Michael D. Kotsovos

Strain-softening of concrete in uniaxial compression

0025-5432/97

STRENGTH AND DEFORMATION CHARACTERISTICS OF REINFORCED CONCRETE WALLS UNDER LOAD REVERSALS

The effects of loding history and repair methods on the structural characteristics of reinforced concrete walls was investigated. Large scale slender wall models were tested to failure, then unloaded, repaired, and retested to destruction under various regimes of cyclic horizontal loading. It was found that, while repairing only the damaged regions of the compressive zone was sufficient to fully restore wall strength, the additional use of epoxy resins to heal major flexural and inclined web cracks led only to a marginal improvement of the structural characteristics, the latter being distinctly inferior to those of the original walls. Such results are in compliance with the concept of the compressive force path and demonstrate that, in contrast to widely held views, the compressive zone is the main contributor to shear resistance.

Modelling of crack closure for finite-element analysis of structural concrete

Abstract An existing finite element (FE) model of structural concrete is extended so as to also allow for crack closure in localized regions of a structure. The model is used to study the behaviour of structural concrete members under various types of loading, encompassing both proportional and sequential loadings. The analysis is found to yield a close fit to experimental values, and to confirm the view that, while neglecting crack closure has a negligible effect on the predicted behaviour of structural concrete under proportional loading, sequential loading usually requires a proper allowance of crack closure if sensible analytical predictions are to be achieved. Moreover, the results support experimental findings which indicate that compliance with the earthquake-resistant design clauses of the Greek version of the new European code of practice may, in fact, cause, rather than safeguard against, brittle types of failure.

Shear-lag revisited: The use of single fourier series for determining the effective breadth in plated structures

Abstract The problem of shear lag is tackled by means of a classical technique which, in its present form, appears to offer a simpler and more direct approach, yielding a closed-form solution for the longitudinal stress distribution in wide flanges of box girders and ensuing effective-breadth ratios. The method is particularly suited to parametric studies and a number of these are undertaken, both for purposes of computational illustration and so as to study cases of practical relevance. Some of the salient features in the latter are highlighted, especially as an aid to engineers at the preliminary design stage of plated structures.

Shear failure of reinforced concrete beams

Abstract It has been suggested in a previous work that the causes of shear failure exhibited by reinforced concrete (RC) beams are associated with the stress conditions in the region of the path along which the compressive force is transmitted from support to support. The work described in this paper presents experimental evidence supporting the above concept. The work is based on a comparative study of the behaviour of concrete beams reinforced in compliance with this concept and that of beams reinforced in compliance with current design procedures.

A mathematical model of the deformational behavior of concrete under generalised stress based on fundamental material properties

The paper presents a description of the effect of internal fracture processes on the deformational behaviour of concrete under increasing load and introduces a mathematical model which describes the stress-strain relationship of the material under short-term generalised states of stress.RésuméLe comportement non linéaire du béton sous contrainte croissante est probablement déterminé par les processus de rupture interne qui prennent la forme d’une extension et d’une propagation des fissures en direction de la contrainte de compression principale maximale. L’extension et la propagation des fissures réduisent les fortes concentrations de contrainte de traction qui se présent en bout de fissure et déterminent la formation de vides à l’intérieur du matériau.En vue d’une description mathématique du comportement en déformation du béton, on a réalisé un modèle du matériau pourvu des propriétés de résistance du béton, et dont la rupture se produit selon des processus qualitativement similaires. Le comportement en déformation du modèle comprend les composantes suivantes:La composante A, définie par les propriétés mécaniques du modèle assimilées à un corps solide continu isotropique,La composante B, déterminée par la réduction des fortes concentrations de contraintes causées par les processus de rupture.La composante C, qui traduit l’effet de la formation de vides intervenant au cours des processus de rupture.La description mathématique des composantes A et B s’est appuyée sur une étude des résultats expérimentaux obtenus sur le béton mis en états de contrainte pluriaxiale alors que celle de la composante C a résulté de considérations théoriques sur les mécanismes de rupture du béton. L’étude des données a montré que les composantes A et B sont indépendantes de l’anisotropie «induite par détérioration» alors que la contrainte C traduit essentiellement l’effet sur la déformation de l’anisotropie «induite par détérioration».

Single Fourier series solutions for rectangular plates under in-plane forces, with particular reference to the basic problem of colinear compression. Part 1: Closed-form solution and convergence study

Abstract This paper considers certain types of classical plane-stress problems, concentrating on their solutions by means of the simple, although usually approximate, single Fourier series technique. While this approach is well known, no systematic studies based on it seem to have been carried out in the past. Such a study is presently undertaken with reference to the problem of a colinearly compressed rectangular plate so as to reach some conclusions regarding (ordinary) convergence rates for stresses, as well as to investigate possible ways of improving convergence. In addition, a truly closed-form solution for the single Fourier series technique is given (thus bypassing the need to repeatedly solve systems of equations) which encompasses completely arbitrary normal and/or shear loading along two opposite edges. In a follow-up paper (Tahan et al., Thin Walled Structures 17 (1) (1993)), the present findings will be used to conduct an investigation of the stress distribution in a plate subject to colinear compression .

Nonlinear finite-element analysis of concrete structures: Performance of a fully three-dimensional brittle model

Abstract This paper constitutes possibly the most thorough study of the generality of any finite-element model to date (whether two- or three-dimensional). The proposed model is three-dimensional (and hence fully triaxial) and brittle in nature, permitting the validation of certain general concepts regarding the failure of concrete in a structure. The wide-ranging applicability of the package is tested on the basis of eight case studies, some stemming from recently completed objectivity and preliminary generality studies (the results of which are presently summarized and, in some cases, expanded), others appearing as additional—and even more challenging—complex test cases.

Deformational behaviour of concrete specimens in uniaxial compression under different boundary conditions

This paper reports experimental data produced by a series of uniaxial compression tests on cylindrical specimens of concrete. The specimens were tested under four different boundary conditions, with their deformational behaviour carefully recorded to study the ensuing strain-softening behaviour.

Size effects in structural concrete: A numerical experiment

Abstract The paper forms part of a programme of investigation into the causes of size effects in structural-concrete behaviour. It is postulated that one of the causes of size effects is load-induced non-symmetrical cracking which is inherent in concrete due to its heterogeneous nature. The validity of this postulate is supported by the results—presently reported—of non-linear finite element analysis, in which the model used to describe cracking is capable of inducing non-symmetrical cracking in symmetrical structural members subjected to symmetrical loading. The results demonstrate that the package used can also provide a close fit to experimental values and, moreover, that the presence of stirrups (designed so as to prevent shear failure) is sufficient to diminish the effect of non-symmetrical cracking and hence eliminate size effects.