3.1. SHORT SUMMARY OF THE STATE OF THE ART
The use of SMAs for strengthening RC structures is a feasible technology to improve their behavior under different circumstances such as cyclic loads, large deformations and the repair of deteriorated structures, among many others. SMAs are materials with a relatively wide use in different fields such as aeronautical engineering and medicine. From a civil engineering point of view, they have been used in specific applications—at research level—
although their use is not yet widespread.
In order to be used in civil and structural engineering, their special shape memory effect (SME) and pseudo-elasticity properties need to be useful in the temperature range of civil engineering. Many SMAs present difficulties when applied in such a temperature range where SME is the required special feature. One SMA with applicability in this range of temperatures is Ni-Ti-Nb alloys.
Strengthening of RC structures has already been undertaken with SMAs in specific applications, mostly for confining purposes in columns. With regard to shear strength reinforcement, a binary alloy such as Ni-Ti has been internally used to reinforce concrete beams, but a ternary alloy such as Ni-Ti-Nb has not been used, to the best of the author’s knowledge, for external shear strengthening of critical RC members.
Chapter 3
3.2. SPECIFIC OBJECTIVES
The main objective of this Ph.D. thesis is to expand knowledge of repair and strengthening in RC structures using SMAs in structural engineering. Specifically, an active external shear strengthening method using Ni-Ti-Nb SMA wires as external reinforcement in shear critical RC beams will be developed.
The suitability of this Ni-Ti-Nb SMA with a wide thermal hysteretic behavior will be studied for generating recovery stresses within the typical temperature range for outdoor civil engineering structural applications.
The practical procedure of the application of this SMA as shear strengthening of RC beams in an experimental campaign, as well as the improvement of shear strength of the tested beams, will lead to the development of a new technology for shear strengthening of RC structures using SMAs. The specific objectives for this purpose are:
- Physical and thermo-mechanical characterization of the available SMA (Ni-Ti-Nb), by means of different experimental tests to determine its properties.
- Experimental application of external strengthening of RC beams without internal shear reinforcement by means of spirals, or other configurations, made from Ni-Ti-Nb wires.
- Characterization of the Ni-Ti-Nb recovery stresses under non-idealized conditions.
- Validation and extension, if needed, of previously developed mechanical models for the determination of the shear strength to take into account the external strengthening in shear.
3.3. METHODOLOGY
To achieve the proposed specific objectives, the following methodology has been applied:
- For the physical and thermo-mechanical characterization of the available SMA, the following laboratory tasks were carried out:
Research objectives
• Determination of composition of the supplied material: 3 mm diameter Ni-Ti-Nb wires by means of Dispersive Energy X-ray Spectroscopy (EDX).
• Thermal characterization of Ni-Ti-Nb. Determination of characteristic transformation temperatures for direct and reverse phase transformation by means of Differential Scanning Calorimetry (DSC) and resistivity tests.
• Mechanical characterization of the Ni-Ti-Nb wires in martensite and austenite phases by means of tensile tests at different ambient temperatures.
• Thermo-mechanical characterization of Ni-Ti-Nb wires by means of determination of recovery stresses during activation (reverse transformation) with constrained strains.
- For the experimental application of external strengthening of RC beams without internal shear reinforcement by means of spirals of Ni-Ti-Nb wires to improve their strength, the following laboratory tasks were carried out:
• Planning an experimental campaign of small-scale tests of RC beams with an external reinforcement of Ni-Ti-Nb wires: design of test specimens and concrete properties determination campaign, design of specific shear strengthening elements, design of test instrumentation and monitoring.
• Fabrication of tests specimens, strengthening them and installing the instrumentation to carry out the experimental tests.
• Conducting of the experimental tests.
• Analysis of the behavior of the strengthened beams.
- For the characterization of the Ni-Ti-Nb recovery stresses under non-idealized conditions, a new campaign of laboratory tests was carried out, including:
• Additional recovery stress tests on Ni-Ti-Nb wires with activation by means of a heat gun.
• Additional recovery stress tests on Ni-Ti-Nb wires with different initial imperfection on the sample.
• Actual recovery stress determination on the wire links installed on the beams.
Chapter 3
- For the validation and extension, if needed, of a previously developed mechanical model to take into account the external strengthening using SMAs the following analytical tasks were carried out:
• Prediction of the shear strength of the tested beams using two different shear models: The Compression Chord Capacity Model (CCCM) and the shear design equations included in Eurocode-2.
• Comparison of the predictions by the applied shear models and the experiment results to assess the performance and applicability of the models.
Characterization of Ni-Ti-Nb wires