What are self-healing materials?
Self-healing materials are materials that can repair minor damage such as hairline cracks by themselves, without anyone having to do anything. The idea is borrowed from nature: just as a wound in the skin closes up, a small crack in the material seals itself again.
How does it work? The main principles:
- Autogenous healing of concrete: ordinary concrete can already seal very fine cracks to some extent by itself. Unreacted cement continues to hydrate when water gets in, and lime (calcium carbonate) forms in the crack. This only works for very narrow cracks.
- Bacterial concrete (bio-concrete): spores of special lime-producing bacteria are added to the concrete, together with nutrients such as calcium lactate. The spores can survive dormant in the concrete for years. When a crack forms and water and oxygen get in, the bacteria become active and convert the nutrients into limestone that seals the crack. This concept was developed at TU Delft, among other places.
- Capsules: small capsules or fibres containing a healing agent (for example a resin) break open when a crack runs through them and fill it.
- Self-healing plastics and coatings: polymers that re-bond when exposed to heat or light, used among other things in lacquers and coatings.
Application
In construction it mainly concerns concrete and mortar for structures where cracks can lead to leaks or corrosion of the reinforcement: basements, tunnels, water basins, retaining walls, car parks and concrete repairs. There are also repair mortars and sprays containing bacteria for treating existing small cracks.
What it can and cannot do:
- Can: make fine cracks watertight and so extend service life, especially in places that are hard to reach for maintenance.
- Cannot: repair structural cracks caused by overloading, settlement or design errors. The material also does not restore strength the way a structural repair does.
- Conditions: moisture is needed to start the healing process, and the crack width must be limited.
Tips:
- With cracks, always find the cause first. A self-healing material will not fix a sinking foundation.
- Self-healing concrete is more expensive than ordinary concrete. It mainly makes sense where leaks or maintenance are costly.
- Ask the supplier for evidence: which crack width is sealed and under what conditions?
Related Terms
- Crack
- Shrinkage crack
- Concrete cancer
- Concrete corrosion
- Reinforced concrete
- Retaining wall
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