Cracks in Concrete Garage Floor (What You Need To Know!)

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Are Cracks in a Garage Floor Normal?

Have you just got the concrete floor of your garage placed. That is great! The concrete will cure in around a month. But if you are not prepared for it, you may get a rude shock. As the concrete cures you may find cracks appearing. Are cracks in a concrete garage floor normal?

A concrete garage floor will surely crack. It always has and it always will. It is the intrinsic nature of a concrete slab on grade. Concrete garage floor cracks are not a cause for concern unless they are structural in nature.

Concrete is one of the most widely used building materials in the world. Infrastructure projects, from dams to bridges, airports to skyscrapers, mostly use concrete.

The architects and the engineers involved are fully aware that concrete cracks, But they do not worry. Why? Because they understand concrete and know how to control, manage and repair cracks in concrete.

In this post I hope to tell you everything that you need to know about concrete, concrete cracks and concrete garage floors. Enough so that you do not stress out when you come across cracks in your concrete garage floor.

Cracks in Concrete Garage Floor

So, let’s start with the basics.

What Is Concrete?

Concrete basically has only three components; cement, water and aggregates.

“Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time.”

Source: Wikipedia

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, but once the concrete mix has been poured the curing process begins. This happens by a chemical process called hydration. The cement reacts with the water to form crystals.

The crystals grow into a matrix enveloping the sand and other aggregates in a tight mesh. This converts the concrete mix into a rock hard solid material.

What Can Cause Concrete to Crack?

A concrete garage floor cracks, either due to stress caused by shrinkage during the curing process or stress caused by ground movement below the slab, later on.

Concrete Shrinkage

The water in the concrete mix evaporates during the curing process. This causes the concrete to shrink. Concrete has very high compressive strength but very poor tensile strength. The concrete slab is not able to withstand the stress created by the shrinkage and cracks.

The shrinkage cracks in concrete slabs is a perfectly normal occurrence. They are also known as hairline cracks and are less than 1/16 th an inch wide. Shrinkage cracks can become visible as soon as the curing process begins.

Another form of shrinkage cracking is called “crazing”. Crazing happens if the surface of the concrete slab dries out very fast, during curing. This can happen due to high air or concrete  temperatures or low humidity (often both). The cracks are very fine and shallow but spread across the entire surface.

The shrinkage cracks in the concrete garage floor do not affect the structural integrity.

Ground Movement

In spite of a well prepared and compacted base, the ground below the slab moves and settles over a period of time. This exerts tensile stress on the garage floor.

Ground settlement under a concrete slab is usually the result of:

  • Change in soil moisture content
  • Poor base preparation & compaction
  • Soil erosion due to rain or plumbing leaks

The cracks caused by ground movement may or may not be serious depending on the reason and the size of the cracks. But you do want to call in your contractor to take a look.

How Do I Keep My Garage Floor from Cracking?

It is impossible to avoid cracks in concrete slabs. But you can take preventive steps to 

  • Minimize the extent & size of cracks
  • Make them less visible
  • Reduce the chances of their beng structural

The preventive steps are:

Base Preparation & Compaction

It is very important to prepare the base for the concrete slab correctly. Start off by marking out the area of the garage floor. 

Remove the top soil by digging a hole in the ground. The depth of the hole should be equal to the sum total of the base and concrete slab thicknesses. For a slab-on-grade allow a few extra inches at the edges. The edges will then function as footings.

The exposed soil should be smoothed and any low areas filled up. Tamp the surface using a hand or mechanical tamper. You should be able to walk on the tamped soil without leaving footprints.

Check out Bully Tools Tamper Thick Steel Plate with Steel Handle on Amazon.

Next put layers of compactible gravel, crushed stone or recycled concrete for providing adequate strength to the base. The layers must be compacted at each stage for a strong and solid base. Build up around 3-4 inches of this base.

A well prepared & compacted base will prevent soil subsidence and reduce the possibility of large cracks in the concrete garage floor.

Slab Thickness & Reinforcement

Of course the strength of a concrete slab is proportional to the thickness. The concrete slab thickness should not be less than 4”. 

You do not need rebars in a concrete slab that is 4” thick. However, do use wire mesh or fibers in the concrete mix. This will add to the tensile strength and reduce the chances of cracks in the concrete garage floor.

Concrete Mix Composition

The concrete mix composition, especially the water-cement ratio, plays a critical role in determining the concrete shrinkage, concrete strength and development of cracks in the concrete slab.

“There’s an inverse relationship between the eventual compressive strength of concrete and the amount of water used in the mix—the higher the water-cement ratio, the lower the strength.”

CONCRETE CONSTRUCTION

To get a compressive strength of at least 3500 psi use concrete mix with a water-cement ratio of 0.5 or less. This will minimize shrinkage cracks too as there is less water to evaporate.

Contractors do not like a low water-cement ratio, as it makes it more difficult to work the concrete slurry. Fortunately, there are concrete mix additives that help reduce the water-cement ratio without reducing the workability.

Sika is a world leader in water-reducing admixtures. Water-reducing admixtures lets you reduce water content in a concrete mix by 5-10% without any effect on the workability.

You can get a high strength, low shrinkage concrete which is also pliable and workable with the right concrete mix additive.

The Curing Process

The complete concrete curing process takes 28 days.

During this process, water is reacting with cement to form crystals and evaporating. The quantity and rate of water evaporation is directly related to concrete shrinkage and cracks developing in the concrete slab.

The concrete and ambient temperature are crucial to get the best results from concrete slabs. Concrete should be placed ideally in conditions between 50 degrees and 75 degrees Fahrenheit.

Moreover, this temperature range must be maintained during the entire concrete curing process.

The crystal growth & water evaporation process is curtailed at low temperatures and accelerated at high temperatures.

During the initial curing process, water bleeds and evaporates from the surface. The  concrete surface will dry out. Moisture loss from the concrete surface must be reduced by fogging or ponding the surface with water.

Without fogging or ponding at this stage, crazing can occur.

Even after the initial curing, water-based curing methods need to be employed to control the rate of water evaporation from the concrete. This can include fogging, sprinkling, ponding or wet covering.

Expansion & Control Joints

Expansion and Control joints in a concrete slab are sometimes misunderstood to be the same thing. They are similar but they are not the same. However, both help “manage” the cracks in the concrete garage floor.

Expansion Joints

Expansion joints are placed before the concrete pour. The purpose of the expansion joint is to give space to the individual slabs to move, expand and contract, without putting stress on the adjoining slab, wall or whatever it abuts.

Concrete is very rigid and brittle. Expansion joints are made out of a material that is flexible. For a concrete garage floor Sakrete concrete expansion joint works quite well.

Control Joints

Control joints are made after the concrete pour, leveling & floating, but while the concrete is still wet and malleable. Concrete has a tendency to crack every 10-12 feet. 

Strategically placed control joints encourage the concrete slab to crack at the control joint, rather than anywhere at random. A control joint in a concrete slab is the weak spot. Cracks due to ground movement are more likely to happen at the weak spot.

The depth of the control joint should be about ¼th the thickness of the concrete slab. In other words the control joint should be 1” deep for a 4” thick concrete garage floor.

Concrete Sealing

Once the concrete slab has fully cured (28 days) it is time to seal it. There are two types of concrete sealers; penetrating & topical. Each has a role to play when it comes to dealing with cracks in a concrete garage floor.

Penetrating Sealer

Concrete is full of pores; a result of water & entrained air in the concrete mix finding their way to the surface and escaping. 

Water and chemicals seep into the concrete pores and hasten concrete degradation. Weak concrete is more likely to crack under stress from ground movement. Moreover, during winters, the water in the pores, will freeze and thaw cyclically and put pressure on the existing cracks to grow.

A penetrating sealer will seep into the tiny pores (capillaries), fill them up, solidify and block them, so that water can not enter.

I recommend Foundation Armor SX5000 Water Based Silane Siloxane Penetrating Concrete Sealer.

Topical Sealer

Topical sealer is helpful in filling up the surface cracks. As a result the crack will not become home to dust, water, mold and insects such as ants. The topical sealer will, however, not hide the crack as it is clear. 

I recommend Foundation Armor AR350 Solvent Based Acrylic Wet Look Concrete Sealer.

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Thank you very much for reading the post. I do hope you found it informative and useful.

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