We start our notes on NDT method for condition assessment of infrastructure facilities by presenting the Ultrasonic Pulse Velocity – UPV method. The UPV is the one of the most basic acoustic techniques, however, it remains an effective and reliable test method for quality control of concrete materials, and detecting damages in structural components. The basic concept on which the UPV is established, is measuring the travel time of acoustic waves in a medium, and correlating it to the elastic properties and density of the material.
The UPV methods have been traditionally used for the quality control of homogeneous materials (e.g. metals and welded connections). In recent year, this technique is increasingly being employed for the conditions assessment of concrete materials. UPV can be used to predict the strength of concrete, dynamic modulus of elasticity, depth of cracks, etc. What makes the UPV an interesting alternative for many existing intrusive tests is that the UPV method is a reliable and less time consuming method. The test is repetitive, which means that it can be performed as many times as needed, to collect reliable data. The test procedure has been standardized as ASTM C 597
Ultrasonic Pulse Velocity: The Concept Behind the Method
The UPV test is based on measuring the velocity of acoustic waves in material. For example, for the case of concrete material, the velocity of acoustic waves is measured and correlated with the quality of concrete (e.g. mechanical or durability performance) when the acoustic waves are propagated through the medium along a given trajectory. The very basic device to perform a UPV test consists of a pulse generator, a transmitter transducer, and a receiver transducer. The transmitting transducer sends a pulse into concrete, and the receiving transducer, at a distance X, receives the generated pulse. The simple kinematic equation, V=X/T can be used to measure the wave velocity, where V is the wave velocity; X is the wave propagation trajectory, and T is the travel time of the wave).
The following video schematically shows the fundamental of the acoustic wave propagation through a medium, how the simple kinematic equation is applied to calculate the wave velocities, and how the internal anomalies, and deficiencies influence the wave trajectory and the resultant travel time and wave velocity.
It is assumed that the velocity of acoustic waves is high in a good quality concrete, and vice versa. One such correlation is reported by Breysse (see Table 1) [Breysse, 2012]. It is obvious that the order of the velocities should be adjusted with regard to the material type (e.g. concrete, masonry, metal, plastic, etc.).
Figure 2 [ACI Committee 228] 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.
Ultrasonic Pulse Velociry – Condition Assessment of Concrete Structures
The most practical form of the UPV measurement in concrete structures is the seismic tomography. This is one of the imaging techniques, which can prepare a two or three dimensional mapping from the present state of concrete structures, quantifying the internal anomalies and deficiencies based on the order of velocities as shown in Table 1. Figure 3 from Rivard et al. (2010) shows the seismic tomography testing contour map for a cross-section of a hydraulic concrete structure. The velocity scale fits to the order of velocity presented in Table 1. The red color represents the poor condition, and the blue color corresponds to the very good condition.
Ultrasonic Pulse Velocity : Limitations
The UPV is a very powerful technique when it is combined with proper knowledge of the test and the structure under investigation. However, like other tests, it has its own limitations. The following describes two most observed limitations of the UPV technique for damage detection in concrete:
- Despite UPV’s advantages, some limitations are observed with the UPV technique for the condition assessment of concrete. Indeed, these limitations come from the non-homogeneous nature of the concrete. Concrete is a composite materials consisted of cement, fine and coarse aggregates, and water, in which the aggregates are distributed randomly. Concrete contains natural micro-cracks, pores and deficiencies either in cementitious paste, in aggregates, or at the inter-facial transition zone between the cementitious paste and aggregate. When the goal is to detect the small size cracks due to a damage mechanism at early ages, the frequency of the detective acoustic waves should be increased. On the other hand, the acoustic waves with very high frequencies are most likely attenuated because of the non-homogeneous nature of concrete. Such sensitivity to the frequency of the detective acoustic waves induces a lack of the precision of the UPV technique for the early age detection of a damage mechanism.
- Sometimes inconsistent results are observed for the UPV tests despite significant damage. Such observation is common for the NDT inspection of cracks related to the Alkali Silica Reaction (ASR) in concrete. This inconsistency of the results might be due to filled cracks with ASR gel. This effect is so-called the bridge effect. Indeed, when the cracks interfaces are filled by the ASR gel, acoustic waves with higher frequencies require in order to detect the cracking effects. This increase of frequency would not be effective anymore when the threshold frequency is reached.