Researchers in the USA are engineering a concrete to repeatedly self-heal when it cracks. The aim is to limit the costs and environmental footprint of regular maintenance. The material also eliminates steel reinforcement, which traditionally tightens the cracks but is susceptible to corrosion, as water and de-icing salts penetrate and weaken the structure.
Resonance frequency measurements have revealed that, on average, the new self-healing material recovers about 80% of the properties of the virgin concrete after being subjected to three per cent tensile strain – enough to catastrophically fracture traditional concrete.
The team at the University of Michigan is exploring this further to achieve 100% recovery and ensure ‘robustness’, so that self-healing can occur anywhere a crack forms, repeatedly (even in the same location) and over the lifetime of the structure.
‘Our aim is to create infrastructures that would be zero maintenance,’ says Dr Victor Li at the University. Self-healing occurs through continuous hydration. Calcium ions from the interior migrate to the surface and hydrate when they come into contact with water (rain), forming calcium silicate hydrates that bind the crack phases together.
Another route is the calcium ions chemically reacting with water and CO2 in the air to form calcium carbonates around the cracks and PVA fibres in the concrete. ‘Calcium carbonates are brittle, but with fibres inside them, they become strong enough to carry loads,’ explains Li.
Cracking mode in concrete
To achieve self-healing, the cracks must be below 50µm. Li and his team have accomplished this by improving on their engineered cementitious composite (ECC), which has already been used in earthquake resistant structures such as the Kitahama Building in Osaka, Japan.
The ECC (bendable concrete) ‘looks and feels like a concrete until you put a load on it [and] it deforms like a metal without fracturing’, explains Li. ‘Cracks are kept at 60-150µm, compared to millimetre-size cracks in normal concrete.’
The ECC is produced by carefully controlling the geometry and distribution of the sand particles and fibres in concrete, the percentage of each ingredient in the total make up, and the order and nature of how they are mixed to adhere to a strict design criteria based on micromechanical theory. A proprietary 20nm coating on the PVA fibres controls interaction with the cement matrix.
The size of the cracks has been minimised via a new cracking mode. Li explains, ‘Normal mode is the Griffiths crack, [where] as the length elongates, the width of the crack opens up. We designed a “flat” crack – even if the length extends, the width does not.
‘We used fracture mechanics so that the energy balance favours the “flat” crack – [that is] the energy to the crack due to load and the energy absorbed by the material as it cracks’.
By modifying the above ECC composition, the size of the microcracks has been reduced further in the new concrete, and the material combines self-healing with ductility (see image left). The exact details are patent pending.
Wetting and drying cycles
Concrete and cement science specialist Charles Fentiman, of Fentiman Consulting in Southwater, UK, says ‘the novelty is keeping the microcracks narrow so that they can heal.
‘Bridge decks have always been a difficult application to maintain as most building materials are rigid, so the movement and load on the structure means that cracks form and you get rapid deterioration. Something that is flexible and self-healing seems like a good solution’.
He says, however, that the researchers need to be more ‘specific about the climatic requirements’. Through laboratory testing, the Michigan team has found that the material requires about one to five cycles of wetting and drying to self-heal – although it can also self-heal if submerged for underwater applications. Tropical conditions (about 55ºC) have been found to accelerate self-healing, while the concrete does not recover its properties where there is continuous air exposure without wetting – a limitation for certain climates.
Li insists that further investigation is required, moving to outdoor trials. ‘The amount of recovery depends on the severity of damage and environmental exposure.’