Aggregates are one of the important constituents of concrete that comprises up to 75 percent of concrete. They are considered as inert fillers in concrete but play important roles in the behaviors both freshly mixed and hardened concrete. Changes in gradation, maximum aggregate size unit weight and moisture content affect the character and performance of concrete.
Economy, workability, durability and strength are important factors in the selection of aggregates. Considering economy increasing the maximum aggregate size reduce cost by reducing cement requirement, which is the most costly ingredient of concrete. And it means lower water if water to cement ratio is kept constant; which in turn reduce the possible shrinkage and creep of concrete in the long term.
Workability is another factor for aggregate selection. Increasing the size of aggregates, elongated and angular shapes reduce workability while more rounded and smaller aggregates are easier to work with.
The strength of a concrete mix largely depends on the strength properties of aggregate and its chemical content.
The rapid development of high-rise buildings, housing complexes, malls, governmental and nongovernmental buildings and road construction has created high demand for aggregate. The demand for aggregate is directly related with the growth in the construction sector which in turn depends on the overall economic development.
Gradation is the particle size distribution of an aggregate as determined a sieve analysis using wire mesh of square openings. There are a number of sieve sizes used for fine aggregates and coarse aggregate. Both fine and coarse aggregates should satisfy certain gradation limits to attain economy, workability, shrinkage and strength requirements.