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23 This chapter summarizes the results of the surveys of agencies and industry. As noted previously, a total of 45 state DOTs responded to the survey, a 90% response rate. In addition, representatives of five Canadian provinces also responded. The agency survey was distributed to members of the AASHTO Committee on Materials and Pavements with copies to members of the Standing Com- mittee on Research. A total of 62 industry representatives from 28 states responded to the industry survey. The industry survey was distributed to the membership of the National Asphalt Pavement Association (NAPA) through their ActionNews newsletter and social media channels. Copies of the agency and industry questionnaires are provided in Appendices A and B, respec- tively. Detailed, tabulated survey responses are provided in Appendices C and D. In the text, the question number is indicated by Q and the number (e.g., Q1). If the results are reported as a percentage of respondents, that is denoted by % (e.g., Q1, %). If the number of respondents is reported, that is indicated by # (e.g., Q1a, #). The survey responses are usually illustrated graphically as well as in text. Pie charts are usually used when the responses are single answer (e.g., yes/no). Bar graphs are usually used when mul- tiple responses can be selected (e.g., changed mix design and roller patterns). When percentages are reported, those reflect the percentage of those who responded to the questionânot all survey respondents answered every question. 3.1 State DOT Survey Responses A total of 45 state DOTs responded to the survey request. The states responding are shown in Figure 6. In addition, five Canadian provinces (Alberta, British Columbia, Manitoba, New Brunswick, and Ontario) also responded; their responses are summarized in Section 3.2. In response to the question asking if the DOTs have established policies or guidelines regard- ing appropriate asphalt lift thicknesses relative to the NMAS of the mixture (Q1), 40 states said yes, one was unsure, and four states said no, as illustrated in Figure 7. Those guidelines are most commonly enacted through design manuals or memos, or through the standard specifications, as shown in Figure 8. The information on the required minimum and maximum lift thicknesses specified that was provided in response to the second and third questions (Q2 and Q3) is quite complicated because there are so many variablesâtype of gradation (fine dense, coarse dense, SMA, or open graded) and mix size (4.75, 9.5, 12.5, 19.0, 25.0, and 37.5 mm). For details, the reader is referred to Appendix C. A broad summary is provided here. First, it can be observed that many states do not use 4.75- or 37.5-mm mixtures. Similarly, fewer states use SMA or open-graded mixes than more conventional dense-graded mixes. This may not be surprising since mixes with NMASs between 9.5 and 25.0 mm could be considered the workhorses in most states. Relatively little C H A P T E R 3 Survey Results: Current Practices Regarding Lift Thicknesses
24 Impact of Asphalt Thickness on Pavement Quality Responded No Response Figure 6. States responding to the agency survey. Figure 7. Does your organization have established policies or guidelines regarding appropriate asphalt lift thicknesses relative to the nominal maximum aggregate size (NMAS) of the mixture? (Q1, percent of respondents, %). new construction, which might use 37.5-mm bases, is occurring, and 4.75-mm mixes are still relatively new or used mostly for pavement preservation. SMAs and open-graded mixes may be considered specialty mixes for specific applications in some states. Another observation regarding minimum and maximum lift thicknesses is that no state reported specifying different lift thicknesses for fine versus coarse mixes (but as the case examples show, some states do have different required lift thicknesses). This suggests no state has fully implemented the recommendations from NCHRP Report 531 (Brown et al., 2004)âor at least few have. The majority of the states responding specify minimum lift thicknesses of at least three times the NMAS (3ÃNMAS) for those mix sizes that they use; about 60% of the responding states use at least 3ÃNMAS for 9.5-mm mixes and nearly 80% of responding states use at least 3ÃNMAS for 12.5-mm mixes. Some states bracket â3ÃNMASâ between their specified minimum and maximum values. For example, Virginia specifies that dense mixes be placed at between 2.5
Survey Results: Current Practices Regarding Lift Thicknesses 25 and 4ÃNMAS for all sizes except 37.5 (which they do not use). A total of 26 states specify differ- ent maximum lift thicknesses for different mix sizes or types. In most cases those differences are for dense-graded versus the other gradation types, not fine versus coarse. When asked if there are situations in which the agency might grant exceptions to the allowable lift thicknesses (Q4), 30 (70%) of the responding states indicated yes, nine (21%) said maybe, and only four (9%) said no. See Figure 9. Reasons for granting exceptions varied widely but included â¢ Constructibility; â¢ Maintaining or correcting cross slope, drainage and gutters, and curb height; Other Specifications Design Memo Pavement Design Manual Figure 8. If yes, how are these policies or guidelines enacted? (Q1a, number of respondents, #). Figure 9. Are there situations in which your agency may grant exceptions to the allowable lift thicknesses? (Q4, %).
26 Impact of Asphalt Thickness on Pavement Quality â¢ For scratch or leveling courses; â¢ To limit drop-offs; â¢ Shoulders and transitions; and â¢ To control costs. Some states indicated they might allow thicker lifts to be placed (especially deeper in the pave- ment) if the contractor demonstrates the ability to achieve density or to speed construction. Agencies were asked what their target densities are for dense-graded mixes (Q5). See Figure 10. Only two indicated using less than 92% of Gmm. One of those states, Kansas, has a lower speci- fication limit (LSL) in their percent within limits (PWL) specs of 91.00% for lifts 50 mm (2 in.) or thinner; they specify an LSL of 92% for lifts thicker than 50 mm (2 in.). (A 2016 specification review under the FHWA Cooperative Agreement with the Asphalt Institute put the number of states with density requirements below 92% much higher at around 12 [Blankenship, 2017].) Seventeen states use a target of 92% and two use 92.5%. Five states target 93%, and three target greater than 93%. A total of 18 states have other targets. Many reported âotherâ because the tar- gets vary with mix type, application, lift thickness, or design air voids. Alabama targets greater than 94% and Georgia targets 95%. Montana and South Carolina, among others, offer incen- tives for densities greater than 94%. A majority of states (30, or 75% of respondents) have the same target densities if static rolling is required, as shown in Figure 11. Of the 10 that do not use the same targets, the reasons include using static rolling only where underground utilities may be damaged, using an approved roll- ing pattern, number of passes or test strip for static rolling, waiving the requirement, or making project-level decisions. Virtually all states use cores to determine density, 39 of 45 (see Figure 12). In many cases, they are used to correlate nuclear gauges but in almost half the states only cores are used for accep- tance. About half (21) the states use nuclear gauges and seven use nonnuclear gauges. A total of 25 states use AASHTO T 166, Standard Method of Test for Bulk Specific Gravity of Compacted Hot Mix Asphalt (HMA) Using Saturated Surface-Dry Specimens, to deter- mine core densities. One state uses AASHTO T 275, paraffin coating, and one routinely uses the vacuum sealing method (AASHTO T 331). Nine states have state methods, most of which are variations on AASHTO T 166 (see Figure 13). Figure 10. Target density requirements (Q5, %).
Figure 11. Are target densities the same if compaction is limited to static mode? (Q6, %). Figure 12. How is pavement density measured in your state? (Q7, #). Figure 13. If you use cores, how does your agency determine the bulk specific gravity of the cores? (Q7b, #).
28 Impact of Asphalt Thickness on Pavement Quality Figure 14. Have you observed paving contractors having more difficulty in obtaining required field densities since implementing the Superpave mix design system? (Q8, %). Agencies were also asked if they perceived that contractors had more difficulty compact- ing Superpave mixtures compared to their historic mix designs (Q8, see Figure 14). A total of 16 states said they had seen increased difficulty when they first implemented Superpave but the situation had improved. Of those states, three had increased lift thicknesses; two revised their density requirements; nine reported more use of fine mixes; six changed mix design parameters like gyration levels and minimum binder contents; two allowed the use of smaller NMASs; and 10 reported other changes (see Figure 15). Among the other changes, most cited changes made by the contractors within the existing specifications; contractors learned how to modify rolling patterns or trains or their mix designs to enable better compaction. Two states said achieving compaction is still an issue. Four states observed problems with certain mixes or applications; more information on this is provided in Question 11. No increased difficulty was noted by 11 states. Six respondents were unsure, and three states have not implemented Superpave. Of the DOTs adopt- ing Superpave, 17 states kept the same density requirements; 18 states changed their requirements. Many states have adjusted lift thicknesses since implementing Superpave (see Figure 16). At least 13 states reported doing so, but 15 states have not. Ten state respondents were unsure; in at least some cases this was reported by people who had not worked at the agency during the transition and thus lacked the requisite institutional knowledge. Three states have not implemented Superpave. When changes had been made (see Figure 17), they were usually based on recommendations from research such as NCHRP Report 531 (seven states). Four states reported seeing problems achieving compaction; two each observed increased permeability or other distresses, including segregation or raveling; and four reported other reasons. The other reasons included allowing the use of mixes with other NMASs, increased lift thicknesses for easier compaction, and crushing of aggregates. The next question explored the use of new tests or technologies by the states (Q10, see Figure 18). Experimental use is considered to be pilot projects or research. Routine implies more common, widespread use. No DOT reported using a workability test routinely and two use workability experimentally or occasionally. Two states use a permeability test routinely; eight states use permeability experimentally. Seven states routinely monitor mat temperature while 18 states are experimenting with the technology. Three states routinely use intelligent compac- tion (IC) and 19 are experimenting. Finally, seven states routinely use other technologies and
Survey Results: Current Practices Regarding Lift Thicknesses 29 Figure 15. If the situation has improved, what changed? (Q8b, #). Figure 16. Has your agency changed recommended or required lift thicknesses since implementing Superpave? (Q8c, %).
30 Impact of Asphalt Thickness on Pavement Quality three are experimenting. These other options include things like the use of material transfer devices, modified mix designs like Superpave 5 in Indiana and balanced mix design in Texas, segregation checks with a nuclear gauge, and minimum mix temperatures. There are a number of factors that can make achieving density problematic. Agencies were asked to identify which they had observed (Q11, see Figure 19). The most common response was low base or mix temperatures, reported by 18 states. The next most common were those com- ponents that tend to stiffen mixes, including polymer modified binders (12), reclaimed asphalt pavement (13), recycled asphalt shingles (5), ground tire rubber (2), and certain binder grades (5, which again often included modified binders). Thin lifts were also commonly cited (12). Highly angular aggregates (10) or certain aggregate types (3) were also noted. The problematic aggregate types included those for high traffic volumes, unstable or tender mixes, and natural sands. Other conditions were mentioned by 12 others including paving on bridge decks, over Figure 17. If lift thicknesses have been changed, what was the reason for the change? (Q9b, #). Figure 18. Does your state use these tests or technologies routinely or experimentally? (Q10, #).
Survey Results: Current Practices Regarding Lift Thicknesses 31 unstable bases or first lifts, high gyration mixes, dry mixes, and some source-specific combina- tions of materials. There are many strategies that reportedly have been used to address some of the difficulties mentioned. These included â¢ Use of warm mix asphalt technologies, â¢ Changes in gradations, â¢ Changes in gyration levels, â¢ Changing temperature restrictions, â¢ Changes in roller patterns or compactor types, â¢ Limiting recycled asphalt pavement (RAP) or changing RAP design requirements, â¢ Increasing binder contents, â¢ Changing allowable binder types, and â¢ Using smaller NMAS mixes. Several respondents noted that contractors have learned what they need to do, within the speci- fications, to achieve densities. As mentioned above, use of WMA technologies is used or allowed in some states to improve workability. A total of 32 states reported allowing the use of WMA at normal mix temperatures as a compaction aid (Q12). Ten states have a permissive specification or do not monitor whether WMA is used. No state responded that they do not allow WMA at normal temperatures, but it is known from other work that there are states that require temperature reductions. When asked if the use of WMA seems to improve the ability to achieve compaction, two states (7%) said it always helps, 13 (46%) said it frequently helps, 13 (46%) said it sometimes helps, and none said it never helps (see Figure 20). States were also asked if they had observed any pavement distresses that they attribute to inadequate compaction during construction (see Figure 21). As expected, the most common Figure 19. Have you observed contractors having increased difficulty obtaining adequate density in mixtures with certain components or in certain applications? (Q11, #).
32 Impact of Asphalt Thickness on Pavement Quality responses were increased permeability (23), increased cracking (23) and decreased durability (31), which could all be related to increased air voids. These problems could also contribute to shorter service lives (32) and increased maintenance needs (29). Other distresses (13) include segrega- tion, raveling, and joint failures. Rutting was not noted as a big problem (4). Ten states (24%) reported conducting research into pavement density and lift thickness and seven (17%) are planning or considering initiating research (Q14, see Figure 22). A total of 16 states (38%) have not conducted this type of research. Somewhat surprisingly, 9 respondents Always Frequently Sometimes Figure 20. Does the use of WMA as a compaction aid seem to improve the ability to get density? (Q12b, %). Figure 21. Have you observed any asphalt pavement performance problems that are perceived to be related in whole or in part to inadequate compaction during construction? (Q13, #).
Survey Results: Current Practices Regarding Lift Thicknesses 33 (21%) were unsure; this may be because they were not in the research divisions in their agencies or had not been around long enough to know about past research. The DOTs did indicate needs for research, as summarized in Figure 23. Understanding the effects of mixture properties (25 responses) and improved test methods or technologies to ensure adequate compaction (23) were the most common answers. Findings ways to achieve adequate density (17), verifying t/NMAS ratios (12), and improving mix compactibility (13) were also popular. Six respondents offered more specific topics including nonnuclear measurement Figure 22. Has your state conducted or sponsored any research related to asphalt pavement density and lift thickness (currently or in the past)? (Q14, %). Figure 23. Do you see a need for additional research into any of these topics? (Q15, #).
34 Impact of Asphalt Thickness on Pavement Quality options, intelligent compaction, improved permeability tests, lightweight deflectometer testing, and ways to determine uniformity of compaction. Finally, respondents were given the opportunity to offer other comments. Some of these included the following quotes: â¢ âContractors have sometimes mentioned that the specified lift thickness is too thin to adequately densify the pavement.â â¢ âOur lift thicknesses can cause difficulty achieving optimum compaction, however, the desire to earn incentives based on ride quality often trumps the problems of inadequate density.â â¢ âOur experience has been that the nationally recommended lift minimums and maximums hold true for the mixes in our state, for the most part. We have done informal research on 4 in. lifts placed in a slot by taking cores and verifying that the density profile is consistent throughout the lift. I believe [my] DOT switched to Superpave in the late 90s, when I was still in college, so I am unaware of what transition pains took place in achieving density when the change was made. I know at some point we switched from a target density as a percentage of maximum achieved in the control strip.â â¢ âMore lift thickness certainly helps density but economics drives thickness for us.â â¢ âIâve always been of the opinion that smaller is better when it comes to NMAS and thicker is better for lift thicknesses. But what are reasonable (cost effective?) limits for each of these parameters?â 3.2 Canadian Responses Five Canadian provincesâAlberta, British Columbia, Manitoba, New Brunswick, and Ontarioâresponded to the survey; three of those have implemented Superpave and one is actively considering implementation. These responses, then, may not be representative of the other provinces. In addition, those provinces that have implemented Superpave mostly did so a little behind some of the states and so may have benefited to some extent by the early lessons learned in the United States. This was, at least, the intent of the AASHTO Lead State Team. A brief summary of the Canadian responses follows. â¢ Three of five provinces responding have policies or guidelines for lift thicknesses in design memos, specifications, tender documents, or other guidelines. â¢ Only Ontario uses dense-graded 4.75-mm mixes, and their maximum lift thickness is 19 mm, 4ÃNMAS, for both fine and coarse gradings. â¢ For the other NMASs, Ontario generally meets the recommended 3ÃNMAS for fine and 4ÃNMAS for coarse graded mixes; Alberta comes close or brackets the recommended ratios but does not differentiate fine and coarse. â¢ Most provinces (three of five reporting) will consider granting exceptions to the required lift thicknesses for various project- and site-specific reasons. â¢ British Columbia has a target density of 92% Gmm but has not implemented Superpave; New Brunswick targets 92.5%; Alberta targets >93% for Superpave but still mostly uses Marshall; Ontario uses PWL specifications and targets 92% for Superpave with 100% pay for densities over 90%. â¢ As in the United States, the provinces reporting rely on cores for acceptance. They use ASTM D2726, AASHTO T 209, or their own, similar methods to measure density. â¢ Alberta and New Brunswick have not observed contractors having more compaction issues with Superpave mixes; Ontario has with certain mixes; British Columbia and Manitoba have not implemented Superpave. â¢ Ontario did change lift thicknesses when implementing Superpave; Alberta and New Brunswick did not.
Survey Results: Current Practices Regarding Lift Thicknesses 35 â¢ New Brunswick routinely assesses workability using visual cues, like checking, cracking, push- ing, and shoving, and they are experimenting with using mat temperature. Ontario is using a permeability test experimentally. â¢ The provinces reported seeing compaction issues with mixes containing modified binders, ground tire rubber (GTR) or certain binder grades, or when placing in thin lifts or lower base or mix temperatures. â¢ Warm mix technologies are sometimes used to aid compaction. â¢ The provinces consistently report issues with excessive permeability, durability, service life, and the need for increased maintenance when density is not achieved. In summary, at this time the Canadian provinces that responded to the survey generally have less experience with Superpave than many U.S. states. Nonetheless, their experience, while varied from province to province, is similar in many regards to the U.S. experience. They are seeing some of the same issues with difficulty compacting some types of mixes, but at least two provinces have not observed more problems with Superpave than with other (Marshall) mixes. Practices for density measurement, target density levels, and lift thicknesses are similar to the states. It is important to recall, however, that only five provinces completed the survey, so these findings may not be truly representative. 3.3 Industry Survey Responses A total of 62 industry representatives responded to the industry survey. Those contractors work in 28 states, as shown in Figure 24. The survey was distributed through NAPA. Some state asphalt pavement associations (SAPAs) also distributed the survey notification to their members, explaining why some states provided a large number of responses. This is known to be the case in Indiana and Massachusetts, and perhaps other states. The number of respondents per state is shown by the number on that state in Figure 24. Because the number of responses per state varies and many states are not represented, these results may not be reflective of the industry as a whole. Since many contractors work in more than one state, the industry respondents were asked to report all of the states in which they work (Q1, see Figure 24) as well as the state where they work 5 1 1 1 1 1 3 15 2 1 33 1 12 5 2 3 2 3 7 2 1 1 1 6 2 1 Figure 24. States represented by industry respondents; number of respondents per state (Q1, #).
36 Impact of Asphalt Thickness on Pavement Quality most often. In Appendix D, industry responses are identified by both all the states they work in as well as the most common one. Their answers could reflect any state in which they work but most likely focus on the state where they most commonly work. Most industry respondents were aware of their statesâ lift thickness guidelines or policies; 55 of 62 (88%) were aware, four (7%) were not, and three (5%) were unsure (Q2, see Figure 25). Most seemed to think these policies were in their statesâ specifications and were less aware of the statesâ pavement design manuals. This is, perhaps, not surprising since many contractors would not be involved in doing pavement designs, as opposed to consultants, so they may not be familiar with pavement design documents. One contractor admitted to not knowing how the requirements were enacted but knowing they exist. More contractors report using cores for quality control testing (54 in Figure 26, Q3) than report using nuclear density gauges (36) or nonnuclear gauges (26). This may be because many states require gauges to be calibrated to cores even if the contractor regularly uses the gauge to monitor density. Some of the comments note that contractors pull the cores, which are provided to the agency for testing. Figure 25. Are you aware of any established policies or guidelines regarding appropriate asphalt lift thicknesses relative to the nominal maximum aggregate size (NMAS) of the mixture in any of the states in which you work? (Q2, %). Figure 26. How do you typically measure pavement density for quality control? (Q3, #).
Survey Results: Current Practices Regarding Lift Thicknesses 37 About a third (20 or 34% as shown in Figure 27) of the industry representatives reported having difficulties attaining density when Superpave was first implemented but having an easier time now (Q4). One (2%) reports continuing to have difficulty, and 19 (31%) have difficulty with certain mixes or applications. Among those indicating the situation has improved (see Figure 28), most said the mix design parameters have been changed (14) and/or finer mixes are being used (13). Seven report using smaller NMAS mixes, six cite other changes, and three say the state changed the minimum lift thickness. The other changes noted include learning how to change the mix designs to make them more compactible, changing compaction procedures, or using WMA technologies. The most commonly cited reason contractors have difficulty achieving density (see Figure 29) is the use of thin lifts (14 of 60 responses). Low base or mix temperatures (9) and when com- pacting over concrete (8) are also commonly reported. Mix stiffeners including polymers (5), certain binder grades (4, all polymer modified), RAP (2), recycled asphalt shingles (RAS) (4), ground tire rubber (2), and highly angular aggregates (4) were also noted. Slag or manufactured aggregates were noted by two respondents as being occasional problems. Figure 27. Have you observed more difficulty in obtaining required field densities since implementing the Superpave mix design system in the states where you work? (Q4, %). Other Smaller NMAS Mix Design Finer Mixes 7 Lift Thickness 6 3 14 13 60 2 4 8 10 12 14 16 Figure 28. If the situation has improved, what changed? (Q4b, #).
38 Impact of Asphalt Thickness on Pavement Quality Most of the industry respondents (50 of 57, 88%, see Figure 30) report that they are able to use WMA technologies as compaction aids. Only one said this was not allowed. When WMA is used as a compaction aid (see Figure 31), 11 respondents (20%) said they always use it, 27 (50%) say it is frequently used, 10 (16%) say it is sometimes used, and eight (14%) never use it as a compaction aid. Overall, then, it seems logical that most contractors find WMA frequently or always helpful as a compaction aid; if not, they would not use it. Contractors do acknowledge that there are pavement performance problems associated with inadequate compaction (see Figure 32). Increased permeability (25), increased cracking (23), and decreased durability (31) have been observed. These lead to shortened service lives (36) and the need for increased maintenance (24). A total of 15 respondents commented that they had seen increased densification under traffic. Nine respondents indicated other; these responses included such things as raveling, spalling, and joint failure. One respondent commented that they have observed many of these issues but they were not necessarily compaction related. However, another respondent said almost all performance problems can be attributed to com- paction problems. Another acknowledged that these problems could be âself-induced due to poor planning, equipment breakdowns and unexpected weather.â Figure 29. Have you observed any increased difficulty obtaining adequate density in mixtures with these components or in these applications in any of the states where you work? (Q4c, #). Figure 30. Do any of the agencies you work for allow the use of warm mix asphalt technologies at conventional temperatures as a compaction aid? (Q5, %).
Survey Results: Current Practices Regarding Lift Thicknesses 39 When asked if they had encountered situations where they would request exceptions to the required lift thicknesses (Q7), the contractors were pretty evenly split. A total of 30 (52%) had encountered those situations and 26 (48%) had not, as in Figure 33. The reasons to request an exception (see Figure 34) included anticipated compaction issues (29), smoothness require- ments (21), late season paving (11), and maintenance of traffic issues (6). Other reasons (5) included anticipated segregation issues, unstable base conditions, short construction times, and drop-off concerns, among others. In most cases, contractors were not aware of any performance issues in those cases where they requested exceptions (19). In 11 cases, they did note problems with low densities, cracking from over-rolling, raveling, surface texture, and profile problems. When asked if they have had to change mix designs, compaction operations, or other pro- cesses to cope with compaction difficulties (Q8), the vast majority of contractors (48, 84%) indicated they have had to make changes in their mix designs, compaction operations, or other processes to facilitate compaction, as shown in Figure 35. Nine contractors (16%) had not made changes. Figure 36 shows how successful these are perceived to be. Increasing compactive effort and changing roller patterns have been largely successful. Others report that use of fine mixes, less angular aggregates, other mix design changes, WMA, more compactive effort, and different Figure 31. If you are allowed to use WMA as a compaction aid, how often do you use it in those states where it is allowed? (Q5b, %). Figure 32. Have you observed any asphalt pavement performance problems that are perceived to be related in whole or in part to inadequate compaction during construction? (Q6, #).
40 Impact of Asphalt Thickness on Pavement Quality Figure 33. Have you encountered situations during project planning or construction where you have requested exceptions or waivers to the lift thickness requirements? (Q7, %). Figure 34. In what situations would you consider requesting changes to the specified lift thicknesses? (Q7b, #). Figure 35. Have you had to change your mix designs, compaction, or other processes to cope with compaction difficulties? (Q8, %).
Survey Results: Current Practices Regarding Lift Thicknesses 41 roller patterns and types of rollers have been only marginally successful. Use of intelligent com- paction and thermal imaging were seen as being not very successful. Contractors would like to see more research into the field validation of t/NMAS ratios (29, see Figure 37), improved test methods or technologies (21), ways to improve mix compactibility (21), and the effects of mix properties on compactibility (18) (Q9). Fewer respondents (12) are interested in exploring ways to achieve adequate density. There were a number of interesting comments in reaction to the question about research needs and a final question asking for additional comments. Some of these additional thoughts are listed below. A number of industry representatives commented on the causes of density problems and effects on performance. â¢ âMost (70%) of our density issues stem from poor execution in the field, 25% from poor field conditions, and 5% from mix production.â â¢ âWe use 15 ton rollers to beat the mix into submission. Meets compaction, not good for the mix.â Figure 36. If yes, what changes have you chosen to make and how successful would you judge them to be? (Q8b, #). Figure 37. Do you see a need for additional research into any of the following? (Q9, #).
42 Impact of Asphalt Thickness on Pavement Quality â¢ âI believe the performance issues in [my state] are related more to low/inadequate asphalt binder content (as allowed by the specifications) and are not the result of inadequate compaction or lift thickness.â â¢ âThe DOT typically specifies lift thickness for surface and intermediate courses that are near the minimum for compaction based on NMAS. This is done for economy. We support this practice since pavement type is largely selected competitively on the basis of initial cost. But, it does make achieving compaction and smoothness more challenging.â â¢ âMany agencies are moving towards thin lifts without considering the need to increase paver speed or decrease production, add rollers or decrease production, without considering the reduced time available to compact, and without considering the increased difficulty in evalu- ating densityâmore influence of underlying layers with nuclear gauges, and more difficulty in obtaining and measuring cores. I would like to see more details on some of the mix changes some agencies are employing for thin lift applications and their effectiveness.â Some respondents commented specifically on the effects of lift thickness. â¢ âMy personal experience as it applies to lift thickness and its effects on pavement perfor- mance is that if the lift thickness is within 2.5â4 [times] the nominal maximum aggregate size, it significantly reduces your chance of issues. These issues can include but are not lim- ited to segregation (physical and thermal), compaction, bond, and workability. All of these issues can lead to an overall failure in performance. This has not been too much of an issue in our state contracted work, but has been more of an issue in our private work (parking lots, town roads, etc.). Any time we have been under 2.5 [times the NMAS], we have noticed these issues arising more frequently. A lot of the time these specifications do not take into account that a paver is designed to scalp high spots and fill in low spots over the underlying surface in order to make it flat. When a high spot is scalped in the underlying surface, it reduces your lift thickness and if not accounted for can be detrimental to the overall quality of the final surface.â â¢ âWe have observed the thicker the mat the easier to achieve density. Mostly running our roll- ers in echelon with rollers with 84 [in.] drums have eliminated most of our density issues, regardless of mat thickness.â â¢ âWe have found that lift thicknesses may be increased with no significant issue obtaining density. The key is ensuring communication and execution of increasing compactive effort to match. The crew has to be very intentional about achieving density and not delay on a single part of the process. Weâve found this to be true for base, binder and surface mixes.â â¢ âWhen used appropriately, lift thickness and NMAS correlate well together. Mixes [designed] with the correct NMAS and placed in the correct lift thickness have sufficient structural aggre- gate skeleton to support the design traffic loads and [can] be compacted well enough to be impermeable to prevent water infiltration, mix stripping, and subgrade degradation.â â¢ âThicker pavement sections tend [to] improve all aspects of pavement performance.â â¢ âWe achieve superior quality when the plans call for 4 or 5 [times] NMAS lift thickness. This is probably the MOST important factor in compaction and ride quality. This will ensure longer pavement life.â Last, a few respondents suggested research needs. â¢ âCorrelation of lab permeability to field nuclear density to minimize destructive testing.â â¢ âBalanced mix design/air void regression mix designs to slightly increase binder content.â â¢ âField validation of increased permeability versus lift thickness would be a good avenue to go. In terms of mix designs, most, if not all, contractors know how to change their mixes to give them the best chance at compaction success.â â¢ âFlip side of [minimum] thickness to [nominal maximum aggregate] size. . . . How thick is too thick?â
Survey Results: Current Practices Regarding Lift Thicknesses 43 3.4 Analysis and Summary of Survey Findings As shown in the preceding sections of this chapter, there are similarities and differences between the agenciesâ and industryâs awareness and perceptions of issues regarding lift thick- nesses and the related issues of density and pavement performance. â¢ Most states and provinces have policies and guidelines regarding allowable lift thicknesses. Most industry representatives are aware of these guidelines, though they may not know exactly how they are promulgated. â¢ Many, but not all, state lift thickness requirements meet the minimum recommended t/NMAS ratio of 3:1 for fine, dense-graded mixtures but very few use a minimum of 4:1 for coarse gradings. â¢ While many contractors have developed ways to accommodate compaction, many also indicate being hampered by these lift thicknesses. â¢ There is a wide variety of target densities specified by the states, ranging from less than 92% to more than 94%. While this might not seem like a huge range, the impacts on performance and service life, as reflected by the literature review, can be significant. â¢ Most states use cores for acceptance and contractors tend to use nuclear gauges, often correlated to cores, for quality control. â¢ Most of the states using cores evaluate the bulk specific gravities using AASHTO T 166 or a modification thereof. â¢ Agency and industry responses to the question about whether contractors had more dif- ficulty obtaining compaction were quite similar. There were initially issues that have been largely resolved. A few instances of difficulties persist. More contractors seem to recog- nize issues with certain materials or applications than do the agencies. However, both groups acknowledge that those materials that tend to stiffen mixes tend to create the most difficulty. â¢ In cases where the situation has improved, both agencies and industry note the use of finer mixes, and changes in the mix design parameters have been helpful. More contractors seem to think the use of smaller NMASs helps. â¢ Use of mat temperature monitoring and intelligent compaction by agencies seems to be increasing, though industry representatives do not seem to think these technologies are very useful for improving densities. â¢ Fewer than half of the responding states changed their lift thicknesses since implementing Superpave. Those that did make changes did so for a variety of reasons including research recommendations and observations of increased difficulty in achieving compaction with Superpave mixes at specified lift thicknesses. Contractors are in favor of these increases in lift thickness though they recognize the importance of controlling costs. Some contractors commented that the use of smaller NMAS mixes at the same lift thickness could help achieve density without increasing costs. â¢ Most states would consider granting exceptions to the minimum lift thicknesses for cause and most contractors have encountered situations where they would request them. Industry acknowledges this sometimes leads to performance problems. â¢ Most contractors have made changes to their mix designs, material selection, or compaction procedures within the specifications to make achieving density somewhat easier. How suc- cessful these changes are perceived to be varies. â¢ While some states have individually researched issues related to density and lift thickness, more have not, though most recognize the need for more research. Industry also sees a need for more research in certain areas (see Chapter 5).