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68 Trackless tack coat consists of a polymer-modified emulsion parallel to the direction of the grooves. This test arrange- with hard base asphalt cement. These results relate directly to ment should generate the lowest ISS, which is in the direc- the viscosity of the residual binders at the test temperature. tion of traffic and, therefore, is more conservative. Figure 73 The influence of tack coat type appears to increase with the (a through c) presents the variation of ISS with surface types increase in the residual application rate. Except for the milled and residual application rates. As shown in these figures and HMA surface, the no-tacked cores failed during extraction due to its high roughness, the milled HMA surface provided due to the poor bonding at the interface. This emphasizes the highest ISSs, followed by the PCC surface. In most cases, the importance of using a tack coat material at the interface the old HMA surface provided greater interface strength than to avoid poor bonding between the layers. To balance the did the new HMA surface. It is noted that differences are aforementioned factors, one should select a tack coat residual more pronounced at low and intermediate residual applica- application rate that would ensure that the ISS is greater than tion rates and less pronounced at high residual application the calculated shear stress at the interface due to traffic and rates. It is likely that the effects of microstructure features thermal loading. that contribute to the surface roughness or texture are less pronounced when they are filled with tack coat materials. 4.6.2 Effects of Surface Type 4.6.3 Effects of Surface Wetness SS-1h emulsified tack coat was evaluated on all four sur- face types. On the other hand, the trackless tack coat and PG The effects of surface wetness on the ISS were evalu- 64-22 asphalt binder were evaluated for two surface types: old ated for old HMA, PCC, and milled surfaces. Figure 74 HMA and grooved PCC surfaces. PCC samples were tested (a through c) presents the effects of surface wetness. Sta- 100 Interface shear Bond Strength (psi) 90 Milled HMA PCC 80 Existing HMA New HMA 70 60 50 40 30 20 10 0 0.00 0.05 0.10 0.15 0.20 Residual Application Rate (gsy) (a) 100 Interface shear Bond Strength (psi) 90 Existing HMA 80 PCC 70 60 50 40 30 20 10 0 0.00 0.05 0.10 0.15 0.20 Residual Application Rate (gsy) (b) Figure 73. Effects of surface types on ISS for (a) SS-1h tack coat (b) PG 64-22 and (c) trackless tack coat.