James A Scherocman

Evaluation of Bond Strength of Tack Coat Materials in Field: Development of Pull-Off Test Device and Methodology

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.

Friction Testing of Tack Coat Surfaces

In response to litigation because of an accident case involving tack coat on a construction project, a field trial was evaluated to determine frictional characteristics of tack-coated surfaces. When a literature search on this topic returned little information, state Department of Transportation materials engineers were surveyed to determine the state of the practice with respect to tack coat operations. On the basis of the survey responses and the existing litigation, a field trial was conducted to evaluate friction numbers on tack coat materials. Variables included residual asphalt content (three levels); test time (three levels); combinations of wet, dry, and flushed surface conditions (seven levels); and replicate testing. It was found that at typical residual asphalt rates reported by states and specified in Louisiana, reduced friction capability existed for up to 7 h after application. With the friction numbers obtained, traffic should be maintained only at controlled low speeds if at all. However, the residual asphalt content appeared heavier than in typical practice. At residual application rates that were typical of practice, friction properties were produced that would allow traffic at moderate speeds. After several days, friction numbers returned to the original condition because of traffic or weather abrasion.

Determination of Moisture in Hot-Mix Asphalt and Relationship with Tender Mixture Behavior in the Laboratory

In recent years, a significant percentage of coarse-graded Superpave® asphalt mixtures have exhibited tender mix behavior. Although the underlying causes of the phenomenon are not well understood, many experts believe the main cause of the tender zone problem is related to the amount of fluids, particularly excess moisture, in the asphalt mixture. Unfortunately, some asphalt technologists consider the accurate determination of moisture in an asphalt mixture to be a difficult prospect. The purpose of the present research was to determine if the tender mix behavior exhibited by some coarse-graded Superpave mixtures is related to moisture content and to develop procedures to accurately measure the moisture content in an asphalt mixture. The research indicated that conventional oven drying (110°C) was an acceptable alternative to the standard distillation procedure (ASTM D1461). The use of the microwave oven procedure for determination of moisture content also appeared to be acceptable, although in the present research there was a significant problem with breakage of the glass containers. The best sampling container for determination of the moisture content after a storage period was a 4-L aluminum paint can. When the paint can was used and sealed, the moisture content after 72 h of storage at room temperature was statistically equal to the moisture content determined immediately after sampling of the asphalt mixture. Other containers studied (paper grocery bag and plastic oven bag) indicated lower moisture content values after storage and a consistent loss of moisture the longer the sample was stored.