Ultrasonic testing on evaluation of concrete residual compressive strength: A review

Residual mechanical strength is a necessary parameter in any diagnosis of recovering structures after fire. Even though concrete is a material traditionally resistant to high temperatures, it can lose a considerable part of its mechanical properties (compressive strength; modulus of elasticity). Numerous are the parameters involved in the residual mechanical strength of post-heating concrete (type of aggregate, strength class of the concrete, moisture content, porosity, the heating and cooling procedure of the specimens, etc.) In order to map these alterations in the concrete, destructive and non-destructive techniques are used. The ultrasonic technique, for example, is one of the most used ones, because it allows estimating the material mechanical properties without damaging it. Nevertheless, it is not always suitable to correlate the UPV with the residual compressive strength, due to other factors that interfere in the behavior of the ultrasonic pulse, specifics to the analyzed concrete. In this scenario, this work makes a rigorous and current bibliographic survey on researches that considered the relationship between ultrasonic pulse velocity and residual compressive strength of concrete exposed to high temperatures. The results from the works presented were evaluated according to the relevance of the mentioned parameters. The analyses showed that regarding concrete under high temperature is not always reasonable to use a general equation between residual ultrasonic pulse velocity and residual compressive strength to correlate them. While the compressive strength showed significant variations on the parameter mentioned above, the UPV did not behaved the same way. Thus, based on the findings from the works evaluated, a relationship between Ultrasonic Pulse Velocity and concrete heating temperature is suggested. This relation will provide more accuracy on the estimation of residual compressive strength of concrete through a non-destructive technique. This estimation will be based on the residual compressive strength of concrete, through the traditionally proposed relationship (residual mechanical strength x heating temperature) given by international codes, using the temperature related to the residual UPV value.