What Is WBM Road?
The water bound macadam, commonly abbreviated as WBM, is a type of road, named after John Macadam. It consists of raw materials like clean and crushed aggregates, screening and binding materials.
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
The materials used for the preparation of WBM road are as follows:
- Coarse aggregates,
- Screenings,
- Binding materials, and
- Water.
1. Coarse aggregates
- Crushed aggregates
- Over burnt bricks
- Crushed slag
- Kankar
- Broken stones
- Laterite
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 |
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 | ||
40 | 0 -15 | ||
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 |
2. Screenings
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 WBM road construction.
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 road construction. 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.
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 road constructed should not be more than 75 mm.4. Rolling
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 screenings5. 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.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.
Also Read
Concrete Road Vs Asphalt Road – Which Is Better?
Cement Concrete Roads – Advantages & Disadvantages
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