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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

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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.