Ultrasonic Testing of Concrete – How it works?
The concept behind the technology is measuring the travel time of acoustic waves in a medium, and correlating them to the elastic properties and density of the material. Travel time of ultrasonic waves reflects internal condition of test area. In general, for a given trajectory, higher travel time is correlated to low quality concrete with more anomalies and deficiencies, while lower travel time is correlated to high quality concrete with fewer anomalies. Once ultrasonic wave spreads within the test area, the wave is reflected in boundary of anomalies resulting in higher travel time. This results in higher transmission time (lower wave speeds) in poor quality concrete and lower transmission time (higher wave speed) in good quality concrete.
Different configurations of transducers can be used to perform a UPV test. This includes direct transmission, semi-direct transmission, and indirect (surface) transmission. Figure above shows different configurations of transducer based on the access to the surface of test area. The ultrasonic velocity is prone to signal travel trajectory that is defined by the transducer configurations. Figure below is a representation of the effect of concrete anomalies and deficiencies on the acoustic wave travel time and the corresponding velocity throughout a given trajectory (ACI 228.2R, 2013).
Couplant | Concrete-Transducer contact
The UPV transducers must be in full contact with the concrete surface; otherwise the air pocket between the transducer and concrete may result in measurement error (i.e. inacurate measurement of transit time). One reason is that only negligible amount of wave energy will be transmitted in a poor contact. Different couplants can be used to eliminate air pockets and to assure good contact (e.g. petroleum jelly, grease, liquid soap, and kaolin-glycerol paste). It is recommended to make the couplant layer as thin as possible.
Applications of UPV Testing for Concrete
Several researchers and engineers have studied the use of ultrasonic testing of concrete in different engineering projects:
1- Pulse Velocity Determination
2- Concrete Quality Assessment (learn more)
3- Establishing Homogenity and Uniformity of Concrete
4- Measurement of Surface Crack Depth (read more)
5- Prediction of Compressive Strength of Concrete (read more)
UPV – Influencing Parameters
In order to conduct a reliable ultrasonic testing of concrete, the surface of concrete should be clean, and free of dust. A suitable couplant is needed to establish an ideal connection between concrete and UPV transducers. Special attention should be given to rebar in concrete, since the wave travel speed in metal is much higher than in concrete. The interpretation of test results in heavily reinforced concrete is somewhat difficult. The direct configuration is the most ideal for getting reliable readings; however, the use of this configuration is mainly limited to laboratory. In summary, the following issues should be addressed before, during, and after performing the test:
1- Concrete Properties (aggregate size, type, and content)
2- Transducer Contact/couplant material
3- Presence of Rebar
4- Sensor Configuration