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119 chapter six sUMMARY OF CURRENT PRACTICE AND EFFECTIvE PRACTICEs OBJECTIvEs OF sYNTHEsIs sTUDY The primary objective of NCHRP Synthesis 20-05 Topic 43-03, Practices for Unbound Aggregate Pavement Layers, was to gather information on the current state of the practice and the state-of-the-art research findings on the following topics: 1. Materials characterization and quality of natural aggre- gate and common recycled materials that relate to performance; 2. Properties of unbound aggregate layers that are used in the design of pavements and how they are determined (need to first determine the method of design); 3. Influence of gradation on permeability; 4. Current practices and innovations in construction, com- paction, and quality assurance (QA) procedures (such as compaction in thicker layers, use of intelligent compac- tion (IC) systems, and the use of tests other than density in evaluating in-place modulus, stiffness, and quality); 5. Performance of different base types in research pave- ment sections; 6. Potential to save energy and hauling costs by better utilizing local aggregates and recycled materials; 7. How states manage storage, transport, and placement of materials to minimize degradation of material prop- erties and performance, including lessons learned; and 8. How states address climatic, subgrade, and drainage considerations in design of aggregate base layers. The previous aggregate base and subbase issues target both flexible and rigid pavement systems and exclude for the purposes of this synthesis gravel and/or unpaved roads. 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 also were excluded from the scope of this synthesis. Relevant information was gathered through literature review, survey of the members of the AASHTO Highway Subcommittee on Materials (including Canadian Provinces), industry input, and selected interviews. REsEARCH FRAMEWORK Gaps in knowledge and current practice were noted along with research needs to address these gaps. Information gath- ered under the scope of this synthesis study findings were documented in this report under the following four chapters. Aggregate Types and Material selection A brief overview of the different types of aggregate materials available as natural resources and mined from sand and gravel pit and quarry operations throughout the United States and Canada was provided. Important aggregate properties and quality aspects that enable a certain aggregate material to pass agency specifications for pavement granular base/subbase use were summarized to establish guidelines for aggregate source selection. The concept of best value granular material utiliza- tion was introduced for pavement projects with the potential to save energy and material hauling costs through examples of recent sustainable construction practices highlighting how local aggregates and recycled materials can be better used in granular base/subbase applications. The use of recycled granular materials in base and subbase layers was discussed in detail. The following two categories of recycled materials were considered: (1) unbound aggregate materials recycled from old pavement base/subbase layers and (2) recycled surface course materials: that is, reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA). The goal was to shed light onto what tests are used by agencies to characterize recycled materials for unbound granular base/subbase acceptance and design. Information was gathered on whether or not the same tests are used for characterizing virgin materials and recycled materials before they are included in unbound aggregate base (UAB)/subbase layer specifications and the potential environmental concerns when using RAP and RCA aggregate materials. Granular Base/subbase Construction Practices Diverse agency specifications and construction practices for UAB layers were discussed from the survey results and lit- erature to summarize aspects such as storage, transportation, and placement (e.g., lift thickness) of materials to minimize deviation from intended use and the degradation of material properties and maximize performance through improved structural load-taking ability. Applications of nonstandard or 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, were described in detail. Beneficial international practices (e.g., South African and Australian practices for constructing thinly surfaced pavements with the most effective utilization
120 of unbound aggregate layers) were documented with proper construction techniques. In addition, new construction prac- tices and performances of recently built test sections by sev- eral state highway agencies in the United States (e.g., Georgia, Louisiana, and Virginia) were summarized in this chapter to demonstrate the advantages of these new unconventional pave- ment types using unbound aggregate layers. Unbound Aggregate Base Characterization for Design Information was gathered from the survey results and research publications on specific unbound aggregate material property inputs obtained from different laboratory and field testing alternatives. All levels of material characterization and qual- ity aspects of different aggregate types (crushed stone, sand and gravel, slag, and other types of recycled materials) were described through aggregate properties and properly evaluated for granular base/subbase strength, deformation, and modulus requirements. Consideration was given to the current agency specifications/design approaches in use and the new character- ization tools and improved models [such as stress-dependent and anisotropic modulus, International Center for Aggregate Research (ICAR) model, and so forth] developed for aggre- gate base/subbase layers. The need for improved characteriza- tion of aggregate materials through nonlinear stress-dependent and anisotropic (directionally dependent) models was docu- mented in this chapter based on the improved predictions of pavement responses through comparisons of the predicted and field-measured values in constructed unbound aggregate layer applications. Finally, information on how highway agencies address cli- matic, subgrade, and drainage considerations in the design of unbound aggregate layers was discussed in this chapter. Influ- ences of aggregate gradation, fines content, and other material properties, such as particle shape and angularity, on permea- bility were topics of specific interest when reviewing and sum- marizing agency survey responses and specifications related to the structural contributions of open-graded aggregate drainage layer applications (e.g., permeable bases and drainable sub- bases commonly used in rigid pavement foundations). Compaction, Quality Control, and Field Performance Detailed findings were presented on different approaches used by transportation agencies for compaction testing on labora- tory samples, field compaction, quality control and quality assurance (QC/QA), and finally, field performance evalua- tions of constructed UAB/subbase layers. Different aspects of compaction and QC of UAB and subbase construction were discussed by introducing the theory of compaction along with the objectives behind compacting unbound aggregate pave- ment layers. This was followed by a review of different types of compactors commonly used for compacting UAB and sub- base layers in the field. The concept of QC was introduced, emphasizing that constructed layer density measurement is the most commonly used field evaluation tool for verifying the adequacy of UAB/subbase construction. Different field techniques used to measure densities of constructed pavement layers were discussed with particular given to the widespread nuclear gauge-based direct density measurement methods. The concept of modulus-based compaction control was introduced by highlighting its potential advantages, such as continuous compaction control for uniformity over the âspot checkingâ compared with density-based compaction control approaches. Different IC approaches currently available were discussed through a review of equipment manufacturers. In addition, experiences of different states in the United States for implementing IC approaches were presented along with their preliminary findings. Finally, other portable devices used for measuring the in situ moduli of constructed pavement layers were discussed. sUMMARY OF sTATE PRACTICEs Use of Unbound Aggregate Base and subbase Layers â¢ All the responding transportation agencies indicated the use of UAB/subbase layers into the design and construc- tion of pavement structures. Flexible pavement base courses appear to be the most common application of unbound aggregate layers. â¢ UAB courses are the most common type of aggregate layers used by transportation agencies (used by 96% of responding agencies), whereas only 24% of responding agencies indicated the use of open graded drainage layers (OGDLs). Another common instance of unbound aggre- gate application in pavements was in working platforms or subgrade replacement and subbase applications. Material selection and Construction Practices â¢ No common practice exists among state and Canadian provincial transportation agencies regarding the fre- quency of acceptance checking of materials obtained from normally used and approved aggregate sources. Only 39% of the responding agencies indicated that aggregate material quality checking was a requirement before every major construction project. â¢ Apart from gradation analysis, no other aggregate material quality test is consistently used by transporta- tion agencies. â¢ Significant differences exist among agencies regard- ing the maximum allowable particle size for aggregates used in unbound aggregate layers. â¢ Ninety-eight percent of the responding agencies do not distinguish between nonplastic and plastic fines when specifying the maximum amount of fines allowed in base and subbase courses.
121 â¢ Sixty-three percent of the responding agencies limit the maximum construction lift thickness to be less than 8 in. (205 mm). Although several research studies have reported on the successful construction of thicker unbound aggregate lifts, transportation agencies have not adopted such thick lift construction as a practice and into their specifications. Unbound Aggregate Base Characterization for Design â¢ Shear strength index tests, such as California Bearing Ratio (CBR) and Hveem stabilometer R-value, are the most commonly used ones, not only for determining strength properties, but also for characterizing modulus and deformation behavior of UAB and subbase layers. â¢ The use of falling weight deflectometer (FWD) is the most common practice among transportation agencies for evaluating the characteristics of in-service unbound aggregate layers. Only 10% of responding agencies have started adopting portable field devices, such as the light weight deflectometers (LWD) and soil stiffness gauge (GeoGauge), for in-place modulus measurement of constructed aggregate base and subbase layers. â¢ Approximately 61% of the responding agencies use the AASHTO 1993 design guide for designing pavements with UAB and subbase layers; 22% of the responding agen- cies use empirical methods (AASHTO 1972, AASHTO 1986, or agency-specified empirical procedures), and 30% of the agencies have adopted the use of the Mechanistic- Empirical Pavement Design Guide (MEPDG). â¢ About 22% of the responding agencies do not use resil- ient modulus as an input for the design of UAB and subbase layers. Although the remaining agencies use aggregate resilient modulus as a key input for pavement design, the common practice is to assign a single modu- lus to the entire aggregate layer without considering any modulus distribution within the layer owing to the load- or stress-dependent nature of unbound aggregate layer modulus characteristics. Only one agency indicated the use of state-of-the-art concepts such as aggregate cross- anisotropy (directional dependency) in pavement design. â¢ More than 50% of the responding agencies do not run laboratory tests to determine the resilient modulus of aggregates and instead use empirical correlations with index properties such as CBR and aggregate gradation parameters. â¢ More than 80% of the responding agencies do not have specific guidelines for including locally available âmar- ginalâ aggregates into the thickness design procedure. Compaction, Quality Control, and Field Performance â¢ Drop-hammerâbased techniques, such as the standard and modified Proctor tests, are used by 91% of the responding agencies for establishing the compaction characteristics of aggregates in the laboratory. â¢ Field moisture and density measurements using nuclear density gauge are commonly used in 89% of the respond- ing agencies. The in-place densities thus determined are compared with laboratory-established compaction char- acteristics to check the degree of compaction (DOC) achieved in constructed aggregate layers. Only 28% of the responding agencies construct test strips to establish roller patterns and check for compaction density growth of aggregate layers. â¢ There is no common practice among transportation agencies regarding the minimum compaction require- ments in constructed aggregate base and subbase layers. Compaction requirements often are based on certain percentages of the laboratory-established dry density values. Such differences in compaction requirements by agencies may potentially lead to significantly different pavement aggregate layer responses (i.e., stiffnesses) resulting in much different rutting performances even when similar pavement configurations are constructed for standard design loads. â¢ More than 50% of the responding agencies expressed interest in implementing non-nuclear density measure- ment methods for construction control of UAB/subbase layers for one reason or another. However, several of them indicated a lack of confidence in the performance of non-nuclear moisture-density measurement alternatives. â¢ Although 37% of the agencies have participated in demonstration projects involving continuous compac- tion control of UAB/subbase layers using IC techniques, only one agency (Texas) has actively implemented IC techniques to construct in-service pavements with UAB/ subbase layers; that agency also reported having such a specification currently adopted for use by practitioners. â¢ Ninety-six percent of the responding agencies do not implement modulus-based compaction control during the construction of unbound aggregate pavement layers, and use achieved layer density and density-based rela- tive compaction as the primary indicator of construction quality. â¢ None of the agencies have incorporated nontraditional compaction techniques, such as the South African âslushingâ method, into unbound aggregate layer con- struction practices. Any application of such technology has been confined to trial and demonstration projects involved with the application of inverted pavements. Recycling Aggregates and Recycled Granular Materials â¢ Thirty-three percent of the responding agencies do not commonly recycle unbound aggregate materials from base and subbase layers of existing pavements, clearly showing a lack of more sustainable construc- tion practices.
122 â¢ Forty-eight percent of the agencies have not incorpo- rated the use of recycled aggregates from existing base and subbase courses into their specifications. â¢ Eighty-three percent of the responding agencies indicated that contractors are not allowed to use locally available âmarginalâ or âout-of-specificationâ aggregates for UAB and subbase layer construction. Modifying the structural designs of pavements to accommodate such aggregate types on a project-basis may significantly reduce the trans- portation costs associated with material procurement. â¢ Sixty-eight percent of the responding agencies reported no environmental concerns associated with the use of recycled materials in the unbound aggregate layer appli- cations. This indicates a possible gap in knowledge with respect to phenomena such as leaching from recycled aggregates. â¢ Sixty-four percent of the respondents do not require any strength, deformation, or modulus characterization of recycled materials such as RCA and RAP before their use in unbound aggregate pavement layers. For agen- cies that require such tests to be conducted on recycled materials, the quality requirements are the same as for virgin aggregates. Climatic Effects and Drainage â¢ Sixty-one percent of the respondents indicated climatic effects on pavement subgrade performance as an issue of major concern. However, only 15% of the respon- dents currently test unbound aggregate materials for suction characteristics and other moisture effects, such as soil water characteristic curve or suction characteris- tics of fines and freeze-thaw durability. â¢ Fifty-nine percent of the responding agencies do not consider the effects of climatic changes on unbound aggregate layer performance during pavement design. For the agencies that take this aspect into consideration, changing the aggregate layer resilient modulus or struc- tural layer coefficient (as defined by the AASHTO 1993 pavement design guide) appear to be the most common practices. â¢ Only 41% of the responding agencies specify different gradations for unbound aggregate applications target- ing drainable versus low permeability aggregate layers. â¢ Fifty percent of the responding agencies indicated that drainage is not one of the primary functions of flex- ible pavement UAB/subbase layers. For the agencies that consider the drainability of dense-graded aggregate layers, limiting the maximum allowable percent fines (material passing sieve No. 200) appears to be the most common practice aimed at facilitating drainage. â¢ Only 4% of the respondents conduct in situ permeability tests to measure the effectiveness of open-graded aggre- gate drainage layers. Equal numbers of agencies rely on laboratory permeability measurements or empirical cor- relations to estimate the permeability of such drainage layers, when used. â¢ Thirty-seven percent of the respondents indicated the construction of filter layers (using aggregates or geo- synthetics) as a common practice for protecting aggre- gate drainage layers from clogging. â¢ Thirty percent of the responding agencies do not con- struct subsurface drainage systems, such as âedge drains,â whereas 26% of the respondents indicated such drainage systems are commonly used. FUTURE REsEARCH AND IMPLEMENTATION Based on information gathered from this comprehensive sur- vey of state and Canadian provincial transportation agencies, the following topics have been identified where significant gaps in knowledge exist, and accordingly, future research and demonstration projects may be required to modify/improve or further develop agency specifications. Use of Locally Available Marginal and Out-of-specification Materials Most agencies currently do not allow the use of marginal and/or out-of-specification materials in the construction of UAB/subbase layers. Future research needs to focus on how existing pavement designs can be modified for accommo- dating mechanistic-empirical (M-E) pavement design con- cepts so that properly mechanistic-based evaluations can be performed for the use of these materials, which would lead to significantly reducing material transportation costs. Labo- ratory and accelerated pavement testing efforts need to be carried out to evaluate and verify adequate pavement perfor- mance using marginal quality aggregates. Use of Modulus-Based Construction Quality Control Although several research and demonstration projects have advocated the benefits of implementing modulus-based con- struction quality control techniques, density-based compac- tion remains the most commonly used approach. The ongoing NCHRP 10-84, âModulus-Based Construction Specification for Compaction of Earthwork and Unbound Aggregate,â will shed more light on the desired characteristics when developing such specifications. Depending on the project findings, accelerated testing of full-scale pavement test sections and demonstration projects will need to be carried out to evaluate the effectiveness of such modulus-based construction specifications. Use of Intelligent Compaction Techniques Almost all research studies and demonstration projects focus- ing on continuous compaction control using IC techniques have
123 advocated the promise shown by this method. Although pilot studies using IC techniques have been conducted in the United States since 2004, state transportation agencies are hesitant to use this technology more actively, which is pri- marily the result of the lack of having available standards and construction specifications. Transportation agencies would benefit from participating in IC demonstration projects, and subsequently developing state-approved standards and construction specifications. Ongoing demonstration research studies and demonstration projects funded by the FHWA can contribute significantly to this cause. Note that IC was selected as a FHWA Every Day Counts (EDC) initiative for 2013 (http:// www.fhwa.dot.gov/hfl/innovator/issue32.cfm). Alternative Base Course Applications such as Inverted Pavements A review of published literature established that there is wide- spread consensus among researchers regarding the benefits of alternative base course applications, such as the inverted pave- ment concept of constructing aggregate layers on a stiff sub- base. The use of inverted pavements with thin asphalt surface courses is common in South Africa. Moreover, other coun- tries, such as France and Australia, also use thick aggregate base courses as the primary structural layer in their pavement systems. However, current agency practices in the United States and Canada do not adopt these alternative construction practices. Optimal use of unbound aggregate layers as the pri- mary structural component in pavement systems will greatly benefit with the construction of cost-effective and long-lasting pavement structures designed for improved performance. It would be beneficial to thoroughly evaluate performances of existing pavement sections constructed using such alternative base courses. KEY LEssONs AND EFFECTIvE PRACTICEs Material selection and Quality Testing â¢ The use of 100% uncrushed aggregates in UABs/subbase layers must be done with caution, realizing their substan- dard strength properties and high rutting potentials. â¢ To ensure ease of construction and adequate compac- tion, the maximum particle size allowed in UAB, sub- base, and drainage layers are best restricted to 1.5 in., 2 in., and 4 in., respectively. â¢ Excessive fines (P200) deteriorate aggregate layer per- formance, especially in the presence of moisture. The maximum amount of fines allowed in UAB/subbase layers are best restricted to 12% unless prior perfor- mance of a material can be documented to show that the material performs satisfactorily at higher fine contents. â¢ The presence of plastic fines in an unbound aggregate layer is best limited. For instances where the presence of plastic fines is unavoidable, different threshold limits can be set for the maximum allowable fines content for nonplastic and plastic fines. â¢ In addition to commonly used tests for evaluating the physical characteristics, the mechanical performance of recycled materials, by-products, and other marginal aggregates needs to be carefully studied. â¢ RAP materials are tested in the laboratory for resilient modulus and permanent deformation behavior before being used in UAB/subbase layers. Several studies have reported high resilient modulus values for RAP accompanied by significantly high permanent deforma- tion accumulations. â¢ The expansive properties of RAP materials contain- ing expansive components such as steel slag are best carefully evaluated before their application in UAB/ subbase layers. â¢ Recycled crushed concrete often can be adequately used in UAB/subbase layers. â¢ Care needs to be taken while blending two different recy- cled aggregate types to ensure that the resulting blend possesses adequate physical, chemical, and mechanical properties. â¢ Recycled materials from unknown sources or those to be used in drainage applications are always tested for potential environmental impacts before being used in UAB/subbase layers. â¢ The use of recycled materials in pavements may be evaluated on a project basis, instead of following generic guidelines. For example, the use of RCA in UAB/subbase layers may or may not be allowed, depending on whether or not the pavement has an underdrain system. Granular Base and subbase Construction Practices â¢ Stockpiling of aggregates using the windrow concept has been proven to be the most efficient practice as far as minimizing segregation is concerned. â¢ From extensive review of the literature as well as cur- rent state practices, this synthesis study finds an opti- mum lift thickness of 12 in. for the construction of UAB/subbase layers. Note that this finding is based on the assumption that the UAB/subbase layer to be constructed is at least 12-in. thick. Moreover, the DOC achieved is contingent upon the use of adequate equip- ment by the contractor. Unbound Aggregate Base Characterization for Design â¢ Compaction and stress-induced anisotropy may be considered during the design and analysis of pavement systems with UAB and subbase layers. â¢ Test procedures (AASHTO T 307 and NCHRP 1-28A) for conducting resilient modulus tests on aggregates have
124 been available for more than a decade. These methods can adequately capture the stress-dependent nature of unbound aggregates and are ready to be implemented in practice. Agencies may incorporate these specifications into practice. â¢ New research efforts are needed for developing har- monized test protocols for quantifying the permanent deformation behavior of aggregates. â¢ It would be useful for stress dependence of unbound aggregate materials to be incorporated into future releases of DARWin-ME, the current AASHTO mechanistic empirical pavement design procedure. â¢ A simplified approach is available for agencies to incor- porate the cross-anisotropy of unbound aggregates into pavement design without the need to conduct state-of- the-art triaxial tests. â¢ Rapid removal of excessive moisture from unbound aggregate layers can be achieved through (1) selec- tion of aggregate materials with low water-retaining ten- dencies and (2) design of suitable subsurface drainage systems. â¢ Aggregate materials may be tested for erosion poten- tial or âerodibilityâ before being used in UAB/subbase layers, particularly under rigid pavements. â¢ Stable open-graded and gap-graded aggregates with low fine (P200) contents are best used in unbound aggregate drainage layers. â¢ Tube suction tests can evaluate the frost susceptibility of aggregates before their application in unbound base/ subbase layers in areas experiencing significant frost penetration. Compaction, Quality Control, and Field Performance â¢ Drop-hammerâbased compaction methods (e.g., AASHTO T 99 and T 180) may not be adequate for coarse-grained aggregates, particularly those with low fines (P200) con- tents. Transportation agencies may need to adopt test procedures similar to ASTM D 7382 to establish the moisture-density curves for unbound aggregates using a vibratory or gyratory compactor. â¢ The use of roller types that are most suitable for the particular material types is critical to ensuring adequate compaction of unbound aggregate pavement layers. â¢ Several research and implementation projects have reported different degrees of success with in-place modulus measurement devices. Although these devices have been used successfully to identify anomalies in construction conditions, extensive calibration for local materials is needed before they can be used as primary tools for quality control. â¢ Most research and implementation projects conducted in the United States involving the use of continuous compaction control and IC to construct UAB/subbase layers have reported considerable success. However, such practices are not common for transportation agen- cies. Encouraging more implementation projects across agencies can help to incorporate continuous compac- tion control and IC into agency practice. â¢ Suction effects and resulting changes in aggregate layer modulus can be considered during the design of UAB/ subbase layers.