Non-destructive testing using ultra-sonic waves is an effective way to determine the quality of concrete and other materials. Ultra-sonic testing is a practical method of locating defects such as voids, honeycombing, delamination, and internal cracking. An advantage of ultra-sonic waves in comparison to radio waves (GPR) is that they can penetrate through metallic materials (by approximately 60%) and cannot travel through air; therefore can be used to locate voids within post-tensioning systems along with other applications. Empirical carry out ultra-sonic surveys on site with experienced operatives using the following equipment and techniques: 

• Pundit Lab – In principle, an electro-acoustical transducer emits a pulse of longitudinal vibrations to a second transducer over a known path length, which creates an electrical signal. The time in which this signal travels between the transducers can then be recorded to determine the quality and consistency of a concrete element.  When the ultra-sonic wave encounters any air it will find a path around it on its route to the receiving transducer, therefore the pulse takes longer to complete its path. In most applications it is likely the concrete will be reinforced, and although it can have minor effect on results the pulse will travel through the steel and not affect the test.  

• Ultra-Sonic Tomography – Unlike the pundit lab technique ultra-sonic tomography does not require two transducers and can be used when access is only available to one face of the element. The unit itself typically has twenty four individual transducers that penetrate ultra-sonic waves into the concrete giving live data to the user in the form of tomography. Each individual scan location can be processed and stitched together to give the onsite engineer a real time understanding of potential defects at the test location. The following applications are an example of what empirical offer and have experience in undertaking: 

• Post Tensioned Quality Assurance – Initial tracing of post-tensioned ducts can be carried out by GPR (Ground Penetrating Radar), however the radio waves will not penetrate through the metal duct. Using the ultra-sonic tomography the ducts can be traced along the length of the tendon that is accessible, and potential areas of defects can be identified. These targeted locations can be intrusively exposed to calibrate and confirm results by trained operatives. 

• Location of Defects within Reinforced Concrete – Tomography is proven to locate voids and honeycombing within concrete to help gain an understanding of concrete quality. An example can be poor compaction of newly poured concrete where air pockets can be present. The technique can identify locations where repairs may be necessary. GPR has been proven to locate larger voids within concrete, however due to the sensitive nature of ultra-sonics it can locate smaller defects such as honeycombing. 

• Testing of Steel Fibre Reinforced Concrete – Due to the presence of steel fibres in this type of construction GPR is typically not suitable due to the fibres attenuating the GPR signal. Up to 60% of the ultra-sonic wave can pass through steel, therefore it is successful in locating voids within fibre reinforced concrete. This technique can also be used to identify the thickness of steel fibre reinforced concrete. 

• Impact Echo – Impact echo tests on concrete involve an operative striking the surface with an impactor which generates a stress wave (sound) in all directions. Similar to ultra-sonics these waves are reflected by internal defects within the concrete element such as cracks, honeycombing, delamination etc.  Applications for this test can include some of the following: 

• Pile integrity testing 

• Concrete element thickness

• Concrete quality determination