ACI 437R-19 Strength Evaluation of Existing Concrete Buildings. Pulse velocity—The principle of pulse velocity is described in 32.3.3. Pulse travel time between the transmitting and receiving transducers is affected by the concrete properties along the travel path and the actual travel path distance. If there is a region of low-quality concrete between the transducers, the travel time increases and a lower velocity value is computed. If there is a relatively small void between the transducers, the pulse diffracts around the void as it travels through the concrete. This increases the actual path length and a lower pulse velocity is computed. While the pulse velocity method can be used to locate abnormal regions, it cannot identify the depth or the nature ol’ the abnormality. (‘ores are often taken to determine the nature of the indicated abnormality. Impact-echo——In the impact-echo method, a short-duration mechanical impact is applied to the concrete surface (Sansalone and (.‘arino 1986). The impact generales stress waves that propagate away from the point of impact. The stress wave that propagates into the concrete is reflected if it encounters an interface between the concrete and a material with different acoustic properties. lithe interface is between concrete and air, almost complete reflection occurs, The reflected stress wave travels back to the surface. where it is again reflected into the concrete and the cycle repeats. A receiving transducer located near the impact point monitors the surt’ace movement resulting from the periodic arrival ol’ the reflected stress wave. The transducer signal is recorded as a function of time from which the depth of the reflecting interlace can be determined. II’ there is no defect. the thickness of the member can be determined, provided the thickness is small compared with the other dimensions. In general. the thickness has to be less than 20 percent of the smallest lateral dimension for the response to be dominated by reflections from the back wall. If this condition is not satisfied, reflections from the side boundaries will interfere with reflections from the back wall and interpretation of results becomes complicated.
Because the stress wave undergoes multiple reliections between the test surface and the internal reflecting interface. the recorded waveform is periodic, lithe waveform is transformed into the frequency domain, the periodic nature of the waveform appears as a dominant peak in the amplitude spectrum (Carino et al. 1986). The frequency of’ that peak can be related to the depth of the reflecting interlace by a simple relationship (Sansalone and Streett 1997).
Impact-echo can be used to measure the thickness of plate-like elements if there is access to only one face (ASTM C1383). A plate-like element is one in which the smallest lateral dimension is at least five times the thickness to be measured. Two procedures are required to measure the thickness. The first is to determine the stress-wave speed in the concrete, and the second is to measure the thickness frequency of the plate and calculate thickness, The wave speed can be established by a surface measurement technique using two transducers (Sansalone and Streett 1997).ACI 437R pdf download.