IRC SP 25 : 2019Guidelines for Design and Construction of WBM Roads

IRC SP 25 specifies guidelines for design and construction of Water Bound Macadam (WBM) roads — the traditional Indian rural road pavement consisting of hand-broken stone aggregate compacted with screening fines and water rolling. WBM is labour-intensive, uses fully local materials, and has been the workhorse pavement layer for rural roads since pre-Independence.

Use IRC SP 25 when: - Designing PMGSY (Pradhan Mantri Gram Sadak Yojana) low-volume rural roads - Constructing village connectivity roads using local labour and materials - Maintenance / overlay of existing WBM roads - Estate / plantation / forest road construction (limited budget) - Sub-base layer beneath bituminous surface for medium-volume rural roads - Temporary haul roads at construction sites

IRC SP 25 has evolved through the years (1984 → 1996 → 2019). The 2019 revision aligns with modern PMGSY practice and acknowledges that WBM is being increasingly replaced by Wet Mix Macadam (WMM) on PMGSY contracts due to faster construction and more reliable density. WBM remains relevant where: - Local labour is abundant and cheap - Mechanical paver / mixer is impractical (remote, hilly) - Initial cost is the dominant constraint

WBM is a base course material; it almost always needs a bituminous surface (premix carpet, surface dressing, or BC) to provide running surface, dust control, and water resistance. WBM as the running surface (without bituminous treatment) is acceptable only for very low traffic farm-to-market or estate roads.

Layer composition: - Compacted thickness: typically 75 mm (single layer); 150 mm (two layers, each 75 mm) - Width: pavement carriageway + 0.5 m side overlap

Aggregate grades (per IRC SP 25 Table):

| Grade | Aggregate size | Nominal compacted thickness | |---|---|---| | Grading 1 | 90-45 mm | 100 mm | | Grading 2 | 63-45 mm | 75 mm | | Grading 3 | 53-22.4 mm | 75 mm |

Grade 2 is the most common — angular, hand-broken stone, 63-45 mm, hard rock (granite, basalt, limestone, quartzite).

Construction sequence (per layer): 1. Prepare formation — sub-grade or sub-base properly prepared, levelled, compacted. 2. Spread coarse aggregate uniformly to loose thickness ~125-130 mm (compacts to 75 mm). 3. Set kerb / edge confinement — prevent lateral spread during rolling. 4. Initial dry rolling — light steel-wheel roller, 2-3 passes; aggregate keys together. 5. Apply screening / binding material — 12.5-19 mm crushed stone screenings, broomed in to fill voids; ~15 % volume of coarse aggregate. 6. Sprinkle water + roll — water added to allow screenings to flow into voids; alternate watering and rolling until screenings fill all voids. 7. Final rolling — heavy steel-wheel roller (8-10 t), 6-8 passes; surface tight, water no longer absorbed. 8. Set period — 24 hours minimum before opening to traffic; longer for layers in road body.

Open to traffic: can serve as running surface temporarily; for permanent road, bituminous surface treatment within 2-4 weeks.

Aggregate quality (Clause 4): - Source: hard rock, igneous / metamorphic preferred (granite, basalt, quartzite); good limestone acceptable - Aggregate impact value (AIV): ≤ 30 % per IS 2386 Part 4 - Los Angeles abrasion: ≤ 40 % - Flakiness + elongation: ≤ 35 % - Water absorption: ≤ 2 %

Screening / binding material: - Type A (preferred): 12.5-19 mm crushed stone - Type B: well-graded sand / quarry dust (acceptable for less critical layers) - Volume: 15 % of coarse aggregate volume

Water for binding: - Quantity: ~0.4 m³ per cubic metre of compacted WBM (varies by porosity) - Quality: per IS 456 Clause 5.4 — clean, no significant chlorides / sulphates - Spray uniformly during rolling, not at end

Compaction (Clause 5.4): - Initial dry rolling: light roller (3-6 t), 4-6 passes - Wet rolling: 8-10 t roller, until surface is tight (visual + tap test) - Final compaction acceptance: no visible undulation under straight edge (3 m), surface uniform appearance

Acceptance tests: - Aggregate gradation and quality at source qualification - Layer thickness: cores or open-cut sampling, every 100 m, ± 10 mm tolerance - Surface tolerance: 12 mm under 3 m straight edge

Bituminous surface treatments over WBM (per IRC SP 72:2015): - Surface dressing (single coat): cheapest, 2-3 year life, 6-13 mm chip + bitumen seal - Premix carpet: 20 mm, 3-5 year life - BC: 25-30 mm, 8-10 year life (for higher-traffic rural roads)

Cost comparison (approximate, ₹/m²): - WBM 75 mm + surface dressing: ₹150-250 - WBM 75 mm + premix carpet: ₹250-350 - WBM 75 mm + 25 mm BC: ₹350-450 - Equivalent WMM 150 mm + 30 mm BC: ₹500-700

• IRC SP 72:2015 — flexible pavement design for low-volume rural roads (the design framework that uses WBM as a layer).
• IRC SP 20:2002 — Rural Roads Manual.
• IRC:36:2010 — earth embankment construction (subgrade and sub-base preparation).
• IRC:73:1980 — geometric design standards for rural highways.
• IRC:111:2009 — dense-graded bituminous mixes (BC over WBM).
• IS 2386 Part 1:1963 — aggregate test methods.
• IS 2386 Part 4:1963 — aggregate mechanical properties (AIV, LA abrasion).
• IS 2720 (Part 4) — soil grain-size analysis.
• IS 2720 (Part 7) — Proctor compaction.
• IS 73:2013 — paving bitumen (for surface treatment over WBM).
• IS 8887 — bitumen emulsion (cold-mix surface treatment).
• MoRTH Specifications for Road and Bridge Works (5th Revision) Section 400 — granular base courses (WBM and WMM).

1. Soft / weak aggregate. Hard-rock requirement is non-negotiable. Soft sandstone, weathered laterite, schist degrade rapidly under traffic and rolling. AIV ≤ 30 % must be verified at source. 2. Insufficient screening / binder. Voids not filled; layer remains loose, water enters and disturbs. Use minimum 15 % screening volume; broom in thoroughly. 3. Excessive water during rolling. Water washes screenings out; layer 'pumps' under roller. Maintain optimum moisture (per layer porosity); add small amounts and roll. 4. Rolling pattern wrong. Edges first, then centre, with overlap; otherwise layer rolls outward and edges loose. Per IRC SP 25 standard pattern. 5. Layer thickness > 75 mm in single lift. Cannot compact uniformly through depth. Place 75 mm at a time; compact each before next. 6. WBM as running surface in heavy rainfall area. Without bituminous seal, WBM erodes — water enters voids, washes screenings, structural integrity lost. Always provide surface dressing or premix carpet within 2-4 weeks of WBM placement. 7. Inadequate edge confinement during rolling. Material spreads laterally; layer thinner than design. Provide kerb or wood plank against edge during rolling. 8. No drainage during construction. Rainwater pools on WBM, doesn't drain through (low permeability of fully-compacted layer); damages layer integrity. 9. Poor source qualification — wrong rock type. Even hard rock from one quarry differs in flakiness, AIV, abrasion from another. Test every quarry, every season (weather affects fines content). 10. Skipping cross-camber on WBM surface. Water needs to drain off; without 2-3 % cross-camber, water pools, edge erosion follows. 11. No quality control on aggregate breakage at source. Hand-broken aggregate has variable size (especially as quarry workers tire). Random sieve check at delivery; reject batches outside grading range. 12. Screening type wrong for application. Type A (crushed stone screenings) is what locks the layer; Type B (sand) is acceptable in low-volume but doesn't lock as well. Choose per traffic / criticality.

WBM construction within a typical PMGSY rural road project:

1. Site preparation — clearing, stripping topsoil, formation cut/fill per IRC:36:2010. 2. Subgrade preparation — top 500 mm to 98 % MDD, CBR ≥ 5 % preferred. 3. Granular sub-base (GSB) — if WMM is the base, GSB is below; if WBM is the base, often skip GSB on low-volume rural roads. 4. Base course (WBM per this code, IRC SP 25): - Source aggregate (test, qualify) - Spread coarse aggregate to loose thickness - Initial dry rolling - Apply screening + water - Wet rolling - Final rolling and surface tolerance 5. Bituminous surface (IRC:111:2009 or IRC SP 100 series for surface dressing): - Surface dressing (cheapest, low-volume) - Premix carpet (medium) - 25 mm BC (higher quality) 6. Side drains + cross-drainage — concurrent with road construction. 7. Quality acceptance: - Aggregate gradation and AIV per delivery - Layer thickness verification - Surface tolerance (3 m straight edge) 8. Defect-liability period — 3-5 years for PMGSY; routine maintenance includes annual seal coat (for surface dressed) and sub-grade drainage cleaning.

WBM vs WMM comparison: - WBM: hand-broken aggregate, manual rolling, 30+ days per km, labour-intensive, fully local materials, lower initial cost, requires careful workmanship, more variable quality - WMM: machine-crushed aggregate, paver-laid, 5-7 days per km, mechanised, plant-mix produced, higher initial cost, more uniform quality, faster

Modern PMGSY-II / III contracts increasingly favour WMM. WBM remains for genuinely remote / low-volume contexts where mechanisation isn't viable.