WBM: material, equipment, procedure, quality control

Water Bound Macadam (WBM) is a base or surfacing course of clean crushed stone aggregate of graded sizes, bound by screenings, binding material and water, and compacted by rolling (IRC:19 and MoRTH Section 404; used in Nepal DoR specifications for base courses).

Materials

• Coarse aggregate: hard, tough crushed stone (Los Angeles abrasion value below 40 percent and aggregate impact value below 30 percent for base; up to 50 and 40 percent for sub-base), flakiness index below 15 percent, in three gradings: Grading 1 (90-45 mm), Grading 2 (63-45 mm) and Grading 3 (53-22.4 mm).
• Screenings: crushed stone of 13.2 mm (Type A) or 11.2 mm (Type B) nominal size to fill the voids in the coarse aggregate; non-plastic moorum or gravel (liquid limit below 20, plasticity index below 6) may be used where economical.
• Binding material: a small quantity of fine material such as crusher dust or moorum (PI below 6 percent), used only if needed to bind the screenings.
• Water: clean and free from salts.

Equipment: three-wheel power roller (8-10 t) or vibratory roller (8-10 t), motor grader or spreader, water tanker, brooms, screening spreaders.

Construction procedure

1. Prepare the base: clean it, trim it to profile and fill depressions; provide lateral confinement by building the shoulders (or kerbs) along both edges to the thickness of the layer.
2. Spread the coarse aggregate to a uniform thickness and the correct camber by stone spreader or manually. The compacted thickness of one layer is 100 mm for Grading 1 and 75 mm for Gradings 2 and 3; the loose depth is about 1.25-1.30 times this.
3. Dry rolling: roll from the edges to the centre with an 8-10 t roller until the aggregate is firmly keyed (no creep ahead of the roller).
4. Apply the screenings gradually in 3 or 4 stages, with dry rolling and sweeping, so that the voids are filled.
5. Sprinkle water on the surface and roll; the slurry formed by the screenings and water fills the voids and binds the stone. Continue wet rolling until a wave of slurry forms ahead of the roller wheels.
6. Apply binding material (if required), sprinkle water, roll, and let the surface dry overnight.
7. Allow the layer to dry and set (1 to 2 days, with no traffic) before laying the next layer or the surface treatment/premix carpet.

Quality control: aggregate tests (LAA, AIV, flakiness, water absorption, soundness), grading of aggregate and screenings, thickness and camber checks with a template, density by sand replacement, surface regularity by straight edge (max 12 mm undulation), and tests every 100-200 m.