--- title: "Water bound macadam materials and construction" subject: "Transportation Engineering II" chapter: 3 key: "semester_7/transportation_engineering_ii:wbm-construction-procedure" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/wbm-construction-procedure site: BCT Engineering --- # Water bound macadam materials and construction Transportation Engineering II · Chapter 3: Road Construction Technology - Asked in: 2075 Chaitra, 8 marks; 2075 Asoj, 8 marks; 2072 Chaitra, 8 marks; 2066 Bhadra, 4 marks (asked 4 times) - Video: [W.B.M (Water Bound Macadam) Roads in Hindi (Highway Eng) Lectures-6 // Diploma, B-TECH, SSC JE, GATE](https://www.youtube.com/watch?v=wOW8beorlwQ) — IQ Cracker - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/wbm-construction-procedure - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-3 ## Question Describe the materials required and construction procedure of water bound macadam (WBM) road. ## Short answer Use clean crushed stone aggregate, screenings, binding material and water; spread aggregate, dry roll, apply screenings in stages with water and wet rolling until slurry forms. Compact from edges to centre, cure, then surface treatment. QC tests include LAA, AIV, flakiness, grading, density and surface regularity. ## Answer **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order in WBM construction? A. Spread coarse aggregate, dry roll, apply screenings in stages, then sprinkle water and roll to form slurry B. Spread aggregate, sprinkle water, then apply screenings C. Apply screenings first, then coarse aggregate, then roll D. Apply binding material first, then roll dry Answer: A. Aggregate is keyed by dry rolling, voids are filled by screenings, then water makes slurry that binds. (Key point: Order of operations) 2. What is the role of screenings in WBM? A. Provide the main load-carrying stone B. Fill the voids between the coarse stones C. Act as the bitumen binder D. Increase the layer thickness to 150 mm Answer: B. Screenings of 13.2 mm stone dust fill voids and, with water, form binding slurry. (Key point: Screenings) 3. Why is dry rolling done from the edges towards the centre until there is no creep? A. To wash the aggregate clean B. To make slurry rise to the surface C. To key the aggregate firmly and confine it before screenings are added D. To cure the layer for 7 days Answer: C. Edges first give confinement; no creep shows the stones are interlocked. (Key point: Dry rolling) 4. When is wet rolling stopped? A. After exactly one pass B. When the surface turns black C. When the layer reaches 28 days of curing D. When a wave of slurry ahead of the roller stops forming Answer: D. A slurry wave ahead of the roller means voids are not yet full; once it stops, the layer is well bound. 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