5.3.4 Ultrasonic Surface Wave (USW

5.3.4 Ultrasonic Surface Wave (USW) The USW method is an offshoot of the spectral analysis of surface waves (SASW) method used to evaluate material properties (elastic moduli) in the near surface zone. SASW utilizes the dispersive properties of surface wave, i.e., the velocity of propagation as a function of wavelength, in layered systems to obtain information about layer thickness and elastic moduli. To quantify the overall deck condition with respect to the concrete elastic moduli, the following expression has been introduced:

USWindex=

�������� ×0+������������������ ×50+��������ℎ ×100 ������������ (14)

where ��������ℎ, ������������������, and �������� = areas with elastic moduli in ranges of >4400 ksi (30 GPa) 2900-4400 ksi (20-30 GPa); and <2900 ksi (20 GPa), respectively. ������������ corresponds to the total surveyed area. Changes in concrete quality described by the elastic modulus measurement using USW is depicted in Figure 51 by plotting the average elastic moduli for the entire deck of the BEAST during multiple rounds of data collection. It is obvious that the overall deck condition, in terms of average elastic moduli, decreases as the deck ages. Similarly, Figure 52 below plots the variation of condition index for the different rounds of USW data collection efforts from BEAST and compares it against the performance curve of the representative bridge. Alternatively, as shown in Figure 53, the defect indexes indicate the locations of the bridge that are rated deficient and are in need of immediate attention. A review of all three plots reveals expansion of zones with low moduli from BEAST commission up to 2 million cycles of live load passage. In fact, many of the low-moduli zones are more of an indication of delamination than an actual measure of concrete modulus.

Figure 51. Average elastic modulus calculated based on USW for the entire BEAST deck area

Figure 52. Condition index calculated based on USW – elastic modulus

Figure 53. Defect index calculated based on USW – elastic modulus