Use of reclaimed asphalt pavements (RAP) and recycled asphalt shingles (RAS) in asphalt paving, although considered as sustainable, is a practice that agencies are reluctant to employ because of the unpredictability of asphalt mixes containing recycled materials. The asphalt binder in RAP/RAS is aged and stiffened, which reduces ductility of the pavement. Consequentially, a pavement can exhibit unsatisfactory fatigue performance and have the potential for early cracking failure. Although methods exist to counteract the brittle behavior of pavements containing RAP/RAS (namely binder-grade bumping, binder-grade dumping and high binder content), they are not accounted for in mechanistic-empirical (ME) pavement design. Additionally, the cost benefits of using RAP/RAS in pavements are not easily calculated. For these reasons, characterization of fatigue performance for asphalt pavements containing RAP/RAS in ME design software needs to be accomplished and a life-cycle cost analysis (LCCA) framework for pavements containing RAP/RAS needs to be developed so that agencies can make informed decisions about RAP/RAS use in asphalt mixtures. In this study, laboratory test results for asphalt mixtures with different combinations of RAP/RAS contents, binder contents, and binder types were used to calculate ME pavement model coefficients to perform forward calculations to determine pavement performance. Using predicted performance from ME models, LCCAs were conducted to determine the cost benefits of using binder-grade bumping/dumping and high binder content in Oregon asphalt mixtures. These strategies are expected to increase RAP/RAS use in asphalt mixtures, reduce life-cycle costs, improve the cracking performance and encourage widespread use of RAP/RAS asphalt mixtures.
Bonding created by the tack coat allows the pavement system to carry heavy truck loads as a monolithic structure and improves the structural integrity. In Oregon and throughout the U.S.A., CSS-1H is the most commonly used tack coat type. However, field observations have revealed that new engineered tack coats, although more expensive, outperform the conventional types in relation to shear resistance. In this study, the impact of these new engineered emulsions on in-situ bond performance was quantified by laboratory testing and numerical modeling. Bonding damage performance of all tack coats was experimentally determined by using direct shear tests. Full-scale moving truck load models were developed and calibrated using the load-displacement parameters obtained from the laboratory shear tests. The impact of adverse construction conditions, such as dust, rain, and tack coat coverage, on tack coat bond damage under heavy truck loads was determined. It was concluded that the presence of dust had relatively the lowest contribution to shear damage. Rain during construction had the highest impact on the damage behavior and tack coat application on a wet surface increases the potential for damage by 20.1%. A 50% coverage of tack coat during construction resulted in 12.8% higher damage levels compared with 100% tack coat coverage of the surface area. Moving load models for heavy trucks caused 2.44 times more bonding damage at the bonded interface compared with the damage created by smaller trucks (F450).
In light of the various quality assurance (QA) issues pertaining to tack coats that occur during construction, there is a need for a means of verifying interlayer bond quality in situ. Despite the immense use of tack coat as a constituent in paving, there are no construction specifications with provisions for the quantification of tack coat bond quality in laboratory or field settings. In this study, a construction QA process for tack coat bond performance was proposed. A novel field tack coat bond strength test device, TackBond, was developed and used for this purpose. The performance of engineered (new tack coat technologies that are tracking less) and conventional tack coats was also evaluated in the laboratory and the field using the developed TackBond test system. The TackBond device was improved in this study by adding features that render it more practical, portable, accurate, and better suited for a variety of pavement surface conditions. Engineered tack coat performance was compared with that of tack coats used conventionally on both milled and overlay surface types. The suitability of the TackBond Test device for capturing the true response of each tack coat was first evaluated by comparing results from TackBond laboratory tests with monotonic direct shear tests (DST) on laboratory-produced samples. Strong correlations between the two test types were achieved. Results of field and laboratory TackBond tests showed that the in situ QA control process developed in this study could be effectively used to improve the in situ tack coat bond performance.
The tack coat bond is known to affect the longevity of asphalt pavements. Proper interlayer bonding prevents successive pavement layers from acting independently of one another and creating non-uniform stress and strain profiles in the pavement structure. Poor bonding between pavement layers can result in various pavement failures such as slippage cracking, debonding, and early fatigue cracking, all of which contribute to a reduced pavement fatigue life. Tack coat application rate and uniformity (that can be achieved by uniform tack coat application and by avoiding/minimizing tracking) are two major factors that control the performance of the tack coat bonding and longevity of the pavement structure. In this study, a wireless scale system (OreTackRate) that can be controlled from a tablet computer was developed to measure tack coat application rate accuracy and uniformity. The developed wireless scale system was recommended to be implemented during construction to validate application rate accuracy and uniformity. In addition, a distributor truck certification process was developed and presented in this study. The developed scale system can also be used to determine whether the applied tack coat is cured at any time point during construction. Residual tack coat application rate can also be measured using OreTackRate during construction. Implementation of all these tests, procedures, and technologies is expected to improve the tack coat uniformity during construction and improve the overall longevity of the pavement structure.
The tack coat is a bonding agent applied between new and existing asphalt pavement layers. Roadway construction operations inherently introduce adverse conditions, such as dust on the pavement surface and nonuniform coverage (streaking), which can compromise tack coat bond quality. Climatic conditions such as heavy rainfall also create issues late in the construction season. Newly developed engineered tack coat emulsions, which utilize stiff asphalt binders and polymer modifiers, are purported to improve interlayer bonding characteristics and reduce the propensity of early fatigue cracking and pavement failure. This study evaluates engineered tack coats against conventionally used tack coats in a laboratory setting to identify their benefits when subjected to different pavement surface types, application rates, and commonly experienced adverse roadway construction conditions. Monotonic direct shear testing was employed to characterize interlayer bonding using two response parameters, interlayer shear strength and interlayer bond energy. This study advances the knowledge of engineered tack coat performance under real-world conditions and also employs a laboratory sample preparation methodology that is representative of pavement construction in the field.
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