WATER BOUND MACADAM (WBM) | MORTH Specification Explained

MORTH SPECIFICATION ( V REVISION) 2013
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WBM road means water bound macadam road. The wearing surface of WBM road consist of clean and crushed aggregates which are mechanically interlocked by rolling operation. The material is bound with filler material (which are also called as screenings) and water, laid on prepared base course.
There are mainly 3 types of materials which are used in the construction of WBM road.
Course Aggregate
Screenings (filler material)
Binding Material
a ) Course Aggregate :
Unlike other course aggregates it consist of mixture of hard and durable crushed aggregates and broken stones. The aggregates used for each layer of the WBM road construction should be properly graded. Below table shows the standard gradation of the aggregates that can be adopted.
The course aggregates used in the construction of WBM road should hold the following properties.
It should be hard and durable.
The course aggregate should be free from flaky and elongated particles.
The course aggregate should be in acceptable shape and size.
b ) Screenings :
The material which is used up to fill up the excess voids present in the compacted layer of course aggregate is called as screenings.
Screening material is nothing but the aggregates of smaller size than the course aggregates. Below table shows the standard grading required of screening for construction of WBM road.
c ) Binding Material :
Binding material which is going to be used for the construction of WBM road should be properly approved by engineer and it should have plasticity index value less than 6.
Construction Procedure of WBM Road.
1 ) Preparation of Foundation for WBM Road :
The subgrade or base course is properly prepared for the required grade and camber of WBM road. The potholes and the depressions on the surface of the road are properly filled up and compacted.
2 ) Provision for Lateral Confinement :
Before laying of aggregates the shoulders having thickness as that of compacted WBM layer should be constructed. They should be constructed with proper quality of murum or earth.
The main purpose of constructing shoulders is that the road surface to be constructed retain in between them and it becomes easy for further laying of course aggregates.
3 ) Spreading of Course Aggregates :
The course aggregates are uniformly spread on the prepared base after the construction of the shoulders. Total number of layers and thickness of WBM road depends upon the details of design pavement.
In general for ordinary roads, single layer of compacted thickness 75 mm may be sufficient. For special roads, 2 layers of 150 mm each compacted thickness may be provided.
If the course aggregate is used of number 1 grade as shown in above course aggregate grading table then it is compacted to thickness of 100 mm.
4) Rolling Operation :
Rolling operation is carried out for compacting the course aggregates. Generally it is done with the help of 3- wheeled power rollers weighing 6-10 tonnes or with the help of vibratory rollers.
Skilled operators should be used for driving the rollers as the fault rolling operations causes formation of corrugations, unequal finish of road surface, wearing of road in few months of construction.
5 ) Application of Screenings :
After the rolling operation is properly finished screenings is applied to properly fill the voids remained after the compaction of aggregates. The screening may be applied in 3 or more layers as per the site conditions.
After uniformly spreading of screening compaction is carried out with the help of dry rollers for each layer of screenings. After compaction brooming of the each layer should be properly done to remove the uncompacted screening material.
6) Sprinkling of Water and Grouting :
After the application of screening the road surface is properly sprinkled with plenty of water. After the water is sprinkled brooming is done to sweep the wet screening properly into the voids.
Rolling operation is further carried out for proper compaction. If the voids are still visible then additional screenings can also be applied and properly compacted.
7 ) Application of Binding Material :
Same procedure is used for the application of binding material as that of screenings. Here after each layer water is sprinkled and rolling operation is carried out.
At the time of rolling operation the wheels of the roller should be constantly watered so as to washout the binding material that gets stuck to the wheels of the rollers.
8 ) Setting and Drying of Surface :
After the final rolling operation the road is allowed to cure or set over-night. The next day if the depressions or voids are visible then again sufficient amount of screenings or binding materials can be used and compaction is done.

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