How do you repair hairline cracks in concrete slabs?

Ready Mix Concrete

Hairline cracks on concrete can be challenging to repair. As time passes, all concrete structures tend to form cracks. What’s important is that these cracks should not be visible, nor should they affect the overall performance of the structure.

A well-made concrete structure should not develop structural cracks quickly. Nevertheless, hairline cracks can be expected. Don’t stress; these cracks are not a sign of bad quality and can be fixed without much effort.

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What causes hairline cracks on concrete?

Both significant structural cracks and hairline cracks are often caused by stress relief. When the tensile stress created is more than the overall capacity to resist, it results in the formation of a crack.

Most structural cracks on concrete surfaces form as a result of pressure and stress. However, hairline cracks can also occur as a result of shrinkage from heat.

Since they ideally happen near the surface, reinforcing your concrete is not going to help prevent them. The only preventative option you have is to use a good mix design that offers low shrinkage tendencies.

However, you may still have to deal with hairline cracks, especially if there is moisture around the structure. When exposed to water, concrete tends to absorb moisture and later dry out in the sun repeatedly.

With time, these drying and wetting cycles will cause your structure to crack the same way steel eventually breaks after being bent back and forth repeatedly.

Excess exposure to heat can also cause hairline cracking on concrete. It is often not the overall intensity of the heat that damages the structure, but the total time the concrete is exposed.

Even though heating causes damage, subsequent cooling also plays a critical role. As the structure cools, it will shrink, and this, in turn, causes hairline cracks to form.

How to repair hairline cracks on concrete

Repairing a hairline crack on a concrete structure involves filling the gap using a material that bonds well to the concrete, restores its original appearance, and prevents liquids from penetrating inside the structure and staining the concrete.

Step 1: cleaning the cracked surface and its interior

The hairline crack needs to be appropriately cleaned to eliminate any dirt or stains on the area. It would be best to clean any external matter that has covered the surface before applying the filling material to achieve a proper bond.

Step 2: make the crack moist.

You need to moist the hairline crack on the surface using water at least seven hours before the application of filling material or grout into the crack.

Moistening this area will help you prevent the concrete from absorbing water from the applied grout. However, you need to ensure that water does not stand or clog the surface of the crack. If this happens, then your grout will dry out before it can properly seal the crack.

Step 3: preparation of sealant

The most commonly used grout to fill concrete cracks is Portland cement mixed with water. You need to add enough water to make a good quality grout mix.

Besides cement grout, you can go with any other filler material or sealant to fill out the cracks. Keep in mind that an ideal adhesive needs to have low surface tension and viscosity. These properties help the adhesive to penetrate deep into the gap to fill it.

Step 4: applying the sealant into the crack

After you properly moisten and clean the cracks, grout is applied. Our professionals from Big D Ready Mix Concrete suggest using either a pointing trowel or a putty knife for best results to apply grout properly.

It would help if you forced the grout inside the crack as much as possible to ensure it fills the gaps. Any excess material falling out can be smoothened and leveled to match the existing surface.

Step 5: curing of concrete cracks

The last step is curing concrete. The sealed cracks need to be patched, dried for at least two hours, and covered using a board or a plastic sheet. It is essential to cover the area for around five days. Every day, you need to lift the covering and sprinkle water on the surface.

Hairline cracks are not a big deal. It is something that can only be controlled and is challenging to eliminate.


Repairing hairline cracks on concrete slabs can be difficult for an average homeowner since making the cracks vanish requires a lot of skill, patience, and a good eye to match the color.

A straightforward definition of a hairline crack is a crack that has not completely opened up. This means that there is very little room to fill it with anything other than low viscosity liquids.

Since these cracks do not affect the overall structural integrity of the slab, concealing and sealing the gap is often the primary goal of such a repair.

Want to learn more about concrete? Get in touch with Big D Ready Mix Concrete today!

At Big D Ready Mix Concrete, we have highly skilled professionals who are always ready to help you regarding all your concrete-related queries.

Not only can we give you valuable information about concrete, but we can also provide you with a high-quality concrete-ready mix for all kinds of construction projects.


We have been serving the Dallas area since 2002. Give us a call at 972-737-7976 or contact us online and one of our professionals will guide you through the process.

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Big D Ready Mix Concrete has been serving clients in the Dallas, TX area since 2002, with over 400 utility mixes, high-strength wall mixes, exposed aggregate mixes, flexural strength mixes, stamp concrete mixes, flowable fill mixes, grout mixes, and trailer pump mixes.

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