Validation of Dynamic Simulation of Slow-Moving Surface Deflection Measurements

The Minnesota Department of Transportation (Mn/DOT) built the Minnesota Road Research Project MnROAD between 1990-1993. The 2.5-mile low volume road and the 3.5-mile mainline consists of a 2-lane roadway that originally contained gravel, hot mix asphalt, and concrete test cells designed for both low volume roads and interstate traffic. The mainline interstate cells are trafficked by public interstate traffic and the low volume road has a Mn/DOT 5-axle tractor-semi-trailer to simulated conditions of rural roads in two load configurations, resulting in the same equivalent axle loads or ESALS. MnROAD is located in a wet freeze zone that has affected both its base and subgrade materials with seasonal frost movements. This movement has slowly deteriorated each test cells ride over time. MnROAD has monitored the frost movements using frost pins and has measured the ride (international ride index – IRI) using high-speed profilers over for the life of the project. This paper investigates the loss of ride from both environmental and traffic loadings and how they have combined to cause the deterioration of ride over the last 10 years at MnROAD. The findings suggest that our current process to develop a mechanistic empirical design is currently missing the fact that seasonal differental frost movements play an important role in pavement performance in northern climates and need to be taken into account.

Falling weight deflectometer (FWD) tests are performed worldwide for assessing the health of pavement structures. Interpretation of FWD-measured surface deflections turns out to be challenging because the behavior of pavement structures is temperature-dependent. In order to investigate the influence of temperature on the overall pavement performance and on the stiffness of individual layers, temperature sensors, asphalt strain gauges, and accelerometers were installed into one rigid (concrete) and two flexible (asphalt) pavement structures, mostly at layer interfaces. Three different methods for installation of the strain gauges are compared. From correspondingly gained experience, it is recommended to install a steel dummy as a place-holder into the surface of hot asphalt layers, immediately after their construction and right before their compaction, and to replace the dummy with the actual sensor right before the installation of the next layer. Concerning the first data obtained from dynamic testing at the field-testing sites, FWD tests performed at different temperatures deliver, as expected, different surface deflections. As for the rigid pavement, sledgehammer strokes onto a metal plate, transmitted to the pavement via a rubber pad, yield accelerometer readings that allow for detection of curling (=temperature-gradient-induced partial loss of contact of the concrete slab from lower layers). In the absence of curling, the here-proposed sledgehammer tests yield accelerometer readings that allow for quantification of the runtime of longitudinal waves through asphalt, cement-stabilized, and unbound layers, such that their stiffness can be quantified using the theory of elastic wave propagation through isotropic media.

This paper presents field trials with polymer modified asphalts (Novophalt and Styrelf). The modified binders were used in wearing surface course, binder course and base course and compared with non modified asphalt. Field evaluation included visual survey, roughness measurements and deflection measurements. Preliminary results of these field trials indicated that no significant improvements in pavement performance were obtained. The one year evaluation (after test track construction) under heavy traffic movements did not show significant rut development for either polymer modified or non modified bituminous sections. However, it is planned to annually investigate and report the extent of improvement in pavement performance which might occur in the future when using these polymer modified asphalt binders.

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.

A modified Newton method for the backcalculation of pavement layer moduli, thicknesses, and stiff layer depth from measured surface deflections is presented. The method is an iterative one and can use any mechanistic analysis program for forward calculations (presently an extended precision CHEVRON program is used for this purpose). This method has been implemented in a new backcalculation program named MICHBACK. Several examples of backcalculation based on theoretical deflection basins generated by the extended precision CHEVRON program are presented, and the effect of errors in input layer thicknesses and stiff layer depth on the backcalculated moduli are studied. Moduli backcalculated by MICHBACK are compared with results obtained by the EVERCALC 3.0 and MODULUS 4.0 programs.

: The report presents influence diagrams for vertical stresses, shear stresses, and surface deflections in three-layer pavement systems, which are intended to provide the essential background and bases for evaluating the character and effectiveness of layered system reinforcing action. The vertical stress and shear stress transmission characteristics and critical regions, and the surface deflection performances are developed for nine three-layer concrete pavement systems for a range of layer modulii ratios; and ratios of layer 2 to layer 1 thickness. The effectiveness of reinforcing action, deflection performances, and shear stress performances in critical regions are treated and compared. Two-layer and three-layer thickness and layer modulii equivalences are developed and treated to illustrate methods for evaluating and for improving shear stress and deflection performance by modifications of layer modulii ratios and layer thicknesses. The major objectives are to develop a 'feeling', intuition, and judgments regarding deflection and shear stress performances of layered pavements, and to develop relationships and methods for evaluating layer thickness and modulii ratios, and criteria for rational and effective design for multi-layer pavement system for airfields and for ensuring satisfactory pavement performances and long life.

To effectively investigate the mechanical performance of asphalt pavements, software named SAFEM with a user friendly graphical user interface (GUI) was developed. It is based on the semi-analytical finite element (FE) method, which is a three-dimensional FE algorithm that only requires a two-dimensional mesh by incorporating the Fourier series in the third dimension. SAFEM is able to analyze static and dynamic responses of asphalt pavements under stationary or moving loads. Besides linear elasticity, a viscoelastic material model for asphalt layers has also been integrated. Verification was carried out to test and explore the functionality of the software package. We showed that SAFEM is an accurate tool for the fast prediction of asphalt pavement responses, such as the surface deflection and the strain/stress at response points. Furthermore, SAFEM provided a flexible, robust platform for the future development in the numerical simulation of asphalt pavements.

ABSTRACTPavement evaluation surveys represent the key element for efficient pavement management and for assuring pavement mission capability. In the Department of Defense (DOD), the Unified Facilities Criteria (UFC) 3-260-03 Airfield Pavement Evaluation provides the current guidance for pavement structural evaluations. During structural surveys, FWD tests are executed at different locations within the same section, with the objective of obtaining a full assessment of the section’s structural capability. The availability of multiple deflection measurements for the same section raises the challenge of identifying the deflection basin best representing the entire section and its use in the backcalculation routine to determine the section’s structural strength. This manuscript proposes a fuzzy-based approach for the selection of a representative basin over multiple deflection basins collected for a specific section. The approach accounted for the spatial variability enclosed within the basin membership function obtained by fuzzy c-mean partitioning. The proposed methodology showed promising results for flexible pavements by offering more robust structural assessment that can account for spatial variability and thus minimise some aspects of mission risk that had a large effect on funding allocation and mission readiness.

The instrumentation data obtained in the multiple-wheel heavy gear load tests conducted at the U.S. Army Engineer Waterways Experiment Station are analyzed, and the results of this analysis are presented. The nonlinear pavement response to loads is illustrated. The patterns of measured stresses, deflections, and deflection basins under different wheel assemblies and loads are delineated, and pavement performance (in terms of coverages to failure) under traffic loads is examined. The validity of the superposition principle is manifested, i.e., the stresses and deflections of multiple-wheel loads can be determined with reasonable accuracy from the actual basin of a single-wheel load by the use of the superposition principle. The application of this principle to pavement analysis is emphasized. Conclusions based on the analysis pertaining to pavement research in the area of multiple-wheel heavy gear loads are drawn.

The main objective of this paper was to provide some useful statistical-based models for flexible pavement behavior. The results of a theoretical analysis, using the finite element (FE) program SAP was utilized in this statistical analysis study. The maximum surface deflection, the maximum horizontal tensile strain (t) at the bottom of AC layer and the maximum compressive strain (c) at the top of subgrade are the most commonly used criteria for flexible pavement design. They were used in this study as the basis for measuring the flexible pavement response. The Statistical Package for Social Sciences (SPSS) version 11.0 was used to relate these parameters to the basic flexible pavement layer properties.Clearly, the properties of the top layer, asphalt concrete (AC) layer, are the key to control the behavior of flexible pavements. Based on that, the models developed in this paper concerned with the thickness and modulus of AC layer. Three different AC thicknesses and eight different AC moduli were used to investigate the response of flexible pavement. Based on the theoretical and statistical work presented in this study, simple statistical-based models for the response of flexible pavements were developed. These models could be easily used by pavement engineers to predict the flexible pavement indicators; maximum surface deflection, maximum horizontal tensile strain at the bottom of asphalt concrete layer and maximum vertical compressive strain at the top of subgrade layer. The presented models emphasized that the thickness of asphalt concrete layer was the most important independent variable for the response of flexible pavement followed by the modulus of AC layer.

Multilayer solutions that calculate stresses, strains, and displacements in pavement structures caused by surface loading have been in existence for several years. However, most elastic layer computer software applications have been developed for mainframe computers. Recently, computational capabilities of the practicing engineer have been greatly enhanced by recent developments in personal computers. Results of this study involved development of an accurate and efficient solution to the multilayer problem. In addition, software was developed for use on a personal computer that included routines for backcalculating pavement layer moduli from measured surface deflections. A convergence technique, employing a combination of closed form and numerical solutions, has been developed to provide accuracy of results at the surface of the pavement and near the load. Special methods of solving the system of boundary equations were developed to obtain the terms needed for the closed form part of the solution. Maximum efficiency has been obtained through an in-depth analysis of the parameters affecting accuracy and their relationship with the numerical integration step. In its current form, the computer solution WESLEA is a fully competent three-dimensional solution that will handle up to five layers with varying interface conditions and a maximum of 20 loads. The fifth layer is semi-infinite and can be made stiff based on the choice of modulus of elasticity. A personal computer-based backcalculation program, WESDEF, has been developed utilizing the multilayer program. An evaluation of the new program is made through comparison of identical deflection data using the BISAR n-layered computer program.

Effect of Restorative Configurations and Occlusal Schemes on Strain Levels in Bone Surrounding Implants.

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.

The structural effects of lean concrete sub-bases on concrete highway pavements

Obtaining reliable in situ stress field data is the precondition of the design and excavation of underground engineering. The deformation rate analysis (DRA) method based on the rock deformation memory effect is an effective method for testing in situ stress. Cyclic loading tests were carried out on dense granite to research the influence of stress level on the identification accuracy of the DRA method. The DRA curves of horizontal and longitudinal strains are compared to find the difference between the accuracy in different directions. The results showed that in the range of 15%-60% uniaxial compression strength, the rock deformation memory effect is obvious, which indicates that the DRA method is applicable in low-stress regions, while the measuring accuracy is not as good as that in high-stress areas. When the specimen is subjected to low pre-stress, the accuracy is higher when using the horizontal strain than the longitudinal strain. The accuracy of both horizontal and longitudinal DRA methods increases with the increase in pre-stress. The measuring accuracy is similar when the pre-stress is higher than 45%UCS. What is more, the discrete element numerical model is calculated to verify the test results and reveal the microscopic mechanism of the DRA method under the control of crack evolution. The characteristics of rock memory are related to the evolution of crack closure, cracking, and expansion. The research results can provide an experimental basis for promoting the theoretical development and field application of the testing in situ stress with the DRA method.