Segregation is generally described as localized concentrations of coarser aggregate with fewer files than the surrounding areas (i.e., rough surface texture). Mix properties in segregated areas are characterized by lower asphalt contents, lower densities, higher air voids, higher permeabilities, lower strengths, or lower stiffness than the design mix. Any or all of these deviations from the job mix formula mix properties can be outside of the specified mix properties and result in premature pavement distresses in the segregated areas. Segregation seen behind the paver can be introduced into the mix at any point from aggregate production at the plant to when the mix exits the paver. Temperature segregation typically occurs between loading the haul truck and the back of the paver screed. The key to reducing or eliminating segregation is derived from knowing how and where segregation occurs and what equipment and practices can be used to solve the problem. The main focus of this synthesis is to provide guidance on how to reduce or eliminate segregation during aggregate production, mix design, asphalt mix production, mix transport and transfer, and placement. The objectives of this synthesis were to compile information on: * Descriptions of segregation * Methods for detecting segregation * Testing of segregated areas * Specifications for controlling segregation * Pavement distresses and pavement condition for pavements with evidence of segregation * How and where mix segregates because of: — Aggregate production — Mix design — Asphalt plant production — Mix transport and transfer to paver — Paving operations. Information was collected through a literature review and an on-line survey (Appendix A). The AASHTO Committee on Construction members were surveyed to determine agency current practices and equipment usage that can influence segregation. There was a 96% response rate (48 of 50 states) from the state agencies (Figure 1). The same on-line survey was submitted to state Asphalt Pavement Association (APA) representatives to collect information from the paving industry’s perspective on successful construction equipment and practices for minimizing or eliminating segregation. Requests for survey participation were sent to the 38 states with an APA organization; not all states have associations. A total of 19 of the 38 APA organizations (50%) returned survey responses (Figure 2). The survey questions collected information on segregation descriptions, segregation specifications (standard sections, advantages, disadvantages, incentives/disincentives, desired changes), and were answered by every respondent. Survey respondents had the option of selecting more than one choice for most of the questions and were also able to choose which questions they wished to answer. Most of the respondents answered the majority of the questions; therefore, the percentages reported in the survey result tables are based on the maximum number of survey respondents, N. The number of responses for each row in each question, n, are also shown in these tables. The information is organized in the following chapters: * Chapter One–Introduction — Briefly outlines the study purpose, objectives, study approach, and report organization. * Chapter Two–Literature Review — Contains the subject background and how mix segregation can be reduced during aggregate production, mix design, and asphalt concrete mix production. Mix transportation, transfer, and placement as well as the expected pavement distresses that can be accelerated in segregated areas are discussed. * Chapter Three–Survey Results — Includes the results, analysis, and conclusions based on the current experiences of agencies and industry representatives. * Chapter Four–Conclusions — Summarizes successful strategies for reducing or eliminating mix and temperature segregation and suggests needs for future research. * References * Appendix A–On-Line Survey Form * Appendix B–Respondents. (Author/publisher)
Samenvatting