Modulus and Thickness of the Pavement Surface Layer from SASW Tests

Transcription

1 TRASPORTATIO RESEARCH RECORD Modls and Thickness of the Pavement Srface Layer from SASW Tests JosE M. RoESSET, DER-WE CHAG, KEETH H. STOKOE II, AD MARWA AoAD The spectral analysis or srface waves (SASW) test can be sed rapid ly in the fi eld ro determine the stiffnc sand thickne s of the pavement srraee layer. The test i eqally applicable t a phalt concrete and porrland cement concrete pavemenls. One of 1he most important featres is that testing can be performed qickly (in appr ximately 5 min at each location). Vale. of Yong's modls and thickness of the srface layer are deiermined sing a straightforward procedre. Analytical tdies are presented to sbstamiate this procedre and to optimize its se. Several case stdies from asphalt c ncr re pavemenrs and one Portland cement concrete pavement are pre ented. The re lt show that this a laptation of the A W Lest provide vales of Yong's modls that are sensitive to the elastic stiffness of the srface layer and also provides reasonable estimates of the thickness or the srface layer. In addition, changes in the stiffness of the srface layer with time and temperatre are easily monitored in sit. Reliable measrements of the in sit conditions of pavements are an important aspect in effectively managing pavement systems. Existing nondestrctive devices for modli measrements, sch as the Dynaflect or falling weight deflectometer, cannot be sed to perform an independent measre of only the srface layer. In addition, these devices can be somewhat insensitive to the modls of the pavement srface layer, specially for the cases of a thin srface layer on the order of a few inches thick or nder those conditions where bedrock is near the srface. Optimm reslts 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 srface waves (SASW) test is very sensitive to the vale of Yong's modls of the srface layer and bedrock conditions do not affect the near-srface measrements. In addition, the thickness of srface layer is not reqired to evalate the measrements bt can be estimated from the field data. As originally proposed, the SASW method (1-3) has been a rather complex nondestrctive method involving the se of srface waves to evalate the modls profile of the entire pavement system. However, if only the stiffness and thickness of the srface layer are reqired, the SASW test can be greatly simplified so that testing can be performed rapidly and vales of modli and thickness can be determined immediately in the field. This adaptation of the SASW test, originally proposed by She et al. ( 4), is based on a theoretically sond procedre that is simple, easy to implement, and does not reqire knowledge of any of the layer thicknesses in the pavement profile. Department of Civil Engineering, University of Texas at Astin, Astin, Tex In the following sections, this adaptation of the SASW test is briefly described along with an analytical stdy of the dispersive properties of srface waves in the pavement srface layer. Typical test reslts from several pavements, inclding one where the Portland cement concrete was cring, are then presented. GEERAL BACKGROUD Evoltion of SASW Method The SASW method (1-3) is an in sit seismic method that is sed for near-srface profiling of pavement sites. The SASW method is a modification of the steady-state Rayleigh wave techniqe introdced in the 195s for the measrement of elastic properties of pavements (5,6). The original techniqe involved testing with blky eqipment and analyzing the data with an empirical approach. These two shortcomings reslted in the method's never gaining wide acceptance. In fact, the empirical basis for data analysis reslted in erroneos reslts nder certain conditions that often occr in pavement systems. Becase of the development of portable, sophisticated electronic eqipment capable of performing accrate, highfreqency data acqisition and complex mathematical maniplations rapidly in the field, the blky eqipment associated with the steady-state techniqe is no longer reqired. In addition, a theoretically sond basis for data analysis has been developed (7-1). These two developments have reslted in the application of the SASW method to nondestrctive pavement testing. One of the important areas in which the SASW method can easily be sed is the rapid determination of the modls and thickness of the pavement srface layer. This application is possible becase of the simplicity of data analysis in a niform top layer of any layered system. Eqipment and Field Testing The general arrangement of the sorce, receivers (accelerometers), and recording eqipment in an SASW test is shown schematically in Figre 1. o boreholes are reqired becase both the sorce and receivers are placed on the pavement srface. A piezoelectric shaker is an effective sorce for generating a grop of srface waves over freqencies ranging from abot 1 to 5 khz. These high freqencies are necessary to sample the srface layer. A digital waveform analyzer

2 54 EB m==o== Microcompter == == Waveform Analyzer == == I I =====..., :: s ;,cii:ivor ::....._._.._ TRASPORTATIO RESEARCH RECORD 126 Particle Motion Vertical Vertical Piezoelectric Accelerometer Accelerometer oise Sorce 1 I 2 f \ d D(variable) :1, d i FIGURE 1 General configration of eqipment sed to evalate the srface layer. copled with a microcompter is sed to captre and process the otpts from the receivers. The vertical accelerometers and sorce are arranged in a linear array. The distance D between receivers (see Figre 1) is sally 6 in. bt may be varied by the operator to optimize the test reslts for a particlar site. (Distances of 3 to 12 in. have been sed in practice.) The distance d, between the sorce and the first receiver is sally kept eqal to D bt may also be increased by the operator to minimize destrctive interference from body wave reflections. However, d/d = 1. is normally a good arrangement, as shown in the following analytical stdies. Srface Wave Dispersion The dispersive property of srface waves permits se of the SASW method. Dispersion refers to the variation of srface wave phase velocity with wavelength (or freqency). Dispersion arises becase srface waves of different wavelengths sample different parts of the pavement profile, as shown in Figre 2. As wavelength increases, particle motion extends to greater depths in the profile. The velocities of srface waves are representative of the material stiffness over depths for which there is significant particle motion. For example, the particle motion of a wave that has a wavelength less than the thickness of the pavement srface layer is confined to this layer (Figre 2b). Therefore, the wave velocity is affected by the stiffness of the srface layer and not by the lower layers. The velocity of a wave with a wavelength of several feet is inflenced by the properties of the srface layer, base, and sbgrade becase a significant portion of the particle motion is in these layers (Figre 2c). Ths, by sing srface waves over a wide range of wavelengths, it is possible to assess material properties over a range of depths. However, to monitor only the stiffness of a srface layer, only wavelengths less than the thickness of this srface layer need to be generated and measred. The overall objective in SASW testing is to make field measrements of srface wave dispersion (i.e., measrements of srface wave velocity VR at varios wavelengths LR) and then to determine the stiffnesses of the layers in the profile. For the case of a niform srface layer, the srface wave I I I Depth Depth a. Material Profile b. Shorter Wavelength, A. 1 c. Longer Wavelength, A. 2 FIGURE 2 Approximate distribtion of vertical particle motion with depth for two srface waves of different wavelengths. phase velocity VR is related to the shear wave velocity Vs of the material by Poisson's ratio v. The ratio of VR to Vs varies from.874 to.955 for vales of Poisson's ratio v ranging from to.5. Therefore, once the srface wave phase velocity of the niform srface layer has been measred, it is a simple matter to calclate the shear wave velocity and, hence, Yong's modls of the srface layer sing the following relationships: G (-y/g). V} E = 2 G (1 + v) where C = v (for v 2::.1), G = shear modls, 'Y = total nit weight, g = acceleration de to gravity, and E = Yong's modls. Becase the vales of modli calclated in Eqations 1 throgh 3 are a reslt of seismic measrements, these vales represent the modli at small strain amplitdes. Modli measred at these strain levels are maximm vales of modli. Additionally, if the material stiffness is freqency dependent (sch as for asphalt concrete), then seismic tests will reslt in higher vales of stiffness than determined by static tests becase of the high freqencies sed in seismic testing. As a reslt, the seismically determined vales shold be adjsted accordingly. AALYTICAL STUDIES To apply the SASW test effectively to measrements of the srface layer, analytical stdies of the dispersive characteristic of waves propagating in a niform layer over a half-space were condcted. Two general cases were stdied: (a) dispersion of plane Rayleigh waves, and (b) dispersion of combined Rayleigh and body waves. The theoretical soltion involving plane Rayleigh wave propagation forms the basis for the simplest analysis procedre sed to interpret SASW field data. However, vertical excitation at a point on the srface of a (1) (2) (3)

3 Roessel er al. 55 layered system creates a grop of seismic waves that are composed of body waves (compression (P) and shear (SV) waves] as well as srface (Rayleigh) waves, which propagate radially away from the sorce. Therefore, simlation of wave dispersion from the combined body and srface waves is important to nderstanding SASW testing, particlarly at distances from the sorce that are small relative to the wavelength (nearfield effect). In the following sections, the dispersion characteristics of both plane Rayleigh wave propagation and combined waves excited by a vertical dynamic load are discssed. The mathematical model consists in both cases of a horizontally layered half-space with homogeneos properties within each layer. The soltion to the differential eqations of motion for each layer permits the stresses and displacements at the top of the layer to be related to the stresses and displacements at the bottom of the layer for a given freqency and wave nmber (or wavelength). The stresses and displacements are given by a system of eqations in terms of a matrix T called the "transfer" or " propagator" matrix. Expressions for the elements of the matrix T can be fond in the literatre (7,11). Alternatively, the stresses at the top and bottom of a layer can be expressed in terms of the displacements at the top and bottom throgh a dynamic stiffness matrix, as sggested by Kasel and Roesset (9). By imposing compatibility of displacements and eqilibrim of stresses at the interfaces between two layers, a series of mltiplications of the transfer matrices T of each layer provides a relationship between the stresses and displacements at the free srface and those at any depth. By assming no excitation at the top and no waves propagating pward within the nderlying half-space, the system of eqations can be redced to a set of homogeneos eqations in terms of a 2 x 2 matrix. To obtain nontrivial soltions, this matrix mst be singlar. The vales of the wave nmbers k (for a fixed freqency) that make the determinant of this matrix eqal to zero provide the wave nmbers of the Rayleigh waves propagating at that freqency throgh the soil profile. For each vale of k, one can then obtain the wavelength 27r/k and the corresponding propagation velocity VR. Using instead the dynamic stiffness matrices of the layers, one can assemble a stiffness matrix for the complete soil profile following the same procedres sed in matrix strctral analysis. Again to obtain the modes of propagation, the determinant of the global stiffness matrix is set eqal to zero. The total stiffness matrix is a tridiagonal matrix in terms of 2 x 2 sbmatrices, and therefore the evalation of the determinant is rather simple. In the case of a niform half-space, the zero determinant matrices in both approaches lead to a freqency (or wavelength) independent soltion for the characteristic eqation. The dispersion crve is ths a straight line as shown in Figre 3. In this special case, propagation velocity is independent of wavelength becase the half-space has a niform stiffness and only plane Rayleigh waves are being considered. For a layer resting on a half-space with different properties, VR will vary with freqency. At very low freqencies (long wavelengths), the velocity will tend to the velocity of the half i- 'g Gi g: td '&..., 3: n.,. [; ' Ill FIGURE 3 Dispersion crve for plane Rayleigh waves propagating in a niform half-space. o 1 1 Dispersion of Plane Rayleigh Waves Q O 1 QI FIGURE 4 Dispersion crve for plane Rayleigh waves propagating in a softer layer over a stiffer half-space. space. At very high freqencies (short wavelengths), the vale of V R will eqal the vale of Rayleigh wave velocity in the top Ia yer. Figre 4 shows a typical dispersion crve for a softer layer overlying a stiffer half-space, with stiffness ratio E 1 / 2 =.25. This system cold represent an asphalt concrete (AC) layer over a thick cemented base. Figre 5 shows a similar dispersion crve for a stiffer layer overlying a softer half-space, with E/E 2 = 4. This system cold represent a rather soft srface layer over a stiff ncemented base and sbgrade with similar stiffnesses or it cold represent a Portland cement concrete (PCC) layer over a thick AC base. In either of the cases shown in Figres 4 and 5, the top layer in the profile appears as thogh it were a niform half-space for waves with very short wavelengths (high-freqency waves). In other words, these short-wavelength srface waves sample only the stiffness of the top layer. As sch, the shear wave velocity, shear modls, and Yong's modls of the top layer may be calclated sing the relationships in Eqations 1 throgh 3. This important point forms the basis of the application of the SASW method presented herein. In addition, the thickness h of the top layer may be estimated sing the critical wavelength Le, as shown in Figre 6.

4 56 TRASPORTATIO RESEARCH RECORD FIGURE S Dispersion crve for plane Rayleigh waves propagating in a stiffer layer over a softer half-space.,. Critical Wavelength, J Le= h, I (Log scale) FIGURE 6 Determination of the srface layer thickness from the dispersion crve for plane Rayleigh waves. Dispersion of Combined Body and Rayleigh Waves The physical phenomenon is more complicated when applying a vertical implse at a point on top of a layered system. Waves generated in this case involve both srface waves that propagate radially otward from the sorce along a cylindrical wave front and body waves that propagate radially otward along a hemispherical wave front. The analytical formlation reqires the following processes: 1. Decomposition of the load into a series of cylindrical fnctions (Bessel fnctions) in the radial direction. Each term of the series corresponds to a wave nmber k. 2. Calclation of displacements and stresses for a given freqency and wave nmber sing the global stiffness matrix of the complete layered system. The reslts are the Green's fnctions. 3. Determination of total displacements and stresses integrating the prodct of the Green's fnctions by the corresponding terms of the load decomposition. Becase the terms of the stiffness matrices of each layer are transcendental fnctions (complex exponentials), the inte- 1 2 grals involved in the calclation of the Green's fnctions are done normally by nmerical integration. Formlations along these lines hcive been implemented by Gazetas and Roesset (12) in Cartesian coordinates and Apse! and Lco (13) in cylindrical coordinates. This procedre is particlarly convenient when dealing with a niform half-space or a small nmber of layers, bt expensive when a large nmber of layers is needed to reprodce the variation of properties with depth. An alternative to this formlation is to se the exact analytical expressions (displacements and stresses) in the two horizontal (or radial and circmferential) directions, and a simpler polynomial expansion in the vertical (z) direction if the thickness of the layers is sfficiently small. The approximation in the z direction leads to mch simpler algebraic expressions for the terms of the stiffness matrices of the layers. By expressing the soltion in terms of the mode shapes of the waves propagating throgh the layered system, Kasel (14) was able to obtain explicit soltions for the displacements cased by harmonic loads at any point in the system. Using Kasel's formlation with an approximate soltion for a half-space at the bottom of a layered stratm (1 J) and with the rle sggested by Shao (15) in dividing atomatically the physical layers into finer sblayers to provide an appropriate thickness for each sblayer, dispersion data for the SASW test can be evalated. For example, the dispersion crve for a niform half-space with shear wave velocity of 1, Poisson's ratio of.25, mass density of 1, and material damping ratio of.2 is shown in Figre 7. By applying a vertical load at a point on the srface of the half-space, the response (amplitde and phase) at five other points spaced 1, 1.2, 1.5, 2, and 3 nits from the sorce was compted; then the srface wave phase velocities were obtained from the phase differences and distances between adjacent receivers. This sorce-receiver configration was chosen to evalate the field SASW setp and to verify the near-field effect. The for dispersion crves are similar over the entire range. For relatively high freqencies (f 2 Hz, which corresponds to LR <.5), the crves match very well and correspond to the stiffness of the half-space. For vales of d 1 /LR less than 2 (f = 2 Hz corresponds to d/lr = 2, d 1 = 1, and LR=.5, as calclated from LR= VR!f), srface i- (J). oo Gi GI (() J:. 11.., == f") GI :;: 't: ::I CJ) 1-1 vs= ==- - d2id1= d2/d1= d2/d1= 2. FIGURE 7 Dispersion crve for combined waves propagating in a niform half-space.

5 Roesset et al. wave phase velocities are smaller (within 1 percent difference) than the phase velocities of the plane Rayleigh wave. This difference is believed to be cased by the copling effect of body waves and Rayleigh waves in the zone near the sorce (often called the near-field effect). Slight flctations can be seen in the crve corresponding to the smallest distance ratio d 2 /d 1 This reslt indicates that larger vales of d 2 /d 1 are preferred. In SASW testing, a ratio of two is commonly sed. PARAMETRIC STUDIES A nmber of stdies were condcted to investigate the appropriate SASW configrations (spacings between sorce and receivers) to minimize the near-field effect. In considering d, and d 2 as the distances from the sorce to the two receivers, LR as the wavelength (compted by dividing the phase velocity by the freqency), and h as the thickness of the top layer, the agreement between the dispersion crves corresponding to plane Rayleigh waves and those compted by taking into accont all the propagating waves is in general a fnction of the ratios d/lr, d 2 1LR or (d 2 - d 1 )/LR, d/h, and dzfh. The reslts are also inflenced to some extent by the stiffness contrast between the pper layer and the half-space. To simlate the pavement system, where the top layer is often stiffer than the nderlying layers, a set of simplified, two-layer systems consisting of a srface layer with shear wave velocities of 1.414, 2, 3, 5, and 1 overlying a half-space with a shear wave velocity of 1 was stdied. These cases correspond to stiffness ratios / 2 of 2, 4, 9, 25, and 1. (Mass density and Poisson's ratio of both layers were assigned as 1and.25, respectively, in all cases.) For each stiffness ratio, vales of the thickness h of the top layer of.1,.25,.5, 1, 2, and 5 were sed. Becase the distance d 1 from the sorce to the first receiver remained 1 in all cases, the ratios d/h were 1, 4, 2, 1,.5, and.2. For vales of SASW spacing ratios d 2 /d 1 of1.2, 1.5, 2, and 3 were stdied. Linear material damping vales of and 2 percent were assmed in the calclation of the plane Rayleigh wave soltion and the discrete Green's soltions, respectively, to differentiate the simplified theoretical SASW interpretation from a more complete representation of the field reslts. Two sets of reslts for stiffness ratios of 4 and 25 are shown in Figres 8 throgh 11. Significant flctations in the dispersion crves reslt from copling of the body and Rayleigh waves. The parametric stdies show that the best reslts are generally obtained when dzf d 1 is of the order of 1.5 to 2. The main complication in this case is that, for wavelengths of the order of the layer thickness, there are reflections at the bottom of the layer that reslt in large oscillations in the complete soltion. These oscillations are more prononced for small vales of d/d 1 and increase with increasing contrast in stiffnesses between the layer and the half-space. Vales of d/ LR between.5 and 2 generally prodce reslts that are very close to those of the plane Rayleigh waves. When the modls of the top layer is mch larger than that of the nderlying material, as wold happen with a PCC layer over an ncemented base and sbgrade, determination of the dispersion crves in the range of wavelengths arond the thickness of the layer is always difficlt becase of the large flctations. ;;.. " l[) l[),--,--,.-.,..,..., TTTim--r m-n Ill.... 3:: fl l[)... Q. Ill t: ::i en l[) ;; Qi l[).. 5l Ill.. Q. 3:: Ill l[) ".. t: ::i en o d2 / d1= 1.2 d2/ d1= 1.5 d2 / d 1 = ,-----,.--,-..,...,...,...,.., rrT"m T"MM"TT...-.rrTTrn 1- ld o Qi Ill.. Q. OCJ Ill... 3:: Ill...; ". o ::i en o Plane R-Wave d2/ d 1 = 1.2 d2/d1= 1.5 d2i d1= 2. d2/d1= ,...,...,., ,...,... d2/ d1 = 1.2 d2 / d1= 1.5 d2/ d 1 = 2. d2/ d 1 = FIG RE 8 Comparison of dispersion crves ha. ed on plane Rayleigh wa es and combined waves for SA.SW conligralions: E, IE = 4, d, = 1 and H =. 1,

6 <D i: '() Qi 5l a. en i; 3: fl '<I-, 't: :I en r ,.-,,.,..,., d2id1= 1.2 d2i d1 = d2/d1= 2. d2i d1 = :.:. "t:: CX) o 5l <D Q. 3: fl :I en O.---r-...,.,MTI" ,,-rnrr-...,..,...,.., n oom ,-,...,...,..TJ 1 o- d2id1= 1.2 d2id1= 1.5 d2/d1= 2. d2id1= <D i: 'g 5l Q. CXl d2id1= 1.2 fl '<! d2id1= d2/d1= 2. :I en ---- d2id1= 3. o i...,...,,,......_,_._.'=-c--'--'-.._._._,, '---'-,-:7;' 2 i:. '() CXl.2 81 <D Q. J fl :I en o 'TTTI d2id1= 1.2 d2id1= 1.5 d2/d1= 2. d2 i d1 = 3. <D i-.,..:. '() Qi ,..-r-rnrrr ,...,...'T"T" ,--.--,...,.T"T.,..., Q. CXl. '<! o :I en o d2i d 1 = 1.2 d2i d 1 = 1.5 d2/d1= 2. d2id1= FIGURE 9 Comparison of dispersion crves based on plane Rayleigh waves and combined waves for SASW configrations: E 1 /E 2 = 4, d 1 = 1, and H = 1., 2., 5.. o,;. : IX) Qi o :J: <D : Q..q: 3: fl.l!! :; CJ) o.---.-rn... --r-"t"t"t"ttttt"-: ,.,...,,--,-,.-,-,,ntt\ o- FIGURE 1 d2i d 1 = 1.2 d2id1= 1.5 d2/d1= 2. d2id1= Comparison of dispersion crves based on plane Rayleigh waves and combined waves for SASW configrations: E/E 2 = 25, d 1 = 1, and H =.1,.25,.5.

7 Roesset et al. 59 o,,:;.!::: co go CD.. '<!" 3: o. 't: :::J CJ) o --n--,...,.,,,:;. ::: tx) Qi CD.. '<!" 3: " CJ) 1 -.a=: co c; g CD.. '<!" 3: " CJ)., : ;.;: ---- ii fl Ii ---- "" l.'. t---"\lf 1 1 Plane R Wave d2id1= 1.2 d2id1= 1.5 d2/ d1 = 2. d2id1= d2/d1= 1.2 d2/ d 1 = 1.5 d2id1= 2. d2/d1= ,..,.., ,.-, d2id1= d2id1= d2/ d 1 = d2i d 1 = FIGURE 11 Comparison of dispersion crves based on plane Rayleigh waves and combined waves for SASW configrations: Ei/E 2 = 25, d 1 = 1, and H = 1., 2., The dispersion crve obtained in the field needs to be smoothed if it is going to be assmed to correspond to a plane Rayleigh wave to backfigre the stiffness and thicknesses of the srface layer. However, this smoothing operation is qite straightforward in most cases. CASE STUDIES This adaptation of the SASW method for determining the modls and thickness of the pavement srface layer has been sed on many pavement sections in Texas, inclding 1 sections at the Texas Transportation Institte Annex of Texas A&M University. In all cases, the thicknesses of the pavement srface layer were known. Reslts from for of these sites are presented in the following sections. ew Highway in Astin, Texas Tests were performed on a new asphalt concrete pavement abot 4 days after placement. The reslting dispersion crve is shown in Figre 12. The srface layer exhibited some freqency dependence as noted by the inclined portion of the initial part of the dispersion crve. The average vale of Vn is abot 4,5 ft/sec, which reslts in a Yong's modls of abot 2.8 x 1 8 psf. The thickness was estimated to be.51 ft as compared with.58 ft measred by cores, a reasonable comparison. The vale of the modls seems too large in comparison with vales determined by conventional laboratory tests. As mentioned earlier, modli measred at strain levels associated with seismic testing are maximm vales. Second, the high freqencies sed in the SASW test reslt in higher vales of stiffness for AC material. Tests performed on cores of this material to evalate the freqency effect are shown in Figre 13. The effect of freqency is significant. If one wanted the modls at 3 Hz (say to compare with the FWD), then the SASW vale wold be divided by a factor of abot 4; hence the modls wold be 7. x 1 7 psf. TTI Annex The experimental dispersion crve calclated from measrements on one test section at the TTI Annex is shown in Figre 14. Using an average vale of 5,2 ft/sec for the srface wave phase velocity, a Poisson's ratio of.33, and a nit weight eqal to 145 pcf, the reslting Yong's modls is determined to be 2. 7 x 1 6 psi for the AC layer. Again, the stiffness of the asphalt concrete is high becase of the small strains and high freqencies involved. The thickness of the srface layer is estimated to be.42 ft. Cores from the site show the thickness is.42 ft, a very good comparison. Monitoring Changes with Time To illstrate the seflness and sensitivity of this approach in testing the srface layer, changes in the stiffness of the layer

8 6 R1-R2= 1 ft Cf).2-5 L c =.51 ft. s: '() Qi Cb ps. 4 <P-, ell Q1, () c R1-R2= 6 in )... <\ Rote 1 :::J (/) 3 v -... Astin, TX Site Temp=88 F 2 '--..._...._....._._ 1 Wavelength, ft FIGURE 12 Dispersion crve measred on new ACP abot 4 days after placement iii a. 18 Cyclic Resonant SASW Torsional colmn In Sit a r; 4... y=.3% Ci;. (/) Sample Temp 73 F 85 F 11 F Freqency, Hz FIGURE 13 Inflence of freqency and temperatre on the small-strain shear modls of asphalt concrete: sample cored from a new highway in Astin, Texas.

9 FIGURE 14 Dispersion crve measred on ACP section at the Texas Transportation Institte Annex in College Station. 7 Srface 6 Temperatre( F) <fl ro 143 3: ro ::::J <.!) 3 2 TTI Annex Site# Wavelength ' (ft) FIGURE 15 In sit measrement of the variation in stiffness of an AC layer with temperatre. with time were measred in sit. The first case, shown in Figre 15, shows the inflence of temperatre on the stiffness of an AC layer at the TTI Annex. The second case, shown in Figre 16, shows the stiffening of a portland cement concrete layer dring cring (16). In both cases, the changes in the srface layer were easily measred with a sensitivity nattainable with any other in sit test. COCLUSIOS A new adaptation of the SASW method to determine the modli of the srface layer of both asphalt concrete and portland cement concrete pavements sing srface waves has been developed. This method may also be sed to provide a reasonable estimate of the thickness of the pavement srface

10 62 TRASPORTATIO RESEARCH RECORD aooo 7.2 <. 6 CD CD ctl :s: Dispersion crve wilho 11 1 rpffar.lion 8 1=.. m.in'" CD <. 3 ctl.. :::J Cf) 2 Flctations cased by wave reflection? :::_:_ "\.,... i.l..,. ---:':-;.., 1=816 min..,\ \ ;. k t,.: 77 min...- :.,. ' i \. : \ (,,_.. - 1=369 min.,....- J'....'' ,_...,... " _..."I.... 1=245 min :......, ' '-...,, =19 min , _ ooo.. H Oo< M'- ''' El Paso, Texas O'----'----'---"----''------''-----''-----''-----' Wavelength, ft FIGURE 16 Dispersion crves measred on a continosly reinforced PCC pavement section dring cring at varios times after addition of water to the mix. layer. The most important featres of the techniqe are the following: 1. Testing can be rapidly performed in the field. At the present time, approximately 5 min is reqired to perform the test at each location. The time reqired to condct the test can be frther redced by atomating the placement of the sorce and receivers. 2. Vales of Yong's modls and estimates of the thickness of the pavement srface layer are available immediately in the field. The calclation of these vales is based on a simple, straightforward procedre that can be easily implemented. 3. Yong's modls vales are calclated sing a theoretically sond procedre based on the dispersive property of srface waves. Analytical stdies presented herein establish the validity of this approach. 4. Unlike other nondestrctive test methods, this techniqe is very sensitive to the modls of the pavement srface layer. 5. Becase of the small strain levels that exist in seismic testing, measred modli correspond to maximm vales. Also, becase of the high freqencies involved, modli of AC material need to be redced to compare with modli evalated by other nondestrctive field tests sch as the falling weight deflectometer or Dynaflect. ACKOWLEDGMETS This work was spported by the Texas State Department of Highways and Pblic Transportation. The athors wish to express their appreciation for this spport. REFERECES 1. S. Heisey, K. H. Stokoe, II, and A. H. Meyer. Modli of Pavement Systems from Spectral Analysis of Srface Waves. In Transportalion Research Record 852, TRB, ational Research Concil, Washington, D.C., 1982, pp S. azarian, K. H. Stakoe, II, and W. R. Hdson. Use of Spectral Analysis of Srface Waves Method for Determination of Modli and Thicknesses of Pavement Systems. In Transportation Research Record 93, TRB, ational Research Concil, Washington, D.C., 1983, pp S. azarian and K. H. Stokoe, II. Use of Srface Waves in Pavement Evalation. In Transportation Research Record 17, TRB, ational Research Concil, Washington, D.C., 1986, pp J.-C. She, G. 1. Rix, and K. H. Stokoe, II. Rapid Determination of Modls and Thickness of Pavement Srface Layer. Paper presented at the 66th Annal Meeting of the Transportation Research Board, Washington, D.C., R. Jones. Srface Wave Techniqe for Measring the Elastic Properties and Thickness of Roads: Theoretical Development. British Jornal of Applied Physics, Vol. 13, 1962, pp R. Jones. Following Changes in the Properties of Road Bases and Sb-Bases by the Srface Wave Propagation Method. Civil Engineering and Pblic Works R eview, May 1963, pp A. H askell. The Dispersion of Srface Waves in Mltil ayered Media. Blletin of the Seismological Society of America, Vol. 43, 1953, pp S. azarian. In Sit Determination of Elastic Modli of Soil Deposits and Pavement Systems by Spectral-Analysis-of-Srface-Waves Method (Practical Aspects). Research Report 368-lF, Center for Transportation Research, The University of Texas at Astin, May 1986, pp E. Kasel and J. M. Roesset. Stiffness Matrices for Layered Soils. Blletin of the Seismological Society of America, Vol. 71, o. 6, Dec. 1981, pp

11 Roesset et al R. J. Apse!. Dynamic Green's F11c1io11sfor L"yered Media and Applications to Bondary Vale /rob/ems. Ph.D. dissertmion. University of California at San Dicg S. W. Hll and E. Kasel. Dynamic Londs in Layered Half 'paccs. Pre, 51/1 11gi111wri11g M1!c/11111ics Division Speciale 11fere11 c1.1, AS E, L11rnmie. Wyo., pp G. Gazetas and J. M. Roe. set. Forced ibrations of trip Footings in Layered oil. Proc., mio11al Sm1cwral 11si11l!eri 11g 11fermce, E, Madison, Wi.. Ag. 1976, pp.! I R. J. Aspel and J. E. Lco. On the Green" Fnctiort f r a Layered Half-, p:icc, Part IJ. 811//eci11 of the Seismological Society of America Vol. 73,. 4, Ag. 1983, pp E. Ka se I. A11 l3xplicir So/111/11 fo r tile Green Fimctio11sfor Dynamic Loads in Layered Media. Research Report R81-13, Massachsetts Institte of Technology, Cambridge, 1981, pp K.-Y. Shao. Dynamic Interpretation of Dynaflect, Falling Weight Defleclometer and Spectral Analysis of Srface Waves Tests on Pavement Systems. Research Report 437-1, Center for Transportation Research, The University of Texas at Astin, 1986, pp G. J. Rix, J. A. Bay, and K. H. Stakoe, II. Assessing In Sit Stiffness of Cring Portland Cement Concrete with Seismic Tests. Presented at the 69th Annal Meeting of the Transportation Research Board, Washington, D.C., 199. Pblication of this paper sponsored by Commillee on Pavement Monitoring, Evalation, and Data Storage.