Assessment of the Residual Strength of Fire-Damaged Steel-Rebars

Abstract

Concrete structures are known to exhibit a good behaviour in fire, thanks to the low thermal diffusivity of the material, which provides an effective protection to steel reinforcement. Moreover, a significant strength recovery occurs when the bars cool down to room temperature, though this markedly depends on their metallurgical properties. Since the surviving structure is still required to bear the noticeably-higher loads assigned by the ultimate limit state, the post-fire strength of the reinforcement has to be carefully weighed up. To this purpose, two different Non-Destructive Techniques are investigated in this study. The first one is the Dynamic Hardness Test (also known as Leeb Test), which is quite sensitive to steel decay. The test can be performed onsite by means of a small specifically-designed device, provided that the surface of the bar has been smoothed prior to testing. The second technique is based on the continuous monitoring of the drilling resistance via a special setup, which allows to measure the thrust to be exerted on the bit in order to keep a constant feed rate. This latter method requires no sample preparation, but the correlation with steel decay is rather uncertain, due to the conflicting effects of the decreasing yield strength and the increasing hardening and strain capacity of fire damaged steel. The pros and cons of these two methods are discussed in the paper, in view of their practical implementation for assessing the post-fire safety of actual structures.