What if cracks in concrete could fix themselves? - Congrui Jin

Concrete is the most widely used

construction material in the world.

It can be found in swathes of city pavements,

bridges that span vast rivers,

and the tallest skyscrapers on earth.

But this sturdy substance

does have a weakness:

it’s prone to catastrophic cracking

that costs tens of billions of dollars

to repair each year.

But what if we could avoid that problem,

by creating concrete that heals itself?

This idea isn’t as far-fetched

as it may seem.

It boils down to an understanding

of how concrete forms,

and how to exploit that process

to our benefit.

Concrete is a combination of coarse stone

and sand particles, called aggregates,

that mix with cement, a powdered blend

of clay and limestone.

When water gets added to this mix,

the cement forms a paste and

coats the aggregates, quickly hardening

through a chemical reaction

called hydration.

Eventually, the resulting material

grows strong enough to prop up buildings

that climb hundreds of meters

into the sky.

While people have been using

a variety of recipes to produce cement

for over 4,000 years,

concrete itself

has a surprisingly short lifespan.

After 20 to 30 years,

natural processes like concrete shrinkage,

excessive freezing and thawing,

and heavy loads can trigger cracking.

And it’s not just big breaks that count:

tiny cracks can be just as dangerous.

Concrete is often used

as a secondary support

around steel reinforcements.

In this concrete, even small cracks

can channel water, oxygen,

and carbon dioxide that corrode the steel

and lead to disastrous collapse.

On structures like bridges and highways

that are constantly in use,

detecting these problems

before they lead to catastrophe

becomes a huge and costly challenge.

But not doing so

would also endanger thousands of lives.

Fortunately, we’re already experimenting

with ways this material

could start fixing itself.

And some of these solutions

are inspired by concrete’s

natural self-healing mechanism.

When water enters these tiny cracks,

it hydrates the concrete’s calcium oxide.

The resulting calcium hydroxide

reacts with carbon dioxide in the air,

starting a process called

autogenous healing, where

microscopic calcium carbonate crystals

form and gradually fill the gap.

Unfortunately, these crystals

can only do so much, healing cracks

that are less than 0.3mm wide.

Material scientists have figured out how

to heal cracks up to twice that size by

adding hidden glue into the concrete mix.

If we put adhesive-filled fibers

and tubes into the mixture,

they’ll snap open when a crack forms,

releasing their sticky contents

and sealing the gap.

But adhesive chemicals often behave

very differently from concrete,

and over time, these adhesives

can lead to even worse cracks.

So perhaps the best way to heal

large cracks is to give concrete

the tools to help itself.

Scientists have discovered that

some bacteria and fungi

can produce minerals,

including the calcium carbonate

found in autogenous healing.

Experimental blends of concrete

include these bacterial or fungal spores

alongside nutrients in their concrete mix,

where they could lie dormant

for hundreds of years.

When cracks finally appear

and water trickles into the concrete,

the spores germinate, grow, and consume

the nutrient soup that surrounds them,

modifying their local environment

to create the perfect conditions

for calcium carbonate to grow.

These crystals gradually fill the gaps,

and after roughly three weeks,

the hard-working microbes

can completely repair cracks

up to almost 1mm wide.

When the cracks seal,

the bacteria or fungi will make spores

and go dormant once more—

ready to start a new cycle of self-healing

when cracks form again.

Although this technique

has been studied extensively,

we still have a ways to go

before incorporating it

in the global production of concrete.

But, these spores have huge potential

to make concrete

more resilient and long-lasting—

which could drastically reduce

the financial and environmental cost

of concrete production.

Eventually, these microorganisms

may force us to reconsider

the way we think about our cities,

bringing our inanimate concrete jungles

to life.