It is sometimes necessary to estimate the strength of concrete in place for early form removal or to investigate the strength of concrete in place because of low cylinder test results. The rebound, penetration and pulse velocity tests are relatively simple to perform but the analysis and interpretation of the data are not so easy because concrete is a complex material. Engineers are therefore cautioned that interpretation of data must always be carried out by specialists in this field rather than by technicians performing the tests.
PULSE VELOCITY TECHNIQUES
The ultrasonic pulse velocity method consists of measuring the time it takes an ultrasonic pulse to pass through the concrete. The time of travel between the initial onset and the reception of the pulse is measured electronically. The length of the path between the sending and receiving transducers, divided by the time of travel, gives the average velocity of wave propagation. This technique can be used successfully for a number of purposes; however, less than satisfactory results have been reported when pulse velocity has been used to estimate the strength in place.
The relationship between pulse velocity and strength is affected by a number of variables such as: age of concrete, surface moisture condition, aggregate-to-cement ratio, type of aggregate, and location of steel reinforcement. Inasmuch as a large number of variables affect the relations between the strength parameters of concrete and its pulse velocity, the use of the latter to predict the compressive and/or flexural strengths of concrete is not recommended. Indeed, serious consideration should be given to the use of pulse velocity as a control test in its own right, and perennial attempts to correlate pulse velocity with strength parameters should be discouraged.