How Concrete Cures
There is a common misconception that concrete goes hard when it has “dried out”; this is almost the opposite of what actually needs to happen when fresh concrete is poured.
The process of concrete going hard is called curing, and it is a little more complicated than you might think. For one thing, it can be days, even weeks after being poured before concrete attains anywhere near its full strength. After that, it will still not be fully cured for years, even decades later.
Of course, this does not mean concrete is not usable for the first decade after being poured—it should have around 90% of its total strength after the first few weeks—but it should serve to highlight how this process is not as simple as drying out what is essentially a mixture of concrete, aggregate, and water.
Why Concrete Goes Hard?
Though the answer to this question is far more complex than we are qualified to tackle, the basics are easy enough. When concrete cures, it undergoes a process called hydration, which is a chemical reaction during which the major compounds in the cement bond with the water molecules.
This is why the idea that concrete is “drying out” is inaccurate since it is exactly the opposite that is happening. In fact, you will find in some cases that concrete structures are hydrated as they are curing—spraying it with a hose, for example—in order to aid this process.
It is also important that the water used when mixing concrete be pure since any impurities could cause alternative reactions to take place that might interfere with the hydration of the cement.
Aggregate is a chemically inert solid that is introduced in the concrete mix and can vary in size from grains of sand to coarse rocks, depending on the situation. The main purpose of aggregate is to save money.
Cement is relatively expensive to produce and would be a prohibitively costly material to use on large projects. By introducing aggregate, it is possible to dramatically increase the amount of concrete you have without significantly weakening the compound.
Aggregate needs to be chemically inert for the same reason that the water used in the concrete mix needs to be pure—to stop unwanted chemical reactions taking place that might interfere with hydration. The physical properties of the aggregate depend on the use-case.
For example, concrete that will form the substrate for another surface can be made using larger, coarser aggregate, since it will not be on show when the project is finished. On the other hand, concrete that will form part of a visible fascia or something where aesthetics are important might require a fine sand as aggregate, since that will allow for a smoother finish.
Aggregates are usually cleaned in advance to ensure they are not introducing any unwanted particles in the concrete mix, but even with the added cost of processing the aggregate before mixing, it is still more cost-effective than mixing concrete without any aggregate.
In some cases, it is necessary to make a higher strength concrete. There are many ways this may be achieved, such as reinforcing the concrete with fibre or rebar frames, but for strengthening the concrete itself, speeding the curing process can help. This can be achieved by heating the concrete with steam, which raises the temperature while keeping the concrete hydrated. This leads to the process being completed more quickly.
Curing Times and the Hoover Dam
We mentioned above that, while the majority of the curing is completed early on, it takes concrete much longer to fully cure. A perfect example of this is the Hoover Dam.
Firstly, it is worth noting that a structure as large of the Hoover Dam could not have been poured all at once. A concrete structure that large would simply have taken too long to cure. Instead, the dam was built in smaller segments, so that each one could cure independently.
But even with this method, it was estimated that it would take the full structure over a century to fully cure. Given that the dam would be holding back 9.2 trillion gallons of water, and that the results of a failure would be catastrophic, it’s not surprising that the engineers behind the dam might have looked for a way to speed things up.
And speed things up they did.
Using a series of steel pipes filled with water, the engineers were able to counteract the chemical heat coming from the curing concrete, speeding up the process. They would later fill these pipes with concrete, adding more strength to the overall structure.
At the peak point of construction, the Hoover Dam received a 4×8 cubic yard bucket of concrete every 78 seconds, which would be used to makeover two hundred and thirty enormous blocks of concrete that the dam is composed of. For something small like a garden path, there really isn’t much thought required to pouring and curing concrete. But when you’re building something big, you need to understand the intricacies of how concrete works.
Many people are surprised to hear about concrete burns, but it makes much more sense when you understand that concrete curing is the result of a chemical reaction, rather than the cement drying out.
Concrete burns do not happen all at once, so merely getting some on your skin will not do much damage as long as you thoroughly wash it off in good time. Leaving concrete in contact with your skin, on the other hand, can result in serious injury.
Remember that the curing process takes water through the chemical reaction of hydration. This results in concrete absorbing any moisture around it. Your skin just so happens to be packed with moisture—even when you feel like your skin is “dry”.
You can probably see where this is going.
The process of hydration creates heat, as the builders of Hoover Dam well knew, which is the first problem your poor skin will face. But beyond that, the concrete will also suck the moisture out of your skin, exacerbating the issue.
As we said, it shouldn’t be a problem if you clean the affected area in good time. But if you leave it, you could be in for some nasty chemical burns!
Concrete and the Environment
Concrete may be a versatile and abundant construction material, but it is losing some of its lustre among people who are concerned for the wellbeing of our climate. It is thought that the production of concrete is responsible for anywhere between four and eight percent of the world’s CO2. When you consider the size of the world, that’s a lot of CO2. It’s also worth noting that this means four to eight percent of the CO2 in our atmosphere is entirely human-made since there are no natural sources of concrete.
Beyond carbon dioxide production, there is also water consumption. We have talked about how important water is to the process of making concrete, but what we haven’t touched on is how much water gets taken up by the manufacture of concrete across the globe. Estimates place concrete as responsible for as much as ten percent of the world’s industrial water usage. This can be a real problem for regions where water shortages are commonplace since those regions still look to build, and concrete is one of the cheapest construction materials to hand.
Further exacerbating concrete’s rap sheet as far as the environment is concerned are the harmful particulates that go into making it, which can cause respiratory problems in those who deal with it on a regular basis. Again, this is a larger problem for poorer countries, where health and safety laws may not be as rigorous as they are in other parts of the world.
Despite all of this, a more cost-effective alternative to concrete has not been found, and concrete remains the most-used construction material for large projects.
There are alternatives that are considerably better for the environment, however, even if they are nowhere near as widely used as cement-based concrete. One of the more popular examples of this is lime-based concrete. As the name suggests, this is a construction material that uses lime rather than cement.
There are several advantages to this. For one thing, it produces considerably less carbon dioxide than cement-based concrete. In fact, it actually reabsorbs CO2, further reducing its effective carbon footprint. Another advantage is that lime concrete tends to be breathable and naturally resistant to mould and algae.
Examples of this kind of construction material include Venetian plaster, Moroccan Tadelakt, and Italian Intonachino.
The process of concrete going harder is one that is far more complex and interesting than many people are aware of. It is also considerably more dangerous than many people appreciate. How long we can sustain our current level of concrete use in the current climate—both politically and environmentally—remains to be seen, given the detrimental effects we know it to have on our environment.
But one thing is for sure, few construction materials have had quite the impact on humanity that concrete has. And, whether it’s some ancient Roman formula using seashells and volcanic ash or a more contemporary mixture of cement and sand, there’s every chance that our concrete structures could outlive their creators.