Post-event damage assessment of concrete using the fluorescent microscopy technique

The fluorescent microscopy technique was used to evaluate post-event damage in the microstructure of concrete cylinders previously subjected to 50%, 70%, and 90% of their ultimate strength (f'c). A 25-mm disk was then sawn from the mid-height of each pre-damaged cylinder and vacuum-impregnated within an epoxy resin containing fluorescent dye. The application of a fluorescent microscopic technique on the polished petrographic specimens obtained from each disk allowed the development of relationships to estimate crack area, crack width, and crack length at each stress level. To correlate damage in the concrete's microstructural system with degradation in mechanical properties, companion concrete cylinders preloaded identically, were reloaded to failure. Strength remained constant compared with intact concrete; the residual strain capacity of damaged concrete showed a decrease of 29% at 0.9f'c. A relationship was established to evaluate the residual strain capacity using a damage index based on the change in the area of microcracks.