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OCR for page 53
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
R² = 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-
R² = 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 ±5°C
-10 0 10 20 30 40 50 60 70 from the material softening point. Test results show that PG
Temperature (ºC) 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
60°C, 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 ºC (poisses)
Trackless
2
R = 0.89
Temperature (TOPT) at SMAX (ºC)
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 (ºC) Tensile Strength at 60 ºC (kPa)
(a) (b)
Figure 57. Relationship between absolute viscosity and softening point
and the optimum test temperature.
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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).
