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.

Sustainable Re-Construction in Small Closed Systems

Rubble in streets of Haiti (image from Gerald Herbert/Associated Press - source).

A year after Haiti's devastating earthquake, re-construction efforts there have come upon significant hurdles.  In the latest issue of the American Ceramic Society Bulletin is a great article detailing these hurdles [1] (info also over at ScienceDaily).  According to the article authors DesRoches et al:

"The unprecedented damage was caused by, to a large extent, the absence of seismic details, poor-quality materials (concrete and steel) and lack of quality control in construction."

And more, it is the debris itself which is limiting progress:

"...we believe that landfilling this volume of debris is impossible.  And, our calculations show typical disposal techniques are simply not practical.  The debris has created a huge logjam that is blocking re-construction.  Land can't be cleared, streets can't be opened and even foot traffic must be re-routed over long distances.  Nevertheless, the concrete and other debris largely remain where it was when the earthquake struck ... we estimate that debris removal could take 20 years or longer to accomplish at current removal rates."

Sustainability has different meanings in different areas of the world.  Improved materials, extended means for waste removal / treatment, and higher quality control are possible in more affluent, large, or capable areas.  Haiti, on the other hand, in the state as described by DesRoches et al, may be better thought of as a small closed system, e.g., such as Easter Island [2].  Sustainable re-construction in a small closed system means:

• Given the inability to ship rubble elsewhere, new construction materials must incorporate a maximum amount of rubble; 
• Given the need to re-use rubble in new construction, the construction materials must possess maximum recycle-ability; and
• Incorporation or utility of complex / process-intensive / energy-intensive materials during re-construction can be minimal.

On its face, the latter point is particularly important for Haiti, given the difficulty of maintaining high levels of quality control.  However, the latter point also reflects the more subtle but important links between thermodynamics and sustainability in small closed systems.  More specifically, that the energy and entropy in a system must be closely in tune with the physical limitations of that system; and that is no more true than when evaluating or designing materials for small closed systems. 

[1] R.R. DesRoches, K.E. Kurtis, and J.J. Gresham.  Breaking the reconstruction logjam: Haiti urged to recycle concrete rubble.  Bulletin of the American Ceramic Society, Vol. 90 (2011), pp. 20-26.
[2] P. Nagarajan.  Collapse of Easter Island: Lessons for sustainability of small islands.  Journal of Developing Societies, Vol. 22 (2006), pp. 287-301.

On Human Factors and Materials Design

A contemporary of mine works at the National Research Council of Canada, studying titanium metal foams for use as part of dental implants.  To paraphrase a story of his:  he had been working for some time on a commercial foam, designing it to have a very specific set of material properties, and rigorously testing it to confirm those properties.  When he visited a dentist to get feedback, the dentist simply rubbed the foam part on his jacket - presumably to get a sense of the friction and anticipated quality of bone in-growth, or possibly to get a sense of how easy it will be to implant - and gave his approval.

Human factors engineering is a discipline that has always fascinated me.  It can be loosely described as engineering while keeping the end user squarely in focus.  The dentist in the above story has experience and knows what makes a good dental implant; and so, basically, human factors engineers will try to design around that.  Ergonomics is a good example of human factors design in the workplace; the urinal fly is another, which enables better use of existing materials.  Kim Vicente has written a great, recent book on the subject, The Human Factor [1], and is definitely worth a read.

The dentist story above also provides an example combining human factors and materials science: good implants require certain properties of friction, wear, stiffness, elasticity, etc., for successful handling, installation, and end use.  In many ways, engineers consider materials as black boxes: they have their respective inputs and outputs that can be summarized using material property space maps.  But also often encountered are less immediately-considered characteristics.  For example, a visual preference of one metal finishing over another, despite the two possessing essentially equivalent properties.  Or, the user's response to particular materials: for example, sports equipment can lead to a measurable improvement in the user's performance, even if the product is only a Placebo effect [2].  Or, the ease with which users will adapt to the new technology.

One of the most interesting cases (to me, anyway) where human factors and materials science interact is the design of prosthetic limbs.  Many of you have by now seen a range of types.  The ideal design and applicability of these devices requires a lot of input from the end user, such as: desired ranges of motion and control, force levels, dexterity, aesthetics, and symmetry.  The latter of these is particularly important in lower-limb prosthetics, e.g. for mechanical property and weight balance between limb pairs, and for loads imparted on the remaining parts of the body.  However, in cases where both limbs in a pair are prosthetic, the design space can be expanded as symmetry becomes less constrained.  One well-known case is the Paralympic athlete Oscar Pistorius, dubbed the "Blade Runner" (Figure 1).

Figure 1: Oscar Pistorius, the "Blade Runner" (image from Engadget).  His prosthetic legs are constructed using carbon fiber, one of the lightest and stiffest materials currently available.

Not surprisingly, there is debate as to whether Oscar Pistorius and other "post-humans" should be allowed to compete, e.g. [3,4], a debate which will continue indefinitely as materials science and the perception of prosthetics progresses.  This therefore provides a great example not only of the state-of-the-art application of human factors engineering and materials science and engineering; it is also an example of the evolutionary nature of both.      

[1] K. Vicente.  The Human Factor.  Vintage Canada, Toronto (2003).
[2] C.J. Beedie.  Placebo Effects in Competitive Sport: Qualitative Data.  Journal of Sports Science and Medicine, Vol. 6 (2007) pp. 21-28. (Available On-Line.)
[3] G. Lippi and C. Mattiuzzi.  (Editorial) Pistorius ineligible for the Olympic Games: The Right Decision.  British Journal of Sports Medicine, Vol. 42 (2008) 160-161.
[4] S. Camporesi.  (Editorial) Oscar Pistorius, Enhancement and Post-Humans.  Journal of Medical Ethics, Vol. 34 (2008) 639.

Nanomaterials in Construction

My previous post painted a general overview of the potential of materials science and engineering, such as in building science and sustainability.  In this post, I'll be presenting a few examples.

Via ScienceDaily: Researchers from Rice University and UCLA have recently reviewed examples combining nanomaterials and construction materials, or Manufactured NanoMaterials (MNMs).  A few of the many examples provided in their 2010 paper include (see [1], below): Carbon nanotubes in concrete for mechanical durability and crack prevention; Titanium dioxide nanoparticles in cement for self-cleaning properties, and in glass for solar energy-collecting properties; Iron oxide nanoparticles in concrete for abrasion resistance; Copper nanoparticles in steels for corrosion resistance; and silver nanoparticles in paintings for antibacterial properties.

Before full-scale application can be implemented, a number of issues must first be addressed.  In [1], for example, the authors outline "12 Principles of Ecologically-Responsible Construction Nanotechnology", which aim to address valid concerns such as MNM toxicity, carcinogenicity, contamination, disposal, and waste.  Also important to consider is the cost of these materials.  It has been suggested (see [1]) that that cost of MNMs will decrease due to 1) small additive ratios of the nanomaterial components, 2) further general development of nanomaterials, and 3) increase in production quantities.

In short, at the research level there is a great deal of nanomaterials science and engineering for construction, and there is a long list of potential as well as demonstrated advantages offered by MNMs.  Reviews on the subject include:

[1] J. Lee, S. Mahendra, and P.J.J. Alvarez.  Nanomaterials in the Construction Industry: A Review of Their Applications and Environmental Health and Safety Considerations.  ACS Nano, Vol. 4 (2010) pp. 3580-3590.
[2] Z. Ge and Z. Gao.  Applications of Nanotechnology and Nanomaterials in Construction.  First International Conference on Construction in Developing Countries (ICCIDC-I) (2008), pp. 235-240.  (Available On-Line.)
[3] W. Zhu, P.J.M. Bartos, and A. Porro.  Application of Nanotechnology in Construction: Summary of a State-Of-The-Art Report.  Materials and Structures, Vol. 37 (2004) pp. 649-658.

On the Potential of Materials Science and Engineering

In conversations I've had with industry, there is some agreement that materials science and engineering as a discipline is not being utilized to its full potential.  These sentiments are particularly strong for some large and broad areas such as sustainability and building science.  Consider a key driver, cost: a) the cheapest engineering materials by weight are concretes, ceramics (e.g. marble and sandstone), low-alloy steels, and some woods (e.g. particle-board); and b) the cheapest engineering materials by volume are concretes, polymer foams, and woods (again, particle-board).  The residence of these materials in buildings is perhaps unexpected.  A second driver, the embodied energy of a material is slightly more abstract; it is generally taken to mean the amount of energy needed to produce that material - a particularly important parameter that has been used to measure sustainability.  The engineering materials with the lowest production energy are again the concretes, ceramics, woods, and low-alloy steels.  The correlation between these two drivers is shown in the Figure below (source: Cambridge Engineering Selector). 
  

Production Energy versus Cost for Engineering Materials (Click on Figure to Enlarge)

The correlation is not linear nor is it complete: there are "gaps" which can be filled by creating new materials, composites, or hybrids between existing materials.  This is the basic theory behind Prof. Emeritus Mike Ashby's Material Property Space Maps (e.g., see paper here): a concept which has gained a lot of ground in materials science and engineering over recent years; and a concept which holds a great deal of information on the potential of materials science and engineering.

Of course, the utility and employment of engineering materials is more complex.  There are supply chains, production volumes, established performance codes and standards, and comfort levels with proven successes and limitations.  There is also an underlying rule of thumb that says it takes about 15 years for new materials to come to market.  But that shouldn't halt the discussion: the Canadian Mortgage and Housing Council (CMHC), for example, has demonstrated their interest in progressive design with new materials in projects like the Canuhome; and the CMHC has expressed to me personally an interest in further developing and showcasing new materials in practical applications.   

I've outlined here one of the primary areas I hope to focus on with this blog.  But there are others which can also be considered within the scope of designable material property spaces.  It is my aim with this blog to focus on this theme within the framework of materials science and engineering, while also bringing to light some of the more interesting new materials and new spins on old ones.

Welcome to Materials Science and Engineering - The Blog

Welcome!  This blog will set out to be a forum for sharing and discussing all things Materials Science and Engineering, with a particular focus on addressing key technological limitations or gaps in need of solutions.  A more detailed Mission Statement and opening post is forthcoming; in the meantime, the links I've added provide some sense of the range I hope to span.