The oldest type of highway pavement used in modern times is known as Macadam after the name of Johan Macadam, a Scottish Engineer.
The term Macadam in the present time means the road surface and base constructed of crushed or broken aggregates cemented together by the action of rolling and water.
The binding action in WBM is achieved by stone screening used as filler in the presence of water. Water bound macadam is constructed in thickness varying from 8 to 10 cm depending upon the design requirements.
The surface course of WBM gets deteriorated very soon under the action of mixed traffic. 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.
Each layer is compacted to 75 to 80% of its loose thickness by smoothed wheel roller. In WBM roads generally, camber is provided of the order of 1 in 36 to 1 in 48.
Material Required for Construction Of WBM Road
For WBM construction following materials are required.
- Coarse aggregate
- Fillers Materials
1. Coarse Aggregates
The hard and durable stone should be used for coarse aggregate. Traprock is one of the best stone for road metal with a specific gravity ranging from 2.8 to 3.1. hard limestone, dolomite, and granite are also satisfactory.
The specific gravity of these stones varies from 2.6 to 2.7. Sandstone and quartz whose specific gravity varies from 2.4 to 2.7 are somewhat less satisfactory especially for wearing course.
The course aggregate further should be free from excess of flaky, elongated, soft and disintegrated particles and dirt. Coarse aggregate used should meet the following specifications:
Size – 90 to 40 mm or 63 to 40 mm or 50 to 25 mm
Loss Angles abrasion value
After 500 revolution – 40% Max
Flakiness index – 15% Max
10 to 12.5 mm size screenings may be used. They should be properly graded. The grading requirements should be as follows:
|S.No.||Types of Screening||Sieve Size||% passing by weight|
|1.||12.5 mm (For grading 1 or 2)||12.5 mm||100|
|2.||10 mm (For grading 2 or 3)||10 mm||100|
3. Fillers Materials
To prevent ravelling of WBM surface layer the filler material should contain sufficient clay content. Plasticity index of filler material may be up to 9.0.
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