Falling Weight Deflectometer Load Research Articles

Evaluation of Transverse Cracks for Semi-Rigid Asphalt Pavements using Deflection Basin Parameters

To evaluate transverse cracks on a semi-rigid asphalt pavement by falling weight deflectometer (FWD), a three-dimensional (3D) dynamic finite-element (FE) model to calculate the deflections of transverse-cracked semi-rigid asphalt pavements under FWD loading was developed and validated by in-situ FWD tests. Then, the effect of crack types and crack width on the deflection basin was investigated for semi-rigid asphalt pavements under different interlayer contact conditions. The relationship between transverse cracks and deflection basin parameters (DBPs) was also analyzed. Finally, the slope ratio to evaluate transverse cracks was proposed and validated by field application. Results show that cracks on pavements will make the deflection basin steeper and the crack width slightly affects the deflections for the same type of crack without interaction between adjacent cracking surfaces. Results also indicate that deflection values increase obviously when the surface-base interaction changes from “Full bonded” to “Full slip.” In addition, results indicate that the slope index ( S1), the shape index ( F2), and the area index (AREA) correlate well with transverse cracks, and the surface cracking, “Reflective cracking 1” (surface and base cracking simultaneously), “Reflective cracking 2” (surface, base, and subbase cracking simultaneously) and the hidden cracking (base cracking, or base and subbase cracking simultaneously) can be identified by the slope ratio. Field application also indicates that the slope ratio can be applied to evaluate the types of transverse cracks on semi-rigid asphalt pavements.

Evaluation of FWD Load Transfer Efficiency Measurement

This report documents an evaluation of load transfer efficiency (LTE) of dowel jointed concrete pavements. Measurement of LTE of transverse joints in concrete pavements is universally conducted using a falling weight deflectometer (FWD) device. LTE is an important parameter affecting pavement performance. Due to the importance of the results for maintenance decisions, the accuracy of the measurement technique is investigated in this report. The availability of instrumented dowel fitted concrete slabs in West Virginia Smart Road (Corridor H, Route 33 as well as Goshen Road, West Virginia) offers a unique opportunity to examine the accuracy of determining the LTE of transverse joints using FWD. For this purpose, FWD tests were conducted on both pavement sites at different times during years 2003, 2004, and 2005. Thirty transverse joints were tested along the slab edges as well as along the wheel-path. Trend analysis was performed to evaluate the effect of design features and site conditions on LTE. Key findings from this study are: (1) LTE was found to be a complex parameter that depends on many factors that include load position, testing time, slab temperature, and load transfer device. (2) Testing time and season were found to have a significant effect on the measured LTE. (3) The slabs fitted with 32 mm (1.25 in.) diameter dowels displayed higher variability of the measured LTE than those fitted with 38 mm (1.5 in.) diameter dowels. (4) Joint opening changes daily and seasonally as the ambient temperature changes. As the amount of joint opening increases due to slab contraction during winter, the measured LTE generally decreases. (5) Poor correlation was found between the deflection-based LTE and the percentage of the load transferred through the load transferring devices mounted across the transverse joint.

Damage Assessment for ME Rehabilitation Design of Modified Asphalt Pavements: Challenges and Findings

The overall objective of this study was to assess the use of Level 1 analysis for mechanistic-empirical (ME) rehabilitation designs of deteriorated polymer-modified asphalt concrete (AC) pavements in Nevada using the AASHTOWare® Pavement ME software. This research also explored the possible implementation of a hybrid approach for AC damage characterization to overcome the challenges associated with the use of the Witczak model for estimating the undamaged dynamic modulus master curve of the existing AC layer. Two rehabilitation field projects were used as part of this study. The experimental plan involved falling weight deflectometer (FWD) testing in the right wheelpath before rehabilitation, analysis of core samples, estimation of an equivalent undamaged dynamic modulus, and estimation of equivalent damaged dynamic modulus from FWD backcalculation. The proposed hybrid approach consisted of conducting laboratory dynamic modulus testing on the collected core samples and estimating an equivalent undamaged dynamic modulus at the same FWD testing temperature and loading frequency. The pre-overlay damage, characterized based on the approach in Pavement ME Design software (i.e., using a Witczak prediction model and backcalculated modulus), showed overly high values that did not match with the collected pre-overlay distress data on either of the rehabilitation projects. Based on the findings from this study, the hybrid approach was recommended for implementation by Nevada Department of Transportation (NDOT) when designing AC overlay over polymer-modified asphalt pavements in Nevada. Recommendations for user inputs were also provided for future consideration in Pavement ME Design software.

Rapid evaluation of a transverse crack on a semi-rigid pavement utilising deflection basin data

To evaluate the transverse crack on a semi-rigid pavement rapidly by a falling weight deflectometer (FWD), three-dimensional dynamic finite-element (FE) models were developed to calculate the deflection response of a transverse-cracked semi-rigid pavement under an FWD load. Models were validated by deflection basin data acquired from in situ FWD tests. Then, the effects of thermal cracking and reflective cracking on deflection basin data were analysed. Four loading locations and seven crack widths were considered in the analysis. Results showed that deflection basins calculated by dynamic FE models coincided well with that measured by in situ FWD test. The largest per cent difference between calculated deflections and measured values was less than 9%. When the distance between loading centre and crack was within 1050 mm, load transfer efficiency (LTE) indexes showed a good performance for distinguishing a reflective crack from a thermal crack. A practical field test method placing the transverse crack between sensor S300 and sensor S600 was recommended and LTE300–600 was proposed as the evaluation index for the rapid determination of crack type.

Influence of interface conditions on the response of transversely isotropic multi-layered medium by impact load.

Multi-layered media are one of the most common phenomena in natural or artificial surroundings, and it is considered as a good candidates for biomedical applications. The transversely isotropic characteristic of materials have been widely recognized in elastic multi-layered media. Furthermore, interface conditions between transversely isotropic elastic multi-layered medium layers plays an important role in the medium's performance. Despite many numerical approaches in these analyses, very few theoretical methods are available in dealing with these two important issues. This study presents a method to compute the response of transversely isotropic elastic multi-layered medium subjected to the impact load, i.e. falling weight deflectometer load (FWD) load, when interfacial conditions are considered. Details of the mathematical derivation, implementation and verification of the proposed analytical solution are presented. The absence of positive exponential functions in the solution leads to a considerable improvement in computation efficiency and stability. Subsequent numerical results demonstrate that both transverse isotropy and interface conditions could substantially contribute to the responses of the elastic multi-layered medium.

Finite element modeling and parametric analysis of viscoelastic and nonlinear pavement responses under dynamic FWD loading

Falling Weight Deflectometer (FWD) is the common non-destructive testing method for in-situ evaluation of pavement condition. This paper aims to develop finite element (FE) models that can simulate FWD loading on pavement system and capture the complexity in material properties, layer interface, and boundary conditions. Parametric analysis was conducted considering the effects of dynamic analysis, temperature gradient, bedrock depth, asphalt layer delamination, viscoelasticity, and unbound material nonlinearity on pavement surface deflections and critical strain responses. Although the parametric analysis findings vary depending on the specific pavement response, the study results illustrate the appropriate selection of analysis type, constitutive models of pavement material, and layer boundary conditions on the accuracy of FE modeling results. In particular, the analysis findings show that delamination in asphalt layers induces the greater strain responses; while neglecting bedrock effect overestimates surface deflections. The developed FE models can directly benefit the use of FWD testing for in-situ pavement condition evaluation, such as pavement performance prediction and/or backcalculation of layer moduli.

Evaluation of Pavement Layer Moduli Using Instrumentation Measurements

Instrumentation embedded in pavements is increasingly being used to measure the critical responses and monitor performance of specially-constructed experimental pavement sections or in-service pavements under controlled wheel loading or live traffic. On the other hand, nondestructive tests (NDT), such as falling weight deflectometer (FWD) testing, are routinely performed to evaluate pavement layer structural properties based on deflections measured on the surface of the pavements. In cases where measurements from the pavement surface only are not sufficient to infer pavement layer modulus values, e.g. rolling wheel deflectometer (RWD) tests typically with only one deflection measurement or complex geometries, a procedure that combines measurements from the embedded instruments and surface deflections would provide an alternative to the traditional backcalculation of pavement layer moduli. This study presents an inverse analysis procedure integrating finite element (FE) models and a population-based optimization technique, Covariance Matrix Adaptation Evolution Strategy (CMA-ES), to determine the pavement layer structural properties. Tests using a lightweight deflectometer (LWD) were conducted on instrumented three-layer scaled flexible pavement test sections. The time histories of the LWD load, surface deflection underneath the LWD load, subgrade deflection and vertical stress were recorded and used in the inverse analysis. While the common practice in backcalculating pavement layer properties still assumes a static FWD load and makes use of only peak values of the load and deflections, dynamic analysis was conducted to simulate the impulse LWD load. Results of the inverse analysis show that consistent pavement layer properties can be obtained based on the LWD surface deflection data and measurements of the embedded instrumentation.

Effect of Distresses on Deflection Basins and Backcalculation Modulus of Asphalt Pavement with Cement-Treated Base

The Falling Weight Deflectometer (FWD) is widely used to evaluate the asphalt pavement structural conditions through the deflection basin and the backcalculation modulus. The FWD test has sometimes been performed on asphalt pavement with distresses. It is necessary to investigate the effect of the distresses on the deflection basin and the backcalculation modulus. In this study, the finite element method (FEM) was used to simulate the asphalt pavement with and without the distresses and the FWD load, and thus produce the defection data. The modulus of the structural layers and subgrade was backcalculated from the FWD data using the MODULUS 6.0 program. Then the effect of the distresses was investigated on the deflection basin and the backcalculation modulus. The results indicate that the cracking and cement-treated base crushing in the asphalt pavement may result in an abnormal deflection basin and make the backcalculated modulus incorrect; therefore, it may be necessary to remove the unusual deflection basin before the backcalculation to obtain accepted backcalculation results, or decrease the number of structural layers to reduce the unreasonable backcalculation results.

Dynamic backcalculation of asphalt pavement layer properties using spectral element method

The response analysis mode of asphalt pavement plays a critical role in properly interpreting falling weight deflectometer (FWD) test results and obtaining reasonable backcalculated layer properties. This study uses the spectral element method (SEM) to analyse the dynamic viscoelastic response of asphalt pavement subjected to FWD loading. The modified Havriliak–Negami (MHN) mode is used to characterise the mechanical properties of asphalt concrete (AC). The MHN model can accurately characterise the viscoelastic and damping behaviour of AC with much fewer number of model coefficients (only five) than the widely used Prony series representation, which makes the model well suited for the dynamic response analysis and backcalculation of layer properties. A computer code is developed to implement the SEM procedure and the results are in good agreement with finite element analysis. The dynamic and quasi-static viscoelastic responses are compared and the results show that the inertia and damping behaviour greatly influences the surface deflection of asphalt pavement under FWD loading, especially for pavements with cemented treated base. Therefore, it is important to consider the dynamic effects in interpreting FWD data. The SEM method is used as the forward analysis engine to backcalculate field FWD data, and the results show that the proposed approach is able to well simulate the deflection history at the pavement surface. The layer properties backcalculated from dynamic viscoelastic analysis are significantly different from those obtained from the traditional backcalculation procedure based on the layered elastic theory.

Effectiveness of FWD to Simulate Traffic Loading in Recycled Pavements

AbstractThe aim of the present research is to investigate the effectiveness of the falling weight deflectometer (FWD) to simulate the traffic loading in the case of asphalt pavements that have been cold in-depth recycled (CIR) using foamed asphalt (FA) as a stabilization treatment for base works. The investigation is related with the need of understanding the behavior of asphalt CIR pavements under repeated traffic loads that can result in the prediction of pavement performance and contribute in the optimization of pavement design or redesign. For this purpose, a testing experiment was conducted to monitor and document the in situ developed strains under both FWD loading and truck loading, and to compare them with the calculated ones from finite-element (FE) linear elastic analysis. The developed strains were measured using a fiber optic sensors (FOS) system. The collected data were processed and analyzed taking into account the local conditions. The analysis results indicated that the maximum tensile str...

Measured Dynamic Response of Asphalt Pavement under FWD Load

To study the real dynamic response regularity of asphalt pavement structure under dynamic load, a road test was conducted for three types of typical asphalt pavement. Dynamic and static deflection basins were established. The characteristics of the dynamic deflection basin were studied. The set-in dynamic strain sensors at the bottom of the surface layer collected the dynamic strain response under falling weight deflectometer (FWD) load. Results show that the static deflection basin is deeper than the dynamic deflection basin, but the range of the latter is larger than that of the former. The characteristics of the two deflection basins differ. The change trends of dynamic response with an increase in FWD load reflect nonlinear characteristics. The dynamic strain of three typical asphalt pavements at the bottom of the surface layer does not induce fatigue failure in the pavement structure. The longitudinal strain is larger than the lateral strain. Different prediction models on dynamic strain on the basis of dynamic deflection basin parameters are established.

Finite Element Model of Pavement Response Under Load Considering Cross-Anisotropy in Unbound Layers

Abstract The effects of cross-anisotropy of unbound layer materials on the stress-strain response of a pavement under 18 kip truck load are determined at three temperatures employing a dynamic finite element model (FEM) in ABAQUS. Viscoelastic behavior of asphalt materials are characterized by Prony series coefficients estimated from a laboratory dynamic modulus test. Nonlinear elastic behavior of base material is incorporated in ABAQUS through a User Defined Material (UMAT) subroutine. Sub-base and subgrade materials are modeled as linear elastic material using the falling weight deflectometer (FWD) back-calculated moduli and Poisson's ratio. Cross-anisotropy is introduced in the FEM model by changing the ratio of horizontal to vertical stiffness (n-value) of unbound layers, namely base, sub-base, and subgrade layers of an instrumented pavement. The FEM model is validated using field collected stress-strain responses under FWD loading from the instrumented pavement section at mile post 141 (MP 141) on Interstate 40 (I-40) near Albuquerque, NM. It is observed that the tensile strain, transverse to the traffic direction, at the bottom of the asphalt concrete (AC) layer is significantly influenced by the cross-anisotropy of unbound layers. The vertical strain in the AC layer is barely affected by the cross-anisotropy. However, vertical strains in the unbound layers are significantly affected by cross-anisotropy. Therefore, it can be postulated that fatigue damage that is caused by tensile strain at the bottom of the AC layer, and unbound layer rutting that is caused by vertical strain, should be evaluated for unbound layer cross-anisotropy in the design and evaluation of pavements. Overall, strain responses due to cross-anisotropy are highly sensitive to temperature.

LONG TERM PERFORMANCE OF EXISTING PORTLAND CEMENT CONCRETE PAVEMENT SECTIONS – CASE STUDY

This paper presents the performance of seventeen rigid pavements made of Portland Cement Concrete (PCC) located in the State of Ohio, USA. The Falling Weight Deflectometer (FWD) testing method was used in a total of 58 km of PCC pavement sections to assess the pavement structural condition. Each section is classified as excellent, good, fair, or poor based on the normalized deflections obtained from FWD tests, spreadability, load transfer, and joint support ratio parameters. The overall performance of the analyzed sections is from good to excellent. The field testing methodology along with the performance data analyses presented in this publication are a valuable tool to evaluate the actual structural condition of pavements and as a result short or medium term rehabilitation programs can be issued to ensure the serviceability of the pavement.

Backcalculation of Asphalt Concrete Modulus Master Curve from Field-Measured Falling Weight Deflectometer Data

A new backcalculation program, DYNABACK-VE, was used to backcalculate pavement layer properties with field data. DYNABACK-VE used a time domain viscoelastic dynamic solution (ViscoWave-II) as a forward routine and a genetic algorithm for backcalculation analysis. The genetic algorithm search method was selected because it had a high potential for converging efficiently to a global solution. The forward solution used continuous integral transforms (Laplace and Hankel) that were more appropriate for transient, nonperiodic signals in the time domain. The algorithm was implemented in C++ and coded for parallel processing with multithreading for achieving better computational efficiency. Field falling weight deflectometer load and defection sensor time histories from three sites (Waverly Road near Lansing, Michigan, and two long-term pavement performance sections) were used for validation. The backcalculated asphalt concrete modulus master curves were compared with those obtained from laboratory testing. Very good agreement was obtained. The new algorithm was capable of backcalculating reliably the master curve of the asphalt concrete layer (four sigmoidal coefficients and two time-temperature shift factors), the elastic moduli for the unbound base or subbase and subgrade materials, and the modulus of stiff layer and the depth to stiff layer, if present. The advantage of the new solution is that it can analyze the response of pavement systems in the time domain and can therefore accommodate time-dependent layer properties and incorporate wave propagation. Also, because the backcalculation is performed in the time domain, the algorithm is not sensitive to truncation in the deflection time histories. This is a significant improvement to the state of the art, since truncation of deflection time histories has prevented frequency domain backcalculation solutions from being successful when measured field data are used.

Inverse Analysis of Pavement Structural Properties Based on Dynamic Finite Element Modeling and Genetic Algorithm

With the movement towards the implementation of mechanistic-empirical pavement design guide (MEPDG), an accurate determination of pavement layer moduli is vital for predicting pavement critical mechanistic responses. A backcalculation procedure is commonly used to estimate the pavement layer moduli based on the non-destructive falling weight deflectometer (FWD) tests. Backcalculation of flexible pavement layer properties is an inverse problem with known input and output signals based upon which unknown parameters of the pavement system are evaluated. In this study, an inverse analysis procedure that combines the finite element analysis and a population-based optimization technique, Genetic Algorithm (GA) has been developed to determine the pavement layer structural properties. A lightweight deflectometer (LWD) was used to infer the moduli of instrumented three-layer scaled flexible pavement models. While the common practice in backcalculating pavement layer properties still assumes a static FWD load and uses only peak values of the load and deflections, dynamic analysis was conducted to simulate the impulse LWD load. The recorded time histories of the LWD load were used as the known inputs into the pavement system while the measured time-histories of surface central deflections and subgrade deflections measured with a linear variable differential transformers (LVDT) were considered as the outputs. As a result, consistent pavement layer moduli can be obtained through this inverse analysis procedure.

Fiber optic sensors for assessing strains in cold in-place recycled pavements

This study aims to investigate aspects related to the assessment of strains in the body of asphalt pavements that have been cold in-depth recycled utilising foamed asphalt as a stabilisation treatment for base works. For this purpose, the critical tensile strains within the pavement structure were measured using a fiber optic sensor (FOS) system at four different test sites recycled utilising the mentioned technique. Initially, an investigation was undertaken to define the critical fatigue failure locations within the structure of the test pavements where the FOS systems were eventually installed. The measured strains were verified and documented through falling weight deflectometer loading. The collected data were analysed thoroughly. The results of the analysis prove that the recycled material, except for its sustainability characteristics, could be also characterised as a material durable against fatigue failure.

Effect of Cross-Anisotropy of Hot-Mix Asphalt Modulus on Falling Weight Deflections and Embedded Sensor Stress–Strain

In this study, the effects of cross-anisotropy on asphalt pavement responses are examined. A dynamic finite element model (FEM) was developed in ABAQUS to simulate pavement responses under falling weight deflectometer (FWD) and truck loads on a pavement section on I-40 at Mile Post 141 in New Mexico. This section was recently instrumented with strain gauges, moisture probes, and pressure cells. Pavement response (i.e., stress, strain, deflection) from the instrumented section is compared with the FEM-predicted values. Two combinations of cross-anisotropy are considered. The first combination considers cross-anisotropy of modulus in every layer of the pavement, and the second combination considers it only in the hot-mix asphalt layer. Time–deflection histories, stress, and strain are predicted from the FEM under FWD and truck loads. Results show that predicted deflections, stress, and strain are highly sensitive to cross-anisotropy. Predicted deflections, stress, and strain increase with a decrease in n-value, defined by the ratio of horizontal to vertical modulus of elasticity. Analyses are performed for two shapes of loading area: semicircle–rectangle and rectangle. Predicted stress and strain are shown to be larger for the rectangular shape of the loading area than for the semicircle–rectangle area. This study recommends including modulus anisotropy in FWD backcalculation and in pavement design.

Viscoelastic Genetic Algorithm for Inverse Analysis of Asphalt Layer Properties from Falling Weight Deflections

The falling weight deflectometer (FWD) is a nondestructive test whose results are typically used for backcalculating in situ layer properties of pavements. Most backcalculation methods assume the pavement to be a layered elastic half-space. However, asphalt pavements behave more like multilayered viscoelastic systems, especially in response to small or short-duration load applications. Hence, although elastic analysis is computationally efficient and well accepted in the engineering community, the theory cannot produce the viscoelastic properties of the asphalt concrete (AC) layer. In this study, a new inverse analysis method is proposed to backcalculate both linear elastic and viscoelastic properties of pavement layers as well as the AC time–temperature shift factor. In this method, the FWD load–response history of a single FWD drop and variation in temperature along the depth of the AC layer during the drop are used for performing the computations. The underlying (viscoelastic) forward solver is approximate and disregards dynamic effects; these factors, in return, make it computationally efficient. A genetic algorithm–based optimization scheme is offered to search for the pavement properties. As an example, two sections from the long-term pavement performance study were selected for investigation. The back-calculation results were positive, which indicates that unless a stiff layer exists close to the surface, it should be possible to infer linear viscoelastic properties from a single FWD drop.

Investigating in situ stress-dependent behaviour of foamed asphalt-treated pavement materials

The present research study aims to investigate whether increases in the applied load have an impact on the modulus and the in situ behaviour of foamed asphalt (FA)-treated materials used as base layer in pavements that have been cold in-depth recycled. For this purpose, falling weight deflectometer (FWD) measurements were conducted, using varying load levels and subsequently back-calculation procedures were performed to estimate the layer moduli for each load level in order to investigate the possible stress-dependent and non-linear behaviour of the FA-treated layer material. The pavement response at each load level and therefore the critical strains in the body of the FA layer were calculated. A fibre optic sensors system was used for in situ strain measurements. The relation between the calculated, as well as, the measured strains and the applied FWD loads was used as another criterion of the possible non-linear behaviour of the FA layer. Analysis results did not indicate an in situ stress-dependent behaviour of the FA material.

Backcalculation of subgrade stiffness under rubblised PCC slabs using multilevel FWD loads

Rubblisation is an effective rehabilitation method for deteriorated Portland cement concrete (PCC) pavements due to its low initial costs, minimal traffic disruption and ability to minimise reflective cracking in asphalt overlays. However, the loss of strength in the PCC slab due to rubblisation creates the demand for a subgrade that is strong enough to handle traffic after rubblisation. This paper presents a research effort to develop an analysis method that allows the realistic estimation of subgrade stiffness after rubblisation from deflection measurements on intact PCC slabs before rubblisation. As a forward model, stress- and strain-dependent nonlinear subgrade models are incorporated into a finite element analysis (FEA). Based on the synthetic database generated from the FEA, different methodologies are developed to predict the coefficients in the nonlinear subgrade model from deflections under multilevel falling weight deflectometer loads.