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77 6.1 Objectives and Scope of Synthesis Study The primary objective of this synthesis was to gather the latest information and present current practices implemented by transportation agencies related to aggregate quality require- ments for pavements. The most recent and relevant research findings on the sources and types of aggregates influencing quality were gathered. Information related to aggregate QA proce- dures established by agencies including testing methods (e.g., AASHTO, ASTM, or agency specific methods) and typical ranges/passing criteria were collected and synthesized. Moreover, aggregate approval processes as well as testing frequencies to certify aggregate sources for use in a certain layer of pavement were examined. Quality indicators of aggregates affecting pave- ment performance were discussed to establish linkages between related laboratory and field tests and the aggregate quality and performance. Environmental, economic, and performance- related concerns considered or experienced by agencies due to utilizing alternative sources of aggregates were presented. The data used to develop this synthesis were gathered through an extensive literature review and comprehensive survey responses collected from 45 U.S. DOTs and eight Canadian pro- vincial agencies. Follow-up interviews with selected agencies were also conducted to illustrate examples of implementing aggregate quality practices. 6.2 Summary of Key Findings The summary of key findings has been organized into four subsections corresponding to the information presented in Chapters 2 through 5 and is described below. 6.2.1 Aggregate Source Selection and Quality Properties Information related to sources, locations, tests, and quality properties of virgin (natural) aggre- gates used in North America were synthesized. Additionally, different quality-related properties for recycled aggregates such as RAP and RCA as well as artificial/byproduct aggregates such as SFS, BFS, and QB were also investigated. Blending of different aggregate sources to meet quality requirements was discussed and the restrictions that transportation agencies have put in place for utilizing different aggregate sources for constructing certain pavement layers were highlighted. The main findings include the following: ⢠More than half of the agencies that participated in the survey questionnaire indicated they have a list of approved aggregate types or sources available that are routinely used in their state or province for selecting/approving aggregate materials for use in different pavement applications. C H A P T E R 6 Conclusions
78 Aggregate Quality Requirements for Pavements ⢠Uniform practice does not exist among different agencies regarding the use of RAP, RCA, SFS, and BFS in pavement construction. Furthermore, transportation agencies fol- low different restrictions considering quality concerns for using recycled and artificial aggregates. ⢠Ninety-four percent (94%) of the responding agencies confirmed the use of RAP in pave- ment applications. Residual asphalt binder property and source properties of RAP aggregates were reported as the common RAP quality indicators that are investigated by transportation agencies. ⢠Forty percent (40%) of the agencies reported that they do not use RCA in pavement construction. ⢠Artificial/byproduct and manufactured aggregates, such as SFS, BFS, and LWA, were reported to be used by 45% of the responding agencies. ⢠Transportation agencies indicated that they commonly blend marginal aggregates and QB with virgin aggregates. Out of the 53 agencies, 77% reported that they blend aggregate from different sources. ⢠Additionally, 15% of the agencies reported the use of marginal aggregates, while 21% of the respondents indicated that they use nontraditional large size aggregates. 6.2.2 Aggregate Quality Assurance and Testing The responsible authority in charge of testing aggregate materials, types, and frequencies of quality assurance tests on aggregates, with a focus on quality assessment, along with aggregate sampling procedures were discussed. Important and common aggregate properties affecting qualities of different aggregate sources, including coarse and fine virgin natural aggregates, RAP, RCA, SFS, and BFS, used in pavement construction were investigated. The main findings include the following: ⢠QA methods have been established by agencies to ensure that the aggregate materials received on the job site meet the criteria for utilizing them in construction of a certain pavement layer. ⢠Using in-house geotechnical or material laboratories was found to be the most common way to test and check qualities of aggregate materials by transportation agencies. ⢠AASHTO test procedures were found to be the preferred aggregate testing methods more commonly adopted and used by agencies when compared with the ASTM and agency devel- oped test procedures. ⢠Sampling requirements and frequencies of quality assurance tests on aggregate materials can vary significantly among different agencies. ⢠Review of National Slag Association publications showed that mineralogical, chemical, and freeze-thaw properties for both SFS and BFS aggregate sources need to be considered as impor- tant quality indicators that affect performance. The findings of the survey showed that only few agencies routinely check these properties. ⢠Transportation agencies have established different procedures to control the quality of blended aggregate products. Some common alternatives include controlling the quality of individual components or the entire blend as one product. 6.2.3 Approval Procedures and Aggregate Quality Classes Aggregate materials approval or certification procedures adopted and commonly used by North American transportation agencies to assess quality classes were discussed. Furthermore, the ranges of aggregate quality classes defined by many agencies and how some of these agencies have established a tiered approach to assign these quality classes to a certain layer of pavement
Conclusions 79 were investigated. Aggregate quality requirements published by different transportation agen- cies were reviewed, with a special focus on practices or criteria put in place to classify differ- ent aggregate types and sources including coarse and fine virgin aggregates. The main findings include the following: ⢠Approximately 72% of all participating agencies use their own laboratory for approving aggregates, while 30% reported that they do not have an approved aggregate list and only test aggregates prior to use on every major construction job. ⢠The outcome of the survey showed that only 44% of the participating agencies classify aggre- gate quality based on utilization in a certain layer of pavement (tiered approach). ⢠Agencies often list and include aggregate gradation as a criterion for aggregate quality classification. Aggregate particle size distribution is in fact a performance-based engineering recipe or requirement for certain layer applications but not a quality aspect. ⢠The findings from the survey showed that transportation agencies follow different methodologies, designations, and criteria for aggregate property evaluation and classification based on required quality levels. Alaska, Illinois, Indiana, Ohio, Ontario, and Texas were found to have some of the most comprehensive and diverse aggregate classification systems, with diverse quality classes associated with each aggregate type and application. 6.2.4 Aggregate Quality and Performance Pavement distresses in different types of pavements and how they relate to aggregate qual- ity properties were discussed. Additionally, aggregate performance tests that are considered by transportation agencies to relate laboratory and field performance were investigated. Finally, environmental and performance-related pavement distresses experienced by different agen- cies and potentially caused due to poor aggregate quality or utilizing RAP, RCA, and artificial/ byproduct aggregates such as SFS and BFS were presented. The summary of key findings of this chapter are presented as follows: ⢠Thirty-nine agencies out of 51 survey question respondents reported that they do not track or keep a record with the intent to relate the aggregate quality aspects to observed pavement performance trends in the field. ⢠Review of the literature proved that standard mechanistic-based as well as innovative pave- ment materials characterization and performance tests such as resilient modulus, permanent deformation, shear strength, or advanced skid resistance (e.g., imaging-based surface texture, or variable speed friction test) have been developed by different research institutions. The survey results showed that more than half of the surveyed agencies reported that they do not implement these performance-based tests in their specifications to check and control the end performances of different qualities of aggregates. ⢠Stripping and polishing were found to be the most common aggregate-related flexible pavement distresses reported by transportation agencies. ⢠Fifty percent (50%) of agencies linked an aggregate quality deficiency to a specific performance- reported ASR as the main quality concern and the resulting cracking as the most common aggregate quality-related rigid pavement distress. ⢠Agencies reported poor performance in different pavement layers, that is, primarily asphalt surface course, asphalt base course, PCC, and surface treatment, resulting from aggregate quality issues. ⢠The use of RAP and polishing resistance-related degradation were reported by participating agencies as the common aggregate quality issues affecting pavement performance. ⢠Mineralogy, plasticity of fines, aggregate particle shape, texture and angularity, and resistance to degradation were reported by researchers as important aggregate properties affecting performance in both bound and unbound pavement layer applications.
80 Aggregate Quality Requirements for Pavements ⢠The use of high percentages of RAP was reported by the survey responding agencies to have caused premature asphalt cracking. Also, the use of RCA in pavement layer applications was offered as a possible cause of high pH and leaching issues. Accordingly, the survey results showed that performance-related concerns have led agencies to limit the increased use of RAP, while environmental and durability-related concerns are the most prevalent when it comes to utilizing RCA, SFS, and BFS in pavement construction. ⢠According to the findings from the reviewed literature, expansion of the RAP material was particularly a concern when RAP contained expansive components like steel slag. 6.3 Conclusions and Gaps in Knowledge The outcome of this synthesis shows that aggregate QA practices and approval procedures can greatly vary from one transportation agency to another, while performance-based aggregate specifi- cations and testing are not common. Clear gaps in knowledge were found to exist on how to develop and implement performance-based aggregate specifications that would potentially benefit agencies, users, and stakeholders. Additionally, based on the survey results and literature review, gaps in knowledge were observed on how to develop new and innovative testing methods to better assess the quality of virgin, recycled, and artificial/byproduct aggregates and aggregate blends. For example, 52 agencies, that is, nearly all survey participants, specified the LAA test for checking the durability of virgin aggregate, while research studies conducted by many universities and transportation agencies concluded that micro-deval test results could better correlate to field performance in terms of aggregate surface friction course requirements. This conclusion shows the ongoing need to develop or adopt new and advanced testing methods that may better relate to the actual field mechanisms of aggregate degradation (e.g., fracture and wear) and, at the same time, adequately assess the effectiveness of the most commonly used laboratory tests such as the LAA and magnesium and sodium sulfate soundness to establish aggregate quality properties. 6.4 Suggestions for Further Research The factual data gathered in this synthesis study indicate that the availability of high quality aggregate materials is decreasing in many parts of North America as sand and gravel mines and rock quarries are depleted or lost to other land uses. Based on the findings of this synthesis, further research and investigation may need to be directed toward developing a comprehensive, consistent, and sustainable aggregate quality management system in North America. Related research activities may focus on establishing implementation strategies for such an aggregate quality management system to achieve satisfactory pavement performance and at the same time reduce detrimental environmental impacts. Sharing success and failure stories among agencies may help eliminate some gaps in knowledge, improve sustainable construction practices for the targeted end performance, and address prevailing aggregate quality issues. Finally, the high quantities of construction and demolition debris produced every year in North America necessitate conducting more research to establish sustainable and alternative aggregate resources for pavement construction. In addition to RCA and RAP, which are the most commonly used, artificial/byproduct aggregates, such as SFS, BFS, QB, and manufactured LWA, as well as local/marginal and large unconventional aggregates (e.g., primary crusher runs) can be further studied especially for their uses in pavement construction platforms and secondary roads. If undertaken, such future research should be directed toward investigating and defining strategies, assessment methods, and performance requirements that will enable pavements to be constructed or rehabilitated with available aggregates used in such a way as to best take advan- tage of their potential in pavement construction.
