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37 1400 Asphalt Cement Cutback Asphalt Emulsified Asphalt Usage % x No. of Responses 1200 1000 800 600 400 200 0 CSS-1h RS-1 RS-2 MS-1 SS-1h PG PG58-28 PG 58-22 PG64-16 PG70-10 RS-1h CRS-1 CRS-2 CRS-2h CRS-3 CMS-2 CSS-1 ST-1P TST-1P STE-1 CQS-1H HFE-90 MC 250 SS-1 EAP AC-5 RC 30 RC 70 MC-30 MC-70 MC-800 HFMS-1 HFMS-2h ODOT70 CQS-1HP Tack Coat Material Type Figure 29. Commonly used tack coat materials. Figures 31 and 32, respectively. Of the agencies, 4% indicated surface conditions yielded similar results as the first three that they do not require tack coats between new HMA lay- surfaces, with CSS-1h, CSS-1, SS-1h, and SS-1 being the most ers, while 2% indicated that no tack is required on old HMA used tack coat materials. surfaces. Figures 35 and 36 list the materials used on PCC pave- For tack coats applied between new, old, and milled HMA ments or diamond-ground PCC pavements, respectively. layers, the commonly used tack coat materials were CSS-1H Again, SS-1, SS-1h, CSS-1, and CSS-1h were the most used (32%34%), SS-1 (30%32%), SS-1h (29%32%), and CSS-1 materials with the high-float emulsions ranking highest (21%27%). PG 64-22 was the most used asphalt cement with among the emulsions. an average of 11%, and RC-70 was the most commonly used cutback asphalt with a usage range of 5% to 7%. The residual application rates for most of the emulsions were within the 4.1.3Findings Related to Tack Coat range of 0.03 to 0.05 gal/yd2. Asphalt cement application rates Application Methods ranged from 0.04 to 0.10 gal/yd2. The range of residual rates The Dilution Process Location for cutback asphalts was 0.03 to 0.05 gal/yd2. Only 27% of the respondents gave feedback for tack coat materials used on top Several agencies allow dilution at multiple locations: 49% of surface treatments or seal coats, as well as asphalt-treated reported that the dilution process occurs while the material base courses (see Figures 33 and 34, respectively). These two is in the supplier's tank (see Figure 37). Another 45% allow 40 35 30 25 % of Respondents 20 15 10 5 0 RC 30 RC 70 RC 250 MC-250 MC-800 ST-1P TST-1P STE-1 EAP HFE-90 RS-1h SS-1h CRS-1 MC-70 CRS-2 CMS-2 CSS-1 CSS-1h PG 70-10 PG 58-22 PG 64-16 PG 64-22 PG 58-22 PG 58-16 HFMS-2h AC-5 RS-1 RS-2 MS-1 SS-1 HFMS-1 Figure 30. Tack coat materials placed between new HMA layers.

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Respondents, % % of Respondents % of Respondents 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 RS-1 RS-1 RS-1 RS-1h RS-1h HFMS-1 RS-2 RS-2 MS-1 HFMS-1 HFMS-1 SS-1 HFMS-2h HFMS-2h SS-1h MS-1 MS-1 CRS-1 SS-1 SS-1 CRS-2 SS-1h SS-1h CRS-1 CRS-1 CRS-2P CRS-2 CRS-2 CMS-2 CMS-2 CMS-2 CSS-1 CSS-1 CSS-1 CSS-1h CSS-1h CSS-1h ST-1P ST-1P TST-1P HFE-90 STE-1 TST-1P Figure 31. Tack coat materials placed on old HMA. HF-150S HFE-90 STE-1 EAP EAP HFE-90 PG 70-10 PG 70-10 EAP PG 64-16 PG 70-10 PG 64-16 PG 64-22 PG 64-16 Figure 32. Tack coat materials placed on milled HMA surfaces. PG 64-22 PG 58-22 PG 64-22 PG 58-22 PG 58-16 PG 58-22 PG 58-16 AC-5 PG 58-16 RC 30 RC 30 AC-5 RC 70 RC 70 RC 30 RC 250 RC 250 RC 70 MC-70 MC-70 RC 250 Figure 33. Tack coat materials used on surface treatments, seal coats, or chip seals. MC-250 MC-250 MC-70 MC-800 MC-250 MC-800 MC-800

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% of Respondents % of Respondents % of Respondents 0 5 10 15 20 25 30 15 20 25 30 35 0 5 10 10 15 20 25 30 0 5 RS-1 RS-1 RS-1 RS-1h RS-1h RS-2 RS-2 RS-2 MS-1 HFMS-2h HFMS-1 SS-1 MS-1 HFMS-2h MS-1 SS-1h SS-1 SS-1 CRS-1 SS-1h SS-1h CRS-2 CRS-1 CRS-1 CRS-2 CMS-2 CRS-2 CMS-2 CSS-1 CMS-2 CSS-1 CSS-1h CSS-1 CSS-1h CSS-1h TST-1P TST-1P TST-1P STE-1 Type Type ST-1P HFE-90 Figure 35. Tack coat materials used on PCC surfaces. HFE-90 HFE-90 702.13 EAP EAP EAP PG 70-10 Figure 34. Tack coat materials used on asphalt-treated base. PG 70-10 PG 70-10 Figure 36. Tack coats used on milled or diamond-ground PCC. PG 64-16 PG 64-16 PG 64-16 PG 64-22 PG 64-22 PG 64-22 PG 58-22 PG 58-22 PG 58-22 PG 58-16 PG 58-16 AC-5 AC-5 RC 70 RC 70 RC 70 RC 250 RC 250 RC 250 MC-250 MC-250 MC-70 MC-800 MC-800 MC-250

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40 60 50 % of Respondents 40 30 20 10 0 Supplier's Contractor's In the Other None Terminal Storage Tank Distributor Allowed Tank Responses Figure 37. Potential sites for dilution of emulsified asphalt. dilution to occur in the distributor tank. Only 15% of the Traffic on Tacked Surfaces respondents do not allow dilution of emulsified asphalt The majority of respondents--78%--stated that highway for tack. traffic is not allowed on tack coat materials prior to HMA placement. Of the respondents who do allow traffic on the Verification of Asphalt Emulsion Dilution Rate tack materials, most stated that the tack coats should be cured Half of the respondents stated that emulsion was sampled first. Some reported a time of 1 to 2 hours before traffic is from the distributor and tested for verification. Another 39% allowed onto the tacked pavement. All of the respondents of the respondents required certification from the supplier. who allowed traffic prior to placement of HMA indicated that Only 29% allowed the certification to be performed by the surface type did not affect the time required before traffic contractor (see Figure 38). was allowed. Of the respondents, 47% allow highway traffic for a max- imum of 24 hours before placing the covering HMA layer; Frequency of Verification of Dilution Rate 18% do not allow highway traffic prior to the placement of Of the respondents, 36% verify the dilution rate of an the subsequent HMA layer; and 6% allow 5 days of trafficking asphalt emulsion (see Figure 39); 26% indicated that the before the tack coat must be covered (see Figure 40). dilution rate is not checked; another 26% indicated criteria different from those queried in the questionnaire. Some of Tack Coat Application Equipment these different criteria included the following: verify the dilu- tion rate for every delivery unit, leave it up to the contractor, By far, most agencies (98%) indicated that an asphalt verify every 2 weeks, verify every 43,000 ft2, and periodically distributor with spray bar was the most common specified test. Only 10% verify the dilution rate daily. application method (see Figure 41); 42% allow an asphalt 60 50 % of Respondents 40 30 20 10 0 Asphalt Contractor Tested Tested Visual ASTM None Other Supplier Certificate from the from the Observation D2995 Certificate Contractor's Distributor Storage Tank Responses Figure 38. Verification of the dilution process.

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41 40 35 % of Respondents 30 25 20 15 10 5 0 Daily Monthly Project by Not Checked Other Project Responses Figure 39. Frequency of verification of dilution. 50 45 40 Respondents, % 35 30 25 20 15 10 5 0 0 4 12 24 120 Hours Figure 40. Time that tack coat can be exposed to traffic before covering with HMA. 100 80 % of Respondents 60 40 20 0 Hand Wand Distributor Spray Paver Spray Bar Other Bar Responses Figure 41. Method for applying tack coat materials.

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42 distributor with hand wand, and 4% require a spray bar Pickup of Tack Material by Truck Tires attached to the paving machine. However, the average per- Of the respondents, 67% indicated that pickup of tack coat centage of use for the asphalt distributor with spray bar was 97% compared with 6% asphalt distributor with hand wand. material is a continuing problem; 38% indicated that the tack Half of the agencies that use a spray bar attached to a paver material is required to be completely set before haul trucks use it 100% of the time, while the remaining half used it for are allowed on it. Few respondents, 13%, allow haul trucks to 1% of their mainline paving areas. Not all agencies provided drive on the tack coat material before breaking (see Figure 42). a percentage of use with their survey response. Other methods specified to reduce pickup include the fol- lowing: tack coat is required to break before haul trucks are allowed, reduce the application rate, clean the surface before Breaking/Setting of Emulsified Asphalts applying tack coat, and minimize the distance that haul Of the respondents, 26% permit haul trucks to drive on trucks are allowed to drive on the tack coat. unbroken emulsion. The majority of respondents, 70%, allow haul trucks on an unset emulsion after it breaks. Percentage of Tack Coat Coverage Out of 53 responses, 74% of the responding agencies allow paving to begin immediately after the tack coat material Tack coat coverage is defined herein as the percentage of breaks, whereas 26% do not allow paving until the emul- the pavement surface area coated by asphalt tack. Most agen- sion sets. cies, 64%, responded that the coverage area is typically above In ranking the factors that affect the break and set times 90%. The percentages of responses are as follows: for an emulsified asphalt, respondents indicated that ambi- ent temperature and pavement temperature were the most 1. 100% coverage (37%), important factors. Other factors that were reported were road 2. 90%100% coverage (27%), surface condition, solar effect, and emulsion temperature. 3. 70%90% coverage (18%), Application rate, dilution rate, wind velocity, and humidity 4. 50%70% coverage (9%), and were considered essentially equivalent in level of importance. 5. Less than 50% coverage (9%). The break/set factors are listed below in order of importance, from highest to lowest: A majority of the agencies, 73%, indicated that no specific requirement was used to regulate the application of tack coat 1. Ambient temperature, material; 25% reported that the amount of spray overlap 2. Pavement surface temperature, between adjacent nozzles on the distributor spray bar is a 3. Dilution rate, specified requirement. Out of the 13 agencies that reported a 4. Application rate, requirement for overlap, 46% use a double-overlap configu- 5. Humidity, ration, while 23% use single- and triple-lap. The remaining 6. Wind velocity, and 8% did not mention which degree of overlap was used. That 7. Others. the angle of the nozzles to the axis of the spray bar is a speci- 100 90 80 % of Respondents 70 60 50 40 30 20 10 0 Completely Sanded Before Break Ongoing Other Set Problem Responses Figure 42. Methods used to prevent pickup of tack coat by haul trucks.

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43 Figure 43. Specified requirements for tack coat application. fied requirement was reported by 12% of respondents. Of (i.e., paving season) was a restriction. The percentages of those who indicated the angle of the nozzles as a requirement, responses are given in Figure 44. the average minimum angle was 23 and the average maxi- Some additional common restrictions for application of mum angle was 32. That the height of the spray bar above tack coat were as follows: the pavement surface is a requirement was reported by 10% of respondents. Figure 43 presents the percent of responses 1. Surfaces must be free of standing water or contamination, for each requirement. 2. Manufacturer's recommendations, 3. Do not apply tack coat unless HMA will be immediately placed, and Environmental Restrictions 4. Cannot apply tack coat materials in foggy conditions In discussing the environmental restrictions placed on the application of the tack coat material, almost half of the Application Rates and Residual Tack Coat respondents, 43%, reported a minimum ambient tempera- Rate Verification ture. The average minimum ambient temperature was 6C. Less than 2% reported a maximum allowable ambient tem- Of the responses, 51% indicated that measuring the change perature of 65C; 38% reported that a minimum pavement in the amount of material in the distributor tank after apply- surface temperature was a restriction. The average minimum ing a given section was the best way to check the application pavement surface temperature was 3C. No agency reported a rates (see Figure 45). Less than 2% of the agencies reported maximum pavement for surface temperature as a restriction. that ASTM D 2995 (19) is used. The differences in the weight Impending rainfall was an environmental restriction for 55% of the asphalt distributor over a given area were about 27%. of respondents. More than 75% of the respondents reported Some of the common methods specified by the respondents, that a wet pavement surface was a restriction, whereas 38% but not queried in the questionnaire, are as follows: meter on indicated that a damp pavement surface was a restriction. the distributor, visually, and dipstick reading before and after Approximately the same number reported that time of year an application on a pavement segment. 100 % of Respondents 80 60 40 20 0 Min. Air Max. Air Min Max. Impending Wet Damp Time of Other Temp. Temp. Surface Surface Weather Surface Surface Year Temp. Temp. Responses Figure 44. Environmental restrictions on tack application.