FREE TRAINING FEATURE

FREE TRAINING FEATURE

Want to learn more about pavement management, in-place recycling, and pavement preservation? Do you feel like you're setup for failure as an agency engineer with little or no money to spend on your roads? Enter your email below to get free instant access to a two-hour video-training session that Blair did for the Northwest Pavement Management Association:

Better Roads Radio

Listen to Stitcher
LinkedIn

More Articles available on LinkedIn

Blair Barnhardt

Thin Asphalt Overlays

Thin Overlays

Many agencies have been successful in using thin overlays for preservation of pavements.  A thin overlay is typically 3/4” to 1 ½” and is usually composed of ‘G’ Mix (4.75 mm) HMA or ‘F’ Mix (9.5 mm) HMA.  These mixes have proven their ability to extend the service life of an existing asphalt pavement and often improve the ride quality and correct service defects.  Furthermore, they are often placed as the final wear course on a recycled hot mixed asphalt pavement.

Extensively cracked and rutted pavements are not good candidates for thin overlays, and should be recycled or rehabilitated prior to the application of the thin overlay.  The pavement thickness should be chosen on the basis of the traffic needs of the pavement and the history of the underlying pavement section.  In any case, the thickness of the asphalt overlay should be a minimum of three times the nominal aggregate size.  Thus for a pavement of a nominal aggregate size of 1/2 “ (12.5 mm), the thickness should be at least 1 ½” (37.5 mm).

Note that thin overlays cool very quickly and at times difficult to compact.  The use of warm mix asphalt (WMA) technology for thin overlays is gaining popularity.  The lower temperatures of the WMA means the mix cools slower, allowing more placement and compaction time.

Ultra Thin Bonded Wearing Course

Ultrathin Bonded Wearing Course (UBWC) is a heavy application of a polymer modified emulsion followed immediately by a thin layer of gap graded hot mixed asphalt.  The emulsion is applied through a spray bar mounted behind the wheels and immediately ahead of the screed on a conventional HMA paving machine.  The hot material immediately flashes the water component of the emulsion to steam which draws the residual asphalt into the hot mix to form a strong cohesive bond with both the pavement and the hot mixed asphalt.

Equipment for this surface treatment is specially modified paving machine equipment equipped with an emulsion tank and spray bar system which places the emulsion on the pavement surface just before the pavement surface is spread (see photos below).

This mounting position of the spray bar eliminates the problem of vehicle and equipment tires from driving through the fresh emulsion.  Rollers used for compaction of the UBWC are the same as required for a regular HMA overlay.

The process has also been referred to as the NovaChip process, a proprietary system for applying such an ultra thin overlay.  The surface produced by this UBWC is very smooth and open textured, thereby significantly reducing tire/pavement noise and improving skid resistance.  The system also provides a waterproof layer and strong bond to existing pavement.  We have been told that the system is especially effective in overlaying insitu concrete pavements.

Thin asphalt overlay on an interstate

Thin asphalt overlay on an interstate

Thin Asphalt Overlay

Thin Asphalt Overlay

Ultra Thin Overlay Bonded Wearing Course

Ultra Thin Overlay Bonded Wearing Course

UTBWC Screed

UBWC Screed