General.

Water Bound Macadam (WBM) is the construction known after the name of John Macadam as mentioned in Article 2.1.2. the term ‘Macadam’ in the present day means, the pavement base course made of crushed or broken aggregate mechanically interlocked by rolling and the voids filled with screening and binding material with the assistance of water. The WBM may be used as a sub base, base compacted layer of WBM layer depends on the size and gradation of the aggregates used. The number of layers and total thickness of WBM construction depends on the design details of the pavement. Three different gradations, namely grading – 1, grading – 2 and grading – 3 with different size ranges have been suggested by the IRC and also MORTH specifications. Among these three, grading – 1 contain the largest sizes of coarse aggregates and grading – 3 the smallest. It has been suggested the grading – 1 layer.

Materials.

The materials used in WBM construction consist of : (1) coarse aggregates, (2) screenings and (3) binding material.

Coarse aggregates.

The coarse aggregates used in WBM generally consists of hard varieties of crushed aggregates or broken stones. However crush-able materials or soft aggregates (such as over burnt or Jhama brick, kankar and late-rite) may be used only in the lower layers of the pavement. Crushed slag obtained from blast furnace having density more than 112 $kg/m^3$ may also be used in the lower layers.

The physical requirements of coarse aggregates for WBM construction specified by the IRC for use in the sub base, base and surface courses are given below:

a] For sub base course, either Los Angeles abrasion value to be less than 50 percent or aggregate impact value to be less than 40 percent.

b] For base course, either Los Angeles abrasion value to be less than 40 percent or aggregate impact value to be less than 30 percent.

c] For surfacing course, either Los Angeles abrasion value to be less than 40 percent or aggregate impact value to be less than 30 percent and flakiness index to be less than 15 percent.

Screenings.

The screenings are used to fill up the voids in the compacted layer of coarse aggregates. The screenings consists of the same material as the coarse aggregates, but of smaller size. The grading requirements of screenings for WBM construction are given in Table 8.3. The type – A screenings are to be used with the coarse aggregate grading – 1; Type – B screenings are to be used with the coarse aggregates grading – 3; either Type – A or Type – B screenings may be used with coarse aggregate grading – 2. There is no need to use screenings in the case of crushable type of coarse aggregates.

Binding material.

Binding material consisting of fine grained material passing 0.425 mm sieve is used in WBM construction to prevent raveling of the stones. Kankar nodules or lime stone dust may also be utilized if locally available. The binding material with plasticity index value 4 to 8 is used in WBM surface course construction. The plasticity index of binding material should be less than 6.0 in the case of WBM layers used as base course or sub base course or when the WBM is provided with bituminous surfacing. If the screenings used consist of crush-able material like moo-rum or soft gravel, there is no need to apply binding material, unless the plasticity index value is less than 4.0.