Falling Weight Deflectometer Tests Research Articles

Distribution Characteristics of Dynamic Deflection Basins of Distressed Asphalt Pavements

AbstractThe objective of this study was to discuss the distribution features of dynamic deflection basins of distressed asphalt pavements using the dynamic finite element method. Based on the results obtained by systematic analysis, the criteria used to filter the effective data of falling weight deflectometer (FWD) deflection basins were presented and also verified by in-situ project study. The results demonstrate that the crack width of 0.2 mm is a critical value to determine the existence of contact behavior between crack contact surfaces. The distribution characteristics of dynamic deflection basins display a tremendous difference between intact and cracked pavements. The established criteria, involving surface deflection and surface modulus indicators, are verified to be reasonable and viable for filtering the FWD testing technology.

Evaluation Study of Subgrade Service Condition Based on Falling Weight Deflectometer

To make appropriate rehabilitation decisions, determining subgrade modulus of the existing semi-rigid asphalt pavements is a crucial task for highway engineers. Falling Weight Deflectometer (FWD), as a nondestructive testing technology, is an effective tool to determine subgrade modulus. The objective of this study was to establish a method to predict subgrade modulus of asphalt pavements based on Falling Weight Deflectometer. With the basic theory of structure dynamics, the finite element model of the asphalt pavement was established to generate a synthetic database of surface dynamic deflection basin parameter (DBP) indicators. Then, the relationship between DBP indicators and subgrade modulus was discussed and the prediction function for determining subgrade modulus was established. The analysis results demonstrate that subgrade modulus can be predicted by the direct deflection indicator d9, shape indicator F8 and structure layer thickness and the predicted precision is satisfied. The relative error between the theoretical value and the predicted values ranges from-13.2% to +15.56% according to subgrade modulus. The mean value of absolute relative error is 2.36%. The research results provide a technology supports to popularize FWD testing technology applied in performing subgrade structure performance evaluation.

Impacts of aggregate base on roadway pavement performances

Three premature failures (related to weak, moisture-susceptible aggregate base) were investigated and the results are presented in the paper. Falling Weight Deflectometer (FWD) tests were conducted to determine the structural load bearing capacity. Laboratory tests were performed to evaluate the aggregate base’s susceptibility to moisture. One successful project that used crushed aggregate base as an interlayer to mitigate reflective cracking was studied. The crushed aggregate base interlayer performed well while other treatments failed. The weak and moisture-susceptible aggregate base, in conjunction with a thin, porous and brittle surface AC, caused premature failures for the three studied projects. It was theorized that the weak base deformed under the traffic load, and this cracked the brittle/thin surface mix. FWD test results for all three projects yielded much higher deflections than the target design deflections, indicating insufficient load bearing capacity. Two key factors for minimizing premature failures are: (1) prevention of water infiltration into the base through good surface mix, and (2) building a base that is moisture resistant. The strategy of using a crushed aggregate base as an interlayer has been utilized on other projects with success, provided it meets two key requirements: (1) Class 1 Texas Triaxial class, after 10days capillarity and (2) the 10-day final dielectric should not exceed 13.

Significance of Subgrade Damping on Vehicle-Pavement Interactions

The falling weight deflectometer (FWD) produces pavement responses by a falling mass drop to evaluate structural parameters of the pavement. A reliable simulation model is helpful to calibrate the apparatus as well as the backcalculation programs. The key points in the finite element simulation of FWD tests were studied including model size, boundary conditions, and analysis time increment. The subgrade in the finite element model was formulated with springs of stiffness coefficient of subgrade and dashpots of the derived damping values. It was concluded that at least six times the radius of relative stiffness is required to avoid boundary cut-off error. A rigorous procedure is presented to derive subgrade reaction and damping for various types of subgrade soil. Subgrade damping plays an important role in simulations of the FWD test. Backcalculation programs without subgrade damping may over-predict structural condition of pavements.

The Use of the Dynamic Cone Penetrometer for Quality Control of Compaction Operations

The use of a new quality control (QC) and quality assurance (QA) specification involving Dynamic Cone Penetrometer (DCP) testing in concert with conventional moisture and density testing is becoming more and more frequent in various parts of the world. The need for this additional testing is essential, as the regular in-situ density tests cannot alone ensure the compliance of the layers constructed with the compaction requirements. Recent analyses of the correlation between the DCP testing and the California Bearing Ratio CBR testing show that QC and QA DCP testing is adequate to verify compaction, stability and vertical uniformity in both cohesive and granular soils. Two examples of DCP usage in two Israeli earthwork projects, one of clayey soils and the other of silty-sand soils, indicate the benefits of this usage along with, though for the clayey example only, Falling Weight Deflectometer (FWD) testing.

Evaluation of the Dynamic Modulus for Asphalt Mixtures with Varying Volumetric Properties

The dynamic modulus |E*| of asphalt mixtures with varying volumetric properties was evaluated in this study. The dynamic modulus test, repeated load permanent deformation test, and beam fatigue test were performed in laboratory to evaluate the stiffness, rutting, and fatigue performance for three types of mixtures, respectively. The Falling Weight Deflectometer (FWD) test was conducted to evaluate field pavement conditions of four sections from two highways in Jiangsu. It is observed from the |E*| master curves that for a specific type of mixture, |E*| decreases with the increase in binder content at the lower reduced frequency, while at the higher reduced frequency a peak |E*| value exists at the approximate optimum binder content obtained from volumetric mix design criteria. It is also found from the tests results that dynamic modulus indicators |E*| and |E*|/sinφ do not show good agreements with fatigue and rutting performance of different mixtures. In addition, it is shown that the Witczak model strongly underestimates |E*| of the mixtures used in this study and should be corrected by a shift factor of 1.8621. Finally, a dynamic modulus prediction model for asphalt mixtures with varying volumetric properties was developed in laboratory and successfully verified using in-situ FWD backcalculation.

도로하부 매설관의 매설심도 및 되메우기 재료가 포장체에 미치는 영향 평가

PURPOSES : Compared to the criteria from advanced countries, Korea has conservative criteria for the buried depth of pipeline (about 30~70cm deeper) causing the waste of cost and time. Therefore, this research investigated the effect of various buried depths of pipeline on pavement performance in order to modify the criteria to be safe but economical. In addition, a recycled aggregate which is effective in economical and environmental aspect was evaluated to be used as a refilling material. METHODS : In this study, total 10 pilot sections which are composed with various combinations of pavement structure, buried depth of pipeline, and refilling material were constructed and the telecom cable was utilized as a buried pipeline. During construction, LFWD (Light Falling Weight Deflectometer) tests were conducted on each layer to measure the structural capacity of underlying layers. After the construction is completed, FWD (Falling Weight Deflectometer) tests and moving load tests were performed on top of the asphalt pavement surface. RESULTS : It was found from the LFWD and FWD test results that as the buried depth decrease, the deflections in subbase and surface layer were increased by 30% and 5~10%, respectively, but the deflection in base layer remained the same. In the moving load test, the longitudinal maximum strain was increased by 30% for 120mm of buried depth case and 5% for 100mm of buried depth case. Regarding the effect of refilling material, it was observed that the deflections in subbase and surface layer were 10% lager in recycled aggregate compared to the sand material. CONCLUSIONS : Based on the testing results, it was found that the change in buried depth and refiliing material would not significantly affect the pavement performance. However, it is noted that the final conclusion should be made based on an intensive structural analysis for the pavement under realistic conditions (i.e., repeated loading and environmental loading) along with the field test results.

Modeling of the FWD Deflection Basin to Evaluate Airport Pavements

AbstractThe falling weight deflectometer (FWD) testing develops a deflection basin on the pavement surface. Depths of this deflection basin from the center of the falling weight are measured at different radial offsets. These deflections are used for the backcalculation of the pavement layer moduli. Most of the available backcalculation software uses the layered elastic theory and static load to calculate moduli from known pavement surface deflections. However, the FWD test load is dynamic, and layer materials may show nonelastic behavior. Layered elastic theory in these types of software cannot characterize dynamic response of the pavement. Also, elastic theory is unable to accurately predict the surface deflection whenever stress developed in any pavement layer exceeds the yield point. For this reason, this study has performed a finite-element analysis of the airport pavement under the FWD test considering the dynamic load and materials plasticity. The analysis presented here includes elastoplastic beha...

Moisture-Sensitive and Stress-Dependent Behavior of Unbound Pavement Materials from in Situ Falling Weight Deflectometer Tests

In an instrumented flexible pavement with a subsurface drainage system, a field study was performed to investigate the influence of water on the response of the pavement structure. The drainage system of the structure was clogged for 3 months; this condition allowed the groundwater to rise and the structure to undergo high moisture conditions. Reopening of the drainage system permitted the structure to approach its previous draining hydrological state. Along with monitoring of subsurface groundwater level and moisture content, the structural response of the pavement was studied by conducting frequent falling weight deflectometer tests with multilevel loads. The stress sensitivity of the unbound layers and the influence of moisture on their stiffness were studied, with the intent of using the data to determine the unbound materials' nonlinear parameters through a backcalculation algorithm. The groundwater level rose rapidly after the drainage system was clogged. This rise in groundwater level significantly affected the overall stiffness of the pavement structure, and the backcalculated stiffness of the unbound layers decreased as their moisture content increased. Furthermore, the unbound layers exhibited stress-dependent behavior to multilevel loads. The subgrade showed a stress-softening response in an unsaturated condition and stress-independent behavior in a saturated state. The granular layer exhibited stress-hardening behavior. Backcalculation of the unbound nonlinear parameters determined by the universal extended k–θ model revealed that the k1 parameter decreased with increasing moisture content for both the unbound granular layer and the unsaturated fine-grained subgrade material.

Responses and Performance of Stabilized Full-Depth Reclaimed Pavements at the Minnesota Road Research Facility

This paper details the construction and analysis of three stabilized full-depth reclamation (SFDR) sections (Cells 2, 3, and 4) constructed at the Minnesota Road Research Facility on I-94 in 2008. Three test sections with different ratios of pulverized asphalt concrete to granular base were constructed, and the performance of full-depth reclaimed pavements stabilized with engineered emulsion was studied. Emulsion content and base structure varied between test sections. Each test section was designed for 3.5 million equivalent single-axle loads for a period of 5 years. As of June 30, 2012, the sections had been subjected to approximately 2.2 million such loads. Responses were measured with strain gauges embedded at the bottom of the hot-mix asphalt and SFDR layers in each test section. The strain gauges indicated that the bottom of both the hot-mix asphalt and the SFDR layers was subject to horizontal tensile strain from falling weight deflectometer testing and heavy vehicle loading. Pavement performance for rutting, cracking, and international roughness index was measured periodically. The results indicated that all three cells were performing well. The only crack in the three cells was in Cell 3; the roughness index values were well within the acceptable range and rutting, although progressing, was still acceptable. The paper concludes with modeled responses and performance predictions from DARWinME and BISAR. Model predictions indicate that an SFDR layer will provide greater structural benefits and increased performance than will similar structures with unstabilized full-depth reclaimed or granular base layers.

Consistency and accuracy of selected FWD backcalculation software for computing layer modulus of airport pavements

Falling weight deflectometer (FWD) data are generally processed through backcalculation software for computing layer modulus of airport pavements. Though a number of backcalculation software are currently available, it is not known that software produces the most consistent and accurate results. Inconsistent and inaccurate backcalculation software can take a significant toll on the overlay and rehabilitation design of airport pavements as modulus being the inputs for such design calculations. Most backcalculation software requires an initial value of modulus, known as seed modulus, to initiate simulations. Unfortunately, in many of the software, the outputs vary with the initial choice of seed modulus (consistency). In addition, depending on the software algorithm, the final output differs from the laboratory measured value of modulus (accuracy). In this study, three backcalculation software programs are evaluated for consistency and accuracy. They are BAKFAA, MODULUS and EVERCALC. The major goal is to determine the most consistent and accurate software for determining layer modulus of airport pavements in New Mexico. Consistency of software is determined by comparing outputs from three sets of input data from FWD tests at identical locations with three magnitudes of loads: 40·03, 53·38 and 71·17 kN. Laboratory tests are conducted for modulus and tensile strength of asphalt and soil samples collected from the same locations of FWD tests. Software accuracy is determined based on the distribution and magnitude of the variance of laboratory and FWD modulus. Laboratory tensile stress is also compared with the backcalculated tensile stress using KENLAYER, a multilayer elastic analysis software. Study results show that EVERCALC software produces more consistent and accurate modulus values than the BAKFAA and MODULUS software. Therefore, EVERCALC is recommended for analyzing layer modulus of airport pavements.

Evaluation of Seasonal Variation in Mechanistic Responses of Flexible Pavements through use of Falling Weight Deflectometer Data

Flexible pavements in cold regions with frequent freeze and thaw cycles are prone to damage at the start of thawing and during the thaw–recovery season every year. In an attempt to limit damage to the pavement, many highway agencies enforce a spring road ban (SRB) on secondary roads during the thawing period. The Alberta Ministry of Transportation in Canada performs an extensive falling weight deflectometer (FWD) test program during the spring to enhance SRB decision making. This study uses FWD test data from 2000 to 2006 for multiple highway sections in Alberta to investigate the seasonal variation in pavement stiffness and its effect on pavement mechanistic responses. FWD data were used to backcalculate the moduli for the pavement layers, which were then used in multilayer elastic models to predict the pavement critical responses in thawing versus recovery periods. Asphalt Institute models were used to relate the pavement critical responses to fatigue cracking life (Nf) and rutting life (Nd) in thawing versus recovery periods. Nf and Nd dropped by as much as 80% for fatigue cracking and 95% for rutting in the thawing period compared with the recovery period. Further analysis showed that a 50% reduction in the load applied during thawing can result in an approximately 90% increase in both Nd and Nf. The Mechanistic– Empirical Pavement Design Guide software was used to simulate SRB by reducing the maximum applied load by 50% in the thawing period. The results showed a 50% increase in pavement life in rutting and in top-down and bottom-up fatigue cracking.

Prediction of Asphalt Pavement Temperature Profile During FWD Testing: Simplified Analytical Solution with Model Validation Based on LTPP Data

A pavement temperature profile is indispensable for analyzing and interpreting falling weight deflectometer (FWD) testing data. This paper presents a simple analytical approach to rapidly predicting the transient temperature profile within the asphalt layer of a flexible pavement on the basis of measured surface temperatures during FWD testing. The proposed analytical approach consists of the classical solution of a one-dimensional heat equation in a homogeneous half-space subjected to specified surface temperatures and a Gaussian-quadrature numerical scheme to approximate the classical solution. Model validation was conducted using a large temperature database. Comparisons between predicted and measured pavement subsurface temperatures suggest that the proposed simplified analytical method generated a reasonable asphalt temperature profile during the short period of FWD testing. The main advantage of the proposed method is that it can be easily applied to assist field engineers in rapidly estimating the temperature profile within the asphalt layer of a multilayered flexible pavement based on measured pavement surface temperatures during FWD testing.

Use of Benkelman Beams for Measuring LTE in Whitetopping: An Alternative Tool to FWD Test

Whitetopping is a rehabilitation or structural strengthening alternative on hot mix asphalt (HMA) pavement. It is constructed on the top of existing HMA pavement. Generally, falling weight deflectometer (FWD) deflection measurements are used to obtain the load transfer efficiency (LTE) of the transverse joints. However, the use of FWD in India has been very limited so far because of its high cost and difficulties encountered in maintaining the equipment. Therefore, a need has been aroused to identify an alternative to FWD test, which can be cost effective and easily available. In this work, Benkelman beam deflection test has been conducted using two Benkelman beams simultaneously placed on two adjacent slabs near transverse joint for measuring LTE of 150 mm thick in-service thin whitetopping constructed at Dahanukar Colony, Kothrud, Pune city. An alternative tool has been developed for measuring LTE in whitetopping.

Sustainable Road Management in Texas: Network-Level Flexible Pavement Structural Condition Analysis Using Data-Mining Techniques

AbstractThe research team recognized the value of network-level falling weight deflectometer (FWD) testing to evaluate the structural condition trends of flexible pavements. However, practical limitations due to the cost of testing, traffic control and safety concerns, and the ability to test a large network may discourage some agencies from conducting the network-level FWD testing. For this reason, the surrogate measure of the structural condition index (SCI) is suggested for use. The main purpose of the research presented in this paper is to investigate data-mining strategies and to develop a prediction method of the structural condition trends for network-level applications, which do not require FWD testing. The research team first evaluated the existing and historical pavement condition, distress, ride, traffic, and other data attributes in the Texas Department of Transportation (TxDOT) Pavement Maintenance Information System (PMIS); applied data-mining strategies to the data; discovered useful patter...

Estimation of tensile strains at the bottom of asphalt concrete layers under wheel loading using deflection basins from falling weight deflectometer tests

The falling weight deflectometer is a pavement analysis tool that is now widely used in the pavement engineering field. Using the backcalculation process and the measured deflection basin, the layers moduli can be determined and a mechanistic analysis of the pavement can be made. A new approach is proposed to bypass the necessity of the backcalculation by allowing a direct estimation of the tensile strain at the bottom of asphalt concrete using the deflection basin. A model based on a finite element theoretical pavement analysis is proposed for this purpose. Complementary models have been developed to use the proposed models without having to determine the layers moduli. The proposed model to estimate the tensile strain at the bottom of the asphalt concrete layers is validated and calibrated using data obtained on an instrumented experimental site.

Analytical Review of the HMA Temperature Correction Factors from Laboratory and Falling Weight Deflectometer Tests

The investigation of temperature dependency of asphalt mixtures and pavements has several applications in pavement engineering. To estimate the modulus of asphalt pavement layers from field deflection tests such as Falling Weight Deflectometer (FWD), it is necessary to adjust the deflections or backcalculated moduli to a reference temperature. The main objective of this study was to compare the temperature dependencies of Hot Mix Asphalt (HMA) from laboratory tests such as dynamic modulus and resilient modulus tests with the empirical models suggested for the correction of asphalt pavement modulus from FWD testing. A database including dynamic modulus of 42 asphalt mixtures as well as resilient modulus of 37 asphalt mixtures from several North American studies were analyzed to investigate the variation of their laboratory moduli with testing temperatures. An exponential model was proposed for the temperature dependency of the dynamic and resilient moduli of asphalt mixtures. Additionally, thirteen temperature correction factor models for FWD testing were reviewed. While existing pavement practices do not consider mixture dependency for temperature correction factors, this study concluded that mixture dependency of temperature correction factor is important and should be considered.

A Fuzzy Approach To Relate FWD Surface Deflection To Laboratory Determined Resilient Moduli

Backcalculation of pavement layer modulus by layered elastic analysis programs is the most common practice for pavement evaluation. These programs have limitations in computational algorithm that include assumptions such as layer properties and falling weight deflectometer (FWD) test conditions. FWD test results are aected by stress nonlinearity, layer thicknesses, temperature and other factors. So, layer moduli backcal- culated by these programs are not always in agreement with laboratory measured resilient modulus. A fuzzy model has been developed using the modified learning from example (MLFE) algorithm. It backcalculates the resilient modulus of pavement layers from surface deflections and layer thicknesses. To train the system, surface deflections and pavement thicknesses have been collected during FWD tests. Layer resilient moduli have been determined by laboratory tests on samples collected from field coring. This model is validated by laboratory test data. To investigate computational accuracy, it is compared to the results of the BAKFAA software and it shows the better accuracy of the fuzzy model.

Structural Coefficient of Open-Graded Friction Course

Open-graded friction courses (OGFCs) are noted for their contributions to driving safety and noise reduction on flexible pavements. The inclusion of OGFCs in pavement thickness design has varied across the United States. Some states equate OGFC with dense-graded materials, whereas other states give no structural value. The demand for more economical pavement cross sections fuels the need to quantify the structural value of OGFC to ensure optimized pavement thicknesses. Two full-scale pavement sections—one with OGFC as the surfacing material and one without—are evaluated to determine the structural coefficient of the OGFC layer. The sections were built with the same total thickness at the National Center for Asphalt Technology Test Track in 2009 and were subjected to more than 9 million equivalent single-axle loads. Deflection data from extensive falling weight deflectometer testing were used to find the OGFC structural coefficient (aOGFC) equal to 0.15. This coefficient translated into a required 12% thickness increase to achieve the same pavement structural number as the control section. To confirm this finding, direct strain was measured in each section to compute the pavement thickness required to achieve equivalent strain levels. The increase in thickness was within 0.05 in. of that predicted through the aOGFC approach. Performance monitoring indicated similar performance levels in terms of fatigue cracking (none) and rutting (approximately 5 mm). Additional monitoring and research is needed to validate the OGFC structural coefficient further and develop standards for incorporating OGFC into mechanistic–empirical pavement design.

The Application and Early-Age Behaviors of Continuously Reinforced Bonded Concrete Overlay of Distressed Jointed Concrete Pavements

Abstract Considerable efforts and money are spent to maintain and repair the distressed jointed concrete pavement (JCP) sections by providing partial-/full-depth repair, under sealing, and patching; however, a lot of distresses reoccur frequently. For severe distresses, overlay was constructed on the JCP. Resurfacing with a bonded concrete overlay (BCO) over the existing pavement to form a monolithic structure has proven to be a cost-effective rehabilitation strategy. Continuously reinforced concrete pavement (CRCP) bonded overlay was constructed for the rehabilitation of a 2000 ft long JCP section on U.S.75 in Sherman, Texas, United States which had been subjected to numerous rehabilitations to construct the guidelines of the CRCP bonded overlay. The pavement design and concrete mix design selected are discussed and introduced in this article. In order to reduce the reflective cracking at the location of the JCP joints, geotextile was placed at the joint location before the overlay construction. For the study on the function of the geotextile, the geotextile was not placed at 20 JCP joints for the comparison. One week after the BCO construction, the investigation for the CRCP cracking was carried out at three stages. The investigated results revealed that the cracks at the transverse steel location are much more than those between the transverse steel. Falling weight deflectometer testing was run on the pavement surface before and after the overlay construction to measure the deflection behavior of the pavement. The performance of the overlay shows that the bonded CRCP is appropriate for the JCP resurfacing.