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A ductile, high energy absorptive and rapid post-tensioning system for extending life of concrete structures

Periodic Reporting for period 1 - SMArtPlate (A ductile, high energy absorptive and rapid post-tensioning system for extending life of concrete structures)

Reporting period: 2017-12-01 to 2019-11-30

Extending infrastructure service life rather than demolishing and reconstructing, contributes significantly to the sustainability of the built environment, one of the major objectives of the EU long-term strategy. In fact, minimising the reconstruction of structures reduces the consumption of cement and natural resources, hence lowering the greenhouse gas emissions and enhancing resource efficiency. Not only does it reduce expenditure, but it also decreases disruption to the function of important structures, such as bridges.
External post-tensioning is a retrofitting technique often used to enhance the serviceability performance of Reinforced Concrete (RC) structures through reducing deformation and increasing stiffness as well as improving durability.
Fixing prestressed Fibre Reinforced Polymer (FRP) plates to the structural element and mobilising their prestressing force to the structure is one of the most popular post-tensioning techniques. In this technique the tensile force generated by a hydraulic jack - secured to the structure at one end and to the FRP bar/laminate at the other end - is used to prestress the FRP. A high concentrated force at the jack-to-structure connection risks the local failure in concrete, which is a practical concern especially in post-tensioning of aged and deteriorated structures. Moreover, in such cases, the high FRP strength cannot be fully exploited since the premature concrete cover detachment will often be the prevailing failure mode. Temperature sensitivity and low ductility of FRP systems are other concerns.
The application of Shape Memory Alloys (SMA) for post-tensioning of structures is a recent promising development. SMA possesses a unique feature that enables it to recover its original shape by increasing its temperature once deformed plastically. The electrical resistivity of the alloy can be used to increase its temperature by passing an electrical current through it. Shape recovery under a restraint condition generates sufficiently large forces in the SMA element, suitable for post-tensioning applications. The emergence of the cost-competitive Iron-based Shape Memory Alloys (Fe-SMA) in recent years is a promising development that broadens the application of SMA’s in structural strengthening practices.
In this project, an innovative rapid post-tensioning retrofitting system is conceptualised, and its feasibility and performance efficiency are validated by employing complex experimental and numerical studies. This innovative system, designated “SMArtPlate”, is a thin prefabricated plate made of an Ultra-Ductile Mortar reinforced by SMA rebars. The high bearing capacity of the Ultra-Ductile Mortar facilitates the use of mechanical fasteners to attach SMArtPlate to the RC structure while its self-controlled crack width ensures an enhanced durability performance compared to the existing techniques.
Three different types of Ultra-Ductile Mortar, namely Engineered Geopolymer Composite (EGC), Engineered Cementitious Composite (ECC) and Ultra High-Performance Fibre Reinforced Concrete (UHPFRC), were developed. Using tensile and compression tests, the mechanical properties of these Ultra-Ductile Mortars were fully characterised. Due to the inferior performance of EGC in comparison with the other two composites, only ECC and UHPFRC were selected for the development of the SMArtPlate.
The thermomechanical behaviour of a Nickel-Titanium-Niobium (NiTiNb) wire and an Iron-based Shape Memory Alloy (Fe-SMA) rebar were characterised, and their full tensile behaviour before and after heat-activation was obtained.
A matrix of pull-out test specimens was prepared using each type of SMA’s and Ultra-Ductile Mortars, and bond-slip tests were carried out. While the Fe-SMA rebar presented sufficient bond to ECC and UHPFRC, the bond strength of NiTiNb wire was not adequate for the development of the SMArtPlate.
The results of heat-activation tests revealed that a much higher electrical power is required to heat-activate an Fe-SMA rebar embedded into a UHPFRC plate than into an SHCC plate. Also lighter in weight, the SHCC was finally selected to develop the SMArtPlate.
The second series of pull-out specimens - the Fe-SMA rebar embedded in thin SHCC plate - was prepared, and their heat-activated constrained stress-recovery and post-activation bond performance were studied.
The test results were fed into a Finite Element (FE) model which was validated against experimental results of RC beams strengthened with Hybrid Composite Plate (HCP). The FE study was focused on the behaviour of RC beams strengthened with SMArtPlate. The connection of the plate to the beam was the main parameter of the study. The results of this study confirm not only the feasibility but also the excellent performance of the SMArtPlate as a rapid post-tensioning solution for RC beams.
A rapid post-tensioning system, SMartPlate, was conceptualised, and its excellent performance was validated by employing complex experimental and numerical studies. The project’s outcome provides researchers, engineers, and contractors with an innovative and robust retrofitting solution. SMArtPlate utilises the synergistic advantages of Ultra-Ductile Fibre Reinforced Mortars and Shape Memory Alloys for retrofitting applications, a progress beyond the state-of-the-art of the existing techniques. The numerical models developed in this project further advance the use of computational mechanics in the field of concrete and composite structures.
While applicable to a variety of RC structures, its rapid application along with its high durability and connection reliability, makes SMartPlate a unique solution for post-tensioning of RC bridge structures, offering a notable socio-economic benefit by minimizing disruption in traffic flow.