Shape Memory Alloy retrofitting of columns for future resilient infrastructures

The Shape Memory Effect describes the capacity of some metallic alloys of recovering plastic deformations through a thermo-elastic transformation, i.e. by being heated above the austenitic finish transformation temperature. Nitinol based Shape Memory Alloys (SMA) that exploit this effect also exhibit high strength, high energy absorption capacity, high damping, good fatigue resistance, good corrosion resistance and excellent re-centring ability.
The SMA NiTiNb, a Nickel, Titanium and Niobium alloy, can recover above 8% strain and has a range of characteristic temperatures that make it suitable for applications in structural engineering, and in particular the one promoted here, i.e. enhancing the capacity of bridge support elements to resist impacts from road vehicles by applying active confinement to the member core.
At present passive damage mitigation techniques, such as concrete or steel jacketing and fibre reinforced polymer fabrics, are most commonly used to retrofit structural elements. Active confinement, however, which is the focus of this work, is much more effective in reducing damage since it acts at an earlier stage in the event, before any lateral loads are applied.

The aim of this research is to investigate the application of the shape memory alloy NiTiNb to retrofit reinforced concrete bridge columns. The novelty of this work lies in addressing columns subjected to a sudden impact by a heavy goods vehicle.
Road vehicle impacts are the second cause of bridge failures in the US and much more frequent than failures due to earthquakes. This study intends to model the mechanical response of bridge columns retrofitted with NiTiNb spirals and quantify their resilience in resisting lateral impact loads with respect to columns in the as-built condition.
The project is structured in three main tasks: the thermo-mechanical characterization of NiTiNb alloy; the development of a set of comparative numerical analyses, using a full scale LS-DYNA finite element model of RC columns hit by heavy goods vehicles before and after being retrofitted with NiTiNb spirals; and a small scale drop test used for validation purposes to give credibility to the FE model predictions.

The retrofitting technique simply consists in winding an alloy wire around a column, constraining the two ends by means of mechanical fasteners and heating the spiral above a characteristic temperature using electrical current or a gas torch.
The outcomes of this research will be immediately applicable to circular RC concrete columns supporting bridge decks, but also to a wide range of structures such as underground car parks, railway bridges and buildings located close to major roads or vulnerable to malicious vehicle impact. The project will contribute toward building resilience in critical national infrastructures and can be used to prevent vulnerability of strategic buildings.

Since the method relies on the concept of active confinement of concrete it would not only prevent the vulnerability of undamaged structural members but also restore the functionality of severely damaged columns. Moreover the technique is suitable also for emergency repair since it does not need mechanical pre-stressing of the wire and can be implemented with significant reduction of labour and time with respect to other retrofitting methods.

This project aims to develop a number of spin-offs in the construction industry and will impact a wide range of beneficiaries in the private and the public sectors from SMA suppliers to contractors, consultants and owners/operators involved in the Built Environment. However, the ultimate beneficiary of this project will be the road users in UK and abroad through the provision of safer, more durable and resilient transport infrastructures.

Recent applications for seismic retrofitting of RC members show that SMA-based active confinement possess superior mechanical performance, reduced installation and maintenance time and can be used for emergency repair. These mechanical and practical advantages together with good fatigue and corrosion resistance suggest that shape memory alloy applications could be effective and competitive for transport infrastructure operators (Highways England, Transport Scotland, Department of Economy and Transport in Wales, Roads Service Northern Ireland) as they would mitigate the consequences of vehicle impact and reduce their asset closure time and the hazardous exposure of their road workers for the necessary remediation work.

By extending the whole life cycle of existing infrastructures, this retrofitting application based on shape memory effect will also support economic growth and productivity and reduce consumption of traditional raw materials such as concrete and reinforcing steel with high environmental impact and industrial wastes.

The proposed study is aligned to the Call for research into "Identifying risk and building resilience into engineered systems", the forth of the 2015 Engineering Grand Challenges set by EPSRC. The project focuses on retrofitting of highway structures and bridge piers for resilience against impacts but the research outcomes will be applicable also to a wide range of structures such as underground car parks and buildings located close to major roads or vulnerable to malicious vehicle impacts.

Another long term expected impact is that on code recommendations and guidelines. The non-linear modelling of composite materials and structure-vehicle interaction is one of those cases where academia is called to lead practice and transfer its knowledge to industry through the development of design codes and standards. This research will provide analytical, numerical and experimental validation of a technologically advanced material in a novel application and will be particularly fitting for RILEM, the network of Laboratories and Experts in Construction Materials, Systems and Structures, which is committed to stimulating new orientations of research and applications and to prepare and widely disseminate guides to good practice, recommendations and pre-standards.

In the short term, direct pathways to industrial applications can also be expected thanks to a number of intermediary organisations such as the Building Research Establishment, (BRE), the Cambridge Centre for Smart Infrastructure, Innovate UK, etc. working in consultancy, research and testing services to support the infrastructure sector, as laid down in the impact plan.

On a more personal level the work provides the opportunity for the PI and PDRA to develop a number of technical skills and competencies, as well enhance their career development through access to the necessary 'soft' skills training opportunities provided by the host university. As part of the Pathways to Impact activities the PI will visit a leading researcher on the protection of bridges from terroristic attacks in the US and will discuss a joint application to US-Ireland grants, see Letter of Support.