Slurry surfacing

GENERAL

Aggregate mixes bound with cold bitumen emulsion and mixed in-situ during placement by specialised plant are generally referred to as “slurry surfacing”. Previously thin layer systems using fine aggregate mixed with anionic emulsion systems without polymer modification were called “slurry seals” or “slurries”. The mixes were applied through a dragged screed box.

Also it has seen the application of multiple stone depth surfacings using cationic emulsion systems which incorporate a polymer in the binder to produce a quick set, quick traffic surfacing. These processes are also applied through a dragged screed box but utilise more sophisticated mixing equipment. The processes are termed “cold overlay” or “micro-surfacing” and are also marketed under proprietary names.

These guidelines am intended to provide the user with an indication of the types of applications, which either require or will benefit from the inclusion of a polymer in the overlay design mix.

The range of polymers used in cold overlays includes:

* SBR Latex
* EVA
* EMA
* Natural Rubber (N.R.) Latex
* SBS

To date there is insufficient information available to allow the polymer types used in slurry surfacings to be ranked with regard to performance. However, each of the polymers appear to improve mix performance in critical applications when incorporated in the slurry mix design. The polymer type and quantity is currently selected by the supplier to produce a mix design appropriate to the service requirement of the job.

APPLICATIONS AND PROPERTIES

Slurry Seal (Thin layer)

For single layer (one stone thickness) overlays, there is no special requirement for mix stability to rutting and polymer addition is optional. However, use of polymers in the mix may be considered worthwhile where there is a risk of bleeding, such as in hot climatic regions. Slurries may be slow set or quick set depending on the constituent materials. Slurry seals are generally unsuitable for shape correction due to the small aggregate size and mix design characteristics.

Microsurfacing (Multiple stone depth)

Microsurfacing systems are generally cationic quick set, quick traffic systems utilising aggregates of nominal size 5 mm to 10 mm. Polymers are used to increase binder stiffness, reduce flow, promote mix strength and improve cold weather performance. Microsurfacing may be used for wearing course applications at nominal depths of around 8 mm or may be designed for significant shape correction such as wheel rut repair. Polymers are an integral part of these systems, and are used to optimise the mix design for the overlay product.