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OCR for page 35
35
Section 4
Findings
This section presents the main findings of NCHRP Project lists the asphalt cements or cutbacks used by the different
9-40. It includes the main findings of the worldwide survey, agencies. Sixty percent of the 15 agencies indicated their use
the results of the experimental program described in the pre- of PG 64-22 asphalt cement. Eleven agencies reported the use
vious chapter, and the theoretical investigation that was used of eight cutback asphalts for tack coat. MC-70 had the high-
to relate laboratory-measured interface bond characteristics to est rate of use with 55% of those respondents using cutback
the field stresses in the pavement structure when subjected to asphalts as tack coat.
vehicular loading. The most commonly used emulsions were slow-setting
SS-1 (41%), SS-1h (39%), CSS-1 (37%), and CSS-1h (41%)
4.1Findings of the Worldwide Survey (see Figure 27). The asphalt content of the emulsions gener-
ally ranged between 50% and 65%. A few extremely diluted
A total of 72 responses were identified as having met the cri- emulsions were used by some respondents. The residual rate
teria for inclusion in this study. Where more than one response reported for the cutback asphalt materials ranged from 50%
was received from the same state, the data were combined, to 87%.
and only one respondent was counted in the analysis. Most The survey questions focused on tack coat materials used
of the survey results were presented in terms of percentage in recent construction projects and the percentage of their
of respondents. Two questions on the questionnaire consid- use. Figure 28 was developed by multiplying the percentage
ered the weight of each option (i.e., percentage of use for each of use of tack coat materials by the number of responses in
option/the importance index of each option). Accordingly, order to evaluate the recent usage. Emulsified asphalts were,
two methods were employed to analyze the data: by far, the most commonly used tack coat material followed
by asphalt cement and then cutback asphalts. SS-1, CSS-
1. Data were presented by means of summing the products 1h, SS-1h, and CSS-1 ranked as the most used emulsified
of the percentage of use and the number of responses,
asphalts (see Figure 29). PG 64-22 was the most commonly
used asphalt cement. RC-70 ranked as the most commonly
Percentage of use × number of responses
used cutback asphalt (Figure 29).
2. Weighted average was used to show the overall importance
of each option,
4.1.2Types of Tack Coat Applied to
Importance index × number of responses Different Pavement Surfaces
Number of responses Most of the respondents indicated that their agencies
monitor the application of tack coats and specify ranges for
dilution rates as well as application rates. Of the respondents,
4.1.1Types and Grades of Commonly Used
4% indicated that the dilution rate is determined by the con-
Tack Coat Materials
tractor, 2% stated that they do not monitor the application
Figure 25 shows that 100% of the responding agencies indi- rates, and 2% stated that the application rates are monitored
cated that asphalt emulsions are permitted by their agency. visually. Figure 30 shows the most common materials used as
The percentage of respondents that use asphalt cement and tack between new HMA layers. Tack coat materials used on
cutback asphalts are 27% and 20%, respectively. Figure 26 old HMA surfaces and milled HMA surfaces are listed in
OCR for page 36
100
80
Respondents,%
60
40
20
0
Asphalt Cement Cutback Asphalt Emulsion
Tack Coat Material Type
Figure 25. Tack coat material types.
100
Asphalt Cements Cutback asphalts
80
Respondents, %
60
40
20
0
MC 250
Pen 85-100
RC 250
RC 800
MC-800
RC 30
RC 70
PG 76-22
PG 82-22
MC-30
MC-70
AC-5
PG70-10
PG70-22
PG70-28
PG52-22
PG58-16
PG 58-22
PG58-28
PG64-16
PG 64-22
PG64-28
PG 67-22
Figure 26. Asphalt cements and cutbacks used as tack coats.
45
40
35
Respondents, %
30
25
20
15
10
5
0
HFMS-1
HFMS-2
HFMS-2h
RS-1
RS-2
MS-2
MS-1
SS-1
SS-1h
SS-1L
EAP&T
STE-1
TST-1P
HFE-90
ODOT702.13
RS-1h
CRS-1
CRS-2
CRS-3
CMS-2
CSS-1
ST-1P
CRS-2P
CSS-1h
CQS-1H
SS-1P
CRS-1h
CRS-2h
CQS-1HP
Tack Coat Material Type
Figure 27. Emulsions used as tack coats.
5000
4500
Usage % X No. of Responses
4000
3500
3000
2500
2000
1500
1000
500
0
Asphalt Cutback Emulsified Asphalt
Tack Coat Material Type
Figure 28. Weighted use for the material used as tack coat.
