Browsing by Author "Ozbolt, Josko (Prof. Dr.-Ing. habil.)"

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Open Access
Chemo-hygro-thermo-mechanical model for simulation of corrosion induced damage in reinforced concrete
(2015) Orsanic, Filip; Ozbolt, Josko (Prof. Dr.-Ing. habil.)
Reinforced concrete (RC) elements exposed to chloride induced corrosion have a shortened service life which presents a major challenge for durability of structures. The extensive costs of repair heighten the importance of developing a model as a numerical assessment tool of corrosion induced damage effects. A three dimensional (3D) chemo-hygro-thermo-mechanical (CHTM) model has been recently developed (Ožbolt et al. 2010, 2011). The model is implemented into a finite element code and is capable of predicting the non-mechanical processes before and after depassivation of steel, for both un-cracked and cracked concrete elements. Here presented work is a further development of the model, focused on predicting the rate of rust production and the corresponding damage (cracking and spalling) of the concrete cover surrounding the embedded steel bars. The extent of the damage and its progress in time are highly dependent on the ingress of corrosion products into pores and cracks. Therefore, the transport of corrosion products is incorporated into the model as a convective diffusion problem. The expansion of rust and the subsequent compressive pressure on the cover is taken into account by developing one-dimensional corrosion contact elements. To ensure a realistic simulation of the corrosion induced damage, the model couples the mechanical with the non-mechanical processes, and vice versa. Validation of the CHTM model's ability for simulating corrosion induced damage is performed on concrete specimens with a single embedded reinforcement bar and for multi-rebar cases, without and with stirrups. Comparison of the numerical results with the existing experimental data shows that the model is capable of realistically capturing the damage of the concrete cover and its effects on the mechanical properties of the investigated RC elements. Hysteretic moisture model for concrete is a further addition to the CHTM model. It accounts for the hysteretic behaviour of concrete which is characterized by the ability to contain different degrees of water saturation for the same value of relative humidity, at a constant temperature. Hence, the water content values are highly dependent on the exposure to cyclic wetting and drying conditions. The hysteretic moisture model is validated directly through existing experimental studies by verifying the calculated scanning curves and the distribution of relative humidity, i.e. the water content in concrete, respectively. The importance of incorporating the moisture hysteresis in the CHTM model is demonstrated by comparing the experimentally and numerically obtained distribution of chloride ions in concrete specimens exposed to cyclic wetting and drying conditions.
Open Access
Explosive spalling and permeability of high performance concrete under fire : numerical and experimental investigations
(2014) Bosnjak, Josipa; Ozbolt, Josko (Prof. Dr.-Ing. habil.)
Explosive spalling of high performance concrete under fire is one of the major concerns in front of the engineering community today. It is associated with violent failure of thin layers of concrete resulting in sudden reduction of load carrying capacity which may lead to complete collapse. High pore pressures due to low permeability and stresses due to thermal gradients are considered to be the governing causes of explosive spalling. However, the failure mechanisms and all influencing parameters are not yet fully understood. The most popular method to prevent spalling is the addition of polypropylene (PP) fibres in concrete. It is generally accepted that the PP fibres leave a porous network after melting at around 160 °C, leading to an increase in permeability, thus allowing the water vapour to escape. However, it seems that there also might be other mechanisms which lead to relief of pore pressure. This work is aimed at investigating the phenomenon of explosive spalling and understanding the causes behind the same. Technical difficulties in measuring during the experiments at high temperatures or fire limit the data that can be obtained. On the other hand, numerical analysis provides a better insight into the governing causes and a quantitative estimate of the relevant properties. Therefore, in this work, the experimental investigation is augmented by extensive numerical parametric studies. Experiments under two typical fire scenarios are conducted on slabs made of plain and concrete with PP fibres to compare the performance of the two mixes as well as to investigate the effect of the heating rate on explosive spalling. Significant influence of PP fibres in mitigating explosive spalling is confirmed by these experiments. In order to measure the permeability of concrete at elevated temperature, a new test setup is developed and validated. Permeability experiments on plain and concrete with addition of PP fibres are performed at temperatures up to 300 °C using the new test setup. The results show that permeability of concrete with PP fibres rises even before the fibres melt, thus indicating that the melting of fibres is not the only mechanism responsible for the permeability increase. To confirm this, microstructure of the specimens before and after heating is studied using a scanning electron microscope. The existing thermo-hygro-mechanical model is validated against experiments and is used to investigate the influence of various parameters on explosive spalling. The parameters studied include: permeability, relative humidity, restraint, load, inhomogeneity, aggregate type, etc. The numerical parametric studies are performed at macro- and meso-scale. Due to the high influence of the local inhomogeneities, analysis at macro-scale could only partially capture the failure mode. It is found that all aspects of explosive spalling can be considered only while performing analysis at meso-scale.