Self Healing Concrete

Self Healing Concrete.

A self-healing material is described as a material that is capable of repairing itself back to the original state. The concept of self-healing concrete (SHC) that happens over time (autogenic) has been observed in many old structures which have remained standing for long periods of time in spite of the fact that they have limited maintenance. This observation concludes that the cracks heal when moisture interacts with non-hydrated cement clinker in the crack. Nevertheless, in present-day constructions the cement is lowered as a result of modern construction methods. Hence, the amount of available non-hydrated cement is less and therefore, the natural healing effect is reduced.

Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures. Specially selected types of the bacteria genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. These self-healing agents can lie dormant within the concrete for up to 200 years. However, when a concrete structure is damaged and water starts to seep through the cracks that appear in the concrete, the spores of the bacteria germinate on contact with the water and nutrients. Having been activated, the bacteria start to feed on the calcium lactate. As the bacteria feeds oxygen is consumed and the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up.

The consumption of oxygen during the bacterial conversion of calcium lactate to limestone has an additional advantage. Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all it increases the durability of steel reinforced concrete constructions. The two self-healing agent parts (the bacterial spores and the calcium lactate-based nutrients) are introduced to the concrete within separate expanded clay pellets 2-4 mm wide, which ensure that the agents will not be activated during the cement-mixing process. Only when cracks open up the pellets and incoming water brings the calcium lactate into contact with the bacteria do these become activated. Testing has shown that when water seeps into the concrete; the bacteria germinate and multiply quickly. They convert the nutrients into limestone within seven days in the laboratory. Outside, in lower temperatures, the process takes several weeks.

A brief comparison of some aspects is tackled below.

Safety: Since cracks in SHC are easy to close with no extra costs being added, the general safety of a particular construction is increased. However, that leaves a question regarding the resistant ability of concrete and whether crack closing would affect its strength. All research conducted so far show that the concrete gains about 25% of its original strength in the healed spot which more than the 15% gained back when the crack is sealed by current methods.

Cost: It is obvious that the initial cost of construction using SHC is higher, however, on the long-term, durable concrete is much more cost-efficient due to the low cost of maintenance, durability and the long life-span of the construction.

Durability: According to research and experimentation bacteria-based SHC is denser and more durable than concrete.

Availability: As it is still under development, this kind of concrete is used on a limited scale and still not commercially wide-spread. Some main obstacles are cost and production.

Effects on Architecture & Design: By increasing the life-span of a construction, architects need to re-consider design standers. A long life-span impacts the design of any construction, as architects must take into consideration future prognosis of two main aspects: (i) the potential function within a particular building (potential technological needs, change of function, change of life style, etc.) (ii) The future function of urban space surrounding a certain building. Hence, architects’ main task is to foresee the upcoming needs and the current ones to design and construct a useful, aesthetic and more importantly, highly-flexible buildings in order to be change function easily.

Environmental impact: Cement industry is one of the main two producers of carbon dioxide (CO2) emissions, which is directly harming our planet. Therefore, by using SHC the carbon dioxide emissions are reduced significantly.

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