Paradigms of shear in structural concrete: Theoretical and experimental investigation

Author: Alexander Beck
Language: English
external pageDOI: 10.3929/ethz-​b-000482684call_made

Abstract

This thesis aims to improve the comprehension of the load-deformation behaviour of reinforced concrete membrane elements with very low amounts of and without vertical reinforcement, subjected to in-plane shear and normal forces. The fundamental assumptions underlying most existing models for the shear behaviour of structural concrete – referred to as paradigms of shear – are identified and reviewed on the basis of a theoretical and experimental investigation. In addition, the cracked membrane model with fixed interlocked cracks (CMM-F) is implemented to support the study. The CMM-F is based on consistent mechanical principles and capable of predicting the load-deformation behaviour of membrane elements with very low amounts of and without transverse reinforcement.

The theoretical background of the shear behaviour of structural concrete is reviewed in the first part. Therein, the most relevant previous works including a discussion of models for members with and without shear reinforcement, selected recent publications, and the shear provisions of design codes are presented. Furthermore, the basic hypotheses – the paradigms – underlying most models for the shear behaviour, are ascertained. The identified paradigms are (i) the hypothesis of rotating stress-free cracks in limit analysis methods and compression field approaches, (ii) a pronounced effect of transverse tensile strains on the concrete compressive strength, known as compression softening, (iii) the dependence of the shear strength on the shear transfer across existing cracks, represented by aggregate interlock models, and (iv) the strict differentiation of models for members with and without transverse reinforcement.

In the second part, an experimental test campaign, conducted to gain more insight into the validity of the basic hypotheses, is presented. Six large-scale membrane elements with very low amounts of vertical reinforcement - three with ρz = 0.22 % and three with ρz = 0.14 % – and one uniaxially reinforced membrane element were tested in the newly developed Large Universal Shell Element Tester. Three specimens were subjected to pure in-plane shear and four specimens were tested under in-plane shear with imposed longitudinal strain, two with and two without gradient over the height. Cutting-edge measurement techniques, namely digital image correlation and fibre optic strain measurements, were applied. They yielded very valuable data of the full-field deformations and the crack patterns on the concrete surfaces, and of the continuous strains along the embedded reinforcing bars.

In the third part, the CMM-F is discussed in detail. The relevant material properties implemented in the model, i. e. the mechanical characterisation of plain concrete and reinforcing steel, and their interaction are outlined at the beginning. After some remarks on the cracking behaviour of membrane elements, the cracked behaviour and thus the model representation and its numerical implementation are presented. The model is validated against experimental results from the literature and compared with other models, e. g. the cracked membrane model with rotating, stress-free cracks.

In the fourth part, the CMM-F is validated against the results from the tests of this study. The correlation of the model with the experimental data yields conclusions as to which aggregate interlock model performs best in connection with the CMM-F and whether the correlation is consistent. The influence of the vertical reinforcement ratio and the imposed longitudinal strains are also examined. Furthermore, the cracking behaviour and the application of rotating compression fields to lightly reinforced membrane elements are investigated. Subsequently, the shear transfer across cracks, i. e. the aggregate interlock stresses are studied: they are calculated from the measured crack kinematics using three aggregate interlock models, and from equilibrium in free body diagrams using the measured reinforcement strains. The evaluated aggregate interlock stresses are compared with model predictions and reviewed by a sensitivity analysis. Thereby, the applicability and reliability of the available aggregate interlock models in large structural members are assessed. Finally, some remarks on compression softening are made.