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OCR for page 69
69 100 Interface shear Bond Strength (psi) 90 80 70 60 50 40 Existing HMA 30 20 PCC 10 0 0.00 0.05 0.10 0.15 0.20 Residual Application Rate (gsy) (c) Figure 73. (Continued). tistically different sets are identified in these figures with ure, laboratory-prepared samples grossly overestimated the an asterisk above the bar. As shown in Figure 74a, statisti- ISS by a factor ranging from 2 to 10 when compared with cally significant sets, shown with an asterisk, are often cases field-extracted cores. In the laboratory, ISS decreased with where wet conditions provided greater ISS than dry condi- tack rate, whereas, in the field, ISS increased with tack rate. tions. This is probably due to unaccounted-for factors such A number of factors may cause this discrepancy, includ- as the presence of coarse aggregates at the surface (higher/ ing the difference in mixing and compaction methods and coarser texture), which increased the friction resistance of application method for the tack coat materials. Difference the interface at the selected coring locations. This indicates in compaction methods may result in differences in air void that, even in the presence of light rain, the placement tem- contents and distributions in the specimen, mix resistance perature of an HMA overlay will cause the water to evapo- to shear loading, and mix density. The most probable factor rate or infiltrate into the underlying layer with no practical appears to be the greater asphalt film thickness at the inter- consequence on the interface bond strength. For the PCC face of the new HMA and the smoother/flatter surface of the surface, SS-1h, SS-1, trackless, and PG 64-22 were evalu- freshly made specimens. ated (see Figure 74b). The use of PG 64-22 did not generate sufficient bond strength at 0.031 and 0.062 gal/yd2 under 4.7Experimental VI: Effects wet conditions, indicating possible negative effect of surface of Texture and Permeability wetness at low tack rates. On the other hand, for SS-1 and on Tack Coat Bond Strength trackless tack coats, surface wetness did not affect the ISS. Only SS-1h was evaluated for the milled surface in dry and The objective of this laboratory experiment was to evaluate wet conditions. The influence of surface wetness did not the effects of surface texture and permeability of the exist- follow a consistent trend (see Figure 74c). ing pavement on tack coat ISS. The details of the mixtures' design, surface texture and permeability measurements, and 4.6.4 Effects of Preparation Methods specimen fabrication were previously reported. The tack coat material used in this experiment was SS-1 emulsion. ISS tests Figure 75 and Table 27 present the measured ISS for labo- were conducted for open-graded friction course (OGFC), ratory-fabricated specimens. For SS-1h, AUT, and PG 64-22, SMA, and sand mixtures. Table 28 presents the mean ISSs it was found that the optimum rate--at which the greatest ISS along with their standard deviations and COVs for the three was achieved--is 0.062 gsy. For CRS-1, as the residual appli- mixtures evaluated. Figure 77 shows the variation of the ISS cation rate increased, the ISS value decreased. On the other with the residual application rate. hand, the trackless material showed continuous increase of For the SMA mixture, the peak ISS was observed at a resid- ISS from 0.031 to 0.155 gsy. ual application rate of 0.031 gsy. The ISS was lower at residual To assess the influence of sample preparation methods, application rate 0.155 gsy than that for the no-tack condition. Figure 76 compares the ISS of laboratory-fabricated sam- For the sand mixture, the peak ISS occurred at the no-tacked ples with that of field-extracted cores for tack coat SS-1h condition. For smooth interface conditions and a new sur- in the case of the new HMA surface. As shown in this fig- face (i.e., still coated with asphalt) such as the one simulated

OCR for page 69
70 100 SS-1h PG 64-22 Interface Shear Bond Strength (psi) 90 80 Wet and Clean 70 Dry and Clean 60 50 40 * 30 * * 20 10 0 0.031 0 .062 0.155 0 .031 0.062 0 .155 Residual Application Rate (gsy) (a) 100 Trackless SS-1h PG 64-22 SS-1 Interface Shear Bond Strength (psi) 90 80 70 * Dry and Clean 60 Wet and Clean 50 40 30 * * * 20 * 10 0 0.031 0 .062 0.155 0 .031 0.062 0 .155 0.031 0 .062 0.155 0 .031 0.062 0 .155 Residual Application Rate (gsy) (b) 100 Dry and Clean Interface Shear Bond Strength (psi) 90 Wet and Clean 80 70 SS-1h * 60 50 40 30 20 10 0 0.031 0.062 0.155 Residual Application Rate (gsy) (c) Figure 74. Effects of surface wetness on ISS for (a) old HMA, (b) PCC, and (c) milled surfaces.

OCR for page 69
71 150 SS-1h Trackless PG 64-22 140 CRS-1 AUT 130 120 110 ISS (psi) 100 90 80 70 60 50 0.000 0.050 0.100 0.150 0.200 Residual Application Rate (gsy) Figure 75. Effects of residual application rate on ISS for lab-compacted samples. Table 27. ISS test results for lab-compacted samples. SS-1h Trackless PG 64-22 CRS-1 AUT 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 0.031 0.062 0.155 90.6 93.2 83.0 117.9 125.4 129.1 76.4 98.8 82.7 67.0 65.1 60.1 93.5 112.3 106.8 ISS (psi) 92.5 97.3 84.6 119.3 123.0 131.2 105.2 99.8 85.2 70.4 69.3 62.4 99.2 115.4 109.4 100.1 107.6 87.9 122.6 124.5 131.7 105.6 101.9 88.2 73.1 69.8 66.0 111.6 119.5 115.4 Mean 94.4 99.4 85.1 119.9 124.3 130.7 95.7 100.2 85.4 70.2 68.1 62.8 101.4 115.7 110.5 S.D. 5.0 7.4 2.5 2.4 1.2 1.4 16.8 1.6 2.7 3.1 2.6 3.0 9.3 3.6 4.4 COV (%) 5.3 7.5 2.9 2.0 1.0 1.1 17.5 1.6 3.2 4.4 3.8 4.7 9.1 3.1 4.0 140 Laboratory-Prepared Interface Shear Strength (psi) 120 Field-Prepared 100 80 60 40 20 0 0 0.05 0.1 0.15 0.2 Residual Application Rate (gal/yd2) Figure 76. Effects of sample preparation methods on the ISS.