Long-term behaviour and ductility analysis of standard and fibre-reinforced concrete beams

Long-term concrete behaviour are two subjects of current interest in many studies published over the last decade. This thesis project is dedicated to the study of both the above mentioned topics, developing a comprehensive work that can further bring a higher degree of knowledge of the subject and the state of the art today. The first part of the work is devoted to the study of creep phenomena on reinforced concrete beams, creating a model that describes the real behavior of the structure, evaluating strains and long-term deformations, which cause phenomena of stress redistribution and delayed breakage over time. In this regard, a numerical model will be presented to evaluate the effect of the phenomena of increased deformations linked to time, such as linear creep, non-linear creep and shrinkage. This model was then applied to a case study, which allowed the evaluation of the degree of reliability of the proposed model. The second part of the work, instead, is dedicated to the study of the phenomenon of ductile breakage of beams subjected to bending. In particular the study concerned the influence of metal fibers on the degree of ductility of the structure. A test campaign was carried out, which made it possible, through an analysis of the results obtained, to assess the effect of the fibres on the breaking of the compressed area of the beams. The laboratory tests on fibre-reinforced concrete beams conducted as part of this study have shown that the presence of metal fibres within the concrete matrix increases the degree of ductility of the structure and also the displacement capacity after breakage.

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.

EPFL2011

Modelling shear–flexure interaction in equivalent frame models of slender reinforced concrete walls

Katrin Beyer, Panagiotis Mergos

Quasi-static cyclic tests on reinforced concrete (RC) walls have shown that shear deformations can constitute a significant ratio of the total deformations when the wall is loaded beyond the elastic regime. For slender RC walls that form a stable flexural mechanism the ratio of shear to flexural deformations remains approximately constant over the entire range of imposed displacement ductilities. This paper proposes a method for incorporating shear-flexure interaction effects in equivalent frame models of slender RC walls by coupling the shear force-shear strain relationship to the curvature and axial strain in the member. The suggested methodology is incorporated in a finite element consisting of two interacting spread inelasticity sub-elements representing flexural and shear response, respectively. The element is implemented in the general finite element code IDARC and validated against experimental results of RC cantilever walls. In a second step, it is applied in inelastic static and dynamic analyses of tall wall and wall-frame systems. It is shown that ignoring shear-flexure interaction may lead to erroneous predictions in particular of local ductility and storey drift demands.

2014

Effect of Deformation History on Punching Resistance of Reinforced Concrete Slabs

Robert Koppitz

Deformation-dependent punching shear resistance often constitutes the decisive design criterion for reinforced concrete slabs supported by columns. The increasing number of aging structures exhibiting insufficient punching resistance and detailing deficiencies, and undergoing changes of usage has resulted in a growing demand for strengthening against brittle punching failure. Hence, the research presented in this thesis aims at gaining a better understanding of the identified problems occurring especially in existing structures, such as pre-deformation. A further objective is the experimental and theoretical validation of the performance of a new strengthening concept. A detailed analysis is carried out of the structural behavior of centrically supported rotation-symmetric slabs subjected to punching. In the first part of this research project a literature review is conducted to characterize the available punching models for new slabs and discuss to what extent they are able to consider the problems linked with existing slabs. Benefits and limitations of current strengthening solutions are illustrated, suggesting local prestressing as a promising concept. In the second part such a concept is analyzed that has been developed to improve the system efficiency and punching resistance by immediately activating the post-installed shear reinforcement composed of carbon fiber-reinforced polymer (CFRP) composites, thus partially unloading the slab. An experimental campaign comprising sixteen slabs verifies that the installation of this strengthening concept leads to a more ductile system behavior and significant increase of punching resistance compared to a non-strengthened slab. In the third part the load-deformation behavior of slabs is analyzed and a modification of an analytical model applied to a slab sector is developed, considering the influence of shear on flexural behavior. An overestimation of the flexural capacity and consequently of the punching resistance is thus avoided. The agreement of the modified sector model results with experimental results is confirmed for a large number of experiments from literature. The model is also capable of predicting the load-rotation responses and punching resistances of slabs strengthened with prestressed CFRP straps. The fourth part of the research concerns the effect of pre-deformation, or load history, on the structural behavior of uniaxial members and biaxial slabs. Single unloading and reloading cycles, which reduce tension stiffening and result in additional deformations, leading to decreased concrete punching resistance, are investigated. In slabs prestressed CFRP straps can compensate the additional deformations caused by load history.