Rock solid

Marine biologist Henk Jonkers first became involved in looking for a solution to the concrete problem back in 2006. ‘I already knew of microbes that could make calcium carbonate,’ says Jonkers, of Delft University in the Netherlands. ‘I was asked to investigate whether they could be used in self-healing concrete.’

The bacteria selected for the study were sourced from ‘soda lakes’ – highly alkaline natural environments that occur in places including Spain, Russia and China. They are ‘endolithic’ bacteria that grow in rocks – a natural environment not unlike concrete. Jonkers says he prefers to work with natural bacteria – and these particular species can be found on dust particles across the world – if conditions are alkaline.

‘They are adapted to highly alkaline conditions – a pH of 10-12, which matches that of fresh concrete,’ he says.

The bacteria have a combination of special properties: they produce calcium carbonate; they exist as spores, making them hardy enough to survive cement mixing; and they thrive in alkaline conditions such as in cement. Before being mixed into concrete, they are encased, along with nutrients or feed - biodegradable calcium lactate derivatives - in dry, clay-like ‘capsules’. Here, they remain dormant unless they come into contact with moisture – such as when micro-cracks appear in the concrete and let in water. The microbes then convert the feed into limestone, which fills the crack and prevents further water ingress.

A number of other teams are now researching bacteria for self-healing concrete, but Jonkers believes his approach still has the edge and claims his bacteria are more effective. Early research concentrated on surfaces – and used a mechanism based on the hydrolysis of urea, he notes: ‘We didn’t want to use this method because it produces ammonia – which is harmful to the environment.’

In 2014, Delft University created a spin-off company – called Green Basilisk – to sell its self-healing agents. They are not yet being used in concrete, but are available as two other products: a liquid spray to repair the exterior of buildings and a repair mortar. The mortar and liquid are made using a batch process, but to start supplying into the lucrative cement market, the company needs to reduce the cost of production by building a larger plant.

‘The cost is currently €8/kg, and we need to bring it down to €3/kg,’ Jonkers says. ‘That’s only possible if we increase production – but I’m confident we will get the green light to build a larger plant within a year.’

The company has also begun its first demonstrator project in the shape of a wastewater treatment tank, which will allow it to monitor the effectiveness of its self-healing agents – and, he hopes, attract further investment. ‘Architects call it “living concrete,” says Jonkers. ‘We call it “bio-adaptive” – because it responds to the environment.’

TU Delft is a member of the HealCon consortium – a pan-European project investigating self-healing concrete. Meanwhile, another member of HealCon has developed a new way to peer inside the concrete and assess the effectiveness of the crack-healing process. Christian Grosse, chair of non-destructive testing at the Technical University of Munich in Germany, says his team has modified ultrasound scanning techniques – such as are used in pregnancy scans – to assess crack formation in ‘real’ structures.

‘In the lab, we use techniques like computer tomography (CT), but this is too expensive to use in the field,’ he says. Instead, the researchers measure the time it takes for ultrasound pulses to travel through the concrete. The pulses move more slowly through cracks – but faster once the cracks have been filled. This helps the team to assess the effectiveness of the repair process.

It’s important to note that the ‘bacterial’ method of repair should more accurately be described as ‘sealing’ rather than ‘healing,’ says Grosse – as it only prevents further ingress of water. ‘There’s a big difference between healing and sealing. Some applications don’t need to regain strength – and for these it’s good enough to use bacteria. But in other applications – where cracks are subject to dynamic loads – we need to use epoxy-modified concrete.’

In this case, capsules containing epoxy resin are mixed into cement. They break open if the concrete cracks – then harden, and return the concrete’s structural strength to its original value. ‘I was very surprised that we were able to regain 100% of the strength,’ Grosse says.

The reason, he says, is that the epoxy is filling not just the crack but the pores as well.

Grosse’s work has inspired other TUM scientists to use bacteria to protect construction materials. In this case, they incorporated a bacterial biofilm into mortar – the paste that sticks individual bricks together – to make the surface highly water repellent.

Bacterial biofilms, such as dental plaque, are usually a problem: they can form on medical devices or implants, which can cause infection. ‘Biofilms are generally considered undesirable and harmful,’ says Oliver Lieleg, professor of biomechanics at TUM. ‘They are something you want to get rid of – so I was excited to find a beneficial use for them.’

In a paper in the journal Advanced Materials (doi: 10.1002/adma.201602123), Lieleg and colleagues explain that the biofilm forms tiny, evenly distributed crystalline ‘spikes’ on the mortar – creating a ‘lotus effect’ that makes it more difficult for water to wet the surface. Untreated mortar has similar spikes, but they are too scattered across the surface for the lotus effect to occur, say the researchers.

The hybrid mortar – containing the biofilm – is currently being tested in Grosse’s department, to assess whether it is suitable for commercial applications. ‘If it is, there should be no problem for companies to produce it on a large scale,’ says Lieleg, adding that freeze-dried biofilm also works well – meaning the biological material can be stored in powder form.

In future, the team will investigate the potential for using biofilms to create hydrophobic surfaces in concrete. For Grosse, the chance to combine these two techniques is exciting.
‘If you have hydrophobic material that prevents ingress of water, plus a technique that seals cracks, that would establish a very tough material with high durability,’ he says.

US-based scientists, meanwhile, have taken a different approach – designing a concrete that can regenerate itself multiple times while maintaining physical strength. It contains no ‘healing agents’, and instead undergoes ‘intrinsic’ healing – relying on chemicals already present in the cement mix to expand into the cracks. When water is added to cement to create concrete, two main hydration products are created: calcium hydroxide and calcium silicate hydrate (CSH, 3CaO.2SiO₂.4H₂O):

‘CSH is like a binder, and provides strength and toughness to the concrete,’ says Mo Li, assistant professor at the University in California, Irvine. ‘We want to promote the formation of new CSH within the cracks.’

Filling a crack with CSH – rather than with calcium carbonate, as in the bacterial method – will help the concrete regain something close to its original physical strength, she says. This repair mechanism effectively repeats the hydration reaction that occurs when cement is turned into concrete – but only within the crack itself. The way that CSH forms within a crack is different to how it is created ‘in bulk’. In a paper presented at a conference in China earlier in 2016, Li explained how nanoparticles are used to ‘seed’ the formation of new CSH within the cracks.

‘We need these nucleation sites within the crack in order to grow CSH into it,’ she says.

A key aspect of the technique is to control how cracks spread through the material. This is done by ‘fixing’ the micromechanical properties of the material, so that it cracks in a controlled way. This is done by adding plastic fibres to the cement. This – in combination with detailed simulation of the material’s fracture mechanics – helps to ensure that any cracks spread throughout the material, and are no wider than 15 microns. Li describes this as a ‘steady state’ crack.

Cracking in this controlled way has a huge advantage: a narrow crack is easier to fill than a wider crack. Similar behaviour is seen in nature – in seashells, for example – where a small amount of a nanoscale protein helps to dissipate energy. ‘That’s why the fracture energy of a seashell is many times larger than it would be for pure calcium carbonate,’ Li says.

The end result is a concrete that can repeatedly heal itself. The raw materials for forming new CSH are unreacted silicates within the concrete. A narrow crack that spreads throughout the material has ‘access’ to all the unreacted silicates concrete – and hence an almost limitless ability to heal itself. This will happen every time new cracks are formed.

The team has begun testing the materials in and outside the laboratory. In the lab, researchers designed a test programme that pre-damaged concrete samples under specific loads – and repeated the process to assess its healing properties. The concrete was loaded to 2% tensile strain – which Li says is ‘a high damage level that can rarely be reached under service loading for concrete infrastructure’. At the same time, it was exposed to ‘wet and dry’ cycles in order to mimic environmental conditions – and to promote self-healing.

Resonance frequency was measured, to assess internal damage, and revealed that recovery was fastest after the first three ‘damage cycles’. After seven cycles, the mechanical strength returned to 96% of its original value. For the first three damage cycles, around 70% of the crack volume was healed each time.

Outdoor testing has just started and involves taking samples to regions within easy reach of the university. ‘Because of where we are, we have both the desert and the sea nearby – so we can see how the samples behave in a diversity of climates,’ Li says.

The developers of self-healing concrete are setting their sights on ‘public’ infrastructure such as bridges and tunnels. But if it becomes a commercial reality, the effects may be felt much closer to home – by never having to replace a driveway or garden path ever again.

1 5th International Conference on Durability of Concrete Structures, 30 Jun-1 Jul 2016, Shenzen University, China