Authors: Mouna Reda, Post doctorate fellow, Department of Civil Engineering, McMaster University and Samir Chidiac, Professor, Civil Engineering, McMaster University

self-healing concrete

Photo: Justus Menke / Unsplash

As winter approaches, Canada’s roads, bridges, sidewalks and buildings are facing a familiar problem: cracks caused by large temperature swings. These cracks weaken infrastructure and cost millions to repair every year.

But what if concrete could heal itself like human skin, keeping our structures, roads and bridges strong and saving millions of dollars?

Concrete is the most widely used construction material, known for its durability and low maintenance. Yet it’s still susceptible to cracking.

Concrete is made by mixing cement, water, aggregate and other chemicals used to enhance its properties. As cement reacts with water, it forms a paste that binds everything together.

During this process, changes in volume, improper placement and finishing, and later environmental factors can create cracks. These cracks allow water, other liquids, gases and harmful chemicals to penetrate the concrete, compromising its strength over time.

This challenge has led researchers to eagerly explore what can be done to heal these cracks. In our research, we are researching how self-healing concrete can make infrastructure more durable.

Self-healing concrete

Cracked concrete under the Spadina Avenue exit ramp on Toronto’s Gardiner Expressway. Photo: Sylvia Mihaljevic

When our skin is cut, it’s able to heal on its own. Inspired by this, researchers started re-imagining concrete with similar abilities.

Traditional concrete is able to mend small cracks when water triggers leftover cement in a process known as autogenous healing. This process, however, is very slow and limited to narrow cracks. Since concrete is man-made, it has limited ability to “self-heal” without a little extra help. This led researchers to develop what is called autonomous healing.

Autonomous healing mimics nature by adding special materials like minerals, polymers, micro-organisms or other healing agents into concrete. These materials react chemically or physically with concrete to fill the cracks.

The first modern concept of self-healing concrete was introduced by American researcher Carolyn M. Dry in the early 1990s. In 2006, Dutch microbiologist Hendrik M. Jonkers developed a special concrete that uses bacteria to heal cracks.

Later, Jonkers and civil engineer Erik Schlangen gained attention with “bio-concrete” that incorporates bacteria in spore form. When moisture enters a crack, the spores activate and produce calcium carbonate, one of the most suitable fillers for concrete.

This process, called microbiologically induced calcite precipitation, can heal cracks up to one millimetre wide. The process, however, is very slow and depends on the presence of calcium and moisture in concrete, which makes applying it on a large scale challenging.

Beyond bacteria

self-healing concrete

Confederation Bridge in Cape Jourimain, N.B. 8canada). Photo: Deana Davis / Unsplash

The limitations of bacteria-based self-healing led researchers to explore chemical-based mechanisms. These healing agents will react with water, air, cement or curing agent to fill in cracks quickly.

Healing agents can work in two ways: some use a single material, like sodium silicate. Others, like dicyclopentadiene, need two materials. For a two-component type, a substance must be added to start the reaction, and both materials must be released at the same time to repair cracks.

This chemical method can repair larger cracks and works faster than the bacteria-based approaches but comes with its own challenges. The biggest question is: How can we ensure the healing agent survives concrete mixing and is only released when a crack forms?

To address this, researchers store the healing agent in protective mediums — either a special network (called a vascular network) or tiny capsules. These storage mediums protect the healing material until a crack forms. When that happens, the capsules or network rupture to release the healing agent and fill the crack.

Vascular networks require an external reservoir to supply the healing agent, which makes them difficult to cast, vulnerable to damage during casting and susceptible to leaks. Because of this, encapsulation has emerged as a promising approach.

Encapsulation as a potential solution

Encapsulation involves coating the active agent with polymeric shells to create micro-capsules. Despite its promise, this technique still faces hurdles. Researchers use different methods to make and test the capsules, and there is no standardized way to compare results or test efficacy. The bond between the capsule and the surrounding concrete poses additional challenges and needs more investigation.

In our lab at McMaster University, we are researching the optimum geometrical and mechanical properties of capsules that are compatible with the surrounding concrete. The capsules should survive concrete harsh mixing conditions, while still rupture upon cracking.

We’re also developing a standarized test method to evaluate the survival capsule rate during mixing, and another test to evaluate the efficiency of the self-healing concrete system. And we’re investigating the feasibility of incorporating both bacteria- and chemical-based capsules for short- and long-term self-healing.

More research is needed to determine which self-healing method works best —bio-concrete, chemical-based concrete or perhaps a combination of both.

Ultimately, finding ways to integrate these solutions into infrastructure will benefit communities around the world. Cracks in concrete don’t just look bad; they lead to deterioration over time and costly repairs. That is why developing concrete that resists cracking or heals itself is so important. The Conversation

This article is republished from The Conversation under a Creative Commons license. Original article.

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