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

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103 • Links between test methods, training and certification pro- grams, and segregation (Table C3) The conclusions are organized as individual tables for quick reference on how and where segregation occurs and how it can be reduced or eliminated: • During mix design and aggregate production (Table C4) • At the asphalt plant (Table C5) • During mix transport and mix transfer (Table C6) The important points identified in each of the literature review and survey sections are assembled in table formats, which can be used for quick guidance in this appendix. • Descriptions, detection, and inspection responsibilities for segregation (Table C1) • Specifications, pavement distresses, and capabilities of pavement management systems to evaluate segregation (Table C2) APPENDIX C Checklist for Reducing or Eliminating Segregation Topic Categories Importance Descriptions of Segregation Localized areas of coarse texture in the finished mat. Descriptions of segregation need to be standardized to help with consistent detection of segregation, particularly visual detection. Longitudinal “streaks” of coarser-texture mix in center of mat behind the paver (e.g., under gear box, at screed extensions). Longitudinal “streaks” of coarser-textured mix on one or both sides of mat behind the paver. Localized areas of very fine texture in the finished mat (smoother surface than uniform textured areas) Localized areas of hotter or cooler temperature mix behind the paver. Transverse sections of cooler temperature mix (e.g., after changing trucks, paver stops) Localized “clumps” of fibers and/or binder or other mix additives Localized binder-rich areas (e.g., “fat spots”) in the finished mat. Detection of Segregation Visual Almost always used; considered the “gold standard” Temperature differences Currently used in addition to visual detection, but automated sensor bars are being more widely used. Ride quality; bump detection Used more frequently by industry; possibly linked to ride quality specifications Early loss of ride quality occurs in segregated areas Texture Not routinely used Longitudinal texture profiles can detect end-of-truck segregation and evaluate effectiveness of material transfer units in reducing this type of segregation GPR, Intelligent Compaction Have possibilities, but have not yet been evaluated for detection of segregated areas Responsibility for Field Inspections Wide mix of agency staff and consultants Well-defined definitions and understanding of segregation on how and where segregation can occur are important for everyone conducting field inspections. TABLE C1 DESCRIPTIONS, DETECTION, AND INSPECTION RESPONSIBILITIES FOR SEGREGATION

104 Topic Categories Importance Specifications Method of detection Visual detection most used followed by temperature differences; no consistent use of temperature differences at this time. Disincentives Remove and replace most frequent disincentive Time needed for additional testing for segregation is a good disincentive; delays construction Percent within limits (PWL) and level of segregation incremental pay factors not frequently used Advantages Defines steps for detection, testing, quantifies segregation Provides ability to reject work Forces contractor “best practices” Disadvantages Can be subjective Too much time needed for testing; delays construction Accepts lots that look segregated Dispute resolution Usually dealt with by the field- or district-level staff Desired changes Less subjective Use temperature measurements Require cores Smaller tolerances for QC/QA testing Pavement Distresses in Segregated Areas Raveling, potholes Most frequently expected distresses Increased roughness Expected at least 35% more often Longitudinal and fatigue cracking in wheel path Expected about 25% more often Pavement Management System Ability to identify pavement distresses resulting from segregation Do not currently have the ability to assess and monitor intermittent pavement distresses TABLE C2 SPECIFICATIONS, PAVEMENT DISTRESSES, AND CAPABILITIES OF PAVEMENT MANAGEMENT SYSTEMS TO EVALUATE SEGREGATION Topic Categories Importance Roadway QC/QA Test Methods in Segregated Areas Nuclear and non-nuclear gauges No consistent method of use: Variations in testing include single point, pair of test points, or longitudinal, transverse, and skewed density profiles. Nuclear density gauges: Overestimates roadway density if coarse textured surfaces are not sanded prior to testing (air gaps alter readings). Clean gauge bottom also needed to keep air gaps at a minimum. Non-nuclear density gauges: Any variations in mix moisture content influence density measurements; gauges not always calibrated with cores. Laboratory QC/QA Testing for Segregated Samples Density tests AASHTO T166: Overestimates density in segregated areas because segregated samples are more permeable. Coated or sealed samples are more likely to provide more accurate density measurements. Gradations Ignition oven testing: Aggregate fracture because of heat needs calibration factor or any increases in coarse sizes as a result of segregation are underestimated. Performance-related testing Not currently used to evaluate the change in mix propertied in segregated areas. Training and Certification Programs Field inspectors Both agency staff and consultant are used for field inspections. Training is important for all field inspection staff so that everyone has a consistent understanding of segregation definitions, detection, causes, and methods for reduction. Asphalt plant and field technicians Topics covered in training need links between the topic and cause segregation at each step. Materials engineers, lab techs Importance of using best field and laboratory practices and methods when determining mix properties in segregated areas needs to be included in training programs. Equipment operator certification programs Loader operator certification and training programs can help reduce segregation. Skills needed for constructing aggregate stockpiles, managing stockpiles at the asphalt plant, and loading the cold feed bins at the asphalt plant. TABLE C3 LINKS BETWEEN TEST METHODS, TRAINING AND CERTIFICATION PROGRAMS, AND SEGREGATION

105 Location Topics How to Reduce Segregation Comments Mix Design Gradation Limit gaps in the gradation Use gradation chart with all of Superpave sieve sizes plotted to see any gaps which can be minimized Bailey method of designing gradation helps minimize gaps Maximum aggregate size Use 9.5-mm maximum size aggregate mixes 12.5 mm or larger maximum size aggregate mix increasingly segregate with increases in maximum size aggregate Physically reblend mixes with larger maximum size aggregates when transferring mix from haul trucks to paver (examples: material transfer units; baffles at back of end dump bed to funnel mix) asphalt SMA and porous Increase additives to prevent drain down Occasionally segregate; asphalt separates (drains down) from aggregate Asphalt content (asphalt film thickness) Minimum voids in mineral aggregate (VMA) establish a minimum acceptable asphalt film thickness Adequate asphalt film thickness is needed to “stick” the aggregate particles together Small increase in asphalt content in gradations with some gapping (example: 0.2% typical dense mix) Low asphalt content mixes such as base courses and stabilized mix can frequently segregate Recycled materials with an asphalt content Keep RAP content to less than 10% QC testing needed to keep track of variations in the recycled material asphalt content RAP at 10% to 25%: Reblend the mix before it starts to move through the paver can help (examples: material transfer vehicle/device baffles at back of end dump bed to funnel mix) Higher contents of asphalt containing recycled material can result in “dry” mixes (low asphalt film thickness) when recycled asphalt content not completely used in effective asphalt content Variations in asphalt content in the recycled material results in variations in asphalt film thickness, which makes it difficult to control segregation Aggregate Production Practices Loader operator training and certification programs Majority of stockpiles are constructed with labor intensive practices with trucks, dozers, and fixed conveyors TABLE C4 HOW AND WHERE SEGREGATION CAN BE REDUCED DURING MIX DESIGN AND AGGREGATE PRODUCTION Location Topics How to Reduce Segregation Comments Asphalt Plant Cold feed bins At least four or more cold feed bins are needed More cold feed bins allow a larger number of aggregate stockpiles to be used with narrower ranges of gradations in each Batch plants Actively dislodge fines that build up on the #1 hot bin Keeps “clumps” of fines from dropping into mixer all at once Drum mix plants Drum: Kickback flights inside the drum Helps improve mixing and drying times, which improves asphalt film thickness Discharge point: Use fixed plow at exit Forces mix to reblend as it exits Discharge point: Set drag slat conveyor at 90o to exit Forces mix to reblend as it exits Silo batchers Keep top chute over center of batcher Keeps coarser aggregates from collecting on one side Fill batcher and drop all at once into the silo There needs to be enough mix so it flattens out when it lands in the silo. This keeps a cone from forming, which segregates mix Close batcher gates quickly Keeps mix from “dribbling” into silo Load out “Build” smaller batches of mix in all haul trucks (multiple drops) Helps keep any possible segregation blended with overall mix gradation DO NOT load mix in single drop Results in coarser particles rolling to entire perimeter of haul truck bed DO NOT load mix continuously as larger haul truck moves forward Results in one continuous cone, which lets coarser aggregates roll down and to the outer edges of the haul truck bed TABLE C5 HOW AND WHERE SEGREGATION CAN BE REDUCED AT THE ASPHALT PLANT

106 Location Topics How to Reduce Segregation Comments Mix Transport Truck types Belly dumps and live bottom: May help reblend mixes with lower levels of segregation Survey indicated these haul trucks help minimize segregate, but this benefit may be due to remixing, which occurs during transfer rather than truck type Truck options Tarps: Some indication they help manage temperature segregation Widespread use most likely because they limit flying debris Insulated truck beds: Help minimize temperature segregation Used by agencies in the colder regions of the country Used by agencies with long haul distances Material Transfer End dumps Use funnel-shape baffles at back of bed Forces mix to reblend as it flows through funnel into paver hopper Windrow elevators (pick-up devices) Deposit sufficient mix in windrow to keep optimum amount of mix supplied to screed Windrows commonly formed by belly dumps DO NOT “dribble” mix out in thin stream When next truck places more mix on top of “dribble,” both mix and temperature segregation is produced Minimize segregation in haul truck bed Limited amount of segregation can be remixed by windrow elevators but more severe levels of segregation will still be transferred to the paver hopper Material transfer vehicles/devices Keep sufficient amount of mix in holding hopper and surge bin Keeps screed auger from being “starved” for mix DO NOT run surge bin low of mix Properly size surge bin to size of paver hopper Needed for proper operation of transfer units Make sure remixing augers in holding hopper or surge bin are running Easily overlooked during set up; needed for most successful reblending of mix DO NOT scrape down cold mix built up on sides of holding hopper or surge bin if the mix is too low This creates both mix and temperature segregation Pavers— At Front Hopper Keep hopper at least half full If wings are flipped, then there is sufficient mix in the hopper to reblend any coarser aggregates deposited by haul trucks DO NOT let hopper run too low before flipping wings Creates end of truck segregation Moving mix Drag slats: ensure coarse aggregate stay on conveyors (may require retrofit) Coarse aggregated rolling off conveyor edges creates longitudinal segregation that is documented to cause premature top-down longitudinal cracking Outboard motors: allow closer spacing of conveyors Helps minimize coarser aggregates from rolling off conveyor Pair of twin augers: newer design,which helps reblend mix in hopper May help reduce some forms of longitudinal segregation, but benefits have not yet been fully documented Independent operation of conveyor speed Different speeds allow the volume of mix supplied to each side of the screed to be balanced DO NOT allow variable speed operation to draw mix down too low on one side of the hopper Can create one-sided longitudinal segregation when wings are flipped Pavers— At Back Near gear box Kicker paddles: pushes mix under gear box Keeps coarser aggregates mixed with more uniform mixUse reverse flow option if available on paver to push mix under gear box Screed augers Keep volume of mix consistent; about 75% of the auger height is about right Lower levels “starve” augers, which segregates mix Head of material Keep optimum height of mix all the way to the edge of the screed Optimum mix level helps keep angle of attack constant, mat thickness uniform, and keeps the screed from dragging and tearing the mat Screed extensions Balance flow of mix to each side of screed Can require independent speed controls for mix delivery from hopper when extensions are different widths on each side Use auger extensions when screed extensions are used Mix needs to be pulled from the center by the augers across the full width of paving. Segregation occurs when the mix is pushed. This happens when auger extensions are NOT used. TABLE C6 HOW AND WHERE SEGREGATION CAN BE REDUCED DURING MIX TRANSPORT AND MIX TRANSFER

Abbreviations used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation

TRANSPORTATION RESEARCH BOARD 5 0 0 F ifth S tre e t, N W W a s h in g to n , D C 2 0 0 0 1 A D D R ESS SER VICE R EQ UESTED NO N-PRO FIT O RG . U.S. PO STAG E PA ID CO LUM BIA, M D PER M IT NO . 88 ISBN: 978-0-309-27182-0 9 7 8 0 3 0 9 2 7 1 8 2 0 9 0 0 0 0 TRB