IRC 27:2009 is the Indian Standard (IRC) for specifications for bituminous macadam. IRC 27:2009 specifies Bituminous Macadam (BM) — a hot-mix asphalt base course widely used in Indian flexible pavement construction for state highways, district roads, and urban roads. BM consists of graded aggregate + bitumen (3.5-4.5% by weight) hot-mixed, laid, and compacted. Three grades: Grade I (40 mm max size) for heavy base, Grade II (25 mm) intermediate, Grade III (20 mm) for light traffic. Mix design via Marshall method (75 blows, stability > 7.5 kN, flow 2.5-4.5 mm). Typical 50-80 mm compacted thickness per layer. BM serves as the base course between granular sub-base (GSB or WBM) and surface wearing course (DBM, BC, or SDBC). Amendment No. 1 (2018) updated bitumen grade references (VG-30 default replacing 80/100) and raised aggregate quality standards. Amendment No. 2 (2022) added provisions for recycled asphalt pavement (RAP) up to 25% in BM mix. BM is less costly than DBM (dense bituminous macadam) but provides less structural capacity — choose by traffic and design requirements.
Specifies the materials, mix design, production, construction, and acceptance criteria for Bituminous Macadam (BM) used as base course in Indian flexible pavements — a hot-mix asphalt bound aggregate layer.
Aggregate grading, bitumen content, mix and laying temperatures for BM base course.
| Reference | Value | Clause |
|---|---|---|
| Bituminous Macadam (BM) — purpose | Profile-correcting / base course | Cl. 1.1 |
| BM — typical compacted layer thickness | 50-100 mm | Cl. 5.1 |
| BM Grading 1 — nominal max aggregate size | 40 mm | Table 2 |
| BM Grading 2 — nominal max aggregate size | 19 mm | Table 2 |
| Bitumen content — Grading 1 (40 mm) | 3.1-3.4% by mass (3.3% typical) | |
| Bitumen content — Grading 2 (19 mm) | 3.3-3.5% by mass | |
| Bitumen grade | VG-30 (typical), VG-40 in hot climates | |
| Mix temperature — at plant | 150-165 °C (VG-30) | Table 4 |
| Laying temperature — minimum at paver | 130 °C (VG-30) | |
| Rolling temperature — break-down (initial) | 120-150 °C | |
| Rolling — final close to ambient | Above 80 °C | Cl. 9.3.3 |
| Aggregate — LA Abrasion max | ≤ 40% | Table 27.2 |
| Aggregate — flakiness + elongation max | ≤ 35% | Table 27.2 |
| Aggregate — water absorption max | ≤ 2% | Table 27.2 |
| Aggregate — soundness loss max (sodium sulphate) | ≤ 12% | Table 27.2 |
| Stripping value — minimum coating retained | ≥ 95% | Table 27.2 |
| Compaction — minimum density (lab Marshall) | ≥ 98% of trial patch density | Cl. 9.3.4 |
| Air voids in field core | 5-10% (target 7%) |
IRC 27 (2009) provides Specifications for Bituminous Macadam (BM) — an open-graded bituminous layer used as binder course below the wearing course in flexible pavements. BM is the more open-graded, lower-binder, more economical alternative to Dense Bituminous Macadam (DBM) per IRC:29:2019.
Use IRC 27 when you are: - Specifying a binder course on low-to-medium traffic roads (rural roads, low-volume SH, district roads) - Doing DPR for PMGSY rural roads where economy is paramount - Specifying base course for less-demanding pavement structures - Considering BM vs DBM — choosing on cost-vs-performance basis - Working on village road / rural road projects where budget constrains material choice - Re-paving low-volume roads with simpler bituminous specifications
BM key characteristics: - More open-graded than DBM (large coarse aggregate, fewer fines) - Lower binder content (~3.5-4.5 %) - Lower stability (Marshall < 6 kN, less than DBM) - Lower cost per cubic metre - Lower performance under heavy traffic - Higher voids (5-10 % vs 3-5 % in DBM)
When BM is appropriate: - Low-traffic roads (AADT < 2,000 commercial vehicles/day) - Rural connectivity roads - Light-duty service roads - Where wearing course will be substantial (full BC on top)
When BM is NOT appropriate: - NH / SH with > 5,000 commercial vehicles/day traffic - Expressway corridors - Heavy industrial roads - Where pavement structural performance is critical
Comparison:
| Property | BM | DBM | |---|---|---| | Max aggregate | 19-26.5 mm | 19-26.5 mm | | Gradation type | Open-graded | Dense-graded | | Binder content | 3.5-4.5 % | 4.0-5.0 % | | Voids in mix | 5-10 % | 3-5 % | | Marshall stability | ≥ 5-7 kN | ≥ 9 kN | | Cost (relative) | 75-85 % of DBM | 100 % (baseline) | | Permeability | Higher | Lower | | Performance under heavy traffic | Limited | Better | | Typical use | Rural, low-traffic | NH/SH, heavy traffic |
Pavement application: - PMGSY rural roads: BM at 50-75 mm + BC at 20 mm = 70-95 mm total bituminous - District roads: BM at 50-65 mm + BC at 25 mm = 75-90 mm - Light NH (T1, T2 traffic categories): BM at 50 mm + DBC at 30 mm = 80 mm - Heavy NH (T3+): DBM at 50-75 mm + BC at 40-50 mm (no BM use)
Why open-graded? - Open-graded BM allows water through (more permeable than DBM) - Concern: water entering through cracks reaches base + subgrade - Mitigation: ensure good drainage of underlying layers - Modern projects increasingly favor DBM over BM for this reason
Construction differences: - BM mix laid hotter (160-180 °C) due to lower binder content → harder mix to compact - BM requires more rolling passes to achieve density - BM mix segregates more easily → care in transport + paving - BM less forgiving of compaction errors
Aggregate gradation envelope (BM, 26.5 mm max — typical): - 37.5 mm sieve: 100 % - 26.5 mm: 95-100 % - 19 mm: 60-95 % - 13.2 mm: 35-65 % - 4.75 mm: 15-32 % - 0.6 mm: 8-18 % - 0.075 mm: 0-10 % - Binder content (Marshall): 3.5-4.5 %
Alternative gradation (BM, 19 mm max): - 26.5 mm: 100 % - 19 mm: 95-100 % - 13.2 mm: 55-85 % - 4.75 mm: 20-38 % - 0.075 mm: 0-10 %
Aggregate properties: - Los Angeles abrasion: ≤ 40 % (more relaxed than DBM/BC) - Aggregate impact value: ≤ 30 % - Crushing value: ≤ 30 % - Soundness: ≤ 12 % (5 cycles) - Polishing value (wet): ≥ 40 BPN (where required) - Flakiness + elongation: ≤ 30 % combined - Stripping resistance: ≥ 95 % retained
Bitumen specification: - VG-10: soft surface (less common for BM) - VG-30: standard for plains BM - Penetration: 50-70 (VG-30) - Softening point: ≥ 47 °C - Cutback bitumen alternative: RC-2 / RC-3 for cold weather BM
Marshall mix design parameters: - Optimum binder content: target air voids 5-7 % - Air voids (Va): 5-10 % (much higher than DBM's 3-5 %) - VMA (Voids in Mineral Aggregate): ≥ 12 % (lower than DBM) - VFB (Voids Filled with Binder): 55-65 % - Stability (Marshall): ≥ 5-7 kN (much lower than DBM's 9 kN) - Flow: 2-4 mm - TSR (moisture susceptibility): ≥ 0.80
Construction temperatures: - Mixing temperature: 160-180 °C (slightly higher than DBM to compensate for low binder) - Transport: minimum 165 °C - Laying: ≥ 150 °C - Compaction completion: complete before mix < 120 °C
Layer thickness: - BM as binder course: 50-65 mm (typical PMGSY) - BM as base course: 65-100 mm (light-traffic) - Layer thickness ≥ 2.5 × max aggregate size - Aggregate 26.5 mm → layer ≥ 65 mm; aggregate 19 mm → layer ≥ 50 mm
Tack coat (where required): - Bitumen emulsion (RS-1 / SS-1) per IS 8887 - Application rate: 0.2-0.4 kg/m² - For BM on stabilised base or DBC under BM
Field density: - Field density ≥ 95 % of Marshall density - Density measurement: nuclear gauge + cores - Density at joints: within 1 % of mid-mat density - More rolling passes needed than DBM due to mix stiffness
Acceptance criteria: - Plant production: aggregate gradation + binder content within ± 2 % - Field density: ≥ 95 % minimum on 90 % of measurements - Layer thickness: ± 6 mm of design - Surface evenness: 3 mm max under 3-m straight-edge (longitudinal)
Modern practice trend: - Many state PWDs + NHAI now prefer DBM over BM for binder course - Reasons: better fatigue resistance, lower permeability, longer service life - IRC 27 remains relevant for PMGSY + rural roads where economy paramount
1. BM specified for heavy traffic. BM at 50-60 mm under BC at 25 mm specified for NH with 8,000 commercial vehicles; rutting + structural failure in 2-3 years. Use DBM or thicker BM + thicker BC for heavy traffic. 2. Field density insufficient. BM is hard to compact; density target missed. More rolling passes + better roller sequence. 3. Aggregate gradation outside envelope. Production produces coarse mix; voids too high; binder content too low for structural performance. Strict gradation control + acceptance testing. 4. Binder content under-spec. Mix dry; aggregate not properly coated; ravelling + stripping. Verify binder content per batch. 5. No wearing course on BM. BM exposed to direct traffic; surface texture rough, water ingress, rapid deterioration. Always cover BM with BC or DBC wearing course. 6. Permeability issue. Surface cracks let water through BM open-graded structure to base + subgrade; structural deterioration. Subgrade + base drainage essential. 7. Mix laid in wet weather. Moisture in mix; stripping; raveling. Avoid laying in rain. 8. Construction temperature too low. BM is hard to compact even hot; below 150 °C compaction inadequate. Reject cold mix. 9. No surface texture check. BM surface used directly without overlay; rough riding surface; complaint. Always overlay with BC. 10. Marshall stability tested but not met. Mix design fails stability requirement; project proceeds anyway. Mandatory stability ≥ 5-7 kN. 11. Modified bitumen used unnecessarily. CRMB on rural road; cost increases without commensurate benefit. CRMB / PMB justified only on high-traffic applications. 12. Layer thickness inadequate. Below 2.5 × max aggregate; aggregate broken during rolling; voids excessive. Strict thickness compliance. 13. No TSR test. Moisture sensitivity not verified; first monsoon stripping. Mandatory TSR ≥ 0.80. 14. Joints construction inadequate. BM cold joints; bonding poor; longitudinal cracking. Hot mix overlap + tack at all joints. 15. No maintenance budget for early-life intervention. BM degrades faster than DBM; surface dressing + crack sealing essential. Schedule surface maintenance year 3-5.
Low-volume road project — IRC 27 touchpoints:
1. DPR / pavement design: structural design per IRC:37:2018; BM specified where economy permits.
2. Mix design (laboratory): - Aggregate gradation within IRC 27 envelope - Bitumen grade VG-30 standard - Marshall test at trial binder contents (3-5 %) - Volumetric verification (Va = 5-10 %, VMA ≥ 12 %) - Stability ≥ 5-7 kN - TSR ≥ 0.80 - Final mix proportion
3. Plant setup: - Standard hot-mix asphalt plant - Aggregate stockpiles managed for gradation - Bitumen storage tank
4. Trial section: - 100-200 m on representative subgrade + base - Validate plant + paver + roller - Cores for density + mix-design conformance
5. Mass production: - Plant mix at 160-180 °C - Transport in covered dumpers - Tack coat (if BM over DBC) - Paver-laid at 150-170 °C - Rolling: multiple passes for required density (more than DBM)
6. Quality control + acceptance: - Plant: aggregate gradation + binder content per batch - Site: layer thickness + density + temperature - Cores: 1 per 1000-2000 m² - Surface evenness check
7. Subsequent wearing course (always required): - BC or DBC layer laid on top per IRC:107:2013 - Bond at interface critical
8. Operations + maintenance: - Annual visual inspection - Surface dressing at year 3-5 (refresh skid + waterproof) - Surface overlay at year 6-10 (resurface) - Full reconstruction at year 15-20 (BM design life shorter than DBM)
IRC 27 is the economy binder-course specification for India's vast network of rural + district roads — invoked on every PMGSY project + many low-volume state-highway upgrades. Modern NH practice has largely moved to DBM (per IRC:29:2019) for better performance under heavier traffic loads.