Waleed Zeiada

Evaluation of Fiber-Reinforced Asphalt Mixtures Using Advanced Material Characterization Tests

The objective of this study was to evaluate the material properties of a conventional (control) and fiber-reinforced asphalt mixtures using advanced material characterization tests. The laboratory experimental program included triaxial shear strength, dynamic (complex) modulus, repeated load permanent deformation, fatigue, crack propagation, and indirect tensile strength tests. The data was used to compare the performance of the fiber-modified mixture to the control. The results showed that the fibers improved the mixture’s performance in several unique ways against the anticipated major pavement distresses: Permanent deformation, fatigue cracking, and thermal cracking.

Using Dynamic Modulus Test to Evaluate Moisture Susceptibility of Asphalt Mixtures

The stripping in of hot-mix asphalt (HMA) is assessed by AASHTO T283 by means of the indirect tensile strength test. The tensile strength ratio (TSR) is used as the criterion for strength retention after sample conditioning. In recent years, the dynamic modulus (E*) test, conducted according to AASHTO TP62-03, has gained wider use in the pavement community for two reasons: it is a major input into the Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures and is being used as a simple performance test indicator. The objective of this study was to assess whether the E* laboratory test can be used as a replacement test property for indirect tensile strength in AASHTO T283. Because the E* test is nondestructive, unlike the indirect tensile strength test, the advantage would be that the same specimens could be used before and after moisture conditioning. The scope of work in this research included conducting a laboratory testing program on several types of asphalt mixtures by means of both test procedures. All mixtures were obtained from construction projects in the field. A unique aspect of this study was that some of the mixtures failed in the field after stripping. The E* tests were used to determine the percent of retained stiffness, a term referred to as E* stiffness ratio (ESR). Results of both TSR and ESR conducted on the same mixtures were compared and statistically analyzed. The analysis indicated that there was no statistically significant difference between the measured TSR and ESR values for the same mixture. The correlation obtained between the two ratios had good measures of accuracy. It was concluded that the ESR can potentially replace TSR testing to assess field moisture damage for asphalt mixtures. The recommendation was to continue the testing program and expand the database for future analysis.

Comparison of conventional, polymer, and rubber asphalt mixtures using viscoelastic continuum damage model

In this study, a laboratory experimental programme was conducted to compare the material properties and fatigue performance characteristics for reference, polymer-modified and rubber-modified gap-graded mixtures. These mixtures were placed on E18 highway between the interchanges Järva Krog and Bergshamra in the Stockholm area of Sweden. The advanced material characterisation tests included dynamic (complex) modulus for stiffness evaluation and the uniaxial tension–compression for fatigue assessment. The data were used to compare the performance of the rubber-modified gap-graded mixture to the reference and polymer-modified gap mixtures using the viscoelastic continuum damage (VECD) approach. Different researchers have successfully applied the VECD model to describe the fatigue behaviour of asphalt concrete mixtures. The damage characteristic (C–S) curves were established for each of the three mixtures. The fatigue behaviour for the three mixtures was ranked based on the C–S curve results and the rubber-modified mixture showed the best fatigue damage resistance followed by the polymer-modified and reference mixtures. The VECD approach provides a more comprehensive analysis to evaluate fatigue resistance compared with the traditional fatigue evaluation using a number of cycles at a given stiffness reduction.

Significance of Confined Dynamic Modulus Laboratory Testing for Asphalt Concrete: Conventional, Gap-Graded, and Open-Graded Mixtures

The objective of this study was to evaluate the effect of confinement on the dynamic modulus E* values for different asphalt mixtures. Traditionally, only unconfined tests have been conducted, mainly driven by the simplicity of the test procedure and laboratory equipment needs. In addition, the moduli of the most widely used, dense-graded (conventional) asphalt mixtures are thought to be less influenced by confined testing. However, the increased use of gap- and open-graded asphalt mixtures necessitates the consideration of confined laboratory testing to obtain their true material properties. After five confinement levels were evaluated, a stress of 138 kPa (20 psi) was selected as a rational confinement level for confined E* laboratory testing, because an increase of confining stress beyond 138 kPa did not significantly increase the moduli of the mixtures. The scope of work included testing and analyses of 26 mixtures: four conventional, 12 gap-graded, and 10 open-graded mixtures. The AASHTO TP62-07 procedure was used to measure E* values of the asphalt mixtures in both unconfined and confined states of stresses. Results for the conventional mixtures showed that there was no significant effect of confinement at lower temperatures, but the confined moduli were higher at the highest test temperature. For the gap- and open-graded mixtures, confined test values were higher than unconfined test values at all test temperatures. Statistical hypothesis testing of the data confirmed these findings.

Effect of Air Voids and Asphalt Content on Fatigue Damage Using the Viscoelastic Continuum Damage Analysis

In this study, a laboratory experimental program was conducted to investigate the effect of asphalt content and air voids on the material properties and fatigue performance characteristics for asphalt concrete mixtures. Two levels of asphalt content (4.2 and 5.2%) and air voids (4.5 and 9.5%) were considered to produce four asphalt concrete mixtures combination. The advanced material characterization tests included: dynamic (complex) modulus for stiffness evaluation and the uniaxial tension-compression for fatigue assessment. The fatigue analysis was performed for each mixture using the simplified viscoelastic continuum damage (S-VECD) approach. The damage characteristic (C-S) curves were established for each of the four mixtures. In order to have more useful information about the fatigue resistance of the four mixtures, the C-S curves were used to obtain the fatigue relationships by performing simulated predictions of the fatigue life at specific conditions. It is found that the S-VECD simulations are able to reflect the observed material trends. Simulations performed with this model also suggest that the impact of air void and asphalt content changes differ between stress controlled and strain controlled loading. The quantification of these differences may have implications in both pavement and material analysis and design.

Refining Conditions of Fatigue Testing of Hot Mix Asphalt

The beam fatigue test of hot-mix asphalt (HMA) has been used for nearly a half century. However, several conflicting results have been recently reported. This study attempts to refine test conditions such as waveform type (haversine versus sinusoidal), incorporating rest periods between loading cycles, and the effect of rest period on the healing of the HMA to minimize (eliminate) gross errors in the data analysis of the fatigue test results. In the deflection-controlled haversine test [ASTM D7460, 2010, “Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,” Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1–14] permanent deformations lead to a new equilibrium neutral position of the beam and the force output follows a sinusoidal waveform. This tends to bend the beam in both directions similar to the deflection-controlled sinusoidal test. This would produce erroneous fatigue results since the test assumptions do not match the actual test conditions. In contrast, the deflection-controlled sinusoidal test [AASHTO T-321, 2012, “Standard Method of Test for Determining the Fatigue Life of Compacted Hot Mix Asphalt (HMA) Subjected to Repeated Flexural Bending,” Annual Book of AASHTO Standards, Vol. 32, Washington, DC, pp. T321-1–T321-11] is more consistent than the deflection-controlled haversine test [ASTM D7460, 2010, “Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,” Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1–14]. When tests, with and without rest periods, are compared for healing studies, it is even more important to use a deflection-controlled sinusoidal test in order to obtain a fair comparison and accurate healing results. Since neither the haversine waveform nor the sinusoidal waveform in the lab exactly simulates field conditions, it is important to use a sinusoidal waveform in order to obtain consistent results. It is recommended that ASTM changes the ASTM D-7460 designation and test procedure to require a deflection-controlled sinusoidal waveform instead of haversine. Implementing the recommended test conditions is a crucial step in studying the concept of HMA healing and; as a result, estimating the endurance limit which plays an important role in designing sustainable pavements.

Characterization of Microdamage Healing in Asphalt Concrete with a Smeared Continuum Damage Approach

The fatigue of asphalt concrete (AC) mixtures is a major source of distress in pavement structures. In most laboratory fatigue studies, testing is carried out by means of continuously repeating load cycles. However, under real traffic conditions, AC is subjected to a succession of load pulses as the traffic passes. Between these loads the material is not subjected to external forces and undergoes changes related to relaxation and healing, which together act to increase the overall fatigue life relative to what is observed under continuous loading. In the study reported in this paper, the effects of a rest period on the fatigue response in uniaxial, on-specimen, displacement-controlled loading was studied at 4.48C, 21.18C, and 388C. Loading consisted of a sinusoidal pulse–rest history with rest periods of 1, 5, or 10 s. Mixtures were used with the same aggregate structure and constituent materials but different asphalt and air void (AV) contents. It was found that the introduction of even short rest periods substantially improved the fatigue response of AC. A viscoelastic damage model for smeared healing was derived and characterized with the measured data. This model was compared with a damage-only continuum formulation and used to identify the impacts of healing, apart from any viscoelastic relaxation effects. It was found that greater AC and lower AV content yielded more favorable healing behaviors. The findings also suggested differences in the active healing mechanisms in pulse–rest and block–rest loading, typically used to study healing in AC. These differences are discussed.

Comparison of Fatigue Damage, Healing, and Endurance Limit with Beam and Uniaxial Fatigue Tests

The concept of an endurance limit assumes a strain value below which the net fatigue damage that occurs during a load cycle is zero. The fact that real traffic loads are separated by rest periods may allow for partial or full healing of the microcracks, which can affect this endurance limit. If the asphalt layer thickness is controlled to keep strains below the endurance limit, the fatigue life of the pavement can be extended considerably. In the study reported in this paper, it was hypothesized that the endurance limit in asphalt concrete developed from the interaction and balance of damage and healing during a load cycle. This hypothesis formed the basis of the testing and analysis program, which evaluated the effects of air voids, asphalt content, rest periods, and temperature on the endurance limit. Two types of fatigue tests were conducted: beam (flexural) and uniaxial. A regression model also was developed on the basis of the results of each test and used to obtain the endurance limit values. This paper compares fatigue damage, healing, and endurance limit results from the two tests under similar conditions. The comparison shows that the beam fatigue test yields less overall fatigue damage and less healing than the uniaxial fatigue test. Beam fatigue yields 8 to 14 times longer fatigue lives, while uniaxial fatigue yields higher healing (10.4 times for the only available case). Because damage and healing combined to govern the endurance limit, the two tests produced close values in which the overall uniaxial endurance limit values were 12% less than the beam fatigue endurance limit values.

Laboratory Validation of Healing-Based Fatigue Endurance Limit for Hot-Mix Asphalt

One of the main requirements of designing perpetual pavements is to determine the endurance limit of hot-mix asphalt (HMA). The endurance limit, as applied to HMA and flexible pavement design, is the strain or stress level below which the HMA would endure indefinite fatigue load repetitions and the pavement would not experience bottom-up fatigue cracking. The purpose of this study is to validate the endurance limit for HMA by using laboratory beam fatigue tests. A rational procedure was developed to determine the endurance limit of HMA attributable to healing that occurs during the rest periods between loading cycles. Relating healing to endurance limit makes this procedure unique compared with previous research projects that investigated these concepts separately. An extensive laboratory testing program was conducted according to AASHTO T 321-03 test procedure as a part of NCHRP Project 9-44A. Six factors that affected the fatigue response of HMA were evaluated: binder grade, binder content, air voids, test temperature, rest period, and applied strain. The endurance limit was determined when no accumulated damage occurred and indicated complete healing during the rest period after each load application. A threshold rest period of about 5 s for a load duration of 0.1 s—beyond which no more healing was gained—was obtained. HMA exhibits endurance limits ranging from 37 to 246 microstrains, depending on mixture properties and environmental conditions. The results of this study can be used to design perpetual pavements that can sustain a large number of loads if traffic volumes and vehicle weights are controlled.

Impact of asphalt concrete fatigue endurance limit definition on pavement performance prediction

Abstract Many well-constructed Hot Mix Asphalt pavements have been in service for 40 or more years without any evidence of fatigue cracking. This field experience suggests that there exists a strain level, known as the fatigue endurance limit (FEL), below which an asphalt concrete pavement will not exhibit fatigue cracks. Several studies have been conducted to define and verify this limit. Each of these methods is associated with certain assumptions regarding the nature of the FEL and heretofore a comprehensive comparison of each has not been made using a consistent set of mixtures. Likewise, the impact of any observed differences in FEL on the predicted pavement performance has not been made. This paper investigates and compares six different methods for identifying the FEL: NCHRP 9–44A approach, simplified viscoelastic continuum damage model, smeared-healing with continuum damage model, plateau value approach, pseudo-strain analysis method, and reduced cycles method. Each method is found to yield different values ranges from approximately 30–170 microstrains at 21.1 °C. The predicted FEL from each of the six methods are then used with the mechanistic empirical design algorithm to evaluate their effects on predicted pavement performance. Simulation outputs show different pavement performance and perpetual pavement structural design thicknesses from each of the methods. The study outcomes are expected to benefit future field verification research of FEL as it provides comprehensive analyses using six different methods. This future verification research may indicate the method that best represents actual perpetual pavement design and performance.