The effect of the method of test on the flexural strength of concrete

Abstract

Summary As a basis for standardizing a method of test for the modulus of rupture of concrete, flexure tests have been carried out on beams of various sizes using central and third-point methods of loading and various rates of loading. Tests were also carried out on small beams sawn from larger ones in order to isolate the effects of specimen size. A reduction of approximately 30 per cent was observed in the modulus of rupture when the depth of the beam was increased from 3 in. to 8 in. for a span-depth ratio of 3, but other effects of size were small. Increasing the rate of increase of stress from 20 to 1,140 lb per sq.in. per min. resulted in an increase of about IS per cent in the modulus of rupture. Centnil loading gave results about 20 to 25 per cent higher than third-point loading but the results were less uniform. The effects of the size of specimen have largely been accounted for by variations in the quality of the concrete in beams of different sizes together with a statistical aspect termed the “weakest link” theory, and an effect due to changes in the rate of increase of stress. The effect of the method of loading can largely be explained by the “weakest link” theory and by considerations of the stress distributions in the comparatively short beams used. As a result of the investigation it should now be possible to relate the results from the standard procedure now adopted at the Road Research Laboratory to those from other laboratories using different procedures.