Concrete is an integral part of many construction jobs and often DIY concrete jobs done around the home. Its excellent compression strength and general ease of use make it versatile and robust, and using rebar helps it to deal with tension stress. So when pouring a simple 4-inch concrete slab do you need rebar?
No, you do not need rebar for a 4-inch slab of concrete on grade. A 4-inch-thick slab cast on the ground and in permanent contact with it will float and rebar is not required. Rebar is recommended on concrete measuring 5 – 6 inches thick.
In this article, we will examine the importance of reinforcing concrete with rebar and the proper steps to go about this. Most importantly, we will look at the circumstances that require these reinforcements, and when they may not be applicable.
Rebar and Precast or Cast-in-Place Concrete
Rebar or reinforcing bar typically contains ridges to help it better adhere to the concrete. These bars are primarily made from steel, though there can be alternative forms.
Concrete with reinforcement can be either prestressed or not. The former involves placing the reinforcements, typically rebar, under significant tension or strain. When the concrete dries, this strain is then released, further compressing the concrete, which helps apply the tensile strength from the rebar to the concrete itself.
Additionally, concrete slabs can either be cast-in-place or precast. Precast concrete is usually manufactured in a plant, providing the exact dimensions and conditions for the casting.
Cast-in-place has more risks associated with it than its precast alternative because it lacks the same level of precision.
As for the concrete mix itself, there are several options available to you, including mixing your own concrete. While these do not have a significant impact on whether you should install rebar or not, they can be an essential part of the overall project.
What Thickness Must Concrete Be for Rebar?
The American Concrete Institute (ACI) lists the factors that play a role in how thick the covering concrete must be to support rebar.
The reinforced concrete that can generally support the thinnest covering is the precast variant. The reason for this is that precast concrete, made in a manufacturing plant, is far more precise than the alternative. Getting the casting of rebar correct is vital to the effectiveness of the reinforcement.
Cast-in-place concrete requires the placement of wet concrete around the rebar, then holding it in place as it sets and dries around it. This is usually done with rebar supports that help hold it at the correct depths, but this does open it up to a certain level of operator error.
Both prestressed and non-prestressed cast-in-place concrete, regardless of the reinforcement used, requires three inches of concrete over the rebar when in permanent contact with the ground (source).
If you didn’t cast it on the ground and it isn’t exposed to the weather, then you can get by with about an inch.
For a precast reinforced concrete slab, the minimum cover required is ⅝-inch for #11 bar and lower. For a cast-in-place concrete slab, this sits around ¾-inch. You cannot achieve this thin level of concrete when exposed to the weather or in contact with the ground.
Three main conditions determine the minimum thickness of concrete covering the rebar. The first is the level of exposure to the elements. Contact with the ground or outdoor weather can increase the required depth of the concrete covering the reinforcement.
The size of the rebar used is another condition that affects the required thickness of the concrete with thicker bars requiring more concrete placed on top.
For example, #11 bar is about 1.41 inches in diameter, while #14 is about 1.693 inches. That seemingly minuscule difference is enough to require ⅝-inch and 1-¼ inches respectively in thickness under the same conditions.
Finally, the role of the concrete impacts the necessary thickness of the covering. Ground slabs and walls require less concrete over the rebar than columns or beams.
For cast-in-place concrete that will be in contact with the ground permanently, the recommended covering concrete thickness is 3 inches.
For cast-in-place slabs less than 5 inches thick, this means that, in most circumstances, there should be no rebar involved. The slabs at this thickness are simply too thin to adequately cover and protect the rebar while still exploiting its reinforcing nature.
One thing that can certainly help is to check your local regulations, which take into consideration the local environment for optimal construction.
Benefits of Rebar
There are several reasons construction workers add rebar to concrete, and each of these is worth considering when making your own decision as to using it or not. Some of these are more situational than others and could apply more to particular circumstances.
The most critical benefit that rebar adds to concrete is structural integrity. While concrete is fantastic at handling compression, it struggles with impact and bending stress. This discrepancy is where the rebar comes into play, providing additional strength to the whole structure (source).
Generally, this added strength is for concrete slabs expected to support heavy loads. Smaller cars and less regular traffic don’t always require reinforcement for the concrete they drive on. In these circumstances, it is possible to get by without any rebar involved at all.
When concrete dries, there is a natural shrinkage that takes place. Adding rebar can reduce the majority of cracking and curling that could otherwise occur during this shrinkage.
The reality is that you won’t be able to prevent all the possible cracks that can occur with concrete, but rebar can undoubtedly keep this down to a minimum.
While reducing the occurrence of cracks, they will still occur, even with rebar, and the environment plays a significant role in how extensive they will be. When this does happen, the rebar helps hold the concrete together, helps prevent major cracking however some still debate on how much it helps on surface cracks.
Risks of Adding Rebar
While rebar can be a welcome additive for supporting and reinforcing concrete, it can also negatively impact its integrity.
Such structural issues mainly happen when someone has improperly placed the rebar within the concrete. If placed too deep or too shallow, even marginally, then the resulting structure may reduce its load-bearing capabilities by up to 20% (source).
Additionally, ensuring the rebar covered with a sufficient layer of concrete is essential for a variety of reasons, and failing to provide adequate covering may damage the rebar over time and weaken the integrity of the reinforced concrete. Let’s look more into the benefits of covering rebar.
The Importance of Adequate Coverage
Ultimately, there are many reasons to opt for rebar. Still, it is also essential to remember that any steel or similar reinforcement used within the concrete will require protection from the elements. If exposed to the air, the rebar may rust and lose integrity, which applies to improperly-placed rebar as well.
Rebar comes in the form of deformed bars with ridges along the edges, which ensures the concrete properly adheres to it. This combination allows the rebar to absorb the stresses experienced by the slab effectively.
The proper thickness of concrete over the rebar creates this tensile transfer and activates the reinforcement. Without the correct amount of cover, this does not happen. Using too little or too much concrete with rebar can have a diminishing effect on the resulting slab.
The alkalinity of concrete and lack of oxygen prevents rebar from corroding when completely covered (source).
When exposed, the rebar can begin to rust, which ultimately weakens its overall resolve. This corrosion can lead to further damages as the rebar can no longer support as much impact stress as before.
Damage like this can occur in steel when the alkalinity drops. The alkalinity decreases when the concrete becomes progressively more carbonated, which happens when it is exposed to air.
Heat can affect the material that makes up rebar, be it steel or otherwise. As such, it is essential to protect these reinforcements with the necessary amount of concrete.
There must be enough cover, depending on your environment and the circumstances that the slab will face, that will prevent the rebar from getting damaged.
In steel, for instance, heat can cause expansion and contraction, which threatens the concrete encasing these reinforced bars. It will also affect the overall integrity and load-bearing capabilities of the rebar.
Welded Wire Mesh
An alternative to rebar is welded wire mesh or fabric mesh, which is a lighter, thinner option for reinforcing concrete slabs. As such, it requires less concrete to hold it in place and still provides additional support as regular rebar would.
The reinforcement that welded wire mesh adds is not as comprehensive as the thicker rebar. It can also be harder to add to concrete and correctly position in place. This costs extra time and money to implement properly.
For thinner concrete slabs, however, this will give the added reinforcement while not taking up as much space as the thicker rebar.
Rebar has many benefits when used to reinforce concrete. However, understanding when it is necessary is also fundamental; otherwise, you risk weakening the integrity of the slab.
For 4-inches and below, you may choose to avoid rebar for these reasons, especially if it will be in contact with the ground. In these instances, it may be worthwhile using welded wire mesh instead of rebar. When in doubt, check your local building regulations for further information.