Ultrasonic Pulse Velocity (UPV) is an effective non-destructive testing (NDT) method for quality control of concrete materials, and detecting damages in structural components. The UPV methods have traditionally been used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. Ultrasonic testing of concrete is an effective way for quality assessment and uniformity, and crack depth estimation. The test procedure has been standardized as the “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016).
How Does Ultrasonic Pulse Velocity Work?
A standard Ultrasonic Pulse Velocity (UPV) equipment consists of two transducers (1 sending and 1 receiving transducer), connected to a pulse generator and data logger.
The Concept Behind Ultrasonic Pulse Velocity
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. The travel time of ultrasonic waves reflects the internal condition of the test area. In general, for a given test path, higher travel time can generally represent low-quality concrete with more anomalies and deficiencies, while shorter transit time is correlated to high-quality concrete with fewer anomalies. Once the ultrasonic wave spreads within the test area, the wave is reflected in the boundary of anomalies resulting in an increase in the transit time of the signal.
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).
Bonding Agent - 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. inaccurate measurement of transit time). One reason is that only a negligible amount of wave energy will be transmitted when there is poor contact. Different bonding agents 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 Ultrasonic Pulse Velocity for Concrete
The Ultrasonic Pulse Velocity (UPV) test can be used to assess different parameters of concrete. It can be used to assess the quality and uniformity of concrete, as well as estimate the concrete strength (when combined with Rebound Hammer). The following summarized the key applications of UPV method in evaluating concrete structures.
UPV – Influencing Parameters
In order to conduct reliable ultrasonic testing of concrete, the surface of concrete should be clean, and free of dust. The following summarizes the key factors to conduct a reliable UPV measurement in the field.
- A suitable bonding agent is needed to establish an ideal connection between concrete and UPV transducers.
- Effect of Steel Rebar: 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.
- UPV Configuration: The direct configuration is the most ideal for getting reliable readings; however, the use of this configuration is mainly limited to concrete specimens in the laboratory.