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OCR for page 66
66 12000 7000 0. 07 l/m 6000 10000 k-modulus (kN/m3) k-modulus (kN/m3) 0. 14 l/m 5000 8000 0. 28 l/m 4000 6000 3000 4000 0. 7 l/m 2000 0. 14 l/m 2000 1000 0. 28 l/m 0 0 1. 0E +0 1 1 .0 E+ 02 1. 0E+ 03 1. 0E +0 4 1 .0E+ 05 1.0E+0 6 1.0E+0 1 1 .0 E+ 02 1. 0E +0 3 1 .0E+04 1. 0E +0 5 1 .0E+06 G* /sin (kPa) G* /sin (kPa ) (a) Trackless (b) CRS-1 Figure 71. Relationship between k-modulus and G*/sin d. rate (see Figure 71). Therefore, it may be concluded that the in the test results were less than 15% for all conditions. As amount of tack coat material influences the ISS but not the presented in this section, test results were analyzed to investi- interface stiffness. The authors postulate that the interface stiff- gate the effects of the variables considered in the test factorial ness modulus may be mainly influenced by surface texture. on ISS. Since the focus of this experiment was on the effects of As shown in Figure 70, the ISS values did not exhibit much surface types and preparation methods, the effects of surface difference for a G*/sin d value below about 100 kPa (14.5 psi) cleanliness were presented in Experiment III. and 1000 kPa (145.03 psi) for trackless and CRS-1, respectively. At higher G*/sin d values, the difference in ISS between the 4.6.1Effects of Tack Coat Type three residual application rates became more pronounced. and Residual Application Rate Further, the trackless material produced greater ISS differ- ences than did CRS-1 at the same G*/sin d values. The rela- Figure 72 (a through d) presents the variation of the ISS tionship shown in Figure 70 between the ISS and G*/sin d with emulsified tack coat types and residual application rates may be used to establish a laboratory design threshold for this for the different surface types (i.e., old HMA surface, PCC sur- parameter in order to ensure that the selected residual appli- face, milled HMA surface, and new HMA). As previously men- cation rate and tack coat material would perform acceptably tioned, only one emulsion (SS-1h) was used on the new HMA in the field. However, setting this limit on G*/sin d would surface and two emulsions (SS-1h and SS-1) were applied on require field validation of tack coat performance and that the milled HMA surface. These results were obtained for clean the required ISS be greater than the predicted shear stress and dry samples with no confinement pressure at 25C. at the interface due to traffic and/or thermal loading. The As shown in Figure 72, all tack coat materials showed that variation of the limit on G*/sin d with surface texture and the ISS increased as the residual application rate increased surface type should also be investigated. The influence of sur- within the evaluated application-rate range (0.031 to 0.155 gal/ face texture on tack coat ISS has been investigated as part of yd2); hence, it was not possible to identify an optimum resid- NCHRP Project 9-40 and is presented in the next section. ual application rate. This may indicate that, under actual field Results implied a direct relationship between the roughness conditions, optimum residual application rates may be greater of the existing surface and the shear strength at the interface; than that commonly predicted from laboratory-based experi- therefore, a milled HMA surface would provide the greatest ments. However, while higher application rates may increase ISS, followed by PCC, old HMA, and new HMA. ISS, excessive tack coat may migrate into the HMA mat dur- ing compaction and service, causing a decrease in the air void content of the mix, and may even cause the appearance of fat 4.6Experiment V: Effects of spots on the HMA surface. One study reported that excess tack Pavement Surface Type and might be picked up by hauling trucks and paving equipment-- Sample Preparation Method causing safety concerns when tracked onto pavement mark- The mean ISSs along with their standard deviations and ings in traffic intersections close to the construction area (42). COVs were obtained for each condition considered in the test For old HMA and PCC surface types, the trackless tack factorial. Triplicate samples were tested for each test condi- coat exhibited the highest shear strength at the residual appli- tion defined by tack coat type, residual application rate, con- cation rates of 0.031 and 0.062 gal/yd2 for both old HMA fining pressure, dusty surface, and wet conditions. The COVs and PCC surfaces, and CRS-1 and SS-1 exhibited the lowest.

OCR for page 66
100 Interface Shear Bond Strength (psi) SS-1h CRS-1 90 80 Trackless PG 64-22 70 60 50 40 30 20 10 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Residual Application Rate (gsy) (a) 100 Interface Shear Bond Strength (psi) 90 Trackless SS-1h 80 PG 64-22 SS-1 70 60 50 40 30 20 10 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Residual Application Rate (gsy) (b) 100 Interface Shear Bond Strength (psi) 90 SS-1h 80 SS-1 70 60 50 40 30 20 10 0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 Residual Application Rate (gsy) (c) 100 Interface Shear Bond Strength (psi) 90 SS-1h 80 70 60 50 40 30 20 10 0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 Residual Application Rate (gsy) (d) Figure 72. Effects of residual application rates and tack coat types on ISS for (a) old HMA surface, (b) PCC surface, (c) milled HMA surface, and (d) new HMA surface.