TRANSPORTATION RESEARCH RECOR D 1260 53 Modulus and Thickness of the Pavement Surface Layer from SASW Tests JosE M. RoESSET, DER-WEN CHANG, KENNETH H. STOKOE II, AND MARWAN AouAD The spectral analysis or surface waves (SASW) test can be used rapid ly in th e fi eld ro determine the st iffn c sand thickne s of the pavement surraee layer. The test i equally applicable t a phalt concrete and porrland cement concrete pavemenls. One of 1he most important features is that testing can be performed quickly (in appr ximately 5 min at each location). Value. of Young's mo dul us and thickness of the surface layer are deiermined using a straightforward procedure. Analytical tudies are presented to substamiate this procedure and to optimize its use. Several case studies from asphalt c ncr re paveme nrs and one Portland cement concrete pavement are pre ented. The re ult show that this a lap- tation of the A W Les t provide va lues of Young's modulus that are sensitive to the elastic stiffness of the surface layer and also provides reasonable estimates of the thickness or t he surface layer. In addition, changes in the stiffness of the surface layer with tim e and temperature are easily monitored in situ. Reliable measurements of the in situ conditions of pavements are an important aspect in effectively managing pavement systems. Existing nondestructive devices for moduli measure- ments, such as the Dynaflect or falling weight deflectometer, cannot be used to perform an independent measure of only the surface layer. In addition, these devices can be somewhat insensitive to the modulus of the pavement surface layer, specially for the cases of a thin surface layer on the order of a few inches thick or under those conditions where bedrock is near the surface. Optimum results are also obtained with these tests when the thickness of the layers in the pavement are known a priori. On the other hand, the spectral analysis of surface waves (SASW) test is very sensitive to the value of Young's modulus of the surface layer and bedrock con- ditions do not affect the near-surface measurements. In addi- tion, the thickness of surface layer is not required to evaluate the measurements but can be estimated from the field data. As originally proposed, the SASW method (1-3) has been a rather complex nondestructive method involving the use of surface waves to evaluate the modulus profile of the entire pavement system. However, if only the stiffness and thickness of the surface layer are required, the SASW test can be greatly simplified so that testing can be performed rapidly and values of moduli and thickness can be determined immediately in the field. This adaptation of the SASW test, originally pro- posed by Sheu et al. ( 4), is based on a theoretically sound procedure that is simple, easy to implement, and does not require knowledge of any of the layer thicknesses in the pave- ment profile. Department of Civil Engineering, University of Texas at Austin, Austin, Tex. 78712. In the following sections, this adaptation of the SASW test is briefly described along with an analytical study of the dis- persive properties of surface waves in the pavement surface layer. Typical test results from several pavements, including one where the Portland cement concrete was curing, are then presented. GENERAL BACKGROUND Evolution of SASW Method The SASW method (1 -3 ) is an in situ seismic method that is used for near-surface profiling of pavement sites. The SASW method is a modification of the steady-state Rayleigh wave technique introduced in the 1950s for the measurement of elastic properties of pavements (5,6). The original tech- nique involved testing with bulky equipment and analyzing the data with an empirical approach. These two shortcomings resulted in the method's never gaining wide acceptance. In fact, the empirical basis for data analysis resulted in erroneous results under certain conditions that often occur in pavement systems. Because of the development of portable, sophisticated elec- tronic equipment capable of performing accurate, high- frequency data acquisition and complex mathematical manip- ulations rapidly in the field, the bulky equipment associated with the steady-state technique is no longer required. In addi- tion, a theoretically sound basis for data analysis has been developed (7-10). These two developments have resulted in the application of the SASW method to nondestructive pave- ment testing. One of the important areas in which the SASW method can easily be used is the rapid determination of the modulus and thickness of the pavement surface layer. This application is possible because of the simplicity of data analysis in a uniform top layer of any layered system. Equipment and Field Testing The general arrangement of the source, receivers (acceler- ometers), and recording equipment in an SASW test is shown schematically in Figure 1. No boreholes are required because both the source and receivers are placed on the pavement surface. A piezoelectric shaker is an effective source for gen- erating a group of surface waves over frequencies ranging from about 1 to 50 kHz. These high frequencies are necessary to sample the surface layer. A digital waveform analyzer