An in-situ study of stress evolution and fracture growth during compression of concrete

We experimentally correlate fracture nucleation with the evolution of aggregate stress tensors during the compression of concrete. The concrete sample, made with portland cement and single-crystal quartz aggregates, was compressed to failure in eight strain increments. X-ray computed tomography was used to determine the extent of fractures and their proximity to aggregates at each strain increment. Three-dimensional X-ray diffraction was simultaneously used to evaluate the full stress tensor in 30 to 40 individual aggregates at each strain increment. We found that a recently-developed mean-field model provides a good prediction of average aggregate stress tensors throughout the sample but does not predict the significant stress heterogeneity observed in individual aggregates, which are likely caused by aggregate interactions and may be responsible for fracture nucleation. Stress variations in aggregates after the nucleation of particular fractures suggest that fractures divided the sample into distinct load paths that experienced either an increase or decrease in compressive stress with further strain. We discuss extensions of the experimental measurements for quantifying specific failure processes in concrete, such as the fracture of the interfacial transition zone. We also discuss the applicability of the measurements to a broad range of other composite materials, and use of the measurements for developing and calibrating theoretical and computational models for composite strength.