What Is WBM Road?
The aggregates are mechanically interlocked by rolling and the screenings and binding materials, are used to fill the voids in the aggregates. All the materials are mixed with water.
The mixture of such materials is laid on a well prepared subgrade, sub-base, or on existing road pavement. Water bound macadam can be used as a sub-base, base course, or surface course. The thickness of each layer in WBM road ranges between 7.5 cm to 10 cm.
According to IRC – 19-1997, a camber of 1 in 48 is provided at the formation level of the subgrade as well as the finished surface if the rainfall in that area is not heavy, and if the rainfall is heavy, a camber of 1 in 36 is suggested.
Materials Used In WBM Road
The materials used for the preparation of water bound macadam are as follows:
- Coarse aggregates,
- Binding materials, and
1. Coarse aggregates
The coarse aggregates used for water bound macadam are as follows:
- Crushed aggregates
- Over burnt bricks
- Crushed slag
- Broken stones
The table given below shows the maximum permissible values of tests for aggregates, required for various layers of pavement.
|Sr. No.||Test||Sub base||Base course||Surfacing course|
|1||Los Angeles abrasion test||60||50||40|
|2||Aggregate impact value test||50||40||30|
|3||Flakiness index test||15||15|
Indian Road Congress (IRC) recommended values for size and grading requirements of coarse aggregates used in water bound macadam are as given in the table below:
|Grading number||Aggregate size range in mm||Sieve size in mm||% passing by weight|
|1||90 to 40||100||100|
|80||65 – 85|
|63||25 – 60|
|20||0 – 5|
|2||63 to 40||80||100|
|63||90 – 100|
|50||30 – 70|
|40||0 – 15|
|20||0 – 5|
|3||50 to 20||63||100|
|50||95 – 100|
|40||35 – 70|
|20||0 – 10|
|10||0 – 5|
The screenings are basically used for filling up the voids in the layer of coarse aggregates. Screenings include aggregates of smaller sizes. The screenings are generally the same material as coarse aggregates used in water bound macadam.
The table below states the grading requirements of screenings for water bound macadam.
|Grading||Size of screenings||Sieve size||% By weight passing|
|A||12.5 mm||12.5 mm||100|
|10 mm||90 – 100|
|4.75 mm||10 – 30|
|150 micron||0 – 8|
|B||10 mm||10 mm||100|
|4.75 mm||85 – 100|
|150 micron||10 – 30|
3. Binding Materials
To avoid raveling of aggregates, fine grained materials such as kankar nodules or lime stone dust are used as binding materials for the construction of water bound macadam.
When WBM is used as a surface course, the binding materials used in it should have a plasticity index between 4 to 9.
When and where crushable materials such as moorum or soft gravel are used as screenings, binding materials are not required.
WBM Road Construction Procedure
The following steps are adopted for the construction of WBM roads:
1. Preparing the foundation for receiving the WBM course
The foundation supporting the layer of WBM is either the subgrade or sub-base course. It is prepared to the required grade and camber. It is cleaned of all the dust particles and loose materials present in it.
The foundation supporting the WBM should be dried. When the existing road is topped black, furrows are cut at an interval of 1m at an angle of 45 degrees to the center line of the carriageway.
2. Lateral confinement of aggregates
The aggregates in WBM are confined because if they are placed on the subgrade, there are possibilities that the aggregates may come out due to the load of traﬃc. It is done by construction of shoulders of thickness similar to the thickness of the compacted layer of WBM (7.5 cm).
3. Spreading coarse aggregates
The aggregates should be evenly and uniformly spread on the prepared base in the required amount. They may be spread manually or mechanically. The thickness of the layers (compacted) of WBM constructed should not be more than 75 mm.
Once the aggregates are spread evenly, the rollers are deployed to carry out the process of compaction. The rollers used shall be of 6 to 10 tonnes capacity.
The compaction of aggregates should start from the edge of the surface. The aggregates are compacted partially then the compaction is paused to permit the application of screenings
5. Application of screenings
After partial compaction of aggregates, screenings are applied so that the voids in the aggregates are filled completely. They are applied gradually over the surface in three or more applications. Again after applying screenings, dry rolling is resumed so that they are completely filled in the voids.
6. Sprinkling and grouting
The surface is sprinkled with water after applying the screenings. The surface is then swept and rolled so that the voids are completely filled. If any voids are left empty, additional screenings may be applied.
7. Application of binding materials
After applying the screenings, binding materials are applied in thin layers. Two or more thin layers of binders are applied.
After the application of each layer of binding materials, water is sprinkled, followed by compaction of layers by rollers of 6 to 10 tonnes capacity.
8. Setting and drying
After all the above-mentioned processes, the layer of WBM is allowed to set overnight. Next day the road is inspected and if any voids are empty, they are filled and compacted.
Advantages Of WBM Road
The advantages of WBM roads are as follows:
- Initially, the construction of WBM road is cheaper.
- Locally available materials are used.
- No specially trained labours required.
- If it is maintained properly, it can take up to 900 tonnes of traﬃc per day.
Disadvantages Of WBM Road
The disadvantages of WBM roads are as follows:
- Higher maintenance cost.
- It is permeable. So it causes softening of subsoil and may get damaged during the rainy season.
- The life span of WBM roads is less.
- It is not able to bear heavy traﬃc loads.