Learn about eleven common types of concrete cracks and what causes them.
Virtually every old-school concrete guy will always tell you that there are only two guarantees with concrete: first, that it will get hard, and second, that it will crack.
Over time, concrete is bound to develop cracks for many different reasons – environmental conditions, design flaws, or construction issues. Sometimes these cracks are serious, and sometimes not so much.
In this article, we’re using our 30+ years of experience repairing concrete to shed some light on different types of concrete cracks, why they happen, and what they mean.
1. Shrinkage Cracks
Shrinkage cracks are common in freshly poured concrete and result from the natural drying and curing process. As water evaporates, the concrete shrinks, leading to small, often fine cracks on the surface.
While shrinkage cracks are generally superficial and do not compromise structural integrity, they may not look the best. Proper curing practices and the use of shrinkage-reducing additives in the concrete mix can help reduce the chance of them happening.
2. Overloading Cracks
Overloading cracks form in concrete when excessive weight is put on a concrete slab. Concrete is a very strong building material overall, but too much weight will crush it.
In residential concrete slabs (driveways, sidewalks, patios, etc.), direct overload on the slab itself is not common. Instead, there is a higher likelihood of excess overload on the ground beneath the slab.
When the ground below a slab becomes soft and wet, like after a heavy rain or snowmelt, the concrete can be pressed down by any excessive weight placed on the slab, resulting in cracks.
This excessive weight can come from dumpsters, RVs, or other types of big, heavy equipment. With that being said, overloading cracks can still occur even without wet, soft ground conditions.
Pro Tip: The PSI rating of a concrete mix, such as 2,000, 3,000, 4,000, or 5,000+, indicates the pounds per square inch required to crush the concrete. A higher PSI rating means a higher strength concrete. For reference, residential concrete is typically 3,000-4,000 PSI.
3. Hairline Cracks
Hairline cracks form due to concrete settling while it’s curing. They are usually very thin cracks, but they can run very deep in the slab, as well.
Because hairline cracks run deep, they pose a risk for more serious cracking once the concrete has hardened.
As water and moisture make their way deep into the cracks and freeze as temperatures drop, the water and moisture expand. This expansion causes the cracks to widen and get a little bit worse after every freeze-thaw cycle.
Pro Tip: Properly maintaining a good seal in a crack can protect against additional damage from freeze-thaw cycles.
4. Settling Slab Cracks
The soil under a new concrete slab is very rarely properly compacted before the new concrete is poured on top. This means that over time, the soil will continue to settle and bring the slab on top down with it.
This isn’t just a concern when new concrete is installed, though. For example, if a utility company has to dig on your property and doesn’t compact the soil after they’ve installed their new lines, the soil will settle quite a bit and leave new voids behind.
This soil movement leaves unsupported weak spots in the slab, and these weak spots are more prone to cracking.
Depending on the way the settling occurs, the cracks could be pinched together or spread open. Cracks in a dip or low spot will typically be pinched tightly together, while a crack at a relatively high spot will be open wider.
5. Heaving Slab Cracks
Freeze-Thaw Cycles
As the ground and soil freeze, they expand and push on nearby structures or concrete, causing structural damage, like cracking. During a freeze-thaw cycle, the ground can expand and lift a significant amount (multiple inches) before it thaws and lowers back to its original position.
The soil movement brought on by temperatures fluctuating above and below freezing causes concrete to crack when it can’t move freely with the ground.
This is an example of why it is important to have proper expansion and control joints installed during the original installation – these joints help control cracking and allow for the free movement of slabs during freeze-thaw cycles.
Tree Roots
Tree roots growing nearby can also push the concrete slab out of place, causing it to crack with the heaving motion.
It’s important to be careful when planting trees near concrete slabs for this reason. Consider consulting an arborist to better understand the size of the root system of your desired tree to better plan its placement on your property.
Using a root barrier system from the get-go, or cutting back the roots when you first notice heaving, can also help protect the concrete from cracking.
6. Foundation Settlement Cracks
Foundation settlement cracks are usually found around the corners or perimeter of buildings in the form of a stair-step pattern within the wall. They happen when the soil or ground underneath a concrete foundation sinks or compresses over time and the foundation settles unevenly with it.
Settlement cracks can occur for different reasons, like soil issues or improper site preparation when the foundation was originally poured.
It’s important to inspect your foundation regularly for cracks or movement and fix any problems that you notice as soon as possible to prevent the damage from worsening.
Pro Tip: If you can fit more than a quarter in the settlement crack, you should have it inspected by a professional.
7. Structural Cracks
Structural cracks are more serious and can compromise the integrity of a concrete structure. They may appear as wider cracks (⅛” or more), often extending through the entire thickness of the concrete.
Structural cracks can widen over time, allow for water leakage, and cause the structure to become unstable. If left alone, structural cracks can get significantly worse and even cause the building to fail.
Structural cracks can result from many different factors, such as:
- Overloading
- Poor construction practices
- Design flaws
- Soil movement
- Water infiltration
Getting in contact with a reputable foundation repair contractor or structural engineer is the best way to diagnose and fix structural cracks.
8. Crazing Cracks
Crazing cracks are fine, interconnected cracks that form on the concrete surface. Because they don’t penetrate deep into the slab, they don’t pose a threat to the concrete itself, although they can affect the concrete’s aesthetics.
Crazing cracks are usually caused by rapid drying of the concrete surface when the concrete is poured, leading to a mismatch in shrinkage between the surface and the underlying layers.
Proper curing techniques and avoiding overly rapid drying can help minimize crazing during installation, but there is not much that can be done after the fact to hide them.
Pro Tip: While not a guaranteed fix, sealing concrete with crazing cracks using a high-quality sealer can help prevent them from worsening.
9. Crusting Cracks
When concrete is curing, the top layer of the slab can dry out quicker than the bottom layer, which can cause the surface to become crusty or cracked. This is more common on sunny, hot, or windy days, as the dry time of the top layer is accelerated even more.
Crusting cracks don’t affect the integrity of the slab, as they are only surface-level, but they don’t look the best. So, while it may not be exactly the look you were going for, they aren’t usually a structural problem.
Crusting cracks can occur on all types of concrete finishes, but it is commonly associated with stamped concrete.
10. Re-Entrant Corner Cracks
Re-entrant corner cracks happen when concrete is poured around corners or rounded objects like columns or manholes. As the concrete shrinks during the drying process, it doesn't shrink evenly, leading to long cracks radiating from the object.
To prevent this issue, it's common practice to install control joints at strategic points when the concrete is poured to coax the cracking into the valley of the joint. Forcing the crack into a joint will help maintain the aesthetics of the concrete surface.
11. Internal Reinforcement Corrosion Cracks
Rebar and wire mesh are commonly used to reinforce concrete. But as moisture penetrates the porous concrete, the steel of the rebar or wire begins to corrode.
The rust increases the size of the steel and puts stress on the concrete from the inside. This internal stress can cause concrete to crack, break down, and spall over time.
What Should You Do About Concrete Cracks?
It’s important to identify potential issues early on and find the necessary solution before concrete cracks have time to worsen. While some cracks may be just cosmetic, others can be a sign of more serious structural problems.
Regularly inspect your interior concrete (floors, garage, foundation walls) and exterior concrete (driveways, patios, pool decks, exterior foundation walls, etc.) concrete for signs of new cracks.
If in doubt about your concrete cracks, reach out to experienced professionals who can help you find the right solution. You may be surprised to find that your concrete cracks are a simple fix!
A-1 Concrete Leveling has been repairing and restoring concrete across the US for over 30 years. Click the link below to request a free inspection and cost estimate for your concrete repair and maintenance needs, including diagnosing and taking care of concrete cracks!
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