Self-Healing Concrete

An example of a severely damaged concrete support beam of a motorway bridge. This pillar has suffered reinforcement corrosion due to ingress of de-icing salts through micro cracks formed in the concrete

Mineral-producing bacteria have been found that could help mend micro-cracking in concrete. Dr Henk Jonkers, a micro-biologist at Delft University, talked to Ingeniaabout research developments in producing bioconcrete that could bring benefits for civil engineering projects.

Self-healing concrete could solve the problem of concrete structures deteriorating well before the end of their service life. Concrete is still one of the main materials used in the construction industry, from the foundation of buildings to the structure of bridges and underground parking lots. Traditional concrete has a flaw, it tends to crack when subjected to tension.

A healing agent that works when bacteria embedded in the concrete convert nutrients into limestone has been under development at the Civil Engineering and Geosciences Faculty in Delft since 2006. The project is part of a wider programme to study the self-healing potential of plastics, polymers, composites, asphalt and metals as well as concrete.

Dr Henk Jonkers, a microbiologist who specialises in the behaviour of bacteria in the environment, has developed self-healing concrete in the laboratory and full-scale outdoor testing will start in 2011. The first self-healing concrete products (successful research results permitting) are expected to hit the market in two years’ time and are expected to increase the lifespan of many civil engineering structures.

Jonkers has worked closely with civil and structural engineers to learn about the properties of concrete and steel reinforcement, and develop the concrete. “For a biologist to work with civil engineers to incorporate living matter into structural concrete material is in itself a great innovation,” he says.

Why the need?

Concrete will continue to be the most important building material for infrastructure but most concrete structures are prone to cracking. Tiny cracks on the surface of the concrete make the whole structure vulnerable because water seeps in to degrade the concrete and corrode the steel reinforcement, greatly reducing the lifespan of a structure.

Concrete can withstand compressive forces very well but not tensile forces. When it is subjected to tension it starts to crack, which is why it is reinforced with steel; to withstand the tensile forces.

Structures built in a high water environment, such as underground basements and marine structures, are particularly vulnerable to corrosion of steel reinforcement. Motorway bridges are also vulnerable because salts used to de-ice the roads penetrate into the cracks in the structures and can accelerate the corrosion of steel reinforcement. In many civil engineering structures tensile forces can lead to cracks and these can occur relatively soon after the structure is built.

Repair of conventional concrete structures usually involves applying a concrete mortar which is bonded to the damaged surface. Sometimes, the mortar needs to be keyed into the existing structure with metal pins to ensure that it does not fall away. Repairs can be particularly time consuming and expensive because it is often very difficult to gain access to the structure to make repairs, especially if they are underground or at a great height.

How does bioconcrete work?

Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures. Specially selected types of the bacteria genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. These self-healing agents can lie dormant within the concrete for up to 200 years.

However, when a concrete structure is damaged and water starts to seep through the cracks that appear in the concrete, the spores of the bacteria germinate on contact with the water and nutrients. Having been activated, the bacteria start to feed on the calcium lactate. As the bacteria feeds oxygen is consumed and the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up. It mimics the process by which bone fractures in the human body are naturally healed by osteoblast cells that mineralise to re-form the bone.

The consumption of oxygen during the bacterial conversion of calcium lactate to limestone has an additional advantage. Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all it increases the durability of steel reinforced concrete constructions.

The two self-healing agent parts (the bacterial spores and the calcium lactate-based nutrients) are introduced to the concrete within separate expanded clay pellets 2-4 mm wide, which ensure that the agents will not be activated during the cement-mixing process. Only when cracks open up the pellets and incoming water brings the calcium lactate into contact with the bacteria do these become activated.

Testing has shown that when water seeps into the concrete, the bacteria germinate and multiply quickly. They convert the nutrients into limestone within seven days in the laboratory. Outside, in lower temperatures, the process takes several weeks.

Dr Henk Jonkers places a sample block of self-healing concrete into a machine that will create cracks on the surface

Finding the right bacteria

The starting point of the research was to find bacteria capable of surviving in an extreme alkaline environment. Cement and water have a pH value of up to 13 when mixed together, usually a hostile environment for life: most organisms die in an environment with a pH value of 10 or above. The search concentrated on microbes that thrive in alkaline environments which can be found in natural environments, such as alkali lakes in Russia, carbonate-rich soils in desert areas of Spain and soda lakes in Egypt.

Samples of endolithic bacteria (bacteria that can live inside stones) were collected along with bacteria found in sediments in the lakes. Strains of the bacteria genus Bacilluswere found to thrive in this high-alkaline environment. Back at Delft University the bacteria from the samples were grown in a flask of water that would then be used as the part of the water mix for the concrete.

Different types of bacteria were incorporated into a small block of concrete. Each concrete block would be left for two months to set hard. Then the block would be pulverised and the remains tested to see whether the bacteria had survived.

It was found that the only group of bacteria that were able to survive were the ones that produced spores comparable to plant seeds. Such spores have extremely thick cell walls that enable them to remain intact for up to 200 years while waiting for a better environment to germinate. They would become activated when the concrete starts to crack, food is available, and water seeps into the structure. This process lowers the pH of the highly alkaline concrete to values in the range (pH 10 to 11.5) where the bacterial spores become activated.

Finding a suitable food source for the bacteria that could survive in the concrete took a long time and many different nutrients were tried until it was discovered that calcium lactate was a carbon source that provides biomass. If it starts to dissolve during the mixing process, calcium lactate does not interfere with the setting time of the concrete.

Interest from industry

When the idea of bacteria-mediated concrete was first mooted by US academics in the late 1990s by the research group of Professor Sookie Bang, testing and application of the theory was not taken forward because there was a lack of interest from the commercial engineering sector for such a product.

The R&D process still has some way to go but several big industry players have created partnerships with Delft University to develop applications of self-healing concrete. Investment funding from industry is now forthcoming.

The concept is to engage with one major player from each concrete sector. Delft is therefore developing self-healing concrete products for specific civil engineering markets that will not be in competition with one another. Products will be developed for sectors such as tunnel-lining, structural basement walls, highway bridges, concrete floors and marine structures. Pure concrete products will take two years to develop and products with steel reinforcement will take four years.

Full-scale testing

Starting this year, there will be full-scale outdoor testing of self-healing concrete structures. A small structure or part of a structure will be built with the self-healing material and observed over two to four years. Structures will be fitted with some panels of self-healing concrete and others with conventional concrete so that the behaviour of the two can be compared. Cracks will be made in the concrete that are much larger than the ones that have healed up in the laboratory to determine how well and fast they heal over time.

Commercial partners have asked whether the process could be used to repair existing structures. To answer this Delft University has just been awarded funding of €420,000 from the Dutch government. Two postdoctorate scientists will spend two years developing a self-healing system to be applied to existing structures.

The research will test two systems. The first technique will see bacteria and nutrients applied to the structure as a self-healing mortar, which can be used to repair large-scale damage. The second technique will see the bacteria and food nutrients dissolved into a liquid that is sprayed onto the surface of the concrete from where it can seep into the cracks.

The Dutch government has also just awarded €450,000 funding for another research project that will be undertaken into concrete basement walls and pre-cast concrete floors which are vulnerable to groundwater.

Meanwhile, work is proceeding to address concerns from industry as to whether the bacteria can survive dormant for the full service life of the concrete structure. Evidence from the soil samples taken from desert areas and stored in museums shows that the soil still contains live bacteria spores after 200 years.

To address other concerns laboratory tests are being carried out to accelerate the ageing process of self-healing concrete. The tests will subject the concrete to extreme environments to simulate changing seasons and extreme temperature cycles, wetter periods and dryer periods.

Some disadvantages

There are two key obstacles that need to be overcome if self-healing concrete is to transform concrete construction in the next decade.

The first issue is that the clay pellets holding the self-healing agent comprise 20% of the volume of the concrete. That 20% would normally comprise harder aggregate such as gravel. The clay is much weaker than normal aggregate and this weakens the concrete by 25% and significantly reduces its compressive strength. In many structures this would not be a problem but in specialised applications where higher compressive strength is needed, such as in high-rise buildings, it will not be viable.

The second disadvantage is the cost of self-healing concrete is about double that of conventional concrete, which is presently about €80 euros per cubic metre. Jonkers says: “At around €160 per cubic metre, self-healing concrete would only be a viable product for certain civil engineering structures where the cost of concrete is much higher on account of being much higher quality, for example tunnel linings and marine structures where safety is a big factor – or in structures where there is limited access available for repair and maintenance. In these cases the increase in cost by introducing the self-healing agents should not be too onerous.”

Added to this, if produced on an industrial scale it is thought that the self healing concrete could come down in cost considerably.

If the life of the structure can be extended by 30%, the doubling in the cost of the actual concrete would still save a lot of money in the longer term. The Delft team is currently working on the development of an improved and more economic version of the bacteria-based healing agent which is expected to raise concrete costs only by a few euros.

Bio concrete mark II

A second self-healing agent that will be much cheaper and also would result in much stronger concrete is being developed in close collaboration with Erik Schlangen, Virginie Wiktor and Arjan Thijssen in Delft. Presently the majority of the extra cost comes from the calcium lactate which is very expensive. The process of embedding the bacteria and nutrients into the pellets is also expensive because it involves a vacuum technique. A sugar-based food nutrient would potentially bring down the cost of the self-healing concrete to €85-90 per cubic metre. But a sugar-based nutrient would not remain intact within expanded clay pellets as calcium lactate does. Much of the sugar would be dissolved and it would delay the setting time of the concrete.

The new self-healing agent being developed would immobilise the sugar-based nutrient during the mixing process. So the team has now developed an alternative self-healing agent with a new shape and form and the way that the bacteria and nutrients would be stored would be totally different.

The new healing agent would comprise only 3-5% of the overall volume and the concrete would therefore be much stronger. The new self-healing agent would be a viable product for most structural concrete applications. The team still has to do a lot of lab testing which will take another year before the new product is ready for full-scale testing.

Jonkers says: “If the cost of the self-healing agent can be brought down sufficiently and the concerns over the long-term effects on the concrete performance properly addressed, then the product could have great potential.”