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Table 28. ISS test results for the OGFC, sand, and SMA mixtures.
OGFC Sand SMA
Residual
application 0.000 0.031 0.062 0.155 0.000 0.031 0.062 0.155 0.000 0.031 0.062 0.155
rate (gsy)
64.6 60.1 57.6 70.6 83.5 71.6 74.0 63.5 62.4 77.7 66.8 39.5
ISS (psi) 65.7 64.1 55.3 70.7 87.6 79.5 73.7 68.4 59.4 91.3 73.0 40.4
73.2 66.4 52.3 75.2 89.3 86.4 70.8 70.3 69.4 91.8 73.2 43.7
Mean 67.8 63.6 55.1 72.2 86.8 79.2 72.8 67.4 63.7 86.9 71.0 41.2
SD 4.7 3.2 2.7 2.6 5.0 7.4 1.8 3.5 5.1 8.0 3.6 2.2
COV (%) 6.9 5.0 4.8 3.6 5.8 9.3 2.4 5.2 8.1 9.2 5.1 5.4
with the sand mixture, tack coat application reduces inter- smooth, voidless, impermeable surface of the sand mix. Once
face shear bond strength. It appears that--for a new, smooth the voids in the surface of the OGFC were filled with tack coat
surface--tack coat acts as a lubricant and, thus, decreases the material, the OGFC showed an increase in ISS to a value that
shear strength at the interface. is slightly higher than that of the sand mix.
For the OGFC mixture, the ISS decreased slightly from the
no-tack condition with an increase in the residual applica-
4.8 Theoretical Investigation
tion rate, reached a minimum at residual application rate of
0.062 gsy, and then increased with an increase in the resid- Peak values of ISS, k-modulus, and displacement at fail-
ual application rate at 0.155 gsy. It appears that the higher ure (dmax) were calculated for each tack coat material and
voids in the surface of the OGFC initially yielded lower shear are presented in Table 29. As previously noted, all tack coat
strength than did that for the sand mix. However, when the materials showed the highest strength at a residual applica-
voids in the surface of the OGFC are filled with asphalt at tion rate of 0.155 gsy. Within the residual application rate
the highest residual application rate (0.155 gsy), the shear range considered, no optimum residual application rate was
strength becomes equivalent to that of the sand. determined. This was attributed to the highly oxidized HMA
One would expect that higher surface texture would yield surface at the PRF site, which required greater tack coat rates
higher ISS, such as a milled surface or an OGFC; however, it than expected. The mean profile depth (MPD) for the old
was observed that the surfaces of the laboratory-compacted HMA surface, which was measured using a road surface pro-
specimens (which are compressed against a smooth, flat steel filer according to ASTM E 1845, was 0.04 in (1.05 mm). While
plate) were flat but with significant voids in the case of higher residual application rates may increase ISS, excessive
the OGFC. These highly permeable voids likely absorbed tack coat may migrate into the HMA mat during compac-
the asphalt from the tacked interface and, thus, reduced the tion, causing a decrease in the air void content of the mix. It
ISS of the OGFC to a lower level than that of the relatively is also observed from the results presented in Table 29 that
Figure 77. Mean interface shear bond strengths for the
OGFC, sand, and SMA mixtures.
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Table 29. Interface shear behaviors for different tack coat types and at three
residual application rates.
Residual
FE Tack Coat ISS ISS COV k dmax
Application Rate
Case ID Material 2 (MPa x 103) (psi) (%) (N/mm3) (mm)
(gal/yd )
1 0.031 76.5 11.1 14.1 0.1916 0.39
2 CRS-1 0.062 129.6 18.8 7.6 0.1845 0.70
3 0.155 148.9 21.6 10.9 0.1304 1.14
4 0.031 117.9 17.1 14.2 0.2297 0.51
5 SS-1h 0.062 139.3 20.2 9.8 0.2826 0.49
6 0.155 415.7 60.3 6.4 0.2769 1.51
7 0.031 150.9 21.9 10.2 0.2688 0.56
8 Trackless 0.062 263.4 38.2 12.3 0.2642 0.99
9 0.155 655.0 95.0 8.3 0.2456 2.67
10 0.031 138.6 20.1 13.0 0.1757 0.79
11 PG 64-22 0.062 154.4 22.4 12.7 0.1898 0.81
12 0.155 258.5 37.5 7.2 0.1411 1.83
the interlayer tangential modulus decreased with the increase face was expected to experience fatigue failure before the end
in residual application rate, which is indicative of greater of its service life. A stress ratio of 0.50 is usually assumed in
deformability and flexibility at the interface. In addition, the laboratory fatigue testing of HMA and tacked interface as an
trackless tack coat exhibited the highest shear strength, and indication of failure (35, 43). It is also hypothesized that, at a
CRS-1 exhibited the lowest. stress ratio of 0.50 or less, the fatigue life at the tacked inter-
The effects of tack coat interface shear bond characteris- face would be infinite (i.e., no fatigue-related distress at the
tics, as measured by the LISST test on pavement responses interface).
at the interface, were investigated using the results of the FE Based on this theoretical approach, Figure 79 (a to f) pres-
model. Figure 78 (a to f) compares the calculated shear stress ents the calculated stress ratio for each tack coat type and
at the interface between the old and HMA overlay with the ISS residual application rate. For Structure A, it is noted that
for the different tack coat material types and residual appli- trackless--at intermediate and high residual application
cation rates. As shown in Figure 78, only two cases (Struc- rates--and SS-1h and PG 64-22--at a high residual appli-
ture A and Structure E with CRS-1 at 0.031 gal/yd2 residual cation rate--passed this criterion. CRS-1 did not meet this
application rate) failed due to a single load application. For criterion at any of the rates evaluated. For Structure B, the
the other structures, none of the evaluated cases failed at the majority of the tack coat types and residual application rates
interface due to a single load application. It is also noted that would be expected to perform satisfactorily against fatigue
the calculated shear stress did not substantially change from damage at the interface. In this case, only CRS-1 and SSh-1h
one tack coat application case to another. However, the cal- at the low residual application rate (0.031 gal/yd2) would be
culated shear stress changed from one pavement design to expected to experience fatigue damage at the interface. It is
another. evident from these results that the performance of tack coat
While the results presented in Figure 78 relate to the shear materials at the interface is primarily dictated by the pave-
response of the interface against a single tire load application, ment design. In other words, the influence of tack coat type
pavement structures are typically subjected to repeated fluc- and residual application rate becomes more relevant in thin
tuating vehicular loads. Such load patterns may cause fatigue pavements and less dominant in thick pavements.
failure at the tacked interface through a process of cyclic Based on the results presented in Figures 78 and 79, Fig-
cumulative damage. To assess the potential for fatigue failure ure 80 presents the variation of the predicted shear stress ratio
at the interface, the stress ratio (which is the ratio of the pre- with the ISS for the different tack coat materials and residual
dicted shear stress at the interface to the ISS) was calculated. application rates. As shown in this figure, a power law model
If the stress ratio was less than 0.50, the interface response is adequate in describing the relationship between the shear
against fatigue failure was assumed to be acceptable. On the stress ratio and the ISS. Utilizing the presented models, it was
other hand, if the stress ratio was greater than 0.50, the inter- determined that the minimum laboratory-measured ISS at the
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74
100
CRS-1 SS-1h Trackless PG 64-22
90
80
70
Shear Stress (psi)
60
ISS
50
Shear Stress
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
Application Rate (gsy)
(a) Structure A
100
CRS-1 SS-1h Trackless PG 64-22
90
80
70
Shear Stress (psi)
60
50
ISS
40
Shear Stress
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
Application Rate (gsy)
(b) Structure B
100
CRS-1 SS-1h Trackless PG 64-22
90
80
70
Shear Stress (psi)
60
ISS
50
Shear Stress
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
Application Rate (gsy)
(c) Structure C
Figure 78. Comparison of the calculated shear stress to the ISS.
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100
CRS-1 SS-1h Trackless PG 64-22
90
80
70
60
Shear Stress (psi)
ISS
50
Shear Stress
40
30
20
10
0
0.03 1 0 .062 0.15 5 0 .031 0.06 2 0 .155 0.03 1 0 .0 62 0.15 5 0 .0 31 0.06 2 0 .1 55
Application Rate (gsy)
(d) Structure D
100
CRS-1 SS-1h Trackless PG 64-22
90
80
70
Shear Stress (psi)
60
ISS
50
Shear Stress
40
30
20
10
0
0.03 1 0 .062 0. 155 0.031 0.062 0.155 0.03 1 0 .062 0. 155 0. 031 0.062 0.155
Application Rate (gsy)
(e) Structure E
100
CRS-1 SS-1h Trackless PG 64-2
2
90
80
70
Shear Stress (psi)
60
ISS
50
Shear Stres s
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
Application Rate (gsy)
(f) Structure F
Figure 78. (Continued).
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1.4
CRS-1 SS-1h Trackless PG 64-22
1.2
1
Shear Stress Ratio
0.8
0.6
Limiting
Ratio
0.4
0.2
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
Application Rate (gsy)
(a) Structure A
0.9
CRS-1 SS-1h Trackless PG 64-22
0.8
0.7
Shear Stress Ratio
0.6
Limiting
0.5
Ratio
0.4
0.3
0.2
0.1
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
Application Rate (gsy)
(b) Structure B
0.45
CRS-1 SS-1h Trackless PG 64-22
0.4
0.35
Shear Stress Ratio
0.3
0.25
0.2
0.15
0.1
0.05
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
Application Rate (gsy)
(c) Structure C
Figure 79. Calculated shear stress ratios for different tack coat types and
residual application rates.
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0.6
CRS-1 SS-1h Trackless PG 64-22
Limiting
0.5
Ratio
Shear Stress Ratio
0.4
0.3
0.2
0.1
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
Application Rate (gsy)
(d) Structure D
1.1
CRS-1 SS-1h Trackless PG 64-22
1
0.9
0.8
Shear Stress Ratio
0.7
0.6
Limiting
0.5
Ratio
0.4
0.3
0.2
0.1
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
Application Rate (gsy)
(e) Structure E
0.4
CRS-1 SS-1h Trackless PG 64-22
0.35
0.3
Shear Stress Ratio
0.25
0.2
0.15
0.1
0.05
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
Application Rate (gsy)
(f) Structure F
Figure 79. (Continued).
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1.4
Design A
Design B
1.2
Design C
y = 15.514x-1.035
1 R² = 0.99
Shear Stress Ratio
0.8
0.6 y = 10.671x-1.047
R² = 0.99
0.4
0.2 y = 3.634x-0.977
R² = 0.9984
0
0 10 20 30 40 50 60 70 80 90 100
ISS (psi)
1.4
Design D
1.2 Design E
Design F
1
Shear Stress Ratio
0.8
y = 12.633x-1.033
y = 7.1991x-1.054 R² = 0.99
0.6 R² = 0.99
0.4
0.2
y = 3.634x-0.977
R² = 0.99
0
0 10 20 30 40 50 60 70 80 90 100
ISS (psi)
Figure 80. Relationship between shear stress ratio and
laboratory-measured ISS.
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interface to achieve a shear stress ratio of 0.50 or lower was Table 30. Recommended tack coat residual
28 psi for Structure A, 19 psi for Structure B, and 8 psi for application rates.
Structure C. Similarly, the minimum laboratory-measured ISS
Surface Type Residual Application Rate (gsy)
at the interface to achieve a shear stress ratio of 0.50 or lower
New Asphalt Mixture 0.035
was 23 psi for Structure D, 13 psi for Structure E, and 8 psi for
Old Asphalt Mixture 0.055
Structure F. These limits can be used in the selection of tack
coat materials and residual application rates based on labora- Milled Asphalt Mixture 0.055
tory DST results to predict performance at the interface in the Portland Cement Concrete 0.045
field. If a single ISS value needs to be specified to prevent failure
at the interface, and considering a safety factor of 1.4 against
variability in measurements and in construction, an ISS value
of 40 psi is recommended. cussions with state dots and industry personnel, Table 30 lists
Based on the results of the FE analysis, findings of the the recommended tack coat residual application rates for the
experimental program for different surface types, and dis- various pavement surfaces.
