The effect of the aggregates characteristics on the performance of Portland cement concrete

Quiroga, Pedro Nel
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Aggregate shape, texture, and grading have a significant effect on the performance of fresh concrete. Aggregate blends with well-shaped, rounded, and smooth particles require less paste for a given slump than blends with flat, elongated, angular, and rough particles. At the same time, uniform gradings with proper amounts of each size, result in aggregate blends with high packing and in concrete with low water demand. Optimized aggregate blends have high packing, requiring low amounts of paste. As a result, they are less expensive and will have less durability problems caused by the paste such as heat generation, porosity, and drying shrinkage. Current ASTM C 33 standard limits the amount of material passing the N 200 sieve (micro fines) to 7 percent. However, manufactured fine aggregate (MFA) usually has between 10 and 20 percent micro fines. These limits, intended for natural sands, force MFA producers to wash aggregate incrementing costs and generating environmental issues. Research at the University of Texas and experience in other countries show that good quality concrete can be made with MFA with high micro fines content. Many proportioning methods such as ACI 211 do not consider high amounts of micro fines, considers partially the effect of shape and texture of aggregates, and do not encourage optimization of aggregate. The effect of shape, texture and grading of aggregates on fresh concrete was evaluated experimentally, quantified by means a proportioning method based on packing density concepts, the Compressible Packing Model (CPM), and analyzed by an empirical tool suggested by Shilstone. The effect of different types and amounts of micro fines was evaluated simultaneously as well as the impact of chemical admixtures and supplementary cementing materials on concrete with high micro fines was also evaluated experimentally. And it was concluded that chemical admixtures and some supplementary cementing materials can be used to improve the workability of concrete with high micro fines without negatively affecting hardened concrete. Guidelines for proportioning and optimizing aggregate blends were made based on Shilstone’s Coarseness Chart and the 0.45 Power Chart and CPM equations and procedures.