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SELF-HEALING CONCRETE: A FUTURE SOLUTION FOR AMERICA’S INFRASTRUCTURE PROBLEM

AMERICA’S INFRASTRUCTURE PROBLEM

As a society I think we can all agree that traffic is horrible. Unfortunately, traffic will only worsen if the government continues to under invest in infrastructure. America’s infrastructure problem is much bigger than just congestion and traffic issues. A large part of the infrastructure problem is aging dams, bridges, and roads. Seventeen percent of American dams are classified as having high-hazard potential. This number is increasing rapidly as well. In 2005 there were 10,213 high hazard dams and in 2015 it increased to 15,500. This staggering number of unsafe dams is due to the fact that most dams in America are 56 years old. American bridges are just as bad because 40 percent are 50 years or older. Again, just like dams many bridges are becoming dangerous. About nine percent of the nation’s bridges are structurally deficient which is an improvement from 2015, however, it is concerning that one in every eleven bridges Americans drive over is not structurally sound [1]. The condition of roads is just as poor as dams and bridges with one out of every five miles of highway pavement is in unsafe condition. To repair all the dilapidated dams, bridges, and roads is not an easy task. To repair all the highways would cost 420 billion dollars, the bridges would cost 123 billion, and the dams would be 45 billion [1]. In total this would end up costing 588 billion dollars which would be a large investment for the government, state, and private sector to make, however, it is affordable. The problem is in another fifty years the United States will have to make the same repairs. Dams, bridges, and roads are built with concrete because it is an amazing support. However, over time it cracks and leaves the reinforcing steel bars in bridges and dams exposed to air and water. Water and air will weaken the steel bars and make the structure of the dam or bridge unsafe. While roads do not have reinforcing bars, cracks in the road make for an unsafe driving condition. Because of this concrete as a life span around 50 years [1]. If the repairs are continued to be made with the same concrete used in today’s infrastructure, the United States will have to make repairs every 50 years. If we keep having to make half trillion-dollar payment every fifty years, the cost could soon not be affordable. To truly fix the infrastructure issue, a new material needs to be used that is just as strong as concrete and lasts longer than concrete. Or improvements need to be made to concrete mix that will prevent or delay cracking.


SELF-HEALING CONCRETE

Civil engineers and researchers are choosing to improve the current concrete mix instead of creating a new material. Their current focus is trying to implement self-healing technology into the concrete, that will allow the concrete to repair itself. There are three types of self-healing currently in development. The first involves using super elastic alloys in the reinforcing bars inside of the concrete. These bars can stretch to extreme lengths under pressure and then after the pressure is released, they return to their natural position. Their ability to return to natural position helps reduce the size of cracks in the concrete as the concrete forms to the bars [2]. By reducing the size of the split, there is more protection from water for the reinforcing bars which means less corrosion. The other two methods involve inserting capsules into the concrete mix that react and release a material that fills the cracks. The first of these two capsule methods uses different chemicals inside of the capsule. Depending on what chemical is stored inside the capsule different reactions will happen, however, all the reactions are triggered by the presence of water because water seeps into the cracks of the concrete and breaks the capsules. One possible reaction will create a substance like liquid glass that will seal over the cracks. Another reaction releases an anti-corrosive substance that will coat the concrete and metal reinforcing bars [3]. Both methods protect the support of the concrete and will prolong the lifespan of roads, bridges, and dams. By increasing the lifespan of our infrastructure, we will have to make repairs less often which means less money being spent on infrastructure.


BACTERIA-BASED SELF-HEALING

Possibly the most interesting of all the methods uses bacteria-based self-healing technology, that works by inserting spore-forming bacteria with a calcium-based nutrient into the concrete mix. However, finding bacteria that could survive in the concrete was not easy. Concrete is an extreme alkaline environment with a pH value of 13. Most organisms cannot survive in an environment with a pH value above 10. Because of this engineers and researchers looked towards organisms that could thrive in alkaline environments. They collected many different bacteria samples from alkaline lakes in Russia and endolithic bacteria, which live in stone. After testing the samples in concrete mix, it was found that the only bacteria that could survive were ones that produced spores because they have an extremely thick cell wall that can protect from the effects of the harsh concrete environment [4]. The spore-forming bacteria and nutrients are put into capsules then into the wet concrete mix. These capsules can lie dorman t in the concrete for up to 200 years, however, when the concrete starts to crack and water seeps in, the capsules will break open. [5]. The bacteria will then start to germinate because of the combination of water and calcium lactate. As the bacteria feed on the calcium lactate they consume oxygen which reacts with calcium ions to from calcium carbonate(CaCO3). Calcium carbonate is also known as limestone. The limestone then solidifies on the cracks, which then seals the crack. This process also lowers the pH of the concrete environment, which helps precipitate more calcium reactions [6].


RESEARCH ON SELF-HEALING TECHNOLOGY

Self-healing concrete is still a relatively new technology so there are no large-scale implications of it. Most testing has been small-scale in laboratories except for the researching done by Cardiff University. The research team at Cardiff wanted to replicate simple structural elements found in many construction sites. For their first large test, they created retaining walls. They made five retaining walls each with different contents. One panel had microcapsules and basic mix, another had shape memory polymers, another had bacterial spores, and the other two were control panels. A variable load was then placed on top of each panel. Fortunately, each of the new self-healing techniques performed better than the control. The microcapsule panel showed visual signs of crack healing. The shape memory polymers wall showed an average reduction in crack width of around twenty percent. The bacterial spore panel also showed signs of crack healing [7]. Overall, the first trial was successful for the self-healing concrete methods. Each technique met expectations. Both healing methods produced the healing material and shrank the crack. The shape memory polymer system reduced the size of the cracks like it was meant to. Also, there were no alarming issues with any of the methods, however, this was just the first large-scale trial and more trials will be needed to insure the viability of these methods.


DRAWBACKS TO SELF-HEALING TECHNOLOGY

In small scale trials, researchers have found problems with self-healing concrete. The capsules with the bacteria are mashed into clay and that clay occupies about twenty percent of the volume in the concrete mix. That means there is twenty percent less sand, gravel, or crushed stone in the concrete mix. Those materials are much stronger than the clay so adding the self-healing concrete weakens the concrete’s strength. Just like other new technology, self-healing concrete is expensive. Conventional concrete costs around 90 dollars per cubic meter and self-healing concrete costs around 180 dollars per cubic meter [4]. Despite the weakened strength and higher cost, self-healing concrete is still beneficial to society. The price per cubic meter for the new concrete is so high because it is being made on a small scale. If production was changed to an industrial scale the price would fall. Even with the higher cost, the healing concrete will save money in the long term because our bridges, dams, and roads would require fewer repairs than normal concrete. The weakened strength is not a major concern for bridges, dams, or roads as the compressive strength of the self-healing concrete is weak and bridges, dams, and roads do not rely heavily on compressive strength. The only infrastructure this weakened compressive strength strongly affects is skyscrapers. Luckily, America’s infrastructure problem does not involve skyscrapers rather roads, bridges, and dams which are all old and in need of immediate repair. These repairs have an estimated cost of 588 billion dollars. However, the repairs cannot use the current concrete mix because we will need to make the same repairs in fifty years. Then we will have to pay another half a trillion dollars and this cycle will continue. Fortunately, innovations in concrete technology particularly bacterial spore concrete have created a possible solution to the problem. This concrete has bacteria that react when they encounter water and create limestone that fills the cracks in the concrete. This prolongs the life of bridges, dams, and roads meaning repairs will happen less often. However, there has not been much large-scale testing of this technology, so it is not proven to work yet. Despite this self-healing concrete is the best option currently available to solve this infrastructure and it is the solution that engineers ne ed to further pursue and develop.


SOURCES
  1. “Failure to Act.” American Society of Civil Engineers. 2016. Accessed 10.29.2017. https://www.infrastructurereportcard.org/wp-content/uploads/2016/10/ASCE-Failure-to-Act-2016-FINAL.pdf
  2. A. Abdulridha, D. Palermo, S. Foo, F. Vecchio. “Behavior and modeling of superelastic shape memory alloy reinforced concrete beams.” Elsevier Ltd. 2.9.2013. Accessed 10.29.2017. https://ac.els-cdn.com/S0141029613000114/1-s2.0-S0141029613000114-main.pdf?_tid=2acc9720-bdcd-11e7-bb4f-00000aacb361&acdnat=1509407634_47beadea83cf491a82462faa7a24caab
  3. L. Wenting, J. Zhengwu, Y. Zhenghong, Z. Nan, Y. Weizhong. “Self-Healing Efficiency of Cementitious Materials Containing Microcapsules Filled with Healing Adhesive.” PLoS One. 11.28.2013. Accessed 10.26.2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3842932/
  4. D. Arnold. “Self-Healing Concrete.” Ingenia. 3.2012. Accessed 10.26.2017 http://www.ingenia.org.uk/Content/ingenia/issues/issue46/arnold.pdf
  5. A. Stewart. “The living concrete that can heal itself.” CNN. 3.7.2016. Accessed 10.29.2017. http://www.cnn.com/2015/05/14/tech/bioconcrete-delft-jonkers/index.html
  6. J. Zhang, R. Wu, Y. Li, J. Zhong. “Screening of bacteria for self-healing of concrete cracks and optimization of the microbial calcium precipitation process.” Applied Microbiology & Biotechnology. Aug2016, Vol. 100 Issue 15. 2016. Accessed 10.29.2017. http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=116709847&site=ehost-live
  7. R. Davis, M. Pilegis, A. Kanellopoulos, T. Sharma. “Multi-scale cementitious self-healing systems and their application in concrete structures.” Cardiff University. 2015. Accessed 10.30.2017. http://orca.cf.ac.uk/93656/1/DundeeM4Lpaper.pdf