The majority of UPV measurements are taken by measuring the transmission time of high frequency Compression Wave (P Wave) energy through concrete, which is the same energy we hear as sound at lower frequencies.
Shear wave energy(S Wave), a side to side motion of particles only propagates in solid materials. UPV system transducers are available that produce and record shear wave energy.
Shear wave energy is also the most common energy used to create Ultrasonic Pulse Echo Array systems for imaging concrete.
To measure the velocity of Compression or Shear wave energy in a structural element a sending and receiving UPV transducer pair is used. Transducers are Piezoelectric and characterised by their frequency and energy.
Before UPV equipment may be used a calibration bar with a known transmission time is used to set the reported Transmission time measurements to only consider the physical transit time between transducers.
The calculation of a Velocity requires the operator to measure and enter the length of the direct signal path between the sending and receiving transducer. By measuring transit time over this distance Velocity is calculated.
UPV hardware is also able to support collecting measurements in a defined area, either at even spacings along a line or every position within a measurement grid.
The assumption is that UPV transducers are placed in the Direct configuration shown below, placed on opposite faces of the element and transmitting directly through the material being tested. Semi-direct placement is also appropriate, working around a corner. The calculation and process described are appropriate with either configuration
When transducers must be placed with an Indirect orientation the measurements can no longer be directly compared, this placement introduces additional uncertainty in the signal path and the measured velocity is typically slightly lower than the same measurement taken with a Direct signal path.
Site testing may use UPV to compare the Velocity of concrete as a measure of consistency, to locate regions of Voiding and Cracking along a signal path or as an indication of Quality.
When checking consistency and looking for defects the main data point interpreted is the comparative velocity between "Good" areas and the rest of the structure. A secondary interpretation would be to use the Amplitude/Signal Strength of the measurements. The lower speed and/or lower signal strength regions indicate potential defects.
Quality measurements use a simple faster is better analysis, Compression Wave Velocity in typical concrete is roughly between 3500-4500 m/s speed.
There is also a commonly used Compressive Strength correlation for Compression wave Velocity, which has the advantage over Schmidt Hammer measurements of considering the internal concrete. Reb-Ultra measurement combines Schmidt and UPV for a better quality correlation.
In the material testing laboratory the Compression and Shear wave Velocities are measured for direct record, or used to further calculate material properties. A common measurement is to use a pair of Compression and Shear Wave Velocity measurement to calculate dynamic E-modulus and Poisson's ratio in rock or concrete samples.
UPV transducers are chosen based upon the minimum sample size which needs to be tested and constrained by a maximum Particle Size which will interfere with the measurements.
54 kHz transducers are most common, but a range of frequencies may be chosen as per the table below, lower frequency is helpful for long transmission lengths, higher frequency for small samples and detecting small defects.
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