Code of practice for construction of reinforced concrete shell roof

IS 2204: Scope Summary and Key Points

Scope (Clause 2.0 & 2.1)

• Purpose: Provides general guidance for construction of reinforced concrete shell roofs.
• Applicability: Covers various shell shapes; design specifics must follow the designer's specifications.
• Metric System: All dimensions and quantities are in metric units.
• Rounding Off: Follow IS 2-1960 for numerical rounding.
• Limitations: Focuses on technical provisions for shell roof construction, not full contractual details.

Key Specifications for Construction Sequence (Clause 5.2)

Additional Notes

• Shell roofs require specialized design and construction.
• The standard encourages international coordination but adapts to Indian building practices.
• Always refer to the latest versions of referenced IS codes.

flowchart TD
    A[Start Construction] --> B[Stage I: Edge Formwork + Reinforcement + Concreting]
    B --> C[Stage II: Remove Edge Formwork, Leave Supports]
    C --> D[Stage III: Full Shell Formwork + Steel + Concreting + Curing]
    D --> E[Stage IV: Remove Full Formwork]
    E --> F[Stage V: Waterproofing & Insulating]

This sequence ensures structural integrity and proper curing for shell roofs.

For detailed design formulas and reinforcement tables, refer to the specific design clauses and IS 456 for concrete and reinforcement specifications.

IS 2204: Shell Dimensions and Types - Key Points

1. Shell Dimensions (Clause 2.1)

• Shells are thin curved surfaces where thickness (t) << radius of curvature (R).
• Typical dimensions include:
  • Radius of curvature (R)
  • Span (L)
  • Thickness (t)

2. Thickness Control (Clause 9.6)

• Shell thickness is controlled using templates of corresponding thickness.
• Thickness must be checked at typical points between thickness guides to ensure uniformity.

3. Shell Roof Definition (Clause 2.2.10)

• Shell roofs are curved surfaces with small thickness relative to other dimensions.

4. Types of Shells (Figures 7-11)

• North-Light Cylindrical Shells (Fig. 7)
• North-Light Conoidal Shells (Fig. 8)
• Hyperbolic Paraboloid Shells (Fig. 9)
• Inverted Umbrella Shells (Fig. 10)
• Segmental Dome Shells (Fig. 11)

Typical Thickness Guidelines:

Summary:

• Thickness must be small relative to radius (t/R < 0.05).
• Use templates for thickness measurement.
• Shell types vary by geometry: cylindrical, conoidal, hyperbolic paraboloid, dome, etc.

graph LR
A[Shell Roof] --> B[Cylindrical Shell]
A --> C[Conoidal Shell]
A --> D[Hyperbolic Paraboloid]
A --> E[Inverted Umbrella]
A --> F[

IS 2204: Key Information to be Provided by Designer

1. Working Drawings (Clause 3.1)

• Orientation & arrangement of shell roofs.
• Exact dimensions and reinforcement details, including positioning.
• Details of:
  • North lighting and skylights.
  • Roof drainage system.
  • Construction and expansion joints.

2. Formwork and Centering (Clause 3.2)

• Designer must approve:
  • Formwork design.
  • Sequence of erection and release.

3. Expansion Joints (Clause 12.1)

• Provide clear gap ≥ 2 cm at expansion joints.
• Structural isolation of roof members.
• Use elastic filler in gap.
• Waterproofing must be continuous across joints.
• Double columns may be necessary.

4. Thermal Insulation (Clause 14.2)

• Designer specifies net weight of insulation.
• Weight must not exceed design assumptions.

Summary Table: Expansion Joint Specification

flowchart TD
    A[Designer] --> B[Working Drawings]
    A --> C[Formwork Design]
    A --> D[Expansion Joint Details]
    A --> E[Thermal Insulation Weight]
    B --> F[Reinforcement & Dimensions]
    B --> G[Lighting & Drainage]
    D --> H[Gap ≥ 2 cm]
    D --> I[Elastic filler]
    D --> J[Waterproofing continuity]

This ensures clarity for builders and maintains structural integrity during construction.

IS 2204 – Reinforcement Key Specifications & Tables

1. Reinforcement Diameter Limits (Clause 4.4.1)

2. Placement & Anchorage (Clause 7.1)

• Reinforcement including edge members and traverses must follow design drawings.
• Ensure monolithic connection by anchoring shell reinforcement into edge members via bond bars.
• Proper anchorage ensures load transfer and structural integrity.

3. Formwork (Clause 6.2)

• Formwork must be designed for easy removal as per Clause 11.
• Supports proper curing and reinforcement placement.

Summary:

• Use ≥5 mm dia bars for shells.
• Max dia depends on shell thickness (10, 12, or 16 mm).
• Reinforcement must be anchored into edge members.
• Follow detailed drawings for reinforcement layout.

flowchart LR
    A[Shell Thickness] --> B{Thickness Range}
    B -->|4-5 cm| C[Max Dia 10 mm]
    B -->|5-6.5 cm| D[Max Dia 12 mm]
    B -->|≥6.5 cm| E[Max Dia 16 mm]
    A --> F[Min Dia 5 mm]
    G[Reinforcement Placement] --> H[Follow Design Drawings]
    H --> I[Anchorage to Edge Members]
    J[Formwork] --> K[Designed for Easy Removal]

This ensures compliance with IS 2204 reinforcement requirements for shell structures.

IS 2204 - Sequence of Construction for Shell Roofs (Clause 5.2)

Key Stages for Single Shell Roofs:

Multiple Shell Roofs:

• Usually 4-5 shells in series.
• Construction stages same as single shells.
• Supports for edge beams, traverses, and shells remain until entire series completion.

Additional Notes:

• Formwork design and sequence must be approved by the designer (Clause 3.2).
• Use air-entraining agents to improve concrete workability without increasing water content (Clause 4.5.3).
• Maintain sequence strictly to ensure stress patterns match design assumptions (Clause 5.1).

flowchart TD
    A[Erection of formwork for edges & traverses] --> B[Reinforcement placement & concreting]
    B --> C[Remove formwork leaving supports]
    C --> D[Erection of formwork for entire shell]
    D --> E[Steel placement & concreting]
    E --> F[Curing]
    F --> G[Removal of formwork]
    G --> H[Waterproofing & insulating]

This sequence ensures structural integrity and adherence to design assumptions for shell roofs.

IS 2204: Key Formwork Guidelines for Shell Roofs

1. Design & Fabrication (Clauses 3.2, 6.1, 6.2)

• Formwork must be carefully designed and fabricated to maintain shape integrity due to thin shell thickness.
• Minimize differential settlement of centering and props.
• Use panelized or mobile formwork units for repetitive construction.
• Surfaces should be hard, smooth, and durable for multiple uses, potentially eliminating plastering.
• Formwork must be removable as per Clause 11.

2. Sequence of Construction (Clause 5.2, Table 5)

• For multiple shells, supports for edge beams and traverses remain until all shells in series are completed.

Important Notes:

• Formwork design must allow easy removal without damage.
• Ensure stable support and alignment to avoid shape distortion.
• Consider economical reuse by using firm construction and smooth surfaces.

flowchart TD
    A[Erection of edge formwork] --> B[Place reinforcement]
    B --> C[Concrete edge members]
    C --> D[Remove edge formwork leaving supports]
    D --> E[Erect full shell formwork]
    E --> F[Place shell reinforcement]
    F --> G[Concrete shell & curing]
    G --> H[Remove shell formwork]
    H --> I[Waterproofing & insulation]

This sequence ensures structural integrity and quality surface finish in shell roof construction.

IS 2204: Key Points on Placing of Reinforcement

• Minimum Cover:

  • 12 mm minimum concrete cover to reinforcement (Clause 7.2).
  • Use precast mortar/concrete pieces to regulate cover accurately.

• Fixing Reinforcement:

  • Bars must be correctly placed and firmly fixed.
  • Use tying or welding to maintain bar spacing (Clause 7.2).

• Anchorage:

  • Shell reinforcement must be anchored into edge members with suitable bond bars for monolithic action (Clause 7.1).

• Avoid Displacement:

  • Provide walkways above concrete level, supported independently, to prevent disturbing reinforcement during concreting (Clause 9.7).

Summary Table: Reinforcement Placement

flowchart LR
    A[Reinforcement Placement] --> B[Minimum Cover 12 mm]
    A --> C[Fixing: Tying/Welding]
    A --> D[Anchorage: Bond Bars]
    A --> E[Displacement Control: Walkways]

This ensures durability, structural integrity, and ease of construction per IS 2204.

IS 2204: Materials - Key Specifications & Guidelines

• General: The standard guides reinforced concrete shell roof construction; designer's specifications override general rules (Clause 2.1).

• Reinforcement: Detailed working drawings must specify reinforcement type, positioning, and arrangement (Clause 3.1).

• Thermal Insulation:

  • Net weight of insulation must not exceed design assumptions (Clause 14.2).
  • Designer must specify insulation weight clearly.

• Finishing: After shell completion, interior surfaces may be given a rubbed finish using abrasive stones (Clause 16.1).

Typical Material Properties (per IS standards referenced):

Notes:

• Use metric units throughout.
• Working drawings must include shell orientation, reinforcement, lighting, drainage, joints.
• Waterproofing details are covered separately (Clause 15).

flowchart LR
    A[Designer] --> B[Working Drawings]
    B --> C[Shell Orientation & Arrangement]
    B --> D[Reinforcement Details]
    B --> E[Lighting & Drainage]
    B --> F[Construction & Expansion Joints]
    B --> G[Thermal Insulation Weight]
    G --> H[Must not exceed design weight]

For detailed material strengths and mix designs, refer to IS 456 and IS 1786 respectively.

Casting of the Shell – Key Points from IS 2204

• Aggregate Size (Clause 8.2):

  • Max aggregate size = 20 mm
  • Aggregates must conform to IS 383-1952 (Coarse and Fine Aggregates for Concrete).

• Shell Thickness (Clause 9.6):

  • Use templates to regulate thickness during casting.
  • Thickness must be accurately checked at typical points between thickness guides.

• Formwork and Erection (Clause 6.1):

  • Formwork must be carefully designed to maintain shape integrity and minimize differential settlement of centering and props.
  • For repeated use, formwork panels or mobile units with hard, smooth surfaces are preferred to avoid plastering.
  • Designers may specify the surface pattern on the intrados for aesthetics or economy.

Summary Table for Shell Casting Parameters

flowchart TD
    A[Design Shell Formwork] --> B[Select Aggregates (Max 20 mm)]
    B --> C[Prepare Templates for Thickness]
    C --> D[Cast Shell with Formwork]
    D --> E[Check Thickness at Points]
    E --> F[Ensure Minimal Settlement of Props]
    F --> G[Finish Surface as Specified]

This ensures accurate thickness, structural integrity, and surface quality during shell casting per IS 2204.

IS 2204: Curing and Protection of Concrete Shell Roofs

Key Specifications (Clause 10.1):

• Initial curing (first 24 hours): Keep surface continuously moist using wet canvas or gunny bags to prevent damage.
• Extended curing (at least 10 days): Maintain continuous moisture by:
  • Sprinkling water periodically, or
  • Covering with wet gunny bags or similar materials, or
  • Using approved curing compounds (subject to engineer's approval).

Rationale:

• Shell roofs have a high surface area to volume ratio, increasing evaporation risk.
• Proper curing prevents early drying, shrinkage cracks, and ensures strength gain.

Additional Recommendations (from general practice):

Summary Diagram of Curing Process

flowchart TD
    A[Fresh Concrete Placed] --> B[Wait until surface hardens enough]
    B --> C[Keep surface moist continuously for 24 hours]
    C --> D[Maintain moisture for at least 10 days]
    D --> E{Method}
    E -->|Sprinkle water| F[Periodic sprinkling]
    E -->|Wet covers| G[Gunny bags or canvas]
    E -->|Curing compounds| H[Approved curing compounds]

Note: Proper curing is critical for durability and strength, especially for shell roofs with thin sections and large exposed surfaces.

Removal of Formwork - IS 2204 Key Points & Formulas

Clause 11.1 - Decentering Procedure:

• Decentering must be gradual, controlled, and without shock.
• Stress pattern during removal should resemble the design stress pattern.
• Adjacent supports may need simultaneous lowering.

General Guide for Decentering Start:

[ \text{Concrete Strength} \geq 2 \times (\text{Max Dead Load Stress}) \times \frac{F_c}{F_b} ]

• (F_c) = Ultimate crushing strength of concrete.
• (F_b) = Ultimate buckling strength of concrete in the shell.

Time Guidelines (if no cube tests):

• Remove shell centering after 14 days.
• Decenter bottom shuttering of edge members and end frames after 21 days.
• Casting edge members a week before the shell allows simultaneous striking.

Important Specifications:

• Formwork must be rigid, smooth, and reusable (Clause 6.1).
• Design and erection should allow easy removal (Clause 6.2).
• Sequence and details of formwork erection/removal must be approved by the designer (Clause 3.2).

Summary Table for Removal Timing

flowchart TD
    A[Concrete Casting] --> B{Concrete Strength ≥ 2 × Max Dead Load Stress × Fc/Fb?}
    B -- No --> Wait
    B -- Yes --> C[Start Gradual Decentering]
    C --> D[Lower Adjacent Supports Simultaneously]
    D --> E[Complete Removal of Formwork]

Note: Always follow designer's approval for removal sequence and timing.

IS 2204: Waterproofing & Insulation Key Points

Thermal Insulation (Clause 14)

• Weight Restriction: Net weight of insulation ≤ design assumption; designer must specify.
• Methods:
  • Lightweight insulating concrete, foam concrete, cork over shell roof.
  • Air gap between shell and rigid waterproofing (e.g., asbestos sheets).
  • Casting shell over rigid thermal/acoustic boards.
  • Spraying insulating coatings (e.g., asbestos) on shell underside.

Waterproofing (Clause 15)

• Weight Restriction: Net weight of waterproofing ≤ design assumption; designer must specify.
• Methods:
  • Bitumen-hessian, bituminous felts, cold bitumen.
  • Aluminium foil lining.
  • Asbestos sheet lining.
• Flexible waterproofing methods are preferred for shell roofs.

Design Reminder:

• Always specify net weight of insulation and waterproofing layers in design to ensure structural adequacy.
• Use lightweight materials to minimize added loads.

Typical Weight Considerations (approximate):

flowchart TD
    A[Shell Roof] --> B[Thermal Insulation Layer]
    B --> C[Waterproofing Layer]
    C --> D[External Environment]

    subgraph Thermal Insulation Options
        B1[Lightweight Concrete]
        B2[Air Gap + Rigid Board]
        B3[Sprayed Coating]
    end

    subgraph Waterproofing Options
        C1[Bitumen-Hessian]
        C2[Aluminium Foil]
        C3[Asbestos Sheets]
    end

IS 2204: Skylights and Roof Openings in Shell Roofs

Key Points from IS 2204:

• Clause 13.1: Skylights should be located at or very near the crown or slope near the crown of the shell roof for structural efficiency.
• Clause 12.1: Expansion joints near openings must have a minimum 2 cm clear gap, filled with elastic filler, ensuring waterproofing continuity.
• Clause 3.1: Detailed working drawings must specify skylight dimensions, reinforcement details, and their exact positioning.

Design Considerations:

• Skylights weaken shell action; place near crown to minimize bending moments.
• Maintain shell thickness relative to radius per Clause 2.2.10.
• Expansion joints must structurally isolate roof members around openings.

Typical Specifications:

Diagram: Skylight Placement on Shell Roof

graph TD
    A[Shell Roof] --> B[Crown]
    B --> C[Skylight near Crown]
    A --> D[Slope]
    D --> E[No Skylight]

For detailed reinforcement and structural design, refer to IS 2210 and shell thickness-radius criteria in IS 2204.

IS 2204 - Thermal Insulation for Reinforced Concrete Shell Roofs

Key Methods (Clause 14.1)

Thermal insulation can be provided by:

• Light-weight insulating concrete, foam concrete, cork applied over the shell roof.
• Air gap between shell and rigid waterproofing (e.g., asbestos sheets).
• Casting shell over rigid insulating boards (thermal + acoustic).
• Spraying insulating materials (e.g., asbestos) on shell underside.

Important Specifications (Clause 14.2)

• The net weight of insulation must not exceed the design assumption.
• The designer must specify the insulation weight explicitly.

Additional Notes:

• The design of shell roofs and insulation is specialized; designer's specifications override general rules.
• Use lightweight materials to minimize load on shell structure.
• No embedded fittings after shell completion without designer approval (Clause 13.2).

Typical Thermal Insulation Weight Limits:

flowchart TD
    A[Shell Roof] --> B[Thermal Insulation]
    B --> C[Light-weight concrete / foam concrete / cork]
    B --> D[Air gap + rigid waterproofing]
    B --> E[Rigid insulating boards]
    B --> F[Sprayed insulating coating]

Summary: Use lightweight insulation methods as per Clause 14.1, ensure insulation weight matches design (Clause 14.2), and follow designer's specific instructions for shell roof thermal insulation.

IS 2204 - Quality Control & Testing for Reinforced Concrete Shell Roofs

Key Points from Clause 9.11:

• Concrete cubes must be taken daily during concreting.
• Cubes are tested to verify concrete quality (strength, uniformity).

General Quality Control Guidelines (from IS practices & related codes):

Typical Cube Strength Testing Procedure:

1. Cast 3 cubes of 150 mm size from each day's concrete.
2. Cure cubes in water at 27±2°C.
3. Test at 7 days and 28 days.
4. Average of 3 cubes should meet design strength.

Important Specifications:

• Use IS 456 and IS 516 for detailed concrete testing methods.
• Follow IS 1199 for sampling and testing concrete.
• Maintain records of tests for quality assurance.

flowchart TD
    A[Start Concreting] --> B[Take Concrete Sample]
    B --> C[Cast 3 Cubes (150mm)]
    C --> D[Cure Cubes (27±2°C)]
    D --> E[Test at 7 & 28 Days]
    E --> F{Strength Meets Design?}
    F -- Yes --> G[Proceed with Work]
    F -- No --> H[Investigate & Rectify]

Summary: Daily cube tests ensure concrete quality in shell roof construction. Follow IS 2204 Clause 9.11 along with IS 456, IS 516, and IS 1199 for comprehensive quality control.