Macadam Roads
-->
Grovel and waterbound macadam (W.B.M.) roads are cheap quality and are mostly used in indian villages. It is the oldest method of construction of road known to mankind.
The road having its wearing surface consisting of clean crushed aggregates mechanically interlocked by rolling and bound together with filter material (screening) and water laid on a prepared base course is called Water Bound Macadam (WBM) road.
The aggregate used for construction of WBM roads is known as macadam. These are superior type of low cost roads. Its thickness ranges from 8 to 30 cm. The thickness depends upon the design requirements. Each layer of 12 to 15 cm thick is provided separately. 30 cm thickness can be arranged in two layers. A camber of 1 in 36 to 48 is recommended for such types of roads.
These roads get deteriorated rapidly under the mixed traffic conditions and therefore WBM is frequently used as base course for important high cost roads such as bituminous, macadam, premix carpet and asphalt concrete roads.
WBM road is considered as mother road for all types of modern road construction.
Construction of WBM Roads
Materials required for construction of these roads are
Coarse aggregate of crushed or broken stone, crushed slag, overburnt bride aggregate or kankar or laterite of superior quality.
Screening : This is used to fill the voids in aggregate and usually of the same material as aggregate. Murum or gravel may be used for this purpose.
Binding material for WBM road should comprise of a suitable material approved by Engr.-in-charge. Its plasticity index value should be less than 6. Its application is not necessary when screening used are of crushable type such as murum or gravel.
All these materials are stacked on both sides of the road. Aggregate be 20% more than required on volume basis. The quantity of screenings is about 30 t0 35% of aggregate.
Methods of Construction
The following are the stages of construction of WBM road.
-->
Preparation of subgrade
It is in the form of a trench. For preparing the sub-grade a trench is dug in conformity with the lines, grade and cross-section after completion of earth-work. No trenching is necessary in rock-cutting areas.
After making a trench a sub-grade is thoroughly compacted by rolling with a road roller of weight not less than 8 tonnes. Water is sprinkled uniformly on the sub-grade on the previous evening.
On clayey sub-grade, a layer of granular material such as sand, murrum, gravel laterite or kankar is spread to a thickness varying from 100 to 150 mm.
Preparation of the Base Course
The base of foundation course consists of 12 to 18 cm size boulders or broken pieces of stones, long size kankar, overburnt bricks or brick soiling. The width of this course is about 60 cm wider than the pavement width, projecting about 30 cm. on each side.
Preparation of intermediate and wearing course
It is laid in one or two layers, depending upon the total designed thickness.
The following steps are taken for the preparation of intermediate and wearing course of water bound macadam road.
Preparing the surface.
Providing edging or earthen kerbs.
Spreading of coarse aggregate.
Dry rolling.
Spreading of screenings.
Wet rolling.
Application of binding material, watering and rolling.
Finishing the surface.
Setting and drying.
Preparation of Shoulders
During curing, the shoulders are prepared by filling earth to the specified cross slope. These are then properly compacted by rolling or tamping.
Opening to Traffic
After drying, the road is opened to traffic. The traffic should be well distributed over the full width of the road by placing obstacles longitudinally in the forms of drums etc.
Defects of Water bound Macadam Roads
In these types of roads, screening sand, clay and murrum act as binding materials. Binding effect of these mostly depends upon the presence of moisture. When a fast moving vehicle passes over this road the binding material in the form of slurry, is easily sucked out by the pneumatic wheel tyres of the vehicle and thus the surface is disintegrated. Under the very high contact pressure of iron tyred bullock carts and due to attraction effects, the aggregates are crushed to powder. Mixed traffic destroys WBM road. This can be improved by surface dressing or painting.
