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IRC 21 : 2000

Standard Specifications and Code of Practice for Road Bridges — Cement Concrete (Plain and Reinforced)

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SupersededSpecializedCode of PracticeTransportation · Bridges and Bridge Engineering
Superseded by IRC 112
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Summary

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 SUPERSEDED by IRC 112 which uses Limit State Method.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.

Key Values
Concrete grade (bridge deck)M25 minimum
Concrete grade (wearing coat)M30 minimum
Permissible stress M25 bending8.3 MPa
Practical Notes
! SUPERSEDED by IRC 112 for all new designs — use limit state method.
! Still needed for assessment and load rating of thousands of existing bridges designed to IRC 21.
! Working stress method gives more conservative designs than limit state — existing bridges have reserves.
! State highway bridges built before 2010 almost certainly used IRC 21 — check original drawings.
! For strengthening/widening existing IRC 21 bridges, check original design assumptions before adding loads.
! Always verify the cement grade and aggregate quality for bridge construction as per the specific project requirements. Substandard materials are a common cause of premature failure.
! Ensure adequate compaction of concrete, especially in heavily reinforced sections like deck slabs and abutments. Voids can significantly reduce strength and durability.
! Pay close attention to the curing of concrete. Extended moist curing is crucial for achieving the designed strength and preventing surface cracks, particularly in exposed bridge elements.
! The 'working stress method' is the basis of this code. While still widely used, engineers should be aware of the 'limit state method' (IRC: 112) for newer designs, which offers a more robust approach.
! Adequate cover to reinforcement is paramount for durability. Consider aggressive environments like coastal areas or industrial zones and increase cover if necessary.
! During construction, monitor concrete temperature. Excessive heat can lead to thermal cracking, especially in massive elements like piers and abutments.
! For reinforced concrete members subjected to shear, check for the necessity of shear reinforcement (stirrups) based on the calculated shear stress and permissible limits.
! When detailing reinforcement, ensure proper anchorage length and laps. Insufficient anchorage can lead to premature failure of the bond.
! The modular ratio (m) is critical for calculating the distribution of stresses between concrete and steel. Ensure the correct value is used based on the concrete grade.
! Always maintain a construction joint properly to ensure continuity of load transfer and prevent seepage.
! For seismic zones, additional reinforcement and design considerations beyond this code may be necessary, referencing relevant seismic codes.
! Regular inspection and maintenance of bridge components are vital. Early detection of distress like cracking or spalling can prevent major structural issues.
Cross-Referenced Codes
IRC 112:2020Code of Practice for Design of Reinforced Con...
→
IRC 6:2017Standard Specifications and Code of Practice ...
→
IS 456:2000Plain and Reinforced Concrete - Code of Pract...
→
RCC bridgeconcrete bridgeworking stressbridge designlegacyIRC
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Similar International Standards
Key Differences
Key Similarities
Parameter Comparison
ParameterIS ValueInternationalSource
⚠ Verify details from original standards before use
Quick Reference Values
Concrete grade (bridge deck)M25 minimum
Concrete grade (wearing coat)M30 minimum
Permissible stress M25 bending8.3 MPa
Permissible stress M30 bending10.0 MPa
Permissible stress Fe 415200 MPa
Permissible stress Fe 500240 MPa
Modular ratio (M25)10.98
Clear cover (moderate)40mm
Minimum Grade of Concrete for Wearing Coat (Bridges)M25
Minimum Grade of Concrete for Deck Slab (Bridges)M20
Minimum Grade of Concrete for Beams and GirdersM15
Minimum Grade of Concrete for Abutments and PiersM15
Nominal Cover to Reinforcement in Bearings (Bridges)50 mm
Nominal Cover to Reinforcement in Deck Slabs (Bridges)30 mm
Nominal Cover to Reinforcement in Beams (Bridges)25 mm
Nominal Cover to Reinforcement in Abutments and Piers30 mm
Maximum Percentage of Reinforcement in Concrete Sections4.0%
Minimum Percentage of Reinforcement in Concrete Sections0.24% (for Fe415 steel)
Characteristic Strength of Concrete (M20)20 N/mm²
Characteristic Strength of Concrete (M25)25 N/mm²
Modulus of Elasticity of Concrete (M20)22,000 N/mm²
Modulus of Elasticity of Concrete (M25)24,000 N/mm²
Modulus of Elasticity of Steel (Mild Steel/High Tensile Steel)200,000 N/mm²
Permissible Bending Stress in Concrete (f cbc) for M155 N/mm²
Permissible Bending Stress in Concrete (f cbc) for M207 N/mm²
Permissible Bending Stress in Concrete (f cbc) for M258.5 N/mm²
Permissible Tensile Stress in Steel (f st) for Mild Steel140 N/mm²
Permissible Tensile Stress in Steel (f st) for High Tensile Steel (10 mm dia)190 N/mm²
Permissible Shear Stress in Concrete (f v) for M150.3 N/mm²
Permissible Shear Stress in Concrete (f v) for M200.4 N/mm²
Key Formulas
Moment of resistance: Mr = R × b × d² (balanced section)
Lever arm: z = d - xu/3
Neutral axis: xu/d = 0.53 (Fe 415, M25 balanced)
x_u = (m * f_st) / (f_cbc + m * f_st) * d
M_u = 0.87 * f_y * A_st * d * (1 - (A_st * f_y) / (b * d * f_c))
Z = d * (1 - 0.42 * x_u / d)
Shear Stress (v) = V / (b * d)
Moment of Resistance (M) = 0.5 * b * D^2 * N * (1 - 0.417 * N) * f_ck
Key Tables
Table 1 — Permissible stresses in concrete
Table 2 — Permissible stresses in steel
Table 1 - Mix Proportions for Different Grades of Concrete
Table 2 - Permissible Stresses in Concrete
Table 3 - Permissible Stresses in Reinforcement
Table 4 - Modular Ratio (m)
Table 5 - Shear Strength of Concrete
Table 6 - Minimum Reinforcement in Slabs
Key Clauses
Cl. 303 — Permissible stresses in concrete
Cl. 304 — Permissible stresses in steel
Cl. 305 — Design of beams and slabs
Cl. 306 — Reinforcement detailing
Cl. 2.1.1 - Materials for Plain and Reinforced Concrete
Cl. 3.1 - Design of Reinforced Concrete Slabs
Cl. 4.1 - Design of Beams and Girders
Cl. 5.1 - Design of Columns
Cl. 6.1 - Design of Bearings
Cl. 7.1 - Design of Plain Concrete
Cl. 8.1 - Durability of Concrete Structures
Cl. 9.1 - Water-Cement Ratio
Can I still design a new bridge using IRC 21?+
No. IRC 112 (Limit State Method) is mandatory for all new bridge designs per MoRTH/NHAI specifications. IRC 21 (Working Stress Method) is superseded. However, it remains essential for assessing, load-rating, and retrofitting existing bridges.
What is the difference between Working Stress and Limit State methods?+
Working Stress (IRC 21): design at service loads with permissible stresses (safety built into material limits). Limit State (IRC 112): design at factored loads with characteristic strengths and partial safety factors. Limit state gives ~15-20% more economical designs.
How do I assess an existing bridge designed to IRC 21?+
Use IRC 21 permissible stresses to check existing capacity. Compare with current loading per IRC 6:2017. If capacity < demand, strengthening is needed. Common methods: external post-tensioning, carbon fibre wrapping, or addition of supplementary girders.
What is the minimum grade of concrete required for different bridge components as per IRC 21:2000?+
IRC 21:2000 specifies minimum concrete grades based on the structural element's importance and stress levels. For instance, wearing coats typically require M25, deck slabs M20, while beams, girders, abutments, and piers can be designed with M15, provided the design calculations confirm its adequacy and durability.
What is the significance of the modular ratio (m) in IRC 21:2000, and how is it determined?+
The modular ratio (m) represents the ratio of the modulus of elasticity of steel to that of concrete. It is crucial for calculating the relative stiffness of steel and concrete in flexural members. IRC 21:2000 provides values for 'm' based on the grade of concrete, which are used in stress distribution calculations under the working stress method.
How does IRC 21:2000 address durability requirements for concrete bridge structures?+
Durability is addressed through requirements related to water-cement ratio, concrete cover to reinforcement, and permissible stresses. The code mandates minimum concrete grades and specifies maximum water-cement ratios to ensure resistance against environmental attack, while adequate cover protects reinforcement from corrosion.
What are the permissible bending stresses for concrete and steel according to IRC 21:2000?+
IRC 21:2000 provides tables for permissible stresses. For concrete, permissible bending compressive stress (f_cbc) varies with the grade of concrete (e.g., 5 N/mm² for M15, 7 N/mm² for M20). For steel reinforcement, permissible tensile stress (f_st) depends on the type and diameter of steel, with higher grades allowing higher stresses.
When is shear reinforcement required in concrete beams as per this code?+
Shear reinforcement (stirrups) is required when the calculated shear stress in a concrete beam exceeds the permissible shear stress for the concrete grade. IRC 21:2000 provides formulas to calculate shear stress and dictates the design of shear reinforcement based on these calculations to ensure the beam can resist shear forces safely.
What is the maximum permissible percentage of reinforcement in a concrete section under this code?+
IRC 21:2000 limits the maximum percentage of reinforcement in concrete sections to 4.0% of the gross cross-sectional area. This limit is in place to ensure adequate concrete can surround the reinforcement for proper bond and to prevent congestion that could hinder proper compaction.
How does the nominal cover to reinforcement vary for different bridge components?+
Nominal cover is critical for protecting reinforcement from corrosion and fire. IRC 21:2000 specifies different cover requirements based on the element's exposure and function. For example, bearings typically require a higher cover (50 mm) than deck slabs (30 mm) or beams (25 mm) to ensure adequate protection.
Can IRC 21:2000 be used for designing bridges in seismic zones?+
While IRC 21:2000 provides fundamental design principles for reinforced concrete bridges, it is primarily based on the working stress method. For bridges in seismic zones, it's essential to consult specific seismic design codes (like IRC: 6) which provide additional requirements for ductility and seismic detailing that go beyond the scope of IRC 21:2000.
What is the role of the water-cement ratio in concrete mix design as per this code?+
The water-cement (w/c) ratio is a critical parameter that significantly influences the strength and durability of concrete. IRC 21:2000 specifies maximum permissible w/c ratios for different grades of concrete to ensure adequate strength development and to minimize permeability, thus enhancing resistance to aggressive environments.
How are bearings designed for bridges using IRC 21:2000?+
IRC 21:2000 covers the design of bearings to transfer loads from the superstructure to the substructure. It specifies requirements for materials, dimensions, and the design of the bearing itself, including considerations for expansion and contraction. Adequate cover to reinforcement within bearing areas is also stipulated for protection.
What are the implications of using High Tensile Steel reinforcement as per IRC 21:2000?+
IRC 21:2000 permits the use of high tensile steel, which offers higher permissible tensile stresses compared to mild steel. This can lead to more economical designs by reducing the amount of steel required. However, its use requires careful detailing and consideration of its properties, including higher modulus of elasticity, in design calculations.