WBM roads, short for water-bound macadam roads, are a type of road construction named after John Macadam. These roads use simple materials like clean and crushed aggregates, screenings, and binding materials. Let’s delve into the details of what makes up a WBM road.
WBM roads consist of layers of materials that are mechanically interlocked. These layers can serve as a sub-base, base course, or surface course, each having a specific thickness ranging from 7.5 cm to 10 cm.
1. Coarse Aggregates : The coarse aggregates used in WBM road construction include crushed aggregates, over burnt bricks, crushed slag, kankar, broken stones, and laterite. Various tests ensure these aggregates meet the required standards for different pavement layers.
Maximum Permissible Values for Tests on Coarse Aggregates
| 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 |
IRC Recommended Values for Size and Grading Requirements
| Grading number | Aggregate size range (mm) | Sieve size (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 : Screenings, smaller aggregates used to fill voids, follow a specific grading requirement outlined in a table.
Grading Requirements of Screenings
| 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 | 10 – 30 |
| – | 150 micron | 10 – 30 | – |
3. Binding Materials : Fine-grained materials like kankar nodules or limestone dust are used as binding materials to prevent aggregate raveling. For WBM surface courses, the binding materials should have a plasticity index between 4 to 9.
The construction of WBM roads involves several steps to ensure a durable and reliable road network.
1. Preparing the Foundation : The foundation, either subgrade or sub-base, is prepared to the required grade and camber, ensuring it is free from dust and loose materials.
2. Lateral Confinement of Aggregates : To prevent aggregates from spilling due to traffic load, shoulders are constructed with a thickness similar to the compacted layer of WBM.
3. Spreading Coarse Aggregates : Coarse aggregates are evenly spread on the prepared base, manually or mechanically, with the compacted layer’s thickness not exceeding 75 mm.
4. Rolling : Rollers with a capacity of 6 to 10 tonnes compact the aggregates partially before applying screenings.
5. Application of Screenings : Screenings are gradually applied to fill voids, followed by dry rolling to ensure complete filling.
6. Sprinkling and Grouting : The surface is sprinkled with water, then swept and rolled to fill any remaining voids. Additional screenings may be applied if necessary.
7. Application of Binding Materials : Thin layers of binding materials are applied, followed by sprinkling water and compaction using rollers.
8. Setting and Drying : After the processes, the WBM layer is allowed to set overnight, inspected, and any remaining voids are filled and compacted.
WBM roads offer several advantages, including cost-effectiveness, use of locally available materials, no need for specialized labor, and the ability to handle up to 900 tonnes of traffic per day initially.
However, WBM roads have their drawbacks, such as higher maintenance costs, permeability leading to subsoil softening in rainy seasons, a shorter lifespan, and limited capacity for heavy traffic loads.
