Reynaldo Roque

Evaluation of Healing Property of Asphalt Mixtures

Although repeated traffic loading causes damage to accumulate in asphalt pavements, the damage heals during rest periods (time between traffic loadings). Consequently, this healing enhances the fatigue life of the pavement. A method was developed to determine the healing rate of asphalt mixtures in terms of recovered dissipated creep strain energy (DCSE) per unit time. The healing properties of four different asphalt mixtures were evaluated with this approach. The test procedure consists of a repeated loading test and periodic resilient modulus tests. A normalized healing rate in terms of DCSE/DCSEapplied was defined to evaluate the healing properties independent of the amount of damage incurred in the mixture. From the test results, it was determined that the healing rates of the asphalt mixtures tested increased dramatically above 10°C and were more affected by the aggregate structural characteristics (i.e., aggregate interlock, film thickness, voids in mineral aggregate) of the mixtures than by polymer...

Evaluation of Laboratory Measured Crack Growth Rate for Asphalt Mixtures

A clear understanding of the cracking mechanisms of asphalt mixtures is needed to identify the most rational approach to analyzing its cracking behavior. Asphalt mixture properties were determined for eight asphalt mixtures of known cracking performance. Crack growth rates were measured in the laboratory for each mixture using the method developed by Roque et al. The crack growth rates were evaluated to determine (a) the validity of the test results, (b) the relationship if any between laboratory crack growth rates and field performance, (c) the relationship between the measured crack growth rates and other mixture properties, and (d) the validity of Paris law of crack propagation for evaluating asphalt mixture performance in the field. Through this evaluation, it was found that Paris law does not appear to incorporate all aspects involved in the mechanism of cracking of asphalt mixtures subjected to generalized loading conditions, such as those encountered in pavements in the field. Therefore, a concept appears to be warranted involving the use of fracture energy as a failure criterion for the initiation and propagation of cracks. This concept may explain both laboratory-measured crack growth rates and field cracking performance.

Effect of styrene butadiene styrene modifier on cracking resistance of asphalt mixture

A laboratory investigation was conducted to evaluate the effects of styrene butadiene styrene (SBS) modification on the cracking resistance and healing characteristics of coarse-graded Superpave® mixtures. Four types of asphalt mixtures with 6.1% and 7.2% design asphalt contents using unmodified and SBS-modified asphalt cement were produced in the laboratory. Tests performed with the Superpave indirect tensile (IDT) test included repeated-load fracture and healing test, strength tests at two loading rates, and longer-term creep tests to failure. The test results showed that the benefit of SBS modifiers to mixture cracking resistance appeared to be primarily derived from a reduced rate of micro-damage accumulation. The reduced rate of damage accumulation was reflected in a lower m value without a reduction in fracture limit or healing rates. It was shown that the benefits of the SBS modifier were clearly identified by using the hot-mix asphalt fracture model, which accounts for the combined effects of m value and fracture energy limit on cracking resistance. It was also determined that the residual dissipated energy as determined from Superpave IDT strength tests appears to be uniquely associated with the presence and benefit of SBS modification and may provide a quick way to make relative comparisons of cracking performance. Longer-term creep test showed that time to crack initiation appeared to provide another parameter uniquely related to the effects of SBS modification. The key to characterizing the effects of SBS modifier on the cracking resistance of asphalt mixture is in the evaluation of the combined effects of creep and failure limits.

Automated procedure for generation of creep compliance master curve for asphalt mixtures

For the design of asphaltic paving mixtures under heavy traffic loading, the Superpave system specifies use of performance-based mixture tests and prediction models to supplement volumetric mix design procedures. Central to the mechanics-based thermal cracking model used in Superpave is the prediction of thermally induced stresses based on a master curve and shift factor concept. The original version of Superpave had procedures for automated construction of the mixture creep compliance master curve from measured mixture properties. However, recent studies have indicated the need for several new modeling techniques, the development of which has resulted in the need for substantially more sophisticated procedures for automated construction of the master curve. This paper details the development of a computer program called MASTER, which automates master curve construction using built-in logic capabilities designed to handle the wide variety of measured responses encountered in practice. MASTER was found to agree closely with manually determined shift factors for 36 field mixtures investigated. The program was also found to be extremely robust, producing rational shift factors even when used to analyze complicated, thermally damaged materials. On the basis of these findings, the procedures developed here are recommended for inclusion in future revisions of the Superpave performance modeling software.

Predicting Viscoelastic Response and Crack Growth in Asphalt Mixtures with the Boundary Element Method

It has long been accepted that cracking of hot-mix asphalt pavements is a major mode of premature failure. Many state agencies have verified that pavement cracking occurred not only in fatigue cracking, in which a crack initiates from the bottom of the asphalt layer, but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. To improve current pavement designs and the cracking resistance of mixtures, it is necessary to understand the mechanisms associated with crack initiation and crack growth in hot-mix asphalt mixtures. However, the complexity of the problem and the lack of simple-to-use analysis tools have been obstacles to a better understanding of hot-mix asphalt fracture mechanics and the development of better hot-mix asphalt fracture models. Until today, the well-known finite element method has been the primary tool used for modeling cracks and their effects in mixtures and pavements. Unfortunately, it is both complex and numerically intensive for fracture mechanics applications. The displacement discontinuity boundary element method is presented, which is a numerical method that has been very successful in many other engineering fields, as a potential method for modeling cracking in hot-mix asphalt mixtures and pavements. A series of examples are provided to illustrate the effectiveness of the method in dealing with cracks, crack propagation, and visco-elasticity in hot-mix asphalt. It was concluded that the method was easy to use, resulted in accurate solutions, required minimal computation time, and significantly simplified the modeling of crack-related problems.

DEVELOPMENT OF EFFICIENT CRACK GROWTH SIMULATOR BASED ON HOT-MIX ASPHALT FRACTURE MECHANICS

It has long been accepted that cracking of hot-mix asphalt (HMA) pavements is a major mode of premature failure. Many state departments of transportation have verified that pavement cracking occurred not only in fatigue cracking in which a crack initiates from the bottom of the asphalt layer but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. Recent work at the University of Florida has led to the development of a crack growth law based on viscoelastic fracture mechanics that is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The model requires the determination of only four fundamental mixture parameters, which can be obtained from less than 1 h of testing using the Superpave® indirect tensile test (IDT). These parameters can account for microdamage, crack propagation, and healing for stated loading conditions, temperatures, and rest periods. The generalization of the HMA crack growth law needed for its successful implementation into a displacement discontinuity boundary element method is described. The resulting HMA boundary element approach is shown to predict the crack propagation of two coarse-graded mixtures under cyclic IDT loading conditions.

Multilayer Boundary-Element Method for Evaluating Top-Down Cracking in Hot-Mix Asphalt Pavements

Cracking in hot-mix asphalt (HMA) pavements is a major mode of premature failure. Recent work at the University of Florida has led to the development of a new viscoelastic fracture mechanics-based crackgrowth law called the HMA fracture mechanics law, which is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The successful simulations of crack growth for generalized pavement conditions depend largely on how well the state of stress can be predicted in and around existing cracks in pavements. Previous work has focused on the adaptation of a displacement-discontinuity boundary-element method for predicting stresses in the Superpave® indirect tensile test (IDT), which then were subsequently used to predict the crack initiation and crack growth in simulated IDT tests that used HMA fracture mechanics. The previous displacement-discontinuity boundary-element formulation is here extended into layered materials. Homogeneous layers are stitched together numerically in welded contact. The ability of the new numerical formulation to model the effects of temperature-induced stiffness gradients on tensile stresses at the top of two cracked pavement sections in Florida is demonstrated. These pavement sections were modeled with and without temperature-induced stiffness gradients. The introduction of stiffness gradients into the HMA layer is shown to increase the magnitude of tensile stresses at the top of the pavement, which is consistent with previous observations.

Use of Two-dimensional Finite Element Analysis to Represent Bending Response of Asphalt Pavement Structures

Use of the finite element method (FEM) and other advanced analysis techniques is suitable for evaluation of pavement response and performance. Three-dimensional FEM analysis provides most accurate representation of pavements; however, it is costly, particularly for predictions that involve continuously changing structures and thousands of load applications of varying magnitudes and positions. Axisymmetric and two-dimensional analyses provide simpler, more cost-effective solutions at the expense of accuracy. This paper describes a study undertaken to evaluate discrepancies between two- and three-dimensional analysis of asphalt pavements, and to determine whether a modified two-dimensional analysis could be used as a reasonable approximation of three-dimensional bending response. Discrepancies between two- and three-dimensional analyses were found to be dependent upon the pavement structure. An approach was developed to use pavement structural characteristics to define an adjustment factor which could be applied to the loading in the two-dimensional case such that two-dimensional analyses would reasonably estimate the critical tensile pavement stresses as computed from three-dimensional analyses. The benefits and limitations of the approach are discussed, and an example of its use in evaluating crack growth is illustrated.