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Figure 59. General description of the Louisiana Interlayer Shear Strength Tester.
= k (2) for SS-1h, CRS-1, and trackless tack coats, respectively.
Triplicate specimens were tested for each test condition
where, defined by tack coat type, residual application rate, confin-
t =interlayer shear stress (ksi);
ing pressure, and dusty and wet conditions. All tests were
x=interlayer displacement within the interface (in); and
performed at a temperature of 25°C. In general, the COVs
k
=interlayer tangential modulus (lb/ft3). in the test results were less than 10%. As shown in the
following sections, results were analyzed to investigate
The k-modulus is computed by dividing the peak stress by the effects of variables considered in the test factorial on
the displacement at failure from the stress versus displacement the ISS.
curve (see Figure 60).
Effect of Emulsified Tack Coat Types
4.4.1Effects of Tack Coat Characteristics and Residual Application Rates
on Interface Shear Strength
Tables 20 and 21 present the statistical analyses of the
Tables 18 and 19 present the mean ISS test results along effects of application rates and tack coat types on ISS based on
with their standard deviations and coefficient of variations a two-tailed t-test at a 95% confidence level. As shown in these
60
Peak stress
50
Interface Shear Stress (psi)
40
k-modulus (lb/ft3)
30
20
10
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Displacement (in)
Figure 60. Typical interface shear stress versus displacement
for trackless at 0.06 gal/yd 2.
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Table 18. ISS of SS-1h emulsified tack coat.
Tack Coat SS-1h
Confinement
Pressure Residual
(psi) Appl. Rate 0.031 0.062 0.155
(gal/yd2)
Surface D1 D W2 W D D W W D D W W
Condition H3
L4
H L H L H L H L H L
11.1 13.5 13.6 12.7 14.6 7.5 16.3 12.7 36.8 32.9 44.0 34.0
ISS (psi) 9.7 12.9 13.1 13.2 12.0 8.0 18.1 11.7 41.7 40.1 37.7 34.5
0
12.9 14.5 15.5 15.0 13.2 7.8 16.8 13.6 40.3 34.9 37.8 34.8
Mean 11.2 13.6 14.0 13.6 13.3 7.8 17.1 12.7 39.6 36.0 39.8 34.4
S.D. 1.6 0.8 1.3 1.2 1.3 0.3 0.9 0.9 2.5 3.7 3.7 0.4
COV 14.2 5.9 9.0 8.8 9.8 3.7 5.4 7.3 6.4 10.3 9.2 1.2
16.5 20.9 13.7 24.5 18.6 10.4 15.2 15.5 40.4 51.9 38.5 47.7
ISS (psi) 15.7 20.2 16.1 22.4 14.9 11.6 17.8 16.6 43.6 44.0 38.5 45.4
18.8 26.1 15.9 19.3 17.6 12.9 19.1 16.3 40.7 43.9 41.2 43.4
20
Mean 17.0 22.4 15.2 22.1 17.0 11.6 17.4 16.2 41.6 46.6 39.4 45.5
S.D. 1.6 3.2 1.4 2.6 1.9 1.2 1.9 0.6 1.8 4.6 1.6 2.1
COV 9.5 14.3 9.0 11.7 11.3 10.6 11.2 3.4 4.2 9.8 4.0 4.7
1
Dry Condition, 2 Wet Condition, 3Clean Condition, 4 Dusty Condition.
Table 19. ISS of CRS-1 and trackless emulsified tack coat.
Tack Coat CRS-1 Trackless
Confinement
Pressure Residual
(psi) Appl.
0.031 0.062 0.155 0.031 0.062 0.155
Rate
(gal/yd2)
Surface D1 D D D D D D D D D D D
Condition H 2
L 3
H L H L H L H L H L
6.9 10.3 12.3 10.3 12.6 24.6 13.3 20.1 28.4 51.3 58.0 60.2
ISS (psi) 6.6 10.0 11.4 13.1 15.6 23.9 16.0 22.4 24.6 49.7 61.0 60.5
0
8.5 7.8 13.3 12.2 14.3 24.1 13.9 23.6 22.2 61.2 68.1 65.1
Mean 7.3 9.4 12.4 11.8 14.2 24.2 14.4 22.0 25.1 54.1 62.4 62.0
S.D. 1.0 1.3 0.9 1.4 1.5 0.4 1.5 1.8 3.1 6.3 5.2 2.7
COV 14.1 14.2 7.6 11.9 10.9 1.5 10.2 8.1 12.3 11.6 8.3 4.4
10.2 16.9 14.0 18.8 12.9 34.2 18.0 34.3 35.2 75.2 65.2 79.6
ISS (psi) 13.3 13.6 11.5 20.3 15.6 33.4 22.6 26.4 34.0 76.9 55.8 74.9
11.0 15.4 11.7 17.5 16.8 34.2 22.2 31.9 38.6 69.7 70.2 75.3
20
Mean 11.5 15.3 12.4 18.9 15.1 33.9 20.9 30.8 35.9 73.9 63.7 76.6
S.D. 1.6 1.6 1.4 1.4 2.0 0.5 2.5 4.0 2.4 3.8 7.3 2.6
COV 14.3 10.5 11.2 7.3 13.1 1.4 12.0 13.1 6.7 5.1 11.5 3.4
1
Dry Condition, 2 Clean Condition, 3 Dusty Condition.
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Table 20. Statistical analysis of the effects of application rates on ISS.
Tack Coat Statistical Test Condition Confinement P-value Results
Application Rates Clean-Dry Unconfined < 0.0001 Significant
Application Rates Clean-Dry Confined < 0.0001 Significant
Application Rates Dusty-Dry Unconfined < 0.0001 Significant
Application Rates Dusty-Dry Confined < 0.0001 Significant
SS-1h
Application Rates Clean-Wet Unconfined < 0.0001 Significant
Application Rates Clean-Wet Confined < 0.0001 Significant
Application Rates Dusty-Wet Unconfined < 0.0001 Significant
Application Rates Dusty-Wet Confined < 0.0001 Significant
Application Rates Clean-Dry Unconfined 0.0010 Significant
Application Rates Clean-Dry Confined 0.0893 Not Significant
CRS-1
Application Rates Dusty-Dry Unconfined < 0.0001 Significant
Application Rates Dusty-Dry Confined < 0.0001 Significant
Application Rates Clean-Dry Unconfined < 0.0001 Significant
Application Rates Clean-Dry Confined < 0.0001 Significant
Trackless
Application Rates Dusty-Dry Unconfined < 0.0001 Significant
Application Rates Dusty-Dry Confined < 0.0001 Significant
tables, all cases except one indicated that tack coat types and around 0.062 gal/yd2. Similar trends were noted at a confine-
application rates had significant effects on the measured ISS. ment pressure of 20 psi. For the residual application rates
Figure 61a presents the variation of ISS with emulsified tested, it was not possible to determine the optimum residual
tack coat types and residual application rates. The results were application rate. This may be attributed to the highly oxidized
obtained from clean and dry specimens with no confinement HMA surface at the PRF site, which required greater tack coat
at 25°C. For each residual application rate, the trackless tack rates than expected. It may also indicate that, under actual
coat exhibited the highest shear strength and CRS-1 exhibited field conditions, optimum residual application rates may be
the lowest. Trackless and SS-1h yielded similar and higher greater than that commonly predicted from laboratory-based
ISSs than CRS-1 at the low residual application rate--that is, experiments. While higher residual application rates may
0.031 gal/yd2. increase ISS, excessive tack coat may migrate into the HMA
All tack coat materials showed the highest strength at a mat during compaction, causing a decrease in the air void
residual application rate of 0.155 gal/yd2. Shear strength of content of the mix. Figure 61b presents the variation of the
SS-1h and trackless consistently increased as residual appli- measured air voids of the overlaid mixture for each residual
cation rate increased. In contrast, measured shear strength application rate. As shown in this figure, the increase in residual
for CRS-1 appeared to stabilize at a residual application rate tack coat application rate was associated with a decrease in air
Table 21. Statistical analysis of the effects of tack coat types on ISS.
Rate Statistical Test Condition Confinement P-value Results
Tack Coat Type Clean-Dry Unconfined 0.0022 Significant
Tack Coat Type Clean-Dry Confined 0.0032 Significant
0.031
Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant
Tack Coat Type Dusty-Dry Confined 0.0027 Significant
Tack Coat Type Clean-Dry Unconfined 0.0004 Significant
Tack Coat Type Clean-Dry Confined < 0.0001 Significant
0.062
Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant
Tack Coat Type Dusty-Dry Confined < 0.0001 Significant
Tack Coat Type Clean-Dry Unconfined < 0.0001 Significant
Tack Coat Type Clean-Dry Confined < 0.0001 Significant
0.155
Tack Coat Type Dusty-Dry Unconfined < 0.0001 Significant
Tack Coat Type Dusty-Dry Confined < 0.0001 Significant
