Ultrasonic Pulse Velocity
UPV utilises compression waves (P wave) and is used in the construction industry to determine the presence of internal defects such as voiding & cracking. It can also be used to determine the depth of concrete structures such as piles. UPV require the use of transducers, a transmitter to “inject” the wave into the structure and a receiver to “listen” for the wave arrival. This wave may either be direct or reflected, depending upon the information required. Voids or cracks are detected by measuring the time taken for the wave from the transmitter to arrive at the receiver. The wave takes longer to “go around” a defect than it does going in a straight line in good material. It is generally preferred that the direct transmission procedure be used, however this is dependant upon both opposing surfaces of the structure being available. In structure such as bored piles, it is the reflected wave that is received. The wave is transmitted and the reflection from the bottom of the pile is received. The shape of the received wave then indicates depth of pile and / or any defects within. Unlike a shear wave tomogragh, transducers are placed individually, Their size can vary from 25mm diameter to 75mm and each requires a defect free flat surface. Coupling gel is commonly used to obtain a good “connection to the structure.
Shear waves (S wave) travel slower than compression waves but react when they come into contact with water or air. They also have the ability to pass through reinforcing and steel without significant impact on the wave. They are therefore extremely useful in detecting voiding behind steel where GPR cannot see. An example of this would be checking Post Tension Cable ducts for air gaps after grouting. Hi frequency GPR also attenuates after 300-400mm where as Shear wave can travel to 1.5 metres before full attenuation. This also makes it a useful tool for measuring wall and footing thicknesses by locating the back wall reflection. Unlike UPV, shear wave tomograghs require no coupling gel for the transducers, however the surface must be flat and clean for all the transducers (between 10 – 40) to make contact.