How to Prepare Pavement for Surface Treatments

Proper surface preparation can extend the life of your surface treatment

The first element of preparation is to plan the work when the weather cooperates. Most chip seal operations, for example, require an air temperature of 70 degrees F or warmer.
The first element of preparation is to plan the work when the weather cooperates. Most chip seal operations, for example, require an air temperature of 70 degrees F or warmer.

"Surface treatment” is a broad term used to describe a number of asphalt / aggregate systems applied to the entire surface of a pavement, usually for a sealing effect against the intrusion of water and air. Since this sealing effect keeps base materials dry and slows the oxidation (hardening) of the asphalt pavement, it can usually extend the service life of a pavement. Because of this benefit, surface treatments are used as part of an overall “pavement preservation” program. In this role they‘re often applied to pavements that are still in good to very good condition in accordance with Pavement Management System guidelines. 

Typical surface treatments include:

  • Fog seals
  • Chip seals (hot & cold)
  • Scrub seals
  • Slurry seals
  • Parking area sealcoats
  • Cape seals
  • Microsurfacing
  • Thin-bonded wearing course
  • Slurry

Most of these treatments involve the use of asphalt emulsions, which are water-based forms of asphalt, requiring a cure or “break” period for the water to evaporate, leaving the asphalt particles to perform their sealing and bonding action. Because they rely on evaporation of water, they’re warm weather, low humidity operations. Therefore, a requirement for a good job is warmer dry weather! As an example, most chip seal operations require an air temperature of 70°F or warmer.

So the first element of preparation is to plan to do the work only when it’s warm enough. Perhaps the only exceptions to this are:

  • Hot-applied chip seals, which do not involve emulsions but rather use a hot asphalt rubber or polymer-asphalt binder
  • Microsurfacing, which contains polymers and cement and relies on more of a “chemical break”
  • Thin-bonded wearing courses, a hot mix process

This article will focus on preparation work necessary for effective surface treatments. It should be noted that even though public agencies usually contract-out surface treatment work to specialty contractors, it’s not uncommon for many of the preparation tasks to be done “in house” by agency crews.  Of course, prep work can also be included in a contract, and in the current economy, with agencies downsizing, there may be a trend in this direction.

Typical Preparations

Typical preparation tasks for a surface treatment include;

  • Cleaning the old pavement surface
  • Removal of pavement markings (raised markers , thermoplastic striping)
  • Masking of “street iron” (manhole covers, utility boxes, drain inlet grates)
  • Sealing of cracks
  • Digouts & patching (of localized problem areas)
  • Leveling & rut-filling
  • Key (wedge) cuts at gutters

Of course “administrative” preparations must also be made – things like press releases and news articles, notifying the neighborhood, posting ‘no parking’ signs, arranging for tow-away service, ensuring access for equipment and haul trucks and arranging for staging areas, if necessary.

Let’s look more closely at the jobsite preparation tasks.

Cleaning the old pavement

The success of any surface treatment depends on the asphalt binder sticking to the old surface. Perhaps the single biggest deterrent to good adhesion is dust. Therefore, the old surface must be clean. As a minimum, power brooms or street sweepers should be used shortly before the surface treatment. Where necessary, water flushing should also be used. Pay special attention to any areas that have been milled as these tend to be dusty surfaces. Parking areas may exhibit oil drip areas. Depending on severity, these may need to be burned off, dug out and patched or sealed with a special oil spot primer.

Removal of pavement markings (raised markers, thermoplastic striping: It’s usually not necessary to remove painted striping, but raised markers and thermoplastic stripes (e.g. crosswalks) and legends (e.g. arrows) should be removed - usually by grinding. In some cases the surface treatment can be placed so as to avoid (and preserve) existing special markings. Temporary markers – usually the peel-and-stick, stand-up reflector type –  should be installed to serve until the permanent lane striping is applied.

Masking of “street iron”: Street iron such as manhole covers and valve box covers should be protected by masking them with paper or special plastic sheeting. Small peel-and stick markers should be attached to the metal to reference the location of the iron to facilitate removal of the masking material after the surface treatment is applied. Reference staking or GPS logging may also be used for this.

Sealing of cracks: Existing cracks wider than 1/4 inch should be sealed with specialty crack sealer material. Both hot and cold applied products can be used. The sealant supplier should be consulted as to the proper product for your location and climate. Different products may be necessary depending on the time of year you’re doing the crack sealing.

In climates that experience extreme fluctuations in temperature (e.g. mountains, desert), pre-routing of transverse cracks should be considered.

When sealing cracks, care should be taken to:

  1. Try to place the sealant only into the crack
  2. Not leave excessive smearing of sealant on the surface of the pavement
  3. Not leave sealant high so as to form a ridge or bump that motorists would feel. (A squeegee tool should be used only where needed to knock down high spots.)

Cracksealing may be done days or weeks ahead of the surface treatment. Cold-applied sealants should be fully cured prior to covering them with any surface treatment.

If sand in used to prevent tracking of the sealant, a thorough sweeping must be done prior to placing the surface treatment.

Digouts & Patching: Although surface treatments are intended for roads still in good condition, there may be some localized pavement problems. These usually take the form of “alligator” cracking or disintegration in a wheel path of the lane, indicating a load-related, structural failure. These areas should be repaired before placing a surface treatment. The usual approach is to dig out the bad material and replace it with new hot mix asphalt (HMA). Cold mix or proprietary “pothole patch” materials should not be used for patching prior to a surface treatment.  They contain volatile elements that would be sealed in by the surface treatment, keeping them from hardening An unstable, soft spot could result..

Digout & patching operations are also sometimes referred to as “mill & fill,” “plugging” or “R&R." Guidelines for proper digout & patch work include:

  1. Layout a perimeter cutline at least 1 foot beyond any visible cracking.
  2. Use straight lines and square corners to create the cut lines.
  3. Try to keep longitudinal cutlines out of the wheelpath.
  4. For patches thicker than 4 inches make cut wide enough for a small roller to fit into the trench for compacting the base material and lower lifts.
  5. Excavate deep enough to permit the new HMA patch to be at least 50% thicker than the old pavement that failed (This will mean removing some of the aggregate base layer.)
  6. After excavating, always level and compact the remaining base material
  7. Use an HMA mix type appropriate for the traffic loading
  8. If  the HMA in the new patch will be more than 4 inches thick, place it in two lifts, if practical.
  9. Each lift thickness should be at least 3 times the size of the large aggregate in the HMA.
  10. Make at least four passes of the roller on each lift while the mix is above 175°F.
  11. Place enough loose HMA so that after four roller passes the surface will be flush with the old pavement
  12. Don’t use vibratory rolling when the roller drum is touching the old (cold) pavement
  13. Check the final patch with a straight edge.

The surface of a new HMA patch will be more “open” (porous) compared to the surrounding old pavement that has been under traffic for years, so an important final step is to apply a tack coat of asphalt emulsion to the surface of the new patch. This will help seal the surface so it won’t absorb the binder in the future surface treatment.

Leveling: Some surface treatments (e.g. chip seals, slurry seals) should not be placed on surfaces with rutting in the wheelpaths.  In the case of chip seals, the watery emulsion that’s spray-applied will simply pond in any ruts and result in the chips being fully imbedded  or “drowned’ in asphalt, resulting in an asphalt-rich, slick wheelpath. In the case of slurry seals, excess slurry mixture will be deposited in the rutted area and will later be unstable under traffic – especially in hot weather. So it’s important that the pavement be checked with a straight edge and excessive rutting be corrected via “leveling” work.

So how is leveling accomplished? The most common approach is to simply “blade lay” an HMA leveling course -- usually with aggregate no larger than 1/2 inch -- and “feathering” the edges by careful hand raking to remove larger aggregate. This is followed by rolling. In more severe ruts, say deeper than 1/2 inch, rolling should be done with a rubber-tired (pneumatic) roller . Care should be taken to minimize any edge ridges as they will reflect through and be visible after the surface treatment. As with digouts & patching, a leveling patch should also be fog sealed with asphalt emulsion and allowed to cure prior to placing any surface treatment.

A rutted pavement may also be leveled by filling the ruts with microsurfacing, or by grinding it down with either a diamond grinder or a milling machine. These operations often involve subcontractors with special equipment, and may not be cost effective. Remember also, that after any milling or grinding, it’s important to thoroughly sweep (and wash?) the surface to remove the dust created by the operation.

Summary

Surface treatments are important “workhorse” strategies used by pavement managers nationwide. Like many operations – including painting your house – their success depends highly on the degree of preparation that’s done on the old pavement. When applied to pavements still in good condition, and with an effort on pre-cleaning, and the proper repair of major defects in the old pavement, these pavement preservation procedures will fulfill their mission of extending the life of a pavement, and protecting this important investment.

Roger D. Smith is a pavement consultant/trainer in California and editor of the California Pavement Preservation Center Newsletter, www.csuchico.edu/cp2c/  


 



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