Around 70 per cent of the world’s population lives in a structure built with concrete - the composite is second only to water for use in construction, and its production is responsible for more than eight per cent of global carbon emissions.
This is why innovation in this space is critical and has led to the introduction of smart concrete.
From cement-based sensors that can detect cracks the naked eye can’t see, to a new form of sustainable concrete that heavily reduces fossil fuel use in cement production, many inroads are being made paving the way to a greener future.
Scientia associate professor Wengui Li, in the School of Civil and Environmental Engineering (UNSW) with expertise in construction materials and structural engineering, is one such researcher looking to conquer concrete.
His focus on concrete initially developed from an interest in seismic design, or helping buildings withstand earthquakes.
“I’ve found that maybe it's better to do some innovation on the material level, and then use the new material for the structures and improve the structure performance,” he said.
“I think in Australia, and over the world, more and more researchers are doing work in concrete materials.”
Currently Li’s team is testing cement-based sensors that can detect microscopic cracks in the hopes they can be repaired before getting worse.
The sensors, made of conductive materials, measure ‘piezoresistivity’, or a material’s change in electrical resistance when pressure or weight is applied. Hooked up to cables, the sensors send data that warn about cracks based on how dramatically the resistivity changes as power surges through the sensors.
Li said the sensors could assist in the maintenance of large apartment buildings.
A 2023 Strata Defects Survey Report published by the Office of the NSW Building Commissioner found 53 per cent of residential apartment buildings had a serious defect, and seven per cent of those defects were structural.
Li said the sensors are better than other measuring methods like strain gauges and optical fibres, which have been used since the 1930s.
“It offers advantages like high sensitivity. The cost is low, the durability is better, it means it can be used for a longer time,” he said.
“It's far better to act early on cracks in structures. Leaving it too late can lead to demolishing.”
Identifying cracks is one thing, but then the repair work begins.
Reinforced concrete is concrete with steel bars inside and is the foundation for most structures in use today. It can have a life span as short as 50 years before it starts to decay.
‘Concrete cancer’ happens when the steel inside reinforced concrete starts to rust. Concrete is porous, and moisture seeping in can cause the steel to expand, with the concrete cracking against the pressure.
Li and collaborating researchers have successfully added a crystalline mixture to concrete that has shown healing in hairline cracks after a fortnight.
He said using self-healing concrete not only means safer structures but “has an added effect of reducing emissions by cutting down on cement production”.
“If we can solve this problem, we can improve the durability and extend the service life of the concrete to more than 100 years,” Li said.
But there is an irony with his self-healing mixture: it must be exposed to water to work.
“It needs to be near moisture, otherwise the hydration reaction can't happen,” he said.
The main carbon emitter when it comes to concrete is the production of cement.
Cement’s key ingredient, clinker, is made by burning things like limestone and clay in extremely high temperatures.
Producing a tonne of cement creates nearly a tonne of CO2.
A typical three-bedroom family home requires about 14 tonnes of cement.
Geopolymer concrete does away with cement and instead uses byproducts from the production of coal and iron mixed with a corrosive liquid.
It’s been used by a major Australian construction company to build an airport and a wharf, but for now it’s a lot more expensive to produce than standard concrete.
“Cost must be measured against sustainability so other alternatives still need to be explored, such as calcined clay, volcanic ash, and other waste materials,” Li said.
“We consume about 1280 kilotons of glass packaging each year, of which only 59 per cent is recycled, with the rest going directly to landfills.
"To maximise the use of waste glass in concrete production, we have developed waste glass powder-based geopolymer concrete with a total glass content of over 80 per cent.”
Researchers in South Korea used concrete to clean up the air in a busy tunnel by coating the road in titanium dioxide and subjecting it to light exposure.
The process is called ‘photocatalysis’, where light is used to speed up a chemical reaction.
The experiment stopped pollution from vehicle exhausts forming into dangerous particles. Instead, they turned into salts that were washed away by rain. These salts can potentially be used for fertiliser.
This is a great result but, like other innovations mentioned, production at scale is expensive and different methods of combining the titanium dioxide with the concrete (like mixing them together instead of spraying) weakened the concrete’s strength in testing.
Li said photocatalytic concrete has great potential to remove air pollutants caused by massive vehicle exhausts, providing clean air in cities and towns.
But it needs more work to improve its efficiency with new hybrid nano-photocatalyst.
Another exciting development from the United States where researchers are turning concrete into an energy storage system by simply adding one extra material.
Popular batteries, like Tesla’s Powerpack, are built with an expensive material called lithium.
American researchers have managed to make a battery with cement and carbon black, two of the cheapest and most widely available materials on Earth.
Carbon black, a byproduct of burning organic materials, is added to the cement mixture as it’s being made. It forms conductive branches in the porous holes made by cement as it reacts to water.
By making two small plates of the ‘wired’ concrete, sealed together but separated by an electrolyte and membrane (key parts of a battery that keep the energy from going haywire), they were able to store a small amount of power.
After proving it works by powering LED lights, the researchers said they were working towards storing a day’s worth of energy for an entire house by building a foundation for one with their invention.
Li said the next 10-20 years will be big for concrete that does more than just provide a foundation.
“After trying to make concrete stronger and more durable, the next research will focus on the multifunctional,” he said.
“Soon, more and more innovations will be moved out of labs and into the field for commercialisation.”