WBM Road : WBM Full Form (Water Bound Macadam)

What Is WBM Road

The full form of WBM is Water Bound Macadam Road which is the most commonly used road construction procedure for more than 100 years. The Water Bound Macadam is named based on the Scottish engineer John Loudoun Macadam who first introduced and construct the WBM road.

Water Bound Macadam is a type of flexible pavement in which the base and surface layer contains crushed stone or broken rock pieces and materials are well interlocked with the help of a mechanical roller.

Then the voids are filled with the help of screening material and binding materials (stone dust) along with water and compaction.

The thickness and the number of each compacted layer in WBM depend upon the design consideration and loading. However, they generally vary from 7.5 cm to 10 cm thick.

The Water Bound Macadam layers can easily disintegrate due to moving vehicle loads and surface water, thus the surface layer is usually prohibited to provide the WBM layer. The road camber of 1 in 36 to 1 in 48 is usually preferable on WBM roads.

Materials Required For the Construction of WBM Road

There are mainly three types of construction materials required for the construction of WBM roads:

Coarse Aggregate
Screening
Binding Material

Coarse Aggregate

Coarse Aggregate consists of a mixture of hard and durable crushed aggregate and broken stones. The aggregate used for every layer of the Water Bound Macadam Road construction should be well graded.

The below tables show the gradation of the aggregates that can be adopted.

The coarse aggregates used in the construction of the WBM road should possess the following properties

It should be strong, hard, and durable.
It should not have an excessive amount of elongated and flaky particles.
It should be in acceptable shape and size as shown in the gradation table.

Grading Number	Size Range in mm	Size of Sieve in mm	Percentage by Weight Passing the sieve
1.	90 to 40	100 80 63 40 20	100 65-85 25-60 0-15 0-5
2.	63 to 40	80 63 50 40 20	100 90-100 35-70 0-15 0-5
3.	50 to 20	63 50 40 20 10	100 95-100 35-70 0-10 0-5

Screening

A screening is a construction material used in WBM road construction work to fill the void or space left between aggregate particles after compaction is done.

Generally, construction materials used for screening purposes have smaller sizes compared to coarse aggregate.

The following table shows the standard grading required for screening for the construction of WBM Road.

To reduce the overall cost of WBM roads, IRC has recommended using non-plastic materials such as kankar, murram, or gravel instead of screening.

Binding Material

A Binding Material that is going to be utilized for the WBM road should be checked and authorized by the working engineer and it should have minimum plasticity. Generally, the Index value is limited to 6.

Class of Grading	Size of Screening in mm	Size of sieve in mm	Percentage by Weight passing the sieve
A	12.5	12.5 10.0 4.75 0.15	100 90-100 10-30 0-8
B	10.0	10.00 4.75 0.15	100 85-100 10-30

Usually, the Binding Material is not required if screening is utilized in the construction of WBM road or crushed rock as they have crushable properties.

Cross Section of WBM Road

Following is the WBM road cross-section drawing,

Select the method of execution of the work of WBM road construction in PWD

WBM Road Road Construction Procedure

The execution of WBM Road includes a series of sequential operations operated with a distinct interval of time. The followings are the steps to be followed during construction.

The construction Procedure of the WBM road is given below,

Preparation of subgrade for WBM road construction
Laying and fixing steel rebars as lateral reinforcement.
Coarse Aggregates Spreading
Compaction by heavy roller.
Screening material spreading on a surface
Grouting work and sprinkling work
Binding materials applied on the surface
Leveling and drying process
Side Shoulder formation
The Road is open to traffic

WBM Road Construction — WBM Road Road Construction Procedure

1. Subgrade or Foundation Preparation work

The subgrade layer contributes as a foundation layer of the road on which the base, subbase, and wear course are rested. The required filling and cutting are done to bring the desired R.L. of ground.
Preparation of this subgrade layer for the grade & camber and, dust & other loose materials are cleaned up.
If necessary then modified subgrade materials are filled in the holes, depression, and uneven surface and then rolled and compacted.
The level of the foundation must possess a high degree of uniformity, required cross-section, and elevation along with a well-drained condition.

2. Providing Lateral Confinement

The lateral reinforcement or confinement is constructed before the laying of different layers of Water Bound Macadammaterials.
Before laying of aggregates the shoulders have a thickness similar to the WBM road properly compacted layers. It should be constructed with the proper quality of murram or earth.
But if the soil is unstable and the road is at a higher elevation or required to be filled, then the retaining wall can be constructed as lateral confinement.
The main purpose of providing lateral confinement is that the road surface to be constructed is retained in between them and it becomes easy for the lay of coarse aggregates.

3. Spreading the Coarse Aggregates

To spreading the coarse aggregates over the surface of the subgrade, the lumped mass of coarse aggregates is stacked at the definite interval of the road through the required length of the road to be constructed.
Then the coarse aggregates are evenly spread on a subgrade base with the help of different types of machinery like backhoe or dozer.
The total number of layers and thickness of aggregates depends on the details of the pavement design. The thickness of every layer should be such that after compaction it gives a 7.5 cm thick layer.
In general, for minor or ordinary roads, the thickness of a single compacted layer should be 75 mm and is sufficient. For major or special roads, two layers of 150 mm each compacted thickness may be provided.
Then, the road profile is checked by placing templates across the road every 6 m. the surface is checked from time to time to ensure the proper camber and grade of the road.

4. Rolling Process

The process of rolling is carried out along the edges first and then moved to the center for the straight section of the road. For super-elevated curves, rolling is finished from the lower end of superelevation and then moved to the high edge.
To carry out rolling in Water Bound Macadam Roads, the vibratory rollers or three-footed power rollers weighing 6 tons to 11 tons are used.
Skilled operators must be used for driving the roller as the fault rolling operations lead to the formation of corrugations, the unequal finish of the road surface, or the wearing of the road in a few months of construction.
The main objective of this rolling is to fix the coarse aggregate
thoroughly over one another.

5. Application of Screening (Fine) Material

The next step is the spreading of the screening material. After the process of rolling is completed, screening material is applied to fill all the voids that remained between the aggregates after the compaction.
The quantity of screening is generally used to fill about 50% of the total voids and the remaining 50% of voids are filled with filler materials.
The process of spreading, booming, and rolling is carried out simultaneously for each layer till the surface of the coarse aggregate seems firm and well-compacted without any voids.
The screening is applied in 3 layers or even more than that as per the site conditions.
After compaction, booming of the each layer must be done to remove the un-compacted screening material.

6. Sprinkling of Water and Grouting

After completion of wet rolling with screening materials, the layer is water sprinkled and then again rolled so that the water erodes the surface finer and seeps and deposits it to the inner voids.
The water sprinkling and process of rolling continued until all the voids are filled and a wave of grout flushed ahead of the roller.
The quantity of water to be sprinkled is commonly dependent upon many factors such as size and nature of aggregate, type of surface desired, etc.
If the voids are still viewable then an additional layer of screenings can also be applied and properly compacted.

7. Application of Binding Materials

The same procedure is used for the application of binding materials as that of screenings. Hereafter each layer of water is sprinkled and a rolling operation is carried out.
At the time of rolling operation, the wheel of the roller should be continuously watered to wash out binding material that gets stuck to the rollers’ wheels.
The binding material is generally stone dust or lime dust depending upon the suitability and requirements of WBM road, which is applied in 2 or more consecutive thin layers.

8. Leveling and Drying of Surface

After the final rolling operation, the WBM is left to cure for a single day, and a day after if any depressions or undulations are visible then again the adequate amount of screenings and binding materials can be spread and compacted.
The WBM road can be changed by using the upper courses as a bituminous mix as well so that the easy rotting and disintegration of the surface are prevented.

9. Preparation of Shoulders

While curing the road, shoulders are constructed alongside by keeping proper slope with earth filling work. They are properly compacted.

10. Open for Traffic

After proper drying and without any depressions, the road is then made up for the traffic.
For a few days, the traffic should be well disturbed over the full width of the road by placing obstacles longitudinally in the form of drums, barricades, etc.

Maintenance of WBM Road

Within a short period, if any type of potholes or rugs is shown on the newly constructed Water Bound Macadam Road, the concerned authority should repair it by filling it with the required material, and placing the material on potholes should be done properly.

To prohibit aggregate from getting loose from the surface course of the road, a thin layer of moist soil should be spread over the surface periodically, particularly after the rainy season.

The unevenness that occurred must be removed from the constructed road through dragging. It may cause the circumstance more badly for the constructed roads.

Fresh materials should be used in place of any broken materials on the constructed road.

Every 2 to 5 years after the constriction, the surface of the constructed road is required to be renewed, keeping in view the volume of traffic.

After the road opens for traffic, the loose mixture must be removed that comes on the top of the road surface. Along with the fresh binding material, the leveled road surface should be added and it should be properly compacted and watered.

The dust problem can be effectively removed by providing a surface dressing of bituminous materials.

Advantages of WBM Road

Followings are the advantages of WBM road,

It is cost-effective as the cost of construction of Water Bound Macadam road is comparatively low.
In the construction of the WBM road, no skilled laborers are required.
Water Bound Macadam roads are constructed with the help of locally and easily available materials.
If WBM roads are maintained perfectly from time to time then they could resist heavy traffic and approximately about 900 tons per day.

Disadvantages of WBM Road

Followings are some disadvantages of WBM road,

The maintenance cost of Water Bound Macadam road is high as compared to the bitumen base course.
The overall life span of Water Bound Macadam road is less than the bitumen road.
A high probability of danger to traffic and inconvenience is there if these roads do not receive proper maintenance periodically.
These roads lead to yielding and softening of the subsoil as these roads are absorbent to rainwater.
When a fast-moving vehicle passes over a WBM road, the stone piece gets disturbed and finally, the road surface disintegrates.

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