For the ultrasonic test at lease two tubes (either plastic or steel, minimum diameter of 50 mm or 2"), are cast in the pile and filled with water. A transmitter of ultrasonic pulses is lowered in one of those pipes and a receiver - in another. Both transmitter and receiver are connected by cables to the CHUM which records the first arrival time (FAT) and the energy attenuation as the probes are simultaneously raised to the top. As long as the FAT and the energy attenuation are roughly constant, one may deduct that the concrete quality is also uniform and the pile is therefore acceptable. On the other hand, if at some level there is a noticeable increase in the FAT and/or in the energy attenuation, it means that the concrete at this level is inferior or defective. In such a case, the test may be re-run with the transmitter and receiver at different levels, a technique which enables the determination of both location and extent of the defect (tomography). Installing a larger number of pipes on the perimeter gives an almost complete coverage of the pile cross-section.
Because of the character of the ultrasonic method, it can detect defects which may escape detection by other integrity testing methods. It is especially suitable for testing large-diameter piles and slurry-wall elements.
If a defect is found, the steel tubes may be pierced at the corresponding depth and the pile repaired by grouting.
The piles can be tested after the concrete has gained some strength, usually at an age of five days or more from casting .