Methods and Practices on Reduction and Elimination of Asphalt Mix Segregation (2015)

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METHODS AND PRACTICES ON REDUCTION AND ELIMINATION OF ASPHALT MIX SEGREGATION Mix segregation is generally described as localized concentrations of coarser aggregate and fewer fines than the surrounding areas (i.e., rough surface texture) and can be introduced into the mix at any point from aggregate production to where the mix exits the paver. Temperature segregation is localized areas of cold mix compared with the majority of mix and typically occurs between loading the haul truck and coming off the back of the paver screed. Segregation can only be reduced or eliminated when agency and contractor staff can consistently recognize and detect segregation, understand where it occurs, and know what successful practices and equipment can be used to reduce or eliminate segregation. This synthesis provides guidance on how to reduce or eliminate segregation during aggregate production, mix design, asphalt mix production, mix transport and transfer, and placement. Information on current practices and equipment used by agencies and the paving industry was collected through a literature review and an on-line survey. The survey was sent to mem- bers of the AASHTO Committee on Construction (96% response rate) and state Asphalt Pave- ment Association (APA) organizations (50% response rate; however, only 38 states have APA organizations). Successful options for reducing or eliminating segregate, as well as caveats for what not to do, are included for the following topics. Descriptions—There are three types of mix segregation that result in coarser pavement sur- face textures (end-of-truck, random, and longitudinal). Mix segregation can also be described as concentrations of fine aggregates, binder-rich areas (“fatty spots”), and areas with “clumps” of other additives (e.g., fibers and polymers). Temperature segregation is frequently described as temperature differences of more than a certain threshold such as 25°F or 50°F. Survey respondents were asked to indicate which descriptions are used to describe segregation in their state. The majority of respondents frequently use descriptions for end-of-truck, random, longitudinal, and temperature segregation. About 25% to 30% of the agencies also use one or more descriptions of other types of mix segregation. Standardized segregation descriptions are essential so that field inspections, specifications, and testing for segregation are consistently and more uniformly applied. Detection Methods—Visual detection of segregation is used by almost all agencies and a number of agencies use one or more methods for measuring temperature differences. Infrared guns and infrared cameras are each used about 20% of the time and about 10% use an infrared sensor bar system (e.g., Pave-IR). Inspection Responsibilities—Field inspection responsibilities can be split between agency and consultant staff. How the responsibilities are split varies widely. One agency uses agency staff about 25% of the time and consultants about 75% of the time. Another agency uses staff to inspect projects 98% of the time and only uses consultants about 2% of the time. Given the subjective nature of visual detection, which is most frequently used to detect segregation, it is very important to have well-established segregation definitions and training programs for field staff, both agency and consultants, so that agency specifications are uniformly under- stood and applied. SUMMARY

2 Testing—Once segregation is detected, the most common action requires additional testing of the potentially segregated areas. Roadway (in-place) density testing and laboratory testing for density, aggregate gradations, and asphalt content are used for standard quality control/ quality assurance. However, these test methods usually require additional roadway surface or laboratory sample preparation because of the coarser texture and higher permeability of seg- regated mixes. A lack of understanding of the required practices or proper selection of labora- tory test methods unintentionally skews test results so that mix properties are either under- or overestimated. This leads to accepting a significant amount of mix that does not actually meet specifications or rejecting mix that is actually acceptable. Training and certification programs for field and laboratory staff would include how, when, and where test methods are to be adjusted to accurately measure segregated mix properties. Training and Certification Programs—Training and certification programs are used by a majority of the agency and industry respondents. Interestingly, these respondents do not believe training for segregation identification is an important component. This observation conflicts with written comments from respondents that frequently identify the subjective nature of segregation detection as a disadvantage. Based on the training and certification program question responses and written comments for these questions, training and certification pro- grams that add information on how and where segregation occurs and highlight successful equipment and practices can help reduce or eliminate segregation. Specifications—Agency specifications for the control of segregation that were identified by the survey respondents were reviewed and summarized. In general, there is currently no consistent approach for agency specifications. Pavement Distresses—Respondents identify the most frequently observed early pavement distresses in segregated areas as raveling (texture changes) and potholes in various stages of formation. The next most common “distress” in segregated areas is a loss of ride quality (i.e., rougher ride). Intermittent longitudinal cracking in the wheel path and intermittent fatigue cracking are also considered early distresses in segregated areas by about one- quarter of all respondents. Both types of cracking happen when the tensile strength of the mix cannot support the traffic loads. Segregated mixes lose tensile strength and the loss of strength increases with the increasing severity of segregation. Written comments about the ability of the agency’s pavement condition survey and pave- ment management system to adequately detect and track cyclic early pavement distresses resulting from segregation are an indication that the methods are not currently set up to collect and track this kind of information. How and Where Asphalt Mix Segregates—Key areas where segregation can be gener- ated, observed, and controlled are at the mix design stage, with aggregate production, at the asphalt plant, when asphalt mix is transported, when the mix is transferred to the paver, and may be associated with the paver equipment and operation. Mix designs—Mixes with gradations without gaps between consecutive sieves are less likely to segregate. Mixes with 9.5-mm maximum size aggregate rarely segregate. Segrega- tion is increasingly more likely as the maximum aggregate size increases. The amount of asphalt in the mix controls the asphalt film thickness on the aggregates and an adequate asphalt film thickness is necessary to keep aggregate particles “stuck” together. When the design asphalt content is too low, the likelihood that the mix will segregate increases. When asphalt- containing recycled materials are used to contribute a portion of the effective binder content in the mix, the properties of the recycled material stockpiles are to be controlled so the effective asphalt content (i.e., asphalt film thickness) is controlled. If this is not controlled, segregation can become an issue. Aggregate production—The majority of aggregate stockpiles are constructed with labor- intensive processes (i.e., loader operators) and fixed location conveyors. The skills of the

3 loader operators are important to reducing or eliminating segregation at this point in the process. Training and certification programs help highlight successful practices and skills that reduce or eliminate segregation. Asphalt plant—The main source of segregation at a batch plant is in the #1 hot aggregate bin. Practices that keep the fines from collecting and building up on the sides of the bin are needed to keep “clumps” of fines from dropping all at once into the mixer. Segregation can be reduced or eliminated at drum mix plants by using kickback flights in the drum, a fixed plow at the discharge point, orienting the drag slat to the silo at 90° to the drum exit, maintaining an optimum amount of mix on the drag slat, using silo batchers, keeping an optimum amount of mix in the silo (about 25% to 75% full), and loading all haul trucks with multiple drops. Mix transport and mix transfer—Insulated truck beds can help reduce temperature segregation. Baffles that form a funnel at the back of end dump haul trucks help reblend mix as it is deposited into the paver hopper. Windrow elevators help reblend mixes and reduce segregation. Material transfer units can be successful at reducing or eliminating segregation when they are operated correctly. Paver—Segregation at the front of the paver can be reduced or eliminated by keeping the paver half full at all times, using outboard motors to move conveyors, adding paver retrofits to limit coarser aggregates from rolling off the sides of the conveyors, and using newer paver designs that use a pair of twin augers rather than conveyors to move the mix to the back of the hopper. Segregation at the back of the paver can be reduced or eliminated using kicker paddles or a reverse flow option to push mix under the gear box, keeping a constant volume of mix (and constant head of mix) supplied to both sides of the screed augers, and using auger extensions when screed extensions are used. Suggestions—There are several gaps in the information gathered about reducing or elimi- nating segregation. Additional research or training program content suggested to fill these gaps are summarized here. • Segregation definitions and descriptions could be standardized to improve consistency in the application of segregation specifications such that agency and contractor personnel have a common understanding of segregation. • Future ground penetrating radar and intelligent compaction roller technologies research programs and pilot projects could evaluate the usefulness of this technology for detecting localized low-density areas resulting from segregation and thereby increase the durability in segregated areas. • Recycled materials that contribute asphalt content to the mix could benefit from quality control practices for ensuring consistent effective asphalt content. This is increasingly important as the allowable recycled content increases. It is also important that successful practices to physically reblend high recycled content mixes that contribute to the effec- tive asphalt content be identified. • Paver equipment characteristics that have the potential to reduce or eliminate segregation would be documented. It is important that the effectiveness of pairs of twin augers in the paver hopper, outboard motors and narrower spacing of hopper conveyors, independent speed controls for hopper conveyors or augers, and using auger extensions when screed extensions are used be evaluated. • Pavement condition surveys and pavement management systems could benefit from being adjusted to detect and track early distresses and loss of pavement life resulting from seg- regation. Raveling is the first distress to be seen in segregated areas. Because raveling is viewed as a noticeable change in the surface texture, longitudinal texture profiles may be useful for tracking emerging pavement distresses in segregated areas.