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53 1.0E+06 4.3.3Measurements of Tack Coat Bond Trackless Strength at the Softening Point Complex Modulus, G* (kPa) 1.0E+05 CRS-1 1.0E+04 Additional LTCQT tests were conducted in the field to eval- y = 430032e-0 .159 x uate the repeatability of the ultimate tensile load, Pult, of the R = 0.99 1.0E+03 four tack coat materials (CRS-1, SS-1h, trackless, PG 64-22) at 1.0E+02 the softening point. For each tack coat material, at least three y = 30875e-0 .1 68 x LTCQT tests were performed. Table 17 presents the mea- R = 0.99 1.0E+01 sured tensile strength at the softening point for the four tack 1.0E+00 coat materials. Test temperature was controlled within 5C -10 0 10 20 30 40 50 60 70 from the material softening point. Test results show that PG Temperature (C) 64-22 and CRS-1 had the highest and lowest tensile strengths Figure 56. Relationship between complex modulus (or ultimate tensile loads), respectively. Tensile strengths for (G*) for unaged residues of trackless and CRS-1 both SS-1h and trackless were similar, and they were ranked and temperature. between those of PG 64-22 and CRS-1. Figure 58 presents the ultimate tensile loads for the four tack coat materials. The ranking of tensile strength is in good agreement with those measurements, the relationship between tack coat bonding presented in Table 17; therefore, it may be concluded that characteristics and the rheology of the material was estab- conducting the tack coat pull-off test at the softening point lished. Figure 57b shows the relationship between the tensile can successfully and consistently evaluate the quality of tack strength and the corresponding absolute viscosity, both at coat application in the field. Following the recommended 60C, for each tack coat material (i.e., residual from emul- testing procedure, the LTCQT has shown acceptable repeat- sion). As expected, the increase in viscosity (i.e., resistance to flow) is associated with an increase in tensile strength. ability for all of the tested tack coat materials. For all four tack Figure 57a presents the relationship between the optimum materials tested, the repeatability of the results was reasonable temperature (TOPT), at which SMAX occurs, and the corre- with an average coefficient of variation less than 11%. sponding softening point for each tack coat material. At the softening point, an applied tack coat is in a rheological state 4.4Experiment III: Development that provides sufficient adhesion to the LTCQT loading plate of a Laboratory Test Procedure for tensile testing. As the temperature is increased, tack coat to Measure the Interface consistency is not sufficient to provide full adhesion in the Bond Strength LTCQT loading plate. Based on these results, it is recom- mended to conduct the LTCQT test at the tack coat material A direct shear device was developed for the character- softening point, which is a property that is readily available ization of ISS of cylindrical specimens (see Figure 59). The and can be easily specified. device, which was developed through an iterative process, is 80 1.E+06 T OPT = 0.55 x (Softening Point) + 19.21 Absolute Viscosity at 60 C (poisses) Trackless 2 R = 0.89 Temperature (TOPT) at SMAX (C) 70 1.E+05 Trackless 60 2 R = 0.46 SS-1h SS-1h 1.E+04 a 50 PG 64-22 PG 64-22 1.E+03 CRS-1 40 CRS-1 30 1.E+02 30 40 50 60 70 80 0 5 10 15 20 Softening Point (C) Tensile Strength at 60 C (kPa) (a) (b) Figure 57. Relationship between absolute viscosity and softening point and the optimum test temperature.

OCR for page 53
54 Table 17. Tensile strength at softening point for four tack coat materials. Softening Standard Material Ultimate Tensile Load Tensile Strength Mean COV Point Deviation Type PULT, (lb) SULT, (psi) (PULT /SULT) (%) (C) (PULT /SULT) 30.9 1.6 43.8 2.2 CRS-1 42.5 35.5 1.8 37.7 / 1.9 4.9 / 0.2 13.0/12.4 40.4 2.1 38.1 1.9 66.9 3.4 PG 64-22 48.5 65.6 3.3 62.8 / 3.2 6.0 / 0.3 9.6/10.2 55.9 2.8 40.3 2.0 49.8 2.5 SS-1h 53.0 44.7 / 2.3 3.9 / 0.2 8.7/9.1 44.4 2.3 44.3 2.3 44.7 2.3 Trackless 76.0 49.8 2.5 44.5 / 2.3 5.4 / 0.3 12.0/11.1 39.1 2.0 referred to as the Louisiana Interlayer Shear Strength Tester a constant normal load up to 100 psi, and accommodates (LISST). It consists of two main parts--a shearing frame and a specimen with 4-in or 6-in diameters. The gap between a reaction frame. Only the shearing frame is allowed to move the shearing and the reaction frame is 0.5 in. A wide range while the reaction frame is stationary. A cylindrical speci- of experiments was conducted in order to evaluate the men is placed inside the shearing and reaction frames and ruggedness and reliability of the LISST. Experiments were is locked in place with collars. Loading is then applied to the conducted comparing the results from this device with shearing frame. As the vertical load is gradually increased, those of the Superpave Shear Tester (SST). ISSs of the shear failure occurs at the interface. LISST and SST were similar when dilation was allowed; The LISST device was designed such that it will fit into any however, those results were significantly different when universal testing machine. It has a nearly frictionless linear dilation was not allowed or was limited in the SST device. bearing to maintain vertical travel and can accommodate Details of these experiments are described in Appendix sensors that measure vertical and horizontal displacements. D. Three shear displacement rates of loading were evalu- The device provides a specimen-locking adjustment, applies ated (i.e., 2 in/min, 0.1 in/min, and 0.02 in/min). Based on these evaluations, a rate of loading of 0.1 in/min was 80 recommended in the testing procedure to simulate the slow rate of loading encountered at the interface in the field. Ultimate Tensile Load (PULT, lb) 70 A test procedure for measuring interface bond strength in 60 the laboratory using the LISST device, written in AASHTO 50 format, is presented in Appendix E. 40 Figure 60 presents a typical test result of shear stress versus 30 displacement curve. The ISS is computed as follows: 20 ISS = PULT A (1) 10 0 where, CRS-1 SS-1h Trackless PG 64-22 ISS=interface shear strength (ksi); Figure 58. Ultimate tensile load (PULT) for tack coat PULT =ultimate load applied to specimen (lb); and materials at the softening point. A =cross-sectional area of test specimen (in2).