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.