The effect of coarse aggregate on the mode of failure of concrete in compression and flexure

<jats:p> Summary </jats:p><jats:p> Experiments have been made to determine the flexural and compressive strengths of concretes containing thirteen different coarse aggregates. In the compression tests, observations were made by an ultrasonic pulse technique to detect the onset of cracking prior to failure. </jats:p><jats:p> The main results were as follows. Concretes containing smooth gravels began to crack at lower compressive stresses than did concretes containing coarser-textured aggregate. The compressive strengths did not show such appreciable differences. Concretes which best resisted pre-cracking in compression also gave highest flexural strengths. The relation between the flexural strength of the concrete and the stress at which cracks first occurred in compression was independent of the type of coarse aggregate; no such unique relation existed between flexural strength and ultimate compressive strength. Tentative conclusions are drawn on the mode of failure of the concrete as follows. </jats:p><jats:p> Pre-cracking in a cube when tested in compression probably arises from local breakdown in the adhesion between the coarse aggregate and the cement. The average compressive stress at which the first cracks occur is determined by those properties of the aggregate which influence the aggregate-cement bond. </jats:p><jats:p> Pre-cracking in compression does not lead to the immediate fracture of the concrete specimen. A contribution to the ultimate strength is provided by the mechanical interlocking of the coarse aggregate after pre-cracking has occurred. Thus the compressive strength of concrete is usually greater than the compressive strength of its comparable mortar. </jats:p><jats:p> In flexure, pre-cracking causes fracture of the concrete specimen and the only properties of the coarse aggregate which influence the flexural strength are those which appear to determine the aggregate-cement bond. In general the flexural strength of concrete is lower than the flexural strength of the corresponding mortar. </jats:p>