Methods and Practices on Reduction and Elimination of Asphalt Mix Segregation (2015) / Chapter Skim
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From page 65... ...
66 chapter four CONCLUSIONS The important points identified in the literature review and survey sections are assembled here in table formats that can be used for quick guidance for: • Descriptions, detection, and inspection responsibilities for segregation (Table 27) ; • Specifications, pavement distresses, and capabilities of pavement management systems to evaluate segregation (Table 28)
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From page 66... ...
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)
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From page 67... ...
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 28 SPECIFICATIONS, PAVEMENT DISTRESSES, AND CAPABILITIES OF PAVEMENT MANAGEMENT SYSTEMS TO EVALUATE SEGREGATION
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From page 68... ...
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)
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From page 69... ...
Location Topics How to Reduce Segregation Comments Mix Design Gradation Limit gaps in the gradation Use gradation chart with all Superpave sieve sizes plotted to see any gaps that should 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 segregates with increases in maximum size aggregate Physically reblend mixes with larger maximum size aggregates when transferring mix from haul trucks to paver (e.g., material transfer units, baffles at back of end dump bed to funnel mix) SMA and porous asphalt Increase additives to prevent drain down Occasionally segregate; asphalt separates (drains down)
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From page 70... ...
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 that segregates mix.
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From page 71... ...
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 segregation, but this benefit may be the result of 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.
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From page 72... ...
Location Topics How to Reduce Segregation Comments 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: ensures that coarse aggregate stays on conveyors (may require retrofit)
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