Modelling the Material Behaviour of Concrete with Polypropylene Fibres under Tunnel Fire and its Influence on the Structural Analysis

Abstract

Behaviour of cut-and-cover tunnels exposed to fire can be analysed based on advanced calculation method according to EN1992-1-2 by using a realistic structural model that takes account of mechanical and thermally-induced-indirect effects acting on the structure simultaneously. In addition, EN 1992-1-2 proposes an implicit constitutive stress-strain model for concrete at elevated temperatures. This model is recommended to be used for the concrete structures under standard fire for the heating rates between 2 and 50 K/min. It is based on the mechanical and thermal strains evaluated from small-scale concrete specimens under low heating rates using steady-state and transient tests. The EN 1992-1-2’s material model is implemented in different structural finite element software packages and used to perform nonlinear (physical) structural analysis for various structures exposed to fire. Considering the cut-and-cover tunnels, in Germany, ZTV-ING fire exposure curve is adopted which has a much higher temperature-increase rate compared to standard fire curve in fire growth stage and reaches the maximum value of 1200°C in only five minutes. In addition, to decrease the risk of spalling, Polypropylene-fibres (PP-fibres) are often added to the concrete mixtures for cut-and-cover tunnels. These may result in different global structural responses in the event of tunnel fire. In this work, different aspects of fire design (advanced calculation method) of cut-and-cover tunnels exposed to fire are investigated. A formula is proposed to evaluate temperatures in different layers of lining of a cut-and-cover tunnel’s cross-section for the affected region as a function of fire exposure curve, width normal to the heat transfer direction and thickness of cross-section. This can be implemented to evaluate temperature without performing thermal analysis and to calculate temperatures in the position of reinforcement for the practical use. Stress analyses for a cylinder specimen under different heating rates carried out in FE software package SOFiSTiK using a volume model and a plate-layered model. This is performed to investigate the so-called structural effects that might arise in a cylinder specimen under low compressive load and high heating rate in transient tests. To investigate, the influence of addition of PP-fibres in the concrete mixture on the thermo-mechanical behaviour of concrete, 214 cylinder specimens were tested using steady-state and transient tests. Two mixes used having 0 and 2 kg/m3 PP-fibres while the other constituents being identical. The hardened concrete should meet the requirements of 1 to 2-year-old concrete in the practice. Comparisons are made in terms of thermo-mechanical behaviour of these two at elevated temperatures for the purified strain results based on three proposed calibration methods. Based on the test results of cylinders, an implicit constitutive material model is proposed for concrete having PP-fibres in its mixture, which is a modified form of EN 1992-1-2’s material model. Further, nonlinear (physical) thermo-mechanical analyses of cut-and-cover tunnel structure exposed to ZTV-ING fire exposure performed in SOFiSTiK. Comments are made on the extent to which the bending structural response of the lining of cut-and-