What is the importance of particle size distribution of aggregate for concrete?

The classification or size distribution of the aggregate is an important characteristic because it determines the paste requirement for workable concrete. This paste requirement is the factor that controls the cost, since cement is the most expensive component. This is typically achieved through proper storage of aggregates and reprocessing stockpiles to counteract excessive segregation (Photo. The permitted percentage of harmful substances for fine and coarse-grained aggregates are listed in Tables 1 and 3 of ASTM C 33, respectively.

Once processed, aggregates are handled and stored to minimize segregation and degradation and prevent contamination. Recycled concrete will have a higher absorption and a lower specific gravity than conventional aggregates. Producing good quality, durable concrete containing a portion of recycled concrete aggregate often requires test concrete mixes and close control of the properties of old recycled concrete, with mix adjustments made, as needed. This is a summary of the most important factors to consider when selecting and dosing concrete aggregate.

Using larger coarse-grated aggregates generally reduces the cost of a concrete mix by reducing requirements for cement, the most expensive ingredient. When specifying void graded aggregates, certain particle sizes are omitted from the aggregate of the size continuum. Optimized grading based on aggregate availability and project requirements will result in cost-effective concrete with good workability and finishability. Good quality aggregate must be clean, hard, strong, have durable particles, and be free of absorbed harmful chemicals, clay coatings, or other contaminants that may affect cement hydration or reduce paste-aggregate bonding.

Each region has its own deficiencies in aggregates, but once a combined aggregate gradation (percentage retained versus alternative aggregate sources or additional aggregate mixture) is plotted, one can consider addressing the elusive ideal gradation that provides the best workability, pumpability, reduction of contraction and economy (Figure. In areas where yard space is limited and storage piles are difficult to keep separate, barriers should be used to prevent mixing and cross-contamination of materials of different sizes. This paper investigates the influence of the classification and shape of aggregates on the performance of the coating mortar, both in the fresh and in the hardened state (mechanical properties and durability). Aggregates greatly influence freshly mixed and hardened concrete properties, mix ratios and economy.

Crushed stone is preferred in pavement mixes, since the higher paste-aggregate bond produces greater flexural strengths (Photo courtesy of PCA).

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