Although concrete is the most used building material in the world, it has a serious flaw. It can crack with ease under tension.
When these cracks become too large, they in turn lead to corrosion of the steel reinforcement inside, which not only results in an unattractive appearance but also undermines the structure’s integrity. That is why engineers often use a greater amount of steel reinforcement within a concrete structure than needed, in order to prevent loss in integrity through corrosion and cracks. This extra steel has no structural use and is an expensive solution since steel prices are high compared to the cost of concrete. A more labor intensive way to deal with cracks is to repair them, but this can be extremely difficult in underground structures or underwater supports.
The ultimate solution is obviously self-healing concrete, and that is exactly what TU Delft researchers in the Netherlands are working on.
Microbial Breakthrough Researchers found that by embedding calcite-precipitating bacteria in the concrete mixture, it is possible to create concrete with self-healing capabilities. As the pH value of concrete is very high (approximately a pH of 11.4 to 13, very basic), only the alkaliphilic (meaning alkaline loving) bacteria are able to survive. They mixed several strains of these bacteria into a cement paste and after a month found the spores of 3 bacteria species which were still viable.
The use of bacterial concrete can theoretically lead to substantial savings and increase long-term endurance for structures using steel reinforced concrete. It will also mean durability issues can be tackled in a new and more economical way when designing and building 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 attractive solution for now.
Studies regarding potential interactions of the bacteria with humans will need to be carried out before widespread adoption, but the results from TU Delft’s research provide a promising basis for creating self-repairing structures in the future. Currently, their research (led by Hendrik Jonkers) is focusing on finding the optimal conditions for the bacteria to produce as much calcite as possible and on improving the distribution of food for the bacteria. They also examined the self-healing ability of bacterial concrete and how this is affected by various deterioration mechanisms involved, such as sulfate exposure and temperature fluctuations.
All of their research is done at the TU Delft’s Microlab where fracture testing equipment, as well as numerical tools for structure information and fracture modeling were available. The Self Healing Concrete project is part of the TU Delft wide Self-healing Materials research program at the Delft Center for Materials (DCMat). The project was nominated for the European Inventor Award, which will be awarded this year.