How does aggregate affect the strength of concrete?

The tensile strength of concrete is severely affected by the increase in aggregate size. By increasing the maximum grain size to 120—180 mm, the reduction in tensile strength is 30 to 50% compared to concretes with a maximum aggregate size of 20 mm. There is also a reduction in the elongation limit. The shape and texture of the aggregate affect the properties of fresh concrete more than hardened concrete.

Concrete is more workable when a smooth, rounded aggregate is used instead of a rough or elongated angular aggregate. Most natural sands and gravel in riverbeds or seashores are smooth and rounded and make excellent aggregates. Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bonding characteristics, but require more cement paste to produce a workable mix. Most stacked coarse-grained aggregates are in the AD state with an uptake of less than one percent, but most fine aggregates are often in the wet state with a surface moisture of up to five percent.

An experienced concrete professional can use your project evaluation to identify the best shape and size of concrete aggregate for each phase of construction. For heavy RCC elements, such as main beam ribs, the nominal maximum aggregate size should generally be restricted to 5 mm less than the minimum clearance distance between the main bars or 5 mm less than the minimum cover to the reinforcement, whichever is smaller. All aggregates contain some porosity and the specific gravity value depends on whether these pores are included in the measurement. Therefore, for a given aggregate-cement ratio in a concrete mix, workability will be higher if rounded aggregates are used compared to others.

When determining the strength of normal concrete, most concrete aggregates are several times stronger than the other components of concrete and are therefore not a factor in the strength of normal strength concrete. Although aggregate is considered inert filler, it is a necessary component that defines the thermal and elastic properties of concrete and dimensional stability. As the aggregate size decreases, the mixture requires more water to suspend the particles and create a viable solution. Aggregate is commonly considered inert filler, accounting for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete.

These different aggregate properties allow designers and contractors the greatest flexibility to meet their design and construction requirements. However, gravel or rounded aggregates have been used for high-strength concrete without any serious problem of poor adhesion. The crushing value of the aggregate provides a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. Since the weight of the aggregate depends on the moisture content of the aggregate, a constant moisture content is required.

Fine aggregates are aggregates, most of which pass through a 4.75 mm IS sieve and contain only a much coarser material, which is allowed according to the specification. The maximum amount of harmful materials in coarse-grained and fine aggregates (crushed or natural) should not exceed the limits specified in the table. The classification or size distribution of the aggregate is an important characteristic because it determines the paste requirement for workable concrete.

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