Water Bound Macadam Road
Water bound macadam road is a road in which the wearing course consists of clean crushed aggregates which are mechanically interlocked by rolling. These aggregates bound together with filler material and water laid on a well-compacted base course.
Water bound macadam road is the most commonly used road construction procedure for over more than 190 years and in the first phase in most of the road projects, water-bound macadam road is constructed, and the surfacing is done with the premix carpet bituminous macadam or cement concrete.
Materials for Water Bound Macadam Road:
In the construction of Water Bound Macadam (WBM) road, there are mainly 3 types of materials which are used such as;
- Coarse Aggregate
- Screenings (Filler Material)
- Binding Material
Coarse aggregates consist of a mixture of hard and durable crushed aggregates and broken stones, and aggregate should be properly graded for each layer of the WBM road construction.
In the construction of water bound macadam road, the coarse aggregates should hold the following properties such as;
- It should be durable and hard.
- The coarse aggregate should be free from elongated particles and flaky particles.
- The coarse aggregate should be in acceptable size and shape.
Physical requirements of course aggregates required for construction of water bound macadam road is given in the fig below;
Screenings: (Filler Material)
Screenings are the material that is used up to fill up the excess voids present in the compacted layer of course aggregate and these materials are the aggregates of a smaller size than the coarse aggregates.
Non-plastic materials are used in order to reduce the overall cost of water bound macadam road.
Binding material should be properly approved by engineer and it should have a plasticity index value less than 6 for the construction of water bound macadam road.
Construction Procedure of Water Bound Macadam Road:
There are the following steps in the construction procedure of WBM road as given below;
Preparation of Foundation for WBM Road:
For the required grade and camber of WBM road, the subgrade or base course is properly prepared and the potholes and the depressions on the surface of the road is properly filled up & compacted.
Provision for Lateral Confinement:
The shoulders having a thickness as that of compacted WBM layer should be constructed before laying of aggregates and with the proper quality of earth, they should be constructed.
The purpose of constructing shoulders is that the road surface to be constructed retains in between them and for the further laying of course aggregates, it becomes easy.
Spreading of Coarse Aggregates:
After the construction of the shoulders, the coarse aggregates are uniformly spread on the prepared base and the total number of layers and thickness of water bound macadam road depends upon the details of pavement design.
Single-layer of compacted thickness 75 mm may be sufficient for ordinary roads and 2 layers of 150 mm each compacted thickness may be provided for special roads.
For compacting the coarse aggregates, the rolling operation is carried out and it is done with the help of vibratory rollers or with the help of 3-wheeled power rollers weighing 6 to 10 tonnes.
For driving the rollers, skilled operators should be used because the fault rolling operations cause the formation of corrugations, the unequal finish of road surface, etc.
Application of Screenings:
The screenings are applied to properly fill the voids remained after the rolling operation is properly finished and in 3 or more layers as per the site conditions screening may be applied.
For each layer of screenings, compaction is carried out with the help of dry rollers, and to remove the uncompacted screening material, brooming of each layer should be properly done after compaction.
The Sprinkling of Water and Grouting:
The road surface is properly sprinkled with plenty of water after the application of screening and brooming is done to sweep the wet screening properly into the voids after the water is sprinkled.
Application of Binding Material:
As screenings, the same procedure is used in this step and rolling operation is carried out after each layer of water is sprinkled.
To washout, the binding material that gets stuck to the wheels of the rollers, the wheels of the roller should be constantly watered at the time of rolling operation.
Setting and Drying of Surface:
The road is allowed to cure or set over-night after the final rolling operation and the next day again sufficient amount of screenings or binding materials can be used and compaction is done if the depressions or voids are visible.
Preparation of Shoulders:
The shoulders are constructed alongside by filling earth to specified cross slope at the time of the curing of the road and compacted properly.
Open for Traffic:
The road is then made open for traffic after proper drying and without any depressions. By placing obstacles longitudinally in the form of drums, barricades, etc the traffic should be well distributed over the full width of the road.
Maintenance of WBM Road:
- By the period of time when the potholes and ruts occur on the road, they should be filled with adequate materials and properly compacted.
- By means of dragging, the corrugations that occurred on the roads should be removed and by fresh materials, broken materials of the roads should be properly restored.
- In 2-5 years or based according to the traffic volume, the surface of the road should be renewed.
- The loose aggregates start coming on the top of the surface of the road so they should be removed and leveled surface should be added by fresh binding material.
Advantages of Water Bound Macadam Road:
There are the following advantages of water bound macadam road such as;
- The construction cost is comparatively low in WBM road.
- No skilled laborers are required in the construction of WBM road.
- From locally available materials they are constructed.
- It can resist a load of traffic of about 900 tonnes per lane per day if these roads are maintained properly and from time to time.
Disadvantages of Water Bound Macadam Road:
There are also some disadvantages of water bound macadam road such as;
- There is a high cost for maintenance on this road.
- There is a very less overall life span of these roads.
- The WBM roads can cause inconvenience and danger to traffic if they are not properly maintained.
- It leads to softening and yielding of subsoil because WBM roads are permeable to rainwater.