Researchers Develop Self-Healing Concrete Method
Researchers from the USC Viterbi School of Engineering recently announced that they have created a new method for developing self-healing concrete, using engineered aggregates in place of natural aggregates.
The study, authored by Associate Professor of Civil and Environmental Engineering Bora Gencturk and Ph.D. candidate Xiaoying Pan, has since been published in Construction and Building Materials.
Creating the Concrete
While concrete is reportedly the second-most used material in the world, it has a low tensile strength that causes it to crack under stress or changes in temperature or humidity. The USC research team wanted to look at creating a concrete that is self-healing, but also prioritize keeping costs low and maintaining other concrete properties, such as strength and production method.
To start, Pan and Gencturk identified current methods of self-healing concrete, including adding bacteria, which is activated by chemical reactions resulting from the crack to heal itself, and placing microcapsules containing healing agents inside the concrete that activate once it is stressed. However, Pan said, these methods are expensive and make them impractical in real-world applications.
Concrete is the most widely used building material in the world — but it also easily cracks under stress.— USC Research (@ResearchAtUSC) March 4, 2022
USC Viterbi researchers are developing self-healing concrete that is low cost and upholds the innate strength of the original material: https://t.co/X8ko6AgFcz pic.twitter.com/IdZezh6aoA
“Normally the price of concrete is around $150 per cubic meter,” she said, “But using bacteria-infused self-healing concrete, that price can increase to $6,000 per cubic meter.”
Additionally, these methods require higher-level expertise or technical knowledge to mix appropriately, which would complicate the construction process or result in structural issues due to weakening the concrete’s original compressive strength.
The researchers then proposed creating support inside the concrete that emulates existing aggregate structures to make concrete to offer a realistic option that is easily adoptable by the industry. The first step was to design engineered aggregates with healing agents that would activate once encountering cracks, as well as providing the best healing result for various types of cracks.
Pan added that self-healing concrete is not a new method, but other approaches are not practical or sustainable on a larger scale.
“Basic reactions could cure cracks a few micrometers in size, but not larger,” she said, adding that the bacteria or microcapsule methods were expensive and “anytime you put holes in concrete, it will naturally create weaknesses.”
USC researchers created a computational model to help them identify which shape and size of engineered aggregate is optimal for varying sizes of cracks. The model reportedly offers suggestions for optimal formation in one large crack or multiple small cracks that connect to one another, but can also be applied to other self-healing concrete approaches outside of USC’s.
“When we have different configurations of concrete structures, those structures tend to have different crack sizes,” Pan said. “In that case, we want to know how to select the best size and shape for the engineered aggregate to create an optimal healing effect.”
“It’s a mathematical model, so it can apply to other self-healing concrete materials that put healing materials within a shell structure.”
According to the release, the next step in the research is to study how the overall strength of the concrete is impacted by different sizes and shapes of engineered aggregate. These models are expected to provide recommendations for providing optimal healing effects, while maintaining structural integrity and strength.
Additionally, researchers will look at different types of materials to work with, including polyurethane, sodium silicate and cement. Pan said cement is a good candidate because it is “cheap and won’t increase the price of concrete dramatically.”
Smaller samples have reportedly been tested with only one or two engineered aggregates inside. The team conducted a test to analyze what happens with water flow through a large crack and if the concrete heals sooner rather than later.
“When concrete is cracked, water can flow through it very fast and bring with it aggressive, corrosive agents,” Pan said. “That can lead to more damage.”
Other Self-Healing Concrete Research
Last year, scientists from the Polytechnic Institute of the Far Eastern Federal University (FEFU) announced their development of a concrete material that can reportedly seal cracks and restore strength independently.
The study of self-healing concrete is especially relevant for construction in seismically hazardous areas where small cracks appear in structures, as well as in areas with high humidity or rainfall. The research was conducted alongside colleagues form Russia, India and Saudi Arabia, and has since been published in the Sustainability journal.
To create the self-healing concrete, scientists replaced ordinary water traditionally used when preparing the mixture with an aqueous concentrate containing the bacteria Bacillus cohnii, which can survive within the pores of the dried cement stone, filling any resulting damages with calcium carbonate (CaCO3).
Once the mixture had cured, scientists tested the material for compression until it cracked and then observed how the bacteria was able to fix the gaps and restore the strength of the material. According to the report, once the bacteria was able to access oxygen and moisture through the cracks, the bacteria became “awakened” and was able to successfully heal cracks with a 0.2-0.6 mm width within 28 days by releasing the CaCO3, which crystallized under the influence of water.
After the concrete slabs returned to its original compressive strength, the bacteria “fell asleep” again.
The university notes that spores of the bacteria Bacillus cohnii can live within concrete for up to 200 years, and in theory, could extend the life of structures for that same period—almost four times the service life of conventional concrete.
To conduct the experiment, scientists grew the bacteria Bacillus cohnii in the laboratory using an agar substrate and culture medium, as to force the bacteria to survive within the conditions of the cement stone pores. Crack repair was evaluated under a microscope, while the chemical composition of the repairing agent was isolated and studied using an electron microscopy in addition to X-ray images.
In reaching this discovery, scientists plan to develop a reinforced concrete next, which would further enhance the material’s properties using different types of bacteria with the potential of speeding up the recovery process.