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.
Effect of Surface Characteristics on Bonding Properties of Bituminous Tack Coat
The evaluation of the influence of surface characteristics on the bonding properties at the interlayer is important to understand better how multi-layered pavements behave under traffic conditions. Three double-layered systems were prepared with dense, porous asphalt concrete and stone mastic asphalt mixtures. The direct shear test was used to measure the bond properties provided by tack coat, whereas the surface characteristics were measured by traditional techniques. Variables included surface type, curing time, curing temperature, emulsion type, and residual application rate. For a given value of applied normal stress, the interface between the dense-graded asphalt mixtures resulted in higher peak shear strength than that between the open- and gap-graded mixtures. An increase in mean texture depth and film thickness led to a decrease in the peak shear strength and the reaction tangential modulus. The surface characteristics were shown to play a key role in interlayer bonding. Curing time was an important factor, indicating that paving could be performed right after the emulsion was broken. Increasing the residual application rate increased shear strength up to a point, then shear strength decreased gradually. The use of tack coat increased the interface bond strength, and an optimum residual application rate existed, at which the shear resistance reached a maximum value. The bonding properties at elevated temperatures were more associated with the surface characteristics than at lower temperatures.
Remotely-Piloted Helicopter Citrus Yield Map Estimation
A remotely-piloted Gas Xcell mini-helicopter with GPS, wireless Ethernet, and a machinevision system was flown over orange trees to take overhead images. The individual tree images were processed to estimate the number of orange pixels in the images. The fruit yield of individualtrees at harvest a month later had an r-squared value of 0.373 with the number of orange pixels onthe irrigated tree images. While it was demonstrated that a remotely-piloted mini-helicopter could beused to acquire useful overhead images of citrus trees, substantial further research is needed toimprove the accuracy of estimates of individual tree yields or health.