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.
Interface Bonding between Hot-Mix Asphalt and various Portland Cement Concrete Surfaces
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. In this study, accelerated pavement testing was performed to quantify the effects of tack coat type, tack coat application rate, and PCC surface texture on the interface bonding and overlay performance. This study builds on and validates previously completed laboratory tests where an HMA overlay was placed on top of existing PCC pavement composed of various surface textures including smooth, transverse tining, and milling. 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; asphalt binder PG 64-22 was applied at 0.04 gal/yd2. In total, 25 test sections were constructed and loaded with the accelerated transportation loading assembly 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 study validated laboratory-determined optimum tack coat application rate, which provided the lowest interface strain and surface rutting in the field. Both PG 64-22 and SS-1hP showed better rutting resistance than RC-70. Milled PCC surface provided lower rutting than transverse-tined and smooth surfaces. The study also showed that PCC cleaning methods played an important role in the HMA–PCC interface bonding.
Influence of filler addition, storage medium and evaluation time on biaxial flexure strength and modulus of adhesive systems
Abstract Objective. This study evaluated the effects of filler addition, storage medium and time on biaxial flexural strength and flexural modulus of six adhesive systems. Materials and methods. The adhesives were either unfilled resins: Single Bond, Prime&Bond 2.1 and One-Step; or filled resins: Single Bond Plus, Prime&Bond NT and One-Step Plus. Resin discs of each product (0.5 mm thick × 6.0 mm diameter) were prepared using silicon molds (n = 10). The discs were stored dry, in water, mineral oil or ethanol for 1 week or 3 months before biaxial flexural testing. Data were statistically analyzed by 3-way ANOVA and Tukey's post-hoc test (α = 0.05). The specimens were tested in a universal testing machine at 1.27 mm/min until failure occurred. Results. In general, the storage in ethanol led to significantly lower flexural strength and flexural modulus values than the other storage conditions for all adhesives in both storage periods. Filler addition increased flexural strength and flexural modulus for Prime&Bond NT, when it was stored dry or in water for 1 week. For Single Bond Plus, the filler addition resulted in higher flexural modulus, when it was stored for 1 week in oil and in ethanol. No significant differences in flexural modulus were observed between One-Step and One-Step Plus in any storage medium. Conclusions. Results suggested that filler addition does not necessarily have to increase the flexural strength and flexural modulus. Ethanol and oil storages tended to yield opposite effects.