A Practical Guide to Understanding Concrete Shrinkage Cracks

You just had a beautiful new concrete driveway installed, in hopes that it would last a lifetime. But a few days later you notice some fine, irregular cracks forming in the surface. What happened? And is your concrete ruined?

This type of cracking, which is caused by the shrinkage of concrete, is actually a common problem that can occur when freshly placed concrete dries during the curing process. Large, flat surfaces such as concrete driveways, patios, and floors are especially vulnerable to shrinkage cracks, particularly if they dry too quickly. But shrinkage cracks can also appear in vertical structures like concrete walls and columns.

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While shrinkage cracks are generally superficial and won’t compromise the structural integrity of your concrete, they can certainly detract from its appearance. Learn more about what causes shrinkage cracking of concrete and what strategies you can take to reduce or prevent it.

When are shrinkage cracks most likely to occur?

Shrinkage cracks in concrete are most likely to show up during the early stages of the curing process, typically within the first 24 to 72 hours after the concrete is poured. As the concrete begins to set and lose moisture, it shrinks. This early cracking, called “plastic shrinkage cracking,” happens while the concrete is still in a plastic state and before it has fully hardened.

But even after the concrete hardens, some shrinkage cracking can occur due to the gradual loss of moisture over time. This type of cracking, called “drying shrinkage cracking,” can take place weeks or even months after the concrete is placed.

What is the main cause of shrinkage in concrete?

Plastic shrinkage cracking is primarily due to rapid evaporation of water from the concrete surface, often due to environmental factors such as high temperatures, low humidity, or strong winds. This loss of moisture causes the concrete to shrink slightly, resulting in the formation of small, hairline cracks on the surface.

Adding too much water to the concrete mix increases the potential for shrinkage. As the excess water evaporates, the concrete shrinks more. Inadequate curing of the concrete is another factor that can increase the likelihood of shrinkage cracking because the surface dries too quickly. (Learn more about how proper curing can reduce the rate of moisture loss in concrete.)

Another common cause of cracking in concrete slabs is a lack of control joints. When properly installed, these joints help to prevent cracking caused by temperature fluctuations and drying shrinkage by allowing for the movement of the slab.

Water highlights the shrinkage cracks on this colored concrete surface.

What do shrinkage cracks look like?

Concrete shrinkage cracks are typically narrow, irregular fissures less than 1/8-inch-wide and ranging in length from a few inches to a few feet. They often appear in a random, spider web-like pattern across the surface and may even be unnoticeable from a distance. Usually the cracks don’t penetrate deeply into the concrete and are only superficial.

Note that cracks caused by plastic shrinkage only occur during the first few days after concrete placement, as the freshly placed concrete dries. Cracks caused by shrinkage after the concrete hardens are typically wider and deeper than plastic shrinkage cracks.

Are shrinkage cracks a defect?

Concrete shrinkage cracks are a common occurrence, and while they can be troubling, they don’t necessarily mean your concrete is defective. However, if the cracks become wider over time or one side of the crack is higher than the other (which could be a sign of subgrade settlement), consider having the problem evaluated by a professional to determine the cause (see Why Does Concrete Crack?).

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How do you stop concrete from cracking when drying?

There are several steps you can take to greatly reduce the likelihood of cracks forming in your concrete as it dries.

• Make sure your contractor uses a concrete mix with the right water-cement ratio. Adding too much water to the concrete can lead to excessive shrinkage and weaker concrete.
• Keep the concrete moist while it cures to slow down the evaporation of water from the surface.
• Avoid rapid drying by protecting the concrete from exposure to wind and direct sunlight, especially during hot weather. (Learn more about curing concrete in hot weather.)
• Ask your contractor to add a shrinkage-reducing admixture to the concrete mix to minimize the amount of drying shrinkage that occurs as the concrete hardens. Adding synthetic fibers to the mix can also be beneficial in controlling shrinkage cracking (see When to Use Fiber-Reinforced Concrete).
• Make sure control joints are installed in the slab at the appropriate intervals.
• Consider installing steel reinforcement to help withstand tension forces caused by drying shrinkage and thermal expansion and contraction.

Because weather can be a major contributing factor to plastic shrinkage cracking, consider delaying your concrete pour if high winds, low relative humidity, and hot temperatures are predicted. If that’s not possible, you can lessen the impact of adverse weather conditions by using windbreaks, sunshades, and wet coverings.

Why does concrete shrink when it cures?

Shrinkage occurs because excess water not needed for the hydration process evaporates from the concrete, leading to a reduction in volume. Although this evaporation is normal, it can sometimes lead to shrinkage cracks if the rate of water loss is too rapid and not properly managed.

What is acceptable shrinkage for concrete?

Some amount of concrete shrinkage is inevitable, but what is considered acceptable typically falls within a range that minimizes the risk of cracking and other shrinkage-related defects.

According to industry standards, drying shrinkage over a 28-day period should generally be below 0.05%, or less than 1/2 inch per 100 feet of concrete. For most residential concrete, however, testing to measure the amount of shrinkage is impractical. Instead, concrete contractors often rely on good construction practices to keep shrinkage to an acceptable level.

How do you fix shrinkage cracks in concrete?

They may not need to be fixed at all if the cracks are barely visible and don't affect the concrete’s structural performance. But if you’re concerned about the appearance of your slab, here are some repair options to consider:

• For narrow cracks, you can fill them with a pourable concrete crack filler that will flow into the cracks to ensure that you fill them completely.
• For wider cracks, use a thicker concrete patching compound to fill the cracks, feathering the material with a trowel so the repair is even with the surrounding surface.
• If the shrinkage cracking is extensive, consider resurfacing the entire slab with a concrete overlay to give it a new, flawless finish.

Before you start any repairs, make sure the cracks are clean and free of debris so the crack filler will adhere properly. For more tips on fixing minor cracks in concrete and selecting the right repair materials, see How to Fix Cracks in Concrete Slabs.

How long does it take concrete to stop shrinking?

Concrete undergoes the most significant shrinkage within the first 28 days after placement, as the concrete cures and loses moisture. However, concrete can continue to shrink for several months, depending on environmental conditions and the concrete mix used. Even after a year, concrete may still continue to shrink but at a much slower rate. Eventually, the concrete reaches a point of stabilization and any additional shrinkage will be minimal.

Are shrinkage cracks in concrete inevitable?

Shrinkage cracks in concrete are very common, but they are not entirely unavoidable. Using strategies such as proper curing, installing control joints, and keeping the water-cement ratio low can greatly reduce the likelihood of shrinkage cracks from forming. To get reassurance that all the proper precautions are being taken, be sure to discuss these issues with your concrete contractor before your next project.

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