Falling Weight Deflectometer Load Research Articles

Conversions of viscoelastic deflections of asphalt pavement under TSD load to elastic ones for pavement condition assessments

The Traffic Speed Deflectometer (TSD) facility has gained popularity in assessing asphalt pavement conditions due to its efficient testing process and minimal disruption to traffic flow. Current applications of TSD test results resemble those developed for the Falling Weight Deflectometer (FWD) test results, such as analysing the deflection basin shape and back-calculating the pavement layer modulus. However, the TSD facility induces more viscoelastic pavement deflections compared to FWD, primarily due to its rolling load. This research focuses on converting TSD’s viscoelastic deflections under a wider range of loading conditions into elastic deflections under FWD load. This conversion aims to facilitate the use of FWD-based approaches for analysing TSD deflection data. The findings indicate that the converted TSD deflections generate more reasonable evaluation results, further emphasizing the necessity of converting TSD deflections to elastic ones. The models proposed in this research serve as valuable references for performing the viscoelastic-elastic deflection conversion.

Influence of Spring Stiffness on the Loading Speed of the Falling Weight Deflectometer (FWD) in Bearing Capacity Evaluation of Cement Concrete Pavement

Introduction: The bearing capacity evaluation of cement concrete runway pavement is one of the most important contents in the airfield operation. Among the methods for estimating bearing capacity, the Falling Weight Deflectometer (FWD) is the most widely used method because it does not damage the pavement structure and spends so much time. The equipment for evaluation should have suitable parameters, such as. magnitude and loading speed, to the actual operating loads. Aims and Objectives: Currently, the equipment to assess the bearing capacity of cement concrete pavements of highways and airports by falling weight has been widely used in the world and is being applied in Vietnam. The loading system consists of a falling block through a spring system acting on the pressure plate under the pavement with a certain falling height. In particular, the spring system plays an important role in controlling the speed of the load on the pavement. In Vietnam, the test standards (Industry Standard 22TCN335-06, Vietnamese Standard TCVN 11365-2016) for assessing the pavement bearing capacity by FWD equipment do not specify the selection of spring stiffness corresponding to different operating speeds of the vehicle. Therefore, it is difficult to use the equipment for pavements with operating speeds different from the standard. In this paper, we study the effect of spring stiffness on the loading speed when considering different loading speeds corresponding to different speeds of vehicles moving in reality. This research is only limited to laboratory experiments. This study aims to determine the dependence of the loading magnitude and loading time on the damping spring stiffness of the dynamic loading equipment, that is, used as a basis for choosing the appropriate damping stiffness of FWD corresponding to actual vehicles. Methods: Among the methods for estimating the bearing capacity, the Falling Weight Deflectometer (FWD) is the most widely used method as it does not cause any damage to the pavement structure and spends much time. The equipment for evaluation should have suitable parameters, such as magnitude and loading speed, to the actual operating loads. Results: This paper presents the results of experimental research that determines the influence of the damping spring stiffness on the loading speed of the dynamic loading equipment on the model of the concrete slab in the laboratory. Conclusion: Based on the comparison of the FWD loading speed when using 02 damping springs with different stiffness, we propose the basis for choosing the damping system that conforms to the evaluation needs of different types of vehicles.

Deflection-Based Approach for Flexible Pavement Design in Thailand

The Department of Highways (DOH), Thailand, has adopted both empirical and mechanistic approaches for flexible pavement analysis and design. Recently, the deflection-based design approach has been comprehensively reviewed by the DOH for the possible adoption of national design standards and practices. One of the key reasons is that Thailand’s road authorities, i.e., the DOH and the Department of Rural Roads (DRR), have considered the falling weight deflectometer (FWD) for the new construction and rehabilitation of road pavements. In addition, the FWD is widely accepted as the non-destructive test for deflection measurement and structural capacity evaluation. Ultimately, the implication of FWD deflections for in-house pavement analysis and design shall be developed and proposed to Thailand’s road authorities. Therefore, this study presents the deflection-based approach of flexible pavement design in Thailand. The FWD and a standard Thai truck were selected as the main loading applications in this study. A typical FWD loading stress of 700–800 kPa was practically adopted by the DOH and compared with a standard 10-wheel 25-ton truck with a tandem axle-dual wheel configuration with a tire pressure of 690 kPa. The layered elastic analysis was performed to calculate the pavement responses. The results suggest that the flexible pavement design based on a deflection-based approach is simple, practical, and conservative.

Numerical analysis and conversion of dynamic and static deflection of asphalt pavement under FWD loading

During the falling weight deflectometer (FWD) evaluation, only calculating the acceptance deflection by adopting the layered elastic system theory has its limitations. In this paper, a three-dimensional finite element method was adopted to carry out the dynamic simulation analysis of typical asphalt pavement structures under FWD test. As a result, a dynamic deflection calculation model of asphalt pavement under FWD loading was established as well as the calculating method, in which a layered elastic system theory is adopted for calculations on the basis of the dynamic-static deflection ratio. Furthermore, an equivalent conversion method of asphalt pavement was proposed based on the deflection equivalent principle. Last, the applicability of the method was examined by comparing the FWD results of the pavement sections tested on-site. The method proposed in this paper to calculate the dynamic deflection of asphalt pavement is critical for the quality acceptance of asphalt pavement construction, modulus back calculation, and asphalt pavement design.

Relationship between backcalculated dynamic modulus, estimated dynamic modulus, and fatigue damage in asphalt concrete

ABSTRACT There are several methods for determining the stiffness of asphalt concrete in an existing pavement. The three primary methods are dynamic modulus testing in the laboratory, predictive equations, and falling weight deflectometer (FWD) testing. The Pavement ME asphalt over asphalt (AC/AC) overlay design procedure uses multiple methods to characterise fatigue damage in the existing asphalt concrete. This paper examines the relationship between backcalculated dynamic modulus, estimated dynamic modulus, and fatigue damage in in-situ asphalt concrete. FWD load magnitude and asphalt temperature, both of which have been shown to affect the relationship between backcalculated and estimated dynamic modulus, are also considered. A statistical analysis is performed using data from the Long-Term Pavement Performance Programme (LTPP). Analyses are performed within individual LTPP sections and using pooled data from multiple pavement sections. It is determined that the relationship between backcalculated dynamic modulus and estimated dynamic modulus is significantly related to the amount of fatigue damage in the in-situ asphalt concrete. However, there is also a threshold of fatigue damage above which the relationship between backcalculated modulus, estimated modulus, and fatigue damage is no longer significant.

Field investigation and numerical analysis of an inverted pavement system in Tennessee, USA

Inverted pavement system has been introduced for many years with field investigations, laboratory studies and numerical simulation works. Field investigations of inverted pavement structures have not been widely conducted and are very limited in the USA. This study presents a comprehensive field study on a full-scale inverted pavement as well as a conventional control pavement section constructed in Tennessee, USA. A combination of multiple non-destructive testing (NDT) methods, including ground penetration radar (GPR), Benkelman beam test, 3D road profiling test by laser crack measurement system (LCMS), and falling weight deflectometer (FWD) tests were utilized to assess the actual thickness, structural capacity and surface conditions of the pavement structures. The road surface profiling test results showed that the inverted pavement outperformed the conventional section in roughness, cracking and rutting conditions. Deflection Basin Parameters (DBPs) such as surface curvature index (SCI), base damage index (BDI), base curvature index (BCI) and W7 (7th sensor’s value of FWD) values were compared to study the layers’ condition of the inverted and conventional pavement sections. In addition, a preliminary simulation analysis by a Finite Element Method (FEM) model was conducted to compare the performance of inverted and conventional pavement sections under FWD load. The simulation results showed that the inverted pavement structure could reduce the potential of crack initiation and propagation due to smaller tensile stress at the bottom of the asphalt concrete layer (AC). Also, the smaller deflection was measured at the top of subgrade soil (SG), which reduced the total deflection of the inverted pavement structure. Furthermore, the cement-treated base (CTB)-constrained unbound aggregate base (UAB) acted as a cushion to relieve the tension from CTB and support the AC layer. The stress-dependent property of UAB in the inverted pavement contributed to the performance and ductility of inverted pavement but had little influence on the conventional pavement.

Evaluation of inverted pavement by structural condition indicators from falling weight deflectometer

Inverted pavement has been a potential alternative to the conventional flexible pavement structure due to its cost-efficient usage of asphalt, comparable performance and durability. For increasing utilization of the inverted pavements, appropriate structural evaluation is necessary. The Falling Weight Deflectometer (FWD) test has been widely accepted for evaluating the structural condition of conventional pavements through pavement deflection basin parameters (DBPs). As a special type of pavement, inverted pavement relies on the stress-dependence of the unbound stone interlayer to withstand the heavy traffic loads, which typically affects a relatively larger pavement area. So far, there have been few systematic studies on the effectiveness of utilizing FWD to evaluate the structural conditions of inverted pavements. This study used a DBPs-based inverted pavement structure assessment system to assess an inverted pavement constructed in Georgia, USA. A nonlinear stress-dependent finite element model of the inverted pavement coupled with the impulse loading mode was built to simulate the realistic FWD loading process and investigate the effect of layer stiffness on the DBPs evaluation system. Furthermore, a full-scale inverted pavement section in Knoxville, TN, USA, was tested by the FWD method to obtain the corresponding deflection basin parameters. The DBPs data were used to evaluate this practical inverted pavement project based on the aforementioned DBPs-based assessment system. The analysis results show that the DBPs-based pavement evaluation method can provide a quick and comprehensive assessment for each layer in the inverted pavement structure without knowing the detailed properties of the layers. The evaluation results were consistent with the data from the testing vehicle equipped with the laser crack measurement system. Thus, the DBPs-based evaluation system is a promising method to assess the condition of the inverted pavement structures, contributing to the Inverted Pavement Management System (IPMS).

Numerical investigation of pavement responses under TSD and FWD loading

The Traffic Speed Deflectometer (TSD) overcomes the limitations of the Falling Weight Deflectometer (FWD) in terms of traffic interruption and testing inefficiency, thus has been used for network-level pavement structural evaluation. However, compared to FWD, which has accumulated plenty of data and comprehensive evaluation methods over decades, the applicability of TSD is yet to verify. It is imperative to compare pavement responses under TSD and FWD to validate the applicability and substitutability of TSD. Based on the difference between TSD and FWD in loading characteristics and asphalt concrete (AC) properties, six 3D-Move models were established successively to explore the influence of different testing factors on pavement responses. The simulation results show that under the same load magnitude, peak deflections and peak strains caused by TSD were always greater than that of FWD due to the lower equivalent load frequency of TSD. Compared to the peak deflection, the deflection basin shape of TSD was more sensitive to the dynamic effects and the viscoelasticity of the AC layer. Besides, AC viscoelasticity and damping caused the lag in TSD deflections and strains. In the layer modulus back-calculation from TSD deflections, the dual circular load and dynamic effect of TSD were crucial, but the AC viscoelasticity could be neglected. The use of FWD-based programs to back-calculate the layer modulus from the TSD deflection would result in a stiffer AC layer and a stiffer subgrade.

Time-domain elasto-dynamic model of a transversely isotropic, layered road structure system with rigid substratum under a FWD load

In this paper, the time-domain elasto-dynamic model of a transversely isotropic, layered road structure system resting on rigid bedrock under a falling weight deflectometer (FWD) load is proposed by using the Laplace-Hankel transformation, and developing an improved propagator matrix method (PMM) with less calculation cost, higher calculation accuracy and better numerical stability. The improved model greatly avoids the problem of data overflow and maintains the computational efficiency of PMM. The present model is then verified by comparing with the corresponding FEM results. A parametric analysis is finally performed to discuss the effects of material anisotropy and rigid substratum on dynamic response of a road system, which is meaningful for design and practice of road engineering with modulus back-calculation.

Moving Wheel versus Impact Deflections and Their Use in Pavement Evaluation

Traffic speed deflection devices (TSDDs) have been developed since around 2000 to allow for safe and efficient structural evaluation of highway networks. One barrier to TSDD implementation is the inherent differences in deflections produced by moving truck loads and by falling weight deflectometer (FWD), the current deflection testing standard. To better understand the differences in data produced by the two devices, FHWA sponsored research into one particular TSDD, the rolling wheel deflectometer (RWD). The study utilized the finite layer program ViscoWave to model both FWD and RWD loads to demonstrate the effect of their inherent differences on pavement deflections and other simulated parameters. In addition, ViscoWave was used to generate theoretical FWD and RWD deflections for a diverse set of pavement structures and subgrade conditions. The resulting deflections were used to develop correlations between the two devices, which were validated with side-by-side FWD and RWD field tests performed on 23 sites. The research determined that the differences between FWD and RWD deflections vary depending on pavement factors and loading characteristics. The two devices produced similar deflections on thicker, stiffer, lower-deflection pavements, while the FWD produced relatively higher deflections on thinner, weaker, higher-deflection pavements. Therefore, use of common FWD data analysis programs will produce different results, such as layer moduli, for TSDD devices. Advanced analysis routines capable of modeling the TSDD’s moving load and loading configurations are needed.

Multiscale Modeling of Asphalt Concrete and Validation through Instrumented Pavement Section

In this study, macroscale responses of asphalt concrete (AC) are predicted from the responses of its corresponding microscale representative volume element (RVE) within a finite element framework using quasi-static and dynamic analyses. Nanoindentation test was performed on the mastic and aggregate phase of an AC sample to determine the viscoelastic and elastic properties of RVE elements. Aggregate-mastic proportions in the RVE were obtained from the morphological image analysis. Macroscale model responses were compared with the AC pavement responses obtained from an instrumented pavement section subjected to falling weight deflectometer loading and a class 9 vehicle. Model responses are very close to the actual responses. The multiscale analyses show that tensile strain in microscale RVE is 5–10 times higher than that in a macroscale element. Furthermore, multiscale analyses also show that variations in the microscale RVE, such as the reduction in the aggregate-mastic ratio or increment in the voids, can increase the maximum tensile strain at the bottom of the AC in macroscale model by around 25%.

Relationships between Asphalt-Layer Moduli under Vehicular Loading and FWD Loading

AbstractThe modulus of the asphalt layer in a pavement structure subjected to vehicular loading is different from that subjected to falling weight deflectometer (FWD) loading. A comprehensive appro...

Relationship between Backcalculated and Estimated Asphalt Concrete Dynamic Modulus with Respect to Falling Weight Deflectometer Load and Temperature

There are several methods for determining the stiffness of asphalt concrete in an existing pavement. The three primary methods are: dynamic modulus testing in the laboratory, predictive equations, and falling weight deflectometer (FWD) testing. Asphalt over asphalt (AC/AC) overlay design procedures allow the use of multiple methods to characterize fatigue damage in the existing asphalt concrete. Therefore, understanding the difference between these methods is critical for AC/AC overlay design. The differences between the methods for determining asphalt concrete stiffness and how these differences are related to FWD load magnitude and asphalt temperature are examined. Data from the Federal Highway Administration’s Long-Term Pavement Performance Program (LTPP) are used in this investigation. It is found that the stiffness determined through FWD testing and backcalculation is generally less than that estimated using the Witczak predictive equation and binder aging models. Furthermore, it is found that both FWD load magnitude and asphalt temperature have a significant effect on the difference between backcalculated and estimated stiffness of asphalt concrete. Backcalculated stiffness increases relative to estimated stiffness as FWD load and temperature increase. These effects must be considered when multiple methods of determining asphalt concrete stiffness are used interchangeably for overlay design.

Mechanistic-Based Approach to Utilize Traffic Speed Deflectometer Measurements in Backcalculation Analysis

Backcalculation analysis of pavement layer moduli is typically conducted based on falling weight deflectometer (FWD) deflection measurements; however, the stationary nature of the FWD requires lane closure and traffic control. In recent years, traffic speed deflection devices such as the traffic speed deflectometer (TSD), which can continuously measure pavement surface deflections at traffic speed, have been introduced. In this study, a mechanistic-based approach was developed to convert TSD deflection measurements into the equivalent FWD deflections. The proposed approach uses 3D-Move software to calculate the theoretical deflection bowls corresponding to FWD and TSD loading configurations. Since 3D-Move requires the definition of the constitutive behaviors of the pavement layers, cores were extracted from 13 sections in Louisiana and were tested in the laboratory to estimate the dynamic complex modulus of asphalt concrete. The 3D-Move generated deflection bowls were validated with field TSD and FWD data with acceptable accuracy. A parametric study was then conducted using the validated 3D-Move model; the parametric study consisted of simulating pavement designs with varying thicknesses and material properties and their corresponding FWD and TSD surface deflections were calculated. The results obtained from the parametric study were then incorporated into a Windows-based software application, which uses artificial neural network as the regression algorithm to convert TSD deflections to their corresponding FWD deflections. This conversion would allow backcalculation of layer moduli using TSD-measured deflections, as equivalent FWD deflections can be used with readily available tools to backcalculate the layer moduli.

Assessing artificial neural network performance for predicting interlayer conditions and layer modulus of multi-layered flexible pavement

The objective of this study is to evaluate the performance of the artificial neural network (ANN) approach for predicting interlayer conditions and layer modulus of a multi-layered flexible pavement structure. To achieve this goal, two ANN based back-calculation models were proposed to predict the interlayer conditions and layer modulus of the pavement structure. The corresponding database built with ANSYS based finite element method computations for four types of a structure subjected to falling weight deflectometer load. In addition, two proposed ANN models were verified by comparing the results of ANN models with the results of PADAL and double multiple regression models. The measured pavement deflection basin data was used for the verifications. The comparing results concluded that there are no significant differences between the results estimated by ANN and double multiple regression models. PADAL modeling results were not accurate due to the inability to reflect the real pavement structure because pavement structure was not completely continuous. The prediction and verification results concluded that the proposed back-calculation model developed with ANN could be used to accurately predict layer modulus and interlayer conditions. In addition, the back-calculation model avoided the back-calculation errors by considering the interlayer condition, which was barely considered by former models reported in the published studies.

Field Characterization of Pavement Materials using Falling Weight Deflectometer and Sensor Data from an Instrumented Pavement Section

This study deals with the backcalculation of the mechanical properties of pavement layers using not only the falling weight deflectometer (FWD) sensor data but also pavement response under that FWD load from the embedded sensors in an instrumentation section. To perform the backcalculation, a layered viscoelastic pavement model incorporating asphalt concrete (AC) cross-anisotropy is developed as the forward model. Field degree of cross-anisotropy in AC is determined at the maximum magnitude frequency obtained through continuous wavelet transform of the material response signal. The material response signal is obtained from the deconvolution between the loading signal and the signal registered at the embedded sensors. An inverse analysis methodology is also developed to calculate the gross vehicle weight (GVW) of any vehicle passing through the instrumentation section using only the backcalculated material properties and pavement responses. From the results, it is observed that inclusion of the AC cross-anisotropy reduces the error norm, and a good agreement is observed with the laboratory dynamic modulus in both horizontal and vertical directions. It is also observed that the maximum magnitude frequency of material response and degree of cross-anisotropy in AC both decrease with an increase in average AC temperature. Furthermore, using the backcalculated material properties and pavement responses, it is possible to determine the GVW with 95% accuracy.

Comparisons between Asphalt Pavement Responses under Vehicular Loading and FWD Loading

The strain responses of asphalt pavement layer under vehicular loading are different from those under falling weight deflectometer (FWD) loading, due to the discrepancies between the two types of loadings. This research aims to evaluate and compare the asphalt layer responses under vehicular loading and FWD loadings. Two full‐scale asphalt pavement structures, namely, flexible pavement and semirigid pavement, were constructed and instrumented with strain gauges. The strain responses of asphalt layers under vehicular and FWD loadings were measured and analyzed. Except for field measurements, the finite element (FE) models of the experimental pavements were established to simulate the pavement responses under a wide range of loading conditions. Field strain measurements indicate that the asphalt layer strain under vehicular loading increases with the rising temperature roughly in an exponential mode, while it decreases with the rising vehicular speed approximately linearly. The strain pulses in the asphalt layer generated by FWD loading are different from those induced by vehicular loading. The asphalt layer strains generated by FWD loading are close to those induced by low vehicular speed (35 km/h). The results from the FE model imply that the asphalt layer strains under FWD loading and vehicular loading are distributed similarly in the depth profile. For flexible pavement, the position of critical strain shifts gradually from the bottom of the asphalt layer to the mid‐depth of the layer, as the temperature increases. For semirigid pavement, the position of critical strain is always located at the intermediate depth of the asphalt layer, regardless of temperatures.

Determination of effective frequency range excited by falling weight deflectometer loading history for asphalt pavement

Falling weight deflectometer (FWD) tests have been widely used to evaluate the layer properties of in-situ asphalt pavement structures. The FWD loading is essentially impulse loading and can excite a wide range of frequency components. This study is to determine the effective frequency range excited by FWD loading through the dynamic back-calculation analysis of FWD data. The FWD deflections were calculated using the spectral element method for asphalt pavements under various field measured FWD loading histories. The calculated deflections were then used as the measured ones to back-calculate the properties of asphalt concrete layers using the dynamic back-calculation procedure. The viscoelastic behaviour of asphalt concrete was characterized using the modified Havriliak-Negami model. For each set of FWD data, several sets of back-calculated model coefficients were obtained by inputting different seed values of model coefficients in the back-calculation procedure. The results show that although all the back-calculated and measured deflection curves are in good agreement, the values of the back-calculated model coefficients can be significantly different and the back-calculated master curves match well only in a specific frequency range that is the effective frequency range excited by FWD loading. The effective frequency ranges for various pavement structures and FWD loading histories were determined. The results show that a higher loading peak level can excite a wider effective frequency range, or more specifically the greater upper limit, regardless of the pavement structures. In general, the effective frequency range excited by a FWD loading history is 5 ~ 65 Hz for asphalt pavement. The existence of effective frequency range indicates that the information contained in FWD data is not sufficient to capture the asphalt concrete properties over the entire frequency domain.

Towards trackbed design with asphalt underlayment using FWD-based numerical model

ABSTRACT Ballasted track is the conventional type of track structure around the world. Although it has several benefits, it often requires notable maintenance to ensure adequate operation. The trackbed design plays a major role on the structural performance of the track, and consequently on its maintenance. Apart from slab track, which requires significant investments, alternative solutions such as the use of asphalt underlayment with ballasted tracks can lead to a satisfactory balance between investment and maintenance. This paper investigates the use of asphalt underlayment in trackbed using a numerical model of the falling weight deflectometer (FWD), for different subgrade moduli. This is to investigate the resulting dynamic sleeper support stiffness due to FWD loading to develop a design chart for trackbed with asphalt underlayment. The paper also illustrates the use of these design charts on theoretical design scenarios and carry out a comparison with other international practices.

Accuracy evaluation of statically backcalculated layer properties of asphalt pavements from falling weight deflectometer data

This study is to evaluate the dynamic effects of falling weight deflectometer (FWD) loading on the surface deflection of asphalt pavement and the accuracy of statically backcalculated layer moduli from FWD data. The dynamic and static deflections were computed using the spectral element method and the layer elastic theory, respectively, for various pavement structures. The static deflection is considerably larger than the dynamic deflection for typical FWD loading and the normalized difference between static and dynamic deflections increases with increasing distance from the load center and decreases with increasing loading duration. The dynamic deflections were utilized to backcalculate the layer moduli using two static backcalculation procedures, MODULUS and EVERCALC. The backcalculated moduli can be significantly different from the actual moduli. The results indicate that the static backcalculation procedure can lead to significant errors in the backcalculated layer moduli by ignoring the dynamic effects of FWD loading.