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.
Material selections in asphalt pavement for wet-freeze climate zones: A review
Abstract The objectives of this review paper are to document current practices in wet-freeze climate zones and introduce emerging technologies that would be largely implemented in wet-freeze climate zones in the near future. A current practice is defined as a procedure that has been shown by research or experience to produce acceptable results and that is established or proposed as a standard suitable for widespread adaptation. An emerging technology is defined as a technique or a material that can improve the asphalt pavement performance, partly replace general materials, and contribute to more sustainable asphalt pavements. This review identified the criteria used to determine locations in the United States and two Canadian provinces with a similar wet-freeze climate. In addition, asphalt binders, asphalt additives/modifiers, asphalt mixture types, reclaimed asphalt mixtures, asphalt mixture test methods, asphalt mixture design methods, and deicers were reviewed. This review concluded with some recommendations in terms of using environmentally friendly materials and selecting cracking and rutting failure criteria to design asphalt mixtures with enhanced performance in wet-freeze climates.
Characterization of Pavement Layer Interfaces.
The aim of this research was to derive constitutive models for the interfaces of flexible pavement layers and to determine the effects of interface condition on pavement performance. A simple constitutive model was derived for the asphalt layer interfaces using data provided by laboratory direct shear tests at several normal load levels. In the model the shear displacement is proportional to the shear stress until the shear strength of the interface is reached. Field shear tests at several levels of normal load were used to study the bond between asphalt surface layers and granular or soil-cement bases. The interface constitutive models were integrated into the ABAQUS finite element model to prove the significant impact of the interface condition on pavement life.