Why Does Concrete Crack? Causes and What Helps

Can anyone pour concrete that never cracks? No. NRMCA’s Concrete in Practice sheet on cracking says all concrete tends to crack, and fully crack-free concrete is not possible. The real goal is to understand the cracks you will get and keep them under control.

Can Concrete Ever Be Completely Crack-Free?

No. All concrete has a tendency to crack, and NRMCA’s CIP 4 sheet, last revised in 1998, says completely crack-free concrete cannot be made. Cracking can be reduced and controlled, though.

The cause is movement. Concrete contracts and expands as moisture and temperature change, and it bends under load. Cracks can form when the design and the pour make no room for that movement.

The same sheet says the majority of cracks usually come from improper design and construction practices. It names omitted or improper joints, poor subgrade preparation, high-slump concrete or extra water added on the job, improper finishing, and inadequate or no curing. The sections below cover them.

Which Kinds of Cracks Appear, and When?

CIP 4 lists seven forms. They are plastic shrinkage cracks, cracks from improper jointing, cracks from continuous external restraint, cracks from a lack of isolation joints, D-cracks, craze cracks and settlement cracks. Three show the range of timing and risk.

What Causes Plastic Shrinkage Cracks?

Plastic shrinkage cracks form when surface moisture evaporates faster than rising bleed water can replace it. The surface dries before the concrete sets, so it cracks while still plastic. NRMCA’s CIP 5 sheet describes them as unsightly but rarely a threat to strength or durability.

Weather sets the evaporation rate. CIP 5 names wind above 5 mph, low relative humidity, and high air or concrete temperature. A vapor retarder under the slab greatly raises the risk. These steps lower it:

• Dampen the subgrade and forms, leaving no standing water.
• Use fog sprays and windbreaks.
• Cover the surface with wet burlap or polyethylene between finishing steps.
• Use cooler concrete in hot weather.
• Start curing as soon as finishing ends.

A Federal Highway Administration pavement guide adds a mix angle. It says concretes with water-cement ratios below about 0.45 tend to be more susceptible to plastic shrinkage cracking. Below about 0.40, concrete can internally desiccate, and water-added curing methods are recommended. Those are pavement-concrete statements, so read them as background for a slab, not as a rule.

How Do Joints and Restraint Lead to Cracks?

Concrete cracks at random when something holds it still as it contracts and no joint takes the movement. CIP 4 gives an example: a wall cast on a footing and restrained along its bottom edge. A missing isolation joint, where a floor meets a wall, column or footing, works the same way.

Contraction joints are grooves about 1/4 to 1/3 of the slab thickness deep. Isolation joints run full depth, with a barrier that prevents bond. Here is one micro-example on a 4 in slab.

1. Spacing. CIP 4 uses 24 to 36 times the thickness: 4 x 24 = 96 in (8 ft) and 4 x 36 = 144 in (12 ft).
2. Depth. A quarter to a third of 4 in is 1.0 to 1.33 in.
3. Check. The Control Joint Spacing Calculator at its 2.5 factor returns 10 ft and about 1 in for the same slab, and it handles your own sizes.

Shape matters too. Panels should be square, and their length should not exceed about 1.5 times their width. An 8 ft wide panel therefore runs at most 12 ft long, since 1.5 x 8 = 12.

Which Preventable Mistakes Cause Most Cracks?

Most cracks trace to design and construction practices, and CIP 4 gives a short prevention list. Start with the ground and the mix, then move to the surface. To size the pour first, use the concrete slab calculator.

• Subgrade. Remove topsoil and soft spots, then compact the subgrade or granular fill. Never place concrete on a frozen subgrade.
• Vapor retarder. One placed directly under a slab increases bleeding and cracking potential. Cover it with 3 to 4 in of compactible granular fill.
• Water. Use moderate slump, not over 5 in, and avoid retempering to raise slump. Extra water added on the job is among the usual causes.
• Finishing. Do not finish with bleed water on the surface, and do not overwork it.
• Steel cover. Keep at least 2 in of cover over reinforcing steel, so salt and moisture stay away from the steel.

How Many Days Should Fresh Concrete Be Cured?

NRMCA says to start curing as soon as possible and keep the concrete moist for at least 3 days. QUIKRETE’s data sheet for Concrete Mix 1101 asks for more: 5 days in warm weather at 70 F or higher, or 7 days in colder weather at 50 to 70 F.

The two figures do not conflict. One is a general minimum, and the other is one manufacturer’s instruction for one product, so other brands differ. Follow the bag you buy. The same sheet warns that hot, dry or windy weather can hinder hydration and lead to shrinkage cracking. It also says to protect fresh concrete from freezing for the first 48 hours.

For help planning the timing, try the concrete curing time calculator.

Which Cracks Deserve a Closer Look?

Closely spaced pattern cracks and D-cracks from freezing and thawing deserve attention, because NRMCA says they may lead to deterioration. Most random cracks that appear at an early age are different. CIP 4 calls them unsightly but rarely a threat to structural integrity or service life.

FAQs About Cracks in Concrete

Sources and Further Reading

This is general guidance and not a diagnosis of any crack in your concrete. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-19.