Scrub Seal Application

Spraying Operations

Distributor Pre-spray Checklist

• Determine the asphalt distributor velocity and pump speed
• Delineate the distributor shot or limits
• Construct paper joints
• Clean and open nozzles
• Ensure emulsion is at the correct application temperature
• Check spray bar height

Application Rates

The application rate that the distributor will be spraying should be based on the emulsion application rate that was calculated in the scrub seal design.  The spray pattern should be inspected right after takeoff to ensure the nozzles are spraying properly and the fan pattern is uniform.  If any streaking, ridging, puddling, or flowing of emulsion off the roadway surface is observed the spraying operation should be stopped immediately.

Construction Joints

Construction joints, such as transverse and longitudinal joints, are important to consider when spraying emulsion. Construction paper should be utilized when starting a shot or stopping a shot. Longitudinal joints should coincide with painted lines and should overlap 2” to 4” for uniform appearance.

Key Elements for Spraying Operations

There are four key components you need to consider before starting any job. You’ll also need to maintain them throughout the job.

• Desired Application Rate (gallons per yard)
• Forward Ground Speed (feet per minute)
• Asphalt Pump Output (gallons per minute)
• Spray Bar Width (feet)

It takes a combination of accurately set components to produce a correct shot rate. All of the components are working together simultaneously and if one of them changes, it will make the others change. The components all working together provide a consistent shot rate regardless of speed or bar width changes.

It’s important that all your components are periodically checked and recalibrated to the manufacturer’s preset standards. If a major component is replaced, you need to follow the manufacturer’s installation and set-up procedures carefully.

Stockpile Operation Checklist

• Proper gradation - the front-end loader should penetrate the stockpile deep enough to have a full range of the aggregate gradation.  Segregation could take place while the aggregate is in the stockpile, where the finer particles tend to shift between the coarse particles making the stockpile show a greater content of coarse particles near the top and outside.
• Contamination -  each load should be free of contamination. The loader operator should not scrape too close the bottom of the stockpile allowing grass or clay balls to be picked up with the aggregate.
• Degradation - the loader should not allow the front wheels to roll over any of the stockpile. This could lead to larger pieces being crushed into smaller particles thus changing the aggregate gradation.
• Full trucks
• Excess dust - if dust becomes a problem, it may be reduced by lightly sprinkling the stockpile with water. Only enough water should be used to reduce the dust.

Aggregate Spreading

After the stockpile is checked, the contractor should check to ensure that he/she has the required number of haul trucks needed to cover the emulsion shot and that they are in position before spreading starts. As the distributor begins its spraying operations, the spreader should move to within a few feet of the starting point. While the joint paper is being removed, the operator should align the spreader and make sure that there are enough discharge gates open to cover the entire shot of binder. As the spreader and truck move forward, the gates should be opened just before reaching the beginning of the emulsion shot. 

For best results, the aggregate should be applied as soon as possible. Visual checks should be conducted early in the spreading process to ensure that the aggregate “curtain” is uniform across the entire width of the discharge hopper. The “curtain” should be only one aggregate particle thick and light should be easily seen through the curtain. Any dark streams suggest a gate is open too wide. Any unusually light streak means not enough aggregate is being released. If the aggregate appears to be stacking as it is placed on the binder, it is being applied too heavily.  

Behind the spreader, the pavement surface should be checked for contaminants and streaking of thin or thick rows of aggregates. If there is evidence of thick and thin alternating streaks running transversely or looks like a ripple effect then it indicates that the spreader speed is too high.

Rolling

Once the aggregate is placed, rolling should start immediately.  The first rollers are the haul trucks that transport the aggregate and will be backing up to connect to the chip spreader.  Speeds should be kept under 25 mph.

Rolling a scrub seal is different than rolling asphalt.  Hot mix asphalt rolling is rearranging the aggregate particles in the mix to create density or compaction.  Scrub seals are not compacted, rather the aggregate is simply repositioned to place the largest side of the aggregate in the binder.  This gives us the best chance of the aggregate sticking in the emulsion.  Scrub seals need to be rolled directly behind the chip spreader. 

There should be enough rollers to cover the entire mat width in one pass (one direction).  They should be in a staggered pattern making a minimum of three passes.  If rollers are unable to keep up with operations, the distributor should stop until rollers catch up.  The rolling pattern will depend on the number and types of rollers.  The rolling pattern should be checked to verify that aggregate orientation is correct and if it is not correct, then additional rolling will be required.

Sweeping Operations

Once rolling is complete, the next phase is sweeping.  Sweeping of newly constructed scrub seals should be conducted preferably the next morning to remove excess chips.  Sweeping should begin at the centerline and sweep the excess aggregate toward the outside edge of the roadway.  It is important to note that joints should be swept during construction to ensure it is clean.