The water-bound macadam construction named after john macadam. In the macadam road construction, the crushed or broken aggregates are bound together by using the rolling procedure.
Binding can be achieved by the use of stone dust as a filler in the presence of water.
The WBM roads are divided into the following types according to the materials used for construction:
- WBM roads with gravel including crushed gravel
- WBM roads with hard broken stones
- WBM roads with soft broken stone and brick residue
Procedure for macadam road construction
a) The thickness of WBM should be from 8cm to 30 cm with each layer of compaction from 7.5cm to 15 cm.
b) The camber provided should be a minimum of 1 in 36 to a maximum of 1 in 48.
- Coarse aggregates:
- the crushed and broken stones should be hard,
- and free from excess of flaky and elongation.
The requirements of coarse aggregates are:
- Aggregate impact value (AIV) = 30%
- Los angles abrasion value (LAA) = 40%
- Flakiness index (FI) = 15%
2. Screening materials of grade A with passing through screening size of 12.5 mm and grade B with a size of screening 10 mm
3. Filler materials of plasticity index (PI) up to 9 shall be used
d) Equipment used
- Aggregate spreader
- Water tankers
- Compaction equipment (road roller)
e) Construction procedure
- 20% of extra broken stones should be provided than estimated.
- The subgrade surface is prepared
- Pavement construction is carried out by lateral confinement and spreading of coarse aggregates
- Dry rolling is done by roller with a load of 6 to 10 tons
- Application of screening materials
- Wet rolling is carried out to fil about 50% of remaining voids
- Filer materials with plasticity index up to 9 are applied in two successive thin layers
- Finishing off is done by 6 mm thick sand or earth spreading, sprinkled with water and rolled.
- Shoulders are prepared
- The WBM is opened for traffic after a few days of construction.
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