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Structural Concrete, Vol. 9, no. 1, March 2008

Non-linear and plastic analysis of RC beams subjected to fire

P. Riva, University of Bergamo, Italy
J.-M. Franssen, Université de Liège, Belgium 

Member analysis is the main verification method adopted for reinforced concrete (RC) beams by most codes. The verification by means of member analysis consists of comparing the design forces (bending moment, axial force and shear force) with the resisting forces, where the former are computed at ambient temperature and the latter are evaluated using simplified methods considering the prescribed fire duration. The main objection that might be raised against member analysis is that, by computing the design forces at ambient temperature, indirect actions arising in the structure owing to thermal expansion are not taken into consideration; the time-dependent response of the structure is also neglected. In this paper, the behaviour of a set of fixed-end rectangular beams with varying axial restraints is discussed. The results are used to illustrate a simplified plastic verification procedure that allows determination not only of the load-carrying capacity of the beams, but also evaluation of the deflections for any given fire duration. 

Structural Concrete, Vol. 9, no. 1, March 2008

Flexural response of reinforced concrete beams exposed to fire

V. Kodur, Michigan State University, USA
M. Dwaikat, Michigan State University, USA 

The flexural response of reinforced concrete (RC) beams exposed to fire is investigated in this paper. A macroscopic finite element model, capable of tracing the behaviour of RC beams from pre-fire stage to collapse in fire is used in the analysis. The model includes the three stages associated with fire resistance analysis, namely establishing the fire temperature - time development, calculating the heat transfer through the structure from fire and the structural analysis. The model is applied to investigate the effect of six parameters, namely the fire scenario, load level, concrete cover thickness, aggregate type, failures criteria and span length on the fire response of RC beams. Through the results of the parametric study, it is shown that the type of failure criterion, load level, fire scenario, concrete cover thickness and aggregate type have significant influence on fire resistance of RC beams. It is also shown that, while the span length has significant influence on the overall fire behaviour, it has a minor effect on the fire resistance of RC beams. 

Structural Concrete, Vol. 9, no. 1, March 2008

Mass transport through concrete walls subjected to high temperature and gas pressure

A. Laghcha, LGCIE, INSA-Lyon, France
G. Debicki, LGCIE, INSA-Lyon, France 
B. Masson, EDF/SEPTEN, France 

The aim of this study is the modelling of mass transport phenomena through a concrete wall, when a gas (dry air plus water vapour) at high temperature and pressure is applied to one face of the wall. The temperature of the heated wall was increased from 20 to 141 C, while the other wall was exposed to ambient conditions. A uni-dimensional numerical analysis was performed, by using the thermohydromechanic model (THM) included in theCode_Aster for the description of non-saturated porous media. Two fluid phases were considered in the material: a liquid phase (water) and a gas phase (dry air plus vapour). The vapour-to-liquid phase change was introduced as well. Owing to the progressive saturation of the wall, the porosity, the shape of the sorption isotherm and the permeability greatly influenced the results. The numerical results are compared with experimental investigation. The tests concerned three concrete cylindrical specimens, which represented core samples extracted from a concrete wall. During the tests, the specimens were subjected to the same boundary conditions found in the wall (front end-section exposed to the autoclave and back end-section exposed to ambient temperature, and the lateral surface sealed and insulated to eliminate lateral hygral and thermal flux). Three different cementitious composites were tested (two concretes with different permeability for the first and second specimens, and one with highly porous mortar for the very permeable 'flaw' created in the third specimen). The numerical results were in good agreement with the tests in terms of phenomenological evolution and flow rate through the concrete, and confirmed the necessity of having reliable data on the thermal - hydromechanical properties of the material, to guarantee the validity of the results.

Structural Concrete, Vol. 9, no. 1, March 2008

Concrete spalling assessment methodologies and polypropylene fibre toxicity analysis in tunnel fires

G.A. Khoury, Imperial College London, U.K., and University of Padua, Italy

Concrete is by far the largest component of tunnels. Given the high relative humidity in tunnels (e.g. 75%) when compared with buildings in general (e.g. 50%), there is a higher risk of the occurrence of explosive spalling in tunnels during a fire, which increases with increase of the level of pore filling with water in the concrete. Tunnel fires described by hydrocarbon-type fire scenarios are also more severe than building fires described by cellulose fire scenarios (e.g. ISO 834 fire scenario) owing to their confined nature. Passive fire protection in tunnels involves the use of thermal barriers and/or polypropylene fibres in the concrete mix. The latter operates on the pore pressure mechanism of explosive spalling. This paper presents the concept and methodology of the separation of pore pressure spalling from thermal stress spalling for the first time in large-scale experiments as part of the NewCon international research project, by the use of thermally stable lightweight aggregate of negligible thermal expansion. This paper also presents the concept of the pressure induced tangential space (PITS) as a mechanism for increased permeability during fire even before the fibre is melted. The prediction of explosive spalling is still not a fully developed science. Prediction methods include large-scale testing, use of nomograms, theoretical models and numerical models. Numerical modelling, in addition to costly large-scale testing, offers a promising way forward. This paper also introduces for the first time the concept of the expert assessment of spalling in tunnels with a tentative example following a risk-based approach for a given concrete, different traffic conditions and initial pre-fire stress in an example separating tunnel wall. Finally, definitive conclusive calculations for a tunnel example in a severe fire indicates negligible toxicity from the combustion of polypropylene fibres used in tunnel concretes to combat explosive spalling. This work was carried out as part of the NewCon international research project.

Structural Concrete, Vol. 9, no. 1, March 2008

Today's concretes exposed to fire-test results and sectional analysis

P. Bamonte, Politecnico di Milano, Italy
P.G. Gambarova, Politecnico di Milano, Italy
A. Meda, University of Bergamo, Italy 

The well-known capacity of concrete to withstand high temperature and fire is put to the test by the most recent, high- and ultra high-performance cementitious composites, since their more closed pore structure favours pressure build-ups in the pores filled with water, turning to vapour at high temperature. The ensuing spalling phenomena can be prevented by adding polymeric fibres to the mix, while material toughness can be improved - at any temperature - by adding metallic fibres. However, concrete mechanical behaviour depends on the thermal field, which is strictly related to the type of fire and to the thermal properties of the material. Hence, special concretes for special structural applications should be thoroughly characterised at high temperature and after cooling, to evaluate their thermal and mechanical properties. These properties are recalled in the first part of this paper, with reference to thermal diffusivity, compressive and tensile strength, elastic modulus and fracture energy. Furthermore, to maximise the benefits coming from the use of better materials, a parallel rethinking of some aspects of structural analysis is needed. With regard to this point, in the second part of the paper some suggestions and proposals are formulated with reference to the analysis of reinforced concrete sections subjected to combined bending and axial force, and some considerations are made on two rather underrated aspects of the analysis: the role of the thermal self-stresses and the increasing slenderness of fire-exposed columns.

Structural Concrete, Vol. 9, no. 1, March 2008

Recent development in fire design of concrete structures

N.P. Høj, HOJ Consulting GmbH, Switzerland

Concrete is known to be an excellent structural material, owing especially to its many favourable properties, to its constituent materials available from many local sources, unlimited forms, easy placement, economy and aesthetics. The question remains whether concrete is also an attractive material in terms of its properties with respect to fire. The present paper aims to discuss this question, reach some conclusions and give some indications for the future. The paper also provides an introduction to selected topics concerning fire design of concrete structures, such as the influence of fire on concrete properties (strength, deformation and spalling), member and structural analysis, and the role of both the restraints and the boundary conditions. Only few technical or scientific details are given in the paper, since the author's main objective is to present some topical ideas, on-going research activities and possible future development. 

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