priming Road Construction

Types of Bitumen Surface in Road

Types of bitumen surface will  depend on various type of designed where applicable. Bitumen surface in road consists of a thin (often less than 7 mm) coating of sprayed bitumen (the binder) into which is embedded a covering layer of aggregate. The bitumen surfacing will normally overlay a compacted gravel or crushed rock base which has been primed with a  cutback bitumen , although occasionally pavements may be  bitumen surfaced  without the prior applications of a primer.

The function of the cover aggregate is to resist abrasion and to transmit the wheel loads directly to the underlying pavement. The  bitumen binder  serves to hold the aggregate in position and prevents it being dislodged by the traffic.

The essential requirements of a bitumen surface are :-

• The surfacing should be hard wearing, skid resistant and smooth running.

• The surfacing should protect the underlying pavement from loss of moisture and unraveling under traffic loads.

• The surfacing should provide a high degree of waterproofing to protect the pavement from the effects of climatic changes.

Types of Bitumen Surface

The various types of bitumen surface work include :-

(1) Priming.

This consists of spraying a primer onto the compacted gravel or crushed rock base. The primer is normally bitumen which has been made more fluid called cut-back bitumen by the addition of kerosene (cutter) so that most of it will soaking. Priming is carried out to prepare the surface for the subsequent seal coat and to assist in waterproofing the pavement.

(2) Primer Seal.

This is a heavier primer which is covered with sand or fine aggregate (up to 7 mm maximum size). The sand provides a wearing surface which can carry traffic for a limited period of time (e.g. up to six months).

(3) Seal Coats.

The most common seal coat consists of a single application of bitumen (binder) which is sprayed onto the primed pavement followed by an application of aggregate which is rolled with rubber tyred rollers to embed the stone in the bitumen. The aggregate particles are all about the same size so that all the stone will protrude approximately the same amount above the surface of the bitumen.

This treatment is also used for resurfacing existing bitumen surfaced roads.Other forms of seal coats include :-

• A single application of bitumen followed by an application of coarse aggregate (e.g. 20 mm or 16 mm) followed then by an application of fine aggregate (10 mm or 7 mm).

• Two or more successive application of bitumen and aggregate in which the rates of application of the bitumen and size of the aggregate are successively reduces.

(4) Special Treatments.

Common special treatments are :-

(a) Dust Laying – the application of low viscosity (fluid) slow curing cut-back bitumen, or a diluted  bitumen emulsion  to a dusty road surface.

(b) Surface Enrichment – the light application of bituminous material, such as cut- back bitumen or diluted  bitumen emulsion  to increase the binder content. If the bitumen surface treatment is very light it is sometimes referred to as a “fog Coat”.

(c) Penetration Macadam – a course of rolled crushed aggregate grouted with bitumen.

(d) Slurry Sealing. – the application of a mixture of bitumen emulsion, water and fine aggregate to an aged and cracked road surface by a spreading box.

Slurry sealing is not strictly a sprayed treatment but is included in this course for convenience.

To achieve a satisfactory bitumen surfacing, all phase of the work, from the preparation of the pavement, through the application of primer, binder and cover aggregate, and the subsequent compaction of the aggregate by rolling, must be carefully designed and supervised.