrete. The coarse aggregate grading limits are given in ASTM C33/ C33M and IS 383 – 1970 – table 2, Clause 4.1 and 4.2 for single size aggregate as well as graded aggregate.
3. Shape of Coarse Aggregate
Coarse aggregates may be round, angular, or irregular shape. Rounded aggregates have lowest water demand due to lower surface area, and also have lowest mortar paste requirement.
These properties make rounded aggregate to yield the most economical mixes for concrete grades up to M35. However, for concrete grades of M40 and above the possibility of bond failure would tilt the balance in favor of angular aggregate with more surface area.
Flaky and elongated coarse aggregate particles not only increase the water demand but also increase the tendency of segregation. Flakiness and elongation also reduce the flexural strength of concrete. Specifications by Ministry of Surface Transport restrict the combined flakiness and elongation to 30% by weight of coarse aggregates.
4. Strength of Coarse Aggregate
Material strength of coarse aggregate is indicated by crushing strength of rock, aggregate crushing value, aggregate impact value, aggregate abrasion value. The IS limits for above tests are Aggregate Crushing value, Aggregate Impact value, and Aggregate abrasion value.
5. Aggregate Absorption
Aggregate absorption is used for applying a correction factor for aggregates in dry condition and determining water demand for concrete in saturated surface dry condition. Aggregate can absorb water up to 2 % by weight when in bone dry state. However, in some cases, the aggregate absorption can be as high as 5%.