The roads whose wearing course consists of clean crushed aggregates mechanically interlocked by rolling and bound together with filler material and layer and laid on a well compacted base course is called water bound macadam road. The strength of a water bound macadam course is due to the mechanical interlocking of the aggregate particles and the cohesion between the aggregate particles due to cementious film of soil moisture binder. The WBM roads are in use in our country both as a finished pavement surface for minor roads and as a good base course for superior pavements carrying heavy traffic.
|Water Bound Macadam|
In water bound macadam roads, the stone pieces used are keyed together by means of sand and clay and no other cementing material is used. The binding effect of sand and clay depends upon the presence of water. When a fast moving vehicle passes over a WBM road, the slurry of sand and clay is sucked out by the pneumatic wheel tyres. The stone pieces get disturbed and finally the road surface is disintegrated. Thus, the WBM roads are not suitable for fast moving vehicles with wheel tyres. These roads are only suitable for slow moving iron wheeled traffic such as tongas, bullock cart etc.
The water bound macadam roads produce dust while in service. In order to minimize the dust nuisance, water should be sprinkled periodically on the road surface and a coat of road oil may be applied so that the dust settles down. The calcium chloride in powdered from may also be spread over the road surface.