Ultrasonic Pulse Velocity (UPV)
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Checking concrete's pulse
CTLGroup's NDT engineers have used measurement of concrete's pulse velocity since 1975; it was the first accurate diagnostic technique developed to evaluate concrete's relative quality and estimate its compressive strength. The pulse velocity technique employs low-frequency mechanical energy to evaluate hardened concrete quickly. The test method is based on physical laws of elastic stress wave propagation in solids.
A repeated pulse is generated and applied to one face of the test object by means of a transmitting piezo electric transducer. The pulses pass through the concrete and are detected at the opposite boundary by a receiving transducer. The test apparatus electronically measures transit time of the pulse through the concrete precisely and accurately. Pulse velocity of concrete is calculated by dividing distance (concrete member thickness) by transit time. Pulse velocity data are interpreted to evaluate relative quality of concrete and to estimate in-place concrete compressive strength. In general, a pulse velocity higher than 3700 meters/second indicates better-quality concrete.
Relative quality and homogeneity of concrete are evaluated by statistical comparison of pulse velocities measured at grid points established on a concrete structure. Relatively low measured pulse velocity can indicate low concrete strength and the presence of defects such as deficient concrete consolidation or large voids.
To estimate compressive strength of in-place concrete, technicicans make pulse velocity measurements on the structure, then calibrate for the particular concrete being tested. In the calibration procedure, compressive strength tests are performed on five to ten cores extracted at selected locations. A correlation between pulse velocity and compressive strength is then established for the particular concrete. This procedure allows evaluation of concrete compressive strength at many locations on the structure where pulse velocity measurements were made. The advantage of using calibrated pulse velocity measurements is that all areas of deficient concrete strength in a suspect placement can be identified with minimal disturbance and damage to the structure.