In the built environment, concrete is the most used material. The widespread use of concrete has made it the second most consumed substance on earth, following water. Due to its durability and ease of sourcing, the large-scale use of concrete in the construction industry will remain well into the arguable future. Consequently, most of the maintenance and restoration work in the built environment is done to predominantly concrete structures such as roads, buildings, bridges and tunnels. Around £10 billion is spent annually, in the UK alone, on maintenance of bridges and an additional £30 billion on other concrete structures.
Self-healing concrete is an innovative solution that mimic’s the biological concept of self-healing. Self-healing concrete is designed to partially or completely ‘heal’ from damages such as cracks, with the aim of restoring the element’s original functionality. This innovation is designed to reduce maintenance procedures, saving costs and eventually make concrete a sustainable construction material. Discussed are the types of self-healing concrete, propose the best self-healing concrete solution with reference to ‘low maintenance solutions’ and elaborate the market opportunities and threats.
Self-healing concrete
The types of self-healing concrete come from the different techniques that have been used to give concrete its ‘self-healing’ aspect;
· Autogenous healing concrete, with un-hydrated cement. When in contact with water the cement is hydrated and the dissolved CO2 reacts with Ca2+ to form (calcium carbonate) CaCO3 crystals.
· Superabsorbent polymers in cement that retain large amounts of fluid and swell when cracks occur (due to humidity exposure), sealing the gap.
· Fly-ash and blast-furnace slag concrete. Due to their low hydration degree, upon cracking the unreacted particles can be activated again to close the crack and regain impermeability.
· Shape-memory polymers that transform into any different shape (previously ‘memorised’ by the material) when heated with a small current.
· Bacteria-based self-healing concrete uses dormant bacteria and organic compounds to convert calcium lactate to calcium carbonate when cracks occur, and the bio-chemical agents are exposed to humidity.
This article will delve into bacteria-based self-healing concrete because this innovation offers a low-cost solution to maintenance as it uses autonomous crack repair. Furthermore, it provides high life-cycle reliability, as the embedded bio-chemical agents can remain inactive for up to 50 years (withstanding mechanical and chemical stresses).
Bacteria-based self-healing concrete
Self-healing in materials is generally classified into two categories; non-autonomic (requires an external trigger) and autonomic (does not require external stimuli). Bacteria-based self-healing concrete is an autonomous material. The principle of this self-healing concrete mechanism is that the bacteria (thermophilic) acts as a catalyst that transforms an available compound (calcium lactate) into a ‘filler material’ (calcium carbonate). Therefore, when a crack occurs the humidity that sips into the concrete structure activates the immobilised bacteria, which converts the present calcium lactate in the matrix to calcium carbonate that then seals the crack.
Ca(C3H5O2) 2 + 7O2 → CaCO3 + 5CO2 + 5H2O
Application in the industry
Bacteria occur everywhere and anywhere in nature. The addition of bacteria in concrete has no effects in the chemical and mechanical structure of the building. The bacteria-based self-healing concrete hydrates to ‘heal’ therefore it’s best suited for marine concrete structures. This concrete is currently in use in particularly vulnerable environments such as tropical and coastal areas. A field application has been set up in Ecuador; where both the concrete canal and irrigation system used bacteria-based self-healing concrete.
Costs (capital vs maintenance)
In accordance with the publication of Dr. Henk Jonkers’ work, the average cost of a cubic metre of the self-healing concrete is £86 (100 Euros) as opposed to ordinary concrete at £60 (70 Euros). This tabulates to a 42% increase in the initial concrete costs, although it is clear that research is still being done to reduce this. However, maintenance costs over a construction lifetime are estimated to cost 5 times the building costs. This translates to a 400% increase on maintenance costs throughout the structure’s lifecycle. Is it then feasible to accommodate the initial capital of self-healing concrete, and reduce on maintenance costs later?
NB: Studies on the potential commercial benefit of self-healing concrete have been done, e.g. the M4L project by Lychgate which surveyed 40 companies. This study showed a 20% increase in the initial capital was acceptable by concrete producers and construction companies with regards to whole-life cost (WLC).
Market opportunities
· Reduced/ nullified maintenance costs.
· Flexibility in its use (just like ordinary concrete).
· Offers a solution to high-risk areas (coastal towns, flood areas and high rainfall areas).
· Self-healing restores better concrete quality as compared to manual repair and maintenance.
Market threats
· The initial capital.
· The self-healing is limited to cracks of up to 0.8mm at a time.
· The possibility of another form of better-quality concrete emerging.
Conclusion
Infrastructure built with self-healing concrete may have a higher initial cost, but the self-healing functionality maintains the quality of infrastructure with minimum or no additional cost accumulation over the life cycle, resulting in a life-cycle cost that could be competitive with that of current concrete infrastructure. Bacteria-based self-healing concrete is the most researched type of self-healing concrete and can be considered a great leap forward towards improved concrete production. Despite this technology being new in the industry, it is used in areas prone to high water damage. Projects in places such as Holland would provide a great starting baseline and a cost-benefit analysis by using this project from the outset.
