Smart or intelligent materials are described as materials that can achieve a controlled, predictable, variation in one or more of their properties as a direct response to an external stimuli and/or a change in their environment. The external changes can be fluctuations in temperature, light, pressure, magnetic or electrical fields, surrounding moisture, or chemicals. Not all smart materials are the same. Some can change their properties due to more than one of the external stimuli at the same time, and some materials offer greater adaptability due to certain external changes than others. However, a common theme is that the changes are reversible, which make these kinds of materials particularly useful.

Given their ability to adapt their properties, smart materials have a wide range of applications not only in technology, manufacturing, science, and medicine, but more specifically, in civil engineering. Whether they are incorporated as part of concrete, plastic, glass or alloys, these materials can be used to create efficiencies in the construction process, increase the lifetime of buildings or structures and enhance their performance over time.

Smart construction materials: Why do we need them?

From the perspective of combating climate change, there is a focus on how buildings and structures (and construction more broadly) impacts sustainability and our environment. This isn’t just about how much carbon dioxide is emitted during the construction process, but how energy efficient a building or structure really is during its lifetime. Materials play a large role in how a building or structure is built, how it performs and how energy efficient it is; and the smarter the material the bigger the positive impact on those metrics.

Natural and man-made materials used during the traditional construction process (concrete, wood, glass etc) can be enhanced, made smarter and more responsive to external changes, so that they can contribute to a more sustainable structure. There are huge benefits in refining efficiencies, improving performances, saving resources and long-term maintenance when smart materials are used.

The technologies behind the materials

There are several technologies behind new smart construction materials. Innovation, research, and development in materials engineering are bringing to market new products that are changing the different components within buildings and other engineering infrastructure projects. Here we look at three materials impacting the industry:

• Shape Memory Alloys: As the name suggests, these materials can change or deform their shape (when stress is applied) and then return to their original shape (when heat is applied). The superelasticity properties can be particularly useful to improve the structural behaviour of bridges and buildings after an impactful event like an earthquake.
   
• Piezoelectric Materials: These can generate electrical energy across their surfaces as a response to mechanical stress. The process is reversible, meaning that they can generate stress or a mechanical response when an electric charge is applied through it and this reversibility makes them very applicable in sensors, energy harvesting and actuators within various building systems.
   
• Electrochromic Materials: They have the ability to alter colour and transparency when an electric voltage is applied to it through a process called electrochromism. As well as being reversible, the changes can be instantaneous, making them a core component of smart glass in windows and building envelopes.

Examples of end-user applications

Smart Concrete: Self-sensing and self-healing

After water, concrete is the most widely used product in construction. Whilst its use has brought about great benefits to structures and buildings world-wide, it’s production accounts for almost 8% of CO2 emissions. Therefore, anything that can help slow down its usage and increase its time in-situ can only be good for the environment.

Cracks and changes in stress within set concrete can and do occur, and over the long term can allow water, chemicals, or other external substances to get inside or bring about other impairments. Undetected or unrectified, this can damage the concrete and the structure itself, making it weaker and increasing the need for maintenance or replacement – increasing the cost and reducing the life-span as a consequence. Smart concrete is an overarching term covering concrete products that have a specific ability; among the most common are self-sensing and self-healing. They can be used to monitor the health of a structure, make changes to the concrete’s composition and take action to rectify the issue.

Self-sensing concrete, as the name suggests, has the ability to monitor its own condition and the stress levels within the structure of which it is part. It is made by adding microscopic carbon fibers and silica fume admixtures to a standard concrete mix to produce concrete that can conduct electricity, changing resistance values (of the electric charge) when damaged. It can be used to monitor vibrations on a structure, replacing vibration sensors which are often used in high-rise buildings, bridges, runways, dams, and other structures. The concrete itself becomes the sensor and is able to detect cracks and other damage caused by high winds, humidity, temperature changes and other environmental conditions.

Self-healing concrete simply has the ability to repair cracks on the concrete face autonomously. It is made by adding a healing agent (like a bacteria and calcium lactate) to a standard concrete mix. The agents are dormant for the most part and only become activated when water is introduced through the cracks themselves. This then enables the mixing of the bacteria and the calcium lactate to produce limestone which seals the crack. Another method is to use micro-capsules containing a sodium silicate healing agent within the concrete mix. When cracks appear, the capsules break up, releasing the chemicals which, when reacted with the calcium hydroxide in the concrete, forms a gel which repairs the cracks.

Both the self-sensing and self-healing concretes have proven themselves in the labs and are being developed for mass, real-life construction usage. These smart materials will play an important role in ensuring structures have increased life-span and durability, which in turn will reduce costs and improve maintainability.

Smart Windows: Switchable, Unbreakable and Warming

Glass is another material that has undergone some major improvements recently which will highly impact its usage and reliability in the long run. For centuries, glass has been used to make windows which allow light and heat into buildings, reduce sound entering or exiting, and provide an overall aesthetically pleasing look. Whilst the shape and architecture of the window itself may have changed over the centuries, the glass component has largely been unchanged until recent technological advances.

One such technology is electrochromics which enables windows to change how much light passes (automatically or manually) through glass by changing the voltage running over it. This switch from transparent to opaque also means that energy usage can be optimised during the day and controlled during the seasons. These smart windows are made up of several layers including an electrochromic layer, ion conductor and ion storage layers, and two thin electrical contacts (electrodes) with a separator in the middle.  When a small voltage is applied to the electrodes, the ions move from one electrode, through the separator to the other electrode. This makes the glass opaque until the voltage is reversed (so the ion goes back to the original electrode) and the glass turns transparent again. This ability clearly has a lot of applications where the need to change daylight (and heat) entering the building is important – schools, hospitals, and offices for example. The control it brings with it could potentially reduce lighting, heating and cooling costs of running these buildings.

Structural glass is another component of a home or office which has changed how the building is occupied, what energy usage it has and how aesthetically pleasing it is. While traditionally, glass panes were incorporated within (load-bearing) steel or timber window frames, their use in modern construction could involve a large piece of glass that is frameless and forms part of the load-bearing element of the structure; a wall, floor, column or roof.

The structural glass itself is much thicker and stronger compared to normal framed window glass, and is made using toughened or laminated glass. With no need for framing, the flexibility that structural glass provides engineers designing residential and commercial offices offers an opportunity to innovate with the structure itself, and its use. Combining structural glass with thermal heating glass and energy saving low-e glass coating also mean that, although sunlight can pour into the building from all angles, it doesn’t become a greenhouse. Modern innovations in glass technology mean that it has become more than an element of a framed window. It can provide changes to the structural make-up of a building and the energy levels used within – making glass a much more versatile smart material.

The future of smart materials in construction

As we have seen, there is now a range of new materials being used in construction which were not viable even 10 years ago.  Looking 10 years hence, engineers will have to respond to the growing need to have materials which can be used to reduce energy waste, be produced economically and have high global applications in buildings and other structures. Greater innovations in nanotechnology, graphene and bioplastics, to name but a few, will bring about new products which will enable civil engineering projects to be built more efficiently, maintained cost-effectively and become more sustainable in the long run.