Shape memory alloy reinforces building
A new shape-memory alloy that only requires one pre-stressing could improve civil engineering. Ceri Jones reports.
A new iron-based shape-memory alloy (SMA) called memory-steel is being produced on an industrial scale to support civil engineering applications. It is more cost-effective than previous SMAs based on expensive materials, such as nickel-titanium. The Fe-SMA is also highly functional, demonstrating high tensile strength, elastic stiffness and shape recovery stress.
The latter enables memory-steel’s use as a prestress material for reinforced concrete, when produced in strips and bars that can be affixed to a building to improve load bearing capacity.
A strip of the Fe-SMA can be heated to an ambient temperature, deformed and fixed in position. Then, when subjected to high heat for a short period the iron will contract, returning the strip to its original shape permanently, negating the need to prestress it – this would save a great deal of money and time, as well as the facilities required to carry out the work.
Traditionally, steel reinforcements need to be hydraulically prestressed – a labour and energy-intensive job requiring manipulation of the steel using ducts to guide tension cables, anchors for force transfer and oil-filled hydraulic jacks. This process takes place onsite, and it is of course very difficult to fit all of the necessary equipment into the building, let alone perform the work. Therefore, while conventional methods work for newbuilds, it is often too complex a process for existing structures.
But as memory-steel bars can be deformed and prestressed once in place, none of this space-hungry work is required. It has proven successful as external end-fixed reinforcements for new concrete structures, or retrofitted to existing ones without the level of invasive work traditionally required.
Proof of concept
The Swiss Federal Laboratories for Materials Science and Technology (Empa) with re-fer AG set out to test and prove the characteristics of SMAs and to develop a material suitable for the construction industry in a project lasting 15 years. They have now reported several successful trials.
When modifying an old building, for instance, taking down a section of wall to add a doorway or lift shaft, the load bearing capacity on the existing reinforcement is increased. Here, a series of memory-steel ribs can be deformed to the shape required, fixed to the wall or ceiling and then heated. A short period of high heat can be applied with an infrared radiator or by passing an electric current through the material.
This will prestress the memory-steel bar and permanently hold it in shape. It offers versatility, as rib-like strips it can be fixed in place with dowels, embedded in concrete, or even secured with a layer of shotcrete.
Bent out of shape
Memory-steel has undergone trials in thermo-mechanical modelling to help simulate construction, short and long-term behaviours of the material post-stressing and in recovery strain, use in shear strengthening of reinforced concrete structures, and mechanical modelling of concrete members embedded with it.
The team hopes that in the future it will be the standard method reinforcing structures with unknown geometry. Currently, hydraulic prestressing creates friction in curved structures, but memory-steel could avoid this as the material contracts more uniformly. When a series of rib bars are embedded into precast concrete, it would allow for better support and strengthening of curved structures without the friction losses.
The product is already being produced on an industrial scale, but next for Empa and re-fer AG is to improve the composition, which is currently Fe–17Mn–5Si–10Cr–4Ni–1(V,C) (mass%).
The team has its sights set on additional civil engineering projects, hoping to use this memory-steel method to manufacture precast concrete parts. The team says, ‘The hydraulic prestressing used up to now creates friction in curved structures, which greatly limits the use of this method. With a memory-steel profile embedded in concrete, highly curved constructions are now also possible – when heated, the profile contracts uniformly over its entire length without friction losses and transfers the stress to the concrete.’
The full paper, Iron-based shape memory alloy strips for strengthening RC members: Material behavior and characterization, was published in ScienceDirect by Moslem Shahverdi, Christoph Czaderski, Masoud Motavalli of Swiss Federal Laboratories for Materials Science and Technology (Empa), and Julien Michels of re-fer AG.