Bio Concrete – 3D Printed Material
Self-healing of Concrete by Bacterial Mineral Precipitation
Image courtesy of Tudelft.nl
Repairing cracks in concrete structures is a time consuming, costly but necessary business. TU Delft is researching how the self-healing capacity of concrete structures can be improved by using calcite-precipitating bacteria and what conditions are necessary for these bacteria to thrive.
Although concrete is the world’s most used building material, it has a serious flaw: it can easily crack when under tension. If these cracks become too large, they will lead to corrosion of the steel reinforcement, which not only results in an unattractive appearance, but also jeopardizes the structure’s mechanical qualities. That is why engineers often use a larger than necessary amount of steel reinforcement within a concrete structure in order to prevent the cracks from becoming too large. This extra steel has no structural use and is an expensive solution as steel prices are high. Another way to deal with cracks is to repair them, but this can be extremely difficult in underground or liquid retaining structures. The ultimate solution would be self-healing concrete, which is exactly what TU Delft researchers are working on.
By embedding calcite-precipitating bacteria in the concrete mixture, it is possible to create concrete that has self-healing capacities. As the pH value of concrete is very high, only the so-called alkaliphilic bacteria are able to survive. We have mixed several of these bacteria into a cement paste and after a month found the spores of three particular bacteria where still viable.
The use of bacterial concrete can in theory lead to substantial savings, especially in steel reinforced concrete. It will also mean durability issues can be tackled in a new and more economical way when designing concrete structures. Bacterial concrete is ideal for constructing underground retainers for hazardous waste, as no humans would have to go near it to repair any occurring cracks. For residential buildings, however, it does seem the traditional repairing of cracks will remain the most economically attractive solution for now.
Currently, our research focuses on creating the right conditions for the bacteria to produce as much calcite as possible and on optimizing the distribution of food for the bacteria. In addition, we are also looking at the self-healing ability of bacterial concrete and how this is affected by the various deterioration mechanisms involved, such as sulfate attacks or temperature fluctuations. All of our research is done at the TU Delft’s Microlab, where fracture testing equipment as well as numerical tools for structure information and fracture modeling are available.
The Self Healing Concrete project is part of the TU Delft wide Self-healing Materials research programme at the Delft Center for Materials (DCMat). Furthermore, we collaborate with the Biotechnology section at the Faculty of Applied Sciences and the South Dakota School of Mines in the United States.
Concrete it has been used since the old times, but has never been more popular than today, as concrete is one of the most widely used materials in the world, but at some point, no matter how it is mixed, it will crack and deteriorate.
Microbiologist Hendrik Jonker thinking about how the body can heal bone through mineralization, looked into whether a similar method could be used with concrete. By mixing it with limestone-producing bacteria, he found that any cracks that formed in the concrete were patched over. For this invention, Jonker was a finalist for the European Inventor Award 2015.
The bacteria, either Bacillus pseudofirmus or Sporosarcina pasteurii, are found naturally in highly alkaline lakes near volcanoes, and are able to survive for up to a staggering 200 years without oxygen or food. They are activated when they come into contact with water and then use the calcium lactate as a food source, producing limestone that, as a result, closes up the cracks.
He calls the material “bioconcrete” that can “self-heal”. In order to keep the bacteria dormant until it is needed, it is placed in small, biodegradable capsules containing the nutrient. When the concrete cracks, and water enters the gaps, it comes into contact with the bacteria and the food source, setting the healing process off. The bacteria then feed on the calcium lactate, joining the calcium with carbonate to form limestone, fixing the crack.
The process has been proven to work effectively, and can even be added to a liquid that could then be sprayed onto existing buildings. The problem, however, as always is the price. It is currently twice the cost of traditional concrete, but Jonker says that this is mainly due to the price of the calcium lactate, and if they can get the bacteria to use a sugar-based nutrient instead, the price would be dramatically reduced.
If they can sort out the pricing, then little would stop the incredible self-healing material from being used in bridges, tunnels, roads and other buildings, with the bacteria laying dormant for centuries and only ‘coming to life’ when needed.
Cardiff University in the first technique uses shape-shifting materials, known as shape-memory polymers, to repair large cracks in concrete. When these materials are heated with a small current, they can transform into a different shape that the material has ‘memorized’. The researchers believe that these materials can be embedded into concrete and used to close cracks or make them smaller.
In the second technique, researchers will pump both organic and inorganic healing agents through a network of thin tunnels in the concrete to help repair damage.
In the third technique, the team will embed tiny capsules, or lightweight aggregates, containing both bacteria and healing agents into the concrete. It is anticipated that once cracks occur, these capsules will release their cargos and, in the case of the bacteria, the nutrients that will enable them to function and produce calcium carbonate, which the researchers envisage will heal the cracks in the concrete.