Reinforcement Couplers and Fire

In the previous British Standard for “normal” concrete design, the fire resistance was partly determined by the concrete cover to the steel reinforcement. As reinforcement couplers tend to be on the main bars and the cover is measured to the links (transverse shear reinforcement) it has typically been assumed that, provided the coupler did not extend further than the links into the concrete cover zone, the coupler’s performance in fire would be adequate. However, the companion code for “special circumstances” provided further details and gave recommended covers to the main bars. It is therefore questionable whether the practice of ensuring the cover to the coupler was no less than the cover to the link met the fundamental intentions of the code writers.

The current European code for fire design of concrete structures defines the fire resistance recommendations not with reference to the clear concrete cover, by rather the ‘axis distance’, which is the distance to the centre of the main reinforcement. As the coupler is typically concentric with the bar its axis distance will be the same as the bars. This might imply that it is acceptable to keep the same axis distance, and hence a potentially significantly reduced clear cover to the coupler. In addition couplers are normally tested to a stress somewhat higher than the yield stress of the bar, so that there is a margin of safety in the coupling at ambient temperatures. The European code makes the simplification that the temperature of the bar is equal to the temperature of the concrete at the level of the centroid of the bar. This is a simplification since the thermal conductivity of steel is considerably greater than that of concrete. Whilst this assumption has been validated for reinforcement, it may not be appropriate for couplers which can be significantly larger than the reinforcing bars in some cases.

Proposed Research

The proposed research is expected to include:

• A review of currently available literature both on the testing and on design practices for reinforcement couplers in fire.
• Initial testing of couplers heated “in air” along with validation of equivalent analytical models.
• Analytical modelling of various types of couplers within concrete and validation of these models with a limited number of tests of couplers in concrete elements subject to increased temperatures.
• Depending on the results of the previous stages, development of a repeatable test that could be used to quantify the structural fire performance of couplers.

Development of design guidance for the use of couplers in the fire limit state.