Water bound macadam materials and construction

Water Bound Macadam (WBM) is a base or surfacing course of clean crushed stone aggregate of graded sizes, bound by stone dust (screenings) 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 for base, aggregate impact value below 30-40 percent), flakiness index below 15 percent, in three grading sizes: Grading 1 (90-45 mm), Grading 2 (63-45 mm), Grading 3 (53-22 mm).
• Screenings: 13.2 mm size crushed stone dust to fill voids (Grading A/B); non-plastic with plasticity index below 6 percent.
• Binding material: a small quantity of clay or fine crusher dust (PI below 6 percent) only if the screenings lack fines.
• 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, trim to profile, fill gaps; provide lateral confinement by building up the shoulders (earth or gravel) to the thickness of the layer before spreading.
2. Spread coarse aggregate in uniform thickness (75 mm compacted per layer for Gradings 2 and 3, 100 mm for Grading 1; loose depth about 1.25-1.30 times) by stone spreader or manually, with camber.
3. Dry rolling: roll from edges to centre with 8-10 t roller until aggregate is firmly keyed (no creep).
4. Apply screenings gradually in 3 or 4 stages with rolling and sweeping so that voids are filled.
5. Sprinkle water on the surface and roll with the roller; slurry formed binds the stone. Continue wet rolling until a slurry of screenings and water forms a wave ahead of the roller wheels.
6. Apply binding material (if required), sprinkle, roll and let the surface dry overnight.
7. Next layer is placed after setting. Allow 1 to 2 days of curing, then give 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.