Does Self-Healing Concrete Really Work?

Around 2016, we started hearing a lot about self-healing concrete. In recent years it has been lauded as one of the top civil engineering innovations. The question is: How well does it really work?

How Does Self-healing Concrete Work?

Concrete is one of the most heavily used building materials. It is used in situations with high stress and exposure to the elements. This makes cracking and degradation of the concrete a major concern. If the cracks are large enough and reach the underlying reinforcement, the strength of the structure can be heavily impacted.

There are many methods for self-healing concrete being developed and researched, but the general concept is embedding a substance into the concrete. When cracks form, this substance is activated and the crack is sealed.

This technology mainly uses microcapsules embedded in the cement. Glass capillaries have also been studied, however they found it is difficult to efficiently fill the capillaries. Microcapsules were introduced in 2001 and are a micron sized stable shell around a solid, liquid, or gas. Cracks forming in the concrete also rupture the microcapsules and the contents are released into the crack.

Previously research focused on cyanoacrylates or epoxy as the encapsulated substance, however high toxicity, high cost, and short shelf life make them not ideal for commercial use. Other studies have also looked at embedding the concrete with bacterial spores that secrete calcium carbonate when exposed to air. The most promising mechanisms using encapsulation use bacteria or minerals.

In this before-and-after comparison, you can see the whitish limestone healing the crack. (Source: Syn.de.Bio)

Really A Top Civil Engineering Innovation?

Most of the research on this new technology has been in the lab. There has been very little field scale research done. A study completed in the U.K. in 2019 attempted to remedy that. Researchers at the University of Cambridge used cast retaining wall panels to test how well the self-healing concrete works.

The walls were filled with 8% by volume encapsulated sodium silicate. The sodium silicate reacts with the calcium hydroxide already present in the concrete to form a calcium-silicate-hydrate gel that fills the cracks. A control wall was also built with no microcapsules.
After a curing period of 35 days, all the walls, including the control group, were mechanically cracked. To simulate the real world, the wall was reloaded after being mechanically cracked. The walls were monitored for six months by measuring the air permeability, crack depth, and microscopic crack width.

Overall, the study found the self-healing concrete using microencapsulated sodium silicate could be used in real world applications. They found the addition of the microcapsules slightly reduced the mechanical strength of the wall; however this was not found to be significant enough to discount the benefits.

The walls showed improved crack width reduction, crack-depth reduction, and recovery in permeability. On average, the crack depths reduced by 20-58% at end of monitoring period. The permeability of the wall was recovered by almost greater than 2.5 orders of magnitude. This indicates that the self-healing wall works well enough to keep water or other fluids from flowing through the wall. Microscopic imaging validated these observations and showed that the observations made in a laboratory are still valid in a real-world setting.