Water Bound Macadam:

Water Bound Macadam (WBM) is a dense and compact course of a road pavement composed of stone aggregates held together by a film consisting of gravel or screenings with a minimum amount of water.

The stone aggregates are of three gradings.

Table : Grading of Stones For WBM

Grading 1 is used in the lower layers, and gradings 2 and 3 are used in the middle and top layers respectively. Stone aggregates, over-burnt brick ballast and Kankar can be used. The loose thickness of the aggregates will be about 120-140 per cent of the compacted thickness. The quantity of screenings and binding material required is about 0.23-0.35 cu m per 10 sq m of area, depending upon the grading selected. The construction follows the sequence of steps as below :

(a) spread the aggregates manually

(b) dry rolling

(c) wet rolling

(d) application of screenings, watering and rolling

(e) application of binding material, watering and rolling

(f)  spreading a thin layer of coarse sand and light-rolling

The rolling operations shall begin with the outside rear wheel covering parts of the shoulder and coarse aggregates, the roller moving forward and backward, the overlap of rear wheel being half its width in each pass. About 4 passes on the aggregates plus four passes after the screenings are applied should be adequate.

WBM is now used only for low volume roads.