Methods and Practices on Reduction and Elimination of Asphalt Mix Segregation (2015) / Chapter Skim
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7 Other terms that have been used to mean end-of-truck segregation include truck to truck (AASHTO 1997) , truckload to truckload (Scherocman 2011)
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8 Longitudinal segregation (Figures 6 and 7) can occur because of improper loading of the bin batcher, under- or over-filling the silos, running the paver screed augers too slowly ("starving" the augers)
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9 aggregate at the inside edges gets dropped under the gear box as the mix moves toward the screed augers (Figure 8)
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10 segregated areas usually exhibit a lack of surrounding acceptable matrix .
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11 sures the distance from the mounted position to the pavement surface every 2 mm (0.8 in.) of longitudinal distance.
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12 striping and tire tracks can significantly influence the image analyses. Temperature Differences Temperature differences can be used to locate non-uniformity in the mix, production, and paving process.
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13 numerous measurements can be taken throughout the paving process. However, the area included in the temperature reported on the device's display can change substantially owing to the distance of the gun to the area being tested and the distance to spot (D:s)
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14 the temperature data, and a global positioning system (GPS) to provide a location reference for temperature anomalies and paver stops (Figure 23)
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15 ment (Al-Qadi and Lahouar 2004)
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16 • Potentially identify areas of low stiffness in new layers that may also indicate segregated areas. • Prevent "over-compaction," which is detrimental to obtaining optimum stiffness.
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17 Detection Method Advantages Disadvantages Visual Detection • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Benchmark method for detection of segregation Detects mix segregation (coarse and fine) Locates binder-rich areas No equipment needed Mix can be evaluated for segregation and can be evaluated during or after construction No cost Subjective Only evaluates surface anomalies Requires experienced field staff, inspectors, contractors Can be wide differences in detection of segregation between experts Fails to locate localized areas of poor density not associated with mix segregation Difficult to see texture changes during night- time paving Does not always identify areas with significantly different mix properties Infrared Gun Identifies areas with temperature differences Effective during day or nighttime paving Easy to use Immediate results obtained Economical Only provides spot-specific measurements Only evaluates surface anomalies Measurements need to be recorded manually Area of mat included in the measurement is device-dependent, distance-dependent, and user-dependent Does not always identify areas with significantly different mix properties Infrared Images or Profiles Detects areas with temperature differences Provides record of temperature variability on entire project Software can indicate significant temperature differences in real time so corrections to the construction process can be made immediately Effective during day or nighttime paving Data needs to be collected during construction Only evaluates surface anomalies or those underlying anomalies that influence surface temperature Technician training and standardized data collection method needed Does not differentiate between mix segregation and temperature segregation Moderate cost Does not always identify areas with significantly different mix properties Quantifies texture changes seen with Testing can only be done after compaction is Surface Texture visual detection of segregation Data can be collected during or after construction Data can be collected at highway speeds using an inertial profiler with a high frequency laser Longitudinal texture profiles can evaluate the successful remixing of segregated mixes by material transfer devices complete Only evaluates surface anomalies Vehicle mounted methods only provide longitudinal profile Static texture measurement methods only provide single point measurements and may require traffic control during testing Does not always identify areas with significantly different mix properties Lighting and environmental conditions can influence results TABLE 2 ADVANTAGES AND DISADVANTAGES FOR METHODS FOR DETECTING SEGREGATION (continued on next page)
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18 gradations, and asphalt content (Table 3)
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19 State References No.
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20 et al.
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21 General dielectric constant ranges for typical asphalt mix materials are: • Aggregates, 2.5 to 5.0 • Asphalt (unmodified or modified) , 2.5 to 3.2 • Air, 1 • Water, 80.4 @ 68°F, 55.3 @ 212°F.
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22 25°F. A Connecticut study reported mixed results for density changes in areas with colder temperatures and the Nebraska research found lower densities were likely when the temperature difference was more than 25°F.
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23 test method and/or test equipment may unintentionally underestimate gradation changes. ➢ Laboratory methods for separating the aggregates and asphalt for cores or loose mix can mask the extent of coarse aggregate segregation.
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24 HOW AND WHERE MIX SEGREGATES Segregation can start as early in the paving process as aggregate production and can continue through the asphalt mix design phase, mix production, and paving operations (AASHTO Joint Task Force 1997; Cleaver 2012)
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25 largest and are thrown around the outer circumference of the pile. Loader operators can help minimize aggregate segregation by blending the fine and coarse areas of the stockpile.
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26 • Roll to the outer edges of the mix in the silos and the edges of the mix in the haul truck bed. • Drop off of the paver hopper conveyors and be deposited under the paver gear box.
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27 improper sampling can lead to incorrectly calculating the asphalt content in the RAP (Advanced Asphalt Technologies 2011)
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28 80 cm (31 in)
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29 plant only needs to keep three stockpiles on site. The limited number of stockpiles implies that there is a wider range of particle sizes in each stockpile, which makes it easier for the coarse and fine aggregates to segregate.
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30 A more recent concern for mix segregation is in how and where materials other than aggregates and asphalt are added into the mix. RAP has been used in mix for years; however, the recent trend is to increase the amount allowed in the mix as well as comingling RAP with RAS.
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31 The uniformity of the film thickness also is improved by limiting the amount of the dust (0.075 mm) material to the lower specification limit.
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32 quickly so the mix is dropped into the silo in a single mass. It is also important that the gates be closed quickly so the mix does not slowly dribble into the silo.
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33 ➢ Segregation is reduced when: – The level of mix in the silo is kept between 25% and 75% full. – Silo gates are quickly opened and closed when loading the haul trucks.
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34 used to haul the mix determines how the mix is transferred to the paver. End Dump Truck Mix Transfer Segregation can be reduced when the end dump haul truck bed is raised before opening the back gate.
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35 that deposits the mix into the paver hopper (Figure 51)
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36 Augers move mix from the outside of the windrow towards the paddles. The paddles rotate downward and push the mix onto the internal conveyor which deposits the mix into the paver hopper.
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37 ➢ Live bottom truck bed designs help reduce segregation. The funnel-shaped sides helps reblend the mix as it is pulled down and moved out the back of the bed.
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38 An MTD is an attachment to the paver designed to receive and hold larger loads of mix in a hopper, then convey it up and into a surge bin in the paver hopper (Figure 56)
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39 hopper as the mix is pulled down and to the back of the paver. The general understanding is that flipping the wings is a bad practice, which is generally true because most paver operators run the hopper almost empty before changing out haul trucks.
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40 Paver Screed Longitudinal segregation on one or both sides of center occurs when the screed augers are operated either too slowly or too fast (Brock et al. 1998; FHWA 2002; Cleaver 2012)
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41 extended on both sides of the screed. Longitudinal segregation on one or both sides of the centerline occurs when the augers are "starved" for mix.
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42 formation (Table 6)
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43 (Henault et al. 2005; Gunter 2012; Sebesta and Scullion 2012)
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44 lower (smoother) by 18 in./mi when an MTV was used and no temperature segregation was documented (Harris 2002)
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45 Texas Over the last 10 years, Texas DOT (TxDOT) has implemented thermal measurements using handheld infrared sensors, density measurements, and requirements for using MTVs to reduce segregation (Rand 2010, 2012)
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46 The automated data collection system continuously collects and records temperature profiles and paver stops. When the contractor uses the automated system, no density profiles are required.
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