Falling Weight Deflectometer Measurements Research Articles

Traffic Speed Deflectometer for Network-Level Pavement Management in Tennessee

The traffic speed deflectometer (TSD) overcomes the limitations of the falling weight deflectometer (FWD) with respect to traffic interruption and test inefficiency and is considered the preferred method for network-level pavement structural evaluation. However, some challenges exist when applying the TSD to routine survey work. This study evaluated the long-term repeatability of TSDs and compared TSD measurements with FWD measurements to examine device-related errors in TSD measurements. In addition, a framework for incorporating TSD measurements into the pavement management system (PMS) was developed by introducing deflection bowl parameters (DBPs). All data analyses were based on TSD data collected in Tennessee. From the current study, the repeatability of the TSD has not been fully validated because of device-related errors. Since TSD measurements and FWD measurements are broadly similar, periodic validation of TSDs by FWDs is recommended for quality control. TSDs provide valuable information about pavement structural conditions that are not reliably or adequately reflected in the current PMS (which focuses mainly on pavement functional conditions). DBPs are a simple and fast method for making initial estimates of pavement structural conditions without the need for layer thickness. In the proposed DBP system, the surface curvature index (SCI_12) and D60 (TSD deflection at 60 in. before the load center) represent the structural strength of the asphalt layer and subgrade, respectively. The combination of pavement functional conditions and TSD-measured structural conditions (PMS+DBPs) may help to make more accurate predictions of pavement performance and to make more appropriate decisions on maintenance and rehabilitation strategies.

An Approach for Adjusting the Laboratory-Determined Dynamic Modulus Master Curve of Asphalt Layers Based on Falling Weight Deflectometer Measurements

Structural assessment is critical for designing asphalt pavement overlays, estimating the remaining life of pavements, and selecting an appropriate rehabilitation strategy for existing pavements. The falling weight deflectometer (FWD) serves as the primary nondestructive test used for evaluating the in situ properties and structural capacity of asphalt pavements. The current procedure involves analyzing the FWD response and estimating layer moduli by assuming an elastic response. However, the response of asphalt layers is viscoelastic (i.e., temperature- and frequency-dependent). This study proposes an approach that combines FWD data with laboratory measurements of the dynamic moduli of field cores to determine the in situ viscoelastic properties of asphalt layers. This approach is implemented by analyzing FWD data from four pavement sections in Qatar. Furthermore, the paper includes a comparative analysis of the response of pavement sections in which the asphalt layers are modeled using dynamic modulus master curves obtained from laboratory tests and those obtained using the approach presented in this study. It was found that using the laboratory-based master curves overestimated pavement performance (i.e., underestimated pavement distresses). It is recommended to use the dynamic moduli from the method presented in this paper for a more accurate estimation of pavement response and performance.

Modulus attenuation of semi-rigid asphalt pavement layer in accelerated pavement testing with MLS66

This research aimed to investigate the attenuation mode of the layer modulus of asphalt pavement in accelerated pavement testing (APT). A full-scale experimental section was constructed and tested using the APT facility. Two non-destructive testing (NDT) methods, named falling weight deflectometer (FWD) technique and portable seismic property analyzer (PSPA) test, were used to obtain the layer moduli of asphalt pavement during the APT test. The variation patterns of layer moduli obtained by FWD and PSPA tests were calculated and compared after the temperature was corrected to 20 ℃. It was found that the variation pattern of surface layer modulus based on field FWD measurements was consistent with the one measured from PSPA tests. That is the modulus of the surface layer increases with the APT load repetitions firstly and then decreases with the rise of the repetitions. The modulus values of the surface layer measured from PSPA tests are obviously larger than those backcalculated based on deflection basins. The ratio of the measured surface layer modulus based on the PSPA test to the backcalculated one based on the FWD test ranges between 2.06 and 2.71. The backcalculated base layer modulus always declines with the increasing loading repetitions. The attenuation patterns of the surface layer modulus and the base layer modulus in the damage stage are described as Ea=421100*N-0.6119 and Eb=128000*N-0.1096, respectively.

Determining pavement structural number with traffic speed deflectometer measurements

Traffic Speed Deflectometer (TSD) overcomes the slow operating speed of the conventional Falling Weight Deflectometer (FWD), but it also introduces new difficulties in estimating the effective structural number (SN) of existing pavements. Due to the different loading mechanisms of TSD and FWD, the FWD-based SN calculation procedure proposed in the AASHTO Pavement Design Guide is incompatible with TSD measurements. To match the existing FWD-based SN records, the authors proposed a new approach to determine the SN with TSD measurements by modifying the AASHTO method. Simulation results show that the difference in AASHTO SN between TSD and FWD essentially came from the difference in the equivalent loading frequency. The asphalt concrete (AC) layer exhibited lower stiffness under TSD than under FWD. The TSD-based AASHTO SN was generally smaller than the FWD-based AASHTO SN (error of 0.74), and the deviation increased with increasing AC thickness. Therefore, an enhanced AASHTO method was proposed for calibrating TSD-based SN by the AC thickness, which made great progress in estimating SN from TSD deflections with an error of 0.17. The proposed method was verified by filed TSD and FWD measurements and was recommended for pavements with AC thickness greater than 4 in..

Temperature Correction of Deflections and Backcalculated Elasticity Moduli Determined from Falling Weight Deflectometer Measurements on Asphalt Pavements

The evaluation of the bearing capacity of asphalt pavements is usually performed by analysing the deflections measured by a Falling Weight Deflectometer (FWD). The deflection changes with the pavement temperature. In evaluation is necessary to consider the thermal gradient of pavement and perform the temperature correction. The article contains an analysis of effects of the pavement temperature on FWD results on the long-term monitored sections. The temperature correction was performed on measured deflections or back-calculated elasticity moduli. The moduli recalculated to the temperature of 20 °C according to both procedures were similar. Comparison of moduli determined by recalculation to moduli backcalculated from the deflection bowls measured at the temperature of 20 °C, has proven smaller differences for the moduli determined from the deflection bowl corrected to the temperature of 20 °C.

Influence of seasonal and diurnal FWD measurements on deflection-based parameters for rigid pavements

ABSTRACT Seasonal and diurnal variations can significantly impact Falling Weight Deflectometer (FWD) measurements and, thus, any deflection-based parameter such as slab elastic modulus (Epcc ), modulus of subgrade reaction (k), and load transfer efficiency (LTE). Also, general guidelines for the FWD measurements are needed to help agencies better interpret the derived parameters. Long-Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP) study data for Jointed Plain Concrete Pavements (JPCP) were analyzed. The results showed that seasonal and diurnal changes impact the derived parameters for the rigid pavements in different climatic regions. PCC slab’s upward curling and full contact with the underlying layer at lower temperatures resulted in higher Epcc and k-values and vice versa. Trends seen for k-values closely followed those for moduli values. Therefore, any guidelines concerning FWD testing should consider PCC layer temperature, which affects its shape at testing time. Also, LTE was found higher at elevated temperatures due to locked joints. Relating testing temperatures with observed trends of parameters helped determine general guidelines for FWD testing. The preferred temperature ranges for FWD testing are 55 to 70°F and 60 to 75ᵒF in freeze and non-freeze regions respectively; testing before noon is recommended in spring and fall seasons in all climates.

Development of an Artificial Neural Network-Based Procedure for the Verification of Traffic Speed Deflectometer Measurements

Since the 1980s, the falling weight deflectometer (FWD) has been the primary deflection-measuring device in the United States to evaluate the structural conditions of in-service pavements. However, the stop and go nature of the FWD limits its application at the network level. In the early 2000s, the traffic speed deflectometer (TSD) was introduced as an alternate deflection-measuring device for network-level applications. TSD collects deflection measurements while traveling at traffic speed, which provides improved spatial coverage and no traffic disturbance. The verification of TSD measurements is of great interest as many agencies move toward widespread implementation. This study aims at developing a reliable and straightforward procedure for the verification of TSD measurements using limited FWD measured deflection measurements. The verification procedure employs a trained artificial neural network (ANN) model to shift TSD deflections to their corresponding FWD deflections. The ANN model was trained and verified based on FWD and TSD measurements from two deflection-testing programs. The developed model accurately predicted FWD measurements with a coefficient of determination (R2) of 0.994. The suitability of the proposed verification procedure was evaluated using statistical and engineering-based measures and showed acceptable accuracy. Results also validated that the proposed method could be used to verify TSD measurements before its use for conducting deflection measurements at the network level.

Seasonal and diurnal variations of back-calculated layer moduli in flexible pavement and FWD testing guidelines

Seasonal and diurnal variations can significantly impact falling weight deflectometer (FWD) measurements and, thus, deflection-based parameters such as back-calculated layer moduli. Therefore, it is necessary to evaluate and quantify these effects. The back-calculated layer moduli data from the Long-Term Pavement Performance Seasonal Monitoring Program study were analyzed within each climatic region. The effects of seasonal and diurnal changes were quantified and related to ambient temperatures were related to the development of general measurement guidelines. The HMA layer moduli showed minimal variation during the spring and fall seasons in every climatic region and were consistently higher based on the deflections measured before noon. However, temperature correction, generally applied to the HMA layer moduli, can eliminate the temperature impacts of the FWD measurements. Because the unbound layer moduli are back-calculated from the single measured deflection basins on the surface, temperature and moisture conditions at the time of measurements can affect the material properties. The base and subgrade moduli showed minimal variation within the temperature ranges of the spring and fall seasons. Therefore, such temperature within each climatic region may result in layer moduli values closest to their representative in-field conditions. In addition, the unbound layer showed no effect of time within a day, suggesting that FWD testing can be conducted at any time during the day. Based on the results, the preferred temperature ranges for FWD measurements are 55–70 °F and 65–75 °F in the freeze and non-freeze regions, respectively. FWD measurements can be conducted at any time during the day. The preferred seasons for testing are spring and fall in all climatic regions.

FBG‐Based Sensing for Structural Health Monitoring of Road Infrastructure

Public road infrastructure is developed all around the world. To save resources, ensure public safety, and provide longer‐lasting road infrastructure, structural health monitoring (SHM) applications for roads have to be researched and developed. Asphalt is one of the largest used surface materials for the road building industry. This material also provides relatively easy fiber optical sensor technology installment, which can be effectively used for SHM applications—road infrastructure monitoring as well as for resource optimization when road building or their repairs are planned. This article focuses on the research of the fiber Bragg grating (FBG) optical temperature and strain sensor applications in road SHM, which is part of the greater interdisciplinary research project started at the Riga Technical University in the year 2017. Experimental work described in this article was realized in one of the largest Latvian road sites where the FBG strain and temperature sensors were installed into asphalt pavement, and experiments were carried out in two main scenarios. Firstly, in a controlled environment with a calibrated falling weight deflectometer (FWD) to test the installed FBG sensors. Secondly, by evaluating the real‐time traffic impact on the measured strain and temperature, where different types of vehicles passed the asphalt span in which the sensors were embedded. The findings in this research illustrate that by gathering and combining data from calibrated FWD measurements, measurements from embedded FBG optical sensors which were providing the essential information of how the pavement structure could sustain the load and information about the traffic intensity on the specific road section, and the structural life of the pavement can be evaluated and predicted. Thus, it enables the optimal pavement future design for necessary requirements and constraints as well as efficient use, maintenance, and timely repairs of the public roads, directly contributing to the overall safety of our transportation system.

Assessing impacts of climate change on flexible pavement service life based on Falling Weight Deflectometer measurements

Flexible pavements are typically designed using historical climate but are challenged by future climate change. Quantifying impacts of climate change on pavement service life can assist road authorities in planning for climate adaptation and, eventually, build climate resilience into road infrastructure design and management. In this study, a novel data-driven methodology is developed in order to quantify impacts of climate change on pavement service life in locations where Falling Weight Deflectometer (FWD) data are continuously measured, by means of: 1) training a supervised model (linear regression or Artificial Neural Networks, ANN) using historical climate data, maintenance, and traffic data as the candidate inputs and pavement layer stiffness back calculated from FWD testing as the outputs; 2) predicting layer stiffness using statistically downscaled future climate projections for three Coupled Model Intercomparison Project Phase 5 global climate models and three greenhouse gas concentration scenarios for four future 20-year periods; and 3) estimating changes in pavement stiffness and service lives due to climate change. A case study performed on a pavement section in Minnesota has shown that pavement layer stiffness will have a long-term reduction under future climate and the investigated pavement will lose up to 22.5% service life at the end of the century (2080–2099) from the 20 years’ service life compared to the baseline climate (1979–1998).

Numerical and Experimental Investigations of Asphalt Pavement Behaviour, Taking into Account Interface Bonding Conditions

The interface bond between layers plays an important role in the behavior of pavement structure. However, this aspect has not yet been adequately considered in the pavement analysis process due to the lack of advanced characterizations of actual condition. In many pavement design procedures, only completely bonded or unbounded interfaces between the layers are considered. For the purpose of the better evaluation of the asphalt pavement behavior, this work focused on its investigation taking into account the actual interface bonding condition between the asphalt layers. Based on the layered theory developed by Burmister (1943), the actual interaction between pavement layers was taken into account by introducing a horizontal shear reaction modulus which represents the interface bonding condition for a given state. The analytical solution was then implemented in a numerical program before doing forward calculations for sensitivity analysis which highlights the influence of the interface bonding conditions on the structural behaviors of asphalt pavement under a static load. Furthermore, the numerical program was applied through an original experimental case study where falling weight deflectometer (FWD) tests were carried out on two full-scale pavement structures with or without a geogrid at the interface between the asphalt layers. Backcalculations of the FWD measurements allowed determining field condition of the interface bond between the asphalt layers. The obtained values of the interface shear modulus in pavement structure with a geogrid are smaller than the ones in pavement structure without geogrid. Moreover, all of these values representing field performance are at the same order of magnitude as those from dynamic interlayer shear testing.

Structural Evaluation of the Transverse Joint of a Concrete Pavement Considering the Degradation Process

Effective maintenance of a concrete pavement requires precise evaluation of the load transfer efficiency (LTE) at the transverse joint, which is simply calculated from the falling weight deflectometer (FWD) deflections at the loaded and unloaded slabs. In this study, the FWD deflections were simulated with the randomly generated elastic moduli of the pavement layers using the three dimensional finite element method considering the degradation process at a joint, which was categorized into five stages, from completely sound, to having broken dowels and a void underneath the joint. A database of FWD deflections, layer elastic moduli and the associated stage was created from the simulation results. The layer moduli were identified from the measured FWD deflections at a joint by searching the closest calculated FWD deflections to those measured from the database. The results of the analysis not only provide the layer moduli, but also the degradation stage of the joint to help engineers determine the appropriate rehabilitation measures. The method was validated on a 16 year old concrete pavement, on which the FWD measurements were performed three times: just before opening to traffic, after 9 years and 16 years of service. The identified layer moduli were not changed after 16 years of service, and some joints degraded owing to dowel corrosion, broken dowels and the existence of voids underneath the joints. The developed method is able to show the entire picture of the state of the joints in a section and also provide information on which parts of a particular joint have deteriorated.

The adjustment of pavement deflections due to temperature variations

ABSTRACT One of the most important steps in the rehabilitation of flexible pavements is the evaluation of their bearing capacity, which is used to assess the moduli of the pavement layers. The evaluation of the bearing capacity is usually carried out by measuring pavement deflections using a Falling Weight Deflectometer (FWD). This evaluation is undertaken in equally spaced points, usually 100 metres apart, with temperature variations throughout the day. Due to the viscoelastic behaviour of the asphalt layers, these FWD measurements are influenced by the pavement temperature, and the moduli can only be compared if they correspond to the same temperature. Thus, this work develops a model to adjust the pavement deflections to a reference temperature based on the characteristics of the pavement; namely: the asphalt layer thickness, subgrade stiffness, the distance between the FWD sensors, the load magnitude, and the pavement temperature.

Comparing Traffic Speed Deflectometer and Falling Weight Deflectometer Data

In recent years the pavement engineering community has shown increasing interest in shifting from a stationary falling weight deflectometer (FWD) to moving testing platforms such as the traffic speed deflectometer (TSD). This paper dealt with comparing TSD measurements against FWD measurements; it focused on the comparison methodology, utilizing experimental data for demonstration. To better account for differences in loading conditions between the two devices a new FWD deflection index was formulated first. This index served as reference/benchmark for assessing the corresponding TSD measurements. Next, a Taylor diagram was proposed for visualizing several comparison statistics. Finally, a modern agreement metric was identified and applied for ranking comparison results across different datasets. Overall, the suggested methodology is deemed generic and highly applicable to future situations, especially for assessing the worth of emerging device upgrades or improved interpretation schemes (or both).

Development of a methodology to backcalculate pavement layer moduli using the traffic speed deflectometer

Backcalculation analysis of pavement layer moduli is typically conducted based on falling weight deflectometer (FWD) measurements; however, the stationary nature of FWD requires lane closure and traffic control. To overcome these limitations, a number of continuous deflection devices were introduced in recent years. The objective of this study was to develop a methodology to incorporate traffic speed deflectometer (TSD) measurements in the backcalculation analysis. To achieve this objective, TSD and FWD measurements were used to train and to validate an artificial neural network (ANN) model that would convert TSD deflection measurements to FWD deflection measurements. The ANN model showed acceptable accuracy with a coefficient of determination of 0.81 and a good agreement between the backcalculated moduli from FWD and TSD measurements. Evaluation of the model showed that the backcalculated layer moduli from TSD could be used in pavement analysis and in structural health monitoring with a reasonable level of accuracy.

Application of Traffic Speed Deflectometer for Network-Level Pavement Management

In this paper, the traffic speed deflectometer (TSD), a device used for network level structural evaluation, is assessed. TSD testing was performed in nine states on a total of 5,928 miles (some repeated) during three time periods: November 2013, May to July 2014, and June to September 2015. This paper presents (1) the results of repeatability and comparison of the TSD with the falling weight deflectometer (FWD), (2) the results of the comparison of TSD measurements with typical pavement management system (PMS) data, and (3) an approach that can be implemented by State Highway Agencies (SHAs) to incorporate indices derived from TSD data into their PMS decision-making process. The results show that repeated TSD measurements follow similar trends and the TSD measurements and FWD measurements on the same pavement sections follow similar trends as well. Comparing TSD measurements with PMS surface condition data confirmed that the TSD provided valuable information about the structural condition of the tested pavement sections that cannot be derived from the already available pavement surface condition as part of an agency’s PMS. An example of how TSD information can be used to refine the triggered maintenance treatment category as part of a network-level PMS analysis is presented for a roughly 75-mile section of I-81 south in Virginia.

Crushed concrete in road structures – two decades of experience

Since the early 1990s, crushed concrete aggregate (CCA)/recycled concrete aggregate has been successfully used in Finnish road constructions. The aim of this paper is to analyse the long-term behaviour of CCA and to compare CCA with conventional mineral aggregates in road structures. The analysed trial sections consist of the road and highway pavements where CCA has been used in the base and subbase layers. The first studied Finnish trial section was constructed in 1995, which has enabled nearly two decades of follow-up studies. In total, five follow-up trial sections were studied. E-modulus of pavement layers was determined using the deflection data from falling weight deflectometer measurements. The studies revealed excellent long-term behaviour in the CCA build pavements and validated the suitability of chosen design parameters in previous guidelines. In this study, the sections consisting of maximum 50 mm particles of CCA in the base or subbase layer gained 10–25% larger bearing capacity in 13–15 years after construction than the reference pavements that were constructed with crushed rock. The increase of the CCA stiffness was faster in the first years after construction and in the 2–5 years little or insignificant increase in such stiffness could be observed.

Use of Embedded Sensors to Evaluate Performance of Traffic Speed Deflection Devices

Abstract Structural adequacy, which is an important aspect of pavement management activities, is typically evaluated using surface pavement deflections measured by the falling weight deflectometer (FWD). Technology that can collect continuous deflection data at traffic speeds can potentially supplant single-point location deflection measurements provided by the FWD for network-level pavement management system (PMS) applications. Previous studies have evaluated the traffic speed deflection devices (TSDD) performance by comparing their results with the FWD measurements. Although appropriate correlations and indices to compensate for the differences in load have been made, a direct comparison under the same loading conditions has not been reported in the literature. The purpose of this paper was to present the results of a field study to evaluate TSDDs by comparing their pavement deflection parameters with those from embedded sensors under actual loads. Two TSDDs were used in this study: the Greenwood traffic speed deflectometer (TSD), and the Applied Research Associates, Inc. (ARA) rolling wheel deflectometer (RWD). The lessons learned from the study are emphasized for further consideration toward developing a calibration methodology for value-added future analyses.

Assessment of the Remaining Life of Bituminous Layers in Road Pavements

In this paper, a mechanistic-empirical approach is presented for the assessment of bearing capacity condition of asphalt pavement layers by Falling Weight Deflectometer measurements and laboratory fatigue tests. The bearing capacity condition ratio was determined using past traffic data and the remaining fatigue life which was determined from multilayer pavement response model. The traffic growth rate was taken into account with finite arithmetic and geometric progressions. Fatigue resistance of layers’ bituminous materials was obtained with indirect tensile fatigue tests. Deduct curve of condition scores was derived with Weibull distribution.

Kinetics-based aging prediction of asphalt mixtures using field deflection data

Asphalt pavements suffer from field aging that accelerates distresses and reduces service life of pavements under the complex in-service environment. The majority of the current methods to study aging are performed in an indirect manner, designed in the laboratory to simulate the field. This study targets the field pavement aging directly and aims at characterising and predicting field aging using the kinetics-based modelling, which contains the field mixture moduli and field aging temperatures. The Falling Weight Deflectometer (FWD) data from the Long-Term Pavement Performance (LTPP) database are employed to provide the field moduli of asphalt mixtures over a long aging period. A pavement temperature model that considers solar radiation, air temperature and wind speed is utilised to determine the temperature fluctuations over the entire aging period and then appropriate field aging temperatures. Eight pavement sections from the four different climate zones are selected and the modulus, temperature, mixture property and climate data are collected. For each pavement section, the rheological properties are first determined to characterise the temperature-dependency of field asphalt mixtures. Then based on the kinetics-based aging prediction models for asphalt pavements, the aging properties are determined to characterise and predict aging of field asphalt mixtures. In the determination of aging properties, it is critical to divide the modulus vs. aging time curve into different segments according to the features of the constant-rate period and fast-rate period. The proposed approaches are validated by comparing the predicted moduli at specified ages to the field measurements by the FWD. It is expected that with more calibration and validation work accomplished in the future, the number of FWD measurements required for nondestructive evaluations could be reduced with the aid of the approaches proposed in this study.