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Structural Concrete, Vol. 7, no. 4, December 2006

Determining the coefficient of concrete strength variation during non-destructive testing

V. Klevtsov, State Research Institute for Concrete, Moscow, Russia 
M. Korevitskaya, State Research Institute for Concrete, Moscow, Russia

All 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.

Structural Concrete, Vol. 7, no. 4, December 2006

Calculation of the coefficients of oxygen permeability of mortar samples using PORECOR analysis

N. Shafiq, University of Technology Petronas, Perak Darul Ridzuan, Malaysia
J. G. Cabrera, University of Leeds, UK

It 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.

Structural Concrete, Vol. 7, no. 3, September 2006

Limit states of cracking in beam-and-block floor systems using pretensioned ribs

R. S. Camposinhos, Instituto Politecnico do Porto, Portugal
A. Serra Neves, University of Porto, Portugal

Beam-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. 

Structural Concrete, Vol. 7, no. 4, December 2006

Crack opening near reinforcement bars in concrete structures

K. Tammo, Lund Institute of Technology, Sweden
S. Thelandersson, Lund Institute of Technology, Sweden

Current 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.

Structural Concrete, Vol. 7, no. 4, December 2006

Nozomi Bridge - a hybrid structure of stress-ribbon deck and truss

N. Ogawa, Chubu Regional Bureau, Ministry of Land, Infrastructure and Transport, Japan
Y. Kamiya, Oriental Construction Co., Ltd, Japan
T. Yoshikawa, Oriental Construction Co., Ltd, Japan
G. Yu, Oriental Construction Co., Ltd, Japan
M. Tsunomoto, Oriental Construction Co., Ltd, Japan

It 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.

 

Structural Concrete, Vol. 7, no. 3, September 2006

Cracking analysis of reinforced concrete tensioned members

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. 

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