Effect of aggregates on the compressive strength and permeability of concrete

Aggregates make up between 60% and 80% of the concrete volume, being surrounded by the cement paste and their properties all have significant impact on the behavior of concrete, during the service life for which it been designed. For this reason, a series of laboratory tests were conducted during this investigation to evaluate the effects of mechanical, durability and geometry properties of aggregates on the compressive strength and permeability of the 28-day concrete specimens. Concrete specimens were made with different types of aggregates, including granite, andesite, silica, limestone, marble and tuff. Studied mechanical properties were compressive, tensile and surface strengths, Los Angeles abrasion index, Schmidt hardness and saturated surface dry density, while permeability, water absorption and porosity were investigated as durability properties. Geometry properties of aggregates, including circularity, solidity and aspect ratio were also measured to investigate the effect of aggregates shape on the concrete compressive strength and permeability. To measure permeability and surface strength, the methods of “Cylindrical chamber” and “Twist-off” were applied, respectively, which were proposed and developed by Naderi. The results obtained revealed that a direct linear correlation, with coefficient of determination higher than 0.89 exists between compressive and surface strengths of the studied concrete specimens and parent rocks. On the other hand, parent rock compressive strength is approximately 140%-270% higher than that of the concrete containing aggregates of the same parent rock, while its permeability is nearly 59%-71% lower than the permeability of the corresponding concrete. It was also observed that concrete compressive strength decreases with increasing its permeability and vice versa. Analysis of the results demonstrated that strong correlations, with coefficients of determination higher than 0.73 exist between concrete compressive strength, concrete permeability and the mentioned mechanical and durability properties of parent rocks and the corresponding aggregates. On the contrary, coefficients of determination lower than 0.1 were obtained for the relationship between geometry properties of aggregates, concrete compressive strength and permeability. This behavior revealed that the impact of aggregates shape on concrete compressive strength and permeability is not significant, compared with the mentioned mechanical and durability properties of the parent rocks and the corresponding aggregates.