March 20, 2009

Dr. Louay Mohammad, CEE Professor and Director of the Engineering Materials Characterization Research Facility of LTRC, serves as the principal investigator on a project titled Optimization of Tack Coat for HMA Placement to determine the optimum application methods, equipment type and calibration procedures, application rates, and asphalt binder materials for the various uses of tack coats and to recommend revisions to relevant AASHTO methods and practices related to the Louisiana Transportation Research Center. The Co-Principal Investigator of this project is Joe Button, head of the Materials & Pavements Division of the Texas Transportation Institute at Texas A&M University.

Asphalt tack coat is a light application of asphalt, usually asphalt diluted with water. It is used to ensure a bond between the surface being paved and the overlying course. A tack coat provides necessary binding between pavement surface layers to make sure the layers of a pavement are critical to transfer radial tensile and shear stresses into the entire pavement structure. On the other hand, no bond or an insufficient bond decreases pavement bearing capacity and may cause slippage. Insufficient stress may accelerate fatigue cracking and lead to total pavement failure. Few guidelines are available for proper selection of tack coat material type, application rate, and placement.

To meet the objectives of this study, two mechanical test devices were developed during the project. The first device characterizes the quality of the bond strength of tack coat materials in tension in the field; while the second one measures the interface shear strength of cylindrical specimens.

Figure 1 shows a new test device, named the Louisiana Tack Coat Quality Tester (LTCQT), which was developed during this project. LTCQT was selected to evaluate the quality of the bond strength of tack coat materials in tension in the field. A user-friendly, menu-driven software and test procedure was also developed to determine reliability and the repeatability of this device in the field. Three emulsions, CRW-1, SS-1h, and trackless and an asphalt cement, PG 64-22, were selected as tack coat materials that were tested over a wide range of temperatures. The tack coat materials tested with LTCQT exhibited a maximum tensile strength, SMAX, at a distinct testing temperature, TOPT, Figure 2. Thus, the response of tack coat material in tension was characterized using SMAX at TOPT. Furthermore, there was a good correlation between the TOPT and the softening point of the tack coat materials evaluated. Therefore, it is recommended to conduct the LTCQT test at the tack coat softening point, which is a readily available property. 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 discriminate between the responses of the evaluated tack coats.

Figure 3 shows the direct shear device that was used to measure the interface shear strength of cylindrical specimens. The device is referred to as the Louisiana Interlayer Shear Strength Tester (LISST). The LISST device was designed such that it will fit into any universal testing machine. It has a nearly frictionless linear bearing to maintain vertical travel and can accommodate sensors that measure vertical and horizontal displacements. The device can also apply a constant normal load up to 689kPa, and accommodates a specimen with 100 or 150-mm diameters. The interface shear strength of emulsified tack coats under a wide range of testing conditions commonly encountered in field application was evaluated using LISST test device. Three types of emulsified tack coats, CRS-1, SS-1h, and Trackless, were considered at three application rates, 0.14, 0.28, and 0.70 l/m2. In addition, a “no tack coat” condition was included in the analysis. The effects of construction conditions such as wet (rainfall) and dusty condition were also evaluated. Laboratory direct shear tests were performed at 25°C. To simulate these test conditions, cores were extracted from a full-scale test site at the Pavement Research Facility of the Louisiana Transportation Research Center. This test site was designed and constructed using conventional tack coat application and paving equipment over an existing HMA pavement surface, Figure 4. Preliminary analysis of the results showed that the trackless tack coat produced the highest shear strength at the three application rates, and SS-1 and CRS-1 resulted in the medium and the lowest strength, respectively. he majority of the cases showed statistically significant difference between clean and dusty condition. owever, no significant difference was found between dry and we conditions.

Article excerpted from December 2008 Civil and Environmental Engineering Newsletter, Julie Mueller, 225-578-9170, jmueller@lsu.edu

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