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: USW index =
𝐴𝐿𝑜𝑤 × 0 + 𝐴𝑀𝑜𝑑𝑒𝑟𝑎𝑡𝑒 × 50 + 𝐴𝐻𝑖𝑔ℎ × 100 𝐴𝑇𝑜𝑡𝑎𝑙
(14)
where 𝐴𝐻𝑔𝑖ℎ , 𝐴𝑀𝑜𝑑𝑒𝑟𝑎𝑡𝑒 , and 𝐴 𝐿𝑜𝑤 = areas with elastic moduli in ranges of >4400 ksi (30 GPa) 2900-4400 ksi (20-30 GPa); and <2900 ksi (20 GPa), respectively. 𝐴𝑇𝑜𝑡𝑎𝑙 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.
