Application of the scanning electron microscope to fracture studies of concrete

Abstract

Concrete, under compression, fails when macrocracks have propagated to the extent that the concrete will not support the applied loads. Many investigators have implied that the failure mechanism of concrete is related to internal microcracking. However, due to the limitations in the techniques employed, the detection of microcracks was somewhat uncertain. The scanning electron microscope (SEM) was chosen as the viewing apparatus because of its distinct advantages to directly observe the formation and/or propagation of microcracks. Microcracks were found to exist in concrete prior to application of compressive stress fields in the form of shrinkage microcracks (initial bond microcracks). As the compressive stress increases these microcracks widen and propagate until failure occurs.