The effect of concrete vertical construction joints on the modulus of rupture

The purpose of this study is to experimentally correlate the compressive strength (f′cf′c) of concrete to the modulus of rupture (fr) for plain concrete beams with a vertical construction joint placed at their center. The ACI code provides a formula for the correlation of fr to f′cf′c, but with the provision that the concrete specimen is monolithic (no joints). It is well known that no concrete structure is built without the use of construction joints, whether planned or un-planned, an engineer would definitely benefit from an equation that could relate the modulus of rupture of concrete as a function of its compressive strength. A better understanding of the effects of construction joints on the modulus of rupture will assist engineers in making rational decisions on how to deal with vertical construction joints, which in turn will lead to ultimate cost savings on large-scale projects.In this study, seven different concrete mix designs were used. From each concrete mix, six plain concrete beams were poured, half of which were monolithic and the other half with a vertical construction joint at the beam center. Four cylinders per design mix were casted for the purpose of obtaining the mix compressive strength. The experimental results indicate that for monolithic beams, the ACI Code always underestimates the modulus of rupture, whereas in the presence of a vertical construction joint, the conducted experiments yield a significant loss in the modulus of rupture of concrete that varies between 24% and 83%. Thus, there is a clear justification for providing dowels at construction joints in order to assure continuity in strength over joints in plain concrete.