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.
E.A. Jordet, A. Aas-Jakobsen AS, Oslo, Norway S. E. Jakobsen, A. Aas-Jakobsen AS, Oslo, Norway
The new Svinesund Bridge is an arch motorway bridge on the border separating Norway and Sweden. The arch is a single, centrally located concrete structure with a span of 247.3 m, which is believed to be the longest span for a single free-standing concrete arch in the world. The bridge decks consist of two steel orthotropic boxes, one on each side of the arch and connected by cross beams. The total length of the bridge is 704 m between abutments. The design is the winning concept of an international competition. The bridge was opened to traffic in 2005, and received the fib Award for Outstanding Structures, Special Mention, in 2006.
Concrete members with plate reinforcement: mechanical bond analysis
T. Ulaga, Walt + Galmarini AG, Zurich, Switzerland T. Vogel, Institute of Structural Engineering (IBK), ETH Zurich, Switzerland
The bond stresses between a concrete body and plate reinforcement are often modelled with a bilinear bond stress - slip relationship. The mechanisms that govern this approach can be investigated on a micro-mechanical level in order to obtain a scientific model basis. As long as the load level is 'low' the theory of elasticity can be used. When the load level is 'high' a crack plane in the concrete body separates the constituents. Owing to aggregate interlock mechanisms, bond stresses still exist. This process can be investigated with the model of the inclined crack opening (MICO). The combination of the cases 'bond at low load' and 'bond at high load' provides a stress - slip diagram which is very similar to the bilinear bond model. The MICO also has the potential to be used for the analysis of shear failure modes in concrete structures. The punching of a flat slab can be considered in order to show the possibilities.
Prestressed CFRP laminates for flexural strengthening of reinforced concrete beams
P. França, IST University, Lisbon, Portugal A. Costa, IST University, Lisbon, Portugal J. Appleton, IST University, Lisbon, Portugal
Significant research on strengthening reinforced concrete (RC) structures with carbon fibre reinforced polymer (CFRP) laminates has been done in recent years. The interest in prestressing this material and the evaluation of the behaviour of the strengthened RC structures is the focus of this paper. A technique of strengthening RC slabs with prestressed CFRP laminates was tested on several T cross-section large-scale RC beams. Comparisons are established between the reference RC beam and the strengthened beams with prestressed and non-prestressed CFRP laminates. To simulate the behaviour of the beams, a non-linear numerical model was used and validated by experimental results. This strengthening technique with prestressed CFRP laminates revealed a substantial improvement, both at serviceability and ultimate states, when compared with the reference beam and with the non-prestressed CFRP laminate strengthened beam.
Behaviour and capacity of CFRP-confined concrete cylinders subjected to monotonic and cyclic axial compressive load
V. Valdmanis, Institute of Polymer Mechanics, University of Latvia, Riga, Latvia L. De Lorenzis, University of Alento, Lecce, Italy T. Rousakis, Democritus University of Thrace, Xanthi, Greece R. Tepfers, Chalmers University of Technology, Göteborg, Sweden
The mechanical behaviour of concrete confined by carbon fibre reinforced polymer (CFRP) sheets is investigated in this study. Two series of tests were conducted on standard concrete cylinders with cube compressive strength ranging from 34.2 to 104.1 MPa, confined by CFRP sheets with 234 GPa elastic modulus and volumetric ratio ranging between 0.45 and 1.35%. Split-disc tests were performed to estimate the tensile properties of the CFRP sheet in the hoop direction. The concrete cylinders were subjected to monotonic and cyclic axial compressive loading with Teflon sheets inserted between concrete and steel bearing platens to reduce friction. The confined cylinder strength, strains and tangent moduli are compared with the values predicted by the recommendations of fib task group 9.3, fib Bulletin 14. It is concluded that, at least for the investigated range of variables, the CFRP tensile strength has to be reduced with a factor 0.50 in the ultimate strength approach in order to obtain accurate strength predictions. For stability control the tangent modulus E2 of the confined concrete in the second pseudo- linear branch of the stress - strain curve (above the unconfined concrete strength) must be estimated and in the tests ranged from about 8 to 20% of the tangent modulus of elasticity E1 of the first branch of the curve.
Serviceability model of corroded reinforced concrete based on the CEB-FIP model code
A. Castel, Laboratory of Materials and Construction Durability, Toulouse, France Th. Vidal, Laboratory of Materials and Construction Durability, Toulouse, France R. François, Laboratory of Materials and Construction Durability, Toulouse, France
In this paper, based on the main assumptions of the CEB-FIP model code, a model of corroded reinforced concrete behaviour is proposed. The model allows the quantification of the coupled effect of the steel cross-section reduction and the loss of the steel - concrete bond on deflection of reinforced concrete beams under service loads. To model the bond degradation, an environmental-damage variable is explicitly introduced into the steel - concrete bond relationship in order to take into account the slip between the steel and the concrete and then the reduction of the concrete tension stiffening. A validation is proposed on two 20-year-old corroded beams tested in flexure.
