Thursday, January 27, 2011

Self-Healing Concrete and Utility of Lessons from Nature

Decomposition of Reinforced Concrete (from Dornob)

To compliment my previous post, here I wanted to take a look at a recent article found via the design site Dornob: The design of a bacteria-concrete hybrid material which has crack self-healing properties.  In short: tiny bacteria (aptly called Bacilla Filla) are genetically programmed to seek out cracks in the concrete; there, the bacteria secrete a combination of calcium carbonate (one of the most basic and widespread building materials) and a "bacterial glue" to fill in the cracks.  When these secretions harden, they have properties essentially matching those of the concrete.

While the potential benefits are far-reaching and the concept intriguing, the science behind such hybrid materials is not new.  A recent review article written by De Muynck et al [1] outlines the science and its long history.  In general, all construction materials undergo degradation with age and exposure to environment ("weathering"), which eventually leads to losses in load-bearing properties.  Adapting lessons from nature, there are a number of known and fairly common bacteria which naturally secrete carbonate mineral compounds; and these bacteria can be controlled to do so depending on the local environment (i.e., acidity, available carbon and calcium, etc.).  Adding the bacteria to the concrete, the result is a self-healing property, or "biocementation", only one of the possibilities offered by "microbially-induced" carbonate formation.

Currently, these kinds of biocementation - i.e., autonomous remediation of cracks and self-healing - are still in the laboratory experiment stages [1] (although other techniques are commercial, such as the application of "calcinogenic" bacteria for surface protection of ornamental stone [2], and the ).  And according to De Muynck et al the largest hurdle towards full-scale application of such a technology would be its cost relative to traditional construction materials and their treatments; after all, typical concrete only costs between 3-30¢ per kg.  Self-healing building materials could, however, add value by reducing the necessity for manual inspection and repair [1].

This example provides an introduction to the advantageous utility of lessons learned from nature, or, more generally, "Biomimicry".  My personal field of expertise surrounds the design, structure, and properties of cellular materials; metal foams, for example, mimic natural cellular materials such as cork and bone, and offer light-weight properties not offered by their fully-dense counterparts.  There is a wide range of biomimicry examples - far too many to cover in detail at once - all of which add an extra dimension to the framework of sustainability and the design of new materials.   

[1] W. De Muynck, N. De Belie, W. Verstraete.  Microbial carbonate precipitation in construction materials - A review.  Ecological Engineering, Vol. 36 (2010) pp. 118-136.
[2] Calcite Bioconcept.

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