All construction materials lose strength and stiffness at elevated temperatures. Based on test results, design codes and guides present simplified values of strength and initial stiffness for various materials at different temperatures.

The codes also provide simplified stress-strain-temperature relationships for steel and concrete, which can be used within advanced models.

Thermal expansion and thermal curvature

All materials will expand, to some extent, when heated. If a non-uniform temperature distribution forms through the section, thermal curvature will occur. Any resistance to the free movement of axial thermal expansion or thermal curvature will induce internal stresses within the member. In addition, due to the assumption of plane sections remaining plane, any non-linear temperature distribution through an element will induce internal thermal stresses.

Creep and transient strains

There are two types of tests to determine the material stress-strain-temperature relationship: steady-state and transient tests. For steady state tests the specimen is heated to a defined temperature and then loaded to failure. In transient tests the load remains constant and the specimen is heated to failure. Transient tests give lower stress values for a given strain but are considered to be more realistic. The heating rate will also influence the stress-strain relationship since there is a component of deformation arising from creep. For steel and concrete, the stress-strain-temperature relationship given in the Eurocodes takes into account classical creep, provided the heating rate remains between 2 and 50ˇăC/minute.

Transient strains experienced by concrete on first heating are extremely important, especially when the concrete is subjected to high compressive forces, and should generally be included within the structural modelling of concrete and steel/concrete composite members.

Concrete Spalling

Spalling of concrete in fire involves the breaking off of layers or pieces of concrete from the surface of the structure, as it is heated. Although a large amount of research has been conducted into spalling, the behaviour is difficult to predict and no definitive design guidance is currently available to estimate the extent and consequence of spalling during a fire.

It is possible to reduce the likelihood of spalling by employing one or more of the following methods:

• Thermal barrier
• Polypropylene fibres
• Moisture content control
• Choice of aggregate
• Air-entraining agent
• Compressive stress control
• Reinforcement (including the use of supplementary reinforcement)

< Back to Introduction