IRC 21:2000 is the Indian Standard (IRC) for standard specifications and code of practice for road bridges — cement concrete (plain and reinforced). IRC 21 was the concrete bridge design code using Working Stress Method. SUPERSEDED by IRC 112 which uses Limit State Method. Still relevant for assessment and retrofitting of older bridges designed to IRC 21.
Specifications and code of practice for design of plain and reinforced cement concrete road bridges using working stress method.
Concrete grades, cover, reinforcement and durability values for plain & reinforced concrete in road bridges.
| Reference | Value | Clause |
|---|---|---|
| Minimum concrete grade — superstructure RCC | M25 | Cl. 304.3 |
| Minimum concrete grade — substructure | M25 | Cl. 304.3 |
| Minimum concrete grade — pre-stressed— see IRC 18 / IRC 112 for PSC | M35 | Cl. 304.3 |
| Minimum cement content — superstructure | 360 kg/m³ | Cl. 304.3.1 (Table 5) |
| Minimum cement content — substructure | 330 kg/m³ | Cl. 304.3.1 (Table 5) |
| Maximum cement content— to limit shrinkage and heat of hydration | 540 kg/m³ | Cl. 304.3.1 |
| Maximum w/c ratio — moderate exposure | 0.45 | Cl. 304.3.1 (Table 5) |
| Maximum w/c ratio — severe / very severe | 0.40 | Cl. 304.3.1 (Table 5) |
| Clear cover — deck slab top | 40 mm | Cl. 304.3.4 (Table 9) |
| Clear cover — main reinforcement (severe exposure) | 50 mm | Cl. 304.3.4 (Table 9) |
| Clear cover — abutments / piers in flowing water | 75 mm | Cl. 304.3.4 |
| Clear cover — foundations against soil | 75 mm | Cl. 304.3.4 |
| Permissible compressive stress — bending (M25 WSM) | 8.3 N/mm² | Cl. 303.2 (Table 6) |
| Permissible bond stress — HYSD (M25) | 1.4 N/mm² | Cl. 303.2 (Table 6) |
| Permissible tensile stress — HYSD steel (Fe415) | 200 N/mm² | Cl. 303.3 (Table 6A) |
| Modulus of elasticity — concrete (M25) | 2.5 × 10⁴ N/mm² | Cl. 303.2.2 |
| Modular ratio m (Fe415, M25)— WSM analysis basis | 10 | Cl. 303.2.3 |
| Maximum aggregate size — RCC | 20 mm typical | Cl. 304.4.1 |
| Slump — bridge deck concrete— varies by placement method | 50–100 mm | Cl. 1714 (MORTH) |
| Curing duration — minimum— extended for blended cements and severe exposure | 14 days | Cl. 1715 (MORTH) |
IRC 21:2000 is Section III — Cement Concrete (Plain and Reinforced) of the IRC standard specifications for road bridges. Together with IRC 18 (prestressed) it covered all concrete bridge design before IRC 112:2020 unified them under a single Eurocode-aligned document.
You reference IRC 21 for: - Maintenance and retrofit of pre-2011 RCC bridges originally designed to IRC 21 - Specification-writing for concrete materials and workmanship (the material requirements remain widely used) - Understanding historical design assumptions on older state PWD bridges - Cross-reference by IRC 22 (composite bridges) for the concrete portion design rules
For new bridge design, use IRC 112:2020. IRC 21 uses working stress method; IRC 112 uses limit state design. The philosophy is fundamentally different.
IRC 21 follows the working stress method — permissible stresses at service loads must stay below allowable values. This was the standard until IRC 112's LSM adoption.
Typical permissible stresses per IRC 21 Clause 303: - Concrete in flexural compression: 8.3 MPa (M25), 10.0 MPa (M30), 13.3 MPa (M40) - Concrete in direct compression: 0.25 × f_ck - Concrete in shear (with stirrups): per Table 7 - Steel (mild): 140 MPa at service - Steel Fe 415: 190 MPa - Steel Fe 500: 240 MPa (introduced in later amendments)
Load combinations per Clause 302: IRC 21 used simpler load combinations than IRC 112 — no partial safety factors on dead/live load (those came with LSM). The margin comes from the permissible stresses being ~50-60% of yield.
Cover requirements: IRC 21 specified 40-50 mm cover for normal bridge work, 65 mm for severe exposure. IRC 112 extended cover classes to match Eurocode exposure class (XC, XS, XD, XF, XA) with more granular depth specifications.
For a simply-supported RCC T-beam bridge (25 m span) per IRC 21:2000:
Step 1 — Dead load: deck slab + T-beam + footpath + parapet + wearing coat. Typically 70-90 kN/m run.
Step 2 — Live load: Class A + Class 70R per IRC 6. Apply impact factor per Clause 211. No factoring at service; just apply impact.
Step 3 — Permissible stress check: Assume 2 T-beams at 4 m c/c. Deck slab 225 mm, T-beam 1800 mm overall × 300 mm web. M25 concrete, Fe 500 steel.
Maximum moment from unfactored DL + LL + Impact at midspan: ~8,000 kN·m total for 2 beams = 4,000 kN·m per beam.
Check compressive stress in concrete at top of flange: σ_c = M × y / I ≤ 8.3 MPa (M25 permissible)
Check tensile stress in steel: σ_s = n × M × (d - y) / I ≤ 240 MPa (Fe 500 permissible)
Where n = modular ratio = E_s / E_c ≈ 8 for M25.
Iterate section / reinforcement until both stresses within limits.
Step 4 — Shear check per Clause 304.
Step 5 — Deflection check per Clause 23.2 of IS 456 (referenced by IRC 21).
Compare with IRC 112 (LSM) approach: - IRC 21: service moment → permissible stress check - IRC 112: factored moment (γ_DL=1.35, γ_LL=1.50) → ultimate capacity check - IRC 112 typically produces 10-20% more reinforcement due to higher safety margin
1. Under-designed for current traffic. Bridges from 1970s-1990s were designed for Class A + Class AA tracked vehicles with lower MSA expectations. Modern traffic exceeds these by 2-4×. Load rating per IRC SP 37 should be done on all bridges older than 20 years.
2. Inadequate cover in coastal exposure. Pre-2000 bridges often used 40 mm cover in coastal areas — now known inadequate. Chloride penetration has corroded reinforcement in many 40-year-old coastal bridges. Periodic NDT (half-cell potential, concrete resistivity) is standard for monitoring.
3. Low-strength concrete specification. IRC 21 permitted M25 minimum; modern IRC 112 Clause 7 recommends M40 minimum for bridge superstructure. Older M25/M30 bridges have higher permeability and faster corrosion progression.
4. No fatigue check. Fatigue analysis was not explicit in IRC 21. Heavy-traffic bridges on NH corridors have shown fatigue cracking in older designs — should be assessed under current IRC 112 fatigue provisions during any major retrofit.
5. Legacy working-stress reinforcement doesn't suit strength-based assessment. When doing load rating per IRC SP 37, converting an IRC 21 design to LSM equivalent needs careful interpretation. The original WSM design may pass LSM checks with different reinforcement distribution than a fresh LSM design would have chosen.
IRC 21:2000 is the classic Indian concrete bridge code. Simple, well-understood, widely used from 1966 through 2015 (across multiple editions). The 2000 revision was the last major update before IRC 112's disruption.
Current status: - Not withdrawn — still officially in the IRC catalog - Not recommended for new design — IRC 112:2020 governs new concrete bridges - Actively referenced for maintenance, load rating, and as historical baseline for older bridges
For bridge engineers working on existing asset maintenance: - Original design drawings are in IRC 21 framework - Load rating must begin by understanding original design capacity - Retrofitting (strengthening, widening) often blends IRC 21 historical detailing with IRC 112 modern analysis - State PWD maintenance contracts typically reference IRC 21 for continuity
For engineers on modern projects: IRC 21 is essentially a reference — cite it for construction specifications (those parts remain robust) but do the structural design per IRC 112.
| Parameter | IS Value | International | Source |
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