Although the best type of surface course is pre-mix carpet for highway maintenance. The surface treatment methods are employed when:
- Intensity of traffic is not very high.
- the pro-mix mixers are not easily available due to long transportation or technical reasons.
- when the cost is high.
The surface treatment may be single or multiple:
i. Single Surface Treatment:
Is wearing course in which the bituminous material is sprayed and the aggregate is placed uniformly over the applied bitumen mineral. The thickness of such layer approximate the nominal size of aggregate used.
ii. Multiple Surface Treatment:
(Double or Triple) is a wearing surface in which a course aggregate is placed on bitumen coat (prime coat) already applied, followed by spraying of bitumen and then by subsequent application of finer aggregate over a second bitumen coat. Generally the minimum size of the smallest aggregate is one of the aggregate used in the preceding application usually thickness of single layer approximately maximum size of aggregate.
Functions of surface treatment:
- to provide long lasting economical surface for granular base road having light and medium traffic volume.
- To prevent entry of surface water into old pavement that have been weathered or cracked.
- It improve the skid resistance of bitumen surface where the surface has polished under traffic.
- To provide temporary cover in case of delayed incomplete pavement.
In Highway Maintenance, For good surface treatment it is necessary that;
- Base course is well prepared to its profile and is made more free from pot holes and ruts.
- Excellence of surface dressing depends upon the correct proportion of binder aggregate.
- Before laying that first surface dressing coat, the base should be made free from all dust loose soil etc.
In all bituminous construction it is necessary that the newly surface posses a bond with the existing base at the interface. It is also necessary that the base is nearly impervious.