Standard Specifications and Code of Practice for Road Bridges, Section IV (Brick, Stone and Cement Concrete Block Masonry) (Second Revision)

The key definitions and general requirements for IRC 40 are outlined in Clause 401 (Definitions) and Clause 400 (Scope). The code covers brick, stone, and cement concrete block masonry for road bridges. Cement types used must conform to specific IS standards as per Table 402.1, including 33 and 43 Grade OPC, Portland slag cement, and pozzolana cements. General requirements include adherence to materials, design considerations, construction workmanship, expansion gaps, and floor protection works as detailed in Clauses 402 to 406. The relevant cement specifications are:

These form the basis for materials and workmanship standards in bridge masonry works.

Sources: Clause 400, Clause 401, Table 402.1

Key specifications for abutments per IRC 40 include:

• Thickness: Must ensure stresses under worst load combinations are within permissible limits (Clause 403.11.1).

• Top Width: As per Table 4 (Clause 403.11.2.1), for slab and girder bridges just below caps:

• For arch bridges, top width must accommodate skewbacks and resist stresses under worst loading (Clause 403.11.2.2).

• Wing walls and returns must have a minimum top width of 450 mm (Clause 403.11.2.3).

• For multiple span arch bridges, abutment piers are provided after every 3rd to 5th span (Clause 403.11.3).

• The slenderness ratio limit: height to base width ratio shall not exceed 6 (Clause 403.11.4).

• Stability checks must consider sliding with friction coefficient 0.6 and overturning (Clauses 403.10, 403.9).

These form the core design parameters for abutments in IRC 40.

Sources: Clause 403.11.1, Clause 403.11.2.1, Clause 403.11.2.2, Clause 403.11.2.3, Clause 403.11.3, Clause 403.11.4, Clause 403.10, Clause 403.9

Ashlar masonry in IRC 40 is governed by detailed bonding, joint thickness, and curing specifications as per Clause 404.3.5 and related sub-clauses. Key points include:

• Bond Stones: Minimum length 450 mm, overlap not less than 150 mm, placed at 1.5 to 1.8 m apart in every course, staggered in layers (Clause 404.3.5.1).
• Headers: Use precast M-15 concrete headers if long headers are unavailable; cast in-situ headers are not permitted (Clause 404.3.5.2).
• Joint Thickness: Minimum 3 mm, maximum 6 mm for Ashlar masonry (Clause 404.3.7).
• Vertical Joints: Must be truly vertical and staggered; distance between vertical joints in adjacent courses not less than half the course height (Clause 404.3.5.4).
• Pointing and Curing: Pointing as per Clause 404.2.5; curing involves keeping masonry moist for 7 days, protecting from rain and rapid drying (Clauses 404.3.7, 404.2.7).

Table of Maximum Joint Thickness:

These requirements ensure structural integrity and proper bonding in Ashlar masonry for bridge works and similar structures.

Sources: Clause 404.3.5, Clause 404.3.7, Clause 404.2.7

The effective height for calculating the slenderness ratio of piers or abutments in IRC 40 is defined as the dimension of the height of the pier or abutment assumed for slenderness calculations, as per Clause 401.8. Reference is made to the Note below Table 13 in IRC:21 Clause 306.1.2 for detailed guidance. Additionally, the effective height relates to the space between the extrados and the horizontal tangent line to the crown (Fig. 4). This effective height is critical for stability and slenderness checks in bridge components.

Sources: Clause 401.8, IRC:21 Clause 306.1.2 Note, Clause 401.9

For Random Rubble Masonry as per IRC 40, the key specifications are:

• Permissible Compressive Stress: According to Clause 403.4.4, permissible compressive stresses for random rubble masonry (IS:1597 Part-I) are 67% of the values given in Table 2 (not fully provided here). Random rubble masonry is limited to return and toe walls up to 5 m height.

• Permissible Tensile Stress: From Table 3 (Clause 403.5), allowable flexural tensile stresses (MPa) for stone masonry with various mortar mixes are:

• Permissible Shear Stress: Clause 403.6 states shear stress shall be the greater of (i) allowable tensile stress from Table 3 or (ii) one-third of compressive stress due to dead load, but not exceeding 0.2 MPa. Note a 20% reduction applies for substructure of arch bridges.

• Permissible Overstress: Clause 403.8 allows up to 25% overstress for eccentric/lateral loads excluding seismic, and up to 50% overstress including seismic forces.

These form the core design limits for random rubble masonry under IRC 40.

Sources: Clause 403.4.4, Clause 403.5, Clause 403.6, Clause 403.8

Key specifications and formulas for Retaining Walls as per IRC 40 include:

• Stability Checks: Stability must be verified for all construction phases and service conditions with factors of safety per Clause 7.6.3.4 of IRC:78 (Clause 403.9).

• Sliding Resistance: Use a coefficient of friction of 0.6 between masonry interfaces unless otherwise specified (Clause 403.10).

• Wall Thickness: Must ensure stresses from worst load combinations remain within permissible limits (Clause 403.11.1).

• Top Width of Piers and Abutments: As per Table 4 (Clause 403.11.2.1):

• Wing Walls and Returns: Minimum top width 450 mm (Clause 403.11.2.3).

• Height to Base Width Ratio: Shall not exceed 6 (Clause 403.11.4).

• Arch Bridges: Top widths must accommodate skewbacks and resist stresses under worst loading (Clause 403.11.2.2).

These guidelines ensure structural adequacy and stability of retaining walls and associated components.

Sources: Clause 403.9, Clause 403.10, Clause 403.11.1, Clause 403.11.2.1, Clause 403.11.2.3, Clause 403.11.4

Design considerations per IRC 40 Clause 403 include:

• General features must follow IRC:5 for dimensions and IRC:6 for loads and stresses, with masonry bridges limited to spans ≤15 m and height ≤10 m (Clause 403.1).

• Load dispersion through masonry footings and arches is at 45° to vertical. For arches, the cushion above the crown must be ≥500 mm with minimum 12% CBR filling; for submersible bridges, a PCC cushion ≥300 mm thick is required (Clause 403.3).

• Permissible compressive stress depends on crushing strength of masonry units and mortar grade, detailed in the following table (Clause 403.4.1):

*Gauged/composite mortar.

• Linear interpolation is allowed for intermediate crushing strengths.

• For well foundations, mortar shall not be leaner than 1:3.

These form the key design bases for masonry bridges under IRC 40.

Sources: Clause 403.1, Clause 403.3, Clause 403.4.1

For piers and abutments in rivers with velocity exceeding 4.5 m/sec carrying abrasive particles, IRC 40 Clause 403.11.5 mandates special precautions:

• Provide an extra 100 mm sacrificial thickness above design thickness up to 0.5 m above High Flood Level (H.F.L.).
• Use minimum 250 mm course height for stone masonry.
• Use bricks with minimum crushing strength of 10 MPa for brick masonry.

Weep holes (Clause 403.11.6) must be provided at max 2 m horizontal and 1 m vertical spacing, staggered, with lowest row 150 mm above low water or ground level. Use 100 mm diameter AC pipes for masonry structures, with slope 1 vertical:20 horizontal towards draining face and a filter layer behind.

Wing walls (Clause 403.11.7) must have minimum 600 mm height at the end away from abutment, extending 600 mm parallel to traffic.

Top widths of piers and abutments (Clause 403.11.2.1) for slab and girder bridges are given in the table below:

The height to base width ratio of piers/abutments shall not exceed 6 (Clause 403.11.4). Stability checks must consider sliding with friction coefficient 0.6 (Clause 403.10).

Sources: Clause 403.11.5, Clause 403.11.6, Clause 403.11.7, Clause 403.11.2.1, Clause 403.11.4, Clause 403.10

Key specifications and formulas for Masonry Arch Bridges per IRC 40 include:

• Span and Height Limits: Masonry bridges are generally limited to spans not exceeding 15 m and masonry height above foundations not exceeding 10 m (Clause 403.1).

• Load Dispersion: Loads on footings and over arches are dispersed at 45° to the vertical. For arch bridges, the load includes road surface and filling up to the extrados, with cushion thickness minimum 500 mm (300 mm PCC for submersible bridges) (Clause 403.3).

• Permissible Compressive Stress: Depends on crushing strength of masonry units and mortar grade, given in Table 2. For arch rings, max permissible stress is 1.6 MPa, except Ashlar stone masonry up to 2.5 MPa (Clauses 403.4.1, 403.4.2).

• Special Precautions: For piers/abutments in rivers with velocity >4.5 m/s, extra 100 mm sacrificial thickness, minimum 250 mm course height for stone masonry, and bricks with crushing strength ≥10 MPa are required (Clause 403.11.5).

• Weep Holes: Spaced max 2 m horizontally and 1 m vertically, 150 mm above low water level, with 100 mm diameter AC pipes (Clause 403.11.6).

• Seismic Restrictions: Masonry arch bridges with spans >6 m are not permitted in Seismic Zones IV and V (Clause 403.13).

• Arch Construction: Full-scale arch templates, radial joints, symmetrical laying, and proper centering and stripping procedures are specified (Clauses 404.5.1 to 404.5.5).

• Expansion Gaps: 40 mm gap every 30 m length, filled with flexible material (Clause 405).

Table 2 (Permissible Compressive Stress in MPa):

*Gauged/composite mortar

This summary is based on IRC 40 Clauses 403, 404, and 405 only.

Sources: Clause 403.1, Clause 403.3, Clause 403.4.1, Clause 403.4.2, Clause 403.11.5, Clause 403.11.6, Clause 403.13, Clause 404.5.1, Clause 404.5.4, Clause 404.5.5, Clause 405

Key specifications for Brick Masonry per IRC 40 include:

• Wetting of bricks: Bricks must be soaked for at least 1 hour and surface dried before laying to ensure proper bond (Clause 404.2.2).

• Laying: Brickwork shall be laid in English Bond, true to line, plumb, and level with whole bricks at faces and junctions (Clause 404.2.3).

• Plastering:

  • Start plastering from top downwards.
  • Use wooden screeds 75 mm wide spaced 2.5 to 4.0 m apart as guides.
  • Wet the wall surface before plastering.
  • Apply mortar between screeds with wooden float, finish with wooden straight edge and wooden float; metal floats are prohibited.
  • Thickness must not be less than specified, with max 3 mm variation (Clause 404.2.6.2).

• Curing:

  • Keep masonry moist for 7 days; top flooded with water at day end.
  • For composite mortar, start curing 2 days after laying, continue 7 days.
  • Protect from rain and rapid drying, especially in hot weather.
  • Plaster curing starts once hardened enough, kept wet for at least 7 days (Clauses 404.2.7.1 to 404.2.7.4).

• Foundation laying:

  • Foundation block top surface hacked, cleaned, wetted.
  • Spread mortar layer not less than 12 mm thick before first brick course (Clause 404.2.8).

These ensure durable, well-bonded brick masonry with proper finishing and protection.

Sources: Clause 404.2.2, Clause 404.2.3, Clause 404.2.6.2, Clauses 404.2.7.1 to 404.2.7.4, Clause 404.2.8

Stone masonry for bridges as per IRC 40 follows IS:1597 (Part-I) for rubble stone and IS:1597 (Part-II) for ashlar masonry (Clause 404.3.1). Generally, coursed rubble stone masonry (first sort) is used, with ashlar masonry for facing if required (Clause 404.3.2). Random rubble masonry is generally not allowed except as per specific clause. Stones must be laid to lines, levels, and shapes shown in plans, with courses of uniform height not less than 160 mm (Clause 404.3.4.1, 404.3.4.5). Stones are wetted before laying to prevent water absorption (Clause 404.3.4.2). Stratified stones must be laid on natural beds with bed joints normal to pressure (Clause 404.3.4.3). Hearting stones are laid on broadest face, minimum 150 mm size, hammered and solidly bedded in mortar; plum stones provided at 900 mm centers for bonding (Clause 404.3.4.4). Mortar used is cement-sand 1:3 (Clause 404.3). For coping, minimum thickness is 150 mm with vertical joints spaced max 1.5 m (Clause 1.5). Mortar mixing, laying, curing, and finishing follow detailed procedures in Clauses 404.1 and 404.2.6. Key points include proper wetting, tooling, curing for 7 days, and no dry or hollow spaces in masonry (Clauses 404.2.6, 404.2.7, 404.3.4).

Sources: Clause 404.3.1, Clause 404.3.2, Clause 404.3.4.1, Clause 404.3.4.2, Clause 404.3.4.3, Clause 404.3.4.4, Clause 404.3.4.5, Clause 1.5, Clause 404.1.2, Clause 404.2.6, Clause 404.2.7

Key specifications and procedures for Concrete Block Masonry per IRC 40 include:

• Block Requirements: Solid concrete blocks must conform to IS:2185 Part-I with minimum compressive strength of 10.5 MPa; hollow or lightweight blocks are not permitted (Clause 402.5.1, 402.5.2).
• Materials: Fine aggregates must conform to IS:383 and IS:2116; water must be clean and within permissible limits per IRC:21 (Clause 402.6).
• Block Size & Density: Minimum thickness and width of blocks are 200 mm each; density not less than 2.2 MT/m³ (Clause 404.4.2).
• Laying Procedure: Bed cleaned, wetted, and covered with fresh mortar; blocks soaked 15 minutes before laying; mortar joints max 15 mm thick; blocks laid in header and stretcher bond with broken joints; courses horizontal and joints vertical (Clause 404.4.3).
• Arch Construction: Full-scale arch template used; voussoirs sized and placed symmetrically; joints radial and beds adjusted for radial planes; no pinning allowed; centering design per IRC:87 (Clause 404.5).
• Expansion Gaps: 40 mm gap every 30 m length of wall, filled with flexible material (Clause 405).

These ensure durability, strength, and proper workmanship in concrete block masonry.

Sources: Clause 402.5.1, Clause 402.5.2, Clause 402.6, Clause 404.4.2, Clause 404.4.3, Clause 404.5, Clause 405

Key specifications for arch construction and centering per IRC 40 include:

• Arch Construction:

  • Full-scale arch shape laid on a leveled platform; stones/blocks cut to size with radial joints parallel to the transverse axis (Clause 404.5.1).
  • Stones laid symmetrically about the crown in full mortar beds; dry fitting before final laying (Clause 404.5.2).
  • For arch rings, permissible compressive stress depends on masonry type and mortar grade, with a maximum of 1.6 MPa generally, and up to 2.5 MPa for ashlar stone masonry (Clause 403.4.2, 1.6 MPa max; Clause 1.6).

• Centering:

  • Design and erection of centering critical for arch success; refer IRC:87 for detailed falsework design (Clause 404.5.4.1).
  • Number of centering sets depends on number of spans; for wide bridges, half-width centering moved sideways may be economical (Clause 404.5.4.2).
  • Stripping centering should be gradual, starting from crown outwards, not before 2-4 weeks (usually 3 weeks for spans under 30 m), ensuring masonry has hardened and haunch filled appropriately (Clause 404.5.5).

• Load Dispersion: Load through arch and filling considered at 45° to vertical; cushion above crown minimum 500 mm (Clause 403.3.2).

• Permissible Compressive Stress Table (Excerpt):

(Refer to full Table 2 in Clause 403.4.1 for detailed values.)

This summary covers arch construction, centering design, permissible stresses, and load dispersion as per IRC 40 clauses 403, 404, and 405.

Sources: Clause 403.3.2, Clause 403.4.1, Clause 404.5.1, Clause 404.5.2, Clause 404.5.4.1, Clause 404.5.4.2, Clause 404.5.5

As per IRC 40, Clause 405 Expansion Gap, the expansion gap is a critical provision in masonry and bridge construction to accommodate thermal and structural movements. Although the exact formula or table for expansion gap sizing is not provided in the retrieved context, Clause 405 specifically addresses this requirement. Typically, expansion gaps are designed based on expected temperature variations, material properties, and structural dimensions to prevent cracking or damage. For precise values and detailed specifications, refer directly to Clause 405 in IRC 40, which covers expansion gap requirements comprehensively.

Sources: Clause 405 Expansion Gap

For Floor Protection Works as per IRC 40, Clause 406, the code refers to IRC:89 for detailed guidelines. Although IRC 40 does not provide explicit formulas or tables for floor protection, it emphasizes referring to IRC:89 for specifications. Additionally, general good practice includes ensuring proper bedding, curing, and use of suitable materials like concrete blocks with minimum thickness and density as per Clause 404.4.2 (minimum 200 mm thickness and density not less than 2.2 M ton/cum). Proper laying and curing methods are also critical, as described in Clause 404.4.3 and 404.4.5. For expansion gaps related to floor and masonry works, Clause 405 specifies a 40 mm gap for every 30 m length, filled with flexible material. For comprehensive design and construction details, consult IRC:89 as recommended.

Sources: Clause 406, Clause 404.4.2, Clause 404.4.3, Clause 405