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Aggregates are an important component of concrete. Not only does it give volume to concrete, it also help reduce shrinkage and cost of concrete.
In terms of volume, aggregates may take up almost 80% of the volume of concrete. Thus its impact on various properties of concrete cannot be underestimated. And gradation of aggregates is one important contributor.
Aggregate gradation determines the void content within the structure of aggregate and consequently the amount of cement paste that is required to fill the void space and ensure a workable concrete.
It is desirable to optimize the aggregate gradation in concrete as cement is the most expensive components of concrete. Therefore one obvious goal is to minimize the void content within the aggregates and thus reduce the volume of cement paste required to achieve a workable, economical and an environmentally sound concrete for a given application. The optimization of aggregate gradation also improves the rheological, mechanical and durability properties of concrete.
Proper aggregate gradation not only ensure a workable concrete mixture that can be compacted easily, but also reduces problems associated with plastic concrete such as potential for segregation, bleeding and loss of entrained air and plastic shrinkage cracking. Furthermore, most concrete that is used in construction of transportation infrastructure is often vibrated to achieve good compaction in concrete.
Segregation in plastic state under vibration particularly is the most vulnerable problem in concrete containing aggregate with poor gradation. Cement paste filling the void space between the aggregate has a tendency to shrink when there is a progressive loss of moisture from concrete, either due to evaporation from surface of concrete or through internal consumption of moisture due to hydration reactions of cement.
Aggregates in concrete, being much stiffer than the hardened cement paste, act to resist the shrinkage behaviour of concrete. Aggregate gradation, which determines the relative proportions of aggregate and cement paste in a concrete, therefore dictates the shrinkage behaviour of concrete and hence long-term durability of concrete.