Scrub Seal Specifications

In this section, we will review the parts of a good specification and some best practices or tips.  Here are the major components needed for a good specification:

• Contract Administration
  • Letting scrub seal contracts in time to permit early season construction
  • Timing the letting of the contract to allow sufficient time for the curing requirements of preconstruction pavement preparation activities
  • Packaging scrub seal contracts in jobs large enough to attract the most qualified contractors
    
    
• Materials
  • Conduct electrostatic testing of scrub seal aggregate source before chip design to ensure that the binder selected for the project is compatible with the potential sources of aggregate.
  • Specify a uniformly graded, high-quality aggregate.
  • Consider using lightweight synthetic aggregate in areas where post-construction vehicle damage is a major concern.
  • Use life-cycle cost analysis to determine the benefit of importing either synthetic aggregate or high-quality natural aggregates to areas where availability of highquality aggregate is limited.
  • Use polymer-modified binders to enhance scrub seal performance.
    
    
• Equipment
  • Use computerized distributors.
  • Preproject analysis of the ability of the scrub seal equipment fleet to keep up with the production rate of the distributor is important.
  • The use of variable nozzles reduces the amount of binder that is sprayed in the wheelpaths.
    
    
• Construction/Application
  • All types of scrub seals are best applied in the warmest, driest weather possible.
  • Ambient air temperature at the time of application should be a minimum of 50°F (10°C) when using emulsions, and 70°F (21°C) when using asphalt cements with a maximum ambient air temperature of 110°F (43°C).
  • The temperature of the surface should be a minimum of 70°F (21°C) and no more than 140°F (54°C) when using emulsions.
  • Complete patches at least 6 months in advance and apply crack seals at least 3 months before the application of scrub seals.
  • Variable nozzles permit the application of a reduced rate of binder in the wheelpaths and combat flooding in the wheelpaths, a defect that makes scrub seals prone to bleeding.
  • Either hand-raking or drag-brooming can correct minor aggregate spread deficiencies such as corrugation, uneven spread, or missed areas.
  • Aggregate should be applied as quickly as possible with both emulsified and asphalt cement binders. Waiting for the emulsion to break reduces the effectiveness of the rollers in achieving the desired embedment depth of the aggregate.
  • Have the most experienced inspector predrive each shot and paint binder rate adjustment on the pavement to facilitate field rate adjustments.
  • In areas where extensive stopping and turning movements take place, the application of a small amount of excess aggregate may reduce scuffing and rolling. The use of a racked-in seal may be a viable engineered solution for determining the precise amount of aggregate for these problematic areas.
  • Rolling guidelines and specifications for roller coverage, rolling patterns, and minimum rolling time or passes achieve full lane coverage and a similar number of passes for all areas of the lane.  Minimum rolling times are generally in the range of 3,000 to 5,000 yd²/hour.
  • The required number of rollers is a function of desired binder distributor production and required rolling time or passes for each shot width on the project.
  • Have rolling follow as closely as practical behind the aggregate spreader.
  • Do not sweep the loose aggregate from the roadway immediately following rolling, because the residual binder has not yet cured enough to bond to the aggregate and underlying road surface. Accordingly, it is important to control the sweeping and not dislodge the embedded aggregate particles from the binder.
  • Maintain traffic control for as long as possible to give the fresh scrub seal the maximum amount of curing time before opening it to traffic.
    
    
• Quality Control
  • Assign experienced personnel who understand the dynamics of scrub seal construction as field QC and QA persons.
  • Regularly calibrate both the distributor and the chip spreader.
  • Evaluate aggregate–binder compatibility tests for local appropriateness and before and during construction.
  • Field test binder at both the distributor and aggregate stockpiles daily to ensure that material has not degraded as a result of handling during transportation.
    
    
• Measurement and Payment
  • Aggregate by the ton and emulsion by the gallon is a typical method of payment.
  • Scrub seal placement can be paid by the square yard.
  • For warranty contracts, a lump sum payment could be made.
  • For term bid contracts, multiple prices could be bid dependent on square yardage.