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.
Quantification of Recycled Asphalt Pavement Blending and Tack Coat Performance to Develop Strategies to Improve Pavement Longevity
approved: _____________________________________________________________________ Erdem Coleri Decreases in pavement program funding levels have encouraged paving agencies to adopt higher levels of recycled materials into their asphalt mixes. In order to fully evaluate the effects of recycled asphalt pavement (RAP) in Hot Mix Asphalt (HMA) mixes, the level of blending between the RAP and virgin binders in the final asphalt mixture needs to be determined. Asphalt mixtures are designed with the assumption that full blending between RAP and virgin binders is attained. However, the actual percentage of binder blending is unknown. Since the binder of asphalt mixtures with RAP is a combination of both the virgin binder and the binder from the RAP material, increasing the total RAP percentage of the asphalt mixture will increase the amount of aged RAP binder in the final mix. As a result, the ductility of the final product will be reduced. However, in addition to the increased stiffness effect, lower blending percentages can also reduce the total binder content of the asphalt mixture and reduce cracking resistance. In this study, the level of blending between RAP and virgin binders at the asphalt mix level was quantified through semi-circular bend (SCB) testing. Solvent extraction and recovery was conducted on RAP to isolate the RAP binder. Isolated RAP binder was manually blended with virgin binder at blending levels of 0%, 50%, and 100% to be able to produce SCB samples with different levels of blending. To quantify the actual blending percentages for two RAP sources in Oregon, the SCB test results of these samples (0%, 50%, and 100% blending cases) were compared to laboratory-produced samples wherein RAP and virgin materials were mixed traditionally. It was concluded that an average blending of 56.5% is to be used for mixture design. Recommendations on the considerations of blending in asphalt mixture design were also provided. Asphalt emulsion, commonly known as tack coat, is the bituminous material applied between pavement lifts to provide an adequate bond between the two surfaces. This material is an interlayer membrane, which serves as a glue between pavement layers, allowing successive layers to adhere together and behave as a monolithic structure. This allows tensile strains at the bottom of the asphalt layer to be minimized, and reduces the chances of premature fatigue cracking and early pavement failure. Several engineered emulsions have been proposed to create a stronger bond compared to the commonly used CSS-1H tack coats of Oregon. This study quantifies the performance benefits of newly engineered emulsions from three different tack coat companies by using rheological and bond strength testing. The use of a simple set of rheological test procedures is proposed to predict the shear strength of the tack coat bond. Suggested rheological tests have the potential to lead to more robust methods of QA/QC to characterize tack coats at the binder level to forecast expected performance in the field. ©Copyright by Sunny Lewis June 6, 2018 All Rights Reserved Quantification of Recycled Asphalt Pavement Blending and Tack Coat Performance to Develop Strategies to Improve Pavement Longevity
GUIDELINES FOR USING PRIME AND TACK COATS
Prime and tack coats have a purpose in the pavement construction process, yet many times they are misused or eliminated during the project. While most of the time no harm appears to occur to the roadway, technical guidance is warranted to assure appropriate usage. The objective of this study was to produce a prime and tack coat guide publication developed for project development and field personnel to provide decision-making guidance on how to use, when to keep, and when to eliminate prime and tack coats. A literature search, which focused on handbooks and technical reports, was conducted to determine the applicability and benefits of prime and tack coat, prime and tack coat effectiveness, materials used and when and where they are used. Central Federal Lands Highway Division's (CFLHD's) current construction specifications were compared with best practices determined from the literature and a phone survey of current practice of state DOTs from the CFLHD region. Finally, a review of the potential harmful and positive environmental effects of the prime and tack coat process, including the various bituminous products used, was undertaken. Based on the information collected, a guideline for CFLHD project development and field personnel was developed. The guideline provides decision-making guidance on how to use, when to keep, and when to eliminate prime and tack coats.