Contribution to the issue of evaluating the compressive strength of concrete exposed to high temperatures using the Schmidt rebound hammer

Concrete properties change as a result of ambient conditions. The internal microstructure changes under high temperatures. As a consequence, this change affects both physical and mechanical properties of concrete and therefore the parameters determined from nondestructive testing. It was determined that for concrete exposed to temperatures in the range of 200 to 400°C the rebound number measured by the rebound hammer is approximately 9% higher after heat load than for wet concrete placed in a standard environment. Temperatures from 600 to 800°C cause a drop in the rebound number, which correlates with a decrease in the concrete’s compressive strength. A strength dependency was established for the N-type Schmidt rebound hammer expressing the relationship between the rebound number and the compressive strength of concrete exposed to heat load, which is characterized by a high correlation between variables (correlation coefficient is 0.98). To estimate the compressive strength of concrete in a structure which has been exposed by fire, can be used both the calculated strength relationship, both the calibration curves for N-type Schmidt rebound hammer calculated by manufacturer, eventually the basic curve from CSN EN 13791. If the calibration equation for N-type Schmidt rebound hammer or basic curve of CSN EN 13791 is taken, the calculated strength for the rebound numbers should be lowered using the corrective coefficient, the value of which is 0.54, respectively 0.65.