News - fib International

Structural Concrete, Vol. 8, no. 1, December 2007

19.08.2011 News

T. Ulaga, Walt + Galmarini AG, Zurich, SwitzerlandT. Vogel, Institute of Structural Engineering (IBK), ETH Zurich, SwitzerlandThe 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. 

Structural Concrete, Vol. 8, no. 1, December 2007

19.08.2011 News

P. França, IST University, Lisbon, PortugalA. Costa, IST University, Lisbon, PortugalJ. Appleton, IST University, Lisbon, PortugalSignificant 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. 

Structural Concrete, Vol. 8, no. 1, December 2007

19.08.2011 News

V. Valdmanis, Institute of Polymer Mechanics, University of Latvia, Riga, LatviaL. De Lorenzis, University of Alento, Lecce, ItalyT. Rousakis, Democritus University of Thrace, Xanthi, GreeceR. Tepfers, Chalmers University of Technology, Göteborg, SwedenThe 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. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

A. Castel, Laboratory of Materials and Construction Durability, Toulouse, FranceTh. Vidal, Laboratory of Materials and Construction Durability, Toulouse, FranceR. François, Laboratory of Materials and Construction Durability, Toulouse, FranceIn 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. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

A. Ansell, Royal Institute of Technology (KTH) Stockholm, SwedenFor shear capacity, the current design procedure used for concrete structures has been found to be inaccurate for some dynamic design load cases. Loads traditionally believed to be highly dynamic are only within the quasi-static range, but it is known that the combination of a high impact velocity and a hard, stiff impact will lead to shear failure. Load cases on concrete structures which can be classified as dynamic are, for example, those relating to weapons and hard impacts from steel objects. The characteristic of a dynamic load has great influence on the overall response and mode of failure of concrete structures. When structures which have been designed to fail in flexure under static loads fail in shear when loaded dynamically, the reason is probably the changing frequency content of the load. On the basis of a literature review, some important conclusions and recommendations are presented in this paper. The results will be used in further evaluation of existing design tools, aiming at accurate and reliable routines for the design of safe and cost-efficient concrete structures. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

A.A. Abbas, Imperial College London, UKM.N. Pavlovic, Imperial College London, UKM.D. Kotsovos, National Technical Univesity of Athens, GreeceThe design of ground-floor slabs (GFS) is largely based on concentrated patch loads (CPL) rather than uniformly distributed loads (UDL) as the former are more critical. This is true provided that the UDL is applied throughout the floor and that the slab is also uniformly supported by the soil beneath (i.e. there are no local soft spots). Clearly, such scenarios will not induce any significant bending stresses in the slab. However, if the UDL is applied only locally on the slab, as is often the case, then bending stresses will occur. A particular case is the arrangement in which two layers of UDL are applied on the floor with an unloaded aisle in between. This results in tensile stresses in the mid-aisle at the top of the slab which must be considered in the design process. A similar situation arises when two spaced CPL are applied, which is common in the case of racking-leg loads. The present article reports on the numerical research work that was carried out to study these effects. The work is based on linear finite-element analysis (LFEA) and the ensuing results are presented herein in the form of design charts. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

G. Xu, Huazhong University of Science & Technology, ChinaJ. Wei, Huazhong University of Science & Technology, ChinaT. Tan, Huazhong University of Science & Technology, ChinaH.Q. Liu, Huazhong University of Science & Technology, ChinaIn this paper, the corrosion influence on bond strength between plain bar and concrete without confinement is studied at the time of cracking and before cracking occurs, and causes of bond strength increment are analysed quantitatively. A new calculation model for corrosion pressure and corrosion depth at the time of cracking is proposed. Considering the change of bar surface and increment of corrosion pressure, a new calculation model for bond strength at the time of cracking and before cracking occurs is given. Additionally, the theoretical model is verified using different test results. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

V.C. Castilho, Universidade Federal de Uberlândia, Brazil M.C.V. Lima, Universidade Federal de Uberlândia, Brazil Genetic algorithms (GA), a search method inspired by Darwin's theory of evolution, offer an optimisation tool that has been used very successfully to solve a variety of engineering problems. The search process it implements starts with a set of one or more chromosomes (initial population) and, by applying selection and reproduction operators, iteratively 'evolves' the population into better ones, until a stopping criterion is reached. This article investigates lattice-reinforced joist slab cost optimisation problems using a GA with continuous variables. The problem considered concerns one-way slabs, continuous over two spans, in which only the in situ concrete characteristics and joist spacing are varied. The design variables are: concrete layer thickness, concrete layer strength, reinforcement, distance between joists and degree of redistribution of the continuous slabs' negative moments. The search for a solution includes an investigation into the use of discrete variables for data representation. To obtain results that allow for a comparative empirical analysis, these problems are also evaluated by a conventional optimisation method. The results indicate that the GA method is a viable optimisation tool for solving lattice-reinforced joist slab cost minimisation problems. 

Structural Concrete, Vol. 8, no. 3, September 2007

19.08.2011 News

E.G. Burdette, University of Tennessee, Knoxville, USAS.C. Howard, University of Tennessee, Knoxville, USA J.H. Deatherage, University of Tennessee, Knoxville, USAD.W. Goodpasture, University of Tennessee, Knoxville, USAThe Tennessee Department of Transportation (TDOT) in the United States has become the national leader in the design and construction of jointless bridges with abutments which are integral with the bridge deck. While TDOT criteria call for limits of length of 152 m (500 ft) and 244 m (800 ft) for steel and concrete bridges, respectively, they have on more than one occasion exceeded these limits significantly. TDOT's desire to address any questions raised about the efficacy of using prestressed concrete piles to support integral abutments and perhaps to extend the limits on bridge length led to the tests reported in this paper. Four full-size abutments, 3.05 m (10 ft) wide, were built and tested in the field. The description and results of these tests are reported and discussed. Of particular interest are the testing of one pile to failure and the cyclic tests performed on one pile. The conclusions drawn were, first, that prestressed concrete piles are appropriate to use to support integral abutments and, second, that the TDOT criteria for bridge lengths are reasonable and conservative. 