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Structural Concrete, Vol. 4, no. 4, December 2003

Size-effect experiments on concrete in compression

Stefan L. Burtscher, University of Technology, Vienna
Johann Kollegger, University of Technology, Vienna

It is well known from literature that heterogeneous granular materials exhibit a size effect under tensile loading. Thus, the strength determined in experiments is not a material property. Some experiments have been performed regarding the size effect in tension whereas very few experiments have been performed in compression. However, compressive loading of concrete is more important because concrete is applied in structural systems to carry load in compression and not in tension. Furthermore, a compressive failure in a load-carrying member is more brittle, and in most cases more dangerous, than a tensile failure. Experimental investigations on the size effect are therefore needed. Experiments from literature on column-like specimens under compressive loading were performed up to a size range of 1:4. The size range of a series of geometrically equal specimens is given by the amplification factor of a characteristic dimension from the smallest to the largest specimen size. Obviously large size ranges are advantageous for experimental studies on size effect. The largest size range (1:16) of experiments on concrete under compressive loading carried out so far will be presented in this paper. This series was performed in one of the largest testing facilities available. A size effect on nominal strength was detected. The results are compared with the two most common size-effect laws.

Structural Concrete, Vol. 4, no. 3, September 2003

Flexural ductility of high-strength concrete beams

L. F. A. Bernardo, Lecturer, University of Beira Interior, Covilhã, Portugal
S. M. R. Lopes, Assistant Professor, FCTUC, University of Coimbra, Portugal

This paper describes an experimental study on the flexural ductility of high-strength concrete beams. Nineteen isostatic beams, simply supported, were tested in the course of this work. Two symmetrical concentrated loads were applied to the beams at intervals of approximately one-third of the span. Ductility was studied by defining certain parameters, which have been termed ductility indexes. The main variables in this study are the compressive strength of the concrete and the longitudinal tensile reinforcement ratio. The results of the tests are examined and discussed, and some conclusions are drawn. The test results were compared with recommendations suggested by codes of practice and again conclusions are drawn.

Structural Concrete, Vol. 4, no. 3, September 2003

Ultimate strength of damaged post-tensioning tendons

Eva M. Eichinger, Vienna University of Technology, Institute for Structural Concrete, Vienna, Austria
Thomas Petraschek, Vienna University of Technology, Institute for Structural Concrete, Vienna, Austria
Johann Kollegger, Vienna University of Technology, Institute for Structural Concrete, Vienna, Austria

In order to assess the influence of the bond action between wires, injection grout and the surrounding concrete on the ultimate load of damaged post-tensioning tendons more accurately, a series of tensile tests on tendons, which were built using old wires from a demolished bridge, was carried out. Wire breakage was included in the tendons at specified patterns. The results of the test series show that tendons with damaged or broken wires are able to carry a very high percentage of the ultimate load of an undamaged tendon. Even in situations where no confinement of stirrups in the surrounding concrete is present, the load-carrying capacity of tendons with broken wires is excellent. If the distance between the wire breakages is about half the anchorage length, the decrease in strength is about 20%. The results of these tests will be used to assess the tendon strength in comparable bridge structures where damage to the tendon is likely to be present.

Structural Concrete, Vol. 4, no. 3, September 2003

The vibration resistance of young and early-age concrete

Anders Ansell, Postdoctoral fellow, Department of Civil and Architectural Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden
Johan Silfwerbrand, Director, Swedish Cement and Concrete Research Institute (CBI), Stockholm, Sweden

During early age, concrete is vulnerable to disturbance from vibrations of large magnitudes. Today, conservative vibration limits are used as standards, and guidelines provide little information. The literature cited in this study contains experiences and results from the construction and civil engineering field, in-situ testing, laboratory testing and computer modelling. On the basis of the reviewed literature, recommended maximum vibration levels for young and early-age concrete are given.

Structural Concrete, Vol. 4, no. 3, September 2003

Investigation of temperature and strain distribution in reinforced-concrete wall of a rapeseed storage silo

K. Diamoutene, Wroclaw University of Technology, Poland
M. Kaminski, Wroclaw University of Technology, Poland

The results of experimental research carried out on reinforced-concrete storage silos and of a numerical analysis of the temperature distribution in the silo wall are presented. Temperature fields in the structure were determined and the finite element method was applied to analyse the forces and moments generated by the thermal fields.

Structural Concrete, Vol. 4, no. 2, June 2003

Theoretical model for the determination of plastic rotation capacity in reinforced concrete beams

R. N. F. do Carmo, Department of Civil Engineering, ISEC-Polytechnic Institute of Coimbra Portugal
S. M. R. Lopes, Department of Civil Engineering, FCTUC-Polo II-University of Coimbra Portugal
L. F A. Bernardo, Department of Civil Engineering, University of Beira Interior Portugal

Evaluating the ducility of reinforced concrete beams is essential in order to avoid a fragile collapse of the structure by ensuring an adequate deformation at ultimate load. This paper presents a theoretical model for the calculation of plastic rotation. Results obtained are compared with those obtained from an experimental programme in which a set of beams were tested to failure. From the comparison, a good agreement between theoretical and experimental results was achieved. The proposed theoretical model considers the influence of certain factors: steel properties, concrete strength, section depth, and the tension stiffening effect. Concrete strength, particularly, is an interesting parameter since for high-strength concrete, the ultimate concrete strain, εcu, decreases as the concrete strength increases.

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