Impact Echo Method (ASTM C1383):
The Impact Echo method utilizes impact generated stress sound waves that propagate through the concrete or masonry mediums to determine the internal flaws and their location relative to the depth of the subject member. The stress pulse is generated by a mechanical impactor and transducer on the surface of the concrete and generates three types of waves – surface waves (R-waves), S- and P-waves as shown in Figure 1 below. The P-wave is used to obtain the information about the test member. With the stress pulse travel time and the known wave velocity, the member thickness and internal flaws can be determined by the patterns presented in the surface displacement waveforms and amplitude spectrum as shown in Figure 1 below. The peak or dominant frequency in the amplitude spectrum indicates the thickness of the member or flaws.
FIGURE 1 – Impact Echo Principle
Ultrasonic Pulse Velocity (UPV), ASTM C597:
Similar to Impact Echo method principle, UPV utilizes ultrasonic pulse generated by an electro-acoustical transducer at one end of the subject member and received by another transducer to assess the homogeneity and quality of the concrete by measuring the pulse velocity (V) based on the time (T) and path travel of the pulse (L, distance between the transducer) through the medium using the equation V=L/T. The higher the pulse velocity, the higher the modulus of elasticity, density and integrity of the concrete. The test is typically used to supplement and confirm flaw detection by the Impact Echo where subject member configuration is the limiting factor. The three method of transmittals are direct, indirect and semi-direct as shown in Figure 2 below.
FIGURE 2 – UPV Method
Ground Penetrating Radar (GPR):
The GPR utilizes an antenna equipped with a transmitter and receiver to transmit electro-magnetic waves with a frequency up to 2600 MHz into the subsurface to detect any change in the material property such as member thickness, embedded items like reinforcing steel, PT strands, electrical conduit, and voids. The reflected energy between materials of differing electrical properties is processed as a pattern to locate the embedded items. Figure 3 below is an example of the processed data on a concrete slab that shows the embedded steel reinforcement, concrete slab thickness and subsurface voids directly beneath the slab.
FIGURE 3 – Example GPR Processed Image of Slab-on-Grade