Four Defining Features of Concrete

Concrete is undoubtedly one of the most important materials in our long history as a species. Thanks to concrete, we have been able to build structures that protect us from the elements, provide renewable energy sources, advance science, and so much more.

It is an integral component in infrastructure, and the only thing that is in more demand in our ever-expanding civilization is water. And, given that concrete needs water to harden, it could be argued that concrete is partly responsible for the demand there, as well.

Despite its importance to us as a species, many people don’t understand concrete beyond a surface—and often mistaken—level of knowledge. But don’t worry, we can help with that. As it turns out, all you need to know about concrete can be broken down into four main properties. So let’s learn what those properties are.

Properties of Concrete

There are many properties of concrete, of course, but the four we are about to get into representing the main aspects of this remarkable construction material.

Strength

It shouldn’t come as a surprise to anyone that concrete is an incredibly strong substance, but knowing the type of strength it possesses is important to properly understand what can and can’t be done with concrete.

The first thing to note is that there is more than one type of strength. Saying concrete is “strong” doesn’t really paint a full picture, since concrete is stronger in some ways than it is in others.

The main strength concrete possesses is compressible. Compressible strength can be thought of as resistance to being crushed. As an example, if you were to take a cube of concrete, sit it on a solid surface, and apply force evenly across the top of the cube, it would require a lot of force to cause the concrete to break apart.

This is different from something like tensile strength, which could be visualized as a long concrete beam being gripped solidly at one end and rotated at the other end, causing the beam to want to twist.

Of course, concrete is by no means weak in these other areas, but it is most impressive when it comes to compressive strength. There are ways in which the strength of concrete can be improved by adjusting the amount of cement in the mixture or by including reinforcements in the structure, such as the rebar frame.

Durability

Concrete is an incredibly durable material. The most common form of concrete used is given a life expectancy of between thirty and a hundred years, depending on the circumstances around its installation. But historically, different versions of cement—the key ingredient in concrete—have lasted for thousands of years.

Concrete is also resistant to weathering, chemicals, and abrasion when it is installed properly, making it uniquely suited to construction, and especially the construction of structures that need to stand the test of time.

This is also one of the reasons that concrete is very commonly used for the making of foundations. The fact that foundations often underground make them far more difficult to maintain, but by using concrete, the amount of maintenance required over the lifespan of the building is significantly reduced.

Appearance

As remarkable as concrete is as a construction material, its appearance is one of its least inspiring properties. The look will depend on the exact ingredients used in any given batch of concrete, but it tends to be a dull gray color that few people would put at the top of their decorating choices.

The texture of the concrete—assuming it has not been polished or coated—is typically rough to the touch. How rough it is will depend on the kind of aggregate that has been used. Concrete is sometimes looked upon as “man-made rock,” which should help give you an idea of the kind of appearance it typically takes.

It is possible to make concrete that is a different color to the standard grey by changing the ingredients slightly. For example, White Portland Cement and a colorless aggregate can produce white concrete, and that concrete can be tinted to produce a specific color.

The texture can also be smoothed over by polishing it, which, when done properly, can produce an almost glass-like finish, which is one of the reasons concrete has increasingly become popular for interior countertops and other areas where it serves as much as an aesthetic feature as it does a structural one.

Hydraulic Properties

One of the commonly misunderstood properties of concrete is the way in which it hardens and the roles that water plays in that process. The common misconception is that water is used to make concrete into a pourable substance, which then hardens as the water evaporates. This has led to the idea that concrete is “drying out” when it sets. This is actually closer to the opposite of what is really happening.

Concrete hardens through a process called hydration and involves chemical reactions between the water and compounds within the cement. These form hydrates, and it is these hydrates give the concrete its strength. Interestingly, the concrete does not gain all of its strength at once.

Different compounds in the cement react at different rates, and each is responsible for some of the strength of the final product. For this reason, the curing process can take as much as a month before it is ready for any excessive force, even though it may appear to be completely hardened after a day or two. In reality, it takes concrete about seven days to gain three-quarters of its strength, and even after a month, it is still not technically cured to full strength—though it should near-enough to use.

One significant bonus to this property of concrete is that it still hardens underwater, which makes it ideal for construction where part of the structure will be underwater. In fact, due to the fact that concrete is made stronger by the water, concrete that has set underwater can sometimes be stronger than concrete that has set in a dry area. In some circumstances, setting concrete may be further hydrated to make it stronger than it would otherwise have been.

Concrete’s Chemical Reactions

The chemical reactions that go on inside setting concrete are what people who are not familiar with the construction material tend to be most ignorant of, but they are very important for getting a good concrete mix and a durable, strong finish. For example, curing concrete too quickly—treating it as though the goal is to “dry it out”—will make it weaker than if you had let it cure naturally.

It is also important to use pure—or as close to pure as you can get—water when mixing your concrete. Water with contaminants in it may interfere with the chemical reactions taking place, also weakening the final result.

The curing time is the piece of the puzzle as far as potentially costly misconceptions about concrete go. For example, a new concrete driveway should be cured enough to safely walk on within twenty-four to forty-eight hours.

However, as we mentioned above, it can take as much as a week before it has attained around three-quarters of what will be its maximum strength. The problem is that three-quarters of its strength may not be enough for a big heavy car, so it is recommended to give it a full month of curing time before parking your vehicle on it, as doing so before could result in the concrete shifting and you being left with four-tire dimples in your driveway.

Aggregates

A big part of how cured concrete looks and acts is the aggregate that is used when making it. Aggregate is essentially a non-reactive medium that is used to bulk up the mixture since cement alone would be prohibitively expensive if used to make the kinds of structures that concrete is used for. The exact ratios of the ingredients of concrete depend on the application the concrete is being used for, but cement typically makes up around ten percent of the final mix.

Aggregate needs to be non-reactive so that it does not interfere with the hydration process, in much the same way that the water needs to be free of contaminants. The aggregate itself is often some variant of stone, such as gravel, but can also be recycled concrete.

Reinforcement

We mentioned above that concrete’s main strength lies in compressive force, which is why it is so well suited to building since the structure at the base of the building has to be able to hold the weight of everything above it. There are areas where the strength of concrete, while still impressive, is not enough, such as tensile or shear strength. In these cases, reinforcement may be used.

The most common example of concrete reinforcement in construction is the rebar frame. In this case, a latticework of metal rods is laced throughout the areas that concrete will be poured. Once the concrete has cured, the metal framework is as much part of the structure as the aggregate and adds to the tensile and shear strength of the concrete.