Standard Specifications and Code of Practice for Road Bridges, Section III: Cement Concrete (Plain and Reinforced) (Third Revision)

The scope of IRC 21 covers the design and construction of road bridges using plain and reinforced cement concrete. It establishes common procedures for structural use of cement concrete in bridges, ensuring stability, soundness, and safety. Key specifications include concrete grades (M15 to M60) with characteristic compressive strengths defined in Table 4, and permissible stresses for concrete grades as per Table 9. Reinforcement grades Fe 240, Fe 415, and Fe 500 are specified with their characteristic strengths and elastic moduli. Exposure conditions (moderate and severe) dictate minimum concrete grades, cement content, and maximum water-cement ratios as detailed in Table 5. The code also specifies acceptance criteria for concrete density, storage of materials, and restrictions such as no mixing or curing with sea water (Clause 302.4.3). Effective span and depth definitions for beams and slabs are provided (Clauses 305.4 and 305.5).

Sources: Clause 302.6.1, Table 4, Clause 303.1, Table 9, Clause 302.5, Table 3, Clause 302.6.2, Table 5, Clause 302.4.3, Clause 305.4, Clause 305.5

Key specifications and formulas for Materials and Concrete Mix Proportioning per IRC 21 include:

• Current Margin for Concrete Mix: Calculated as 1.64 times the standard deviation of at least 40 batches over 5 days to 6 months (Clause 1.64). If insufficient data, use initial margins from Table 6.

• Nominal Mix Proportions (Table 7):

• Concrete Grades (Table 4): Characteristic compressive strength at 28 days ranges from M15 (15 MPa) to M55 (55 MPa).

• Minimum Cement Content and Max Water-Cement Ratio (Table 5): Varies by member type and exposure (moderate/severe). For example, RCC members under severe exposure require minimum M35 grade, 380 kg/m³ cement, and max w/c ratio 0.40.

• Batching Accuracy Limits: Cement and water ±3%, aggregate ±3%, admixture ±5% (Clause 302.9.1).

• Acceptance Criteria: Mean strength of 4 consecutive samples must exceed characteristic strength by 3 MPa; no sample less than characteristic minus 3 MPa (Clause 302.11.1).

• Workability Slump Ranges: Low 25-50 mm, Medium 50-100 mm, High 100-150 mm (Clause 302.11.2).

• Sampling Frequency (Table 8): Number of samples depends on concrete volume, e.g., 1 sample for 1-5 m³, 4+ samples for >50 m³.

These form the core framework for concrete mix design, production, and quality control under IRC 21.

Sources: Clause 1.64, Clause 302.7.2, Clause 302.7.3, Table 7, Table 4, Table 5, Clause 302.9.1, Clause 302.11.1, Clause 302.11.2, Table 8

Key specifications for permissible stresses and crack control per IRC 21 are as follows:

Permissible Stresses in Steel Reinforcement (Table 10):

Permissible Tensile Stresses in Plain Concrete (Table 11):

Note: For concrete cast in one lift with proper joint preparation, permissible tensile stresses may be increased up to 1.25 times the values in Table 11 (Clause 1.25).

Crack Control Detailing (Clause 303.4.1):

• Slabs: max bar diameter 25 mm, spacing 150 mm
• Beams (including flanges in voided slabs/box beams): max bar diameter 32 mm, spacing 150 mm
• Columns: max bar diameter 32 mm, spacing 300 mm

Design Surface Crack Width (Appendix 1):

• Calculated by: Design crack width = 1 + 2(2g_r - C_min)(h - x) [variables defined in Appendix 1]
• Permissible crack width under sustained loads:
  • Severe exposure: ≤ 0.2 mm
  • Moderate exposure: ≤ 0.3 mm (Sustained load = dead load + 50% live load)

These ensure structural durability and serviceability by limiting crack widths and stresses in materials.

Sources: Clause 303.4.1, Clause 303.4.2, Clause 303.5, Clause 1.25, Table 10, Table 11, Appendix 1 (303.4.2)

The general design requirements per IRC 21 include key permissible stresses, concrete grades, reinforcement specifications, and detailing rules as follows:

• Permissible Stresses (Table 9): | Concrete Grade | Ec (GPa) | Permissible Direct Compressive Stress (MPa) | Permissible Flexural Compressive Stress (MPa) | |---|---|---|---| | M15 | 26 | 3.75 | 5 | | M20 | 27.5 | 5 | 6.67 | | M25 | 29 | 6.25 | 8.33 | | M30 | 30.5 | 7.5 | 10 | | M35 | 31.5 | 8.75 | 11.67 | | M40 | 32.5 | 10 | 13.33 | | M45 | 33.5 | 11.25 | 15 | | M50 | 35 | 12.5 | 16.67 | | M55 | 36 | 13.75 | 18.3 | | M60 | 37 | 15 | 20 |

• Concrete Grades (Table 4): M15 to M55 with characteristic compressive strengths from 15 to 55 MPa.

• Minimum Cover: 40 mm generally; 50 mm for severe exposure; 75 mm for foundations under severe exposure (Clause 304.3).

• Reinforcement Grades: Fe 240, Fe 415, Fe 500 with characteristic strengths 240, 415, 500 MPa respectively and modulus of elasticity 200 GPa (Clause 302.5).

• Bar Sizes: Max 40 mm diameter; min 8 mm for all bars; min 12 mm for longitudinal column bars (Clause 304.4).

• Spacing: Horizontal spacing not less than max of bar diameter or 10 mm plus max aggregate size; vertical spacing min 12 mm or max aggregate size or bar diameter (Clause 304.5).

• Design Basis: Elastic theory with modular ratio from Table 9; tensile strength of concrete ignored unless otherwise permitted (Clause 304.2).

• Exposure Conditions: Minimum concrete grade, cement content, and max water-cement ratio vary with moderate/severe exposure (Table 5).

These requirements ensure structural safety, durability, and constructability in reinforced concrete design per IRC 21 (Clauses 303, 304, 302.5, 302.6).

Sources: Clause 303, Table 9, Clause 304, Clause 302.5, Clause 302.6, Table 4, Table 5

Key formulas and specifications for reinforcement detailing and curtailment per IRC 21 include:

• Anchorage Length (la) for Bars in Tension:

  • la = a_l * l_o, where a_l = 1 for straight ends, 0.7 for end hooks (if cover ≥ 34 mm), per Clause 304.6.2.2.
  • Minimum la: 12 times bar diameter (¢) or 300 mm for straight ends; 6¢ or 150 mm for end hooks, whichever is greater.
  • Reduced anchorage length allowed if excess reinforcement is provided (factor c_2 = A_provided / A_required ≥ 1/3).

• Anchorage Length for Bars in Compression: Same as tension bars but hooks not effective (Clause 304.6.2.3).

• Anchorage over Bearings: Bottom bars anchorage length may be reduced to 2/3 at simple supports (Clause 304.6.3.1).

• Curtailment of Bars: Allowed if staggered optimally, bent up at flat angle over anchorage length, and area limits per Clause 305.7 are met (Clause 304.6.3.2).

• Lap Splices: Length la = K * l_o, with K varying by percentage of spliced bars (Clause 304.6.6.3). Values of K can be reduced by 20% if axial spacing > 10¢ or edge distance > 5¢.

• Shear Stirrups Anchorage: Adequate anchorage when stirrups bent around bars at ≥90° and extended 8 times bar diameter or bent 180° and extended 4 times diameter (Clause 304.6.5).

• Spacing and Cover: Minimum clear cover 40 mm (50 mm in severe exposure), bar diameter limits, and spacing rules per Clauses 304.3 and 304.5.

• Tables:

• Lap Length Factor K Table (approximate):

These provisions ensure proper bond, anchorage, and load transfer in reinforced concrete members, optimizing reinforcement use and structural safety.

Sources: Clause 304.6.2.2, Clause 304.6.2.3, Clause 304.6.3.1, Clause 304.6.3.2, Clause 304.6.5, Clause 304.6.6.3, Clause 304.3, Clause 304.5

Key design provisions for footings and bases per IRC 21 include:

• Allowable Bearing Pressure (Clause 307.1): [ \sigma_o = \frac{P}{A_1} ] where ( \sigma_o ) is permissible compressive stress in concrete, ( A_1 ) is the dispersed concentric area (max width of dispersion limited to twice the footing height), and ( A_2 ) is the loaded area. The footing projection beyond the column face shall be at least 150 mm.

• Bending Moment (Clause 307.2.1): Moments at any section are the moment of forces over the entire area on one side; critical section is at the column face.

• Shear Strength (Clause 307.2.2.1): Governed by the severer of: (a) Wide beam action with critical section at effective depth from column face. (b) Two-way slab action with critical section perimeter not closer than half effective depth to loaded area.

• Tensile Reinforcement (Clause 307.2.4): Uniform across width, with more concentration near column in a band equal to short side. Reinforcement in band width = (2/(β+1)) × total reinforcement in short direction, where β = long side/short side ratio.

• Pile Caps (Clause 307.2.5): Designed by truss analogy or bending theory. Minimum thickness = 0.5 × pile spacing for two rows. Punching shear checked on perimeter around column/pile; max punching shear = 0.16 fck.

These summarize the key formulas, tables, and specifications for footing/base design in IRC 21.

Sources: Clause 307.1, Clause 307.2.1, Clause 307.2.2.1, Clause 307.2.4, Clause 307.2.5.1, Clause 307.2.5.5

For pile caps design and shear checks as per IRC 21, key points are:

• Design Methods (Clause 307.2.5):

  • Pile caps can be designed by truss analogy or bending theory.
  • Minimum thickness of pile cap when using truss analogy is 0.5 times the pile spacing for two rows of piles.
  • In truss analogy, 80% of reinforcement is concentrated in strips linking pile heads; no shear check is required.

• Shear Checks (Clause 307.2.5.4):

  • Critical shear section is at a distance of effective depth (d) from column face for one-way action, and half effective depth (d/2) for two-way action.
  • Shear from piles is considered based on pile center location relative to the section (full, none, or interpolated shear).

• Punching Shear (Clause 307.2.5.5):

  • Punching shear checked around column and piles on specified perimeters.
  • Punching shear stress shall not exceed 0.16 fck.

• Reinforcement Distribution (Clause 307.2.4):

  • Tensile reinforcement is uniformly distributed, with more concentration near the column in a band equal to the short side of footing.
  • Reinforcement in band width = 2 / (β + 1) × total reinforcement in short direction, where β = long side / short side.

These provisions ensure safe design of pile caps against bending, shear, and punching shear per IRC 21.

Sources: Clause 307.2.4, Clause 307.2.5.1, Clause 307.2.5.2, Clause 307.2.5.4, Clause 307.2.5.5

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Key specifications for Box Girders and Soffit Slab Reinforcement per IRC 21 are as follows:

• Effective Compression Flange Width: Use Clause 305.15.2 for calculating stresses at any section of the box girder (Clause 309.1).

• Soffit Slab Thickness: Minimum thickness shall be the greater of 200 mm or one-twentieth of the clear span between main girders. Deck slab and webs thickness shall not be less than Clause 305.2 requirements (Clause 309.2).

• Soffit Slab Reinforcement: Minimum reinforcement in transverse direction is 0.5% of flange section for Fe 240 bars or 0.30% for Fe 415/500 bars, distributed equally on both surfaces with max spacing 300 mm. Bars must be bent up into exterior girder stems at least 10 times bar diameter. Longitudinal reinforcement from stress considerations may be equally distributed top and bottom, ensuring minimum reinforcement per Clause 305.10 (Clause 309.3).

• Top Flange Transverse Reinforcement: Must extend to exterior face of outside girders; at least one-third anchored with 90° bends or extended sufficiently for bond (Clause 309.4).

• Reinforcement for Torsion: Longitudinal and transverse reinforcement required when torsion is present, with closed stirrups spaced not exceeding the smaller of (X+Y)/4 or 300 mm, and longitudinal bars at corners not less than stirrup diameter or 12 mm (Clauses 304.7.2.3 to 304.7.2.5).

• Minimum Deck Slab Thickness: 200 mm, with possible 50 mm reduction at cantilever tip if detailed properly (Clause 305.2).

These clauses provide the essential formulas and reinforcement detailing for box girders and soffit slabs in bridge superstructures.

Sources: Clause 309.1, Clause 309.2, Clause 309.3, Clause 309.4, Clauses 304.7.2.3 to 304.7.2.5, Clause 305.2, Clause 305.10

Key formulas and specifications for load distribution on beams and slabs per IRC 21 are as follows:

• Distribution of Live Loads on Longitudinal Beams (Clause 305.12):

  • Loads more than 5.5 m from supports are distributed between longitudinals for shears same as bending moments (Clause 305.12.1).
  • Methods include reactions assuming unyielding supports, Courbon's method, or rational grid analysis.

• Effective Width of Compression Flange for T and L Beams (Clause 305.15.2):

  • T-beams: be = b_w + (1/5)l
  • L-beams: be = b_w + (1/10)l where b_w = web thickness, l = distance between points of zero moment (approx. 0.7 effective span).

• Effective Width of Slab for Concentrated Loads (Clause 305.16.2(1)(i)):

• Effective Span (Clause 305.4):

  • For free supports on line bearings, effective span = center-to-center bearing distance.
  • For free supports not on line bearings, effective span = min(l_c + d, l), where l_c = clear span, d = effective depth.

• Effective Depth (Clause 305.5):

  • Depth from compression edge to centroid of tension reinforcement.

• Load Dispersion on Transverse Beams (Clause 305.13 & 305.14):

  • Dispersion through wearing coat, deck slab, and filling is not considered.
  • Load distribution between longitudinals for intermediate transverse beams follows Clause 305.12.1 methods.

These provisions ensure accurate bending moment and shear force calculations for beams and slabs under live loads in bridge superstructures.

Sources: Clause 305.12, Clause 305.15.2, Clause 305.16.2, Clause 305.4, Clause 305.5, Clause 305.13, Clause 305.14

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Key specifications for Construction Joints per IRC 21 include: For vertical joints, a stopping board must be fixed with sufficient lateral rigidity to prevent displacement during compaction, and concreting should continue up to the board which is removed only after the vertical form removal period (Clause 310.5.3.2). Before resuming concreting on hardened surfaces, the surface must be hacked, cleaned, wetted, and covered with neat cement grout followed by a 10 mm thick cement-sand layer; concreting resumes immediately (Clause 310.5.4). For curing, concrete must be kept constantly wet for a minimum of 14 days (5 days for rapid hardening cement), using ponding or wet coverings like sacking or hessian (Clause 310.7). Protection from sun, drying winds, cold, water, shocks, and vibrations is essential during early hardening. These ensure proper bond at joints and adequate hydration for strength development.

Sources: Clause 310.5.3.2, Clause 310.5.4, Clause 310.7

For shrinkage and temperature reinforcement in IRC 21, key specifications and formulas are as follows:

• Thermal expansion coefficients (Clause 304.9.1):

  • Reinforced concrete: 117 × 10⁻⁷ per °C
  • Plain concrete: 108 × 10⁻⁷ per °C

• Shrinkage coefficient: 2 × 10⁻⁴ (Clause 304.9.1)

• Design provisions (Clause 304.9.2): Every simply supported span must allow rotation and longitudinal expansion due to design loads and forces.

• Crack width control (Appendix 1, 303.4.2):

  • Design crack width formula:

    [ w = 1 + 2(2a - C_{min})(h - x) ]

    where a = perpendicular distance to nearest longitudinal bar, C_min = minimum cover, h = overall depth, x = neutral axis depth.

  • Permissible crack width under sustained loads:

    • Severe exposure: ≤ 0.2 mm
    • Moderate exposure: ≤ 0.3 mm

These ensure adequate reinforcement to control shrinkage and temperature stresses, preventing cracking and structural distress.

Sources: Clause 304.9.1, Clause 304.9.2, Appendix 1 (303.4.2)

Key specifications and formulas for articulations and bearing design per IRC 21 are as follows:

• Articulations (Clause 308.2):

  • Bearings at articulations must prevent concentrated edge stresses and allow angular rotation of cantilevers and suspended spans without damage (Clause 308.2.1).
  • Reinforcement slope and arrangement at articulations should follow the shapes shown in Fig. 13(a) and (b) (Clause 308.2.2).

• Bearing Design:

  • Cross girders monolithic with deck slab shall be provided at bearings; thickness and depth requirements ensure proper support and inspection access (Clause 305.3).

• Shear and Torsion (Clause 304.7):

  • Design shear stress: t = V / (b.d), where V = design shear, b = web breadth, d = effective depth (Clause 304.7.1.1.1).
  • Maximum permissible shear stress t_max per concrete grade is given in Table 12A:

• Permissible shear stress in concrete without shear reinforcement is given in Table 12B (partial):

• Torsion Reinforcement (Clause 304.7.2.4):
  • Longitudinal reinforcement resists equivalent bending moment M_I = M + M_t, where M_t = T * (overall depth / breadth).
  • Transverse reinforcement consists of two-legged closed hoops enclosing corner bars.

These provisions ensure articulation flexibility and bearing durability while controlling stresses in pile caps, beams, and slabs.

Sources: Clause 308.2.1, Clause 308.2.2, Clause 305.3, Clause 304.7.1.1.1, Table 12A, Table 12B, Clause 304.7.2.4

Key inspection and maintenance provisions from IRC 21 include water quality limits, concrete and reinforcement specifications, construction joint treatment, curing, and testing protocols:

• Water Quality (Clause 8.1 & Table 2):

  • Neutralization of 100 ml water sample should not require more than 25 ml of 0.02 N H2SO4.
  • Permissible solids limits (mg/l): Organic 200, Inorganic 3000, Sulphates (SO3) 400, Chlorides (CI) 2000 (non-embedded steel) / 500 (prestressed/reinforced concrete), Suspended matter 2000.
  • Sea water use for mixing/curing is prohibited due to harmful salts.

• Reinforcement (Clause 302.5 & Table 3):

  • Grades: Fe 240 (240 MPa), Fe 415 (415 MPa), Fe 500 (500 MPa) with elastic modulus 200 GPa.

• Concrete Grades and Mix (Clause 302.6 & Tables 4, 5):

  • Grades M15 to M55 with characteristic compressive strengths from 15 to 55 MPa.
  • Minimum cement content and max water-cement ratio vary by exposure (moderate/severe) and member type (PCC, RCC, PSC).
  • Cement content max 540 kg/m3; SO3 max 4% by cement mass; chloride limits per exposure.

• Construction Joints (Clause 310.5.3.2 & 310.5.4):

  • Use stopping boards with lateral rigidity; remove laitance before resuming concreting; apply neat cement grout and 10 mm cement-sand layer on hardened surfaces.

• Curing (Clause 310.7):

  • Keep concrete wet for minimum 14 days (5 days for rapid hardening cement).
  • Protect from sun, drying winds, cold, shocks, vibrations, and traffic.

• Testing and Inspection (Clause 310.11):

  • Tests on cement, aggregates, water, and reinforcement per IS codes before and during construction.
  • Load tests may be conducted if required.

• Crack Width Control (Appendix 1):

  • Design crack width calculated by formula considering cover, strain, and reinforcement.
  • Permissible crack width: 0.2 mm (severe exposure), 0.3 mm (moderate exposure).

These provisions ensure durability, structural integrity, and quality control during inspection and maintenance of road bridges.

Sources: Clause 8.1, Table 2, Clause 302.5, Table 3, Clause 302.6, Tables 4 and 5, Clause 310.5.3.2, Clause 310.5.4, Clause 310.7, Clause 310.11, Appendix 1