Fire Resistance in Concrete Structures

Overview

  • This section covers the design of reinforced concrete structures to withstand fire, focusing on:
    • Methods of design
    • Thermal and mechanical properties of concrete

Behaviour of Concrete Structures in Fire

  • General Properties:

    • Concrete is non-combustible with low thermal conductivity.
    • Typically, concrete remains intact in fires, providing protection to reinforcing steel.

  • Factors Affecting Behaviour:

    • Applied loads on structures
    • Elevated temperatures in concrete and reinforcing steel.
    • Mechanical properties of steel/concrete at high temperatures.

  • Fire Effects:

    • Increase in temperature leads to deformation and possible failure.
    • Lightweight and high-strength concrete show different characteristics.

  • Concrete Types:

    • Lightweight Concrete:
    • Made with normal cement and lightweight aggregates, low thermal conductivity, good fire resistance.
    • High-Strength Concrete:
    • Compressive strength of 50 - 120 MPa, higher rate of strength loss at temperatures up to 400°C, risk of explosive spalling.

Spalling

  • Spalling occurs when high moisture content concrete is heated, causing the outer layers to peel away.
  • High-strength concrete is more susceptible to spalling compared to normal concrete.
  • Preventative methods include adding fine polypropylene fibres (0.15-0.3%) to the mix, which melt and allow water vapour to escape.

Masonry in Fire

  • Concrete Masonry: Often hollow concrete blocks; exhibit good fire resistance.
  • Brick Masonry: Also performs well under fire, with risk of thermal bowing in tall walls leading to collapse.

Fire Resistance Ratings

  • Fire resistance design must ensure that provided resistance exceeds design fire severity:
    • Verification methods include:
    • Time domain: compares fire resistance ratings to code-specified scores.
    • Strength domain: compares load-bearing capacity with expected loads.
    • Temperature domain: compares critical temperature with maximum achieved during fires (less common).
  • Generic ratings for concrete members detailed in standards, with minimum sizes and cover specified.

Design Methods for Fire Exposure

  • Fire Exposure Calculation:

    • Fire exposure can be standard or real.
    • Use of computer programs recommended for thermal gradients in concrete exposed to realistic fire conditions.

  • Assumptions in Thermal Calculations:

    • Heat transfer is primarily a function of concrete properties.
    • The reinforcement’s temperature is assumed to match that of surrounding concrete.

Mechanical Properties of Concrete at Elevated Temperatures

  • Strain components during heating:
    • Thermal strain, stress-related strain, creep strain, and transient strain all contribute to total deformation.
  • Design Values:
    • Tensile strength is typically zero at elevated temperatures.
    • Modulus of Elasticity decreases with temperature across different concrete types.

Member Design Under Fire Conditions

  • Structural design strategies focusing on:
    1. Simple supported beams or slabs (normal temperature on compressive zones).
    2. Continuous systems requiring more complex evaluations.
    3. Use of computer programs for detailed analysis in larger, complex structures.
  • Consideration of average temperature and effects on compressive strength is critical in member design.

Worked Example

  • Example of designing a simply-supported concrete slab with known span, load, geometry, and reinforcing.
  • Calculations include cold and fire load considerations, temperature impacts, and strength evaluations to ensure design adequacy against fire conditions.