1.1 RESEARCH MOTIVATION
Strengthening of reinforced concrete (RC) members in structural engineering is a methodology to address deficiencies from several causes, e.g. design mistakes, changes in the use of a structure, repairing damaged structures or new code requirements, among many others. These deficiencies could lead to shear failures of RC members (a type of failure associated with brittle collapses) which could cause sudden material and human losses. It is therefore clearly necessary that this type of failure be avoided and, for this reason, shear strengthening of existing structures is sometimes required.
Strengthening technologies for critical shear beams may be classified into two categories:
passive strengthening and active strengthening methods. In both cases, the strengthening increases structure safety (by means of increasing strength), but when using passive strengthening methods, e.g. common strengthening with fiber reinforced polymers (FRP), it is necessary for the strengthened structure to increase its deformation and level of damage before engaging the strengthening material. Alternatively, before undertaking strengthening, the structure should be partially, or totally, unloaded. This way the strengthening material could contribute as soon as the structure is reloaded. When using active strengthening methods, the structure is prestressed, or actively confined when the strengthening material
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is correctly installed. However, active strengthening methods generally require hydraulic jacks and anchorages, so it is often necessary to maintain a large work area to accommodate these auxiliary elements. These drawbacks may be overcome using a material that does not require prestressing elements to be activated. One of these is shape memory alloys (SMAs) that only require a simple action such as raising the temperature to be activated.
SMAs are materials that have the ability to achieve high deformations and to recover such deformations after unloading or upon heating. These properties of pseudo-elasticity and shape memory effect are useful for structural engineering. The shape memory effect is the property by which the material, after being subjected to a process of loading and unloading with apparently “permanent” deformations, can return to its previous form by raising its temperature to a certain level. This effect is the result of the reversible phase transformation that SMAs undergo, known as martensitic transformation, involving austenite and martensite solid phases of SMAs and it can be produced by changes in temperature or by the action of stresses for the envisaged application. Alloys with this shape memory effect include Ni-Ti binary alloys.
One of the drawbacks of these materials is that the phase transformations take place at typical service temperatures in civil engineering structures and this makes them inappropriate for proper performance. Nevertheless, there are SMAs, such as Ni-Ti-Nb, that may be stable in this range of temperatures. Its development as a strengthening material would be valuable and its application in civil engineering and building structures be useful.
This Ph.D. thesis has been developed with the framework of the following projects:
“BIA2015-64672-C4-3-R: Development of strengthening techniques with advanced materials for concrete structures and their mechanical behavior models to extend their lifetime”, co-funded by the Agencia Estatal de Investigación (Spanish Government Research Agency) and the European Regional Development Fund (ERDF), and “BIA2012-31432:
Smart materials in structural concrete. Application of Shape Memory Alloys as shear reinforcement in lineal members” co-funded by the Ministerio de Economía y Competitividad (Ministry of Finance and Competitiveness – MINECO) and ERDF.
Introduction
1.2 RESEARCH SCOPE AND SIGNIFICANCE
The main objective of this Ph.D. thesis is to develop a new technology for shear strengthening of RC members by means of shape memory alloys. Specifically, rectangular RC beams have been strengthened using pseudo-rectangular spirals of Ni-Ti-Nb wires. The proposed technology uses the shape memory effect to actively confine or prestress the strengthened concrete member, meaning the strengthening material immediately begins to actively work upon installation and activation. The experimental results show a promising performance of the proposed technology, successfully increasing the shear strength and deflections of the retrofitted beams measured at failure.
The research significance of the work developed in the thesis is it being the first reported practical application of an SMA for shear strengthening of RC members using Ni-Ti-Nb wires. No hydraulic jack has been used to develop the prestraining forces. The specific characteristics of this alloy make it stable at service temperatures of civil engineering. The development of a shear strengthening technology with this SMA will be the main contribution of this research.
1.3 OUTLINE AND CONTENTS OF THE THESIS
This document is organized in eight chapters. After this first introductory chapter, the second looks at a state-of-the-art analysis from two different perspectives: shear strength of RC members and shape memory alloys applied to structural engineering. The third chapter presents the specific objectives of the thesis in each phase of the planned work and in each knowledge field.
Chapter four presents the experimental campaign for the thermo-mechanical characterization of the SMA used (Ni-Ti-Nb wires of 3 mm diameter). Different mechanical and thermal properties were tested in a load frame with a thermal chamber: tensile tests in austenite and martensite, recovery stress tests, material composition determination and tests to determine phase transformation temperatures were carried out. The laboratory tests were carried out in different loading conditions (monotonic, cyclic) and at different temperatures to verify the material behavior in the typical temperature range of structural engineering.
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The results of the tests are set out and analyzed. The conclusions of the thermo-mechanical properties of the SMA are stated.
Chapter five presents the experimental campaign for active shear strengthening of small-scale RC beams using the previously characterized Ni-Ti-Nb wires. This experimental campaign had different stages: the design and casting of the beams; the design and fabrication of different shear strengthening elements from the Ni-Ti-Nb wires; the instrumentation of the beams and SMA elements to acquire data from tests, and the performance of the tests in a load frame in two different phases. The experimental results of these tests are also presented, and the acquired data analyzed. The conclusions of this work are also presented.
Chapter six includes another experimental campaign planned and performed to study the behavior of the Ni-Ti-Nb wires under actual non-idealized conditions of the wires after installation around the RC beams. Additional recovery stress tests were carried out with activation similar to on-site conditions (heat gun) instead of inside a thermal chamber.
Moreover, further tests were carried out to study the effect of initial imperfections of the placement of the wires around the RC beams (non-perfectly straight wires) in the generation of recovery stresses. The results of the tests are set out and analyzed. The conclusions of the thermo-mechanical properties of the SMA under actual non-idealized conditions are stated.
Chapter seven compares the strengthened RC beams experiment results to the predictions from two different shear models: The Compression Chord Capacity Model (CCCM) and the shear equations included in Eurocode 2 (a widely used model in Europe). The specific behavior of SMAs (developing recovery stresses when restrained during the activation process) is taken into account in the models as a stress in the transverse reinforcement or yield strength in the design of the beams. The agreement and differences between the predictions and experiment results are analyzed. The conclusions regarding to the validation of those two existing shear design models are presented.
Chapter eight presents the overall conclusions of the work performed in the thesis and some future research lines are indicated.
References of all cited works and an appendix of the test results for the beam experimental campaign and main model calculations are included in the final sections.
State of the art