52 TRANSPORTATION RESEARCH RECORD 1355 Evaluation of a Dual-Load Nondestrutive Testing System To Better Disriminate Near-Surfae Layer Moduli REYNALDO ROQUE, PEDRO ROMERO, AND BYRON E. RUTH Theoretial analyses were onduted to illustrate the inability of the existing single-load falling weight defletometer (FWD) to disriminate among near-surfae layer moduli of flexible pavement systems. Comprehensive analyses were also onduted to show that this defiieny an be overome by using a dual-load FWD when the loads are spaed suffiiently apart to indue a onave downward urvature in the pavement surfae between the loads (transverse defletion basin). The analyses showed that the asphalt onrete modulus is strongly, and almost uniquely, related to the urvature of this transverse defletion basin, whereas the base ourse modulus is strongly, and almost uniquely, related to the shape of the longitudinal defletion basin. This strong orrespondene was shown to hold true for a broad range of pavement geometries and layer moduli. Stress and deformation analyses were onduted to show that the dual-load system works beause the shape of the transverse defletion basin is most strongly influened by the bending moments indued within the asphalt onrete layer between the loads, and by the relatively large hanges in vertial ompression that are indued in the asphalt onrete layer within this zone. Neither of these effets is observed in the base ourse, whih explains the lak of influene of the base ourse on the shape of the transverse defletion basin. Finally, an analysis was onduted to selet load radii and spaing, and defletion sensor positions that optimize the apabilities of a dual-load system to disriminate among near-surfae layer moduli. It was shown that a set of relatively simple equations an be developed to determine (bakalulate) pavement layer moduli obtained from surfae defletion measurements using the dualload system proposed. Nondestrutive testing is now a ommonly aepted method for pavement strutural evaluation. The surfae defletions produed under a load are routinely used for determining pavement layer moduli in analysis and design. Of the numerous devies that have been developed for this purpose, the falling weight defletometer (FWD) is probably the most widely used. Its advantages inlude simpliity, apability to use variable loads, and the laim that the loading indued by the instrument losely simulates a moving wheel load. However, several disadvantages are also assoiated with the instrument. Ruth et al. (1) and Badu-Tweneboah et al. (2) showed that the defletion basin resulting from the single-load FWD did not allow for aurate disrimination of different pavement layer moduli, partiularly the moduli of near-surfae layers. R. Roque and P. Romero, The Pennsylvania Transportation Institute, Pennsylvania State University, Researh Offie Building, University Park, Pa. 1682. B. E. Ruth, University of Florida, 346 Weil Hall, Gainesville, Fla. 32611. They also showed that defletions resulting from a dual-load system suh as the dynaflet allowed for better disrimination of near-surfae layer moduli when appropriate defletion measurements were obtained. They used a modified sensor onfiguration that defines defletion basins in both the longitudinal and transverse diretions. However, the relatively small and fixed load levels used by the dynaflet system are a distint disadvantage, partiularly when determining effetive layer moduli, whih may depend on the load level used. In addition, the semirigid, nonirular loads are hard to model with existing analysis programs and prevent measurements from being obtained diretly under the load. These observations imply, however, that a superior system an and should be designed to provide optimal disrimination for eah layer. A dual-load FWD would have these apabilities. The two loads would result in improved disrimination of pavement layer moduli while maintaining the advantage of using variable load levels similar to design wheel loads. OBJECTIVES The work reported in this paper was part of a omprehensive study onduted for the Florida Department of Transportation. The objetives were as follows: 1. To determine whether a dual-load nondestrutive testing system provides for better disrimination of near-surfae layer moduli than the existing single-load FWD; 2. To identify a dual-load system onfiguration (load radii and spaing, and defletion sensor positions) that optimizes the apabilities of the dual-load system to disriminate among near-surfae layer moduli; 3. To develop analysis proedures (bakalulation) to determine layer moduli using surfae defletion measurements that would be obtained from the dual-load system onfigured. All three objetives were met, but this paper deals primarily with the first two objetives. The development ofrelationships for modulus predition and their integration into a omputer program was a study in itself and was onsidered beyond the sope of this paper. The speifi objetives of this paper are as follows: 1. To illustrate the inability of the existing single-load FWD to disriminate among the near-surfae layer moduli of flexible pavement systems (asphalt onrete, base, and subbase);
Roque et al. 2. To show that this defiieny an be overome by using a dual-load FWD where the loads are spaed suffiiently apart to indue a onave downward urvature in the pavement surfae between the loads; 3. To show why the dual-load system is more effetive in isolating the effets of the asphalt onrete from the effets of the base ourse on the surfae defletions; 4. To show that the improved disrimination of the dualload system holds true for a broad range of pavement geometries and pavement layer moduli ; and 5. To present the analyses and rationale used to selet the dual-load system onfiguration (load radii and spaing, and defletion sensor positions) that optimizes the apabilities of the dual-load system to disriminate among near-surfae layer moduli. SCOPE Only flexible pavement systems were onsidered in this study. All analyses were onduted using the elasti layer omputer program BISAR (3). Therefore, the layer moduli being onsidered for determination are effetive layer moduli for response predition using elasti layer analysis. The evaluations reported here are based on analyses performed on a range of pavement geometries and layer moduli typially enountered in North Ameria. The range of pavement geometries were identified using the Strategi Highway Researh Program general pavement setions data base. A broad range of pavement layer moduli was seleted. EVALUATION OF EXISTING FWD An evaluation of the defletion basins for a typial pavement struture learly illustrates the inability of the existing singleload FWD to disriminate among the near-surfae layer moduli of flexible pavement systems (asphalt onrete, base, and subbase). Figures 1 and 2 show that for a typial pavement struture (6-in. asphalt onrete, 1-in. base ourse, 1-in. Base Modulus 3 KSI *eo KSI +oo KSI AC: 8 KSI BS: VARIABLE SB: 1 in LOAD: 9 lb RADIUS: 6. in -14 -j----1r----t-~-+--!-~+l--r~""t""~.-----.-~~--.---l 1 2 3 4 5 6 Distane (inhes) FIGURE 2 Effet of hanges in the base ourse modulus on predited defletions for a single-load system. subbase, and semi-infinite subgrade), hanges in the asphalt onrete modulus affet the same portion of the defletion basin as hanges in the base ourse modulus. Therefore, two different ombinations of asphalt onrete and base ourse moduli may result in the same defletion basin. Figure 3 shows that reduing the base ourse modulus for a speifi pavement struture by one-third has roughly the same effet on the defletion basin as does reduing the asphalt onrete modulus by one-half. It would be diffiult to reliably determine the orret moduli of these near-surfae layers on the basis of the measured surfae defletions from the single-load FWD. One ould attempt to use an asphalt onrete-temperature relationship to bound the problem, but temperature relationships are extremely rough at best beause the modulus of the asphalt onrete will depend heavily on many other fators, inluding the degree of age-hardening and the harateristis of the speifi mixture. 53 ;: g -8 iii C-1... AO : VARIABLE BS : 6 KSI SB: 1 in LOAD: 9 lb RADIUS: 6. In 1 2 3 4 5 Distane (inhes) FIGURE 1 Effet of hanges in the asphalt onrete modulus on predited defletions for a single-load system. 6 Modulus Redution ---Referene +1/3 Base *1/2 AC '. ' -4 1-------' :. ;: -6 (.) -8 GI iii C-1................. _,,...;i(--~...--=-:"'f' AC: 8 In BS: 1 In SB: 1 in REFERENCE STRUCTURE AC: 6 KSI BS: 6 KSI LOAD: 9 lb _ 14 -i--+----li---t---'r--.----r--ra.-- Dl_U~S:~6~.~in,----.--.---l 1 2 3 4 5 Distane (Inhes) FIGURE 3 Defletion basins aused by reduing the asphalt onrete or base ourse modulus for single-load system. 6
54 TRANSPORTATION RESEARCH RECORD 1355 Although the results in this example are speifi to the pavement struture used (i.e., the effet of hanging the layer moduli on the defletion basin will be different for different pavement strutures), they are fairly representative of what ours for the range of pavement strutures typially enountered (asphalt onrete from 3 to 9 in.; base ourse from 8 to 16 in.). The problem of disriminating near-surfae layer moduli using the single-load FWD beomes more diffiult for asphalt onrete layers thinner than the 6-in. layer used in the example..!? -4 e._ -6 :;::; -8 GI : C-1 Base Modulus 3 KSI *eo KSI +go KSI 12 KSI AC: 8 KSI BS: VARIABLE SB: 1 in... '--------------i EVALUATION OF DUAL-LOAD SYSTEMS An evaluation of the transverse and longitudinal defletion basins for a dual-load system on the same pavement struture mentioned previously (see Figure 1) learly illustrates the superiority of the dual-load system in independently isolating the effets of the asphalt onrete modulus and the base ourse modulus on the surfae defletions. Figure 4 shows a plan view of the dual-load system. A load spaing of 4 in. was hosen for the analysis. The effets of varying the asphalt onrete modulus and the base ourse modulus on the transverse defletion basin are shown in Figures 5 and 6. Figure 5.; Longltudlnal Diretion 3 ' 4 5 6 7 8 ~ - ----- --- ---- -----. ~ 2 ~ :; ~ Defletion Sensors Load 2 FIGURE 4 Plan view of the dual-load system. o ~---~----~---------~ AC Modulus ~3 KSI....,.... AC: 6 In AC: VARIABLE +BOO KSI BS: 6 KSI SB: 2 in *12 KSI :!" -4 1----~...... --! e._ -6 :;::; -8............................... GI : C-1 LOAD; 2 K llooo I~ RADIUS: 6. In -14 -+----+---+---+---+----+---+----+----< 5 1 15 2 25 3 35 Distane (inhes) FIGURE S Effet of hanges in asphalt onrete modulus on transverse defletion basin for dual-load system: 4-in. spaing. 4 LOAD; 2 X llooo RADIUS: G:O 1n - 14->-----t---+----+---+----+---+----+---r 5 1 15 2 25 3 35 Distane (inhes) FIGURE 6 Effet of hanges in base ourse modulus on transverse defletion basin for dual-load system: 4-in. spaing. learly shows that the asphalt onrete modulus is strongly refleted in the shape of the transverse defletion basin of the dual-load system. As the asphalt onrete modulus varies from 3, psi to 1,2, psi, the defletion underneath the loads hanges signifiantly, whereas the defletion immediately between the two loads remains onstant. On the other hand, Figure 6 shows that the base ourse modulus has a relatively small influene on the shape of the transverse defletion basin. As the base ourse modulus varies from 3, psi to 12, psi the defletion hange is relatively uniform at all points along the defletion basin. Therefore, the shape of the transverse defletion basin appears to provide a lear way to disriminate between the effets of the asphalt onrete modulus and the base ourse modulus. Later in this paper it will be shown that the strong relationship between the asphalt onrete modulus and the shape of the transverse defletion basin for a dual-load system was found to hold true for a broad range of pavement geometries and layer moduli. An evaluation of the longitudinal defletion basins shown in Figures 7 and 8 learly demonstrates that the base ourse modulus is strongly refleted in the shape of the longitudinal defletion basin, whereas the asphalt onrete modulus has a relatively small effet on the longitudinal defletions. Figure 7 shows that there is almost no hange in the longitudinal defletion basin as the asphalt onrete modulus varies from 3, psi to 1,2, psi. Figure 8 shows that the defletions near the transverse enterline between the two loads derease as the base ourse modulus varies from 3, psi to 12, psi. The figure also shows that the defletions beyond 3 in. away from the loads remain relatively onstant as the base ourse modulus hanges. Therefore, the shape of the longitudinal defletion basin appears to provide a lear way to disriminate between the effets of the asphalt onrete modulus and the base ourse modulus. Later in this paper it will be shown that the relationship between the base ourse modulus and the shape of the longitudinal defletion basin for a dual-load system was found to hold true for a broad range of pavement geometries and layer moduli. 4
Roque et al. 55 AC Modulus -3 KS/ - +eoo KS/ *12 KS/..!!? -4 e._.. -6 - -6 LAYER MOOULUS -4..!!? e._.. -6-8 ~ -1 - Modulus Redution -Original +1/2AC *1/3 Base SB: 1 In REFERENCE STRUCTURE AC: 6 KS/ BS: 6 KS/ SB: 35 KS/ SG: 25 KS/ Distane (inhes) FIGURE 7 Effet of hanges in asphalt onrete modulus on longitudinal defletion basin for dual-load system: 4-in. spaing. -14 LOAD : 2 X 9 lb RAO/ US: 6. In -16 5 1 15 2 25 3 35 4 Distane (inhes) FIGURE 9 Transverse defletion basin aused by reduing asphalt onrete or base ourse modulus for dual-load system: 4-in. spaing. Base Modulus +3 KS/ *eo KS/ +9 KS/..!!? -4 *12 KS/._.. E -6......... -8-1 AD: 2 X lb RADIUS: 6. In 1 SB: 1 in AC: 8 KS/ BS: VARIABLE SB: 35 KS/ SG: 25 KS/ 2 3 4 5 6 Distane (inhes) FIGURE 8 Effet of hanges in base ourse modulus on longitudinal defletion basin for dual-load system: 4-in. spaing...!!? -4 e._.. -6 ::: -8 Modulus Redution -Original +112 AC *1/3 Base LAYEfl THICKNESS REFERENCE STRUCTURE LAYER MDOULUS C: 6 in -1 - '. SS: 1 in AC: 6 KS/ SB: 1 in BS: 6 KS/ LOAD: 2 X 9 lb SG : SEMI INF SB: 35 KS/ RADIUS: 6. In SG: 25 KS/ -4-~-1----'I---+~-+'-~..----.~--...~..,..-~..----.~--...~~ 1 2 3 4 5 Distane (inhes) FIGURE 1 Longitudinal defletion basin aused by reduing the asphalt onrete modulus or base ourse modulus for dual-load system: 4-in. spaing. 6 In sharp ontrast to the single-load FWD, Figures 9 and 1 show that two different ombinations of asphalt onrete and base ourse moduli will not result in the same defletion basins for a dual-load system. Whereas for the single-load FWD, reduing the base ourse modulus by one-third had roughly the same effet on the defletion basin as reduing the asphalt onrete modulus by one-half (see Figure 3), the same modulus hanges resulted in distintly different hanges in the transverse and longitudinal defletion basins for the dual-load system. Figure 9 shows that reduing the asphalt onrete modulus by one-half inreased the defletions only under the load, whereas a redution in base ourse modulus of one-third inreased the transverse defletions uniformly. Figure 1 shows that only the redution in base ourse modulus affeted the longitudinal defletion basin. ANALYSIS OF PAVEMENT RESPONSE INDUCED BY DUAL-LOAD SYSTEM Stress and deformation analyses were onduted on a typial pavement struture to determine why the dual-load system works so well in isolating the independent effets of the asphalt onrete modulus and the base ourse modulus. An understanding of the system would allow onfiguration of a system that optimizes the apabilities to disriminate among the effets of near surfae layer moduli. Defletion measurements obtained from suh a system would optimize our hanes of determining near-surfae layer moduli aurately and reliably. An evaluation of the stresses and deformations indued along a transverse ross-setion of a typial pavement sub-
56 TRANSPORTATION RESEARCH RECORD 1355 jeted to a dual-load system demonstrates why the system works. Figure 11 shows that when the loads are spaed 4- in. apart, the surfae deformations between the loads result in a onave downward defletion basin. As shown in the figure, this results in signifiant bending moments in the asphalt onrete layer between loads. Basi mehanis demonstrates that the urvature resulting from these bending moments depends on the stiffness (modulus and thikness) of the asphalt onrete layer. Figure 11 also shows that the bending moments indued in the base ourse are negligible so that the stiffness of the base ourse should have a negligible influene on the urvature of the surfae, whih agrees with the findings presented previously. An evaluation of the vertial ompressive stress distribution in the asphalt onrete and the base ourse layers shows that the effet of the base ourse modulus on the shape of the surfae defletion basin should be negligible ompared to the effet of the asphalt onrete modulus. Figures 12 and 13 show vertial stress distributions at different depths along the transverse ross-setion of the pavement for base moduli of 3, 5 'iii S:4 Ill :ll!: 3 "' iii u t: 2 ~ 1 5 *MIDDLE AC LAYER MIDDLE BS LAYER Loation +BOTTOM AC LAYER -1!-BOTTOM BS LAYER LA YER MODULUS AC: 6 KSI BS: 6 KSI LOAD: 2 X 9 lb RADIUS: 6. in 15 2 25 Distane (inhes) 3 35 FIGURE 13 Vertial stress distributions indued by a dual-load system: 4-in. spaing, high modulus base. 4 - t - 6in 1in BENDING MOMENT AND LAYER STIFFNESS CONTROL SURFACE CURVATURE 4 psi COMPRESSION 19 lb E = 6 KSI E = 3 KSI psi and 6, psi, respetively. In both ases, the transverse stress distribution within the base layer is relatively uniform ompared with the stress distribution within the asphalt onrete layer. Note that it is the variation in stresses that results in hanges in the shape of the defletion basin. Changes in the base modulus will result in hanges in total defletions, but the stress distributions imply that these hanges should be uniform. One again, this agrees with the findings presented earlier. SEMI INF. j 2 psi FIGURE 11 Stresses and deformations indued by a dual-load system. 5 'iii.9:4-111 :ll!: 3 "' iii.. u ;2 ~ 1 ----,-~, 5 *MIDDLE AC LAYER MIDDLE BS LAYER 1 Loation +BOTTOM AC LAYER -1!-BOTTOM BS LAYER AC: 6 KSI BS: 3 KSI LOAD: 2 X 9 lb RADIUS: 6. in 2 25 Distane (inhes) E = 25 KSI 3 35 FIGURE 12 Vertial stress distributions indued by a dual-load system: 4-in. spaing, low-modulus base. 4 DETERMINATION OF OPTIMAL LOAD-SPACING AND LOAD RADIUS The analyses presented previously learly indiate that the asphalt onrete modulus is and should be strongly related to the shape of the transverse defletion basin for a dual-load system. This implies that load spaings, whih produe sharper (rather than flatter) onave downward transverse defletion basins between the loads, will optimize the system's apability to disriminate among the effets of the near-surfae layer moduli. This will allow for more aurate and reliable determination of these moduli. Furthermore, the system's defletion sensors must be positioned to define both the transverse and longitudinal defletion basins aurately enough to detet the independent hanges aused by the different pavement layers. The key to obtaining sharper defletion basins for optimal disrimination among the surfae layer moduli is to position the loads suffiiently far apart to ause signifiant bending moments in the region immediately between the loads. If the loads are too lose, suh that strong interations develop between the loads, these moments may never develop or the entire surfae between the two loads may be in a state of horizontal ompression. On the other hand, if the loads are spaed too far apart, the loads may at independently of eah other, whih would essentially result in two single-load systems. In either ase, the advantages of the dual-load system would be lost.
Roque et al. Given that the shape of the transverse defletion basin would be influened not only by the load spaing, but also by the pavement geometry and layer moduli, a omprehensive analysis was onduted to determine an optimal load spaing that would result in appropriate defletion basins for a broad range of pavement strutures. The values shown in Table 1 were onsidered in the analysis. The following onstraints limited the ranges of aeptable load radii and spaings: The load levels used for testing should be representative of the load levels attained in the field. The load level hosen will govern the radii used to obtain average pressures similar to the ones obtained in the field. The enter-to-enter spaing of the loads should be kept as lose as possible to make the onstrution of a dual-load system possible. A spaing greater than 4 in. was onsidered impratial. Assuming 9, lbs per load, the resulting average stress under eah load would be as follows: 179 psi for a 4. in. radius, 114 psi for a 5. in. radius, and 79.5 psi for a 6. in. radius. These ranges were onsidered to be aeptable in pressures, suh that radii less than 4. in. or greater than 6. in. were not onsidered for evaluation. Elasti layer analyses were onduted for the range of pavement strutures listed in Table 1, using load spaings of 2, 3, and 4 in. and load radii of 4 and 6 in. Typial results of the analyses are shown in Figures 14 and 15, whih show transverse defletion basins for a 4-in. and 8-in. asphalt onrete pavement, respetively. Both figures indiate that load radius had little effet on the shape of the defletion basin. Figure 14 shows that for the thinner pavement setion, all three load spaings resulted in fairly sharp transverse defletion basins, whih would allow for aurate disrimination among near-surfae layer moduli. This was typial for the thinner (lower stiffness) setions investigated, and indiated that there was no advantage of using one load spaing over another. For the thiker (higher stiffness) setions investigated, it was found that wider load spaings were required to obtain defletion basins with reasonably sharp urvatures. A typial example is shown in Figure 15, whih shows that the 4-in. spaing offers a slight advantage over the 3-in. spaing and a signifiant advantage over the 2-in. spaing in produing measurable defletion differenes along the transverse axis. --!!!.!.-1 :;::: g _15 :; O LAYER MODU WS LOAD: 2 X 9 lb AC: 4 in -5 AC: 4 KSI BS: 6 KSI Plate Radius/Load Spaing -4 14 +6'/4' *4'/2 +5 /2 *4'/3' -+-5 /3 5 -+-~--'r-~--.-~~.,..-~---.~~-.-~~...-~--r~---1 5 1 15 2 25 3 35 Distane (inhes) FIGURE 14 Comparison of transverse defletion basins indued by different load-spaing and radii on 4-in. pavement setion. :;::: t) o -.-LA~YE_R_T_H-IC_K_N_E_S_S~LA-Y-ER~M-OD_U_L_U_S.,..-~~~-l.O~A--:2_X_OO oo-~ AC: 8 in -5 - GI -15 - -................ :; ->-- AC: 8 KSI BS: 6 KSI Plate Radius/Load Sp11~lng -4 /4 +6'/4' *4'/2 e /2 *4"/3' -+-e"/3' 5 -+-~~1--~--.-~~.,..-~---.~~-.-~~...-~-+~---1 5 1 15 2 25 3 35 Distane (inhes) FIGURE 15 Comparison of transverse defletion basins indued by different load spaing and radii on 8-in. pavement setion. Based on these analyses, and the fat that spaings greater than 4 in. were onsidered impratial, a spaing of 4 in. was seleted for the optimal system onfiguration. A load radius of 6 in. was seleted for further evaluation. However, any load radius from 4 to 6 in. may be onsidered aeptable, beause the radius of the load was found to have little influ- 4 4 57 TABLE 1 Pavement Layer Thikness and Moduli Considered for Analysis La: er Moduli (ksil Thiknesses (in) Asphalt Conrete 2, 4, 6, 8, 12 4, 6, 8, 12 Base Course 2, 4, 6, 8, 12 5 ' 1, 2 Sub base ** 5' 1, 15 Sub grade 5, 15, 25, 5 semi-infinite **The subbase modulus was varied between the subgrade modulus and the base modulus (i.e. it was never allowed to be greater than the base modulus or less than the subgrade modulus).
58 ene on the shape of the defletion basins. A 6-in. radius would allow for larger loads to be used without overstressing and possibly damaging the surfae layer during testing. EVALUATION OF THE DUAL-LOAD SYSTEM CONFIGURATION An evaluation of the defletion basins resulting from the dualload system onfigured previously ( 4-in. load spaing, 6-in. radius loads) was onduted for the range in pavement strutures and layer moduli generally enountered in North Ameria. These analyses showed that the relationships between the shape of the transverse defletion basin and the asphalt onrete modulus, and between the shape of the longitudinal defletion basin and the base ourse modulus, held true for almost all pavement strutures investigated. Elasti layer analyses were onduted using BISAR to determine the transverse and longitudinal defletion basins resulting from the dual-load system onfigured previously for every ombination of the layer thiknesses and of layer moduli shown in Table 1. An analysis was onduted to determine whether there was a strong orrespondene between the shape of the transverse defletion basin and the asphalt onrete modulus. The differene in defletions between the point immediately underneath the load (Dl) and the point immediately between the loads (D3) was used as the parameter to represent the shape of the transverse defletion basin. The following relationship was found for a typial pavement struture (6-in. asphalt onrete, 1-in. base, 1-in. subbase, semiinfinite sub grade). EAC = ef7.22-o.ssv1-d3jl Rz 84.4 perent (1) where EAC = Dl D3 asphalt onrete modulus (ksi), surfae defletion diretly under one of the loads ( x 1-3 in.), surfae defletion exatly between the two loads ( x 1-3 in.). Similarly, the base ourse modulus was found to be related to the shape of the longitudinal defletion basin for the range of pavement strutures investigated. The relationship involved interations with the subbase and subgrade moduli, suh that a simple orrelation as shown in Equation 1 ould not be obtained. It should be emphasized that Equation 1 is not intended to predit asphalt onrete modulus diretly but only to show the strong orrelation between the asphalt onrete and the shape of the transverse defletion basin as would be measured TRANSPORTATION RESEARCH RECORD 1355 by the dual-load system. Preditive equations for all layer moduli, whih aount for the effets of layer thiknesses and interations among layer moduli, were developed suessfully based these analyses. However, presentation and development of these equations is beyond the sope of this paper. CONCLUSIONS AND RECOMMENDATIONS The following onlusions were reahed on the basis of the results of the analyses presented in this paper: 1. A dual-load nondestrutive testing system provides for better disrimination of near-surfae layer moduli than the existing single-load FWD. 2. The dual-load system works beause the shape of the transverse defletion basin indued between the two loads is most strongly influened by the bending moments indued within the asphalt onrete layer between the loads and by the relatively large hanges in vertial ompression whih are indued in the asphalt onrete layer between the loads. 3. A dual-load spaing of 4 in. was found to provide for optimal disrimination of near-surfae layer moduli for the broad range of pavement geometries and layer moduli generally enountered in North Ameria. 4. Beause of the relatively strong and diret orrelations between different layer moduli and surfae defletions for a dual-load system, it appears that a set of relatively simple regression equations an be developed to determine layer moduli. It is reommended that the dual-load system onfigured in this paper be onstruted and implemented for field testing. REFERENCES 1. B. E. Ruth, M. Tia, K. Badu-Tweneboah. Strutural Charaterization and Stress Analysis of Flexible Pavement Systems. Final report, State Projet 997-7351-1. Florida Department of Transportation; University of Florida, Gainesville, July 1987. 2. K. Badu-Tweneboah, C. W. Manzione, B. E. Ruth, and W. G. Miley. Predition of Flexible Pavement Layer Moduli from Dynaflet and FWD Defletions. In Nondestrutive Testing of Pavements and Bakalu/ation of Moduli (A. J. Bush and G. Y. Baladi, eds.), ASTM STP 126, ASTM, Philadelphia, Pa., 1989. 3. D. L. DeJong, M. G. F. Peutz, and A. R. Korswagen. Computer Program BISAR, Layered Systems Under Normal and Tangential Loads. Kononklijke-Shell Laboratorium, Amsterdam, The Netherlands, 1973. Publiation of this paper sponsored by Committee on Strength and Deformation Charateristis of Pavement Setions.