WBM Road Construction | Water Bound Macadam Road

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 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.

wbm road or water bound macadam road

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.

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Materials Used

The materials used for the preparation of WBM road are as follows:

  1. Coarse aggregates,
  2. Screenings,
  3. Binding materials, and
  4. Water.

1. Coarse aggregates

The coarse aggregates used for WBM road construction are as follows:

  • Crushed aggregates
  • Over burnt bricks
  • Crushed slag
  • Kankar
  • Broken stones
  • Laterite

The table given below shows the maximum permissible values of tests for aggregates, required for various layers of pavement.

Sr. No.TestSub baseBase courseSurfacing course
1Los Angeles abrasion test605040
2Aggregate impact value test504030
3Flakiness index test 1515

Indian Road Congress (IRC) recommended values for size and grading requirements of coarse aggregates used in WBM road construction are as given in the table below:

Grading numberAggregate size range in mmSieve size in mm% passing by weight
190 to 40100100
  8065 – 85
  6325 – 60
  400 -15
  200 – 5
263 to 4080100
  6390 – 100
  5030 – 70
  400 – 15
  200 – 5
350 to 2063100
  5095 – 100
  4035 – 70
  200 – 10
  100 – 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.

The table below states the grading requirements of screenings for water bound macadam.

GradingSize of screeningsSieve size% By weight passing
A12.5 mm12.5 mm100
  10 mm90 – 100
  4.75 mm10 – 30
  150 micron0 – 8
B10 mm10 mm100
  4.75 mm85 – 100
  150 micron10 – 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.

wbm road construction  or water bound macadam road

The foundation supporting the WBM road 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 road constructed should not be more than 75 mm.

wbm road construction or water bound macadam road

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 screenings

wbm road construction or water bound macadam road

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.

wbm road construction or water bound macadam road

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.

wbm road construction or water bound macadam road

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:

  1. Initially, the construction of WBM road is cheaper.
  2. Locally available materials are used.
  3. No specially trained labours required.
  4. 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:

  1. Higher maintenance cost.
  2. It is permeable. So it causes softening of subsoil and may get damaged during the rainy season.
  3. The life span of WBM roads is less.
  4. 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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