Construction Procedure:
a) Preparation of Foundation: The foundation for receiving the layer of WBM may be either the sub-grade or sub-base or base course. The depressions and pot holes on the existing road surface are filled up and the corrugations are removed by scarifying and reshaping the surface to the required grade and reshaping camber as necessary.
b) Spreading of coarse aggregate: The coarse aggregate is spread uniformly on the prepared base. The WBM course is normally constructed to compacted thickness of 7.5cm.
c) Rolling: After spreading the coarse aggregates properly, compaction is done by a three wheeled power roller of capacity 6 to 10 tonnes or alternatively by an equivalent vibratory roller; the weight of the roller depends on the type of coarse aggregates. The rolling is done until adequate compaction is achieved.
d) Application of Screenings: After the coarse aggregates are rolled adequately, the dry screenings are applied gradually over the surface to fill the interstices (Voids) in three or more applications. Dry rolling is continued as the screenings are being spread and brooming is carried out.
e) Sprinkling and Grouting: After the application of screenings, the surface is sprinkled with water, swept and rolled. Wet screenings are swept into the voids using hand brooms. Additional screenings are applied and rolled till the coarse aggregates are well bonded and firmly set.
f) Application of binding materials: After the application of screening and rolling, binding material is applied at a uniform and slow rate at two or more successive thin layers. After each application of binding material, the surface is copiously sprinkled with water and wet slurry swept with brooms to fill the voids. This is followed by rolling with a 6 to 10 tonnes roller and water is applied to the wheels to wash down the binding material that sticks to the roller. When crushable type screenings like moorum or gravel are used, there is no need to apply binding materials, except in the surface course.
g) Setting and Drying: After final compaction, the WBM course is allowed to set over night. On the next day, the hungry spots are located and are filled with screenings and binding material, lightly sprinkled with water and rolled. No traffic is allowed till WBM layer sets and dries out.