Introduction of WBM Road
The WBM full form is water-bound macadam roads. Water bound macadam are the roads which have its wearing surface consisting of the clean and crushed aggregates which are mechanically interlocked by the process of rolling and bound with each other by the filler material (screening), and water which is laid on the subgrade or base course is known as water Bound Macadam roads.
Water bound macadam roads may be used as subgrade or surface course. The water-bound macadam roads construction layers thickness varies from 7.5 cm to 10 cm. It depends upon the type of gradation of Aggregates used.
In this article, you will get to know about the construction of Water-bound macadam Road, and it’s Advantages and Disadvantages.
Material Required for Construction of WBM Road
The materials which are required for the construction of Water Bound Macadam (WBM) roads
- Coarse Aggregates
- Binding Material
- The coarse aggregates are generally used in the construction of water-bound macadam roads which consists of hard varieties of the crushed aggregates are the broken stones.
- The coarse aggregates for Macadam road construction layers should be graded as far as possible to below grading table.
|Size range in (mm)||Sieve Size in (mm)|
Percentage by weight passing the sieve
90 to 40
63 to 40
50 to 20
The coarse aggregates which are used for the construction of WBM roads must have the following properties.
- The coarse aggregate should be durable.
- It should be hard enough to resist the load.
- The coarse aggregate should have acceptable shape.
- It should be free from flaky and Elongated particles.
- The Indian Road Congress has specified the physical requirements for the coarse Aggregates for the Construction of WBM roads.
- As per IRC the Los Angeles abrasion value of the coarse aggregate for the construction road construction layers of Sub-base, Base course and surface course are 60%,50% and 40%
- The Aggregate impact value of the sub-base, base course and surface course are 50%,40% and 30%
- The material which is used in the construction of WBM road to fill up the voids or gap in the compacted layer of the coarse aggregate is known as screenings.
- It is nothing but the aggregates which is of the smaller size than the coarse Aggregates which are used to fill the voids present in the coarse aggregates.
- The coarse Aggregates are compacted with the help of roller, but there are some voids or gaps are present in the Aggregates. It is very necessary that these voids should be filled with the help of fine aggregate.
- If the voids between the coarse Aggregates are not filled and compacted properly, it will reduce the load-bearing capacity of the road. That’s why screening is very important after the compaction of the coarse Aggregates.
- The Grading requirements for the screening of the WBM roads are as follows.
|Grading Classification||Size of Screenings in (mm)||Sieve Size in (mm)|
Percentage by weight passing the sieve
- The Indian Road Congress has recommended the use of non-plastic material like kankar nodules, moorum or gravel. This material should satisfy the following conditions.
- The liquid limit of this material should be less than 20%.
- The plasticity index should be less than 6%.
- The portion of fine passing 0.075 mm size sieve should be less than 10%.
- The binding material which is used for the construction of water macadam roads should comprise a suitable material which is approved by the Engineer.
- The binding material which is used in the WBM road should have plasticity index value less than 6.
- Binding Material Application is not necessary when the screening used are the crushable type such as moorum or gravel.
Construction Procedure of WBM Road
List of construction procedure of water-bound macadam road as below.
- Preparation of the Foundation for Receiving the Water-Bound Macadam Course
- Provision of the Lateral Confinement
- Spreading of the Coarse Aggregates
- Application of the Screenings
- Sprinkling and Grouting
- Application of the Binding Material
- Setting and Drying
- Preparation of Shoulder
- Opening to Traffic
Preparation of the Foundation for Receiving the Water-Bound Macadam Course
- First, upon, the subgrade of the base course is prepared to the required grade and the camber.
- The depressions and the potholes which exist on the road surface are filled up.
- The corrugations are removed by reshaping the surface to the required grade and the camber, whereas it is necessary.
- If the WBM road is to be provided on the existing surface, the furrows of depth 50mm and width 50mm are cut at some meter interval and 45 degrees to the centre line of the carriage way before laying the coarse aggregate.
Provision of the Lateral Confinement
- For providing the lateral confinement of coarse aggregate, it is necessary that the shoulder having thickness equal to the compacted WBM layer should be constructed in advance.
- Good Earth or moorum should be used in the construction of the Shoulders.
- Shoulders are properly prepared and rolled so as to retain the road structure between them.
- The practice of construction of WBM road in a bench section excavated in the finished formation must be totally discouraged.
Spreading of the Coarse Aggregates
- The aggregates which are stored along the roadside are then sped uniformly and evenly upon the prepared base in the required quantities.
- The number of road construction layers and the total thickness of the WBM road depends upon the design of the pavement and the type of traffic.
- Generally, the thickness of 75mm is given for the ordinary road, which is of a single layer.
- For the important roads, there are two layers of equivalent 150 mm compacted thickness is provided.
- If the coarse aggregates of grading number 1 are used then the layer is compacted to the thickness of 100 mm.
- After spreading the course aggregate, then one of the most important processes is done, which is rolling.
- Rolling is done with the help of three-wheeled power roller of the total weight of 6 to 10 tonnes.
- The process of the Rolling is done under skilled supervision because the proper finishing of the water macadam Road surface depends upon the process of rolling.
- The roller should pass equally over the entire surface of the water macadam Road. The speed of the roller should be slow and uniform.
- The process of the Rolling of water macadam Road should start from the edge of the road, and it should be taken to the crown.
- The care should be taken while the process of the Rolling of the road that each successive strip must overlap with the previous strip.
- The Rolling process should be done under skilled supervision otherwise, it will affect the finishing of the road.
- The faulty rolling can cause the formation of corrugations on the road surface, and there will be unequal finish.
- In case of the Superelevation of the road, The Rolling process should be done from the inner or lower edge of the road towards the outer edge of the road.
Application of the Screening
- After the completion of the rolling process of coarse aggregate, the screenings are filled in the voids.
- Screenings are applied generally in three or more layers. It should be done uniformly on the road.
- Each road construction layer should be compacted by dry rolling otherwise, the road starts wearing if the proper compaction is not done.
Sprinkling and Grouting
- After the application of the screening, the road surface is sprinkled with plenty of water.
- The road surface will Swift and rolled with the help of Roller.
- The hand brooms are used to sweep the weight screening in the voids.
- The additional screening material is applied and rolled till the coarse aggregates are will Bond with each other.
- This will help in the proper interlocking of the coarse aggregate and increase its load-bearing capacity.
Application of the Binding Material
- The binding material is applied at a uniform and slow rate over the surface of the rolled compacted layer of coarse aggregate.
- Generally, the binding material is applied on the road surface in two or three layers.
- After each application of the bonding material layer, the surface is sprinkled with water.
- The weight slurry formed is swept with the help of hand brooms for a mechanical broom to fill the void.
- Then the Rolling is done with 6 to 10-tonne capacity roller.
- During the process of rolling, the binding material gets stick to the wheels of Roller. The water is applied to the wheel of the roller to wash out the binding material.
Setting and Drying
- The road surface of the water-bound macadam is then allowed to cure after rolling for 24 hours.
- If the depressions are found on the next day, then they are filled with screening or the binding material.
Preparation of Shoulder
- During the process of curing the shoulders are prepared by filling the earth to the specified cross slope.
- Shoulders are then properly compacted by Rolling or tamping.
Opening to Traffic
- After drying the WBM road, it is open for the traffic.
Also, Read: Procedure for Rcc Concrete
Advantages of WBM Road
- The construction cost of WBM road is low.
- There is no skilled labour required for the construction of WBM roads.
- The construction of the water-bound macadam Road is done by the locally available material.
- The capacity of water-bound macadam Road to take the traffic load of about 900 tonnes per lane per day.
Disadvantages of WBM Roads
- The maintenance cost of water bound macadam road is high.
- WBM Road is less durable as compared to other roads.
- WBM road can cause inconvenience and danger to the traffic if it is not maintained properly.
- WBM roads are permeable to the rainwater which can lead to the softening and yielding of the subsoil.
Maintenance of the Water Bound Macadam Roads.
- The Water Bound Macadam roads are not more durable as compared to the other roads.
- WBM roads required regular maintenance.
- There may be chances of occurring potholes on the road surface of WBM roads, it should be filled with the binding material.
- The corrugations should be removed and filled by the binding material.
- The loose aggregate in the WBM roads will come on the surface, it should be compacted and level properly.