Writting assignment 3
"Bio self-healing concrete, an extraordinary solution for an ordinary problem"


AN EXTRAORDINARY SOLUTION FOR AN ORDINARY PROBLEM: SELF-HEALING CONCRETE

Many structures exposed to environmental and human interaction conditions tend to deteriorate through time due to materials failing processes (aging, corrosion, cracking, weakening, etc.). Concrete, the base material for the developing of our cities, is no exception to this condition. Concrete’s main failure is by cracking, causing severe reduction in the structure’s stiffness, as well as promoting the corrosion of steel bars that usually support concrete structures, and so, reducing its service life. The solution to this problem relies in human intervention that involves the removal or restoration of the affected part. But more often the solution consists in preventing the damage by constant inspection and maintenance techniques that tend to be expensive. However, recent studies have led to the invention of a bio self-healing concrete, that based on the healing properties of nature, repairs its own cracks by the application of mineral-precipitating bacteria that fills the crack in a relatively short time, offering a solution for this historical problem, while saving money, time and material and making the concrete more reliable by extending its service life. [1]

ADDRESING THE PROBLEM: EXPENSIVE AND TIME CONSUMING CRACKING CONCRETE

What is concrete?
Concrete is a mixture of paste and aggregates (rock). The paste is composed of Portland cement and water that coats the surface of fine and coarse aggregates. Hydration is the chemical process through which the paste hardens and strengthens. This allows the concrete to be such a remarkable material, it is plastic and malleable when newly mixed, and then, strong and durable when hardened. In making concrete, proportions are a key factor. A typical mix is about 15-20% cement, 60-75% aggregate (like sand, gravel, etc.) and 12-20% water. Though it may have low quantities of other substances such as chlorides, sulfates, alkalis, etc. [2]
Concrete’s flaws:
Concrete is the world’s most used building material it has serious weaknesses. Concrete users know they face five main problems when working with this product: discoloration (occasioned by adding extra water, poor workmanship, different cement materials or the adding of calcium chlorides to the mix); scaling or peeling (when water seeps into porous, non-air concrete); crazing (occurs when the cement paste comes to the surface and shrinks); curling (shrinkage of the bottom concrete when hardening dur to moisture and temperature conditions); and, the most relevant and consequential of all, cracking (can result of drying shrinkage, thermal contraction, subgrade settlement and applied loads, among others). [3]
Cracking:
As mentioned above, cracking is concrete’s most severe flaw, mainly because it has deeper and worse consequences, affecting the whole structure. Also, cracking is considered the hardest to control and solve between concrete’s issues because of the time and money it requires. This inconvenience is caused, mainly by tension and pressure. When the cracks become too large, they lead to the corrosion of the steel reinforcement, which results not only in ugly appearance, but most important in the damage of the structure’s mechanical qualities. Concrete cracking is commonly treated in two ways. First, by prevention, engineers usually employ large quantities of steel reinforcement within the concrete structure to avoid, or at least, delay the crack becoming too large. This extra steel is structurally useless and very expensive (starting at 300$ per ton). And, second, by repairing them, which requires human labor, and the stop of the structure’s normal operation. Besides this repairing is not always possible when considering underground constructions, liquid retaining structures or toxic-substances containers. This, of course, represents a waste of time and money, both for the solver and the victim of the problem. [4] So, concrete is a very useful and versatile construction element, which has always attained to civil engineering, but now, to solve its essential flaw, chemical and materials engineers bring a new project: Self-healing concrete, which, theoretically and experimentally, deals with cracking from another perspective. [5]

A REVOLUTIONARY SOLUTION: THE TECHNOLOGY BEHIND THE SELF-HEALING CONCRETE

Bio self-healing concrete originates from the necessity of improving cracking-solving methods, by going beyond civil engineering and entering the field of chemical reactions and biological processes. The project for developing this material is led by the Delft University of Technology, (in association with South Dakota School of Mines) who turned concrete into an intelligent material.
Intelligent materials:
But, how can concrete become an intelligent material? Well, intelligent materials are not necessarily electronic devices. The conditions to be labeled in this category are sensing, processing and actuating automatically when detecting a change in their environmental conditions. To become such a material, concrete is being mixed with specific types of bacteria (Bacillus), along with Calcium lactate, phosphorous and nitrogen, to originate the bio self-healing concrete that produces limestone biologically to heal the cracks that appear on its surface. These specific bacteria can lie dormant (asleep) within concrete for 200 years, but when the concrete cracks and water seeps in the spores of the bacteria germinate. Once they are activated, they start feeding with the Calcium lactate, consuming the Oxygen and turning it into insoluble limestone. Then the limestone solidifies on the cracked surface, similar to the osteoblast cells that help repair broken bones in the human body. [4]
Testing:
Though still under study, this product has already been tested in and outside a laboratory. It has been proved that once the bacteria enter in contact with water and its nutrients through the crack of the concrete, it germinates and multiply quickly, converting the nutrients into limestone within seven days (in the laboratory). When exposed to outer environments with lower temperatures the process may take several weeks. To find the right bacteria has been the most challenging part of the testing process because is very hard to find an organism capable of living in an environment with such a high pH as concrete (13). Finally, the bacteria of the genus Bacillus subtilis JC3 were chosen to be the most suitable, after relating it with endolithic bacteria (bacteria which can live in stones) found on Egypt, Russia and Spain. [6]
Further experimentation:
However, a second self-healing agent is being studied because it would result in a cheaper process, as well as in a stronger concrete. Presently, the most expensive side of creating the self-healing concrete comes from the high cost of the Calcium lactate and in the method of embedding the bacteria in the small pellets of the concrete. So, the group is trying to find out a method to localize the nutrients (sugar-based) such that the bacteria could be stored in a different way, to minimize the volume, they occupy. Besides, commercial partners are giving funding to the Delft research group to find out if the process could be used in already existing structures. This conducted the experiments to develop a spray that contains the bacteria and nutrients dissolved into a liquid state, that allows to coat the surface of the structure with it and see if it can repair the cracks when recognizing them. [6]

APLICATIONS AND ADVANTAGES

Though self-healing concrete can be used for any kind of concrete structure, it is particularly ideal for the construction of underground retainers of hazardous waste, structural basement walls and marine structures. This is because those are particularly difficult, even dangerous areas to work in. Using this new form of concrete allows the construction company to save money, mainly by reducing the quantity of steel needed, and the salaries of so many workers dedicated to fixing a damaged structure. Also, it reduces the wasted time and money of the structure’s owner, as he won’t have to close, block or evacuate the construction during “repairing time”.
Collateral benefits:
Besides reducing the costs of concrete structures, the self-healing concrete also reports other beneficial outcomes. In the process of conversion from Calcium lactate to limestone, the bacteria also consume Oxygen, which is the essential element in the corrosion of steel, protecting the inner steel structure, as well. Also, it helps the environment. 7% of the CO2 that goes to the atmosphere due to human activity is caused by concrete production. By creating a way of preserving concrete and expanding its service life, the production of it is reduced, and with it the CO2 emission.
Bio self-healing concrete seems then to be the solution engineers have searched through history to solve such a common problem as the fracture of their constructions. It offers an amazing solution without changing the material itself, but bettering it to expand its service life and versatility. It also widens the applications of concrete and directs its development towards the fields of Chemical engineering, Materials engineering and Bioengineering.

CONCLUSION

In conclusion, bio self-healing concrete is a project that can change the way we build today. By copying natural processes the research group of the Delft Tech University created a concrete with the capability of repairing itself by mixing it with the bacteria Bacillus suptilis JC3, Calcium lactate, Nitrogen and Phosphorous. The key element are mainly the bacteria and the Calcium lactate they convert into limestone when activated by water. When the concrete cracks, which represents the main weakness of this material, it allows water to seep in, activating the dormant bacteria, that starts working to repair it. This process takes about 28 days to cover the crack, and avoids further cracking. It also results a beneficial creation because it lowers the cost of concrete structures and reduces the use of steel. Also, it is more suitable for assemblies underwater, or very elevated pressures, as well as toxic substances containers. Last, but not least, this invention shows what the combination of different engineering disciplines can achieve when working together: basing the project on healing processes of plants an even of bones in the human body(Bioengineering), focusing on the process of creating a new product by combination of simpler ones (Chemical engineering), and giving place to a solid, tough working material (Civil engineering). Creating this intelligent material, represents a new type of durable and sustainable concrete with a wide range of potential applications. [7]

SOURCES

  1. H. Mihashi, "Development of engineered self-healing and self-repairing concrete," Journal of Advance Concrete Technology, vol. 10, p. 16, 2012.
  2. "Portland Cement Association," 2016. [Online]. Available: http://www.cement.org/cement-concrete-basics/how-concrete-is-made. [Accessed 29 10 2016].
  3. J. Rodriguez, "The balance," 2014. [Online]. Available: https://www.thebalance.com/prevent-c-admixture-845054. [Accessed 29 10 2016].
  4. J. Henk, "Delft University of Technology," 2016. [Online]. Available: http://www.citg.tudelft.nl/en/research/projects/self-healing-concrete/. [Accessed 29 10 2016].
  5. A. Berenjian, "Materials and Processing Research Projects," The University of Waikato, 2015. [Online]. Available: http://sci.waikato.ac.nz/study/qualifications/be/projects/mape. [Accessed 29 10 2016].
  6. Ingenia, "Self-Healing Concrete," 2011. [Online]. Available: www.ingenia.org.uk/Content/ingenia/issues/issue46/arnold.pdf. [Accessed 29 10 2016].
  7. J. Henk, "Development of bacteria-based self healing concrete," Delft University of Technology, p. 6, 2008.