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51 This chapter summarizes the overall findings of the synthesis. Last, it points out gaps in the state of the knowledge and topics that could be the subject of additional research, as observed by survey respondents and the literature review. 5.1 Conclusions This synthesis study involved a comprehensive literature review, survey of state and provincial transportation agencies, and survey of paving contractors and state paving associations to assess the state of the practice regarding the effects of lift thickness on the performance of asphalt pavements. The major findings of this effort include the following. 5.1.1 Findings from Literature Review Asphalt pavement density has been recognized as one of the most important factors affect- ing asphalt pavement performance. Some engineers maintain it is the single most important factorâand there is evidence to back up that claim. Field experience shows that poorly com- pacted pavements can densify under traffic, resulting in rutting. The high air void content in a poorly compacted mat allows air to flow through the pavement, increasing binder oxidation and contributing to increased cracking. The void content and related pavement permeability also allow water to enter the pavement, exacerbating moisture damage. Other distresses, like raveling, potholes, and poor ride quality (high IRI), can also result. One of the key factors affecting the ability to achieve adequate density is the ratio of the asphalt lift thickness, t, to the size of the aggregate in the mixture. In the past this ratio was sometimes related to the mix maximum aggregate size, while more recently it was expressed in terms of the mix nominal maximum aggregate size, NMAS. Lifts need to be thick enough to provide room for the aggregates to reorient and densify but not so thick that the bottom of the lift does not feel the compactive force and therefore does not densify. The t/NMAS ratios recommended by various researchers differ somewhat, especially in older references. To help ensure that adequate density can be achieved, a great deal of recent literature recommends that the t/NMAS ratio be at least 3:1 for fine graded mixes and at least 4:1 for coarse graded mixes. Some researchers recommend even higher ratios, especially for coarse mixtures. Thin lifts make it more difficult to achieve adequate compaction, especially at longitudinal joints. Permeability, or the ability for water to enter the pavement, depends not only on the total volume of air voids, but on how interconnected those voids are, as the connections create chan- nels for the water to flow through. Larger air voids, such as those in coarse graded mixtures, are more likely to be interconnected, which is why a higher t/NMAS ratio is needed to facilitate C H A P T E R 5 Conclusions and Further Research
52 Impact of Asphalt Thickness on Pavement Quality compaction of coarse mixes. As air void content increases, density decreases and the likelihood of excessive permeability increases. Many laboratory and field studies show that there is an air void content at which a mix becomes dramatically more permeable. The point at which this occurs depends on a number of variables, including the nominal maximum aggregate size and shape of the gradation curve (fine versus coarse). While the numbers vary, there is substantial agreement that most mixes are relatively impermeable below about 5% to 6% air voids. Finer mixes can remain impermeable at higher air void contents than coarse graded mixes. The same can be said for smaller NMAS mixes compared to larger NMAS mixes because there is less likeli- hood that the voids will be interconnected with smaller sized aggregates. Permeability problems could exist with earlier mix designs but came to the forefront during the implementation of Superpave when changes in the material selection and mix designs, espe- cially the use of coarse mixes, made compaction more difficult. Early Superpave mixtures in the mid to late 1990s were frequently coarse graded because designing rut-resistant asphalt mixtures was a primary goal of Superpave. Coarse graded mixes were generally perceived to be more resistant to permanent deformation than fine graded mixes. Other features of Superpave mixes, such as use of more angular aggregates and more use of modified binders, tended to make stiff mixes that were hard to compact. As experience with Superpave grew, the material requirements and design parameters were refined and more states returned to the use of fine graded mixes. There is research and experience suggesting that fine graded and smaller NMAS mixtures can perform very well in terms of rutting and other performance measures. With the changes brought about by the implementation of Superpave, smaller NMAS mixes can be very rut resis- tant and strong. Both fine gradations and smaller aggregate sizes can facilitate compaction in thinner lifts than are necessary for coarse graded and large NMAS mixtures. Thus, there may be options for economically increasing lift thickness ratios by using smaller NMAS or finer mixes, which would then be easier to compact, in the same lifts or to maintain the same total thickness or pavement strength. There is little published research on the cost implications of increasing lift thickness, though obviously thicker lifts require more material and therefore have higher initial costs. One study used life cycle cost analysis to explore the benefits of thicker lifts on performance. Literature reviewed in that study indicated the higher densities possible with thicker lifts have substantial positive impacts on fatigue life, rutting resistance, and service life, off-setting the higher initial costs. Other research shows positive impacts of higher density on resilient modulus, beam fatigue, and durability, which all contribute to longer service lives. 5.1.2 Findings from Agency and Industry Surveys A total of 45 states responded to a survey for this synthesis. Five Canadian provinces responded to the same survey. The National Asphalt Pavement Association helped to publicize a similar survey for industry, and a total of 62 industry representatives from state paving associations and contractors responded. Those industry representatives work in 28 different states. The survey results show that most states and provinces have guidelines regarding allowable lift thicknesses and most industry respondents are aware of their existence. The survey of the states, however, shows that many states use t/NMAS ratios below the recommended levels, especially for coarse mixes. In fact, few states differentiate between fine and coarse mixtures, though many do account for different sizes or types of mixes. The recommended 4Ãt/NMAS ratio for coarse graded mixes is rarely specified. The industry respondents almost all agreed that thicker lifts are easier to compact and perform better. A majority of states would consider granting exceptions to the minimum lift thickness requirements. Exceptions may be granted for a variety of reasons including constructability
Conclusions and Further Research 53 issues, maintaining cross-slopes and curb heights, scratch courses, to limit drop-offs, and to control costs. About half the contractors responding indicated they would ask for exceptions in some cases such as anticipated compaction issues, smoothness requirements, late season paving, and maintenance of traffic issues. In a few cases, contractors did acknowledge these exceptions may have contributed to performance issues, but most were not aware of any problems. Most states use pavement cores for density acceptance and use AASHTO T 166 or a similar method to determine the core density. Contractors use either cores, nuclear gauges, or non- nuclear gauges for quality control. The target densities for many states fall below the levels that might help ensure the mat is not permeable, with some density targets below 92% (8% air voids). However, a few states success- fully use targets of 94% or greater. A majority of the states responding acknowledged that achieving density was more difficult after the implementation of Superpave. (Three states and three provinces responding have not implemented Superpave at this time.) In more than half of those states, however, the situa- tion has improved since the early days because of changes in mix design parameters, lift thick- nesses, and the use of finer gradations, among other factors. Contractors largely agreed that the situation has improved and they have learned how to deal with difficult mixes. Agencies and industry agreed that some mix components, particularly those that stiffen mixes, and some construction conditions, such as thin lifts and low temperatures, can make achieving density more problematic. A number of states are experimenting with and a few have implemented new tests or technolo- gies to assess adequate compaction; these include things like permeability and workability tests, warm mix asphalt technologies, thermal imaging, and intelligent compaction. States and industry agree that there are still research needs relative to lift thicknesses, density, and performance. There is a perceived need to further verify recommended t/NMAS ratios in the field. Others see a need to better understand the material and mix properties that impact compactibility. None of the states responding said that they had data correlating lift thickness to performance aside from what was reported in the literature review. The impacts of lift thickness on service life are apparently not being documented except in a few cases of limited research. 5.1.3 Lessons Learned from Case Examples The Florida DOT was the first to come forward with its issues regarding high permeabilities in its early Superpave mixes. It implemented and shared its experiences with a number of changes to its specifications and practices that have been quite successful at improving permeabilities, achieving adequate densities, and overall improving the level of service on its roadway network. Increasing the lift thickness and using fine graded mixtures are two key changes. Florida DOT Materials and Construction personnel make a point of communicating on a regular basis, which helps address problems as they arise. The South Carolina DOT has used thick lifts (t/NMAS â¥ 8) on several high-volume paving projects and for full depth patching projects. The prime motivation for this is speed of con- struction. The thick lifts were accompanied by mix design changes (increased binder con- tents, warm mix) to make the mix more compactible. Cores taken on these projects showed uniform density from top to bottom and the field performance after several years has been very good. South Carolina sponsored a section at the NCAT Test Track to test how thick a lift can be placed. Construction in late August 2018 seemed successful with 7 in. (175 mm) of a 12.5 mixâa t/NMAS of 14. The results of trafficking this mix on the track will be of great interest.
54 Impact of Asphalt Thickness on Pavement Quality At the other extreme, the Maryland DOT has put in place definitions of thin lifts and proce- dures for determining density of thin lifts. A test strip is constructed, and nuclear gauge readings are correlated to cores. The gauge is then used to monitor the compaction of thin lifts without additional coring. The thin lift specification has been well received and reportedly works well. The specification is also used for variable depth paving and other applications. FHWA is working with a number of states to explore various means of achieving higher pave- ment densities. At least two of these states to date have constructed test sections using differing NMAS mixes to yield different t/NMAS ratios. It is unknown if other states will do this in future phases of the study. The results to date indicate that it is possible to achieve higher densities in a variety of ways. Using a smaller NMAS (higher t/NMAS) led to a higher density in one case and a similar density in the other case, but all of the densities were quite high, so the capacity for improvement was limited. Last, a veteran mix designer shared his experiences with coping with hard-to-compact mix- tures. He has adjusted by using finer gradations and the Bailey method in mix design as well as more compactive effort in the field. He advocates for the use of thicker lifts and different lift thicknesses for fine and coarse mixes. To avoid cost increases, this designer recommends using a smaller NMAS mix in the same lift thickness. This synthesis has been prepared to summarize the literature and practical experience related to the interrelated effects of lift thickness and density on pavement performance. Given the substantial effect of asphalt pavement density on performance, ways to make mixtures more compactible could have a great effect on improving performance. Increasing lift thickness or tailoring the thickness, mixture nominal aggregate size, and gradation to the application are two of the key ways to facilitate compaction. 5.2 Gaps in the Knowledge and Future Research Needs Despite an extensive amount of research into the effects of lift thickness and mat density on pavement properties, most particularly on permeability, there are still unanswered questions that could perhaps be addressed through more research. NCHRP Report 531 documented evidence that lift thicknesses should be 3ÃNMAS for fine graded mixes and 4ÃNMAS for coarse graded mixes. These values have been confirmed or even increased in other research, and yet, both agencies and industry expressed a need for further field validation of these levels. The effects of various mix properties on compactibility are not completely understood. Both agencies and industry would like to understand the relationships better. This in turn might help us to understand how to make mixes more compactible and how to more easily obtain adequate density in the field, which are of particular interest to industry but also to agencies. Agencies expressed interest in improved test methods or technologies to ensure good compaction, as did industry. The behavior of asphalt materials, their compaction, and ultimate performance of the pave- ment are complex issues with many interrelated factors. It is widely agreed, however, as docu- mented in this synthesis, that achieving adequate density is the most critical factor affecting the performance of asphalt pavements. What is more, the relationship of lift thickness to the nominal aggregate size of the mixture is known to have a major effect on the ability to achieve compaction. Better understanding these relationships and how to deal with them to produce high-quality pavements suited to their specific applications would benefit both agencies and industry.