Written on 19 août 2011.
J. Thomas, Indian Institute of Science, Bangalore, IndiaA. Ramaswamy, Indian Institute of Science, Bangalore, IndiaA total of twelve partially prestressed concrete T-beams were cast without and with fibres in partial and full depth. These beams were tested to assess the effect of fibre addition in different zones in the cross-section, namely, flange, web or the entire cross-section, on the load-deflection performance of the flexure critical beam specimens. An attempt has been made to predict the load-deflection characteristics of the beam specimens utilising the tension softening effects in the stress-strain behaviour of fibre-reinforced concrete. The proposed model accounts for the interaction of the matrix with the fibre pull-out mechanism. The details of the constitutive material properties used and the proposed layer-based analysis are presented in this paper. Comparisons with test data show good agreement.
Written on 19 août 2011.
K. Ganesh Babu, Indian Institute of Technology, Chennai, IndiaP. Dinakar, Indian Institute of Technology, Chennai, IndiaThe use of mineral admixtures is widely accepted, because of several improvements possible in concrete composites and the overall economy. Of these metakaolin is one mineral admixture that appears to have significant potential for the production of high-strength and high-performance concretes. Although available information on metakaolin concretes is limited, the present paper attempts to quantify the 28-day cementitious efficiency of metakaolin in concrete at various replacement levels from the data available in the literature. It was seen that the efficiency of metakaolin in concretes can also be defined through a procedure adopted earlier for other cementitious materials like fly ash, silica fume and ground granulated blast-furnace slag (GGBS). The overall strength efficiency was found to be a combination of two parameters, one depending on the age and the other depending on the percentage of replacement, as is the case with other pozzolans.
Written on 19 août 2011.
R. Tepfers, Chalmers University of Technology, Göteborg, SwedenThe bond clauses in the CEB/FIP Model Code 1990 (MC90) were written for steel reinforcement in concrete. Since then, fibre reinforced polymer (FRP) reinforcing bars have been introduced as an alternative. Reinforcing steel is a homogeneous material; FRP reinforcements, however, are made of a combination of different materials with various shapes. All these FRPs perform differently under different conditions. For example, in severe environments, where steel is not durable, FRPs may perform well. To make use of fibre composites, it is necessary to bring these materials into codes of practice and the easiest way is to adapt the code clauses for steel reinforced concrete to FRP reinforced concrete. The clauses should include boxed values open for individual coefficients of different FRPs. The necessary coefficients for the code models should be determined using systems of appropriate and coupled test methods. This paper covers a code proposal for the design bond stress. Proposals for the design lengths and tension stiffening code clauses will be presented separately.
Written on 19 août 2011.
L. Bing, Shanghai Jiao Tong University, People's Republic of ChinaL. Guanglu, Tongji University, Shanghai, People's Republic of ChinaZ. Long, Tianjin Steel Wire and Steel Cable Group Co., Ltd, People's Republic of ChinaThe failure of a thin-webbed girder of post-tensioned concrete was investigated. In a foggy environment containing hydrochloric acid, the girder ruptured in the midspan as a result of steel strand corrosion. The causes and mechanisms of the strand fracture were identified by careful observations of the fracture surface of the strands and corrosion on the surface of the strands. Scanning electron microscopy (SEM) analysis of the fracture morphology of the strands, after mechanical fracture, showed that hydrogen in the environment had an effect on the strands. The investigation revealed that failure of the girder occurred because, while being attacked by the corrosive media, stress accelerated surface corrosion of the strands such that the cross-sections of the wires in the strands were severely weakened, consequently leading to the fracture of the strands.
Written on 19 août 2011.
Z. Achillides, National Technical University of Athens, Greece K. Pilakoutas, University of Sheffield, UKIn this paper a computational modelling approach is used to investigate the bond behaviour of fibre-reinforced plastic (FRP) bars in concrete. Two finite element packages (ANSYS and ABAQUS) are used to model the bond interaction of FRP reinforcing bars in cubes and beams. The main purpose of this work is to develop additional understanding of how FRP bars 'cooperate' with concrete to sustain the pullout load. Two modelling approaches are presented. In the first approach, a spring describing the behaviour of short embedment lengths in pullout tests was used for predicting the behaviour of longer embedment lengths. In the second approach, spring characteristics obtained from an experimentally determined bond stress against anchorage length envelope are used in FE modelling of beams. Both approaches showed good agreement between analytical and experimental results. However, further development on the analytical modelling of the bond interaction is required, in order to consider the effect of all parameters that influence bond.
Written on 19 août 2011.
K. U. Muthu, M.S. Ramaiah Institute of Technology, Bangalor, India K. Amarnath, M.S. Ramaiah Institute of Technology, Bangalor, India A. Ibrahim, Universiti Teknologi MARA, Selangor, Malaysia H. Mattarneh, Universiti Teknologi MARA, Selangor, Malaysia This paper presents results of an experimental investigation into the load-deflection behaviour of restrained reinforced concrete slab strips subjected to uniformly distributed loads beyond the yield load. A method incorporating compressive membrane action is proposed to predict the behaviour. Results indicate that the proposed method can predict the actual trends for certain scenarios. Previously reported methods have been studied and the results compared with those from the current work.
Written on 19 août 2011.
Jianzhuang Xiao, Tongi University, P.R. ChinaJiabin Li, Tongi University, P.R. ChinaChuanzeng Zhang, University of Siegen, GermanyThis paper presents a statistical analysis of the compressive strength of recycled aggregate concrete (RAC) with recycled coarse aggregate (RCA) coming from one single source. The statistical analysis includes the mean value, the standard deviation and the coefficient of variation in addition to the probabilistic distribution. In order to obtain the compressive strength variations, 371 cube specimens with different RCA replacement percentages of 0, 30%, 50% and 100%, respectively, were tested under identical laboratory conditions. With the statistical data, both the normal and the lognormal model were fitted to describe the probabilistic distribution of the compressive strength. It was found that the statistical characteristics of the compressive strength of recycled aggregate concrete do not differ very much from those of normal concrete (NC) with similar strength, irrespective of the recycled coarse aggregate replacement percentage.
Written on 19 août 2011.
A.W. Beeby, University of Leeds, UKC. Ålander, Ruukki Metals, FinlandJ. Cairns, Heriot-Watt University, UK U. Mayer, University of Stuttgart, GermanyS. Lettow, University of Stuttgart, GermanyD. Ferretti, University of Parma, ItalyI. Iori, University of Parma, ItalyP. Gambarova, Milan University of Technology, ItalyP. Bamonte, Milan University of Technology, ItalyE. Giuriani, University of Brescia, ItalyG. Plizzari, University of Bergamo, ItalyS. Pantazopoulou, Demokritus University of Thrace, GreeceS. Tastani, Demokritus University of Thrace, GreeceProfessor Beeby's 2004 paper questioned the basis of the widely cited "classical" model for estimation of crack widths in reinforced concrete elements. It provides a valuable reminder of the need to verify the predictions of theoretical models against experimental data, a task which he has performed to the benefit of our understanding on several occasions in the past. Professor Beeby will therefore not be surprised to learn that others have subjected his own ideas to similar scrutiny. In the several contributions below, members of fib Task Group 4.5 "Bond Models" raise questions over the validity of conclusions in the paper, based on a wider range of experimental data and more recent developments in theoretical modelling. (John Cairns, Convenor, fib Task Group 4.5 "Bond Models")
Written on 19 août 2011.
R. do Carmo, Instituto Superior de Engenharia de Coimbra, Coimbra, PortugalS. M. R. Lopes, Universidade de Coimbra, Coimbra, PortugalDuctility, particularly the plastic rotation capacity of critical regions, conditions the available degree of moment redistribution and the ability to exploit the additional resistance of hyperstatic structures. A theoretical model for calculating plastic rotation capacity, considering the influence of the main factors, is presented. Special attention has been paid to the influence of the tensile reinforcement ratio, the shear force and the confinement of compressed concrete on plastic rotation capacity. Theoretical results are compared with those obtained using an experimental programme designed to study the influence of these factors. Some extrapolations are made on the basis of the model, and conclusions are drawn.
Written on 19 août 2011.
A. Khennane, University of Southern Queensland, AustraliaA fundamental task in the design of reinforced concrete structures is to search for minimum cost through the variation and placement of the quantities of the relatively expensive steel reinforcement without jeopardising the safety of the structure. The use of nonlinear finite element software can assist greatly in achieving an economical and safe design. However, commercially available finite element software is not designed for this task as most packages have been developed to be used as verification rather than design tools. "Home-written" software can be designed to achieve this task, however it may suffer from serious drawbacks such as bugs, lack of user friendliness, lack of generality, and unproven reliability. This present study shows that if a given software package comes with a scripting interface, it can be transformed easily from a verification tool to a performance design tool. This is illustrated with the use of ABAQUS, but it can be adapted to any other software with a scripting interface.
Written on 19 août 2011.
R. W. Howells, Cardiff School of Engineering, Cardiff University, UKR. J. Lark, Cardiff School of Engineering, Cardiff University, UKB. I. G. Barr, Cardiff School of Engineering, Cardiff University, UKThis paper addresses the influence that changing environmental effects have on the strains developed in a major pre-stressed concrete viaduct. The changes in strain, resulting from variations in temperature and relative humidity, were analysed over a period of one year. It was found that changes in temperature had a greater influence on the strain behaviour of the structure than changes in relative humidity, with strain variations of up to 75 microstrain (με) being recorded. A further study was conducted on the effect that changing temperature has on the strain behaviour of the viaduct during each of the four seasons. It was observed that the variation of temperature over the course of a week resulted in strains of up to 15 με. Finally, in both studies, a linear relationship was observed between strain and temperature, although these relationships differ slightly depending on the season, and the segment location within the span.
Written on 19 août 2011.
B. Elsener, Institute of Materials Chemistry and Corrosion, ETH Zurich, SwitzerlandElectrically isolated tendons with plastic ducts for internal grouted post-tensioning were developed about 15 years ago. This new generation of tendons offers enhanced corrosion protection of the steel strands and the possibility to monitor the corrosion protection by simple non-destructive measurements (electrical impedance). This paper reports practical experience on quality control and long-term monitoring of two flyovers with electrically isolated tendons in Switzerland. The results of impedance measurements are rationalised on the basis of a simple model of a capacitance C (of the polymer duct) in parallel to a resistance R (leaks, defects) that both have a clear physical meaning and depend on the length of the tendon. The penetration of (chloride-containing) water at defects of the duct will lead to a decrease of the resistance R of that tendon. Thus for the first time the corrosion protection of the structurally important post-tensioned tendons can be monitored during the whole service life of the structure.
