Joe P. Mahoney

Top-Down Cracking in Washington State Asphalt Concrete Wearing Courses

For years, pavement engineers within the Washington State Department of Transportation (WSDOT) have observed that longitudinal and fatigue (multiple-interconnected) cracks in their thicker asphalt concrete (AC) pavements appeared to crack from the top of the wearing course downward. Often, the cracks stop at the interface between the wearing course and the underlying bituminous layers (a depth of about 50 mm). Studies done elsewhere in the United States and internationally have reported similar results. The results of extensive coring that WSDOT routinely collects in its pavement rehabilitation process were compared in a study. WSDOT normally cores AC pavements to determine thickness for use in mechanistic-empirical design. In addition to coring for AC thickness, specific information noting surface-initiated (top-down) cracking with the crack depth or full-depth cracking was noted. WSDOT observed top-down cracking occurring in the thicker sections, with thinner sections cracking full depth. Top-down cracking generally started within 3 to 8 years of paving for pavement sections that were structurally adequate and were designed for adequate equivalent single-axle loads.

CONSTRUCTION-RELATED ASPHALT CONCRETE PAVEMENT TEMPERATURE AND DENSITY DIFFERENTIALS

The Washington State Department of Transportation (Washington State DOT) examined temperature differentials in hot-mix asphalt paving over four construction seasons. From those studies it was found that low-density areas can be caused by temperature differentials in the mat. The study summarized is based on an examination of 17 projects during the 2000 Washington State DOT paving season to determine density differentials in the mat with a “density profile.” A density profile is a series of density readings taken in a longitudinal direction over a 15-m (50-ft) section through a low-temperature area. From this collection of density readings, the density range (the difference between the maximum and the minimum readings) and the density drop (the difference between the average and the minimum readings) are determined. The density range and drop are used to determine if low-temperature areas result in inadequate compaction. The criteria set forth by the Washington State DOT included temperature differentials greater than or equal to 14°C (25°F), a maximum density range of 96 kg/m3 (6.0 lb/ft3), and a maximum density drop of 48 kg/m3 (3.0 lb/ft3). Evaluation of the density profiles showed that when the temperature differential exceeded 14°C (25°F), 89% of the density profiles failed to meet the density criteria, but only 19% failed to meet the density criteria when the temperature differential was less than 14°C (25°F). It was found that pavements that experienced large temperature differentials during placement produced substantial density differentials.

EVERFE: RIGID PAVEMENT THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS TOOL

The interactive and computational features of EverFE, a new rigid pavement three-dimensional (3D) finite element (FE) analysis tool, are presented. To date, the use of 3D FE analysis has been hampered by (a) the difficulty of model generation and result interpretation, (b) the inability of many programs to adequately model joint shear transfer due to aggregate interlock and dowel action, and (c) the large computational requirements of conventional solution techniques employed by available programs. The development of EverFE is motivated by the desire to make 3D FE analysis feasible for routine analysis of rigid pavements. The intuitive graphical user interface employed by EverFE, which greatly simplifies model generation and result interpretation, is demonstrated using a sample problem. A novel technique for modeling aggregate interlock joint shear transfer that rationally incorporates nonlinearities is developed and presented, as is a new method for modeling dowel joint shear transfer. The solution strategy employed by EverFE that allows realistic 3D models to be simulated on desktop computers is briefly described, and its performance is presented.

EXPERIMENTAL VERIFICATION OF RIGID PAVEMENT JOINT LOAD TRANSFER MODELING WITH EVERFE

The joint load transfer modeling capabilities of EverFE, a recently developed rigid pavement three-dimensional finite element analysis tool, are verified through comparisons with available experimental data. Dowel joint load transfer is examined via comparison of displacements predicted by EverFE with results from laboratory tests of two small-scale doweled pavement systems, and dowel looseness is shown to be a probable cause for experimentally observed differential joint displacements. Results of finite element analyses using EverFE’s nonlinear, two-phase aggregate interlock constitutive model are shown to agree well with available experimental data. A parametric study is performed that examines the effect of joint opening on aggregate inter-lock load transfer and illustrates the importance of considering nonlinearities in joint load transfer when predicting pavement response. Recommendations for future research on joint load transfer modeling are also discussed.

Calibration of NCHRP 1-37A Software for the Washington State Department of Transportation: Rigid Pavement Portion

A significant amount of Washington State Department of Transportation (WSDOT) portland cement concrete (PCC) pavement that was placed in the 1960s is nearing the end of its serviceable life and must soon be rehabilitated or replaced. Initial WSDOT estimates place the cost of the anticipated work at more than

Field Investigation into Effects of Vehicle Speed and Tire Pressure on Asphalt Concrete Pavement Strains

600 million. A tool to predict PCC pavement deterioration and ultimate failure is needed to prioritize rehabilitation and reconstruction efforts best. The software associated with NCHRP Project 1-37A was chosen as a promising tool worthy of assessment for this application. The urgency of the situation necessitated its use, despite the lack of formal calibration guidance, some software bugs, and isolated model inconsistencies. A procedure was developed and used to calibrate the rigid pavement portion of the NCHRP 1-37A software to data obtained from the Washington State Pavement Management System (WSPMS). Significant findings resulted: (a) the rigid pavement portion of the software was calibrated succ...

Construction-Related Temperature Differentials in Asphalt Concrete Pavement: Identification and Assessment

An asphalt concrete section on a test track in the PACCAR Technical Center in Mount Vernon, Washington, was fitted with strain gauges at the surface and in pavement cores and tested using an instrumented truck operated at different speeds and with different tire pressures. The field test results are presented. The results indicate that the effects of both vehicle speed and tire pressure-contact area on pavement strains are significant: increasing vehicle speed from 2.7 Km/hr (1.7 mi/hr) to 64 km/hr (40 mi/hr) caused a decrease of approximately 30 to 40 percent in longitudinal strains at the bottom of the asphalt concrete layer, which was 137 mm (5.4 in.) thick. The speed effect on transverse strains is lower, causing only a 15 to 30 percent decrease. Reducing tire pressure from 620 kPa (90 psi) to 214 kPa (30 psi) caused a decrease of approximately 20 to 45 percent in the horizontal strains at the bottom of the asphalt concrete layer. The pressure effect on surface strains was significantly lower, causing...

Asphalt Concrete Overlay Design Case Studies

Numerous Washington State Department of Transportation (WSDOT) paving projects have experienced a cyclic occurrence of premature failure of open-textured asphalt concrete (AC) pavement sections by fatigue cracking, raveling, or both, generally called “cyclic segregation” or “endof-load segregation.” This resulted in an initial study in which mat temperature differentials were observed during laydown. In turn, this led to the current study and the reported results. Pavement temperature differentials result from placement of a cooler portion of the hot-mix mass into the mat. This cooler mass generally constitutes the crust, which can develop during hot-mix transport from the mixing plant to the job site. Placement of this cooler hot mix can create pavement areas near cessation temperature that tend to resist proper compaction (they may also exhibit tearing or roughness or appear to be open textured). These areas were observed to have decreased densities and a higher percentage of air voids (higher air voids). Four 1998 WSDOT paving projects were examined to determine the existence and extent of mat temperature differentials and associated material characteristics. An infrared camera was used to identify cooler portions of the mat, which were then sampled along with normal-temperature pavement sections. Gradation and asphalt content analysis showed no significant aggregate segregation within the cooler areas. However, these cooler portions of the mat consistently showed higher air voids than the surrounding pavement. On the basis of numerous studies that have related AC deterioration and high air voids in a mix, it is known that the areas of a mat with higher air voids may experience premature failure compared with the time to failure of the mat as a whole.

Three-Dimensional Finite Element Analysis of Jointed Plain Concrete Pavement with EverFE2.2

During the late 1980s, the Washington State Department of Transportation (WSDOT), the University of Washington, and the Washington State Transportation Center developed a mechanistic-empirical flexible overlay design procedure. Following development, WSDOT implemented this overlay design procedure and has been evaluating flexible overlay projects for approximately the past 8 years. WSDOT rehabilitates about 100 projects each year; approximately 20 to 30 percent of the total projects are designed using the WSDOT overlay design procedure and the AASHTO overlay design procedure (using DARWin). These two procedures are discussed in general, and two case studies illustrate each of the overlay design procedures. Also included is the backcalculation of layer moduli from falling weight deflectometer data.

Online Tools for Hot-Mix Asphalt Monitoring

The features and concepts underlying EverFE2.2, a freely available three-dimensional finite element program for the analysis of jointed plain concrete pavements, are detailed. The functionality of EverFE has been greatly extended since its original release: multiple tied slab or shoulder units can be modeled, dowel misalignment or mislocation can be specified per dowel, nonlinear thermal or shrinkage gradients can be treated, and nonlinear horizontal shear stress transfer between the slabs and base can be simulated. Improvements have been made to the user interface, including easier load creation, user-specified mesh refinement, and expanded visualization capabilities. These new features are detailed, and the concepts behind the implementation of EverFE2.2 are explained. In addition, the results of two parametric studies are reported. The first study considers the effects of dowel locking and slab-base shear transfer and demonstrates that these factors can significantly affect the stresses in slabs subjected to both uniform shrinkage and thermal gradients. The second study examines transverse joint mislocation and dowel looseness on joint load transfer. As expected, joint load transfer is greatly reduced by dowel looseness. However, while transverse joint mislocation can significantly reduce peak dowel shears, it has relatively little effect on total load transferred across the joint for the models considered.