Combined Use of Non-Destructive Tests for Assessment of Strength of Concrete in Structure

Results of an experimental investigation on the effects of concrete materials-, mix- and workmanship-related variables, on the Rebound Number and Ultrasonic Pulse Velocity of concrete, are presented. The investigations aimed at developing a method of combined use of both the non-destructive tests for assessment of strength of concrete with greater accuracy. Workmanship variables included different lengths of moist curing, incomplete compaction and intentionally induced flaws. Rebound Hammer readings increased with the compressive strength of concrete. Ultrasonic pulse velocity values were greatly influenced by the cements and aggregate, extent of moist curing and presence of flaws and voids in concrete, more than their influence on the measured strengths. This demonstrates the limitation of using ultrasonic pulse velocity tests for estimating compressive strength of concrete.IS: 13311 advocates combined use of Ultrasonic pulse velocity (UPV) and Rebound Hammer tests for assessment of concrete strength in structures with greater reliability. However, the approach is qualitative. Adopting such an approach in a quantitative manner, multiple regressions of both Rebound Numbers and Ultrasonic Pulse Velocity on compressive strength of concrete, led to a series of graphs for better assessment of strength.