By Pierce Hoenigman, Fall 2021.

Concrete is a material that dates back to the ancient world, famously used by the Romans in their constructions. The incredibly strong Roman recipe is now lost to history, though refinements over the past few centuries have resurrected the material as central to construction. Still, our modern recipe has its flaws. Small cracks that begin to form as a concrete surface ages soon begin to cause problems; in places that get below freezing, water can seep into these cracks and freeze, expanding and worsening the cracks. Direct sunlight and high temperatures can also contribute to crack formation.[1] These processes can compromise the strength of the concrete, since cracks allow water to collect around the steel reinforcement bars (more commonly referred to as rebar), causing them to rust. While concrete is exemplary in its compressive strength (the amount of pressure it can withstand), rebar is added to increase its tensile strength (the amount of stretching it can withstand). Therefore, any compromises could cause dire problems for the overall structure.

Concrete is often repaired in order to prevent structural collapse, though the costs are astoundingly high; it is easy to repair cosmetically with a new layer, yet that does little to address deeper faults in the structure. While the cost to produce concrete lies somewhere between $65 and $80 per cubic meter, the cost to repair and maintain it is estimated to be $147 per cubic meter.[2] Therefore, there is an ongoing search for solutions to the concrete maintenance problem. A radical approach that has recently been the subject of much research (and many sensationalist headlines) is bioconcrete.

Bioconcrete is not one specific solution, but describes an umbrella of methods to produce an effective self-healing material by embedding bacteria into a concrete mixture. Yet, while researchers have suggested a variety of approaches, certain ones have proven to be more effective than others in recent tests. One of these more successful methods uses Bacillus, Arthrobacter, or Rhodococcus bacteria embedded with organic acid salts.[2] These bacterial strains are activated by oxygen and consume oxygen and organic matter to produce carbonate minerals that fill in cracks.[2] Additionally, in consuming the oxygen, the bacteria prevent it from reaching the rebar and causing it to rust.[2] Through this two-pronged result of filling in cracks and depriving the concrete of oxygen, bacteria can help safeguard the integrity of the structure.[2]

Unfortunately, while the method embedding bacteria in organic acid salts has been proven to be the most effective for healing concrete, no one has yet figured out the best way to get the bacteria into the mixture without compromising them. Illustratively, Bacillus bacteria are versatile and can withstand the basic environment of dry concrete, but do not thrive in the even more basic conditions while the concrete is still wet.[2] One suggested approach is to inject small criss-crossing tubes of bacteria and organic acid salt through the concrete, so that if a crack forms, the sustenance and bacteria flow together and begin the healing process.[2] However, this method has encountered issues with getting the contents of the two tubes to flow and combine. Another issue is the lumpy surface that this process creates.[2] An alternative solution is to simply mix in the bacteria with the concrete, but while they may survive under these conditions for a short time, studies have shown that most of them die off shortly after the concrete has been poured.[1] A third solution is to protect the bacteria from the high pH environment of the wet concrete by embedding them within capsules made of ceramic or glass.[2] Unfortunately, this method isolates them from the cracks as the capsules may not break open after the concrete sets. Therefore, an effective bacterial concrete needs to contain bacterial capsules that are strong enough to withstand the originally alkaline environment but weak enough to crack over time, allowing the bacteria inside to work on the damaged concrete.[2]

Despite the challenges associated with incorporating bacteria into the concrete, bacterial concrete shows promise when compared with the durability of traditional concrete. For example, one study found that bioconcrete reduces surface permeability, especially under higher temperatures.[1] Water absorption in a sample with one species of Bacillus was half of that in regular concrete as well.[2] These advantages in durability could greatly lower the frequency and subsequently the cost of crack repair, extending the lifetime of a concrete specimen before it has to be replaced. Still, while bioconcrete has the advantage of resistance to the elements, there are conflicting reports on its strength when compared with traditional concrete. Though no long-term studies have been done, comparative tests on mortar after a number of weeks show that adding Bacillus species can decrease concrete’s compressive strength by 15 to 63 percent.[2] On the other hand, the capsule method of injecting Bacillus into concrete has been found to increase its compressive strength by 24 percent.[2] The capsule method is therefore likely the most effective procedure for producing bioconcrete and proves that the new material can in practice outcompete its traditional competitors in strength.

While bioconcrete appears to be a promising solution to long-term concrete damage, there are some issues in bringing it to the construction industry. For one, a lot more research is needed in order to come to a conclusion about the proper method of inserting bacteria into the mix. Further, long-term tests are also needed to confirm the strength comparison between bioconcrete and regular concrete. Even with sufficient research, it is likely that the entrenchment of traditional concrete will continue to dominate the market for some time to come. Bioconcrete is more complicated to produce and requires more expensive materials. Therefore, it will take time for bioconcrete to become competitive in price with traditional concrete. Bioconcrete may not take over by next year or even by 2030, but this emerging technology will likely have a large impact on the cities of the future.

[1] Khitab, Anwar et al. 2016. “Sustainable Construction with Advanced Biomaterials.” Science International 28, no. 3 (May-June): 2351-2356. http://www.sci-int.com/pdf/636303641718553865.pdf.

[2] Seifan, Mostafa et al. 2016. “Bioconcrete: next generation of self-healing concrete.” Applied Microbiology and Biotechnology 100, no. 2 (January): 2591-2602. https://doi.org/10.1007/s00253-016-7316-z.

[3] Eagle, Robert. 2013. “Self-healing concrete.” Photograph. 5028×3266. Institute of Making, UCL, https://www.flickr.com/photos/uclnews/8518049675.