The results presented in this paper are part of NCHRP Project 9-40 on the Optimization of Tack Coat for Hot-Mix Asphalt Placement. This paper presents the development of a new test device, the Louisiana Tack Coat Quality Tester (LTCQT), for evaluating the quality of the bond strength of tack coat in the field. LTCQT is a modification of the ATacker device. A test matrix was developed to evaluate the reliability and the repeatability of the LTCQT in the field. Three emulsified tack coats (CRS-1, SS-1h, and Trackless) and an asphalt cement (PG 64-22) were evaluated over a wide range of temperatures and at a residual application rate of 0.23 L/m2. Two key test parameters were determined to characterize the mechanical responses of tack coats: the optimum testing temperature and the maximum tensile strength test. Results indicated that the LTCQT can successfully be used in the field to measure the quality of the bond strength of tack coat and to distinguish between the responses of the evaluated tack coats. A good correlation was observed between the absolute viscosity of residual tack coat material and the tack coat tensile strength. This study shows that the softening point can be an adequate parameter to determine the optimum temperature for the tack coat pull-off test, and therefore pull-off testing at the softening point temperature of the residual binder material is recommended for field tack coat evaluation.

No abstract

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 408C) 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.

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.

No abstract

No abstract

The objective of this study was to quantify the effects of tack coat type, tack coat application rate, and surface type (i.e., hot-mix asphalt versus portland cement concrete) including milled versus unmilled surfaces on the interface shear strength based on full-scale test application. The variation of interface shear strength between field- and laboratory-prepared samples was also investigated. To achieve this objective, five types of tack coat materials were applied at three application rates on four types of surfaces at the Pavement Research Facility site of the Louisiana Transportation Research Center. Samples were cored from the constructed test lanes, and the interface shear strength was measured using the Louisiana Interface Shear Strength Tester. Results of this study showed that a direct relationship was observed between the roughness of the existing surface and the developed shear strength at the interface. A small amount of water seemed to negatively affect interface shear strength with PG 64-22 used as a tack coat material. However, the effect of surface wetness on interface shear strength was less evident for emulsion-based tack coat materials. Laboratory-prepared samples grossly overestimated the interface shear strength when compared with field-extracted cores. While a decreasing trend was observed in the laboratory, an increasing trend in the measured interface shear strength was observed in the field.

No abstract