Why Does Concrete Crack? Common Causes and How to Prevent It
Quick answer: Concrete cracks mainly because it shrinks as it dries and cures, and because it's strong in compression but weak in tension. Small hairline cracks are normal. Most larger cracks come from a poorly compacted base, missing or badly spaced control joints, too much water in the mix, drying too fast, freeze-thaw damage, or heavy loads too early. Good base prep, the right mix, properly placed joints, and proper curing prevent most problems.
There's an old saying in the trade: there are two kinds of concrete, concrete that has cracked and concrete that will. The goal isn't to stop every crack. It's to control where cracks happen and prevent the ones that matter.
Is It Normal for New Concrete to Crack?
Yes, small hairline cracks are common and usually cosmetic. Concrete shrinks slightly as water leaves the mix, and that shrinkage creates stress. Control joints are cut so the concrete cracks in straight, planned lines instead of randomly across the slab.
Cracks become a concern when they are wide, growing, uneven (one side higher than the other), or letting water through.
What Causes Concrete to Crack?
Shrinkage: Concrete loses volume as it dries. Without control joints, it relieves that stress by cracking.
Too much water in the mix: Extra water makes concrete easier to place but weaker and more prone to shrinkage.
Drying too fast: Hot sun, wind, or low humidity can dry the surface before it gains strength, causing surface cracking and crazing.
A poor base: Soft, uneven, or poorly compacted soil lets the slab settle unevenly.
Freeze-thaw cycles: Water that gets into concrete expands when it freezes. Concrete without enough entrained air is especially vulnerable.
Heavy loads too early: Driving or parking before the concrete has enough strength can crack the slab.
Tree roots and drainage problems: Roots and water pooling under the slab can lift or wash out the base.
How Do You Prevent Concrete From Cracking?
Start with the base. Compact the granular base properly and make sure water drains away from the slab.
Use the right mix. For exterior flatwork in Ontario, that typically means 32 MPa with 5% to 8% air entrainment. Learn more in Understanding Concrete Mix Strengths: MPa Explained.
Don't add extra water on site. It weakens the concrete. Volumetric mixing lets the water content be metered at the truck so the mix stays consistent.
Place control joints correctly. Joints should be cut to at least one quarter of the slab depth. A common rule is to space joints, in feet, about 2 to 2.5 times the slab thickness in inches. For a 4-inch slab, that's roughly every 8 to 10 feet. Keep panels close to square.
Cure it properly. Keep new concrete moist and above 10°C for at least the first 7 days. See How Long Does Concrete Take to Cure?
Protect it from early loads. Wait about 7 days before driving on it and 28 days for heavy vehicles.
Seal it. Sealing every 1 to 2 years helps keep water and salt out of exterior concrete.
What Are Control Joints?
Control joints (also called contraction joints) are grooves tooled or saw-cut into the slab. They create a weak point so that when the concrete shrinks, it cracks neatly inside the joint instead of across the surface. Saw-cut joints need to be made as soon as the concrete can be cut without ravelling, usually within hours of finishing.
Does Rebar or Wire Mesh Stop Cracks?
Reinforcement doesn't stop concrete from cracking, but it helps hold cracks tightly together and limits how much they open or shift. Whether you need rebar or mesh depends on the project, the load, and your engineer or municipality's requirements.
Which Concrete Cracks Should I Worry About?
Hairline cracks: usually cosmetic and normal
Cracks along control joints: working as intended
Wide or widening cracks: may point to base or drainage problems
Cracks with one side higher: usually settlement or heaving and worth having inspected
Crazing (fine spiderweb cracks on the surface): usually from fast drying or over-finishing; mostly cosmetic
Can You Fix Cracked Concrete?
Small cracks can be cleaned and filled with a concrete crack filler or sealant to keep water out. Larger or structural cracks should be assessed by a contractor, since the cause, often the base, needs to be fixed before a repair will last.
Frequently Asked Questions
Why does new concrete crack?
New concrete shrinks as it dries and cures. Without control joints, or when it dries too fast, that shrinkage causes cracking.
Is it normal for a concrete driveway to crack?
Small hairline cracks are normal. Wide cracks, cracks with uneven edges, or cracks that keep growing may signal a base or drainage problem.
How do you stop concrete from cracking?
Use a well-compacted base, the right mix without extra water, properly spaced control joints, and proper curing for at least 7 days.
How far apart should control joints be?
A common rule is about 2 to 2.5 times the slab thickness in inches, measured in feet. For a 4-inch slab, that's roughly every 8 to 10 feet.
How deep should control joints be?
Control joints should be at least one quarter of the slab thickness. For a 4-inch slab, that's about 1 inch deep.
Does adding water to concrete cause cracks?
Yes. Extra water weakens concrete and increases shrinkage, which makes cracking more likely.
Does rebar prevent concrete from cracking?
Rebar doesn't prevent cracks, but it holds them tightly together and limits movement.
Can cold weather crack concrete?
Yes. Fresh concrete that freezes can be damaged, and concrete without enough entrained air is more vulnerable to freeze-thaw cracking.
How do I fix small cracks in concrete?
Clean the crack and fill it with a concrete crack filler or flexible sealant to keep water out. Larger cracks should be inspected by a contractor.
Where can I get concrete delivered in the GTA?
Bathurst Mobile Ready Mix supplies and delivers fresh, on-site mixed concrete across Toronto and the GTA. Call 416-656-1937 for a quote.
Get It Right From the Start
The best crack prevention starts with fresh, properly proportioned concrete. Bathurst Mobile Ready Mix mixes concrete on site, measured to the job, across Toronto, Scarborough, Markham, Aurora, Oshawa, and the GTA.
Call 416-656-1937 or email admin@bathurstreadymix.ca for a quote.
Related reading: How Long Does Concrete Take to Cure? · Concrete vs Asphalt Driveway · Understanding Concrete Mix Strengths: MPa Explained · Best Practices for Pouring Ready-Mix Concrete in Winter