Practices for Unbound Aggregate Pavement Layers (2013)

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3 chapter one INTRODUCTION INTRODUCTION AND BACKGROUND As defined by the ASTM International in ASTM D 8-11, an aggregate is “a granular material of mineral composition such as sand, gravel, shell, slag, or crushed stone, used with a cementing medium to form mortars or concrete, or alone as in base courses, railroad ballasts, etc.” According to the National Stone, Sand and Gravel Association (NSSGA), nearly two billion metric tons of natural aggregate were pro- duced from sand and gravel pits and stone quarries in 2010 at a value of approximately $17 billion, contributing $40 bil- lion to the gross domestic product of the United States (http:// nssga.org/ssgReview/index.cfm). Large quantities of produced sand, gravel, crushed stone, and, increasingly, industrial by-products and reclaimed construction materials go into the construction of transportation infrastructure for building road base, riprap, cement concrete, and asphalt concrete to pro- vide bulk, strength, durability, and wear resistance in these applications. According to U.S. Geological Survey (USGS) reports, production and use of aggregates in the United States declined during the economic downturn in the years 2008 to 2010. The demand for all types and uses of aggregates in 2007 and 2008 was on the order of 2.5 to 3 billion tons (2.2 to 2.7 billion metric tons) per year, and Meininger and Stokowski (2011) have predicted the demand might return to such usage levels when construction volumes return. According to NCHRP Report 598 (Saeed 2008), unbound aggregate layers in flexible and rigid pavements generally serve to provide (1) a working platform, (2) structural layers for the pavement system, (3) drainage layers, (4) frost-free layers, and (5) “select fill” material (sometimes as part of the working platform). As a working platform, unbound aggregate layers often are constructed on soft, unstable sub- grade soils or base to provide sufficient stability and ade- quate immediate support for equipment mobility and paving operations without excessive rutting. In flexible pavements, dense-graded unbound aggregate base (UAB) and subbase layers serve as major structural components of the pavement system to provide load distribution (that is, dissipation of high wheel load stresses with depth) and ensure adequate support and stability for the asphalt surfacing. In contrast, open-graded granular layers commonly are constructed in both rigid and flexible pavements primarily for drainage and frost-protection purposes. Note that UAB/subbase layers used in rigid pave- ment structures primarily provide uniform support conditions to the concrete slabs; the structural contribution of such layers often is not the primary design aspect. The availability and cost of asphalt cement is directly related to the supply of petroleum and refining. Portland cement and steel require high fuel input for manufacturing, so use of asphalt contributes significantly to carbon dioxide emissions. Chehovits and Galehouse (2010) presented data from Chappat and Bilal (2003) to emphasize the significantly lower energy usage and greenhouse gas (GHG) emissions associated with aggregate production compared with other construction materials. From the data provided by Chappat and Bilal (2003), the energy consumption for aggregate production (per ton) ranges from 25,850 to 34,470 BTU/t (30 to 40 MJ/t), compared with 4.2 MBTU/t (4,900 MJ/t) for asphalt binder production. Similarly, the GHG emissions for aggregate pro- duction range from 5 to 20 lb. CO2/t (2.5 to 10 kg CO2/t) compared with 442 lb. CO2/t (221 kg CO2/t) for asphalt binder production. Given the higher cost of the cementitious portions of pavement layers and the subsequent adverse impact on nat- ural resources, land use, and the environment, more effective and widespread use of unbound aggregate layers in pavement construction should result in significant conservation of energy and increased service life of transportation infrastructure. An international scanning program sponsored by FHWA, AASHTO, and NCHRP in 2002 observed pavement design and construction practices in France, South Africa, and Australia and subsequently recommended the initiation of demonstration projects with deep subbase and deep base designs as cost-effective and sustainable pavement alterna- tives (Beatty et al. 2002). Properly designed and constructed unbound aggregate layers have the potential to improve pave- ment performance and longevity while also addressing today’s issues of the costs of other pavement materials, the need to save energy and reduce GHG associated with the construction and reconstruction of pavements. Pavement projects using granular layers need to be sustainable and cost-effective by (1) making more effective use of locally available materials through beneficiation and use of marginal aggregate materials (aggregates that do not satisfy all material quality control (QC) requirements but may become allowable upon slight adjust- ment in material quality threshold parameters); (2) increasing effective use of recycled aggregate products, such as recycled concrete aggregate (RCA) and reclaimed asphalt pavement (RAP), in pavement construction; and (3) targeting long life and improvement in pavement performance. North American transportation agencies have diverse specifications and construction practices for aggregate base

4 and subbase layers. Sharing experiences and effective prac- tices for unbound aggregate layers among transportation agen- cies would lead to better design and construction practices. For example, in flexible pavements, and especially for the most common applications of thinly surfaced low- to moderate- volume roads, it is critical that the unbound aggregates compo- nent of these transportation facilities is properly characterized by incorporating recent advances into solutions for a more accurate pavement analysis and improved field performance. Important new findings from major research studies [for exam- ple, from the International Center for Aggregates Research (ICAR)] provide proposed improvements in the design mod- els and the compaction of unbound aggregate lifts in thicker layers (Allen et al. 1998; Adu-Osei et al. 2001; Tutumluer et al. 2001; Ashtiani and Little 2009). Furthermore, recent suc- cessful demonstration projects promoting more widespread use of intelligent compaction (IC) systems and the use of field tests other than just density in evaluating in-place stiffness and quality have received much attention through national and pooled fund studies (www.intelligentcompaction.com). Future use of modulus-based continuous compaction control approaches is being studied through an ongoing NCHRP study (NCHRP 10-84: Modulus-Based Construction Specification for Compaction of Earthwork and Unbound Aggregate) to potentially provide guidelines for standards and construction specifications for improved pavement construction and utili- zation practices with unbound aggregate layers. Interest has also developed in domestic and foreign innova- tive construction practices, such as the “inverted pavement” concept of a granular base over a stiff, often cement-treated, subbase layer at depth. Such innovative practices fully empha- size the importance of unbound aggregates in terms of their functional usage and address potential and economic benefits from use in the construction of sustainable pavement infra- structure. In addition to such well-documented practices high- lighted through international technology scanning programs, several test sections have been built, in Georgia, Louisiana, and Virginia, that apply the “inverted pavement” concept (Metcalf et al. 1998; Beatty et al. 2002; Titi et al. 2003; Lewis et al. 2012; Weingart 2012). This important synthesis topic—“Practices for Unbound Aggregate Pavement Layers”—has consistently generated top priority rankings in recent ICAR/FHWA Technical Work- ing Group meetings, clearly highlighting the need to organize and compress available information from current practices and recent advances in this field. Thus, this synthesis report concerns the full range of aggregate base and subbase issues for both flexible and rigid pavement systems in the following areas: • Materials characterization and quality of natural aggre- gates and common recycled materials that relate to performance; • Properties of unbound aggregate layers that are used in the design of pavements and how they are determined (different methods used by transportation agencies to design UAB/subbase layers is first determined); • Aggregate properties that influence construction, com- paction and performance; • Current practices and innovations in construction, com- paction, and QC and quality assurance (QA) procedures (such as compaction in thicker layers, use of IC systems, measuring and ensuring in situ drainage characteristics, and the use of tests other than just density in evaluat- ing in-place modulus, stiffness, and quality, as well as measurements to ensure adequate in situ drainage characteristics); • Performance trends of in-service pavements and experi- mental test sections, such as the Long-Term Pavement Performance (LTPP) and the Minnesota Department of Transportation’s (MnDOT’s) MnRoad, with different unbound base/subbase types, and climatic, subgrade, and drainage considerations in design of aggregate base; • Role of unbound aggregates in sustainability and the potential to save energy and material hauling costs by better using local and marginal aggregates and recycled materials; • How states manage storage, transport, and placement of aggregates to minimize segregation and degradation of material properties and maximize performance: lessons learned. Significant benefits in consistency of UAB properties or performance could be derived from broader application and implementation of major findings from this synthesis. This type of work can also result in internal reviews within state transportation agencies of their processes and lead to imple- mentation of new and improved construction practices, such as thicker lift aggregate bases, inverted pavement construc- tion, IC, and innovative QC approaches, such as the “Percent Within Limits” (PWL) method, defined by the FHWA as: “the percentage of the lot falling above the lower specification limit (LSL), beneath the upper specification limit (USL), or between the USL and LSL” (http://www.fhwa.dot.gov/pavement/pwl/). Such advances can bring sustainability and offer economical and environment friendly green alternatives for road construc- tion. This synthesis presents an extensive overview of the cur- rent states of practices concerning the design and construction of UAB/subbase layers, along with latest research findings in the corresponding areas. Suggestions for “effective practices” are provided for areas in which significant gaps between research findings and current practices are observed. SYNTHESIS OBJECTIVES AND STUDY APPROACH This study was initiated to gather and summarize informa- tion on existing practices for the design and construction of unbound aggregate pavement layers around the United States and Canada. The main objective of this synthesis study was to summarize the state of the art in design and state of the practice in construction of unbound aggregate pavement

5 layers, as used by different transportation agencies. Agency surveys and reviews of research publications have been conducted to identify effective practices in characterization, design, placement, compaction, QC, and performance for unbound aggregate layers; the results have been compiled into this synthesis report. Therefore, this synthesis report primar- ily concerns the full range of UAB and subbase issues for asphalt, concrete, and rehabilitated pavements only and does not include unbound aggregate layer applications in unsurfaced pavements and gravel roads. In addition, other broader topics in the areas of chemical admixture (such as lime, cement, fly ash, or bitumen) and/or mechanical additive (geosynthetic, fiber, and so forth) stabilization of aggregates are excluded from the scope because such aggregate stabilization topics are subjects of separate synthesis studies. For example, the ongoing NCHRP 4-36 research study “Characterization of Cementitiously Stabilized Layers for Use in Pavement Design and Analysis” aims to recommend performance-related pro- cedures for characterizing cementitiously stabilized pave- ment layers for use in pavement design and analysis and incorporation in the Mechanistic–Empirical Pavement Design Guide (MEPDG) (http://apps.trb.org/cmsfeed/TRBNetProject Display.asp?ProjectID=2494). Similarly, NCHRP Synthe- sis 435 (Topic 40-01) “Recycled Materials and Byproducts in Highway Applications” aims to provide guidelines to states for revising their specifications to incorporate the use of recycled materials and other industrial by-products for pavement construction applications. Information has been gathered through literature review on state, local, and international practices concerning design and construction of unbound aggregate pavement layers as well as through a comprehensive survey of the members of the AASHTO Highway Subcommittee on Materials (including Canadian provinces), and selected interviews. The survey ques- tionnaire and a list of respondents are provided in the appen- dices of this synthesis report. The survey questionnaire had separate parts, which together were relevant to agencies with different experiences and needs regarding different designs and construction practices for unbound aggregate pavement layers. The information was requested to encompass all engineering aspects highlighted in the Summary of this synthesis report, primarily in the following categories: 1. Use of UAB and subbase layers; 2. Material selection and construction practices; 3. Characterization of UAB for design; 4. Compaction, QC, and field performance; 5. Recycling aggregates and recycled granular materials; and 6. Climatic effects and drainage. Information was also gathered regarding possible special provisions governing the use of recycled materials in unbound aggregate layer applications. The questionnaire was purposely designed to be comprehensive and at the same time brief in an attempt to increase the response rate. In addition, summaries of agency documents and research publications have been obtained as examples of current effec- tive practices and recent advances and innovative techniques for improving pavement performance with UAB/subbase layers. Gaps in knowledge and current practices have been identified along with research needs to address these gaps. As a result, this synthesis report also provides information for poten- tial harmonization of specifications (particularly on a regional basis) to ultimately benefit both North American transporta- tion agencies and material producers without adverse impacts on pavement performance. Figure 1 shows a map of the United States with all the surveyed states highlighted. Note that four Canadian provincial agencies (Alberta, Newfoundland and Labrador, Ontario, and Saskatchewan) also responded to the survey questionnaire. Accordingly, information gathered from a total of 46 North American transportation agencies has been summarized in this synthesis. Transportation Agency Use of Unbound Aggregate Base and Subbase Layers The comprehensive synthesis survey questionnaire (see Appen- dix A) on Practices for Unbound Aggregate Pavement Layers was sent to all 50 U.S. states, the District of Columbia, Puerto Rico, and nine Canadian provincial transportation agencies. A total of 46 agencies responded to the survey in a timely manner and answered the first question in the general category, indicat- ing that it was common practice for their agency to incorporate unbound aggregate layers into the design and construction of asphalt, concrete, and composite pavement structures, not including unbound aggregate layer applications in unsurfaced pavements and gravel roads in the survey focus. In accordance, Figure 2 shows in percentages the types of unbound aggregate layers commonly constructed by the responding transporta- tion agencies. A great majority of the responses included con- struction of both the UAB (96%) and subbase (65%) courses in pavement layers. Nearly half of the responding agencies indicated they commonly built working platforms, and about one-fourth of all respondents often constructed open-graded drainage layers in their pavements. Note that the “others” cate- gory in the survey summary plots presented in this synthesis comprise “miscellaneous” responses reported by the surveyed agencies in lieu of the alternatives included in the question- naire. A summary of all agency responses to the questionnaire is provided in Appendix C of this report. Figure 3 shows in percentages the types of pavement struc- tures incorporating unbound aggregate layers commonly designed and constructed by responding transportation agen- cies. All responding agencies routinely build flexible pave- ments with UABs. About 70% of the respondents construct rigid pavements with a granular base or subbase (note that it is unclear from the survey how many of the remaining 30% of the respondents construct rigid pavements on a regular basis; some agencies may construct rigid pavements on sta- bilized bases/subbases only). Accordingly, when it comes to

6 rehabilitated pavements, the number of overlaid pavements using unbound aggregate layers in the rehabilitation process is only 30%. Interestingly, about one-fifth of all respond- ing agencies also construct other types of pavements, such as composite and inverted, that have granular base and/or sub- base layer(s). Figure 4 shows in percentages the primary functionalities of unbound aggregate layers designed and constructed in pavement systems. Nearly all of the responding transportation agencies, as many as 94%, build dense-graded base courses as primary structural layers (only Virginia, Rhode Island, and Pennsylvania reported not using dense-graded base courses as primary structural layers). Consistently with Figure 2, 24% of those agencies construct open-graded aggregate lay- ers under rigid pavements for uniform support and to pro- vide drainage. This demonstrates that untreated open-graded drainage layers are not considered for flexible pavements by any agency. Furthermore, similar to the findings highlighted in Figure 2, about half of the responding agencies (52%) FIGURE 1 Map of the United States and Canada showing all surveyed agencies. Note the map does not show Alaska (response received), Puerto Rico (no response received), or Newfoundland and Labrador (response received). 96% 65% 24% 46% (44) (30) (11) (21) 0 10 20 30 40 0% 20% 40% 60% 80% 100% Base course Subbase course Open graded drainage layer Pavement working platforms for subgrade stability applications Number of Responses Percentage of Survey Respondents 46 survey respondents FIGURE 2 Types of unbound aggregate layers commonly constructed by transportation agencies.

7 100% 70% 30% 22% (46) (32) (14) (10) 0 10 20 30 40 0% 20% 40% 60% 80% 100% Flexible pavements (comprising hot mix or warm mix asphalt surface layer) Rigid pavements (comprising Portland / hydraulic cement concrete slabs) Rehabilitated pavements (asphalt overlay over concrete, etc.) Others such as composite pavements, inverted pavements, etc. Number of Responses Percentage of Survey Respondents 46 survey respondents FIGURE 3 Types of pavement structures incorporating unbound aggregate layers commonly designed and constructed by transportation agencies. 94% 24% 52% 9% (43) (11) (24) (4) 0 10 20 30 40 0% 20% 40% 60% 80% 100% Dense graded base courses as primary structural layers Open graded layers under rigid pavements for uniform support and providing drainage Pavement construction platforms to protect weak subgrade layers from excessive rutting under heavy construction equipment loading Others Number of Responses Percentage of Survey Respondents 46 survey respondents FIGURE 4 Primary functionalities of unbound aggregate layers intended to serve in pavement systems designed and constructed by transportation agencies. build pavement construction platforms to protect weak sub- grade layers from excessive rutting under heavy construc- tion equipment loading. The pavement working platform, which is often referred to as the aggregate cover or subgrade replacement owing to its permanent foundation use in pave- ment construction, is clearly the second most common use of unbound aggregate layers after the structural base and sub- base course application. OUTLINE OF CHAPTERS This synthesis contains six chapters. Chapter two provides a brief overview of different types of aggregate materials and their important properties and quality aspects that relate to agency acceptance criteria for good performance in pave- ment applications. Sustainable aggregate utilization practices in pavement construction are also highlighted, with special emphasis on how to make best use of local, marginal, and recycled aggregate materials in granular layers and how agen- cies could test and characterize recycled materials for unbound granular base/subbase acceptance and design. Chapter three summarizes aspects such as storage, transportation, material handling, placement methods, and lift thickness of aggregate materials adopted by transportation agencies to minimize segregation, degradation of material properties, and maxi- mize performance through improved structural load-carrying ability and superior drainage characteristics. Applications of unconventional pavement types using unbound aggregate layers and related construction practices, such as the inverted pavement concept of a granular layer over a stiff layer at depth, are also described in chapter three. Gaps in knowledge Key Lessons • The use of UAB/subbase layers is a common prac- tice across all transportation agencies in the United States and Canada. • A synthesis summarizing the current state of the art and state of the practice concerning unbound aggregate pavement layers will help significantly in identifying desirable practices for the design and con- struction of better-performing sustainable pavement systems.

8 concerning the “effective practices” for UAB and subbase layer construction, along with research needs to address these gaps, are described. Chapter four reviews UAB/subbase structural pavement layer requirements by first defining typical load-transfer mechanisms and describing the related aggregate tests and characterization models for strength, modulus, and perma- nent deformation behavior; it is hoped this information will facilitate better designs of pavement systems and ultimately ensure adequate performance under traffic loading. Agency specifications and design approaches in use are reviewed, as are the new characterization tools and improved models (such as stress-dependent and anisotropic modulus, ICAR model, and so forth) developed for aggregate base/subbase layers through comparisons of the predicted and field-measured values in constructed unbound aggregate layer applications. Chapter four also reviews significant climatic effects, mois- ture or pavement drainage and temperature, and their signifi- cance on the design and performance of pavement systems with unbound aggregate layers. Chapter five presents detailed findings from the literature review and extensive survey results on different approaches used by transportation agencies for compaction testing on lab- oratory samples, field compaction, QC/QA, and field perfor- mance evaluations of constructed UAB/subbase layers. Finally, chapter six provides a summary of the key findings of the syn- thesis report, including the state of the practice for unbound aggregate material selection and sustainable utilization, charac- terization, design, construction, compaction and QC, as well as performance evaluations. Chapter six also provides a summary of opportunities for additional research needs. There are six appendices of this synthesis report. Appendix A presents the complete survey questionnaire that was sent to highway agencies in the United States and Canada. Appendix B lists the complete survey respondent information. Appen- dix C provides a detailed compilation of the survey responses. Appendices D and E present reviews of current unbound aggre- gate material resilient modulus and permanent deformation models, respectively. Finally, Appendix F provides additional information gathered from 14 state highway agencies through a follow-up survey on resilient modulus testing. Note that the terms “unbound” and “bound” have been used interchangeably in this synthesis report to highlight the particulate nature of aggregate base and subbase layers when constructed without the application of any binding or stabiliz- ing agent. REFERENCES Adu-Osei, A., D.N. Little, and R.L. Lytton, Structural Char- acteristics of Unbound Aggregate Bases to Meet AASHTO 2002 Design Requirements, International Center for Aggre- gates Research (ICAR) Report 502-1, Texas Transportation Institute, The Texas A&M University System, College Station, 2001. Allen, J.J., J.L. Bueno, M.E. Kalinski, M.L. Myers, and K.H. Stokoe II, Increased Single-Lift Thicknesses for Unbound Aggregate Base Courses, ICAR Report No. 501-5, Inter- national Center for Aggregate Research, The University of Texas at Austin, 1998. Ashtiani, R.S. and D.N. 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