David Hernando

Performance evaluation of alternative polymer-modified asphalt binders

Styrene–butadiene–styrene (SBS) polymer-modified asphalt (PMA) binder is widely known to provide superior performance, particularly in terms of cracking, as compared to unmodified asphalt binder. In recent years, alternative PMA binders have been developed to meet PG 76-22 specifications and requirements from the multiple stress creep recovery (MSCR) test. However, existing tests/requirements may not be sufficient to ensure the quality of alternative PMA binders. This study evaluated seven types of alternative PMA binders using different tests, including existing Superpave PG binder tests, the MSCR test, and a newly developed binder fracture energy (BFE) test. Results showed that four alternative PMA binders can potentially have equivalent performance to SBS binder. Three deficient binders were identified in both the MCSR and the BFE tests. Superpave PG binder tests, however, distinguished only one as deficient. Compared to the MSCR test, which provides a qualitative assessment (pass/fail criterion), the BFE test brings the added advantage of providing a quantitative assessment of relative binder performance based on fracture energy density values. Findings appeared to indicate that the BFE test can be used as an effective tool for binder specification by state highway agencies.

Estimating the rainfall erosivity factor from monthly precipitation data in the Madrid Region (Spain)

Abstract The need for continuous recording rain gauges makes it difficult to determine the rainfall erosivity factor (Rfactor) of the Universal Soil Loss Equation in regions without good spatial and temporal data coverage. In particular, the R-factor is only known at 16 rain gauge stations in the Madrid Region (Spain). The objectives of this study were to identify a readily available estimate of the R-factor for the Madrid Region and to evaluate the effect of rainfall record length on estimate precision and accuracy. Five estimators based on monthly precipitation were considered: total annual rainfall (P), Fournier index (F), modified Fournier index (MFI), precipitation concentration index (PCI) and a regression equation provided by the Spanish Nature Conservation Institute (RICONA). Regression results from 8 calibration stations showed that MFI was the best estimator in terms of coefficient of determination and root mean squared error, closely followed by P. Analysis of the effect of record length indicated that little improvement was obtained for MFI and P over 5- year intervals. Finally, validation in 8 additional stations supported that the equation R = 1.05·MFI computed for a record length of 5 years provided a simple, precise and accurate estimate of the R-factor in the Madrid Region.

Cracking performance characterisation of asphalt mixtures containing reclaimed asphalt pavement with hybrid binder

This study investigated the effect of reclaimed asphalt pavement (RAP) on the rheological properties of a hybrid binder and the fracture properties and cracking performance of resultant mixtures. Fracture properties of the same mixtures containing up to 40% RAP with a standard polymer-modified asphalt (PMA) binder were used as benchmarks. RAP mixtures with hybrid binder exhibited lower resilient modulus, higher failure strain and higher fracture energy density than PMA RAP mixtures. Results indicated that rubber particles might not dissolve in the base binder; therefore, hybrid binder had more “free” soft binder to blend with the RAP whereas the more integrated polymer network present in PMA binder resulted in stiffer mixtures. Furthermore, PMA RAP mixtures exhibited lower damage accumulation rate than hybrid RAP mixtures. Eventually, hybrid and PMA RAP mixtures yielded comparable energy ratio values, which indicate both binder types can be interchangeably used in mixtures with up to 40% RAP for satisfactory cracking performance.

Development of a new methodology to effectively predict the fracture properties of RAP mixtures

The existing design method for reclaimed asphalt pavement (RAP) mixtures assumes virgin and RAP binders fully blend. However, full blending may not occur and the impact of partial blending on mixture cracking performance is still unclear. A previous study revealed that RAP gradation, which is not currently considered from the standpoint of binder blending, controls the distribution of RAP binder within a mixture and consequently affects cracking performance. The objective of this study was to develop a methodology that overcomes the uncertainty of blending and effectively predicts fracture properties of RAP mixtures. This methodology is based on the evaluation of the interstitial component (IC) of a mixture (i.e. the fine portion that governs cracking performance) by means of a direct tension test named interstitial component direct tension (ICDT) test. Two RAP sources and four RAP contents were considered. ICDT specimens were produced by blending the fine portion of RAP and virgin aggregate with virgin binder in the same way and corresponding proportions as in RAP mixtures. Binder and mixture fracture properties from the previous study were used for comparison. Mixture and IC exhibited almost similar reduction in fracture energy density with increasing RAP content, whereas fully blended binder exhibited a less pronounced reduction. This indicated that IC better simulated the actual blending that occurred in mixtures. Mixtures with coarsely graded RAP (less RAP content in the IC) exhibited better fracture properties; thus, the key to satisfactory cracking performance appeared to be minimising the amount of RAP in IC. Consequently, the stiffening effect of RAP on the fine portion that controls cracking performance should be directly evaluated, instead of placing focus on the fully blended binder or the whole mixture. The ICDT test was proven to be a valuable tool to predict fracture properties of RAP mixtures.

Fracture Tolerance of Asphalt Binder at Intermediate Temperatures

AbstractLoad-induced fatigue cracking is one of the primary distress modes in asphalt pavements at intermediate temperatures. The asphalt community is still searching for a reliable methodology to ...

Identification of Potential Location and Extent of Localized Interface Debonding in the Wheelpath of Asphalt Pavements

Debonding between asphalt layers is usually modeled as a global or “smeared” phenomenon across the entire lane. However, field evidence indicates debonding may be limited to a portion of the interface. The objective of this study was to determine the potential location and extent of localized interface debonding in asphalt pavements so that more realistic interface conditions and pavement responses can be employed in future performance predictions. A parametric study was conducted to locate stress states potentially conducive to interface debonding. Factors considered included asphalt concrete (AC) layer thickness, AC-to-base stiffness ratio, interface compliance, tire size, and traffic wander. The parametric study showed existence of a zone of high shear stress coupled with low confinement for a broad range of depths (1–3 in. below the surface) and extending to 2 in. from the tire edge. Given the drop in confinement immediately outside the tire edge and that shear stress magnitude in this zone was similar to shear strength values reported in the literature, it was concluded that the repetition of these critical stress state conditions can cause localized debonding of an interface located about 2 in. below the pavement surface. Existence of a potential zone of localized interface debonding around the edge of a tire can promote a debonded strip below the wheelpath, which is consistent with field observations. The width of the debonded strip can extend to 42 in. Future research efforts should examine the stress redistribution associated with the presence of a debonded strip below the wheelpath.

Localized Debonding as a Potential Mechanism for Near-Surface Cracking

Research on the potential link between debonding and cracking has primarily focused on changes in location and magnitude of maximum tensile stress/strain under the load center for various interface bonding conditions. Prior modeling efforts have evaluated smeared bonding conditions, which represented the entire interface as either bonded, partially bonded or debonded. However, field evidence indicates debonding is a local phenomenon that starts at a critical location and gradually progresses through a portion of the interface. A parametric study identified a critical zone of high shear stress coupled with low confinement where the onset of debonding is likely. This critical zone is located around the mid-depth of the asphalt layer and extends for nearly 5 cm from the edge of the tire. Localized debonding should be introduced at the identified critical zone to better evaluate stress redistribution resulting from interface failure. Localized debonding was shown to potentially lead to stress states of shear-induced tension in the portion of the interface near the tire edge, which can explain the initiation of near-surface longitudinal cracking.

Cracking mechanisms in asphalt mixtures

Results of recent experiments, advanced modeling efforts, and field performance evaluation were compared to fundamental assumptions associated with traditional fatigue and critical condition approaches. Findings revealed that the critical condition approach most accurately represents the actual mechanisms of pavement cracking. A top-down cracking (TDC) performance prediction model developed based on the hot mix asphalt-fracture mechanics (HMA-FM) was employed to illustrate the full potential of the critical condition approach. The work presented supports serious consideration of a paradigm shift from the traditional fatigue approach to the critical condition approach. Such a shift should enhance understanding of mechanisms and lead to more effective material specifications and pavement design systems.