The uniaxial compressive strength of concrete: revisited

This paper re-examines common notions and conventions regarding the compressive strength of concrete in general and of the uniaxial compressive strength of concrete in particular. A distinction is introduced between the strength of the specimen and the strength of the concrete as a material, and the commonly measured and adopted strength is shown to be the specimen’s strength, wrongly interpreted as the material’s strength. the two major damage modes of concrete specimens (with the formation of either longitudinal cracks or shear bands) are discussed. Such failure modes are wrongly considered as features of concrete behavior in uniaxial compression, but this is not the case. Longitudinal cracking is due to lateral expansion (Poisson’s effect) and occurs at a relatively low applied load in absence of friction at specimen’s top and bottom boundaries. Shear failure (accompanied by the formation of an inclined shear band) is related to the shear envelope parameters that are related to the concrete mixture, but the applied ultimate pressure is not the concrete uniaxial compressive strength. Hence, though caused by applied compressive loading, these failure modes are little/hardly related to the concrete material intended as the ultimate uniaxial stress (strength) corresponding to a maximum value of the uniaxial compressive strain. Using the shear envelope parameters has been proven to yield a very good prediction of the applied compressive loading of the specimen in the limit state, as a demonstration that the applied pressure at specimen’s failure resulting from the formation of inclined fracture bands is the specimen’s failure strength, and not the material’s compressive strength! Reasons are given against the existence of a uniaxial compressive strength failure for concrete, and a piece of evidence in this direction is provided by concrete specimens subjected to pure hydrostatic compression, that do not fail at all. The entire issue requires, therefore, a deep revisiting and re-thinking, to provide correct measures for representing concrete response under compression in analysis and design.