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|Project #||FHWA #||Title||Authors||Report Completion Date|
|Show Abstract||R55||FHWA-ICT-09-035||Tack Coat Optimization for HMA Overlays: Accelerated Pavement Test Report||I.L. Al-Qadi, S.H. Carpenter, Z. Leng, H. Ozer, J.S. Trepanier||2/1/2009|
|Interface bonding between hot-mix asphalt (HMA) overlays and Portland cement concrete (PCC) pavements is one of the most significant factors affecting overlay service life. This study was performed to quantify the effects of HMA type, tack coat type, tack coat application rate, and PCC surface texture on the interface bonding and overlay performance through laboratory testing, accelerated pavement testing (APT), and numerical modeling. This report presents the outcomes of the APT, as well as the numerical modeling of pavement interfaces. The results of the laboratory testing are covered in a companion report. The APT built on and validated previously completed laboratory tests where an HMA overlay was placed on top of an existing PCC pavement having various surface textures including smooth, transverse tined, longitudinal tined, and milled. In addition, zebra sections were included to evaluate the effect of non-uniform tack coat application. Asphalt emulsion SS-1hP and cutback asphalt RC-70 were applied at three residual application rates, 0.02, 0.04, and 0.09 gal/yd2 (0.09, 0.18, and 0.41 L/m2); asphalt binder PG64-22 was applied at 0.04 gal/yd2 (0.41 L/m2). Two HMA designs, standard binder mix and moisture sensitive binder mix, were used along with the three tack coats. Twenty-five pavement test sections were constructed and loaded with the Accelerated Transportation Loading ASsembly (ATLAS) at the centerline. The tensile strains at the bottom of HMA, to quantify potential interface slippage, were measured for selective sections, and primary HMA rutting was analyzed for all sections. The APT results validated laboratory determined optimum tack coat application rate, which provided the lowest interface strain and surface rutting in the field. Both PG64-22 and SS-1hP showed better rutting resistance than RC-70. Milled PCC surface provided lower rutting than transverse-tined and smooth surfaces. The field testing also showed that PCC cleaning methods play an important role in the HMA-PCC bonding. Higher rutting depth was measured in sections with uneven tack coat distribution compared to that with uniform tack coat distribution. Field testing results don’t show enough evidence to prove that using a moisture-sensitive mix will result in higher HMA surface rutting.|
|Show Abstract||R55||FHWA-ICT-08-023||Tack Coat Optimization for HMA Overlays: Laboratory Testing||I.L. Al-Qadi, S.H. Carpenter, Z. Leng, H. Ozer, J.S. Trepanier||9/1/2008|
|Interface bonding between hot-mix asphalt (HMA) overlays and Portland cement concrete (PCC) pavements can be one of the most significant factors affecting overlay service life. Various factors may affect the bonding condition at the interface, including HMA material, tack coat material, tack coat application rate, PCC surface texture, temperature, and moisture conditions. The objective of this study is to quantify the impact of these parameters on the permanent deformation of the HMA overlay. This study includes three major components to achieve the objective: laboratory testing, numerical modeling, and accelerated pavement testing. This report presents and analyzes the laboratory testing results. A direct shear test device was built and utilized to investigate the characteristics of the HMA-PCC interface and to determine the interface shear strength in the lab. Tests were run in monotonic mode at a constant loading rate of 0.47 in/min (12 mm/min). Test specimens were prepared using field PCC cores, laboratory prepared HMA, and tack coat materials provided by the supplier. Parameters affecting the interface performance that were evaluated include HMA material type (SM-9.5 surface mix and IM-19.5A binder mix), tack coat type (SS-1h and SS-1hP emulsions, and RC-70 cutback), tack coat application rate, PCC surface texture, temperature, and moisture conditions. Test results showed that the asphalt emulsions SS-1h and SS-1hP produced greater interface bonding strength than the cutback asphalt RC-70. The SM-9.5 surface mix was found to have better interface strength than the IM-19.0A binder mix. The HMA tested produced the same trend of interface shear strength with tack coat application rate for various tack coat types. The optimum residual tack coat application rate for the SS-1hP emulsion using IM-19.0A binder mix was 0.04 gal/yd2 (0.18 L/m2) in the lab. The direction of tining on the PCC produced no effect on interface shear strength at 20 oC. However, the milled concrete surface provided greater interface shear strength than both tined and smooth PCC surfaces for the same tack coat application rate. At the optimum tack coat application rate, the smooth PCC surface produced higher interface shear strength than the tined surface. As temperature increased, interface bonding strength decreased. Moisture conditioning significantly decreased the interface shear strength. This reduction was more pronounced when a stripping-vulnerable binder mix IM-19.0B was used.|