Lutfi Raad

FIELD AGING EFFECTS ON FATIGUE OF ASPHALT CONCRETE AND ASPHALT-RUBBER CONCRETE

A study was undertaken to investigate the influence of field aging on the fatigue performance of asphalt concrete and asphalt-rubber concrete. Two California mixes were investigated: conventional asphalt concrete dense graded (CAC-DG) and asphalt-rubber hot-mix gap graded (ARHM-GG). Laboratory fatigue tests were conducted on beam specimens obtained from a 10-year-old pavement section in southern California. Both stiffness and fatigue were determined using controlled-strain fatigue beam tests performed at 22°C and -2°C. Results were compared with published data for the original (unaged) materials. Stiffness and fatigue data were also used in pavement analysis to assess the influence of aging on predicted fatigue performance. Results indicate that field aging reduced the beam fatigue resistance of CAC-DG and, to a lesser extent, ARHM-GG. Aging effects on beam fatigue life were more severe at -2°C than at 22°C. The influence of aging on predicted pavement fatigue life depends not only on the stiffness of the mix and its fatigue properties but also on the stiffness or layer moduli of the pavement components. For new pavement construction and overlaid pavement sections, longer fatigue life predictions were obtained for ARHM-GG than CAC-DG for both aged and unaged conditions. Aging of CAC-DG could be detrimental to pavement fatigue. In comparison, aging of ARHM-GG showed increased fatigue life performance.

Fatigue Behavior of Rubber-Modified Pavements

Over the last 18 years, a number of rubberized pavement projects have been built in Alaska. Initial laboratory and field investigations sponsored by the Alaska Department of Transportation and Public Facilities (AKDOT&PF) and conducted by Raad et al. indicated improved fatigue performance of the rubberized sections in comparison with conventional asphalt concrete pavements. The results of a follow-up investigation to develop design equations for rubberized pavements in Alaska are presented. Laboratory studies were conducted on field specimens using the flexural fatigue test in the controlled-displacement mode. Specifically, the rubberized mixes included asphalt-rubber concrete with AC-2.5 (wet-process) and PlusRide RUMAC with AC-5. Tests were performed for a range of temperatures varying between 22°C and –29°C. Fatigue relationships were developed in terms of repeated flexure strain, dynamic flexure stiffness of the mix, and repetitions to failure. Relationships for the dynamic flexure stiffness as a function of temperature were also developed. Dissipated energy associated with repeated flexure stress and strain was determined and used to assess the damage behavior of conventional and rubberized mixes. The proposed fatigue equations were used to compare the behavior of the rubberized mixes with conventional AC-5 mixes at 20°C and 0°C. Results of the analysis show that at 20°C, asphalt-rubber and AC-5 mixes exhibit essentially similar fatigue resistance, whereas PlusRide has the least fatigue life. However, at 0°C, the fatigue resistance of PlusRide and asphalt-rubber exceeds that of the conventional AC-5 mix. The fatigue equations were also used to compare the fatigue life of conventional and rubberized pavements for different surface layer temperatures and foundation support conditions.

An eigen-mode method in kinematic shakedown analysis

Abstract The kinematic shakedown theorem is formulated for some deformation processes as a kinematic extremum problem based on a polyhedral load domain and a deformation mode domain. An eigen-mode method is used to construct the deformation mode domain. Every kinematically admissible strain field within some time interval can be derived from the deformation domain and used in the proposed shakedown formulation to determine the safety load factor. Several simple shakedown problems are examined by using this proposed methodology. These numerical results are discussed and compared with available analytical and other numerical results.

Stability of Pavement Structures Under Long Term Repeated Loading

This paper addresses the stability of pavement structures under long term repeated loading using the shakedown theory. The accumulation of plastic strains in a given system may increase under repeated load applications, leading to incremental collapse, or plastic strains may cease to increase with time, resulting in a stable response or shakedown. An improved numerical method using finite element formulation coupled with an optimization technique is introduced. The method takes into consideration the variable elastic coefficients of both the coarse-grained and fine-grained layers in the pavement structure through the application of an extension of the classical shakedown theorem. The proposed method is used to illustrate the influence of layer properties on the shakedown behavior of general pavement systems.

THERMAL CRACKING MODELS FOR AC AND MODIFIED AC MIXES IN ALASKA

Low-temperature cracking is a major distress mode in Alaskan pavements because of the extreme temperature conditions—which range, in some instances, from about −50°C in winter to more than 40°C in summer. The use of asphalt modifiers in Alaskan pavements occurred over the past 15 years. These modifiers include Styrene-Butadiene-Styrene polymers, Styrene-Butadiene-Rubber polymers, ULTRAPAVE, and CRM [both the dry process (PlusRide) and the wet process]. Field observations and laboratory studies in Alaska and elsewhere indicate that the use of these modifiers would improve the low-temperature cracking resistance of pavements. The degree to which these modifiers provide beneficial effects for Alaskan pavements needs to be evaluated. The objectives of this research were (1) To characterize asphalt and polymer modified asphalt from a number of selected sites using Superpave PG grading system and to conduct thermal stress restrained specimen tests (TSRST) and Superpave IDT laboratory tests on field specimens; (2) To compare low-temperature cracking performance using field surveys; (3) To verify the applicability of the Superpave thermal cracking model (TCMODEL) and other available models for predicting low temperature cracking; and (4) To recommend guidelines for predicting minimum pavement temperatures in Alaska. Results of this study indicate, in general, significant improvement in low-temperature cracking resistance when polymer modifiers are used. Comparisons between predicted and observed low-temperature cracking using available crack propagation models, including Superpave TCMODEL, were poor. An improved regression model was developed using minimum air temperature, TSRST fracture temperature and strength, and pavement age to fit the observed field data for both conventional and polymer modified sections.

Workability and Performance of Polymer-modified Asphalt Aggregate Mixtures in Cold Regions

Polymer-modified asphalts have been used in cold regions for about 15 years to address problems with rutting, cracking and premature aging. However, due to the cold climate and remote locations construction problems are sometimes encountered. This paper deals with workability of polymer-modified mixes while assuring that the desired pavement performance is achieved. The construction problems arise with possible poor compatibility of the base asphalt and the polymer, the storage stability of the asphalt–polymer mixture and cold construction temperatures. These properties were tested for several polymer-modified asphalt combinations. A set of products that were compatible, storage stable and had improved temperature susceptibility were selected and further tested in asphalt-aggregate mixtures. A Georgia Wheel rutting test and the Thermal Stress Restrained Specimen Test were performed. A questionnaire study was also conducted to collect experiences and specifications in cold regions. Tests indicate that polymer-modified asphalts should always be the end result of an extensive product development program. The polymer modification improved the performance of all base asphalts in certain polymer–asphalt combinations. However, some otherwise acceptable binders smoked excessively when the temperature was elevated to the recommended mixing temperature. This issue warrants further investigation.

The Influence of Granular Base Characteristics on Upper Bound Shakedown of Pavement Structures

ABSTRACT In this paper a numerical methodology for determining the upper bound shakedown load is presented. This methodology is used to study the shakedown load of typical pavement sections with granular bases. Results obtained from a matrix of numerical runs are used to establish empirical correlations between shakedown limit (i.e. applied pressure on the surface of the pavement at shakedown) and other system properties. Correlations are also established for the critical response parameters of the pavement components at shakedown. These correlations are used in a sensitivity analysis to determine the relative influence of the base characteristics on shakedown limit. The effect of loss of base modulus and shear strength as a result of increased pore water pressure under saturated conditions of the base is also investigated.

Numerical Application of Shakedown Theory to Pavements with Anisotropic Layer Properties

A general numerical method for shakedown of pavements with anisotropic soil layers can be a useful development. The proposed method can be used to calculate rigorous bound solutions for soils whose cohesion varies with direction. To achieve this objective, the conventional isotropic Mohr-Coulomb yield criterion was generalized to include the effect of anisotropy, which is caused by the variation of cohesion with direction. The numerical formulation of the lower bound theorem using the modified anisotropic yield criterion was then developed. It was found that by using a suitable linear approximation of the yield surface, the application of the bound theorem led to a linear programming problem. The proposed numerical method was applied in the analysis of full-depth asphalt concrete pavements overlying clay subgrade. The analyses were conducted for different asphalt concrete temperatures and nonlinear stress-dependent subgrade properties. Results were compared for isotropic and anisotropic shear-strength parameters.

Performance of Rubberized Asphalt Mixes in Alaska

A study was conducted to compare the fatigue, thermal cracking, and permanent deformation resistance of several Alaskan crumb rubber–modified (CRM) asphalt mixtures with that of conventional mixes. Alaska Department of Transportation and Public Facilities–designated sites in Fairbanks and Anchorage were sampled to conduct flexural beam fatigue tests, thermal stress restrained specimen tests (TSRSTs) and Georgia loaded wheel tests (GLWTs) on both rubberized and conventional mixes. Condition surveys were also conducted at the sampled sites to assess the field performance of these mixes in terms of fatigue, rutting, and low-temperature cracking. Fatigue testing results combined with multilayer elastic analyses for typical Alaskan conditions indicated an enhanced fatigue resistance for the CRM mixes when compared with conventional mixes. However, field performance observations at both conventional and CRM sections indicated no signs of fatigue distress, suggesting similar field performance for both types of mixes. TSRST results indicated an improved thermal cracking resistance for the CRM mixes, especially when the wet process was used. These results were consistent with observed field thermal cracking performance. At a given site, the section with the largest transverse crack spacing had the mix with the coldest TSRST fracture temperature. Conventional mixes outperformed CRM mixes in resisting permanent deformation, in both the lab and the field. GLWT results indicated that CRM mixes deform more and at a faster rate than conventional mixes. This may be due to the dense-graded aggregate used in some of the highly modified CRM mixes, where aggregate-to-aggregate contact and interlock were absent.

Traction Performance of Transit and Paratransit Vehicles in Winter

The traction performance of transit and paratransit vehicles during the winter is an important factor in public transportation system operations. Vehicle traction forces are significantly reduced on snowy or icy surfaces, specifically during stopping, starting, cornering, and hill climbing. Reduced traction increases stopping distances and decreases controllability when a vehicle stops in an emergency situation. This study evaluated the traction performance of transit and paratransit vehicles on snowy and icy surfaces. Field tests were conducted in Fairbanks, Alaska, using three types of vehicles—a 41-passenger transit bus, a 32-passenger transit bus, and a 9-passenger paratransit vehicle. Each vehicle was tested for different combinations of tire types, including highway tires, snow tires, studded-siped tires, highway three-rib tires, all-season tires, and snow-siped tires. Tests of winter traction performance evaluated stopping distance, starting traction, hill climbing, cornering, and controllability. For similar tire combinations and surface conditions, the tested transit and paratransit vehicles had different traction performance. Results indicate that winter traction performance is significantly influenced by vehicle type, tire combination, and road surface (compacted snow or ice). Research findings and recommendations for tire combinations best suited for winter traction are presented.