Construction of Water Bound Macadam ( WBM ) Roads

The term ‘ Macadam ‘ in the present day means, the pavement base course made of crush or broken aggregate mechanically interlock by rolling and the voids fill with screening and binding material with the assistance of water. WBM roads may also be use as a sub base, base course or even surfacing course of low volume roads. Thus thickness of each compact layer of WBM layer depends on the size and gradation of the aggregates use. Thus, the number of layers and total thickness of WBM construction depends on the design details of the pavement.

Material use in Water Bound Macadam Base ( WBM roads )

The materials use in WBM construction thus consists of the following constituents –

  • Coarse aggregate
  • Screenings
  • Binding material

Coarse aggregates

The coarse aggregates use in WBM roads generally consists of hard varieties of crush aggregates or broken stones. However crush able materials or soft aggregates ( such as over burnt or Jharna brick, kankar and laterite ) may be use only in the lower layers of the pavement. Crush slag obtain from blast furnace having density more than 112 kg/m^3 may also be use in the lower layers.

CoursesLos Angeles abrasion valueAggregate impact valueFlakiness index
Sub base courseLess than 50%Less than 40% ——————
Base courseLess than 40%Less than 30%——————
Surfacing courseLess than 40%Less than 30%Less than 15%


The screenings are use to fill up the voids in the compact layer of coarse aggregates. The screenings consists of the same material as the coarse aggregates, but of smaller size. There is no need to use screenings in case of crush able type of coarse aggregates.


Binding Material

Binding Material consisting of fine grain material passing 0.425 mm sieve is use in WBM construction thus to prevent raveling of the stones. Kankar nodules or lime stone dust may also be utilize, if locally available. The binding material with plasticity index value 4 to 8 is therefore use in WBM surface course construction. The plasticity index of binding material should be less than 6.0 in the case of WBM layers use as base course or sub base course or when the WBM is provide with bituminous surfacing. If the screenings use consists of crush able material like moorum or soft gravel, there is no need to apply binding material, unless the plasticity index value is less than 4.0.

Quantity of materials

  • However, Approximate loose quantities of materials require for 100 mm thick WBM grading -1 for sub base course layer in m^3 per 10 m^2 area :-
TypeSize range, mmSieve size, mm
coarse aggregate90 to 45 mm1.21 to 1.43
stone screening type-A13.2 mm0.27 to 0.30
  • Therefore approximate loose quantities of materials require for 75 mm thick WBM grading -2 for base course layer in m^3 per 10 m^2 area :-
Types of Agg.Size range, mmSieve size, mm
Coarse aggregate63 to 45 mm0.91 to 1.07
Stone screening type-A13.2 mm0.12 to 0.15 for base coarse
0.10 to 0.12 for surface course
  • Thus, Approximate loose quantities of materials require for 75 mm thick WBM grading -2 for base course layer in m^3 per 10 m^2 area :-
TypeSize range, mmSieve size, mm
Coarse aggregate63 to 45 mm0.91 to 1.07
Stone screening type-B11.2 mm0.20 to 0.22 for base coarse
0.16 to 0.18 for surface course
  • The approximate loose quantities of materials require thus 75 mm thick WBM grading -3 for base course layer in m^3 per 10 m^2 area :-
Types of Agg.Size range, mmSieve size, mm
Coarse aggregate53 to 22.4 mm0.91 to 1.07
Stone screening type-B11.2 mm0.18 to 0.21 for base coarse
0.14 to 0.17 for surface course

Construction Steps of WBM roads

Step 1

The foundation for receiving the new layer of WBM roads may be either the sub grade or sub base or base course. This layer is thus prepare by scarifying and filling the depressions and patching the pot holes, the receiving surface is brought to the required grade and camber.

Step 2

The dust and other loose materials are clean.

Step 3

Lateral confinement is to be provide before starting WBM roads construction. This may be do by constructing the shoulders in advance to a thickness equal to that of the compact WBM roads layer to be lay and therefore by trimming the inner sides of the shoulder vertically.

Step 4

In order to it, coarse aggregates spread-ed uniformly to proper profile to uniform thickness upon the prepare foundation and check by templates.

Step 5

The layer is compact by a three wheel power roller of 8 to 10 tonnes capacity or also by an equivalent vibratory roller. Rolling is starting from the edges, the roller being run forward and backward until the edges are compact. The run of the roller is then gradually shift towards the center line of the road, uniformly overlapping each preceding rear wheel track by one half width. This process is repeat by rolling from either edge towards the center line until adequate compaction is achieve. On super elevated portions of the road or on divide roads, thus rolling is commence from the inner or lower edge and progress gradually towards the outer or upper edge of the pavement. If require, slight sprinkling of water may be do during rolling.

Step 6

Afterwards the coarse aggregates are roll adequately, the dry screenings are apply gradually over the surface to fill the interstices in three or more applications, dry rolling is continued as the screenings are being spread and broom carried out.

Step 7

Afterwards the application of screenings, the surface is sprinkle with water, sweep and roll, wet screenings are sweep into the voids using hand brooms. Additional screenings are apply and roll till the coarse aggregates are well bond and firmly set.

Step 8

Binding material is apply at a uniform and slow rate at two or more successive thin layers, after each application of binding material, the surface is copiously sprinkle with water and thus wet slurry sweep with brooms to fill the voids. Thus this is follow by rolling with a 8 to 10 tonnes roller and water is apply to the wheels to wash down the binding materials that sticks to the roller.

Step 9

After final compaction, then the layer allowed to dry over night. On the next day the “hungry” spots are locate and therefore filled with screenings or binding materials, lightly sprinkle with water if necessary and roll. No traffic is allow till the WBM roads layer sets and dries out. In the case of WBM roads base course, the layer is allow to dry completely without permitting traffic to ply and then the bituminous surfacing is lay. Limit construction traffic may also be permit to ply over the WBM roads layer taking proper care not to damage the layer.

Step 10

After the WBM roads layer is fully dry, steps are thus taken to lay the next layer, in the case of WBM surface course the road is open to traffic.

Rectification of defects

The surface evenness of longitudinal direction generally checked by 3.0 m straight edge along the middle of each traffic lane. The cross profile therefore checked using camber templates at intervals of 10 m. Where the surface undulations exceed the specify limits, the defective area with a minimum of 10 m^2 area around the spot is scarify, reshape with additional aggregate/screening material as require and re-compact properly as specify earlier.

Quality control checks during the construction of WBM roads

  • The Los Angeles abrasion value or aggregate impact value and flakiness index tests are therefore to be conduct on the coarse aggregates at a rate of one test per 200 m^3 to check whether the specify requirements are fulfill.
  • Grading of the coarse aggregates and screenings however to be check at a rate of one test per 100 m^3.
  • The surface levels of compact WBM layer therefore checked, allowable tolerance is (+15 mm and -15 mm ).

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