Reinforced concrete is one of the most ubiquitous building materials in the world. On its own, however, concrete is actually much more brittle than you might expect, and hardly useful in any but a very few limited applications. When reinforced with steel, however, concrete can be used for slabs, walls, beams, columns, foundations, frames, and more.
Concrete is only strong against forces of compression and has low tensile strength and ductility. Reinforcement materials are needed to withstand shear and tensile forces on the concrete. Steel is used because it bonds well with concrete and expands and contracts due to temperature at similar rates.
When you dig deeper into the science of how steel and concrete behave individually, you’ll quickly see that their properties complement each other in ways that make them uniquely suited to be used together. Their combined properties are beneficial in ways that make reinforced concrete the wonder material responsible for impressive structures, like the Hoover Dam.
Does Concrete Have to be Reinforced with Steel?
Concrete looks extremely strong. It’s basically a rock you grow from a powdered formula. In some senses concrete is indeed very strong, but only if pressure is applied in one specific direction. When force is applied in any other direction, as is most often the case for most building applications, concrete is surprisingly brittle.
There are three fundamental types of stress:
- compression (pushing together),
- tension (pulling apart), and
- shear (sliding along a line or plane).
Concrete is strong against forces or compression, but weak against tension and shear forces. Steel, on the other hand, is strong against all three types of stress.
Concrete is strong against compression forces. This is why it is such a powerful base. Even in ancient times, Roman builders were able to use early forms of concrete (which was not reinforced in any way) for structures such as domes, aqueducts, arenas, and colosseums.
In all of these early examples, concrete was used only in ways that took advantage of concrete’s strength against compression forces. The weight of the structure only pushed down on the concrete, which pushed the concrete together, and which the concrete could easily support.
The fact that ancient Roman structures such as the Colosseum and the Parthenon have stood for thousands of years is a testament to concrete’s strength against compression. Even a cylinder made from a cement mix with a lot of water can withstand 1000 pounds (450 kilos) of compression pressure. Other mixes can withstand even more pressure.
Tension is effectively the opposite of compression in that it is a force that pulls the object apart. Concrete is weak against tension forces, meaning it has a low tensile strength.
When a cylinder made from the same high-water mixture of concrete described above was tested by hanging a weight from it, the sample broke when about 80 pounds (36 kilos) was suspended. This means that concrete is less than 10 percent as strong against tension forces as it is against those of compression.
It may not be immediately obvious why this is a problem for concrete’s use as a building material. It seems to only indicate that concrete should not be used as a rope. When you look at the internal stresses within the concrete, however, you’ll see that when there is compression, there is often also tension.
Imagine a horizontal concrete beam, on which pressure is applied down from the top. This would be similar to walking on a concrete 2nd story floor. On the top of the concrete beam, the force is compression, as the concrete is pressed together. On the bottom, however, as the beam bows, the concrete is pulled apart by a force of tension. This is where plain concrete fails.
Concrete is also weak against shear forces, which cause the material to move along a line or plane. A non-reinforced concrete wall would crumble if it experienced too much shear force from:
- Shear Stress
As we can see, plain concrete is useful if you only apply weight directly down onto it, such as the base of a statue. Modern buildings, however, have to withstand pressure from many types of sources in all types of direction. Without reinforcement, plain concrete will simply fail under these conditions.
Types of Failure
When plain concrete fails, it does so suddenly. One moment the concrete is intact, and the next moment, when the force is greater than the concrete can withstand, it crumbles or breaks into pieces. This sudden breaking is known as brittle mode failure.
The main disadvantage of this type of failure is that there are no visual warning signs. Unless you know the specific strength of the material and are actively measuring the amount of stress applied to the material (conditions which are absolutely unfeasible outside of a laboratory setting) there is no way of predicting failure.
Reinforced concrete, on the other hand, experiences ductile mode failure. This means that cracks begin to form before the concrete completely shatters. This is because though the concrete has been stretched further than it can stand alone, the steel rebar still holds the structure together.
If the structure is only subject to compressive forces (such as a slab of flooring) these cracks might not be a big deal. Unless water is likely to infiltrate the crack and undermine the structure by rusting the rebar or expanding the fissure when freezing, the cracks will simply be pressed together by further compression. In other situations, cracks signify the need to repair the area.
Why Steel is Used
As we’ve learned, plain concrete is only useful in very limited applications because it is strong against compression forces, but weak against tension and shear forces. In order to be as versatile as it is, concrete needs to be reinforced by some material that overcomes these weaknesses. Steel is used to reinforce concrete more often than any other material.
The reason steel is used to reinforce concrete is because steel has several properties that make it particularly suited for this application.
Steel is Highly Ductile
Ductility is a measure of how much deformation a material can undergo before breaking. Concrete has very low ductility. If you twist a chunk of concrete with enough force, it will crumble in your hands. Wood, for example, is somewhat ductile, in that you can bend it a little bit before it will break. Steel, though, is highly ductile. If you bend it, it will simply stay bent.
Steel ductility is useful before the cement is poured because it can be bent into whatever shape will best support the form that is to be poured. Because of this, it’s easy to create a grid of reinforcing steel rebar in whatever shape is needed by the design of the building.
Steel’s ductility is also useful once it is a component of the reinforced concrete. When enough force is applied to the structure to deform it, the concrete may crack, but the steel rebar will maintain intact in the deformed shape. Often the steel is still able to support the structure until it can be repaired or replaced.
Concrete and Steel Have Similar Coefficients of Thermal Expansion
When solids are heated, the molecules within the materials move faster. These more active atoms take up more space the faster they move, so each molecule, and therefore the material as a whole expands. The opposite happens when a solid is cooled. The net result is that solids expand when heated and shrink in size when cooled.
While this is universally true among solids, it happens at different rates for different materials. In an extremely fortuitous coincidence, steel and concrete have very similar coefficients of thermal expansion. This means that when they are subject to heat (or cold) they expand (or shrink) at essentially the same rate.
If this were not the case, steel would be a poor choice to reinforce concrete. Imagine a corn dog, for example. If when cooked the hot dog doubled in size while the cornbread only grew a little bit, the hot dog would quickly burst through the cornmeal. Conversely, if the cornbread expanded quicker than the hot dog, there would be a large pocket of air around the cooked hot dog.
While either of these scenarios would result in a structurally weak corn dog, this is not what happens in the case of concrete reinforced with steel. The two materials expand and contract at nearly the same rate, ensuring that they stay bonded firmly at any temperature.
Steel Undergoes the Same Strain as the Concrete
The bond between concrete and steel is so strong that reinforced concrete acts as a new, stronger material than simply the combination of concrete and steel. This is further enhanced by creating rebar that has plenty of ridges around which the cement will find solid purchase as it dries.
Other reasons steel is used include:
- Easy to Weld
- Easy to Recycle
- Cheap and Highly Available.
1. Steel is Easy to Weld
Because reinforced concrete is used in so many different situations, it’s often necessary to construct rather elaborate internal frameworks of steel rebar before pouring the cement. Even if the shape isn’t unique, the size of the project may require rebar to span lengths far greater than can be feasibly manufactured.
In these scenarios, steel rebar can be welded so that the support is securely where it is needed. Steel is one of the most commonly welded metals as it melts easily without burning through or transferring heat too far from the weld site. This process also doesn’t have any negative effects on the properties that make it such a good choice for reinforcing concrete.
2. Steel is Easy to Recycle
Reinforced concrete is made to last for many years, making it a great building material for structures that are meant to last. When the time does come for deconstruction, though, you will be pleased to learn that it is also easy to recycle.
With the proper equipment, it is easy to pulverize reinforced concrete to separate the steel rebar from the concrete. The concrete can be further crushed and reused as part of the mixture of coarse and fine aggregates that make up 60 to 75 percent of cement mix. The steel can be melted down and reformed as new steel rebar to reinforce the next project.
3. Steel is Cheap and Highly Available
It is rather fortuitous that the metal that has so many advantageous properties for reinforcing concrete is also inexpensive and plentiful. If it were gold or diamonds that had all of these compatible features, it probably wouldn’t be as helpful.
Steel, however, is readily available at relatively low cost.
Prestressed and Post-stressed Concrete
As strong as reinforced concrete is, it is still possible for it to crack. While this ductile mode of failure does not immediately result in the structure collapsing (as brittle mode failure would), it is the first phase in a destructive process known as “spalling”.
When water seeps into cracks in reinforced concrete, it can damage the structural integrity of the building in three ways.
1. Because liquid can fill any pocket it is allowed into, it is easy for water to seep into and fill any cracks in the reinforced concrete. If the temperature, then drops below 32 degrees Fahrenheit (0 degrees Celsius) it will freeze.
When water freezes it does so by forming a structure of interlocking ice crystals. These ice crystals take up more space than liquid water molecules, meaning that ice takes up more space than water. This means that as the water freezes, it pushes on the concrete and expands the cracks even wider.
When the ice then melts, the crack is wider, allowing more water to fill the gap, which then freezes to expand even further. This cycle not only physically pushes the concrete apart, but it allows more and more water to penetrate the structure, increasing the amount of damage caused by the other 2 forms of damage.
2. Eventually the cracks will be wide and deep enough for water and air to reach the steel rebar embedded in the reinforced concrete. This exposure can result in the rebar rusting. In the presence of water, oxygen from the air interacts with the iron in the steel to form rust.
The flaky coating on the surface of the rusting rebar does nothing to protect the interior layers of iron from the corrosion process (the way that the formation of a layer of patina prevents the further corrosion of copper surfaces), so the rebar can be continually degraded until it can no longer withstand the tension forces acting on the structure.
A telltale sign that this type of corrosion is happening is if the concrete appears to be stained brown. This color comes from particles of the rust turning the water brown and draining through the cracks in the reinforced concrete.
3. When water infiltrates the reinforced concrete, it can alter the pH balance of the environment and cause chemical reactions within the concrete. This risk is heightened by the fact that on road surfaces and bridges the use of salt to de-ice roads in winter means the infiltrating water is more likely to be highly alkaline.
These alkalines in the water can react with the silica in the concrete’s aggregates to cause the formation of new crystals. These new crystals take up room and physically force the reinforced concrete apart in the same way that the freezing ice did in example 1. The difference is that the crystals do not melt, so the concrete is pushed apart continuously.
Clearly, it would be better if reinforced concrete isn’t allowed to crack. Because steel is so ductile, however, it will stretch or bend, allowing the surrounding concrete to crack. This is, of course, unless something is done to prevent the steel from acting this way.
In order to prevent cracking, steel rebar can be stretched before the cement is poured. This is known as prestressing (or pretensioning) because it adds tension force to the steel before the reinforced concrete is formed. By doing this, the steel then is in a constant state of pulling itself back toward its natural shape, pulling the surrounding concrete inward, a compression force.
Keeping the concrete in this prestressed condition actually makes it stronger because concrete is strong against compressive forces. It’s sort of like a muscle, which is stronger when taut.
By prestressing reinforced concrete, the material is stronger in two ways.
- Cracks are less likely to form. Because the steel is already pulling the concrete together, it is not allowed to stretch as far as it would if the steel were not prestressed.
- Any cracks that do form are continuously pulled closed by the force of the steel trying to revert to its relaxed state. This limits the amount of water that is allowed to penetrate and corrode the reinforced concrete.
Post Stressed Concrete
The same effect can be achieved by tightening the steel after the concrete has begun to harden. Concrete seems to harden in a matter of hours, but it actually takes about a month to properly cure and continues to harden and strengthen for at least five years after it is poured.
Not only does prestressed and post-stressed concrete result in less cracking, it is actually so much stronger than regular reinforced concrete that smaller and thinner sections of prestressed or post-stressed concrete can carry the same load as unstressed reinforced concrete.
Why Not Just Use Steel?
As you look at the specifics of how reinforced concrete works, you may begin to wonder why we are bothering to use concrete in the process at all. Concrete, after all, is only strong against compression forces, whereas steel is strong against:
In fact, steel is 100 to 140 times stronger than concrete when it comes to tensile strength.
Plain concrete is not very useful on its own. It’s only reinforced concrete, and preferably prestressed (or post-stressed) concrete that is the wonder building material we think of when we picture modern architecture. Since concrete is actually relatively useless without its steel reinforcement, then, why not just build with steel?
Concrete offers many benefits for construction that make it a better building material than just plain steel.
As we’ve seen, when steel is exposed to air and moisture, it rusts. While treatments exist to prevent this oxidation, they require far more maintenance than is feasible. Steel rebar, for example, is often treated before the cement is poured to protect it from the elements, even though it will soon be encased in concrete. Even so, as we’ve seen, it still can rust.
Concrete, on the other hand, is fairly resistant to corrosion. Cracks must first form, and it often takes several years of water infiltrating, freezing, and refreezing in order to compromise the structural integrity of the reinforced concrete. As long as regular inspections are performed, this gives ample time for the corroding section to be repaired or replaced.
Steel is very heavy and would need to be transported to the construction site in full. Concrete, on the other hand, is about a third as dense as steel, and can be transported in its far lighter composite parts.
The benefits of this is twofold. The first benefit is transportation. Steel would need to be transported to the construction site and then welded together to form the structure. This would be very costly as steel is heavy. Concrete, on the other hand, can be transported much easier as its composite parts, then mixed and poured on site, hardening into the final form.
The second benefit is the weight of the final structure. Because concrete is a third as dense as steel (and even contains as much as 5 to 10 percent trapped air) the total weight of a building made of reinforced concrete is much less than one made entirely of steel. Reinforced concrete is typically about 1 to 4 percent steel, so it ultimately weighs a lot less.
Steel, though relatively cheap and abundant, is a lot more expensive than concrete. It simply makes sense to reinforce concrete with steel because you can get the benefits of steel’s strength while retaining the low cost and ease of use of concrete.
The History of Reinforced Concrete
While the use of early forms of cement have been documented in ancient cultures that date back many thousands of years, it was the ancient Romans who introduced the earliest form of concrete as we know it today. While quarrying limestone for mortar, the Romans accidently discovered a silica and alumina bearing mineral on the slopes of Mount Vesuvius.
When mixed with limestone and burned, it produced a cement that could in turn be mixed with water and sand to produce a mortar which was harder, stronger, and more adhesive than ordinary lime mortar. This mixture could harden under water as well as in the air, much like concrete today. In 2000 BCE the Romans used a type of concrete called pozzolana, which used volcanic ash, to build the Colosseum and Pantheon in Rome.
Then, from about 400 to 1750 CE, there is no evidence of the use of concrete. This effectively became the “Dark Ages” of concrete, which spanned from the fall of the Roman Empire until an English engineer, John Smeaton, rediscovered how to make “hydraulic” cement when building a lighthouse in Plymouth, England.
Reinforced concrete was invented and patented by Frenchman Joseph Monier in 1867 CE, but he only applied the technique to cement flowerpots. Reinforced concrete didn’t become a widely used building material until twisted rebar and prestressed concrete were developed in the 1880s.
The first concrete road was poured in 1891 in Bellefontaine, Ohio. The Hoover Dam, the biggest concrete structure ever attempted up to that point, was built in 1936. American architect Frank Lloyd Wright built many iconic concrete buildings in the 1950s. Brutalism, an architecture style that emphasized exposed concrete, was popular from the 1950s to the 1970s.
Concrete is an amazing building material that was discovered thousands of years ago, then forgotten. It is an incredibly useful building material because it can be mixed from powder to create stonelike structures of any shape.
Its usefulness is limited, however, by the fact that concrete is only strong against compression forces, and crumbles easily when subject to tension and shear forces. By reinforcing concrete, however, you can create a material that is much stronger than its components. Steel is particularly well suited as reinforcement because it bonds well to concrete and expands at the same rate.
When combined, steel and concrete form a new building material, reinforced concrete. This new material is more useful than either of its individual components on their own because it combines the strength of steel with the ease of use and relative low weight of concrete.