This study investigated the effect of fire on the ultimate load-bearing capacity of reinforced concrete (RC) slabs. The structural response of RC circular specimens subjected to static load conditions after exposure to a hydrocarbon fire on one side of the specimen was examined. Two fire exposure times were considered (60 and 120 min) in addition to reference non-exposed specimens. The static response was evaluated in the damaged specimens in residual conditions after natural cooling from the elevated temperatures. The temperature distribution across the thickness of the slabs and their load–displacement response was measured. The decrease in the stiffness of the slabs due to the thermal exposure was studied by means of direct ultrasonic pulse velocity (UPV) measurements made before and after the fire tests. The decrease in global stiffness was partially accounted for by UPV measurements. The experimental results showed two peaks in the load-deflection response of the slabs. The first peak was related to an arching mechanism introduced by the specific set-up used. The second peak, corresponding to the ultimate load, occurred due to tensile membrane action at large deflections. While the former was strongly affected by the fire exposure, with the load being halved after the 120-min exposure, the latter was not greatly affected by either the presence of fire or the exposure time. Simplified mechanical models were used to explain the behavior of the RC slabs during the tests.

Reinforced concrete (RC) submerged floating tunnels (SFTs) represent a possible solution for crossing wide, deep fjords, and is considered for the E39 highway route along the Norwegian west coast. With regard to SFTs, the specific accidental scenario that is under investigation is the combined action of fire and subsequent internal explosion, as this is a crucial safety design condition for this type of structure. To assess the structural performance of reinforced concrete structures under combined fire and blast actions, gas burner equipment and a shock tube device were used to generate high temperature and blast loading, respectively, on RC circular slabs. A proper set of instruments consisting of thermocouples embedded in the specimens, accelerometers and ultrasonic pulse velocity (UPV) equipment made it possible to capture the behaviour of the slabs under the combined fire and blast actions and to distinguish the specific role of fire and blast. Simplified numerical tools such as an equivalent elastic single degree of freedom (SDOF) model and a linear elastic finite element (FE) model were used to interpret the experimental results. By considering all combinations of three fire exposure times and two shock waves, the effect of damage accumulation from a combined action of fire and subsequent internal explosion was mapped. A reliable benchmark for numerical models was obtained.

Advanced numerical modeling of high-strength concrete (fc>60 MPa) structures designed to withstand severe thermal conditions requires detailed and reliable information on the mechanical properties of the material exposed to elevated temperatures. The only uniaxial compressive strength variation with temperature is not enough to satisfy the large number of parameters often required by advanced nonlinear constitutive models. For this reason, a complete experimental investigation is required. The paper takes a commonly used high-strength concrete (fc=73 MPa) as an example to describe a comprehensive experimental approach instrumental to the parameter definition and calibration of common constitutive models for concrete. The present study not only studied the overall compressive and tensile behavior of the case study material, but also investigated the effect of elevated temperatures on the specific fracture energy and the evolution of internal damage, in residual conditions after a single thermal cycle at 200°C, 400°C, and 600°C.

Ferry-free coastal route E39 is a project that aims to design a coastal highway without ferry connections. Wide and deep fjords along the Norwegian coast make submerged floating tunnels (SFT) an alternative to conventional structures. In the unfortunate situation of accidental events, the SFT reinforced concrete (RC) structure may become damaged, affecting its load carrying capacity. RC shells can be a representative component of the SFT concrete structure, and it is relevant to study if RC shells can carry the combination of fire and subsequent blast load. An experimental program has been performed in conjunction with numerical simulations in order to enable the risk analysis and the feasibility study of SFT in the E39 project. A shock tube equipment has been used to evaluate the influence of high temperature on the blast load carrying resistance. In addition, static and material tests have been performed for further understanding of the slab behaviour. Nonlinear analyses for static loading and preliminary dynamic analyses have been performed, in combination with standard fire curve load effects. Temperature-dependent material properties have been used based on the results from the material tests. This paper gives an exemplary overview of (sometimes preliminary) results. Both the experimental and numerical results have revealed that high temperature leads to a significant internal damage of the material, affecting the response of RC slabs. In addition, statics tests results for slabs subjected to high temperatures show a significant reduction of the load carrying capacity.