Assessment of a fire-damaged concrete overpass: the Verona bus crash case study

The assessment of fire-damaged infrastructures entails the implementation of quick diagnostic techniques and reliable interpretation procedures to determine the residual safety margin and any possible need for repair works. In this perspective, several tailored Non-Destructive Test methods have been developed in the last two decades, providing immediate results, with no need for time-consuming laboratory analyses. Moreover, matching their indications with the calculated effects of a tentative fire scenario allows harmonizing distinct pieces of evidence in the coherent physical framework of fire dynamics and heat transfer. This approach was followed in the investigations on a concrete overpass in Verona (Italy) after a coach violently impacted one supporting pillar and caught fire in 2017. Technical specifications of the vehicle made it possible to bound the acceptable ranges for fire load and maximum Rate of Heat Release, while surveillance video footage indicated the duration of the burning stage. Some established NDT methods (evaluation of discoloration, de-hydroxylation, rebar hardness) were implemented, together with advanced ultrasonic tests based on pulse refraction and pulse-echo tomography. The results clearly showed the extension of the most damaged area at the intrados of the box girders and validated the maximum heating depth, as predicted by numerical analysis of the heat transient ensuing from the localized fire model.