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.

What is WBM used for?

So nowadays the WBM is used as a base course for the superior type of pavement such as bituminous or cement concrete surfacing. Generally, the layers in WBM are laid in 12 to 15 cm thickness. The total thickness may be up to 30 to 35 cm depending upon the design requirements.

Which is the defect in WBM road?

In dry weather, dust is formed and in rainy season mud is formed on WBM road. Fine particles are sucked up by fast-moving vehicle from the road surface resulting in loss of binding particles in the surface.

What is density of WBM?

Effect on Maximum Dry Density (MDD): it is observed that WBM-A exhibited MDD of 20.60kN/m3 and WBM-B exhibited MDD of 22.66kN/m3, indicating 10% increase in MDD for WBM –B after partial replacement by steel slag, specific gravity of steel slag is higher compared to conventional aggregates.

Why is Wmm better than WBM?

The WMM roads are said to be more durable. ... WMM roads are constructed at a faster rate. The consumption of the water is less in case of the WMM roads. Stone aggregates used in WBM is larger in size which varies from 90 mm to 20 mm depending upon the grade but in case of the WMM, size varies from 4.75 mm to 20 mm.

What Is Water Bound Macadam Road?

The WBM’s full meaning is water-bound macadam road. Appropriate points for road maintenance and road construction process are interpreted for road construction. Highway and road layout design, its implementation, and design of fixed dimensions are based on the traffic on the roads.

India has the largest road network in the world with 3.3 million kilometers This road network includes national highways, state highways, district roads, and village roads. Apart from this, recently constructed expressways and international highways are also included.

The method of different types of road construction is as follows:

Soil Stabilization Roads
Bituminous or Black Top Roads
Water Bound Macadam Roads
Cement Concrete Roads
Earth Road and Gravel Roads

What Is WBM Road?

WBM Road was provided by Scottish engineer John Loudon Macadam. It has been used as a road construction method for the last 160 years.

In WBM road, wearing coarse is made by crushing aggregates with filler material mechanically by heavy compaction. It uses sand as a filler material. This wearing coarse is laid on the base course.

WBM road is the first choice for road construction in rural areas. WBM road is the most used road in rural areas. Because the road is paved when funds are available.

The design of the WBM road determines the thickness of each compacted layer corresponding to the traffic load. According to that design, the layer of WBM road is formed. The thickness of the WBM road is usually kept from 7.5 cm to 10 cm.

WBM can easily transfer a load of vehicles from the road surface. In addition, it is able to easily dispose of rainwater. Camber is provided in WBM Road. The road camber is given in WBM roads from 1 inch in 36 to 1 in 48.

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Materials Required For WBM Road Construction

The materials used in the WBM road construction mainly consist of the following materials.

Coarse Aggregate
Screenings (filler material)
Binding Material

1. Coarse Aggregate

In the construction of the WBM road, hard, durable aggregates and broken stones are used as coarse aggregates. The coarse aggregate used for each level in each construction step of the WBM road is selected by proper grading.

The aggregate used for the construction layers of WBM Road is selected according to the grading table given below,

Grading Number	Size Range in (mm)	Sieve Size in (mm)	Percentage By Weight Passing the Sieve
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	35-70
		40	0-15
		20	0-5
3	50 to 20	63	100
		50	95-100
		40	35-70
		20	0-10
		10	0-5

The aggregate used for WBM road should have some of the following properties.
The coarse aggregate for WBM road must have sufficient durability.
The coarse aggregate should be hard. Which is able to resist loads.
The amount of elongated and flaky particles as coarse aggregate for WBM road should be less.
The gradation for WBM roads should have a range of sizes as shown in the gradation table.

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2. Screening Materials

After compacting the coarse aggregate layer, additional spaces remain between the coarse aggregate. Filling material is used to fill these gaps. Which is called screening.

Aggregates smaller than coarse aggregates are used as screening material.

The screening material required for the construction of WBM Road is taken as per the standardized table given below.

Grading Classification	Size of Screenings in (mm)	Sieve Size in (mm)	Percentage by weight passing the sieve
A	12.5	12.5	100
		10	90-100
		4.75	30-Oct
		0.15	0-8
B	10	10	100
		4.75	85-100
		0.15	10-30

According to the Indian Road Congress, WBM has recommended the use of non-plastic materials as road construction. Which should comply with certain conditions as follows,

The liquid limit of this type of material should be less than 20%.
The plasticity index of this material should also be less than 6%.

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3. Binding Material

The use of binding material in the construction of WBM Road should be checked and authorized by an authorized engineer working on site. The plasticity of the binding material should be tested.

Binding material is not required if material like murram or gravel is used as screening material for the construction of WBM road. Because this content has a crashable property.

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Construction Procedure of WBM Road

1. Preparation of foundation for WBM Subgrade or base courses :

Surfaces are prepared for the required camber in the subgrade or base course. And depressions and pot-holes on existing road surfaces are filled.

Dust and other loose materials from the existing road surface are cleaned for a subgrade layer.

Excess debris is removed by shaping the existing road surface to the required grade and camber

2. Provision of the Lateral Confinement

To provide the lateral confinement required for coarse aggregate for WBM Road, a shoulder equal to the compacted WBM layer is constructed in advance.

Good quality materials or moorings should be used in the construction of shoulders. Shoulders are prepared by proper compaction. So that the material of the road structure between wbm road can be maintained.

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3. Spreading of the Coarse Aggregates

The mass of coarse aggregates for the surface of the subgrade is stacked at the required length of the road at certain intervals.

Coarse aggregates are spread evenly on a subgrade base using machinery like dozers.

The total number of layers and the overall thickness are kept according to the design for WBM road. The aggregate is spread in such a way in the thickness of each layer. So that its thickness after compaction is 7.5 cm.

Generally, a thickness of 75mm for a single layer thickness is reserved for normal roads.

Typically for rural roads, the thickness of the compacted layer of coarse aggregates in a single layer is kept at 75 mm. When for important roads, the thickness of the compact of coarse aggregates is kept at 150 mm. Which is provided in two levels.

4. Rolling Process

The rolling process for WBM Road is started from the edge of the road. Then compaction is done by rolling in the center for the straight section of the road. For the superelevation curve of WBM road, compaction is done by the rolling process from the lower end of the rolling to the upper edge.

For WBM road construction, vibration rollers or power rollers weighing 6 to 11 tons are used for compaction by rolling.

Skilled operators are used to operating the roller for compaction of WBM Road.

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5. Application of the Screening

After the process of compaction is completed by rolling the coarse aggregate, the screening material is filled in the space in the coarse aggregate. Screening is applied in three or more layers in the constriction of the WBM road. It provides a uniform surface for the road.

For WBM road construction each layer should be compacted by dry rolling in sufficient quantity. If for some reason proper compaction is not done by rolling then the wearing process of the rod will start soon.

6. Sprinkling and Grouting

Plenty of water is sprinkled on the surface of the screening road. The filling material will swift and roll into the road surface by vibration using a roller.

The weight screening material in the voids in the coarse aagregate is then cleaned using a hand broom. The re-screening material is then applied. This process is repeated until the mass of aggregate remains void between each other.

This process will increase the strength of the coarse aggregate layer. Therefore the load-bearing capacity of the road will be increased.

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7. Application of the Binding Material

The binding material is applied to the surface of the compacted layer of coarse aggregate. It is applied on compacted coarse surface in two or three layers at slow rate and uniform layer.

After applying the bonding material layer as a layer of WBM, each surface is sprayed with water. This is done using a hand broom to fill the void.

Rollers of 6 to 10-ton capacity are used for compaction by rolling for wbm road construction. During the rolling process, the sprinkler system is turned on in the wheel of the roller to prevent the binding material from sticking to the roller wheels.

8. Setting and Drying

The surface of the WBM road is sprayed and rolling for 24 hours. If depression appears on the road surface the next day, it is filled with screening or binding material.

9. Preparation of Shoulder

The shoulders for the road are filled with earth. The shoulders of the road are prepared by rolling and spreading water in proper proportions.

10. Opening to Traffic

After the surface of WBM Road has dried, it is opened for traffic.

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Advantages & Disadvantages of WBM Road

Advantages of WBM Road

The construction cost of WBM road is less than other types of road.
The construction of WBM roads requires less skilled manpower.
Materials for building WBM are easily available locally.
The carrying capacity of the WBM road can carry a traffic load of about 900 tons of lanes per day.

Disadvantages of WBM Roads

WBM roads are less durable than other roads.
WBM Road maintenance can cause inconvenience and danger to traffic if not carried out periodically.
WBM Road enters the rainwater. Which increases the cost of road maintenance.
The maintenance cost of WBM Road is high. Because it has to be maintained periodically.

Maintenance of the Water Bound Macadam Roads

Regular maintenance is required as water bound Macadam roads are not more sustainable.
The surface of WBM roads is prone to potholes during the rainy season. The void on its surface should be filled with the appropriate quality binding material.
Corrugations Material should be removed from the road surface and filled with the suitable quality binding material.
Loose aggregate in WBM paths should be properly compacted on the surface.

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