Why Does Concrete Require Reinforcement?
Concrete is a fantastic building material used all over the world. It’s easy to make, affordable, durable and very strong. But for all it’s strengths it has one very big weakness, tensile strength. Concrete has a very high compressive strength. Which means it can carry a lot of weight. But it’s tensile strength is very low. This make it prone to structural cracks and failures. But when we add reinforcement to the concrete that weakness can be overcome. So why does concrete require reinforcement? To eliminate it’s naturally low tensile strength and prevent structural cracks.
It’s because of reinforcement materials like steel rebar that we can build huge concrete structures like buildings, bridges and dams. In fact, almost all modern concrete structures use some sort of reinforcement. Whether it be fiber, micro rebar, wire mesh or steel, some type of reinforcement is probably inside every footing, wall, and slab. Even small finished concrete products like cast fireplace surrounds, countertops, planters and furniture use reinforcement to increase their tensile strength. Without it they would easily crack.
Ahead we’ll discuss in detail why concrete requires reinforcement and which types are best.
In this case, stress refers to the forces that work against the concrete. In general there are 2 main types of stress that affect concrete.
- Compression. This is a pushing together stress. If you have a block of concrete and place 100 lbs on it, that’s compressive strength. This is a natural strength of concrete.
- Tensile. This is a pulling apart stress. If you have a 10 ft. concrete beam that’s supported on both ends by posts but nothing in the middle will it crack in the center or not? This is a measure of tensile strength. A natural weakness of concrete.
On it’s own, concrete has a very low tensile strength. In the example above, that 10 ft. beam would eventually crack under it’s own weight without reinforcement. But when we add steel rebar, concrete can easily span that distance even with weight placed atop it.
>Structural engineers design the rebar that goes into concrete based on the load and stresses it will have to endure. This will also take into account movement in the soil,from wind or vibrations. Concrete can become very strong when properly reinforced.
Concrete Is A Composite Building Material
Because if it’s reliance on reinforcement, concrete can be considered a composite material. On it’s own it’s not generally strong enough to build with.
Without some type of reinforcement to increase concrete’s tensile strength, you can’t build much with it. You may assume that concrete can still be used on it’s own because of it’s high compressive strength, but you’d generally be wrong. Rarely does a structural member experience just compression. In reality, almost all structures experience a mixture of stresses. The ground can shift which creates a void beneath a footing. Wind can blow a building from side to side creating lateral pressures. Vibrations can shake a building and it’s foundation. All of these things are considered forms of tensile stress. Without reinforcement, concrete may be able to handle the weight of the structure, but can still fail due to outside stresses.
Concrete alone does not make a good structural material. In the real world, there are just too many types of stress that it can’t resist by itself. To combat this we add reinforcement to improve its strength and overcome it’s inherent weaknesses.
Reinforcement within concrete creates a composite material. Concrete provides the compressive strength while reinforcement provides strength against tensile stress.
Concrete is made by mixing cement, sand and aggregate stone with water. The water chemically reacts with cement which forms a paste that dries and hardens over a period of time called curing. Generally this takes 28 days to complete. During this time, concrete dries, heats up, hardens and grows very strong with a high compressive strength. But a very low amount of tensile strength.
While cement and water are the active ingredients, sand and aggregate stone provide strength to the concrete. Without these two ingredients, cement is much weaker and prone to cracks.
Concrete is a porous material that absorbs water. As the concrete cures, internal water evaporates out which creates lots of small tubes throughout the structure. Water is easily absorbed into these tubes like a sponge.
While concrete’s structure gives it a lot of compressive strength, it has very low tensile strength. These internal tubes and pores can easily crack. For example, even a thick concrete beam supported at both ends has very little strength in the middle and can eventually crack even under it’s own weight.
Concrete’s strength is measured in psi or pounds per square inch. This is a measurement of it’s compression strength. A 3500 psi bag of concrete can resist 3500 pounds per square inch without cracking. But this 3500 psi has nothing to do with it’s tensile strength.
The compressive strength of concrete can be adjusted by changing the type, size and amounts of it’s aggregate. However none of this will do much to alter it’s compressive strength.
Adding concrete reinforcement like wire mesh, chemicals, fiber and rebar are the only way to significantly increase it’s tensile strength.
What Concrete Reinforcement Doesn’t Do
As a rule, concrete reinforcement does not increase concrete compressive strength. For example, if a project calls for 4000 psi concrete, adding reinforcement to 3000 psi concrete won’t make the psi any higher. Reinforcement has nothing to do with the amount of compression,or weight, the concrete can handle. To increase this you have to use other additives or change the mix.
Reinforced concrete helps prevent large cracks that can cause the structure to fail. But they don’t prevent all cracks. Small surface levels cracks are normal and don’t necessarily weaken the concrete. When cracks do get deep, reinforcement helps keep them tight which can prevent concrete from breaking apart.
In addition to using concrete reinforcement, you should also make sure to do the following.
- Consistent Slab Thickness.
- Proper Joint Spacing.
- Take Measures To Resist Shrinkage.
- Build A Solid & Level Base.
- Cure The Concrete Properly.
- Use The Correct Mixture.
- Don’t Use Too Much Water.
Reinforcing materials are a must when building most concrete structures. But they’re only part of the plan. You also need a strong concrete mix, a proper base, and a good installation.
Concrete Requires Reinforcement To Build Large Structures
Why does concrete require reinforcement? An engineer would probably answer, to overcome the inherent weakness of concrete which is a very low tensile strength. By adding reinforcement such as rebar, we can add properties to a concrete structure that makes it able to withstand outside forces. And these forces are very common such as shifting earth, vibration or wind.
However, in the real world to a non engineer, the answer is simpler. We need concrete reinforcement to build cool stuff. Bridges, dams, homes and buildings all need reinforcement. And so do small scale projects such as concrete countertops, cast fireplace surrounds, furniture or planters. Concrete alone is generally not a great building material because it’s very easy to crack.
Although concrete has been used for centuries all over the world to build homes and buildings. Those structures were typically small or built in a way that overcame the concrete weaknesses. In other words, the designs were very limited. These days, we can do almost anything with concrete because we include the proper reinforcement.
Common Forms Of Concrete Reinforcement
There are quite a few types of concrete reinforcement materials on the market. The most common by far being steel rebar. You should choose a material that suites the project your working on and the stress your structure will have to endure. Obviously the type of stress placed on a bridge or building is very different from a countertop. Although the reason your adding reinforcement is generally the same.
The primary purpose of rebar is to increase concrete’s tensile strength. This additional strength helps it resist cracking and compensates for it’s inherent weakness. With greater tensile strength, concrete is able to resist breaking under tension which means it can safely span larger distances. It’s because of rebar that we have many of the bridges, dams and buildings you see today. Most large scale construction and residential structures wouldn’t be possible without concrete reinforcement.
Rebar provides additional tensile strength to a concrete structure. It helps distribute weight and tie the structure together which prevents cracks and structural damage.
Concrete by itself is a very strong and durable building material. It’s been used all over the world to build structures for centuries. But those structures had big limitations until rebar was invented. These days we can build just about anything out of concrete thanks to the inclusion of rebar.
Concrete with a compression strength of 4,000 psi (pounds per square inch) might only have a tensile strength of just 400 psi. That’s very low in comparison. However, by including rebar, tensile strength can actually get higher than concrete’s compression strength.
Rebar also helps deal with expansion and contraction forces that occur naturally with changes in temperature. Rebar helps hold a concrete structure together as it tries to expand and contract.
The rebar inside of a concrete structure can be just as important as what’s on the surface. When you see a concrete building, foundation, bridge or wall, know that inside is generally a well engineered rebar structure that gives it strength. It’s the rebar that makes most modern structures possible.
The learn more about rebar check out our article here.
The addition of fibers can increase the strength of concrete. These concrete reinforcement fibers are added into the concrete while it’s wet and being mixed. This helps ensure an even distribution of fibers throughout the concrete.
While fibers don’t increase concrete’s tensile strength as much as rebar, there’s still a benefit. And they can be used along side rebar for even more durability.
Fibers are very common in applications like slabs and sprayed concrete. When the concrete isn’t thick enough for rebar, fibers can help. They provide some strength without increasing the thickness of the structure.
Micro rebar is a fairly new product that’s only been around for a few decades. It’s made of 1 inch steel that’s twisted to form small ridges. The micro rebar is added into the wet mix just like fiber but adds a lot more strength. In some applications micro rebar is strong enough to replace traditional rebar.
When used in slabs, micro rebar can even be strong enough to eliminate the need for relief cuts. This can be a huge benefit when pouring an interior concrete floor. Concrete expands and contracts with variations in heat. Because of this, slabs needs joints to prevent cracking. But micro rebar can resist these pressures which eliminates the need for joints.
Wire mesh comes in sheets made from thin wire that’s been welded together to form a grid. Sheets of wire mesh are tied together and placed inside the slab.
The strength gain from wire mesh is less than rebar and micro rebar but more than fiber. However you can use fiber with wire mesh. Generally there’s no need to combine micro rebar with wire mesh although I have seen it used along side traditional steel rebar.
Wire mesh is very common inside thin slabs where there isn’t enough thickness for rebar. It’s cheaper and requires less labor than rebar because it comes in large sheets. Sheets simple need to be cut to size and tied together. It’s a very fast and easy process.
Make sure when using wire mesh that you place it in the center of the slab. For example, place it 2 inches deep in a 4 inch slab. This is the typical thickness of sidewalks and patios.
Summary: What Is Concrete Made Of?
Concrete is a fantastic building material used all over the world. It’s easy to make, affordable, durable and very strong. But for all it’s strengths it has one very big weakness, tensile strength. Concrete has a very high compressive strength. Which means it can carry a lot of weight. But it’s tensile strength is very low. This make it prone to structural cracks and failures. But when we add reinforcement that weakness can be overcome. So why does concrete require reinforcement? To eliminate it’s naturally low tensile strength and prevent structural cracks.
It’s because of reinforcement materials like steel rebar that we can build huge concrete structures like buildings, bridges and dams. Whether it be fiber, micro rebar, wire mesh or steel, some type of reinforcement is probably inside every concrete footing, wall, and slab. Even small finished concrete products like cast fireplace surrounds, countertops, planters and furniture use reinforcement to increase their tensile strength. Without it they would easily crack.
If you have any questions or comments about concrete reinforcement E-mail us any time.