Reinforced concrete and fire resistance

Reinforced concrete and fire resistance

György L. Balázs (Budapest University of Technology and Economics)

One of the highest responsibilities of civil engineers and architects is to provide enough strength and deformation capacity for buildings and any engineering constructions that can resist hazard situations. Fire is a typical hazard case. Design should indicate that the selection of construction materials as well as structural systems are able to give enough safety in fire. That means enough time should be provided for humans to leave the building within a defined time limit such as: 30, 60 or 90 minutes.

Many accidents are caused by fire, therefore indicating the importance of design for fire safety. To meet the fire resistance of reinforced concrete structures, the fire resistance of all the constituents must be known: hydrated cement stone, aggregate, bond between aggregate and hydrated cement stone, fibres. Whenever the internal non-hydrated water in concrete reaches 100 °C, internal pressure is produced in the concrete and may lead to explosive spalling of the concrete cover.
The stress-strain diagram of concrete in hot state indicates considerable reduction of the strength above 500 °C. In addition to that, there is a reduction of the modulus of elasticity, while, however, there is an increase of strain capacity. By temperature increase the stress-strain diagram of steel reinforcement in hot state indicates decrease of yield strength and the strength decrease of the yield plateau disappears.There is clear experimental evidence that the increase of slag content of cement reduces the proneness to cracking.

The definition of REI classes is the following:

• R means Load bearing. The ability of a construction element to preserve its’ mechanical characteristics and the relevant load bearing capacity during a normal fire.
• E means Integrity. In other words, the structure’s ability to NOT allow the passage or production of gas or vapor to the area NOT exposed to the fire.
• I means Thermal Insulation. To clarify, the ability of a structure to reduce within a temperature limit the transfer of heat to the unexposed, i.e., cold side. The temperature limit is usually 140°C.

As a short summary:

1. Fire resistance can be reached by:

• careful selection of all concrete components and
• selection of an appropriate structural system.

2. Important parameters for fire resistance are:

• Type and amount of cement
• Type and bond of aggregate
• type and amount of reinforcement in concrete
• internal water content of concrete at the time of the fire outbreak.