BEST PRACTICES FOR IMPLEMENTATION OF TACK COAT : PART 1 , LABORATORY STUDY Prepared
Tack coat is a light layer of diluted asphalt that is applied to hot mix asphalt concrete (HMA) or Portland cement concrete (PCC) pavement surfaces to ensure good interface bonding between layers. Interface bonding is affected by several factors; including tack coat (type, application rate, curing time, application temperature, and asphalt residue content), pavement surface characteristics (asphalt content, aggregate type and gradation, and surface texture), and environmental conditions. This study evaluated interface bonding between two HMA layers by conducting a laboratory shear performance test. The main objective of this study was to evaluate the performance of various tack coats and determine the optimum residual application rates for three pavement surfaces: unmilled aged nontrafficked, milled aged, and unmilled aged trafficked HMA. The study also examined the influences of tack coat curing time, temperature, HMA type, and surface texture on the performance of tack coats. The study considered four tack coat materials: SS-1hp, high float emulsion (HFE), SS-1vh (very hard, no-track emulsion), and straight asphalt (PG 64-22). The tack coat was optimized at residual rates of 0.00, 0.02, 0.04, 0.06, and 0.08 gal/yd2 (0.00, 0.09, 0.18, 0.26, and 0.36 L/m2). Three curing times were considered: 0.25, 2, and 24 hr. Two overlay mixes (9.5-mm surface mix and 4.75-mm surface mix) were used. Prior to testing, the specimens were conditioned at four temperatures: 5°F, 41°F, 77°F, and 113°F (–15°C, 5°C, 25°C, and 45°C). The study found that the optimum tack coat residual rate was 0.04 gal/yd2 (0.18 L/m2) for trafficked and nontrafficked unmilled aged HMA surfaces, while the optimum residual rate for milled HMA was 0.06 gal/yd2 (0.26 L/m2). SS-1vh tack coat showed superior performance over the other tested tack coats. The optimum curing time was determined to be 2 hr. Milling the surface improved interface shear strength. The interface shear resistance was greater when the surface nominal maximum aggregate size (NMAS) increased from 4.75 mm to 9.5 mm. Increasing the temperature resulted in a reduction in shear strength.
Influence of compaction effectiveness on interlayer bonding of asphalt layers
The paper presents the results of research on the interlayer bonding of asphalt layers compacted in the laboratory and in the field. To measure the interlayer bonding at a temperature of 20°C the Leutner test apparatus was used and load was applied at a rate of 50 mm/minute. Three types of interlayer interfaces were tested: no tack coat and tack coats made of two types of the pure bitumen emulsions. Four compaction techniques were used in laboratory conditions: rolling with and without vibrations, gyratory compaction and compaction with slab compactor, including both hot-to-cold and hot-to-hot. The fields specimens cored from pavements differed in terms of relative compaction. It was found out that interlayer bond strength between the layers of asphalt dependeds strongly on the applied compaction technique, as well as the compaction effort. In some cases efficient compaction produced a maximum shear force without tack coat. The type of emulsion used as the tack coat was also found to be highly relevant to the interlayer bonding.
Statistical investigation of two different interlayer shear test methods
The performance of multi-layered pavement systems strongly depends on interlayer bonding. In order to evaluate this parameter, some countries have developed their own test method. However, different devices generally provide different results due to the test conditions. In this article, the statistical investigation of two interlayer shear test methods is shown considering different asphalt materials and different specimen preparation techniques. Both test methods provided comparable results showing the same ranking in interlayer shear stress for different test conditions and the same level of precision in terms of scatter of the results. Moreover, the repeatability limit was found to be dependent on the test method, the pavement material and the compaction technique. It is worth noting that this extensive experimental investigation paves the way for the incoming European Standardization process involving the two shear test methods.