Three researchers have made great strides toward potential innovations in non-invasive (i.e. non-human intervention) repair solutions for concrete and asphalt pavement.

concrete

Victor Li, professor of civil and environmental engineering at the University of Michigan, has developed a concrete mix that can not only repair itself but also bend. A property that could potentially mitigate the major weakness (and repair cost) of concrete cracks. Cracks lead to failure when moisture and other contaminants penetrate the material, expand and contract, and attack steel rebars and meshes.

“Concrete is as brittle as glass. So if you try to push it or bend it, it breaks, “says Li.” And once it cracks, everything will be bad. “

“It essentially forms an adhesive and connects the two sides of the microcracks and basically heals them.”
– Victor Li, professor of civil and environmental engineering at the University of Michigan

Li’s methodology focuses on a layered structure in the concrete mix. His inspiration was a phenomenon that he first observed in an abalone shell, in which mother-of-pearl, also known as mother-of-pearl, lies in shimmering layers. “If you look… Under the microscope you can see that this material is heavily layered,” he says. “Some people call it the” brick layer “technique.”

This is how it works; Li’s concrete mix contains coated microfibers. When the concrete is stressed, these fibers shift and distribute the stress. The material cracks, but only marginally, and shifts and readjusts in a relative interlocking pattern. The composites in the mix are called Engineered Cementious Composites (ECCs).

The self-healing part occurs when water and air penetrate the material and react with “unused cement particles” in the concrete.

“It essentially forms an adhesive and connects the two sides of the microcracks and basically heals them,” says Li. “Many grains of cement in concrete material never get fully hydrated. That is, they are never consumed when converting cement into concrete after mixing water in it. They just stay dormant. When exposed to water and air, these cement grains can heal themselves. “

According to Li, one of the main reasons this blend was developed is for durability. “We want to make sure we don’t need a lot of maintenance,” he says.

He aggressively estimates that this material would extend the life of a bridge by 100 years, “with very little repair requirements”.

According to Li, the other benefit of his specific design is sustainability. “Obviously, if we can reduce the need for repairs, we’re using less material over the life of a bridge,” he says. “We emit less carbon dioxide. We use less energy. In this way we reduce the CO2 and energy footprint of our infrastructures. “

Bioconcrete

Making “Bioconcrete”: Add bacillus bacteria that can withstand harsh environments for long periods of time and a food source in the form of calcium lactate. Both are added to the mixture via biodegradable capsules. Water that penetrates through cracks in the concrete dissolves the capsules and activates the bacteria and calcium lactate. The bacteria feed on and the resulting reaction creates limestone / calcite which expands to fill and connect any cracks. (Photo source: Delft University of Technology)

Another approach to self-healing concrete was developed by Henk Jonkers, professor at the Technical University of Delft in the Netherlands. He developed what he calls “bioconcrete” – in a sense a “living” mixture that heals itself with bacteria.

Jonkers’ concept was to create something that would combat the issues raised in his research – minimizing cracks and potential corrosion, but using a much more reactionary methodology.

This is a simple process: add bacillus bacteria that can withstand harsh environments for long periods of time and a source of food in the form of calcium lactate. Both are added to the mixture via biodegradable capsules.

Water that penetrates through cracks in the concrete dissolves the capsules and activates the bacteria and calcium lactate. The bacteria feed on and the resulting reaction creates limestone / calcite which expands to fill and connect any cracks.

The result of Jonkers’ work is clearly a very sustainable product. But it is also a foray into biological solutions to problems that are traditionally seen as unsolvable by natural processes.

asphalt

A now popular demonstration at a 2012 TED lecture by Erik Schlangen, Professor of Civil Engineering at Delft University of Technology, showed how an asphalt pavement can be repaired if the mixture contains steel wool fibers and is heated more or less by microwaves.

Production of “Biokonkret”: Erik Schlangen, Professor of Civil Engineering at the Technical University of Delft, has shown how asphalt can be repaired with steel wool fibers heated by microwaves. (Photo source: Delft University of Technology)

It was an impressive presentation and excited many people about the possibilities. In the meantime, Schlangen continues to test the technology.

However, the term self-healing is difficult in that it implies that no external effort is required. More recent self-healing concrete research corresponds to this concept, as these solutions do not require any external intervention, only rainwater. This is a great solution provided the material is not in a drought affected area.

The definition of self-healing can safely be expanded to include minimal interference. Therefore, being able to perform repairs by placing a machine over a cracked section of asphalt, applying heat with the microwave, and letting it cool is quite promising.

However, a disadvantage of microwave heating is that it is extremely expensive and requires a lot of energy. It also requires shielding; to avoid possible malfunctions of the equipment and to protect the people in the vicinity. It would also take a lot of fine-tuning to get the right depth, which can be difficult to pinpoint. In addition, unknown materials in the asphalt can react adversely and possibly cause explosions.

In addition to microwave heating, induction heating has been used in bridge decks and is currently being tested for other asphalt repair applications. However, similar to inductive hobs, inductive heating requires electrically conductive materials to be included in the mixture.

– Chris Hill

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