Photo: Maria Orlova

Does a garage slab need rebar?

A garage slab or the garage concrete floor does not need rebars, provided it is 4” or less in thickness. Other forms of concrete reinforcement will however reduce cracks. Rebar is certainly required if the garage slab is 6” or thicker. This is the short & sweet answer.

garagemadesimple.com - Is Rebar For Garage Slab Important? (What You Need To Know!)

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garagemadesimple.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com . The website is also an affiliate of a few other brands. When you think of concrete you almost always think of concrete reinforced with rebars. The fact is you are pretty much right! Concrete is a very strong building material. However, while concrete has very high “compressive” strength, it has very poor “tensile” strength. Hence, the rebars. But then why the question, “Does a garage slab need rebar?” A garage slab or the garage concrete floor does not need rebars, provided it is 4” or less in thickness. Other forms of concrete reinforcement will however reduce cracks. Rebar is certainly required if the garage slab is 6” or thicker. This is the short & sweet answer. Rebars cost money, both in material and labor cost. So many contractors do like to put rebar in the garage slab. Often they have valid reasons (beyond higher profit, of course!). So is rebar for garage slab important? What do you need to know to have a proper discussion with your contractor or even to convince yourself. Read this post and you will have a lot more information than the short & sweet answer given above.

What is Concrete?

Cement is a key ingredient of Concrete but cement by itself is not concrete.

The concrete mix

“.. is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.” Source: The Portland Cement Association The concrete mix is a slurry and can be poured into almost any structural element such as slabs, beams, columns etc. Over a period of 4 weeks this pliable slurry cures into a rock solid mass with great strength…compressive strength to be precise. Strength of a building material is measured in PSI (pounds per square inch). This is the load the material, in this case concrete, can withstand before breaking or crumbling. Compressive Strength is the ability of concrete to withstand a load that is pressing into the concrete. Concrete slabs, such as in garage floors have pretty high compressive strength. The compressive strength of concrete depends on a host of factors. Water to Cement Ratio in the concrete mix is possibly the most important factor. As a thumb rule the compressive strength of concrete goes up as the water to cement ratio goes down. International Code Council (ICC) specifies “Minimum Specified Compressive Strength” for garage floor slabs and recommends that the water to cement ratio be kept at 0.5 or lower. At this compressive strength level the garage slab can take all the load of your cars, furniture, what have you. Tensile strength, on the other hand, is the ability of concrete to resist breaking or cracking under tension. In simpler words, high tensile strength is flexibility and low tensile strength is brittleness. Concrete with a compressive strength of 4000 psi may have tensile strength of just 400 psi. Rebar or any other type of reinforcement in garage concrete slabs are required mainly to handle the flexural or tensile stresses. Such stresses may be caused by uneven shifting of the grade below or large temperature fluctuations.

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To learn more about concrete garage floors check out my earlier blog post 7 Important Things To Know About A Concrete Garage Floor.

What Happens if You Do Not Use Rebar in Concrete Slabs?

Rebars in concrete slabs serve one of the following two functions:

Rebars Provide Structural Tensile Strength

As mentioned earlier, concrete has great compressive strength but poor tensile strength. Concrete reinforced with rebar is necessary to increase the ability of the slab to withstand flexural or tensile stresses. In the case of a garage slab on grade, tensile stresses are created in the slab if the subbase is poorly prepared. The grade itself may become unstable due to soil erosion or seismic activity. For example, in case of heavy rains or even mild earthquake tremors, the grade below the subbase and the garage slab may sink unevenly. If the slab has not been reinforced using rebars or other reinforcing elements, the concrete slab may suffer structural damage and cracking. Rebars for providing structural tensile strength are most effective when they are placed in the middle of the lower half of the concrete slab. However, they serve no purpose if they are right at the bottom of the concrete slab.

Fig – 1

Rebars Limit the Crack Width in Concrete

Concrete, almost always, cracks. The cracks are the result of stresses caused by concrete shrinkage during curing, expansion & contraction during temperature fluctuations, uneven sinking of subbase and grade etc. Rebars or any form of reinforcement can not stop the occurrence of cracks. They have no role in the crack development process. However, rebars, along with contraction & expansion joints, can and will limit the expansion and growth of the cracks in the garage concrete floor.

Why Rebar for Garage Slabs (4″ or thinner) are Not Required?

When the concrete contractors (or you) say that they (or you) are pouring a 4″ thick garage slab, what they (or you) mean is that 4″ is the “nominal” size. In other words, the height of the slab form is 4″.

Real Garage Slab Thickness

So the number quoted for a garage concrete slab thickness is the “nominal” thickness. This is not the “real” thickness. “Concrete floors and slabs on grade are called out by their “nominal” thickness. Just like a 2×4, a four inch thick slab is only 3-1/2” thick. This is so the edges can be formed by using a 2×4 and the area to be prepared can be graded off the same way, by using a 2×4.” Source: Hansen Pole Buildings Slabs generally cure to a thickness less than poured and often in a non-uniform manner across the area of the slab.

Rebar Diameter

Standard Rebar size starts at #3 (3/8″) diameter. In most residential projects #4 (1/2″) diameter and #5 (5/8″) diameter are used. In commercial and large scale projects much higher # rebars will be used based on the engineering specifications.

Embedding of Rebar in Concrete Slab

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If a Rebar is not properly embedded in the concrete then its effectiveness in enhancing the tensile strength of the concrete slab is severely diminished.

Concrete Gripping the Rebar

The Rebar is not smooth. Instead it has ridges. The role of the ridges is to let the concrete “grip” the rebar. This gripping is what allows the transfer of tensile stress from the concrete to the rebar. If the rebar is not embedded properly in the concrete then there will be no “gripping” and no transfer of tensile stress from concrete to rebar.

Rebar Exposed to the Elements

Moreover, if the rebar is not embedded deep enough in the concrete, it may get exposed to the elements, especially moisture, salt, air and chemicals in the atmosphere. A rebar that gets exposed to the elements may rust and weaken. Adequate coverage of the rebar with concrete is essential. Else the rebar may do more damage than good to the garage slab on grade. The thumb rule of structural engineering regarding rebar reinforcement is that the rebar should be embedded at least 2″ on all sides. Quite obviously a 4″ thick garage slab (real thickness 3 1/2″) is just not thick enough to embed a #4 (1/2″) diameter rebar by 2″ on all sides. A rebar in a 4″ (nominal thickness) garage slab would be an absolute waste of money.

How do you Strengthen a 4″ Thick Garage Slab Without Rebar?

The best ways to provide the required structural strength (both compressive and tensile) in a standard concrete garage slab on grade is: Ensure the concrete meets the “minimum specified compressive strength” by ICC. Compressive strength of 3,000 to 3,500 psi is adequate for a garage slab. To achieve the minimum compressive strength above ICC recommendations the water to cement ratio should be 0.5 or less. Ensure that the subbase uses the right grade of gravel and is compacted well. A stable strong subbase will significantly minimize the flexural load on the garage slab on grade Use a thin welded wire mesh as reinforcement to limit the expansion and growth of cracks in the garage slab. Wire Mesh is made from thin stainless steel wires formed into a grid pattern.

Wire Mesh

A Wire Mesh does the same job as Rebar, except that it is much thinner and therefore can add less strength than rebar. However, they are ideal for a 4″ thick garage slab.

Fiber Mesh

Fiber Mesh, a recent development, is an alternative to Wire Mesh. Different types of fibers such as glass, steel, synthetic or natural fibers are distributed evenly in the wet concrete. It saves time and money. However, many contractors are of the opinion that Fiber Mesh is not as effective as Wire Mesh. Thank you very much for reading the post. I do hope you found it informative and useful.

garagemadesimple.com - Is Rebar For Garage Slab Important? (What You Need To Know!)

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