DEFLECTION BOWL PARAMETERS CORRELATION PILOT STUDY OF A LIGHT AND STANDARD FALLING WEIGHT DEFLECTOMETER

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1 DEFLECTION BOWL PARAMETERS CORRELATION PILOT STUDY OF A LIGHT AND STANDARD FALLING WEIGHT DEFLECTOMETER E Horak*, and T Khumalo** * Professor and head of department of Civil Engineering, University of Pretoria, Pretoria. ** Final year student research project for in civil engineering degree, University of Pretoria and presently Engineer in training with Ndodana Consulting Engineers, Centurion, Pretoria Abstract The falling weight deflectometer (FWD) is used world wide as a well established and valuable non-destructive road testing device for pavement structural analyses. The FWD is used mostly for rehabilitation design investigations and for pavement management system (PMS) monitoring on a network basis. A new light weight falling weight deflectometer (LFWD) has recently been developed and is used as a handheld portable apparatus. Various correlation studies with other non-destructive road testing devices have been done world wide and proved that the LFWD is very useful for construction quality control and assurance purposes. The lighter drop-weight has a shallower zone of influence on pavement layer systems and therefore makes it ideal for individual layer structural evaluation during construction. The use of an approximation of the classical Boussinesq theory of an elastic half space to calculate the surface modulus with the FWD and LFWD provides valuable engineering parameters for pavement structural evaluation. A more theoretically correct approach of using multi-layered linear elastic theory in back-calculation procedures can also provide elastic moduli for the pavement structures. As an alternative to this mechanistic or theoretical approach considerable use is made of an empirical- semi-mechanistic analysis technique in South Africa where deflection bowl parameters measured with the FWD are correlated with pavement layer structural strength. This paper reports on the first pilot correlation study done in South Africa of such measured deflection bowl parameters determined with the normal FWD and with the LFWD. It discusses the usefulness and relevance of the LFWD and makes recommendations for use in road construction monitoring and evaluation.

2 2 DEFLECTION BOWL PARAMETERS CORRELATION PILOT STUDY OF A LIGHT AND STANDARD FALLING WEIGHT DEFLECTOMETER 1. Introduction Deflection measurements of pavement structures are used to do structural analyses for the purpose of rehabilitation design as well as for network monitoring of pavement networks. The older equipment like the Benkelman beam and La Croix deflectograph were used extensively in the past and various empirical relations were developed for analysis and overlay design by organisations like Shell, the Asphalt Institute, and TRRL (Jordaan, 1988). In most cases only the maximum deflection were utilised and the shape of the deflection bowl and the significance of its relationship with the pavement structural response was basically ignored and wasted. Since the 1980s significant improvement of non-destructive deflection measuring devices resulted in the ability to measure the whole deflection bowl accurately. It also enabled an appreciation of the value of the whole deflection bowl in structural analysis of roads and pavements (Horak, 1988). The extensive use of the modified Benkelman beam, the road surface deflectometer (RSD), with accelerated pavement testing (APT) devices, like the heavy vehicle simulator (HVS) in South Africa (SA), coupled with the use of the in depth deflection measurements with the multi-depth deflectometer (MDD), helped to give credibility to the back-calculation of elastic moduli with various multi-layered linear elastic computer models. The extensive test programmes of the HVS in SA helped to correlate such back-calculated elastic moduli with pavement performance and deterioration modelling and helped to increase the credibility and use of back-calculated elastic moduli derived from surface deflection measurements. (Horak, et al, 1992). The falling weight deflectometer (FWD) was introduced to SA in the mid 1980s (Coetzee et al, 1989 and Horak et al, 1989) under the name impulse deflection meter (IDM) and soon became well established in SA. The extensive test programme of the HVS resulted in the verification and promotion of the mechanistic-empirical design and analysis procedure in SA (Horak et al, 1992). Horak (1988) initially investigated the use of various deflection bowl parameters and established various semi-empirical mechanistic relationships which could be used in pavement structural evaluations. This work has subsequently been developed further and refined by other researchers in this field (Rohde and van Wijk,, 1996, Joubert, 1993, Horak 1987 and Horak et al, 1992)

3 3 A new generation hand held or portable light weight falling weight deflectometer (LFWD) has recently become available in SA after extensive development and verification testing overseas. The LFWD is briefly described here and relevant correlation studies with other nondestructive deflection devices are reported here. The LFWD was initially acquired to be used for a research project in a neighbouring country and was only briefly available for a very short study in SA prior to commissioning. The opportunity was used to do a short pilot study with the LFWD in correlation with the FWD on one specific pavement type. The pilot correlation study focussed on the possible use of the deflection bowl parameters of the LFWD in a similar fashion to that used and developed for the FWD. The calculation of the surface modulus is also standard output generated by both the FWD and the LFWD. These two instrument linked moduli values were also correlated. The results of this pilot study are reported on in this paper and conclusions and recommendations made regarding future work. 2. The Light Falling Weight Deflectometer (LFWD) The LFWD is a scaled down version of the normal FWD. It comes in two versions namely the pocket PC version and the notebook PC version. Recorded data is transmitted wireless from the instrument to the PC using Blue-tooth technology. It weighs less than 20kg. (Hoffmann et al, 2003). It is highly portable and can easily be carried around and used on construction sites as illustrated in Figure 1. Release mechanism Shaft with preset drop height Dropped weights Wheeled transporter Additional geophones Solid load plate with load cell and geophone Figure 1. The Light Falling Weight Deflectometer (LFWD)

4 4 Figures 2, to follow, illustrates further detail of the basic components of the LFWD. It consists of the following parts: Up to three sensors with the associated electronics, which measure deflections. The electronic equipment in the LFWD is dust-proof and watertight for safe outdoor use. -Housing protecting the sensors -Falling weight (sliding hammer), which varies from 10kg to 20kg. It has a rechargeable battery pack, providing approximately 2000 measurements or, the equivalent of more than twelve hours of continuous operation Figure 2 Detailed Illustration of the LFWD components A trigger handle for adjustment of the drop height. The trigger handle is fitted with a safety lock. 200mm and 300mm diameter loading plates Load cell, which measures the peak value of the impact force. The key value of the impact force is based on the actual measurements of the load cell. The load range is between 1kN and 15kN and the load pulse range is 15 to 20ms.

5 5 The LFWD device measures both the force and the deflections with a velocity transducer. The centre deflection is measured and two more readings further away from the load centre can be measured with additional geophones (normally spaced at 300mm). The LFWD automatically measures and records the deflection bowl and has software which estimates an elastic stiffness calculated similar to the one used to calculate the surface modulus (Craig, 1997;.Nazzal, 2003 and Ullidtz, 1987) of a layered medium assuming a uniform Poisson s ratio, and constant loading on an elastic half-space. This is in essence an approximation of the Boussinesq elastic half-space equation (Ullidtz, 1987) The data collection software displays the approximated surface modulus and the time history graph from both the deflection sensor and the load sensor. The elastic surface modulus is calculated and viewed in real time. Both Poisson's ratio and stress distribution factor parameters may be entered in the user set-up. The measurement data can be printed as a report or can be transferred for further processing in spreadsheet or text form. A complete analysis of the LFWD field data can provide an estimate of the linear elastic response of the individual layer materials making up the pavement structure. Therefore, it is well suited for application in quality control/ quality assurance (Qc/Q A ) procedures for the construction of pavement layers and other geo-materials. However, there are currently limited published data relating to its efficiency (Fleming et al., 2000). 3. Correlations between the LFWD and other non-destructive testing equipment To date the LFWD has been correlated with a number of other non-destructive testing devices (Gurp et al, 2000 and Hoffman et al, 2003). Chen et al (2001) even correlated the LFWD successfully with the Dynamic Cone Penetrometer (DCP). However, only non-destructive testing devices relying on elastic response will be referred to and briefly discussed here. The plate load test (PLT) is used for site investigation and for proof testing of pavement structure layers in some parts of the USA and some European countries. Plate loading tests can be used to estimate the modulus of subgrade reaction (k). Determination of the subgrade modulus is made in the field on selected subgrade soil at its natural moisture content. The modulus of subgrade reaction can be calculated using the following relation (Yoder and Witczak, 1975): k= p/ Where:

6 6 p = unit load on plate (N) = deflection of the plate (mm) The loading of the plate is normally a static load, but in some countries, it can be done dynamically. According to the German code, to relate the stiffness moduli calculated from the plate load test (PLT) and the German Dynamic Plate Test (GDP) the following equation can be used (Livneh and Goldberg, 2001): 300 E( PLT ) E( LFWD) Where: E(PLT) is the German reloading stiffness elastic modulus in MPa obtained by PLT test and E(LFWD) is the LFWD elastic surface modulus. Nassal (2003) did a correlation study between the PLT and the LFWD on cement treated soils, lime treated soils, unstabilised fine-grained soils and granular soils. The relationship for moduli (surface modulus for LFWD) thus developed is as follows: E(PLT) = (E(LFWD)) with an R 2 value of Fleming et al. (1988) demonstrated that a correlation ratio between the elastic surface moduli determined with the German Dynamic Plate (GDP) and the FWD is about 0.5. However, Fleming (1998, 2001) reported that his extensive field-stiffness measurements on in-situ construction sites showed a relatively consistent correlation of 0.6 between the stiffness moduli of the GDP and the FWD. Livenh and Goldberg (2001) suggested that the GDP (LFWD) stiffness moduli are about 0.3 to 0.4 times the conventional FWD surface moduli. Fleming et al. (2000) conducted field tests to correlate the moduli determined with three main types of LFWD available on the market at present with that of the FWD. Their results showed that the resilient surface modulus, E FWD correlated well with moduli obtained from the LFWD. However they found that the correlation coefficients are LFWD instrument specific and should first be established before use with confidence. Fleming (2001) reported that a number of factors influence the measured stiffness of the LFWD including differences in mass, transducer type and software analysis (which records the maximum deflection as that at the time of the peak force). Nazzal (2003) found that the best model to predict the FWD back-calculated resilient surface moduli, E(FWD) in MPa from the LFWD surface modulus, E(LFWD) in MPa is: E(FWD)= 0.97*E(LFWD) for 12.5 MPa < E(LFWD) < 8.65 MPa

7 7 With R 2 = 0.94, significance level < 99.9% and standard error = 3.31 In comparing his studies with those of Fleming (2000), Nazzal (2003) found that his correlations agreed with those of Fleming (2000) for a variety of material types. According to Rahimzadeh (2004) the relationship between FWD and LFWD was found to be material type and thickness dependent. The FWD is regarded as the most appropriate device for setting the standard, because not only is the loading most representative of real traffic loading, but it can also be used for assessment of all pavement layers as construction proceeds. Either the FWD or the LFWD can be used for measurement of stiffness as long as the same plate rigidity factor is assumed ( /2 for a flexible plate). If the LFWD default setting (rigid plate, rigidity factor of 2) is assumed, then a correction factor must be applied, such that E(LFWD) = E(FWD). 4. Description and use of deflection bowl parameters as used in South Africa When a pavement deflects under a load, the influence of the load extends over an area measurable 1 to 2meter away from the point of loading in three dimensions. This is illustrated in Figure 3 for a uniform circular and truck dual axle loading situation. This deflected area tends to form a uniform circular deflected indentation called a deflection bowl. The size and shape of the deflection bowl vary and depend on different factors such as pavement composition and structural strength, size of load contact area, load magnitude and duration of loading, the measuring device used, temperature, etc. (Horak, 1987 and 1988 and Lacante, 1992). In this investigation the impulse loading of the FWD is used as reference measuring device (Coetzee and van Wijk, 1989) simulating a moving loaded wheel situation. The deflection measured in this way has mainly elastic deflection response and not plastic response characteristics like measured under the Benkelman beam device (Horak, 1988).

8 8 Figure 3. Illustration of deflection bowl shapes under various forms of loading (Source: Horak, 1988) In Figure 4 typical deflection bowl shapes are illustrated for various pavement structural compositions but with the same loading situation. The various deflection bowls in Figure 4 illustrate the influence that the structural strength and thickness of the base layer have on the region close to the point of loading. It serves to show that the deflection bowl has various regions with specific pavement structural associations. Horak (1988) described various deflection bowl parameters which can be determined from such measured deflection bowls. The use of measured deflection bowl parameters in the evaluation of the structural capacity of a pavement has subsequently been suggested and used by several researchers (Horak et al. 1989, Maree and Rohde, 1996).

9 9 Distance from point of loading (mm) Figure 4. Typical shape of deflection bowl for various base layer pavement structures In Table 1 various deflection bowl parameters and their formula are summarised. The association of the various deflection bowl parameters with the pavement structure and structural elements are also briefly summarised. Of these nine deflection bowl parameters listed in Table 1, Horak (1988) found that only the first five gave good correlations with the relevant pavement structural condition and individual pavement layer associations. Maree and Bellekens (1991) analysed various pavement structures (granular, bituminous and cemented base pavements) as measured with the FWD. Pavement structures were analysed mechanistically, remaining lives determined and correlated with measured deflection basin parameters. The remaining life is expressed in terms of standard or equivalent 80kN axle repetitions (E80s). These relationships and correlations are shown in Figure 5 to follow for granular, bituminous and cemented base pavements These deflection bowl parameters have been refined and promoted with success as semiempirical-mechanistic indicators of the structural strength and condition of the pavement. (Horak et al, 1989; Rohde and van Wijk, 1996 and Joubert, 1995). These curves and associated criteria have subsequently been included in the TRH12 guideline for rehabilitation design and analysis in SA (CSRA, 1996).

10 10 Table 1: Deflection Bowl Parameters (Horak et al, 1989) Parameter Formula Structural indicator 1.Maximum deflection D 0 as measured D 0 gives an indication of all structural layers with about 70% contribution by the subgrade 2. Radius of Curvature (RoC) RoC gives an indication of RoC= the structural condition of the surfacing and base condition 3.Base Layer Index (BLI) 4.Middle Layer Index (MLI) 5. Lower Layer Index (LLI) (200) 2 /2D 0 (1- D 0 /D 200 ) BLI=D 0 -D 300 MLI=D 300 -D 600 LLI=D 600 -D Spreadability, S S={[(D 0 +D 1 +D 2 +D 3 )/5]100}/D 0 Where D1, D2, D3 spaced at 300mm 7. Area, A A=6[1+2(D 1 /D 0 ) +2(D 2 /D 0 ) + D 3 /D 0 ] BLI gives an indication of primarily the base layer structural condition MLI gives an indication of the subbase and probably selected layer structural condition LLI gives an indication of the lower structural layers like the selected and the subgrade layers Supposed to reflect the structural response of the whole pavement structure, but with weak correlations The same as above 8.Shape factors F 1 =(D 0 -D 2 )/D 1 F 2 =(D 1 -D 3 )/D 2 The F2 shape factor seemed to give better correlations with subgrade moduli while F1 gave weak correlations 9. Slope of Deflection SD= tan -1 (D 0 -D 600 )/600 Weak correlations observed Structural indicators ranging from sound, warning and severe were developed for these first five deflection bowl parameters shown in Table 1. Ranges for such structural indicators can be set for specific pavement base types and traffic classes by using the correlations shown in Figure 5. These criteria are normally used in association with other survey techniques such as visual surveys, Dynamic Cone Penetrometers (DCP) and field material sampling, etc. which enhances the confidence in the rehabilitation investigation and analysis..

11 11 Maximum Deflection BLI MLI LLI Figure 5. Correlation between deflection bowl parameters and remaining life (Source: Maree and Bellekens, 1991)

12 12 The evaluation of the pavement with such set criteria for the deflection bowl parameters is figuratively speaking like using a sieve to identify areas along the length of the road where problems may occur over the length of the road. It also enables a first stab at identifying the most probable structural layer which may be the cause of the distress at such locations. Such a first analysis with deflection bowl parameters then helps to direct further testing and detailed investigations to confirm the cause of failure. The LFWD described before has 3 sensors (spaced 300mm apart) and therefore only measures the associated deflection bowl up to 600mm away from the point of loading. Therefore for deflection bowls measured with the LFWD only the following deflection bowl parameters can be calculated: Maximum deflection, Radius of curvature, Base Layer Index, Middle Layer Index, Shape factor (F1) and Slope of deflection (SD). In the following section more detail of the pilot correlation study is given on the use of these deflection bowl parameters. 5. Comparative pilot study 5.1. Test procedure An initial comparative study was done on deflection bowl parameters measured with both the FWD and the LFWD when the LFWD became briefly available between delivery and deployment in a neighbouring country. The comparative tests were done on a recently constructed road with a granular base pavement. At least ten measuring points were marked on the stretch of road to ensure measurements with the FWD and LFWD would be directly comparable. The measurement methodology followed is described briefly as follows. A 40kN load was used with the FWD and the drop height was set at 850mm. A 300mm diameter plate was used. This resulted in an average loading pressure of 566kPa. A 25kN drop weight was also used which resulted in a loading pressure of 354kPa average. The FWD loading plate is a split plate with the joint in line with the direction of travel and measuring geophones. This loading plate has better load transfer than the solid circular loading plates in that it can accommodate unevenness due to rutting with a closer hugging of the deformed road surface. However, with this being a newly constructed road there was no evidence of rutting. The FWD was positioned on the test lane such that the loading point was at the center of the lane. The nine sensors of the FWD were positioned in a straight line parallel to the length of the road. They were placed at 0, 200, 300, 450, 600, 900, 1200, 1500 and 1900mm from the

13 13 center of the loading point with the standard lowering beam. The mass was dropped 6 times at each station and the resulting deflections of the pavement were measured and recorded. The force, pressure, air and pavement surface temperatures were also recorded. As mentioned before for the second pass, a 25kN load was used while the plate size and drop height were kept the same. For the first pass with the LFWD, a 20kg mass was used and the drop height was set at maximum 850mm The plate used was the 200mm diameter plate. In this case it is a solid load plate and not jointed as the FWD. This loading set-up caused a loading force of 16.8kN average and a load pressure of 535kPa average. The LFWD was positioned at the same measuring stations as used for the FWD in the middle of the lane. Three sensors were positioned in a straight line at 0, 300 and 600mm from the center of the loading plate. They were also positioned parallel to the direction of the road length. The LFWD mass was then dropped three times and at each drop the pocket PC (with blue chip technology) recorded the measured deflections. The pressure and force were also recorded. Three drops were done at each station to check the repeatability. The three measurements were then averaged. For the second pass, a 10kg mass was used. The drop height was kept at 850mm and the plate size was kept the same. In this case the loading pressure resulted in 313kPa average. For the third pass, the mass and drop height were kept the same while a 300mm diameter plate was used. However the last LFWD setup was done on only half the common measuring points and the result was that this set-up had too few measuring points to be useful in the correlation study. In Figure 6 a typical set of deflection measurements and variation in measuring set-ups are shown for both the LFWD and FWD. As can be seen in Figure 6 higher loads for both FWD and LFWD result in higher deflections. This difference is more prominent closer to the point of loading and peters out towards the deflection bowl extremities. The LFWD shows a truncated deflection bowl due to the limited geophone measuring points used. However, the deflections for the LFWD were consistently lower that that measured with the FWD. This is clearly due to the lower load imposed by the LFWD. When nearly the same load pressure (535kPa with the LFWD and 354kPa with the FWD) were used the deflections at the point of loading were close, but the shape of the deflection bowls differed significantly. The variance in loading plate diameter for the LFWD did not show significant differences in deflection further away from the point of loading.

14 14 Deflection (micrometer) LFWD, 535kPa LFWD, 313kPa FWD, 566kPa FWD, 354kPa Distance from point of loading (mm) Figure 6: Deflections measured versus distance from measuring point for FWD and LFWD settings 5.2. Correlations of surface moduli In Figure 7 to follow the surface moduli calculated from two measuring settings for the FWD and LFWD are shown. Both the FWD and LFWD results show classical stress stiffening behaviour of the subgrade (Ullidtz, 1987) based on the shape of the lines from 450mm onwards. The surface moduli increases as depth increase and surface moduli are calculated further away from the point of loading. Various combinations of FWD and LFWD settings were correlated. Correlation of the FWD surface moduli and LFWD surface moduli showed that E FWD, 566kPa and the E LFWD, 535kPa had the best correlation. The regression model was as follows: E FWD = E LFWD, with R 2 = The sample size is very small and not the whole surface moduli curve of the FWD can be used due to the difference in number of measuring points due to the LFWD limitations. Therefore this correlation should only be treated as an early indication that good correlations may be possible if a larger sample can be obtained for granular based pavements.

15 15 Surface moduli calculated for FWD and LFWD Surface moduli (MPa) LFWD, 535kPa LFWD, 313kPa FWD, 566kPa FWD, 354kPa Distance from point of loading (mm) Figure 7. Surface moduli for LFWD and FWD measurements 5.3 Correlations of deflection bowl parameters A correlation was done of the deflection bowl parameters of the LFWD and the FWD using linear regression analysis to relate the FWD and LFWD associated deflection bowl parameters directly with each other. The coefficient of correlation, R 2, was determined for each correlation. A significance level of 95% was used in these analyses. The results of the regression analysis conducted for this study are shown in Table 2. Only the best correlation coefficient results are summarized here for various deflection bowl parameters for various instrument settings. Table 2. Summary of regression results Deflection bowl parameter FWD settings LFWD settings Correlation formula correlation Coefficient (R 2 ) Ymax 566kPa 535kPa Ymax (FWD) = Ymax (LFWD) 0.58 MLI 354kPa 313kPa MLI (FWD) = MLI (LFWD) 0.97 BLI 354kPa 535kPa BLI (FWD) = BLI (LFWD) 0.28 RoC 354kPa 535kPa RoC(FWD) = RoC (LFWD) 0.78 SD 566kPa 535kPa SD (FWD) = SD (LFWD) 0.37 F1 354kPa 535kPa F1 (FWD) = F1 (LFWD) 0.96 As indicated previously only the deflection bowl parameters using deflections measured up to 600mm from the point of loading could be used in this correlation study due to the

16 16 measurement set-up of the LFWD. Only the deflection bowl parameters MLI, RoC and the shape factor F1 had good coefficients of correlation. The lower, but still reasonable, coefficient of correlation of Ymax is expected due to the difference in loading magnitude and the associated depth of influence of the FWD and the LFWD loading situations. The low coefficient of correlation for BLI is more difficult to explain. It seems that the closeness of the 300mm measuring point to the edge of the loading plate in both cases may be the reason for some inaccuracies in that sensor at 300mm. However, this sample is very small and only for one type of pavement (granular base) and a larger sample in future tests can lead to better correlation results for the BLI parameter. 6. Conclusions and recommendations The LFWD proved to be a truly light weight FWD and is highly transportable. It is very easy to operate and changes from the 10kg to the 20kg drop weights or loading plates (200mm and 300mm) are quick and easy to do. The blue-tooth technology proved to be very efficient in transferring recorded deflection data from the device to the handheld or laptop PC. The basic information provided by the LFWD clearly proved to be very useful for construction quality control and assurance purposes. The correlations previously established with other nondestructive testing devices will also provide for useful information to convert LFWD information to material engineering information and ways to analyse such data. It is suggested that such correlation studies be done on various pavement types in SA to establish such SA correlations which can be used with confidence locally. The limited initial availability of the LFWD resulted in this limited pilot correlation study using the normal FWD versus the LFWD on only one type of pavement, a granular based pavement. Surface moduli is currently still under utilized in SA in structural evaluations. The small test sample though did show that there is potential for very good correlations between FWD and LFWD surface moduli calculations. This is an area that needs further investigation as the surface modulus can give an early indication of constructed layer quality and can potentially be monitored by means of the LFWD. Such ongoing research is currently carried out by other students at the University of Pretoria and additional information may become available soon. However, the main focus of this pilot study was to determine which deflection bowl parameters can be used with the LFWD measurements with confidence and whether there are good correlations with those determined with the FWD. It was found that the three geophones

17 17 of the LFWD have a maximum reach of 600mm from the point of loading and therefore only the following deflection bowl parameters can be calculated with confidence: Maximum deflection Radius of curvature (RoC) Base layer index (BLI) Middle layer index (MLI) Shape factor (F1) and Slope of deflection (SD) Of these only RoC, MLI and F1 had good correlation coefficients. It was expected that maximum deflection may give weaker correlations due to the difference in loading weights and shallow depth of influence of the LFWD versus the FWD. It is not clear why RoC had good correlations as the 200mm deflection is interpolated with the LFWD measuring setup. BLI on the other hand use actual measured values and did not have a good correlation. This weaker correlation could be due to measurements being influenced by the edge of the loading plate. Nevertheless the limited sample of results on a granular base pavement showed very good promise. Clearly the LFWD can also be used in a semi-empirical mechanistic analysis of pavement structures and has considerable quality control and assurance application during construction. It is therefore suggested that the size of the testing sample be increased, not only for granular based pavement structures, but also for bituminous based and cemented based pavement structures. The correlations should also be extended from deflection bowl parameters to various forms of elastic moduli calculations and back-calculations to enhance the use of the LFWD in the structural and material evaluation in SA. The correlation of surface moduli determined with the FWD and LFWD should also be investigated as this seems to be a good way of determining engineering properties of pavement layers during construction. References Chen, D.H., Wang, J.N. and Bilyen, J Application of the DCP in the Evaluation of Base and Subgrade Layers. 80 th Annual Meeting of the Transportation Research Board, January 2001, Washington, D.C.

18 18 Coetzee NF, van Wijk AJ and Maree JH (1989) Impact Deflection Measurements. Proceedings of the Fifth Conference on Aspahlt Pavements in Southern Africa. Swaziland, 1989 Craig, R.F Soil Mechanics, 6 th Edition, Chapman and Hall, London, United Kingdom. Committee of State Road Authorities (CSRA) (1996) Guidelines for rehabilitation design of flexible pavements. Technical Recommendations for Highways 12 (TRH 12), Department of Transport (DoT), Pretoria. Fleming, P.R Field Measurement of Stiffness Modulus For Pavement Foundations Transportation Research Board 1755, Submitted to the 2001 Annual Meeting of the Transportation Research Board For Presentation and Publication. Washington, D.C. Gurp, C., Groenendijk, J. and Benving, E Experience With Various Types of Foundation Tests Proceedings of the Fifth International Conference on Unbound Aggregate In Roads, Nottingham, United Kingdom. Hoffman, O., Guzina, B.B. and Drescher, A Enhancements and Verification Tests For Portable Deflectometers, Final Report Minnesota Department of Transportation, St-Paul, MN. Horak E (1987) The use of surface deflection basin measurements in the mechanistic analysis of flexible pavements. Proceedings of the Fifth International Confenernce on the Structural design of Asphalt Pavements. Ann Arbor, Michigan, USA, Horak E (1988). Aspects of Deflection Basin Parameters used in a Mechanistic Rehabilitation Design Procedure for Flexible Pavements in South Africa. PhD thesis, Department of Civil Engineering at the University of Pretoria, Pretoria, South Africa. Horak E, Maree JH and van Wijk AJ (1989) Procedures for using Impulse Deflectometer (IDM) measurements in the structural evaluation of pavements. Proceedings of the Annual Transportation Convention Vol 5A, Pretoria, South Africa. Horak E, Kleyn EG, du Plessis JA, de Villiers EM and Thomson AJ (1992). The impact and management of the Heavy Vehicle Simulator (HVS) fleet in South Africa. Proceedings of the 7 th International Conference on Asphalt Pavements, Nottingham, England. August 1992.

19 19 Jordaan GJ (1988) Analysis and development of some pavement rehabilitation design methods. PhD Thesis, Department of Civil engineering, University of Pretoria, Pretoria, South Africa. Joubert PB (1995) Structural Classification of pavements through the use of FWD deflection basin parameters. Proceedings of the Annual Transportation Convention, Pretoria, South Africa. Lacante, S.C Comparative Study of Deflection Basins Measured on Road Structures With Various Non-Destructive Measuring Devices. Thesis for MTech. Technikon of Pretoria, Pretoria, South Africa. Livneh,M: Goldberg,Y (2001). Quality Assessment During Road Formation and Foundation Construction: Use of Falling-Weight Deflectometer And Light Drop Weight; Transportation Research Record 1755, Submitted to the 2001 Annual Meeting of the Transportation Research Board For Presentation and publication, pp Maree JH and Bellekens RJL (1991) The effect of asphalt overlays on the resilient deflection bowl response of typical pavement structures. Research report RP 90/102. for the Department of Transport. Chief Directorate National Roads, Pretoria, South Africa. Rahimzadeh,B; Jones,M; Thom,N. (2004). Performance Testing of Unbound Materials Within the Pavement Foundation, 6th International Symposium on Pavement Unbound- UNBAR6, Nottingham, 6-8 July 2004 Rohde, G.T. and Van Wijk, A.J A Mechanistic Procedure To Determine Basin Parameter Criteria. Southern African Transportation Conference, Pretoria, South Africa. Ullidtz P (1987) Pavement analysis. Elsevier Science Publishers Amsterdam Netherlands. ISBN Yoder, E.J. and Witczak, M Principles of Pavement Design. John Wiley and Sons, New York. Horak E and Khumalo T (2006) A pilot correlation study between deflection bowl parameters measured with a light weight and standard falling weight deflectometer. South African Road Federation and International Road Federation Conference, South Africa.