Samir N. Shoukry

Performance Evaluation of Backcalculation Algorithms Through Three-Dimensional Finite-Element Modeling of Pavement Structures

Currently, the only means of assessing the performance of a backcalculation algorithm is to compare its results with the values obtained through laboratory measurements of core samples. Discrepancies are attributed to the inability of replicating in laboratory tests the same consolidation and stress conditions encountered in the field. In this paper, an explicit three-dimensional (3D) finite-element model (FEM) is used to back-calculate the layer moduli of rigid, flexible, and composite pavement structures. The modeling approach accounts for the dynamic nature of the falling weight deflectometer (FWD) load, friction at pavement-layer interfaces, and the 3D geometrical characteristics of the pavement structure. The FWD-measured and FEM-generated deflection basins are matched by adjusting the values of layer moduli used in the FEM. The backcalculated moduli using 3D FEM are compared with the results obtained from three backcalculation programs: MODCOMP, MODULUS, and EVERCALC. The results indicate that the three programs have an excellent capability of predicting the FEM-generated subgrade modulus values for flexible, composite, and rigid pavements provided that a correction factor is used. A correction factor for backcalculated subgrade modulus is estimated for each program and pavement type. The mechanistically evaluated correction factors were found to be in close agreement with the experience-based values recommended for flexible and rigid pavements in the American Association of State Highway Officials pavement design guide.

Universal Pavement Distress Evaluator Based on Fuzzy Sets

Setting priorities for pavement maintenance and rehabilitation depends on the availability of a universal scale for assessing the condition of every element in the network. The condition of a pavement section has traditionally been assessed by several condition indexes. The present serviceability index (PSI) is one common evaluator used to describe the functional condition with respect to ride quality. Pavement condition index is another index commonly used to describe the extent of distress on a pavement section. During the decision-making process, both classes of indexes are needed to evaluate the overall status of a pavement section in comparison to other sections in the network. Traditionally, regression techniques were used for the development of functions that relate condition indexes to the information recorded in the pavement management database. This approach produces mathematical functions that are limited to a particular database. The functions so developed may also suffer from inaccuracies due to errors in data collection and recording. There is a need for a more generalized approach for the evaluation of pavement conditions to enable efficient management of large transportation networks. The development of a universal measure capable of formally assessing the condition of a pavement section within the universe of pavement conditions is described. This is accomplished by the fusion of a set of fuzzy membership functions that describe different parameters in the database with the perception of each parameter’s significance. The model output is the fuzzy distress index (FDI), which combines the extent of structural distress with traditional performance parameters such as roughness to describe the overall status of the pavement section. The behavior of FDI over time is examined for a random sample of pavement sections and is compared with trends in the corresponding PSI values (PSI was used only because it was readily available in the database). The results indicate that the flexible, universal FDI is a consistent and accurate measure of the overall pavement condition. The set of generated membership functions describing the different extents of every distress type can be easily standardized over the 50 states, allowing the model to be implemented on any pavement at any location. Also, the parameter weights used in the assessment may be easily adjusted (increased or decreased) to reflect changes in maintenance policies or budget availability at the local, state, or national decision-making level. Moreover, the concept allows for the omission of any number of parameters that might not be available in a particular pavement management database.

Characteristics of Concrete Contact Stresses in Doweled Transverse Joints

The triaxial state of concrete contact stresses that develop at dowel-concrete interfaces in dowel jointed concrete pavements is examined using detailed nonlinear three dimensional finite element (3DFE) analysis. The results are validated through comparisons with: (a) measured strains from laboratory-loaded doweled joint specimens; (b) computed bearing pressure using closed-form solution; and (c) field-measured dowel bending due to slab curling in an instrumented highway section in West Virginia, USA. A parametric study is conducted to investigate the influence of some design parameters on the magnitudes of concrete contact stresses. It is shown that the triaxial contact stresses include a tensile stress component that develop in concrete on both sides of a loaded dowel bar and may initiate small horizontal cracks that reduce load transfer efficiency of the doweled connection. A modified dowel design that reduces the intensity of concrete contact stresses is developed and its laboratory test results are described. The modified dowel design is being field tested in the newly constructed Robert C. Byrds instrumented Highway section in Elkins, West Virginia, USA.

Backcalculation of Thermally Deformed Concrete Pavements

Nonlinear explicit three-dimensional finite element (3-D FE) modeling is used to investigate the performance of the falling weight deflectometer (FWD) test in the evaluation of layer moduli of jointed plain concrete pavements (JPCP) subjected to nonlinear thermal gradient through the slab thickness. Concrete slab separation from the base, in-plane friction at the concrete-base interface, the gravitational forces, and the interface characteristics between dowel bars and surrounding concrete are all represented in the 3-D FE model. Experimental verification of the model is obtained through comparison of the 3-D FE generated response to (a) the FWD measured deflection basin and (b) the measured response of an instrumented rigid pavement section located in Ohio to a loaded truck moving at 21.8 m/s (48 mph). Several cases of linear and nonlinear thermal gradients are applied to the model, and deflection basins are obtained. Two backcalculation programs, MODULUS 5.0 and EVERCALC 4.0, are used for prediction of the layer moduli in each case, and the values are compared. The results indicate that thermal curling of the slab due to negative thermal gradient has little effect on the accuracy of backcalculated moduli. Warping of the slab due to positive thermal gradient greatly influences the measured FWD deflection basin and leads to significant errors in the backcalculated moduli. These errors may be minimized if the time an FWD test is conducted falls between the late afternoon and midmorning (from 5:30 p.m. to 9:30 a.m. during summer in West Virginia).

Microstructure Modeling of Particulate Reinforced Metal Matrix Composites.

The study of microscopic and macroscopic response of a particulate reinforced MMC using finite element analysis is the aim of the current study. In this regard, two types of microstructure models are subjected to FE analysis. In the first part of the work, a technique is presented for the generation of artificial microstructure containing spherical and ellipsoid shaped inclusions. The problem of detection of ellipsoidal intersection is tackled using newly available algorithms. The FE analysis of the artificial microstructure and a summary of the results form the second part of the study. It is seen that the results from the newly developed models agree very well with the published results and that the microstructure generation technique can be reused in many computational micromechanics problems with minimum modifications.

Early age cracking of reinforced concrete bridge decks

This paper describes the instrumentation and test results of a reinforced concrete bridge deck constructed on three-span continuous steel girders in Evansville, West Virginia. An instrumentation system consisting of 232 sensors is developed and implemented specifically to measure strains and temperature in concrete deck, strains in longitudinal and transverse rebar, the overall contraction and expansion of concrete deck, and crack openings. Data from all sensors are automatically collected every 30 minutes starting at the time of placing the deck. The results indicated that elevated longitudinal stress due to constrained drying shrinkage is the main factor responsible for crack initiation during the first two days after concrete placement. Several factors contribute to the deterioration of the deck: (1) thermal stresses developed in the deck because of the constraining effects of the stay-in-place forms and shear studs; (2) non-uniform curing of the concrete along the deck; (3) in-plane temperature variat...

Nonlinear Temperature Gradient Effects in Dowel Jointed Concrete Slabs

The effect of nonlinearity in Temperature Gradient Profile (TGP) on dowel jointed concrete slabs is examined using nonlinear Three-Dimensional Finite Element (3DFE) modeling. The models thermal response is validated versus field-measured data collected from a heavily instrumented section on the Robert Byrd Highway (Route 33) near Elkins, West Virginia, USA. Results indicate that TGP nonlinearity has its maximum effect on the longitudinal stress 0.28 m away from the transverse joint, i.e. near the end of the embedded dowel bars. A TGP that includes a uniform temperature drop is shown to induce a large magnitude of tensile thermal stress at mid slab. This stress is unaffected by the extent of nonlinearity in TGP and depends on the magnitude of temperature drop and the difference between slab top and bottom temperatures. It is shown that dowel bar bending, due to slab curling, introduces a significant edge restraint to slab contraction and expansion due to ambient temperature changes. Mid-slab longitudinal stress is shown to be minimal for a 4.6 m long slab.

Validation of 3DFE Model of Jointed Concrete Pavement Response to Temperature Variations

The most appropriate method to validate the thermoelastic response of a 3D finite element (3DFE) model of dowel jointed concrete pavements subjected to temperature variation is to compare its results with field-measured data. However, field-measured strains in a concrete slab are not only due to temperature variation but they also include components due to construction curling, shrinkage and moisture. Such nonlinear strain contributors are not simulated in concrete constitutive models currently used in 3DFE; thus, field-measured strains cannot be directly used to validate the thermoelastic response of 3DFE models before some data reduction. This paper presents a data reduction technique that renders strains suitable for 3DFE model response validation. The field-measured data were obtained from an intensively instrumented pavement section in West Virginia whose instrumentation plans are described. Data sets of field-measured strains together with the associated temperature profiles are presented in this study. The data can be used by engineers and researchers to validate the thermoelastic response of any 3DFE model of jointed concrete pavement.

Effect of thermal stresses on mid-slab cracking in dowel jointed concrete pavements

Mid-slab cracking is considered to be one of the main causes of pavement deterioration and enormous funds are spent each year on repairing and maintaining cracked pavements. In this study, a nonlinear three-dimensional finite element (3DFE) analysis, which includes detailed consideration of slab constraints by dowel bars, is used to analyse the problem of premature transverse cracking in jointed concrete pavements. The 3DFE model response to ambient temperature variations is validated versus field-measured data obtained from instrumented concrete slabs constructed in Goshen Road near Morgantown, West Virginia, USA. The modelling results indicate that the combination of ambient temperature drop and slab curling induces slab constraints that lead to the development of mid-slab transverse cracks. The slab length is shown to be a critical parameter that governs the magnitude of the maximum thermal stress induced at maximum mid-slab. It is shown in this paper that 4.57 m is the optimal slab length to avoid mid-slab cracking, a conclusion that agrees with recent observations obtained from the analysis of LTPP (Long Term Pavement Performance).

Multi-Fiber Unit Cell for Prediction of Residual Stresses in Continuous Fiber Composites

The mechanical properties of Continuous Fiber Metal Matrix Composite (CFMMC) materials are often affected by the residual stresses that arise during their fabrication process as a result of the mismatch between the Coefficients of Thermal Expansion (CTE) of the fibers and matrix. Three-dimensional finite element Unit Cell Models (UCM) are commonly used to predict such residual stress fields. However, the boundary conditions chosen in the past for these models neglect the effects of interactions between neighboring fibers, which results in poor correlations with experimental results. Previous research shows that the UCM approach usually overestimates the residual stress levels by about 30%. In this paper, two new three-dimensional finite element Multi Fiber Models (MFM) are developed in which the boundary conditions are shifted away from the fiber-matrix interface, in order to account for the effects of neighboring fibers on the stress distribution over such interfaces. One model assumes a hexagonal packing pattern of the neighboring fibers around the fiber-matrix interface where the residual stresses are calculated, whereas the other assumes that the neighboring fibers are packed in a square pattern. The proposed models are examined for two different scenarios regarding the contact surface between the fiber and the matrix, one where there is no bond over the interface and the other where the interface is perfectly bonded. The residual stress predictions of the new MFM models are compared to those of conventional UCM models by using an Alumina/Titanium (Al 2 O 3 /Ti-6Al-4Va) material system to represent, as a test case, a typical CFMMC material. The results indicate that the residual stresses predicted by the MFM models correlate much better with published experimental results than those provided by the UCM models. The effects of the fiber volume fraction and the fiber-matrix bond integrity on the magnitude and distribution of residual stresses are examined for both hexagonal and square packing patterns of neighboring fibers. The analysis demonstrates that all these factors can influence significantly the field of residual stresses that develops in the matrix when the material is cooled-down from its processing temperature. If the fiber-volume fraction is assumed to increase, from 10% to 30% for example, the MFM models predict, as expected, a reduction in the magnitude of such residual stresses, which for the scenario of perfectly bonded interfaces can be as high as 49% for the square fiber packing pattern or 41% for the hexagonal pattern. The numerical results show that the hexagonal fiber-packing pattern predicts, in general, higher residual stresses than the square packing system, so that it should be recommended for design as the more conservative approach.