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

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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. 

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

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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. 

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

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The dynamic shear capacity of concrete structures

A. Ansell, Royal Institute of Technology (KTH) Stockholm, Sweden

For 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

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Aisle-width considerations in ground-floor slabs

A.A. Abbas, Imperial College London, UK
M.N. Pavlovic, Imperial College London, UK
M.D. Kotsovos, National Technical Univesity of Athens, Greece

The 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

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Modelling bond strength of corroded plain bar reinforcement in concrete

G. Xu, Huazhong University of Science & Technology, China
J. Wei, Huazhong University of Science & Technology, China
T. Tan, Huazhong University of Science & Technology, China
H.Q. Liu, Huazhong University of Science & Technology, China

In 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

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Cost optimisation of lattice-reinforced joist slabs using genetic algorithms

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. 

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