Shrinkage Cracks in Concrete Slab: The Ultimate Guide
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Shrinkage Cracks in Concrete Slab: The Process Explained
Concrete is a widely used construction material as it has an impressive compressive strength. And yet there will always be shrinkage cracks in concrete slab. Why does that happen?
Before concrete becomes a rock hard slab it is a concrete mix slurry consisting of cement, water, aggregates, sand and a bit of admixture.
The concrete mix cures through a chemical reaction called hydration, where water reacts with cement to form a crystalline network.
This lattice binds the sand and aggregates to give concrete its strength and durability.
The physical changes that take place when concrete cures are also important. These changes include:
- Setting: The initial setting of concrete occurs within minutes of mixing. The slurry begins to solidify and the concrete mix becomes hard enough to support its own weight.
- Hardening: The hardening of concrete is a continuous, never ending process. However, concrete is typically considered to have fully cured in 4 weeks as it achieves 99% of its full strength.
- Shrinkage: Concrete shrinks as it cures. This shrinkage is caused by the loss of water from the concrete during the curing process.
Concrete cures through hydration to gain strength. Hydration causes excess water to evaporate causing concrete to shrink and crack.
Types of Shrinkage Cracks in Concrete Slab
There are two main types of shrinkage cracks in concrete slabs; plastic shrinkage cracks and drying shrinkage cracks.
| Shrinkage Crack Type | Cause | Effect |
|---|---|---|
| Plastic Shrinkage Cracks | Rapid evaporation of water from the surface of the concrete | Results in hairline cracks that mar the looks but have little impact on structural integrity. |
| Drying Shrinkage Cracks | Gradual loss of water from the concrete over time | Results in larger cracks that can let water penetrate and damage concrete from within. |
Plastic Shrinkage Cracks
Plastic shrinkage cracks occur when the concrete is still in a plastic state, shortly after it is placed. This type of cracking is caused by the rapid evaporation of water from the surface of the concrete.
The water evaporation causes the top layer of the concrete to shrink, and the tensile stresses that are created causes cracks to form.
Drying Shrinkage Cracks
Drying shrinkage cracks occur after the concrete has hardened. This type of cracking is caused by the gradual loss of water from the concrete over time.
As the water evaporates, the concrete shrinks, and the tensile stresses that are created cause cracks to form.
The following are some of the factors that contribute to the severity of shrinkage cracks in concrete slab:
- Use of too much water in the concrete mix. This will result in a concrete mix that is too wet, and the excess water will evaporate more quickly, causing plastic shrinkage cracks.
- Hot weather. Hot weather accelerates the evaporation of water from concrete, which can increase the risk of plastic shrinkage cracking.
- Low concrete strength. Concrete with low strength is more likely to crack due to shrinkage.
- Improper curing. Concrete that is not cured properly will not develop its full strength, and it will be more likely to crack due to shrinkage.
- Poor soil support system. A poor soil support system preparation can cause the concrete to shrink unevenly and lead to cracking.
- Design and construction defects. Cracks can also be caused by design and construction defects, such as inadequate concrete thickness or improper placement of reinforcement.
Detrimental Effects of Shrinkage Cracks in Concrete Slab
Shrinkage cracks in concrete slab can have a number of detrimental effects, including:
- Reduced strength and durability. Shrinkage cracks can weaken the concrete and make it more susceptible to damage from weathering, erosion and chemicals.
- Allow water to seep into the concrete. Water that seeps through the shrinkage cracks in concrete slab can weaken the concrete from within. It can also corrode the steel wire mesh or rebars.
- Create a pathway for pests and rodents. Larger shrinkage cracks can be home to pests such as ants and spiders.
- Cracks are, of course, an eyesore. Shrinkage cracks take away the beauty of smooth unblemished concrete slab, are unsightly and may even reduce the value of your home.
The severity of the detrimental effects of shrinkage cracks will depend on the size, number, and location of the cracks.
Small, hairline cracks, which is often the case, mar the visual beauty of a concrete slab, but do not have any significant impact on the strength or durability of the concrete slab.
However, larger cracks can grow and compromise the structural integrity of the concrete slab.
How to Reduce Shrinkage Cracks in Concrete Slab
There are a number of things that you can do to reduce shrinkage cracks in concrete slab:
- Use the correct amount of water in the concrete mix.
- Cure the concrete properly.
- Use a concrete mix with high strength.
- Avoid hot weather when placing concrete.
- Prepare the subgrade properly.
- Design and construct the concrete slab correctly.
Dealing with Shrinkage Cracks in Concrete Slab
You can be quite sure that you will have shrinkage cracks in a concrete slab, say a driveway or a garage floor. What are you going to do? How are you going to deal with these concrete shrinkage cracks?
The way to deal with shrinkage cracks in concrete slab depends on the size of the cracks. Here is how to deal with them:
Hairline Cracks
Hairline cracks do not impact the structural integrity of the concrete slab and do not require any repair.
It is best to seal them with a high quality penetrating sealer. The sealer will block the concrete pores from within and prevent water ingress.
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A topical sealer is essential for protecting concrete surfaces from water permeation that can damage the concrete from within and compromise its lifespan.
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To cover up the hairline cracks for visual appeal you can use one of the several types of paints or coatings.
Cracks less than 1/2 inch Wide
- Caulk and sealant: Several caulk and sealant options are available for cracks less than half an inch wide. Simply caulk the crack, then use a trowel to spread the caulk evenly. A caulking option will dry quickly, within a couple of hours.
- Epoxy sealant: An epoxy sealant can also be used on smaller cracks, but for larger fractures in your concrete, you’ll want to stick exclusively with an epoxy remedy.
Deeper Cracks
- Concrete crack chaser: Depending on the depth and/or width of the crack, a crack can be v-cut with a concrete crack chaser and filled with a suitable material.
- Epoxy sealant: For deeper cracks, use an epoxy sealant.
Larger Fractures in Concrete
- Epoxy remedy: For larger fractures in concrete slabs, you’ll want to stick exclusively with an epoxy remedy.
Thank you very much for reading this post. I do hope you found it informative and helpful.
