Falling Weight Deflectometer Load Research Articles

Repeatability of Asphalt Strain Measurements under Falling Weight Deflectometer Loading

Embedded instrumentation has become an important tool at accelerated load facilities as states begin implementing mechanistic–empirical pavement design and analysis methodologies. Instrumentation can provide valuable information to help validate mechanistic models and develop a deeper understanding of pavement response under a wide range of conditions. Before these types of investigations are conducted, however, it is critical to determine if the response measurements are accurate and precise. Although both accuracy and precision are important, this investigation centers on establishing the practical level of within-gauge precision for asphalt strain measurements made at the National Center for Asphalt Technology Pavement test track. Ten test sections, each containing 12 strain gauges, were included in this study. Falling weight deflectometer testing was conducted on strain gauges, and the absolute differences were calculated to determine individual gauge variability. It was found that within-gauge variability was not significantly affected by gauge orientation, load level, or pavement condition. Overall, strain readings with differences less than 12 μ∈ were set as a practical benchmark for the test track for within-gauge variability. It is recommended that, as the sections continue to deteriorate, further testing be conducted to better quantify the relationship between variability and pavement distress.

Investigation of thin flexible pavement response between traffic and the falling weight deflectometer (FWD)

The Falling Weight Deflectometer (FWD) is commonly used to evaluate structural characteristics of pavements. The FWD simulates vehicular loads at typical highway speeds and the data is used to back-calculate layer moduli and design overlays (among other functions). In routine testing, the only data obtained is on the surface of the pavement, which leaves potentially significant behaviors within the pavement undetected. The focus of this paper is to present the analysis of a heavily instrumented, full-scale flexible pavement that was subjected to vehicular traffic and FWD loads under controlled testing conditions. Statistical analysis and finite element modeling separately demonstrated that the responses produced by the FWD were, in general, lower than those from corresponding vehicular traffic, and this behavior was more pronounced with depth. Finite element modeling resulted in traffic asphalt strain, base pressure, and subgrade pressure responses exceeding FWD responses by factors of 1.27, 1.76, and 2.43, respectively. Similarly, a statistical analysis resulted in factors of 1.36, 1.66, and 2.39, respectively. Additional analysis showed the deflection from FWD loading would have to be increased between 1.3 to 1.6 times to produce corresponding vehicular traffic stress states.

Trends in Deflection with Application of Repeated Loads

Falling weight deflectometer (FWD) testing is now considered routine for the evaluation of pavement structures. Averaging of FWD load and deflection data collected at the same location and similar drop loads is common before further analysis. The stated reason for this averaging is to decrease the effect of the random error inherent in FWD deflection sensors. Implicit in this methodology is the assumption that there is no significant trend in deflection results with successive load applications. FWD data from the Long-Term Pavement Performance program were analyzed to determine whether a significant trend exists between deflection and drop number. In a majority of data sets, a statistically significant trend does exist for at least one deflection sensor. The trends are most common for the highest load level and the center deflection sensor. These trends are less often practically significant but are common enough that averaging deflection data can be expected to increase total error in a large minority of cases.

Evaluation of Long-Term Performance of Pavement Drainage Layers on I-469 in Indiana

Subsurface drainage is important for long-term pavement performance. Rational procedures to analyze and evaluate the design, reliability, and effectiveness of subsurface drainage systems are needed in order for their use to be recommended with confidence. Three pavement subdrainage test sections were constructed in 1995 on the eastbound driving lane of I-469 in Indiana, at the northern junction with I-69, between Stations 150+05 and 173+40. Presented are the original laboratory characterization and mechanistic evaluation for permanent deformation and stability of the test sections employing finite element analysis. Triaxial tests were conducted on all pavement layers of the sections. Falling weight deflectome-ter evaluations in 1995 and 1998 are also presented. Such measurements are not available after 1998 because compliance with Indiana Department of Transportation safety regulations is required at that location. Finite element analyses were conducted by using laboratory-measured material properties to predict pavement response to falling weight deflec-tometer loads, compare predicted and measured deflections, examine layer shear stability for shear stress and strength, and predict rutting. Long-term pavement performance indicators up until 2007 (including international roughness index and ground penetration radar), after 12 years of heavy truck traffic, are also presented. Finite element analysis predicted very well the deflections measured by the falling weight deflectometer and accumulated rutting of the three test sections. Comparisons of shear stresses and strengths indicated that the sections were stable. All long-term evaluations indicated that all drainage layers in the study sections have performed their function adequately and protected the subgrade.

Engineering Framework for Self-Consistent Analysis of Falling Weight Deflectometer Data

In pavement engineering practice, the falling weight deflectometer (FWD) is a recognized nondestructive tool to evaluate the mechanical characteristics of layered pavement systems. Unfortunately, elastostatic backcalculation remains the norm in the interpretation of FWD data, even though its underlying (static) analysis is inconsistent with the dynamic nature of the FWD test. Because of wave propagation effects, which are especially pronounced in the presence of a shallow stiff layer, the peak pavement deflections induced by the FWD loading can differ significantly from their static counterparts and thus compromise the conventional backcalculation of pavement moduli. In this study a frequency domain–based preprocessing procedure was developed to extract the static pavement response from the transient FWD records to provide a more consistent input for the elastostatic recovery of pavement profiles. To ensure the fidelity of the educed static deflections, the key drawbacks of typical field FWD data, namely, the baseline offset and the low-frequency noise pollution, were examined and remedied. For pavement engineering applications, the featured preprocessing procedure for extracting the static deflections from dynamic FWD records was implemented in a user-friendly graphical environment, GopherCalc. By comparing the results of the proposed and existing backanalyses applied to both synthetically generated (elastodynamic) data and field records, it was found that the proposed procedure had the potential for mitigating systematic errors associated with the dynamic nature of the FWD test while retaining the computationally effective elastostatic backcalculation scheme.

Elastic Nonlinear Finite Element Analysis of a Flexible Pavement Subjected to Varying Falling Weight Deflectometer Loads

A nonlinear finite element method (FEM) model has been developed to model thin-surfaced, unbound granular pavements. The FEM model incorporates a nonlinear anisotropic material model for the granular material and a nonlinear material model for the subgrade. The coefficients for the material models were determined from laboratory repeated load triaxial tests. A test pavement at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) was instrumented with an inductive coil soil strain system to measure vertical compressive strains and pressure cells to measure the vertical compressive stresses. The test pavement was subjected to loading by a falling weight deflectometer (FWD) device at four different load levels. The initial FEM model configuration was undertaken at one load level using the surface deflection measurements from the FWD; however, there were significant differences between the computed and measured stress and strain values. The scalar coefficients for the granular and subgrade materials were adjusted so that the measured and computed stress and strain values matched within a reasonable tolerance (10%). After the calibration process was finished, it was found that the computed and measured surface deflections also matched. The model was rerun with the three remaining load cases; the computed and measured stress, strain, and surface deflection values were in close agreement. The range of computed stiffness values varied with both the pavement and load level. The results from this analysis showed the need to use a full nonlinear model to obtain realistic results when FWD data are modeled.

Assessment of Pavement Layer Condition with Use of Multiload-Level Falling Weight Deflectometer Deflections

A condition assessment procedure for pavement layers that uses multiload-level falling weight deflectometer (FWD) deflections is presented. A dynamic finite element program that incorporates a stress-dependent soil model was used to generate the synthetic deflection database. On the basis of the data in this database, the relationships between surface deflections and critical pavement responses, such as the stresses and strains in each layer, have been established. The FWD deflection data, distress survey results, temperature, and laboratory testing results used to develop this procedure were collected from the Long-Term Pavement Performance project database. Research efforts also focused on the effect of the FWD load level on the condition assessment procedure. The results indicate that the proposed procedure can estimate the asphalt concrete (AC), base, and subgrade layer conditions. The AC layer modulus and the tensile strain at the bottom of the AC layer were found to be better indicators of the condition of the AC layer than the deflection basin parameter. It was also found that the structurally adjusted base damage index and base curvature index were good indicators for prediction of the stiffness characteristics of the aggregate base and subgrade, respectively. An FWD test with a load of 71.2 kN or less does not improve the accuracy of this procedure. The results from the study of the nonlinear behavior of a pavement structure indicate that the deflection ratio obtained from multiload-level deflections can predict the type and quality of the base and subgrade materials.

Assessment of Pavement Layer Condition with Use of Multiload-Level Falling Weight Deflectometer Deflections

A condition assessment procedure for pavement layers that uses multiload-level falling weight deflectometer (FWD) deflections is presented. A dynamic finite element program that incorporates a stress-dependent soil model was used to generate the synthetic deflection database. On the basis of the data in this database, the relationships between surface deflections and critical pavement responses, such as the stresses and strains in each layer, have been established. The FWD deflection data, distress survey results, temperature, and laboratory testing results used to develop this procedure were collected from the Long-Term Pavement Performance project database. Research efforts also focused on the effect of the FWD load level on the condition assessment procedure. The results indicate that the proposed procedure can estimate the asphalt concrete (AC), base, and subgrade layer conditions. The AC layer modulus and the tensile strain at the bottom of the AC layer were found to be better indicators of the condition of the AC layer than the deflection basin parameter. It was also found that the structurally adjusted base damage index and base curvature index were good indicators for prediction of the stiffness characteristics of the aggregate base and subgrade, respectively. An FWD test with a load of 71.2 kN or less does not improve the accuracy of this procedure. The results from the study of the nonlinear behavior of a pavement structure indicate that the deflection ratio obtained from multiload-level deflections can predict the type and quality of the base and subgrade materials.

Temperature Correction of Multiload-Level Falling Weight Deflectometer Deflections

A new temperature correction procedure was developed for multiload-level falling weight deflectometer (FWD) deflections for flexible pavements in North Carolina. In this procedure, temperature correction factors were dependent on the radial offset distance from the FWD load plate. Temperature and FWD multiload-level deflection data used in developing this procedure were collected from 11 pavements in three different climatic regions of North Carolina. The effect of the FWD load level on this temperature correction procedure was investigated. Research efforts focused on improving the accuracy of the current temperature correction procedure of the North Carolina Department of Transportation. The measured deflection and temperature data were also used to validate the long-term pavement performance (LTPP) temperature correction procedure. It was found that the effective pavement temperature prediction algorithm in the LTPP procedure is relatively accurate and that the temperature-deflection correction procedure undercorrects the deflections at higher temperatures in pavements with an asphalt concrete layer thicker than 242 mm. The main reason for this deficiency is that the LTPP procedure was developed from the national database and cannot fully consider the local variation in mixture characteristics.

Measurement of Vertical Compressive Stress Pulse in Flexible Pavements: Representation for Dynamic Loading Tests

Testing at Virginia Smart Road allowed determination of the vertical compressive stress pulse induced by a moving truck and by falling weight deflectometer (FWD) loading at different locations beneath the pavement surface. Testing was performed on 12 different flexible pavement sections. Stress and temperature were measured using pressure cells and thermocouples, respectively, that had been installed during construction of the road. Target testing speeds were 8 km/h, 24 km/h, 40 km/h, and 72 km/h. The considered depths below the pavement surface were 40 mm, 190 mm, 267 mm, 419 mm, and 597 mm. A haversine or normalized bellshape equation was found to be a good representation of the measured normalized vertical compressive stress pulse for a moving vehicle. Haversine duration times varied from 0.02 s for a vehicle speed of 70 km/h at a depth of 40 mm to 1 s for a vehicle speed of 10 km/h at a depth of 597 mm. For the FWD loading, a haversine with a duration of 0.03 s was found to approximate the induced stress pulse at any depth below the pavement surface. Currently, laboratory dynamic testing on hot-mix asphalt (HMA) specimens is performed using a haversine wave at loading duration of 0.1 s. Because HMA is a viscoelastic material, the loading time affects its properties and, therefore, it is recommended that the loading time of HMA dynamic tests be reduced to 0.03 s to better match loading times obtained from moving trucks at average speed and from FWD testing.

New Relationships Between Falling Weight Deflectometer Deflections and Asphalt Pavement Layer Condition Indicators

New relationships have been identified between the layer condition indicators of flexible pavements and falling weight deflectometer (FWD) deflections. Synthetic databases were generated using dynamic finite element analysis with nonlinear material models. The sensitivity of various deflection basin parameters (DBPs) to layer conditions was comprehensively examined on the basis of the developed databases. Three types of layer condition indicators were identified in the study, including DBPs, effective layer moduli, and stresses and strains. The DBPs identified from the sensitivity study were used in developing new relationships between the selected condition indicators and FWD deflections by applying regression and artificial neural network techniques. Even though these relationships include the complicated dynamic effect of FWD loading and nonlinear behavior of unbound materials, the time to obtain results from these procedures is insignificant, thus making the procedures suitable for field implementation.

Dynamic Interpretation of Falling Weight Deflectometer Test Results: Spectral Element Method

The use of spectral analysis as a means of analyzing the dynamic impact of falling weight deflectometer (FWD) load pulses on pavements is covered. The spectral element technique is utilized. Only forward analyses of pavement dynamics are presented, with the emphasis on the suitability of the method for solving inverse problems. LAMDA (layered media dynamic analysis), a newly developed spectral element program, is utilized for the simulation of the interaction between the FWD load pulse and the pavement structure. In LAMDA, the formulation of the wave propagation, reflection, and refraction in a layer is done in a closed form. The assembling of the elements (in the multilayer system) is carried out in a manner similar to that in the finite element method. Consequently, the size of the mesh of a pavement structure is as large as the number of the layers involved. This reduces the computational requirements substantially and hence enables utilization of LAMDA in iterative algorithms for backcalculation purposes.

Interpretation of pavement deflection measurement data using an elastodynamic stochastic approach

Pavement deflection measurements, together with backcalculation procedures, are widely used to estimate the layer moduli of pavement-subgrade systems. Sensitivity analysis of a sample problem indicates that conclusions drawn from static analyses with regards to deflection sensitivity to variation in layer moduli may apply when characterizing uncertainty associated with the interpretation of the falling weight deflectometer (FWD) data. The uncertainty associated with the values of the backcalculated parameters from deflection data is investigated in this paper using an elastodynamic, stochastic finite element approach. The results of the simulations indicate that, in order to properly estimate surface layer moduli, loading frequencies higher than that of excitation by typical FWD loading are required. The low sensitivity of deflection uncertainty to random variations in surface modulus, when compared with that associated with subgrade modulus, is demonstrated to contribute to high variations in backcalculated surface modulus from measured surface deflections. Although focus is placed on uncertainties in elastic modulus and deflection, the methodology presented in the paper can be used to quantify uncertainties associated with other layer properties and pavement responses.Key words: stochastic, finite element, pavement deflection, elastodynamic, backcalculation, layer moduli, falling weight deflectometer test.

Field Study of In Situ Subgrade Soil Response Under Flexible Pavements

Falling weight deflectometer (FWD) and truck tests were performed on three instrumented flexible pavement sections at the Minnesota Road Research project. The purposes of the study were (1) to investigate sub-grade soil response under FWD and moving truck loads and (2) to estimate in situ resilient modulus of the subgrade soil. The truck tests were performed at various speeds ranging from 16 to 78 km/h. The subgrade deformations and the vertical pressures on the top of the subgrade soils were measured from in situ displacement and soil pressure gauges. The experimental results showed that the deformations and the vertical pressures, in general, did not show significant dependency of truck speed within the above speed range. However, a slight decrease of the vertical pressure with increase of speed was observed for a thin conventional pavement section, while the vertical pressure in a relatively thick pavement section appeared to be less sensitive to speed. The results from FWD tests indicated that the subgrade deformation was linearly related to the FWD loads up to approximately 40 kN. Furthermore, a method is presented to estimate in situ subgrade modulus using the linear elastic theory and the measurements from the in situ sensors. The estimated modulus is comparable with the laboratory results at a low deviator stress level and is lower than modulus obtained from the backcalculation using FWD deflection basins.

Deflection Response Models for Cracked Rigid Pavements

The static elastic layer model is customarily used for analyzing deflection measurements and back-calculating pavement layer moduli. Estimation of in-situ layer moduli by means of a mechanistically based iterative technique is known as back-calculation. Despite the fact that the falling weight deflectometer (FWD) load induces a dynamic load, dynamic/impact analysis routines are seldom, because of the mathematical complexity. Dynamic deflection prediction models are developed in this study, which can be used in back-calculation routines. The first step in accomplishing this is to select an appropriate analysis model for which ABAQUS, a general purpose finite-element program, is used. Static and dynamic deflection responses from the ABAQUS model of an uncracked pavement are validated. Following this first step, cracks and joints along with other features of the pavement are modeled in addition to nonlinear behavior of pavement materials. Deflection responses of cracked pavement under a series of increasing loads, assuming linear and nonlinear behavior of base, subbase, and subgrade materials, are computed and compared. After finalizing the model, a synthetic deflection database, specifically with FWD load, is developed relying on a fractional factorial design layout in which thicknesses, layer moduli, and cracks are allowed to vary over a range. Making use of this database, regression equations that predict surface deflection bowls in terms of layer moduli and thicknesses are developed. Deflection equations are validated using field data from two in-service pavements.