I often get asked about cracks in concrete foundations. Many home owners are nervous when they see cracks in concrete and wonder if they are bad or dangerous. While it’s a natural reaction to be concerned when you see something cracked the reality is that 95% of cracks in concrete are harmless and nothing to worry about.
Why Does Concrete Crack?
Concrete cracks when it is subjected to tensile stress (forces that pull apart on something vs forces that push together something). Concrete has very strong compressive strength and very poor tensile strength. That’s why concrete is reinforced with steel reinforcing called rebar. So you end up with a composite material, the concrete takes the compressive loads and the steel rebar takes the tensile forces.
There are many factors that cause concrete to crack. Some of the factors that can cause concrete cracks are:
- Drying Shrinkage – as concrete cures and goes through it’s chemical reaction between the water and cement particles it begins to “dry” out. As with most materials that dry out they begin to shrink causing hairline cracks to appear.
- Thermal Shrinkage – all materials shrink and expand with changes in temperature. As concrete cools down (the chemical reaction that’s created when water and cement is mixed creates heat) it shrinks. The shrinking from cooling down can also cause hairline cracks to appear.
- Restraint – This topic is a bit harder to explain but it’s caused when concrete is confined and prevented from moving due to the structure. If the concrete wants to move due to loading, thermal or other effects and it’s restrained from doing so it will crack.
- Settlement – If the ground around a foundation settles it can cause the concrete to crack. Settlement can cause significant damage under certain circumstances.
- Loads – As I mentioned when concrete is subjected to loads that cause tension it will crack the concrete. Tension can be caused by many different loading conditions and it’s best dealt with by designing reinforced concrete that uses rebar.
- Corrosion – When steel rebar corrodes inside of concrete it expands and causing the concrete around it to crack.
Which Concrete Cracks Are Bad?
Obviously this is a topic that has many answers and not easily answered in a simple way. However, what you need to know is that ALL concrete cracks. In fact, reinforced concrete must crack before the rebar can take hold and do it’s job. Most cracks are so narrow that you can’t see them unless you look very closely with good light.
So which cracks are bad? Typically if you see a concrete crack that’s less than 1/8 inch wide then I’d say you can totally ignore it. If you find a crack that’s between 1/8 inch and 1/4 inch wide it is most likely nothing to worry about. In this situation it’s important to look at the concrete on both sides of the crack and try to determine whether or not there has been any relative movement between the sides, up/down, left/right, in/out. This would be an indication of settlement and therefore a potential problem.
If you have a concrete crack that’s 1/4 inch wide or wider then I recommend you contact a local structural engineer. Cracks of that width indicate significant movement within the foundation and warrants further investigation.
Minor Concrete Crack Repairs
Minor cracks in concrete can be repaired easily by DIY home owners. Concrete cracks can be filled / sealed using a number of products. I happen to like concrete crack fillers from Sika. You can also use hydraulic cement (it expands slightly as it cures) to fill cracks. The idea behind filling the cracks is to seal them up to prevent moisture and air from moving into the concrete.
Cracks in concrete are a fact of life. ALL concrete cracks and you should understand this issue and realize that most concrete cracks are safe. If you find a wide crack and evidence of movement then contact a local structural engineer for further investigation. I hope this article has cleared up this confusing and mis-understood subject.