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,
最终 得到的材料很牢固 足以支撑
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,
过度冰冻 冰雪融化 重负载等自然过程 都能引起破裂
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
也能进入水 氧气 和二氧化碳
can channel water, oxygen,
and carbon dioxide that corrode the steel
and lead to disastrous collapse.
在类似桥梁 公路 这经常用建筑
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,
may force us to reconsider the way we think
about our cities, bringing our inanimate concrete jungles to life.