Guidelines for Design and Construction of Fibre Reinforced Concrete for Pavements (First Revision)

Scope of IRC SP 46 - Key Points

IRC SP 46 provides guidelines for design, specifications, and testing of Fibre Reinforced Concrete (FRC) pavements.

Key Specifications & Notations:

• Fibre Diameter: Macro fibres ≥ 0.2 mm diameter.
• Flexural Strength Parameters:
  • ( f_{ct} ) = Peak flexural strength (MPa)
  • ( f_{ctk} ) = Characteristic flexural strength (MPa)
  • ( f_{ctm} ) = Mean flexural strength (MPa)
  • ( f_{e150} ) = Equivalent flexural strength in post-crack regime (MPa)
• Modulus of Sub-grade Reaction: ( K ) (MPa/m)
• Modulus of Elasticity of Concrete: ( E ) (MPa)

S-N (Stress-Number of Cycles) Relationship for Fatigue:

[ \log_{10} N = \frac{1.00 - SR}{0.04761} ]

• Where ( SR ) = Stress Ratio
• At ( SR = 0.714 ), ( N = 1 \times 10^6 ) cycles (endurance limit)
• For ( SR \leq 0.714 ), nonlinear curve relation applies: [ N = \left[\frac{4.0381}{SR - 0.65}\right]^{3.333} ]

Reference Codes and Standards:

• IS codes for cement, concrete, admixtures (e.g. IS 1489, IS 3812, IS 9103)
• ASTM standards for FRC testing (e.g., ASTM C1609, ASTM C1116)
• ACI guides (e.g., ACI 544 series)
• European Norms (EN 206-1, EN 14651)

Summary Diagram of Scope

graph TD
    A[Fibre Reinforced Concrete Pavement Design] --> B[Material Specs]
    A --> C[Mechanical Properties]
    A --> D[Fatigue & Endurance]
    A --> E[Testing & Standards]

    B --> B1[Fibre Diameter ≥ 0.2 mm]
    B --> B2[Cement & SCM as per IS Codes]

    C --> C1[Flexural Strength: \(f_{ct

IRC SP 46: Applications of Fiber Reinforced Concrete (FRC)

Although IRC SP 46 does not provide explicit clauses on FRC applications, based on standard engineering practice and FRC characteristics, here are key points:

Applications of FRC:

• Pavements & Runways: Enhanced crack resistance and durability.
• Precast Elements: Improved toughness and impact resistance.
• Industrial Floors: Better abrasion resistance.
• Shotcrete: For tunnel linings and slope stabilization.
• Structural Components: Secondary reinforcement to control micro-cracking.

Unit Weight of FRC:

• Typically similar to normal concrete: ~2400 kg/m³ (varies slightly with fiber content).

Production & Delivery:

• Use standard concrete batching with fibers added during mixing.
• Ensure uniform fiber dispersion to avoid balling.
• Use suitable mixing time (3-5 minutes post fiber addition).

Properties of Hardened FRC:

• Tensile Strength: Increased due to fiber bridging.
• Flexural Toughness: Enhanced energy absorption.
• Crack Control: Fibers reduce crack width and propagation.
• Durability: Improved resistance to freeze-thaw and impact.

Typical Formula for Flexural Strength of FRC:

[ f_{flex} = f_{flex, plain} + k \times V_f \times f_{fiber} ]

Where:

• ( f_{flex} ) = flexural strength of FRC
• ( f_{flex, plain} ) = flexural strength of plain concrete
• ( V_f ) = volume fraction of fibers
• ( f_{fiber} ) = tensile strength of fibers
• ( k ) = efficiency factor (depends on fiber type)

Summary Table: FRC Properties vs Plain Concrete

flowchart LR
    A[Concrete Mix] --> B[Add Fibers]
   

IRC SP 46: Production and Delivery of Fiber Reinforced Concrete (FRC)

Key Specifications (Clause 4.1)

• Fibres:
  • Steel fibres must have ultimate tensile strength ≥ 800 MPa.
  • Fibre shapes: straight or deformed (manufacturer declares shape).
  • Fibres can be:
    • Loose or
    • Collated (glued with water-soluble adhesive to reduce balling).
  • Coated fibres (e.g., zinc) require manufacturer declaration of coating type and possible alkaline reactions.

Applications (Clause 3.11)

• FRC pavements can open to traffic earlier if flexural strength reaches about 50% of specified value within 1-3 days by adjusting mix and fiber content.

Unit Weight

• Typically close to conventional concrete unit weight (~2400 kg/m³), depending on fibre type and dosage.

Practical Notes:

• Ensure fibre dosage and mixing method prevent balling.
• Adjust mix design to achieve early flexural strength for rapid opening.

Fibre Types (Fig. 1 Summary)

• Steel fibres (straight/deformed)
• Polymeric fibres:
  • Mono filament
  • Fibrillated
  • Macro fibres

flowchart LR
    A[Concrete Mix] --> B[Fibres Added]
    B --> C{Fibre Type}
    C --> D[Steel Fibres (≥800 MPa)]
    C --> E[Polymeric Fibres]
    D --> F[Loose or Collated]
    F --> G[Mixing]
    E --> G
    G --> H[Uniform Dispersion]
    H --> I[FRC Production]

For detailed mix design and fibre dosage, refer to IS codes and manufacturer guidelines.

Key Formulas and Tables for Fibres (IRC SP 46):

1. Fibre Dosage Calculation

The nominal fibre dosage (kg/m³) based on volume fraction (%) and specific gravity:
[ \text{Dosage (kg/m}^3) = 10 \times \text{Fibre Volume (%)} \times \text{Specific Gravity} ]

2. Fibre Dosages Table (Table 3)

3. Fibre Specifications and Quality Parameters (Appendix E)

• Fibre Length (l): mm
• Equivalent Diameter (d): mm or μm
• Aspect Ratio (A = l/d)
• Ultimate Tensile Strength: ≥ 800 MPa for steel fibres
• Elongation at Break: %
• Melting Point: °C
• Specific Gravity

4. Polypropylene Fibre

• Required characteristic 28-day equivalent flexural strength = 1.6 MPa

5. Additional Notes

• Steel fibres may be straight or deformed; shape declared by manufacturer.
• Fibres can be loose or collated (collated reduces balling).
• Coatings (e.g., zinc) on steel fibres must be declared with possible alkaline reactions.

flowchart LR
    A[Fibre Volume %] --> B[Calculate Dosage]
    B --> C[Dosage (kg/m3) = 10 x Volume % x Specific Gravity]
    C --> D[

Key Specifications & Tables for Aggregate Size (IRC SP 46):

1. Maximum Aggregate Size

• Nominal max size usually 20 mm.
• As per IRC:15, max size shall not exceed 31.5 mm.
• Depends on pavement thickness & fibre spacing.

2. Aggregate Grading (Table 1: % Passing for Macro Fibre FRC)

3. Mix Proportions for FRC (Table 2)

IRC SP 46: Mix Proportioning for Fiber Reinforced Concrete (FRC)

Key Points from Clause 4.3:

• Objective: Achieve desired fresh and hardened concrete properties, considering fibers as an additional constituent.
• Approach: Any method (trial, experience-based) is permitted to meet performance goals.
• Mix Adjustment: Mix design for FRC differs from normal concrete due to fiber inclusion.
• Workability Range: Mix must suit transportation, laying, and compaction methods.
• Admixture Dosage: Use two dosage levels for plasticizers:
  • Minimum dosage: Ensures baseline workability.
  • Provisional dosage: Adjusted based on site conditions.

Typical Mix Proportions (Appendix C - Reference)

(Note: Fiber content varies based on fiber type and performance requirements.)

General Mix Proportioning Formula:

[ \text{Water-Cement Ratio (w/c)} = \frac{\text{Weight of Water}}{\text{Weight of Cement}} ]

• Adjust w/c for workability and strength.
• Include fibers as volume fraction, affecting aggregate and water content.

flowchart LR
    A[Select Target Strength & Workability] --> B[Choose Cement, Aggregates]
    B --> C[Add Fibers (Volume %)]
    C --> D[Adjust Water & Admixtures]
    D --> E[Trial Mix & Testing]
    E --> F{Meets Performance?}
    F -- Yes --> G[Finalize Mix]
    F -- No --> B

Summary:

• Treat fibers as an additional constituent.
• Adjust mix for workability, strength, and fiber performance.
• Use trial mixes with varying plasticizer dosages.
• Refer Appendix C

Workability (Slump) of Concrete - IRC SP 46 Key Points

• Definition: Workability covers the range of concrete fluidity from dry to wet.

• Measurement:

  • Most common: Slump Cone Test (IS:1199)
  • Accuracy:
    • Low slump (20-75 mm): ±20 mm
    • Medium slump (50-100 mm): ±20 mm
    • High slump (>100 mm): ±30 mm
  • For low slump concrete (7-20 sec Vebe time), Degree of Compacting or Vee-Bee test is better.

• Slump Requirements for Slip-Form Paving:

  • Slump range: 20 to 50 mm depending on concrete & ambient temperature, and laying speed.
  • At 16°C ambient: almost no slump (<10 mm) may be needed.
  • At 30°C ambient: up to 50 mm slump may be required.
  • Workability window is narrow; frequent adjustments may be necessary.

Typical Slump Ranges for Slip-Form Paving (from IRC SP 46)

Workability Tests Reference Standards

• IS:1199 (Slump test)
• ISO 1920-2, EN 12350-2 to 5, DIN 1048

flowchart LR
    A[Concrete Mix] --> B{Workability Test Type}
    B -->|Slump 20-100 mm| C[Slump Cone Test (IS:1199)]
    B -->|Slump < 20 mm| D[Degree of Compacting or Vee-Bee Test]
    C --> E[Measure Slump (mm)]
    D --> F[Measure Vebe Time (sec)]

Summary: Use slump cone test for general workability measurement with ±20-30 mm accuracy. For low slump concrete, prefer Vee-Bee or degree of compacting methods. Adjust slump based on temperature and laying speed to maintain shape in

Dosing of Fibres as per IRC SP 46 Clause 4.5

• Volume fraction of fibre (Vf) = (Volume of fibre / Gross volume of compacted concrete) × 100%
• Dosage formula (kg/m³):

[ \text{Dosage} = 10 \times V_f (%) \times \text{Specific Gravity of fibre} ]

Fibre Dosage Table (Clause 4.5.7)

Notes:

• Fibre dosage in % refers to volume fraction of fibres in concrete.
• Steel fibres have higher specific gravity (~7.85), polypropylene fibres lower (~0.91).
• Proper mixing ensures homogeneity of fibre distribution (not detailed in the clause).

flowchart LR
    A[Volume fraction of fibre (Vf%)] --> B[Calculate dosage]
    B --> C[Dosage (kg/m³) = 10 × Vf × Specific Gravity]
    C --> D{Select Fibre Type}
    D --> E[Steel Fibre]
    D --> F[Polypropylene Fibre]
    E --> G[Refer Table for kg/m³ dosage]
    F --> G

This ensures correct fibre quantity for enhanced concrete performance.

Mixing of Fibre Reinforced Concrete (FRC) as per IRC SP 46

• Optimum Mixing Time (Clause 4.6.2):
  • Must be determined by trial for each batch.
  • Depends on fibre content, mixer rpm, and blade wear.
  • Ensures uniform fibre dispersion and workability.

Key Points for Mixing FRC:

Recommended Procedure:

1. Add coarse and fine aggregates, cement, and part of water.
2. Mix for 1-2 minutes.
3. Add fibres gradually to avoid balling.
4. Add remaining water and admixtures.
5. Mix until uniform fibre distribution is achieved.

flowchart LR
    A[Start: Add aggregates + cement + partial water] --> B[Mix 1-2 mins]
    B --> C[Add fibres gradually]
    C --> D[Add remaining water + admixtures]
    D --> E[Mix until uniform]
    E --> F[Check workability & fibre dispersion]

Note: No explicit tables/formulas for mixing time in IRC SP 46; rely on trial mixes for optimization.

Fibre Balling in FRC (IRC SP 46, Clause 4.7)

• Definition: Fibre balling is the clumping or entangling of fibres into lumps, causing air entrapment and lack of aggregate bonding.
• Effect: Fibre balls reduce concrete quality by creating weak zones with trapped air and no aggregate.

Causes & Prevention (Clause 4.7.1)

• Fibres entering the mixer must be dispersed immediately.
• Mixer movement should carry fibres away faster than the rate of addition.
• Avoid fibre stacking on vanes or partially filled drums.
• Control dosing and mixing parameters to eliminate balling.

Specifications:

• FRC must be free from fibre balls at delivery (Clause 4.7).
• Fibres should be dispensed clump-free for uniform distribution (Clause 4.5.2).

Practical Guidelines for Mixing to Avoid Fibre Balling:

flowchart LR
    A[Fibre Addition] --> B[Immediate Dispersion]
    B --> C[Mixer Movement]
    C --> D[Uniform Fibre Distribution]
    D --> E[FRC Free of Fibre Balls]
    B -->|If slow| F[Fibre Stacking]
    F --> G[Fibre Ball Formation]
    G --> H[Air Entrapment & Weak Zones]

Summary: Engineer dosing and mixing operations to ensure fibres are dispersed immediately and uniformly, preventing fibre balling and ensuring high-quality FRC.

Unit Weight of Fibre Reinforced Concrete (FRC) - IRC SP 46

• Reference: Clause 4.8 & IS:1199 (Method for sampling and testing concrete)
• Determination: Unit weight of FRC is measured the same way as conventional concrete by weighing a known volume of fresh concrete.
• Significance:
  • Lower unit weight → Possible air entrainment or poor mix design.
  • Large variations → Non-uniform fibre distribution.

Typical Mix Proportions (Clause 4.5):

Formula for Unit Weight (Fresh FRC):

[ \gamma = \frac{W}{V} ]

Where:

• (\gamma) = Unit weight (kg/m³)
• (W) = Weight of fresh concrete sample (kg)
• (V) = Volume of the sample (m³)

Notes:

• Ensure uniform fibre dispersion to avoid density variations.
• Use IS:1199 for standard test procedures.
• Refer Appendix D for design examples and Clause 4.11 for hardened state properties.

flowchart TD
    A[Prepare Fresh FRC Sample] --> B[Measure Volume V]
    B --> C[Weigh Sample W]
    C --> D[Calculate Unit Weight γ = W/V]
    D --> E{Check Unit Weight}
    E -->|Lower than expected| F[Check Air Entrapment]
    E -->|Large Variation| G[Check Fibre Dispersion]
    E -->|Normal| H[Proceed with Testing]

This ensures quality control in FRC mix design and uniformity in pavement applications.

Homogeneity of Fibre Dosage - IRC SP 46

Key Points from Clause 4.9:

• Fibre content must be checked from at least 6 samples.
• Each sample should be from fresh concrete with a volume of 10 litres.

Fibre Dosage Calculation (Clause 4.5.7):

[ \text{Dosage (kg/m}^3) = 10 \times (\text{Fibre Volume %}) \times (\text{Specific Gravity}) ]

• Fibre volume % = (Volume of fibre / Gross volume of compacted concrete) × 100
• Dosage is specified in kg/m³ for accurate batching.

Fibre Dosage Table (Nominal values):

Additional Specifications:

• Fibres must be weighed accurately: -0% to +6% tolerance.
• Microfibres packs should match one batch quantity.

This ensures uniform fibre distribution and consistent concrete performance.

IRC SP 46 - Placing and Finishing of Steel Fibre Reinforced Concrete (SFRC)

Key Specifications & Practices:

• Compaction:

  • Use surface/screed vibrators carefully to avoid over-vibration.
  • Avoid vertical immersion (needle/poker) vibrators; they disturb fibre orientation.
  • Use horizontal immersion vibrators (gang type) for slip-form paving to align fibres horizontally, enhancing flexural strength.

• Finishing:

  • Use conventional tools: steel bar/roller/flat screed with rounded edges.
  • Follow with a steel float behind the paver.
  • Press protruding fibres into green concrete; cut or grind any exposed fibres after finishing.

• General Handling:

  • Follow conventional concrete pavement methods as per IRC:15.
  • Minor refinements in technique and workmanship are needed for SFRC.

Additional References:

• IRC:15 for conventional pavement practices.
• ACI 544.3 for fibre orientation and vibration effects.

flowchart LR
    A[Transporting Concrete] --> B[Placing Concrete]
    B --> C[Compaction]
    C -->|Surface/Screed Vibrators| D[Proper Fibre Orientation]
    C -->|Avoid Vertical Immersion Vibrators| E[Prevent Fibre Misalignment]
    D --> F[Finishing]
    F --> G[Press Fibres Into Surface]
    G --> H[Float and Texture]
    H --> I[Cut/Grind Exposed Fibres]

This ensures strong, durable SFRC pavement with enhanced flexural performance.

Properties of Fiber Reinforced Concrete (FRC) in Hardened State
(As per IRC SP 46, Clause 4.11 and related clauses)

Key Specifications:

• Steel Fibres:

  • Ultimate tensile strength ≥ 800 MPa
  • Fibre shapes: Straight or deformed (manufacturer declares shape)
  • Supplied as loose or collated (collated reduces balling)
  • Coatings (e.g., zinc) must be declared with any chemical compatibility info

• Early Strength:

  • FRC pavement can be opened to traffic earlier if flexural strength reaches ~50% of specified value at 1-3 days (Clause 3.11).

Typical Properties in Hardened State:

Important Formula for Flexural Strength (Modulus of Rupture):

[ f_r = \frac{PL}{bd^2} ]

Where:

• (P) = Load at failure (N)
• (L) = Span length (mm)
• (b) = Width of specimen (mm)
• (d) = Depth of specimen (mm)

Summary Diagram of Fibre Types

graph LR
A[Fibres] --> B[Steel Fibres]
A --> C[Polymeric Fibres]
B --> B1[Straight]
B --> B2[Deformed]
C --> C1[Mono Filament]
C --> C2[Fibrillated]
C --> C3[Macro Fibres]

Note: For detailed mix design and mechanical properties, refer to manufacturer datasheets and perform site-specific trials.

Key Specifications & Formulas for Specifying Fibre Reinforced Concrete (FRC) per IRC SP 46

1. Fibres (Clause 4.1)

• Steel fibres:

  • Ultimate tensile strength ≥ 800 MPa
  • Shape: straight or deformed (manufacturer declared)
  • Supplied loose or collated (collated reduces balling)
  • Coating (e.g., zinc) details and chemical stability must be declared

• Polymeric fibres: Mono filament, fibrillated, macro fibres (see Fig. 1 in code)

2. Flexural Strength (Clause 3.11 & B.11)

• FRC pavement can be opened to traffic early if flexural strength ≈ 50% of specified value at 1–3 days.

• Flexural stress (peak and equivalent residual):

  [ f_{pk} = \text{Peak flexural strength} ]

  [ f_{eq} = \frac{\text{Area under load-deflection curve}}{\delta_{lim}} ]

  Where:

  • (\delta_{lim} = \frac{l}{150}) (deflection limit, l = span length)
  • (f_{eq}) = Equivalent (post-cracking residual) flexural strength

• Re150 ratio (to be ≥ designed value):
  [ Re_{150} = \frac{f_{eq,m}}{f_{ctm}} ]

  • (f_{eq,m}) = mean equivalent flexural strength
  • (f_{ctm}) = mean flexural tensile strength of plain concrete

3. Test Report Requirements (Clause 10.1)

Include:

• Mix details, fibre type/dosage (kg/m³)
• Casting/testing dates
• Curing/storage conditions
• Specimen dimensions & condition
• Load-deflection curve
• Flexural strength values (peak & equivalent)
• Number of specimens & deviations

4. Typical Mix Proportions

Refer Appendix C (Clause 4.3.8) for guidance on fibre dosage and mix design.

flowchart LR
    A[Fibre Selection] --> B[Mix