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.
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.