*4.3.3 Biological self-healing concrete*

A promising approach to mitigate durability problems and control microcracking of concrete is to incorporate biological elements that self-heal the material. This technique was originally developed by Dr. H. Jonkers at Delft University of Technology, who incorporate into the concrete a bacterium that has the ability to segregate calcium carbonate, sealing concrete microcracks [110]. This can represent an increment of 20% in the lifetime of a concrete structure [111]. If the same structure but with traditional concrete will need rehabilitation, and then more concrete, during its life to endure the same period of time than the self-healing concrete. Therefore, the amount of CO2 emitted into the environment decreases compared to structures built with conventional concrete. Indeed, Van Belleghem et al. [112] report that the lifetime of structures with self-repairing concrete can reduce the environmental impact by 56–75%.

Due to the biological nature of this technology and its interaction with the components of the concrete mix and the local environmental conditions, geodependence is relevant to the performance of the self-repair process. Indeed, if the ideal conditions of moisture, salinity and temperatures are not present, then the bacteria will not have sufficient stimulus to secrete calcium carbonate and the crack sealing will not be produced [113]. Similarly, cements with a higher amount of pozzolan decrease calcium carbonate precipitation which reduce the self-healing capacity of concrete [114].

In collaboration with Dr. Jonkers, bacterial growth, spore formation and production of concrete samples were developed with two types of Porland pozzolanic cements commonly used in Chile [115]. The *Bacillus pseudofirmus* bacteria solution was impregnated into expanded clays and incorporated into the concrete as a partial replacement of fine aggregate. These specimens were subjected to three different environments simulated in the laboratory: fresh water, salt water and 90% relative humidity, all at 20°C. The results at 28 days show some areas where the self-healing was 100%. Although in other cases the recovery was lower, about 90% of the specimens showed traces of calcium carbonate as a sign of the activation of the self-healing process (**Figure 5**).

The results confirm that biological self-healing of concrete can be produced with Chilean materials and regional environments simulated in the laboratory. In particular, the aggregates and cements used in the research were different from those used in the investigations of Jonkers [23, 116, 117].

**Figure 5.** *Concrete sample with self-healing process activated [115].*
