Does Aggregate Add Strength to Concrete?

Aggregates are a major component of concrete, making up between 60-80% of its volume and 70-85% of its mass. Not only do they play a key role in the strength, thermal and elastic properties, dimensional stability and volume stability of concrete, but they also have a direct impact on its workability. Rough, angular particles are more tightly packed, have more surface area and greater inter-particle friction than smooth, rounded particles. This reduces the workability of plastic concrete and requires a slightly higher cement content to coat the particles.

To ensure optimal performance, aggregates must be solid, clean, hard, durable and free from excessive fines or contaminants that could affect cement hydration or alter the paste-aggregate bond. Regular monitoring of the moisture content of coarse-grained and fine aggregates is essential for consistency and uniformity from batch to batch. Optimizing the grading based on aggregate availability and project requirements will result in cost-effective concrete with good workability and finishability. The ratio between coarse-grained and fine aggregates will vary depending on the characteristics of each aggregate, the method of placement and the desired finish.

For a given weight, larger maximum sizes of aggregate require less surface area of coarse-grained aggregates than smaller maximum sizes. This means that a smaller maximum size of the coarse-grained aggregate would require a higher content of fine aggregate to coat the particles and maintain the cohesion of the concrete mix. The classification or size distribution of the aggregate is an important characteristic because it determines the paste requirement for workable concrete. Well-graded aggregate is best, however, aggregates rated at maximum density can give concrete a rough texture.

Each region has its own deficiencies in aggregates, but once a combined aggregate gradation is plotted (the percentage retained versus other physical and mineralogical properties of the aggregate) must be known before mixing the concrete to obtain a desirable mix. The density of the aggregates is necessary in the dosing of the mixture to establish weight-volume ratios. The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete. If there is a deficiency in a locally available fine aggregate, air entrainment, additional cement or a supplemental cementitious material (SCM) may be added to address these deficiencies.