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(a) (b)
Figure 16. Tack coat application (a) 0.155 gal/yd2 with 100% coverage and (b) 0.031 gal/yd2 with 50%
coverage (SS-1h).
order to eliminate construction traffic on tacked surfaces. distribution conducted at the test site. For SS-1h with 100%
Subsequent to completion of HMA overlay placement, each coverage, trackless, and CRS-1, target application rates of
lane was marked based on previously documented reference 0.062- and 0.155- gal/yd2 were achieved with relatively low
points identifying the various test sections within each lane errors, although errors for trackless and SS-1h exceeded the
(see Figure 17f). 10% error limitation specified by ASTM 2995D. For the 0.031
gal/yd2 target application rate, errors were relatively higher
than those of other application rates, but it is noted that coef-
3.2.5Quality Testing of Tack
ficient of variation (COV) values for 0.031 gal/yd2 rate were
Coat Application
relatively low and showed high consistency. In summary, it is
The calibration of the distributor truck was a lengthy pro- noted that the measured application rates were slightly differ-
cess in this project and required multiple calibration runs to ent than the target values; however, the measured rates met
ensure the accuracy and uniformity of tack coat application. the objectives of the test matrix to simulate low, medium, and
This difficulty highlights the importance of regularly check- high levels. On the other hand, for SS-1h with 50% coverage,
ing the calibration of the distributor in practice. The proce- it was observed that high errors occurred at all application
dure outlined in Test Method A of ASTM D 2995, Standard rates. Figure 19 shows a comparison of 50% to 100% cover-
Practice for Estimating Application Rate of Bituminous Distribu- age from two cores extracted from the test facility.
tors, was followed. The surface of each pavement was initially
cleaned. Square (1 ft by 1 ft) textile pads were attached to the
3.2.6 Specimen Coring and Conditioning
surface of the pavement using a two-sided adhesive tape. The
geometrical layout of the pads is illustrated in Figure 18. Two A minimum of six test specimens were obtained from
pads were aligned in the transverse direction relative to the lane. each test section using a Simco® 255 Pavement Test Core
At least 2 ft were given to accommodate the space needed Drill. The core barrel was positioned over the area in which
for the wheels of the truck during the spray process. Once a sample was to be extracted, and water was allowed to flow
the pads were positioned correctly, the tack coat distributor down the inside of the barrel in order to reduce friction (see
truck applied the material to the section. Figure 20). The core barrel was then driven to the bottom-
For emulsion tack coats, the pads were allowed to remain most layer in order to remove the sample undisturbed. Sam-
in position for 3 hours to ensure that all water had evapo- ples were cored all the way through to avoid pre-stressing of
rated. After this period, the weight of each pad was measured. the samples. The sample was then removed from the core
The final weight, minus the initial weight of the pads with no barrel, labeled, and packaged for transportation. It is noted
tack, represented the residual asphalt cement and was used that a manual corer (i.e., Milwaukee Dynodrill B-1000) was
in the computation of the residual application rate. Table 5 used for weaker samples that required smaller amounts of
presents the results of these measurements for the tack coat torque.
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(a) (b)
(c) (d)
(e) (f)
Figure 17. Overlay construction at the test site.
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Right Wheel of Truck
Left Wheel of Truck
7 6 5 4 3 2 1
1.0 ft 3.0 ft 2.0 ft
13.0 ft
Figure 18. Tack coat rate measurement pad layout for
each section.
Table 5. Tack coat distribution test results at the PRF site.
Target Residual Measured Residual Application Rate
Tack Coat Application Rate Average Standard Deviation
COV Error
(gal/yd2) (gal/yd2) (gal/yd2) (%)
0.031 0.062 0.007 11.8 102.3
SS-1h 50 % 0.062 0.071 0.009 10.8 16.0
0.155 0.166 0.099 59.6 45.8
0.031 0.044 0.004 8.8 42.0
SS-1h 100 % 0.062 0.073 0.007 9.1 19.6
0.155 0.139 0.022 16.6 10.8
0.031 0.040 0.002 6.1 28.7
*
Trackless 0.062 0.068 0.004 7.7 10.8
0.155 0.177 0.011 5.9 14.9
0.031 0.035 0.004 15.3 20.8
CRS-1* 0.062 0.062 0.004 5.7 3.9
0.155 0.152 0.007 4.5 3.6
* Trackless and CRS-1 were distributed with 100% coverage.
(a) 100% Coverage Surface (b) 50% Coverage Surface
Figure 19. Typical tack coat surface coverage on old HMA surface (0.155 gal/yd2 residual application rate).
