What is WBM Road?- Definition and Construction Procedure

What is WBM Road?

Full form of WBM is Water Bound Macadam. It is the layer of broken stone aggregates bound together by stone dust or screening material and water applied during construction and compacted by heavy smoothed wheel roller. It is the oldest type of highway pavement used in modern times is known as Macadam after the name of Johan Macadam, A Scottish Engineer.

What is WBM Road?- Definition and Construction Procedure

Construction Procedure of WBM Road

1. Preparation of Subgrade 

 The site is cleared and weak spots corrected and rolled. To check the spreading of aggregate beyond the carriageway or to provide confinement to the loose aggregate while compacting by rolling either a trench cut in the subgrade and proper camber is provided or earth or brick bunds are made. on the clayey subgrade, a blanket of granular material of a thickness of about 10 cm is provided. The subgrade should be well-drained and checked for stability etc.

2. Spreading of Aggregates  

The coarse aggregate is spread on the prepared surface of subgrade evenly in layers. The thickness of each layer should be such that it gives 7.5 cm thick layer on compaction. The profile of the road is checked by placing templates across the road every 6 meters. To ensure proper camber and grade the surface is checked from time to time.

What is WBM Road?- Definition and Construction Procedure

3. Dry-Rolling 

The objective of this rolling is to key the coarse aggregate thoroughly. After spreading the coarse aggregate and checking all irregularities, the rolling is done by 6 to 10 tonnes, 3 wheeled power roller. Usually rolling is started from the edges with roller running forwards and backwards till the aggregate is fully compacted.

What is WBM Road?- Definition and Construction Procedure

4. Application of Screening 

The next step is the application and keying of screening. after the coarse aggregates are set and keyed thoroughly by the rolling, the screenings are spread uniformly and rolled. generally spreading, brooming and rolling operations are carried out simultaneously. The quantity of screening is generally used to fill about 50% of the total voids and the rest 50% voids are filled with filler materials.

What is WBM Road?- Definition and Construction Procedure

5. Wet Rolling  

After spreading the screening, the surface of the layer is sprinkled with water and rolled again. The sprinkling of water and rolling is continued till all the voids are filled and a wave of grout flushed ahead of the roller. this indicates that all voids are filled with chocking material. The quantity of water and screening are generally dependent upon many factors such as size and nature of aggregate, type of surface desired, etc.

What is WBM Road?- Definition and Construction Procedure

6. Application of Fillers  

The filler material is applied in two successive thin layers. the plasticity index of the filler material should not be more than 9. after the application of the filler, water is sprinkled on the surface and the slurry is allowed to fill the voids. Now rolling is done with 6 to 10-tonne roller. to wash down the binding material off the wheels of the roller water is poured on them.

7. Surface Finishing  

The section prepared as discussed above is allowed to dry overnight and then a  thin layer about 0.6 cm thick of sand or earth spread over the surface. the surface lightly sprinkled with water and rolled again.

8. Shoulders  

These are made of the same cross slope as that of pavement and are compacted by rolling. After proper drying and set of the WBM, It is opened to the traffic.

Also, read -

1. Material Required for Construction of WBM Road
2What Is Super Elevation?

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