Bitumen, usually in the form of an emulsion, is sprayed onto the road surface at an appropriate rate from the spray bar at the rear of a large tanker containing the bitumen emulsion.
Chippings of an appropriate size are immediately applied to the bitumen by a large spreader which usually tows behind it a lorry containing the chippings.
The bitumen part of the surface dressing as well as securing the chippings to the existing surface will seal the old road surface, thus preventing the ingress of water.
Water if allowed to enter the road fabric, will cause severe damage to the strength/load carrying ability of the road and cause early road failure.
The chippings will restore texture to road surfaces that have become smooth with traffic wear. The correct texture on road surface is one of the most important aspects in preventing skidding accidents.
Only premium quality aggregate should be used in surface dressing, i.e. aggregate that is known will not become smooth under trafficking.
Surface dressing will not add any strength to the road pavement, but it does keep an already strong road in a strong condition for longer by sealing water out.
The object of surface dressing is to create a stable mosaic of chippings securely attached to the road surface; this provides a comprehensive seal to prevent the ingress of water in to the road pavement, and a fresh hard wearing skid resistant surface.
This is achieved by spraying the correct amount of bitumen onto the road surface followed by the appropriate amount of the correct size of chippings according to the softness of the road surface.
Once you have established the hardness of the road surface, and you know the amount and type of traffic, you need to consult TRL – Road Note RN39 (7th edition)
"Design Guide for Road Surface Dressing" to design the appropriate dressing for the site.
This will mean determining the correct size of chipping and the correct rate of spread binder.
According to site conditions, it will also be necessary to consider the Polished Stone Value (PSV) and Aggregate Abrasion Value (AAV) of the aggregate source, and also the possibility of the use of an enhanced polymer modified binder.