Revolutionizing Construction & Combating Climate Change: The Promising Future of Self-Healing Concrete

Dallas O’Connor

Concrete is the most used material in the world. It is highly durable, strong, versatile, and cost-effective. It withstands diverse weather conditions, resists erosion, and can be shaped into various forms which makes it ideal for a wide range of construction projects. However, it is not without flaws. These shortcomings present challenges not just for engineers but for society as a whole. Fortunately, current advancements in technology may offer solutions to these issues—one of the most promising being self-healing concrete.

To discuss a topic as innovative and complex as self-healing concrete, it is essential to establish a clear understanding of the basics. As a future structural engineer, understanding the behavior and mechanisms of concrete is crucial. Even for those with a strong interest in the topic, it can be challenging to fully comprehend. With that in mind, I will explain concrete in the most digestible method possible. 

Concrete is primarily composed of four ingredients: coarse aggregate (such as gravel), fine aggregate (such as sand), water, and cement (see Figure 1). These components are then mixed, poured into whatever mold it was designed for, and eventually cures (becomes solid). To clarify, cement is a powdery substance made by heating limestone and clay in kilns (ovens) up to 2,700 degrees Fahrenheit which leads us to the first major problem with concrete. Concrete production is responsible for roughly 9 percent of all global CO2 emissions. The process that causes the majority of these tremendous emissions is the production of the cement. Unfortunately, cement is one of the most energy intensive products in the world because it is heated almost exclusively using fossil fuels. To make matters even worse, carbon dioxide is also a byproduct of the chemical reaction involved to produce cement.1

Figure 1: Concrete Ingredients

Carbon emissions aside, concrete poses challenges to engineers like myself in many different ways. Concrete is a brittle material that is extremely strong in compression, but extremely weak in tension. To clarify this concept, I will describe a simple experiment. Picture a concrete cylinder as shown in Figure 2. First, we are going to place the cylinder on a table and apply a downward force until the concrete cracks or is destroyed. We conducted phase 1 of this experiment and saw that the concrete failed at roughly 1000 pounds of force. Next, to test the concrete’s tensile strength, we are going to keep placing pebbles in a bucket attached to the concrete cylinder, as shown in Figure 3, until it is destroyed. This time, the concrete failed when roughly 80 pounds of pebbles were placed in the bucket. While concrete strength depends on many different factors, such as mix design, size, and shape, for the sake of understanding concrete’s weaknesses, this serves as a fundamental example.

Figure 2: Concrete in Tension and Compression

Figure 3: Tensile Strength Experiment

Understanding the mechanisms in which concrete cracks is critical for structural engineers. There are many ways in which concrete cracks, but all of them can be simplified to some sort of tensile force being applied to the concrete that exceeds its designed tensile capacity. This is why reinforcing steel, commonly known as rebar, is incorporated into concrete. Steel has a high tensile strength which, when binded with concrete, provides additional tensile strength to prevent cracks from forming. However, completely preventing cracks is impractical and would require additional concrete or steel, significantly increasing costs. Hence, structural engineers design concrete with allowable sized cracks and attempt to reduce the size of cracks that form. But if concrete isn’t designed well, isn’t properly mixed, or is exposed to natural hazards like earthquakes, cracks can cause gradual damage to the point of failure. That is because when concrete cracks, water, carbon dioxide, and other harmful substances can enter which can increase the crack size and cause corrosion to the rebar resulting in significant loss of strength. This is the other major problem of concrete despite its significant usage. 

In fact, according to a study published by iScience, in 2020 26 Gigatons of concrete was produced which is 26 billion metric tons or 57.3 trillion pounds. The need for concrete is not only increasing each year but despite the mitigation efforts which have contributed to carbon emission savings, the increase in production has greatly outweighed the impact of these efforts.2  Futhermore, with the explanation of concrete’s gradual cracking used previously, it is evident that concrete structures are not able to last for centuries. In reality, its life span usually doesn’t even exceed 100 years.3 This is extremely problematic considering 42 percent of all bridges in the United States are already over 50 years old.4 Due to this aging infrastructure, the Infrastructure Investment and Jobs Act was passed in 2021 which included $550 billion in new spending in order to upgrade vital infrastructure such as bridges, roads, railways, and airports.5 This strengthens the notion that concrete usage will not be decreasing anytime soon and the need for a significant improvement is necessary. Ultimately, constructing a sustainable future requires materials that do not compromise the environment in the process.

However, there is hope with promising research on self-healing concrete. Self-healing concrete is a revolutionary material that can repair its own cracks without needing manual intervention. By incorporating special mechanisms, it can automatically repair any damage, which helps prevent the gradual degradation that typically leads to failure. This innovation not only increases the durability and lifespan of concrete structures but also reduces the need for frequent repairs, ultimately cutting down on the carbon emissions associated with manufacturing and maintaining concrete. Researchers have explored several approaches to self-healing concrete. Among these, bacterial and crystalline self-healing methods show significant potential.

Bacterial self-healing concrete is a groundbreaking material designed to heal cracks as they form. This innovative approach uses specific bacteria that can produce calcium carbonate (limestone) when exposed to water and carbon dioxide to fill cracks and restore the concrete's strength, essentially enabling the material to "heal" itself. 

The process starts with the integration of bacteria and nutrients into the concrete mixture as noted in research published in the Journal of Building Engineering when it was stated that “to ensure that the living cells can potentially grow and react, an organic food … must also be supplied in close proximity to the bacteria.” These nutrients serve as a food source for the bacteria, ensuring they can activate and produce the material needed to seal cracks.4

There are two primary methods for incorporating bacteria into concrete: direct addition and encapsulation. In direct addition, bacteria and nutrients are mixed directly into the wet concrete before it sets. This approach ensures even distribution of the bacteria throughout the material. However, the high alkaline environment of concrete may shorten the lifespan of the bacteria, requiring careful selection of strains that can survive these conditions. Encapsulation is a method in which the bacteria and their nutrients are enclosed in small, protective capsules that are added to the concrete mix as seen in Figure 4. These capsules, also known as microcapsules, usually have an average size ranging from 0.1 to 0.5 millimeters.6 These capsules shield the bacteria from the harsh conditions within the concrete and allow them to remain dormant. This method is especially useful for long-term applications, as it helps preserve bacterial viability over the lifespan of the structure. Then, when a crack forms, these capsules rupture from the disruption of the concrete, releasing the bacteria into the affected area. In both cases, the bacteria react with water and nutrients to produce carbon dioxide, which combines with calcium ions in the concrete to form limestone. This limestone fills the cracks, halting further degradation of the concrete.

Figure 4: Bacteria Capsules

Similarly, crystalline self-healing concrete is a technology that also enhances the ability of concrete to repair itself when cracks form. This is achieved by adding a special ingredient called crystalline admixture (CA) to the concrete mix. CA is a material made from reactive chemicals that interact with water, forming crystalline structures within the concrete as seen in Figure 5. When a crack forms in the concrete, water enters the crack and reacts with the CA, triggering a chemical process that produces an insoluble precipitate, mainly calcium silicate hydrate which is a gel like substance.7 This is the same substance that is created when cement reacts with water. This material then fills the cracks and seals them as seen in Figure 6.

Figure 5: Crystalline Structure

Figure 6: Crack Filled by CA

The key difference between crystalline and bacterial self-healing concrete lies in their mechanisms and complexity. Crystalline systems use chemical reactions to form crystals that seal cracks, activated solely by water, while bacterial systems rely on living bacteria that require both water and nutrients to produce material for repair. Crystalline concrete is simpler to implement, as it avoids the challenges of maintaining bacterial viability and nutrient supply. In contrast, bacterial concrete introduces biological complexity, necessitating careful handling and additional resources for its activation.

Despite its promise, self-healing concrete still faces several limitations that hinder widespread adoption. Current systems are most effective for sealing small cracks (typically under 0.5 mm), leaving larger structural issues unaddressed. Bacterial self-healing concrete, in particular, involves high production and transportation costs due to the need for specialized bacteria and nutrients. Crystalline methods, while simpler, may require precise environmental conditions for optimal performance. Both systems face challenges with long-term durability and consistent activation over a structure's lifespan. I was fortunate enough to discuss self-healing concrete with Professor Weina Meng at Stevens Institute of Technology in the Department of Civil, Environmental and Ocean Engineering. She has condcuted extended research on concrete materials and has even assisted in MCC research which is multifunctional cementitious composite that has the ability to heal itself. When asked in what ways self-healing concrete needs to improve in order for it to be used consistently she commented that “for now the limitations for the self healing technologies is the applicability for scalability.” She went on to explain that self-healing concrete needs to be able to be used for any application in order for its use to be justified. Therefore, future advancements must focus on enhancing crack-sealing capacity, reducing costs, and ensuring reliable performance under diverse real-world conditions. 

Self-healing concrete offers a promising path toward a more sustainable future by significantly reducing carbon emissions and combating climate change. Its ability to extend the lifespan of structures by 30–40% minimizes the need for repairs and replacements, saving resources and energy.8 These innovations not only decrease emissions but also align with global goals for a greener planet, highlighting self-healing concrete as a transformative solution for building a more sustainable world.

References

1. Fischetti, M., Bockelman, N., & Srubar, W. (2023). Solving Cement’s Massive Carbon Problem. Scientific American. https://doi.org/10.1038/scientificamerican0223-52

2. Watari, T., Cao, Z., Serrenho, A. C., & Cullen, J. (2023). Growing role of concrete in sand and climate crises. iScience, 26(5), 106782. https://doi.org/10.1016/j.isci.2023.106782

3. Arkin, Daniel, and Daniel Arkin. 2023. “How Long Does Concrete Last? The Truth About Precast Concrete.” Premier Precast. January 9, 2023. https://premierprecast.com/concrete-lifespan/.

4. Nodehi, Mehrab, Togay Ozbakkaloglu, and Aliakbar Gholampour. 2022. “A Systematic Review of Bacteria-based Self-healing Concrete: Biomineralization, Mechanical, and Durability Properties.” Journal of Building Engineering 49 (January): 104038. https://doi.org/10.1016/j.jobe.2022.104038.

5. McBride, James. 2023. “The State of U.S. Infrastructure.” Council on Foreign Relations, September 20, 2023. https://www.cfr.org/backgrounder/state-us-infrastructure.

6. Cousins, Stephen. 2019. “Concrete That Repairs Its Own Cracks Is Tantalisingly Close.” RIBA Journal. April 3, 2019. https://www.ribaj.com/products/self-healing-concrete-cambridge-university-research-stephen-cousins.

7. Zhang, Guang-Zhu, Cen Liu, Xiang Ma, and Xiao-Kun Yu. 2023. “The Effects of Crystalline Admixture on the Self-Healing Performance and Mechanical Properties of Mortar With Internally Added Superabsorbent Polymer.” Materials 16 (14): 5052. https://doi.org/10.3390/ma16145052.

8. Basilisk. 2022. “Sustainable Concrete for a Better Environment - Basilisk Self-Healing Concrete.” Basilisk Self-Healing Concrete. March 21, 2022. https://basiliskconcrete.com/en/sustainable-concrete-for-a-better-environment/.