Mostafa A. Elseifi

Flexible pavement responses to different loading amplitudes considering layer interface condition and lateral shear forces

A three-dimensional (3D) finite element (FE) parametric study was conducted to quantify the viscoelastic pavement responses due to different tire configurations: dual and wide-base tires, at three temperatures (5, 25 and 40°C) and two speeds (8 and 72 km/h). Three factors affecting pavement responses were investigated: type of moving wheel loading amplitude (continuous, trapezoidal), interface layer condition (simple-friction and elastic-stick models) and lateral surface forces. It was found that the continuous loading amplitude, which has an asymmetric stress magnitude and considers the difference between the entrance and exit of the tire, can simulate pavement responses to moving wheel vehicular loading more accurately than the currently used trapezoidal loading amplitude. The elastic-stick model resulted in a sensible improvement for predicting pavement responses to dual tire, while the simple-friction model is more comparable to field measurements in the case of the wide-base tire. The shear force was found to positively improve the prediction of the calculated strain at the bottom of the wearing surface and to a lesser degree at the bottom of the hot mix asphalt (HMA) base layer. This study concludes that using continuous loading amplitude and non-uniform pressure distribution to simulate a moving wheel, surface shear forces and appropriate layer interface friction may significantly improve the capability of FE models to predict pavement response to vehicular loading. Results have been successfully validated against field measurements.

Validity of Asphalt Binder Film Thickness Concept in Hot-Mix Asphalt

Despite the possible benefits of implementing asphalt binder film thickness into current specifications to address durability problems, most of the related research has been theoretical and only a few attempts have been made to measure this property experimentally. The objective of this study was to investigate the concept of asphalt binder film thickness experimentally on the basis of measurements obtained by image analysis techniques, reflective light microscopy, and scanning electron microscopy. The results of the experimental program were used to gain insight into the concept of asphalt binder film thickness and its validity. Experimental results indicated that asphalt binder films coating large aggregates do not actually exist in hot-mix asphalt. Instead, what are referred to as asphalt binder films surrounding large aggregates are actually asphalt mastic films. These films are highly irregular in shape and have a thickness greater than 100 μm in the mixture considered in this study. The asphalt binder films in the mastic were observed at a thickness of 2 μm in the mixtures considered. However, these entities do not represent asphalt binder coatings around aggregates; they are only part of a blend with fine aggregates and mineral fillers. Microscopic analysis showed that air voids usually appear near the boundary between large aggregates and asphalt mastic.

Field and Theoretical Evaluation of Thermal Fatigue Cracking in Flexible Pavements

Thermal cracking in flexible pavement occurs when the tensile stress exceeds the tensile strength of hot-mix asphalt at a given temperature or when fluctuating stresses and strains caused by temperature variation lead to a buildup of irrecoverable deformations over time. The objective of this study was twofold: (a) to quantify the measured strain magnitude associated with thermal fatigue through field measurements and (b) to present a three-dimensional, finite element (FE) model that accurately simulated thermal fatigue in flexible pavement. Results of the experimental program indicated that pavement response to thermal loading was associated with a high strain range, reaching a maximum recorded value of 350 um/m. This finding confirms the hypothesis that the criticality of thermal fatigue arises from the high stress-strain level exhibited in each cycle rather than its frequency, which is usually the critical factor in load-associated fatigue cracking. Moreover, the developed FE model accurately simulated...

A Simplified Overlay Design Model against Reflective Cracking Utilizing Service Life Prediction

ABSTRACT Although it is the major mode of failure in rehabilitated pavement structures, reflection of cracking seldom has been considered in the overlay design process mainly due to its complexity. This paper presents the development of an overlay design procedure to predict the service life of rehabilitated flexible pavement structures against reflective cracking. A simple equation was derived based on three-dimensional (3D) finite element (FE) models and by utilizing linear elastic fracture mechanics (LEFM) principles. The FE models simulate a variety of rehabilitated cracked pavement structures. A detailed sensitivity analysis was performed to establish the accuracy of the FE models. Then, accurate simulation of the crack singularity was achieved by modeling several contour integral evaluations along the crack front. Both crack initiation and propagation phases were considered in the formulation. The crack initiation phase is described using a traditional fatigue law developed by the Belgium Road Research Center, and the crack propagation phase is described using Paris-Erdogan phenomenological law. Three contour lines were used around the crack front to calculate the path-independent J-integral. Calculations of the stress intensity factors based on the J-integral are presented. An example is presented to demonstrate the use of the developed design equation in a routine overlay design.

Quantification of Pavement Damage Caused by Dual and Wide-Base Tires

A study conducted in 2001 on the heavily instrumented Virginia Smart Road measured pavement responses to a new generation of single wide-base tire (445/50R22.5) and to dual tires (275/80R22.5). The new single wide-base tire has a wider tread and a greater load-carrying capacity than conventional wide-base tires. The potential fatigue damage resulting from different tire configurations was evaluated. After successful field testing, a finite element (FE) parametric study was conducted to investigate different failure mechanisms that were not evaluated in the field. In this study, dual tires and two new generations of wide-base tires (445/50R22.5 and 455/55R22.5) were evaluated. The main difference between the two generations of wide-base tires is that the 455/55R22.5 is wider than the 445/50R22.5; hence, it further reduces the contact stress at the pavement surface under the same nominal tire pressure. In the developed FE models, geometry and dimensions were selected to simulate accurately the axle configurations typically used in North America; actual tire tread sizes and applicable contact pressure for each tread were considered; laboratory-measured pavement material properties were incorporated; and models were calibrated and properly validated against stress and strain measurements obtained from the experimental program. Four failure mechanisms were considered: fatigue cracking, primary rutting, secondary rutting, and top-down cracking. Results indicated that the new generations of wide-base tire would cause the same or relatively greater pavement damage than conventional dual tires. Because overall truck weight is reduced by approximately 450 kg when wide-base tires are used, it is reasonable to implement the load limits currently applied to the dual-tire assembly on the 455/55R22.5 wide-base tire.

New Generation of Wide-Base Tires: Impact on Trucking Operations, Environment, and Pavements

Since the new generation of wide-base tires was introduced in 2000, researchers from a variety of disciplines have evaluated the impact of the new tires on trucking operations, road infrastructure, and the environment. This paper outlines the status of wide-base tire technology with respect to established benefits, concerns to be addressed, and the potential use of this product in trucking operations. The objective of this paper is to serve as a benchmark for researchers and truck fleet managers with respect to the use of this new technology during the gradual transition from dual tires to the new generation of wide-base tires. For trucking operations, the new generation of wide-base tires provides substantial benefits in fuel efficiency, hauling capacity, tire cost and repair, ride, comfort, and vehicle stability. The new generation of wide-base tires is also comparable to conventional dual-tire assemblies in truck operation and safety. Since the new generation of wide-base tires has only been on the market for a short period of time, the potential for recapping these tires is not well-documented. As for environmental impact, the new generation of wide-base tires provides substantial benefits in gas emission reduction, noise reduction, and tire recycling at the end of service life. With respect to pavement and for primary road applications, pavement damage similar to damage rendered by conventional dual tires is expected, given that the probability of fatigue cracking is usually low in these pavement classes.

Viscosity Determination of Hot-Poured Bituminous Sealants

Hot-poured bituminous sealants are typically selected on the basis of empirical standard tests such as penetration, resilience, flow, and bond to cement concrete briquettes (ASTM D3405). Yet there is no indication of the pertinence of these standard tests to predict field performance. To bridge the gap between sealant fundamental properties and field performance, performance-based guidelines for selection of hot-poured crack sealants are currently being developed. A procedure to measure sealant viscosity is proposed as part of that effort. Using a sealant with an appropriate consistency at the recommended installation temperature would provide a better crack filling and would ensure appropriate bond strength. Therefore, to ensure that sealant-crack wall adhesion is achieved and that the sealant penetrates hot-mix asphalt during installation, a testing procedure for bituminous-based crack sealant viscosity at installation temperature is suggested. This paper proposes use of a rotational viscometer to measu...

VISCOELASTIC MODELING OF STRAIGHT RUN AND MODIFIED BINDERS USING THE MATCHING FUNCTION APPROACH

Two models are proposed to describe the rheological behavior of straight run and modified binders in the linear viscoelastic region. These models characterize the absolute value of the complex shear modulus (| G *|) and the phase angle ( i ). They allow for the establishment of master curves based on measurements made at a limited number of loading times and temperatures. A matching function approach was used to develop the models, which were validated experimentally by characterizing the dynamic mechanical properties of polymer-modified and straight run binders at intermediate and high service temperatures. There was good agreement between the measured and predicted values for the complex shear modulus. The phase angle model describes unmodified binders with less than 5% error. Although the model does not simulate the plateau region observed for polymer-modified binders, the error in this case was less than 10%. The models were successfully used to estimate other viscoelastic functions such as the storage and loss shear moduli, and the relaxation spectrum.

Development of a viscosity specification for hot-poured bituminous sealants

Current crack sealant specifications focus on using simple empirical tests such as penetration, resilience, flow, and bonding to cement concrete briquettes (ASTM D 6690) [1] to measure the ability of the material to resist cohesive and adhesive failures. There is, however, no indication of the pertinence of these standard tests to predict the success of field installation and sealant performance. In an effort to bridge the gap between sealant fundamental properties and field performance, performance-based guidelines for the selection of hot-poured crack sealants are currently being developed. This paper proposes a new viscosity test procedure to help assess the propensity of sealants to wet the crack surface during installation. The proposed procedure calls for the use of a Brookfield rotational viscometer equipped with a modified spindle rod and an SC4-27 spindle at a speed of 60 r/min. Sealants are heated 20 min at the recommended installation temperature and the viscosity is measured after 30 s of spindle rotation in the hot sealant. These experimental conditions provide viscosities representative of sealant viscosity at shear rates during field application. The repeatability for within laboratory and between laboratories was found to be 5.4 and 17 %, respectively. This repeatability is comparable with the corresponding variability of the SuperPave viscosity test for asphalt binders.

Mechanism and modeling of transverse cracking development in continuously reinforced concrete pavement

The relationship between the initiation and spacing of transverse cracking in continuously reinforced concrete pavement (CRCP) and transverse steel reinforcement has been investigated. Field evaluations of CRCP test sections included surface condition inspection using digital video, ground penetrating radar (GPR) survey to determine the relative location of transverse bars with respect to transverse cracks, falling weight deflectometer (FWD) testing to evaluate the crack load transfer efficiency (LTE), and ground-truth coring. Field-testing results suggested that the mean crack spacing was identical to the design spacing of transverse steel bars. A three-dimensional finite element (FE) model was then developed to evaluate the mechanisms that contribute to the initiation of transverse cracking in CRCP. Results of the FE model indicated that two controlling mechanisms may contribute to the initiation of transverse cracking in CRCP: build-up of uniform compressive longitudinal stress at the pavement surface and tensile stress concentration in the vicinity of the transverse steel bars. In general, a close correlation appears to exist between the spacing of transverse cracking and the design spacing of transverse steel bars.