Written on 19 août 2011.
R. Kowalski, Warsaw University of Technology, Warsaw, PolandThis paper describes some tests performed on concrete specimens heated up to high temperature and then cooled down in water. The tests were carried out on cylindrical specimens, 103 mm in diameter and 200 mm in height, made of ordinary concrete with siliceous aggregate. The specimens were heated up to 330, 430 and 550 degrees C and then cooled down to room temperature in five various ways. A first group of specimens was cooled in air, while a second was immersed in water for 5, 10, 15 or 20 minutes, respectively; after the rest period in water the specimens were cooled in air. The next day, after the heating and cooling process, the residual concrete compressive strength was measured. Rapid cooling of heated-up concrete had the highest influence on the strength degradation when the maximum temperature peaked at 330 degrees C while, in the case of thermal cycles peaking at 550 degrees C, the strength degradation was less dependent on the method of cooling. It was concluded that rapid cooling of heated-up concrete can cause a significant strength degradation, especially in the range of low temperatures.
Written on 19 août 2011.
R.M. Fernandes Canha, University of São Paulo at São Carlos, BrazilA.L. H. de Cresce El Debs, University of São Paulo at São Carlos, BrazilM.K. El Debs, University of São Paulo at São Carlos, Brazil This paper presents a theoretical and experimental analysis of socket base connections of precast concrete structures, with the emphasis on pedestal walls. The experimental programme included five specimens subjected to loads with large eccentricities, changing the type of the interface in contact with cast-in-place concrete and the load eccentricities. Three specimens had smooth interfaces and two specimens had rough interfaces. The experimental results indicated the need to revalue the principal design models for this connection. In the case of smooth walls, the friction portion that contributes to the socket connection strength was verified and a design model was proposed and adjusted to the experimental results. Based on the present experimental results, the following conclusions can be drawn: (a) the Leonhardt and Mönnig behaviour model is suitable to represent connections with smooth interfaces; (b) the proposed design model for smooth interfaces provides the closest predictions of the experimental results; (c) the Leonhardt and Mönnig behaviour model is not suitable for rough interfaces; and (d) for rough interfaces, the vertical reinforcement can be designed by bending theory.
Written on 19 août 2011.
V. Klevtsov, State Research Institute for Concrete, Moscow, Russia M. Korevitskaya, State Research Institute for Concrete, Moscow, RussiaAll non-destructive methods for concrete strength control are indirect methods, which is why concrete strength on each specific area is determined with some error. In this paper, probably for the first time, a method to account for this error is presented. Theoretical grounds for the method and its experimental confirmation are also presented.
Written on 19 août 2011.
N. Shafiq, University of Technology Petronas, Perak Darul Ridzuan, MalaysiaJ. G. Cabrera, University of Leeds, UKIt has been established that the durability of cement-based composites is generally controlled by the transport characteristics of fluid in their pore network. Experience has shown that the monitoring of transport properties, such as the coefficient of permeability, in the laboratory is a very exhaustive task. The pore network in a cementitious composite, which provides passage for fluid transportation, is a major controlling factor for transport characteristics. In the last few years, many efforts have been made to correlate the microstructure and the coefficient of fluid permeability, and/or diffusion, for a cementitious composite. In this study, a set of ordinary Portland cement (OPC) and OPC/fly ash mortar samples equilibrated in different relative humidity were analysed using PORECOR analysis and the coefficient of oxygen permeability was calculated and compared with the coefficient of permeability of another set of the same samples tested in the laboratory, and valid statistical correlation was obtained.
Written on 19 août 2011.
R. S. Camposinhos, Instituto Politecnico do Porto, PortugalA. Serra Neves, University of Porto, PortugalBeam-and-block floor systems using pretensioned ribs enable very efficient industrialised production. By ensuring that the ribs are produced under a strict quality assurance programme and that they are safely transported, assembled and properly detailed, an equally monolithic structural performance is achieved. However, calculation methods to control cracking in these structural elements must ensure the durability according to the surrounding environment's severity and predefined exposure classes. The design of beam-and-block floor systems is particularly affected by serviceability limit states. This fact is closely linked to the verification conditions for crack control. Therefore, new design models have been developed to take into account the peculiarities and characteristics of these floor systems. The aim is to find practical and effective crack-control methods for composite beam-and-block floor systems with pretensioned ribs or beams. The verification of the limit state of crack widths has so far been disregarded in the design of this type of lightweight slab. In fact, the current procedure is simply to compare the tensile stress in the lower fibre of the beams with the characteristic strength of the rib's concrete. This paper presents a method to assess crack control. The procedure implies the quantification of a limit bending moment depending on the physical and geometric characteristics of the sections. Simplified calculation methods and verification rules are presented, which allow the establishment of tables and design charts for the indirect verification of this limit state through bar size and spacing limits.
Written on 19 août 2011.
K. Tammo, Lund Institute of Technology, SwedenS. Thelandersson, Lund Institute of Technology, SwedenCurrent concrete codes impose limitations of crack widths at the concrete surface. To investigate how the crack width at the surface affects risk for reinforcement corrosion, the related crack width close to the bar should be determined. An experimental study to investigate how concrete cover affects the crack width at the reinforcement level is presented. Axially loaded concrete prisms with a central 16 mm reinforcement bar were tested. Three different concrete covers, 30, 50 and 70 mm, were used. By continuous monitoring of strains and load, it was also possible to see how the crack width near the bar is affected by crack spacing. The test results showed that the crack width at the concrete surface is more than twice the crack width at the level of reinforcement. The influence of concrete cover seems to be rather small for the crack width at reinforcement level. It was also found that the crack width close to the bar is not affected by short term cyclic variations of five cycles or less with steel stresses in the range 200 - 400 MPa.
Written on 19 août 2011.
N. Ogawa, Chubu Regional Bureau, Ministry of Land, Infrastructure and Transport, JapanY. Kamiya, Oriental Construction Co., Ltd, JapanT. Yoshikawa, Oriental Construction Co., Ltd, JapanG. Yu, Oriental Construction Co., Ltd, JapanM. Tsunomoto, Oriental Construction Co., Ltd, JapanIt is well known that one drawback for stress-ribbon bridges can be that significant horizontal reactions at the abutment can be generated and that they have low flexural stiffness. The latter prevents them from being used as roadway bridges. A new hybrid structure of stress-ribbon deck and truss has been proposed and was adopted in the construction of Nozomi Bridge in Japan, a roadway bridge opened to traffic in 2003. Static and dynamic behaviours of the hybrid structure have been studied analytically and experimentally. The results show that the hybrid bridge has advantages over the stress-ribbon deck bridge as it generates much less horizontal force in suspension cables and has higher flexural stiffness, suitable for use as a roadway bridge, and that the hybrid bridge has advantages over the truss bridge since it can be constructed without extensive falsework and without large erection equipment. In this paper, at first, the hybrid structure is described, then analytical and test results are used to show its static and dynamic characteristics, and finally the construction of Nozomi Bridge is outlined.
Written on 19 août 2011.
N. De Belie, Ghent University, BelgiumW. De Muynck, Ghent University, BelgiumW. Verstraete, Ghent University, BelgiumBiological techniques for cleaning and repair of concrete and stone can be an ecological alternative to traditional conservation techniques. Weathered concrete samples fouled by lichens were treated with Thiobacillus bacteria and an appropriate nutrient, by submersion or sprinkling. The general effect of cleaning was documented by the use of colorimetry and microscopy. For remediation of decayed concrete, biomineralisation of mortar samples of different porosity by ureolytic sludge was tested. For the most porous mortar samples and when urea, nutrient broth and an external calcium source were provided, the amount of water absorbed after 200 hours was decreased by a factor of five compared to untreated samples. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses revealed a dense layer of calcite and vaterite crystals.
Written on 19 août 2011.
S. Khalfallah, University of Jijel, Algeria An analytical model, which can simulate the cracking response of reinforced concrete tensioned members, is presented. This model is principally based on the bond stress-slip repartition function that was successfully applied to the analysis of tensile reinforced concrete members. The formalism of repartition functions of loads, strains and slip between steel and surrounding concrete, has allowed analysis of the cracking phenomenon and its influence on the structural behaviour of reinforced concrete members. The tension stiffening effect accompanying the cracking mechanism is taken into account in a more appropriate manner. The phenomenon of localisation of micro-cracking owing to the softening behaviour of concrete in tension is based on the conservative concept of the cracking fracture energy assuming that the band of strain localisation is an intrinsic parameter of the material. The predictive results show that the model is very satisfactory, allowing the evaluation of each material contribution especially in the cracked range. Moreover, a correlation between the obtained and experimental results is observed through the local and global responses.
Written on 19 août 2011.
T. Yoshioka, Oriental Construction, JapanT. Mori, P.S. Mitsubishi Construction, JapanA. Kasuga, Sumitomo Mitsui Construction, JapanT. Miyagawa, Kyoto University, JapanWith a theme of "Concrete Structures in the 21st Century", the First fib Congress was successfully held in Osaka, Japan. Organised jointly by the Japan Prestressed Concrete Engineering Association and the Japan Concrete Institute, the Congress ran from 13th October to 19th October 2002. This report introduces the Congress and confirms that it has enabled us to envisage the prospects for concrete structures in the 21st century.
Written on 19 août 2011.
A. Kasuga, Sumitomo Mitsui Construction Co., Ltd., Tokyo, JapanExtradosed bridges are similar to cable-stayed bridges in that stay cables are used for strengthening. The concept of extradosed bridges has been taken up in Japan, where several bridges of this type have now been constructed. This paper shows five extradosed bridges that were designed and built by the author, and explicates the difference between extradosed and cable-stayed bridges in terms of structural aspects. In addition, the method of design for stay cables in Japanese specifications is introduced.
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. 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. Proposals for the design bond stress code clauses have been presented in an earlier paper. This paper covers proposals for the design lengths and tension stiffening code clauses.
