Objectives and characteristics
Due to their texture and waterproofing qualities, surface dressed pavement have good surface water drainage-, texture- and adhesion characteristics which leads to significant lower risks of aquaplaning.
Surface dressings are also very cost-effective. It is a technique in tune with sustainable development objectives and in particular: economy, speed of execution, low energy consumption, flexibility of use and safety.
The mechanical characteristics of the mineral aggregate chippings needs to promote adhesion between the road surface and the vehicle tyres. This demands a high resistance to both abrasion (Los Angeles) and polishing (Micro Deval) of the chippings. Similarly, for the long term, the accelerated polishing coefficient is to be as low as possible.
The role of the bitumen emulsion in the surface dressing is to ensure the adherence of the chippings to the road surface and to waterproof the pavement; conventionally cationic and fast breaking bitumen emulsions are used. These characteristics ensure an optimal binder – aggregate adhesion as well as a fast realization of coating. Depending on the anticipated service life of the dressing applied, the base binder of the emulsion is either a standard grade bitumen, possibly slightly fluxed, or a polymer modified bitumen. Modification of the bitumen gives an increased binder cohesiveness providing better resistance to the shear forces generated by the traffic.
Choice of structure and design
Surface dressing design is essentially a function of expected traffic and the quality of the existing surface. The most common and versatile technique is a single layer surface dressing, suitable for almost all traffic; aggregate sizes are most often 4/6 or 6/10 mm. Beyond a certain amount of traffic and in cases of inhomogeneous existing pavement surface, a double surface dressing is required. An intermediate solution, in between these two basic options, is the single-layer double-chipped surface dressing.
Surface dressing applications require well defined materials as well as precise application dosage rates, both for the bitumen emulsion and for the aggregate chippings: sprayers, gritters and compactors. It is also very important to note that the installation of a surface dressing can only be carried on a clean surface, to ensure optimal bonding.
Spray equipment have specific characteristics, which allow the emulsion to be transported and stored at a chosen temperature, be sprayed and applied evenly. The uniformity of spraying, both in cross section and profile length is most often provided by the use of calculators that take into account variations in width and speed of the spraying machines.
Gritters or chipping machines are either carried (on dump trucks) or autonomous machines equipped with a feed hopper. They are also equipped with devices to adapt the aggregate chipping flow to the speed of the machine.
Some manufacturers offer “bi-spreader” type of materials, which allow the synchronized application of the bitumen emulsion and chippings. One of the advantages of this type of material lies in the very short time between the spreading of the bitumen emulsion and that of the aggregate chipping, thus optimally promoting adhesion of the mineral aggregate material before breaking of the emulsion takes place.
Compaction of such surface dressing layers is carried out exclusively by tyre compactors, which ensure the placement of the chippings and their entrenchment in the binder film, finally completed by actual traffic once the treated road is put back into service.
Surface dressings offer excellent value for money and are therefore increasingly used for the maintenance of higher trafficked pavements; in these cases, the use of double layer designs based on modified bitumen emulsion is strongly recommended.
In urban areas, in order to reduce rolling noise, reinforce the resistance to traffic shear forces and avoid the drawbacks associated with chipping loss, the use of a combined coating (Cape Seal) can be considered, combining a single-layer surface dressing with a micro surfacing treatment.