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3 Asphalt surfaced roadways comprise the vast majority of the nationâs roadway network. As states continually deal with critical infrastructure needs and limited budgets, ensuring a long service life from asphalt pavements is critical. It is widely recognized that the most important factor affecting asphalt pavement life is density. Therefore, understanding how the lift thickness and nominal maximum aggregate size (NMAS) in the asphalt mix affect the ability to adequately densify the pavement is crucial. The ratio of the lift thickness to nominal maximum aggregate size (NMAS) is critical to achiev- ing adequate asphalt mixture compaction during construction, which in turn has a profound impact on asphalt pavement performance and durability. If the lift thickness is inadequate, there is not enough room for the aggregate particles to be reoriented and densified; this can be exacerbated by rapid cooling of thin lifts, which shortens the time available for compaction. If the lift if too thick, the material at the bottom of the lift may be beyond the zone of influence of the compactor and thus may not be adequately densified. While the need for a balance of lift thickness to NMAS had been recognized historically, the issue was subject to increased scrutiny during the early days of SuperpaveÂ® implementation. Superpave gradation limits included a ârestricted zoneâ in the intermediate size range. This zone was intended to prevent tender mixes, which were often observed when gradations exhibited a âsand hump.â The Superpave gradation control points, restricted zone, and emphasis on elimi- nating rutting encouraged the use of coarse graded mixtures. These mixes were frequently more difficult to compact than previously used, often finer, gradations. Low mat densities contributed to various distresses, including rutting, raveling, moisture damage, and more. In addition, low density can lead to excessive permeability, which can worsen durability and moisture damage issues by allowing the infiltration of air and water into the pavement. The early experience with coarse mixes brought increased attention to the interrelated issues of lift thickness, mix size, density, and permeability. Numerous research efforts were con- ducted across the country by individual states, groups of states, and national organizations. Many avenues were explored to make mixes more readily compactible, including increasing the lift thickness to help retain heat in the mix and allow adequate room for aggregates to be reoriented into a denser mat. NCHRP Report 531, published in 2004, documented the key relationships between lift thickness, air voids or density, and permeability (Brown et al., 2004). The authors recommended, among other things, a ratio of lift thickness to NMAS of at least 3:1 for fine graded mixtures and at least 4:1 for coarse graded mixtures. Several other research efforts confirmed the benefits of increased lift thicknesses, but the recommendations have not been universally adopted. This synthesis assembles and summarizes pertinent literature on the effects of lift thickness on asphalt pavement performance. It also includes survey responses from 45 state departments C H A P T E R 1 Introduction
4 Impact of Asphalt Thickness on Pavement Quality of transportation (90%) and more than 60 industry representatives to assess the current state of the practice and recent experience relative to the effects of lift thickness. The synthesis explores agency policies regarding lift thickness and density requirements for surface mixes, the rationales for these policies, and their impacts on pavement performance. It provides background on agency requirements for mix sizes relative to structural design, when and why exceptions are made to these requirements, performance of pavements versus ratio of lift thickness to NMAS, and more. 1.1 Background Adequate compaction of asphalt mixtures during the construction process has long been rec- ognized as critical to the satisfactory performance of asphalt pavements. In fact, it is frequently called the single most important factor influencing performance (Hughes, 1989). Achieving proper density can improve the performance of a marginal mixture, but inadequate compaction can result in premature failure of a good mixture (Hughes, 1989). Compaction has been defined as âthe process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangementâ (Leiva and West, 2008). The intent of compaction is to increase the density of the mixture, reducing air voids that allow the infiltration of air and water into the pavement, and bringing aggregate particles into close contact to increase stability. As air voids decrease, density increases. Some air voids are needed in a pavement to allow for the expansion of asphalt binder at high service temperatures, but an excess of air voids allows water and air to flow through the pavement. Excess air intrusion can increase the rate of oxidation of the asphalt binder (Brown, 1982), increasing its brittleness and increasing the chances of cracking. Moisture entering the pavement can cause stripping of the asphalt binder film, which reduces the strength and dura- bility of the pavement. Reaching the proper range of densities (or air voids) has a positive effect on stability, durability, fatigue and thermal cracking resistance, flexibility, impermeability, and resistance to moisture damage (Hughes, 1984). The ability to adequately compact an asphalt mixture depends on a number of factors, includ- ing the environmental conditions at the time of construction (air and base temperatures), mixture and structural properties (aggregate size, shape or texture and gradation, binder type and content, mix temperature, lift thickness, and more), and construction operations (number and type of rollers, roller patterns, etc.) (Leiva and West, 2008). Some of these are within the contractorâs control or the purview of agency specifications and design practices. Other factors cannot be controlled easily, such as air or base temperature, but sometimes can be accommo- dated by changing other elements of the mix production or construction operations (Decker, 2006). For example, the mix production temperature may be increased to accommodate low ambient temperatures or longer haul distances. Breakdown rollers can stay close to the paver to compact the mix quickly before it cools. The ratio of the lift thickness (t) to the NMAS has been recognized by many researchers and practitioners as a key factor affecting the ability to compact a given mixture in a given applica- tion. Hughes (1989) pointed out that there are three aspects of lift thickness that are impor- tant during construction: the total lift thickness, which affects cooling of the mat; the relation between the lift thickness and maximum aggregate size; and the uniformity of the lift thickness. Uneven lift thicknesses, such as when paving over a rutted underlying layer, can result in vari- able densities across the mat as the roller bridges over the ruts or in the underlying profile being reproduced (reflected) in the new surface. As noted, there are many factors that influence the compactibility of an asphalt mix and many of these can be compounded in a given situation. Therefore, while the emphasis of this synthesis
Introduction 5 is on lift thickness, some other options are presented to facilitate compaction if the lift thickness cannot be increased. Of course, lift thickness and mixture type also affect pavement structural design, but the focus of this synthesis is the effect of lift thickness during construction on the compactibility of the mixâthe ability to densify the mat sufficiently and uniformlyâand the resultant pavement performance. 1.2 Synthesis Approach The intent of this synthesis is to explore agency policies regarding lift thickness and density requirements, the rationales for these policies, and their effects on pavement performance. This is accomplished by a review of pertinent literature and agency specifications as well as surveys of state and provincial departments of transportation and industry. The surveys and follow-up interviews investigate how decisions are made regarding mix type and size relative to structural design, when and why exceptions are made to the lift thickness requirements, performance of the pavements constructed in accordance with the policies and those where exceptions have been made, and other related issues. The surveys also document related factors, such as the density requirements in force, the methods of measuring density, compaction tempera- ture requirements, and the use of warm mix asphalt additives (with or without a temperature adjustment). The emphasis of this synthesis is on state and provincial agency policies, practices, and pave- ment performance. A secondary focus is on a survey of paving contractors to explore the reasons why or situations in which they would seek exceptions to the policies, their experiences with compacting asphalt mixtures, and steps they can take to achieve adequate compaction with dif- ficult mixtures. The second chapter of this synthesis report summarizes the literature review. More than 120 papers, reports, articles, and other documents related to aspects of compaction, lift thick- ness, and performance were reviewed. The Transport Research International Documentation (TRID) database was the main source of references. Other databases, Google, and the reference lists in individual reports were also used to identify relevant documents. Because early Superpave mixtures were often difficult to compact and led to increased emphasis on lift thickness, a brief history of the evolution of Superpave is presented in Chapter 2. The third chapter summarizes the agency and industry responses to the survey questions. Representatives of 45 states and 5 provinces responded to the agency survey. There were 62 responses to the industry survey. Copies of the questionnaires and the detailed, tabulated survey responses are in the appendices. Chapter 4 includes case examples describing experiences of agencies and industry regarding the effects of lift thickness and approaches to dealing with related issues. Chapter 5 summarizes the overall findings of the synthesis, lessons learned, and gaps in the knowledge.