In “Making the Impossible Possible: Concrete,” I examine the history of concrete. For 12 millennia, humans have been using concrete to strengthen floors and walls. And as we tweaked its list of ingredients, making it more robust, those walls became roofs, buildings, and much, much more. Today, its evolution continues with groundbreaking advancements like self-healing concrete technology, paving the way for even more resilient structures of the future.
Concrete and the environment
The downside to concrete is that it’s an energy-intensive building material. Its main ingredient—sand—is the second-most consumed natural resource on the earth. At first blush, this shouldn’t be an issue because sand is everywhere—beaches, playgrounds, ditches, deserts, rivers, lakes, oceans. It might be everywhere, but that doesn’t mean all sand is created equal. Because it isn’t.
The windblown sand in deserts, such as the Sahara, is plentiful. It’s also round and smooth, so it does a poor job of sticking together, an essential characteristic of concrete. In contrast, sand on beaches, riverbeds, floodplains, lakes—basically wherever there’s water (or the hint of water)—is more angular. Meaning the individual grains have more surface area to lock together, making it a far superior material for concrete. It’s so superior that some parts of the world are running out of it.
Concrete contributes up to 8 percent of total carbon dioxide emissions. All told, this comes to about 3 billion metric tons. Almost half of these emissions result from manufacturing clinker, which involves heating limestone and clay to 2,600°F, grinding them to a fine powder, and mixing them with additives to make cement. The remaining half comes from mining the limestone, clay, and additives and trucking them to be processed and then to the construction sites. Water and air quality are also impacted, especially at the mines and cement factories.
The final ingredient in concrete is water. Used throughout the entire production process, from clinker to concrete, it accounts for about 9 percent of global industrial water withdrawals. In a world impacted by climate change, with many regions drought-stricken or on the verge, the production of concrete siphons away vast amounts of water, exacerbating the stresses that agriculture and community water systems place on water supplies.
A final downside to concrete is its durability. The Romans built their structures with concrete, and many are still standing 2,000 years later. In contrast, modern concrete can last at most a century. Why? Because modern concrete contains reinforcing bar (rebar) to boost its strength. Unfortunately, concrete is prone to cracking, potentially exposing the rebar to water. Should this happen, the rebar can rust, and eventually the surrounding concrete crumbles away.
Despite all these detractions, concrete is here to stay. We’re producing more of it today than we did 50 years ago, and those production levels will continue rising well into the future. Estimates vary as to exactly how much we currently produce—from 4 billion tons to 30 billion tons—but suffice it say, it’s a lot of concrete, whatever the actual total is!
Concrete might be a dirty building material, but that doesn’t mean it has to stay this way. The good news is that building materials engineers have been focusing on ways to make concrete less energy demanding, thereby reducing its carbon footprint while enhancing its durability. These techniques address energy use (e.g., solar or wind power), clinker and aggregate substitution, polymer rebar, and self-healing concrete.
Autogenous self-healing concrete
What’s interesting about self-healing concrete is that it’s always been around. Regardless of how well concrete is mixed, a portion of cement remains unhydrated. When a tiny crack forms and the unhydrated cement comes into contact with water, the resulting chemical reaction produces calcium carbonate (aka limestone).
This is called autogenic self-healing because the concrete uses the surrounding material to heal itself. Unfortunately, as the concrete ages, the proportion of unhydrated cement declines, as does this natural reaction. But what if we enhanced this autogenic process? Could structures last well beyond a century?
Researchers are testing different additives, including microscopic fibers, polymers, and minerals, to answer these questions. Fibers prevent microcracks from getting too large; thus, enabling concrete to heal itself. Polymers (or hydrogels) serve a twofold purpose: they interfere with microcracks, blocking them like micro-linebackers, and they are super absorbent. Capable of ballooning several times their size, they gobble up any water that’s present. Then, the water is released back into the concrete at a rate that encourages intrinsic self-healing. Minerals enhance and extend concrete’s self-healing capabilities. Fly ash, blast furnace slag, rice husk ash, waste plastic, and feldspar are among dozens of materials, natural and synthetic, being considered.
Autonomous self-healing concrete
There’s another kind of self-healing system called autonomic, in which healing contents are packaged like pills and kept separate from the concrete. These agents are delivered in one of two ways. The first technique embeds the agent in a material that withstands the harsh mixing process yet breaks when ruptured by microcracks. Despite these riddle-like challenges, there are a surprising number of materials that can do just that, including diatomaceous earth, gelatin, silica, plant fibers, glass, and ceramic.
Similarly, several healing agents can be employed, like epoxies, cyanoacrylates (super glues), and even bacteria. All are showing promise, but none of them are as sexy as bacteria. Besides sparking the creation of bioconcrete and perhaps a revolution in the construction industry, bacteria may give us a badly needed boost with climate mitigation. Here’s how:
We mix these little critters with their favorite food, calcium lactate, then encase them in a hydrogel that tolerates the high alkaline environment. If left undisturbed, bacteria can lie dormant for potentially 200 years. If exposed to water, they reactivate. Hungry from their long slumber, bacteria snarf the food and produce calcium carbonate as a byproduct. Besides filling microcracks, they absorb carbon dioxide beyond what concrete naturally absorbs.
The second technique employs a microvascular network of hollow tubes containing one or more healing agents that react to different conditions. Also, these tubes align themselves so that they connect the interior to the exterior. This way, healing agents can also be applied to the surface, which the tubes absorb and shuttle where needed. This last feature gives microvascular networks a leg up over capsules because they can deliver healing agents indefinitely, whereas capsules are finite in number.
The benefits of self-healing concrete are many. Its comprehensive strength is enhanced, allowing structures to withstand greater loads. Unlike regular concrete, it requires less maintenance and repairs, which helps offset the increase in upfront costs. Because it has low permeability and heightened resistance to freeze-thaw cycles, its durability is considerably greater than regular concrete, so structures can last decades, perhaps even centuries longer. Longer-lasting structures mean they need to be replaced less frequently, and the overall demand for concrete diminishes.
Setting aside the elevated costs, this is a burgeoning technology, so only a few companies are working with it. Also, it doesn’t have any standardized codes because it’s still so new. However, our urban environments will continue expanding, and as sustainable building practices, such as self-healing concrete, become more mainstream, these issues will disappear. Over the next four years alone, its market growth is anticipated to rise 8 percent to $10.8 billion.
Between 1880 and 2020, seven of the hottest years on record have happened since 2014, with 2020 taking silver. Global temperatures go hand in hand with greenhouse gas emissions. To control one, we need to control the other. Considering that the construction industry accounts for 38 percent of these emissions, any advancements in this industry will have immediate benefits in our fight to mitigate climate change.
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