IRC 18:2000 is the Indian Standard (IRC) for design criteria for prestressed concrete road bridges (post-tensioned). IRC:18 focuses on the design of post-tensioned prestressed concrete road bridges, detailing the application of prestressing forces to achieve enhanced structural performance. It mandates stringent checks for serviceability limit states like deflections and cracking, alongside ultimate limit states for ultimate strength and stability. The code emphasizes thorough consideration of creep, shrinkage, and relaxation effects on the prestressing force over time. Designers must adhere to specific material requirements, anchorage zone designs, and stress limitations in concrete and steel to ensure long-term structural integrity and rider comfort. The document serves as a crucial reference for engineers involved in the conceptualization, detailed design, and construction of prestressed concrete bridges.
This IRC code provides comprehensive guidelines and criteria for the design of post-tensioned prestressed concrete road bridges. It covers aspects ranging from material properties, prestressing forces, load calculations, stress limitations, and detailed design procedures for various bridge components. The code aims to ensure the safety, serviceability, and durability of such bridges under anticipated traffic and environmental conditions.
Key design parameters for post-tensioned PSC bridges, including material grades, stress limits, prestress losses, and durability requirements.
| Reference | Value | Clause |
|---|---|---|
| Min. Grade of Concrete for PSC— For post-tensioned prestressed concrete components. | M40 | Cl. 5.2.1 |
| Min. Cement Content (Severe Exposure)— For PSC members in 'Severe' exposure. | 360 kg/m³ | Cl. 5.3.1 (Ref. IRC:21) |
| Max. Water-Cement Ratio (Severe Exposure)— For PSC members in 'Severe' exposure. | 0.40 | Cl. 5.3.1 (Ref. IRC:21) |
| Min. Cover to Tendons (Severe Exposure)— For PSC beams. Verify against latest IRC:21. | 50 mm | Cl. 5.9.2 (Ref. IRC:21) |
| Min. Cover to Tendons (Moderate Exposure)— For PSC beams. Verify against latest IRC:21. | 40 mm | Cl. 5.9.2 (Ref. IRC:21) |
| Permissible Compressive Stress at Transfer— f_ck is characteristic cube strength at 28 days. | 0.50 f_ck | Cl. 6.2.2.1 |
| Permissible Compressive Stress at Service— Under serviceability limit state loads. | 0.33 f_ck | Cl. 6.2.2.2 |
| Permissible Tensile Stress (Type 1 Structure)— No tensile stress permitted under service loads. | 0 MPa | Cl. 6.2.3.1 |
| Permissible Tensile Stress (Type 2, M40)— For limited prestressing where some tension is allowed. | 3.6 MPa | Cl. 6.2.3.2 (Table 2) |
| Max. Jacking Stress in Tendon— f_pk is characteristic tensile strength of tendon. | ≤ 80% of f_pk | Cl. 5.5.3 |
| Initial Stress after Anchorage— Stress in tendon immediately after anchoring. | ≤ 70% of f_pk | Cl. 5.5.3 |
| Loss due to Shrinkage (Post-Tensioned)— Total residual shrinkage strain (ε_cs). | 2.0 x 10⁻⁴ | Cl. 6.3.4.2 |
| Loss due to Relaxation (Initial Stress 0.7 f_pk)— For stress-relieved strands. Value is approx. 5%. | 70 N/mm² | Cl. 6.3.4.4 (Table 4) |
| Friction Coefficient (μ) - Metal Sheathing— For wires/strands in metal ducts. | 0.25 | Cl. 6.3.3.2 (Table 3) |
| Wobble Coefficient (k) - Metal Sheathing— For wires/strands in metal ducts. | 0.0015 per metre | Cl. 6.3.3.2 (Table 3) |
| Assumed Anchorage Slip (Δ)— Typical value, verify with prestressing system supplier. | 6 mm | Cl. 6.3.3.3 |
| Min. Horizontal Spacing of Ducts— Whichever is greater. | Duct dia. or 40mm or (Agg. size + 5mm) | Cl. 11.1.1 |
| Min. Vertical Spacing of Ducts— Whichever is greater. | Duct dia. or 50mm | Cl. 11.1.1 |
| Max. Shear Stress (τ_c,max) for M40— From Table 10 of IRC:21:2000. | 4.0 MPa | Cl. 6.4.2.2 (Ref. IRC:21) |
| Modulus of Elasticity of Concrete (E_c)— Short-term static modulus. f_ck in N/mm². | 5700 √f_ck MPa | Cl. 5.4.2 (Ref. IRC:21) |
IRC 18:2000 specifies design criteria for prestressed concrete road bridges (post-tensioned). Historically the go-to PSC bridge code before IRC 112:2020 unified RCC and PSC design under Eurocode framework.
You reference IRC 18 when: - Maintaining or retrofitting pre-2011 post-tensioned bridges originally designed to IRC 18 - Designing PSC I-girder, T-girder, and box girder bridges (IRC 112 covers modern design, but many project specifications and engineer libraries still reference IRC 18 for workmanship) - Specifying prestressing materials and procedures for segmental construction - Working with older state PWD projects that predate IRC 112 adoption
For new PSC bridge design, use IRC 112:2020 — its limit-state framework is current Indian best practice. IRC 18 remains referenced for historical continuity and for some specific construction details.
IRC 18 covers both pre-tensioned and post-tensioned systems:
Pre-tensioned (typically factory-produced): - Strands stressed before concrete is cast - Concrete bonds to stressed strands directly - Most common for I-girders, precast beams up to 25 m span - Producers: Larsen & Toubro, Shapoorji Pallonji, Kalindee Rail Nirman
Post-tensioned (site-cast): - Strands threaded through ducts after concrete casts - Stressed against bearing plates - Used for long spans (25-40+ m), box girders, balanced cantilever construction - Requires specialized contractors and quality control
Typical prestressing strands per IS 6006 / IS 14268: - 7-wire low-relaxation strand, 12.7 mm diameter, 1860 MPa ultimate - 15.2 mm diameter for heavier tendons - 7 or 19 strands per tendon typical for bridge post-tensioning
IRC 18:2000 uses permissible stress (working stress) method — an older approach where stresses at service load must stay below permissible limits.
IRC 112:2020 switched to limit-state design — more rational, aligns with international practice. This is a fundamental philosophy change.
If you are working on: - Pre-2015 bridges under IRC 18: continue with permissible stress method for assessment; convert to LSM only if doing major retrofit - Post-2015 bridges: use IRC 112 LSM approach - Maintenance inspections of older bridges: evaluate per the code they were designed to; identify concerns with modern analysis as supplementary
Permissible stresses per IRC 18 Clause 16: - Concrete in compression: 0.33 × f_ck at service - Concrete in tension: 0 for Class 1 (no tension), 1.5 × √f_ck for Class 2, 1.8 × √f_ck for Class 3 - Steel in tension (at service): 0.8 × f_pu
1. Corroded post-tensioning tendons. PSC bridges from 1980s-1990s often show tendon corrosion — poor grouting of ducts, bleeding of grout, water ingress through cracks. Inspection programmes now include acoustic emission testing, GPR scanning, and endoscopy at selected anchorages.
2. Anchorage zone cracking. High-pressure prestressing forces concentrated at anchorages cause splitting cracks if reinforcement was inadequate. Spiral or mat reinforcement at anchorages per IRC 18 Clause 19 is mandatory.
3. Creep and shrinkage underestimation. IRC 18 uses simpler creep-shrinkage models than IRC 112. Long-term deflections of bridges designed pre-2000 often exceed original predictions by 20-40%. Monitoring and re-levelling is standard for older PSC bridges.
4. Fatigue under heavy-truck traffic. Bridges designed for 1980s traffic levels now carry 2-3× the axle loads. Fatigue on prestressing tendons, particularly near supports, is now a significant concern. Load rating per IRC SP 37 should be redone every 10-15 years on heavily-trafficked corridors.
IRC 18:2000 is officially still in force but largely superseded by IRC 112:2020 for new design. It remains in the IRC catalog for continuity with historical projects and as reference for construction specifications and detailing.
Field reality: - ~50% of India's PSC bridges were designed under IRC 18 during 1985-2015. These bridges are now 10-40 years old. Maintenance teams must reference IRC 18 to understand the original design assumptions. - Retrofit projects typically use IRC 112 for the new calculations while preserving original IRC 18 construction details where possible. - State PWDs in transition: smaller states still reference IRC 18 in some tender documents; NHAI and major state highway corridors use IRC 112 exclusively.
For any new PSC bridge design: default to IRC 112. Only use IRC 18 if the client specifically requires it (rare, usually for consistency with an adjacent existing structure) or if you're maintaining/retrofitting a pre-2011 bridge.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Load Factors/Partial Safety Factors | |||
| Concrete Grades | |||
| Prestressing Steel Strength | |||
| Deflection Limits | |||
| Creep and Shrinkage Coefficients |