Not usually. Hairline shrinkage cracks are normal. Wide cracks, cracks with one side higher than the other, or cracks that keep growing should be investigated.
Cracking in Concrete.
All concrete moves as it dries and as temperatures change. Most cracks are cosmetic and predictable. Knowing the cause helps you prevent the ones that matter.
At a glance:
~0.5 mm/m : Typical drying shrinkage
¼ depth : Control joint depth
First hours : When plastic shrinkage cracks form
Why Concrete Cracks.
Concrete shrinks as it loses water. It also expands and contracts with temperature, and bends under load. When that movement is restrained by the ground, forms or reinforcing, the concrete cracks to relieve the stress.
Some cracking is normal and does not affect strength. Good design and placement control where cracks form and how wide they get.
Common Types of Cracking:
Plastic shrinkage cracks: Short, often parallel cracks that appear while the concrete is still soft. Caused by the surface drying faster than bleed water can replace it, usually in sun, wind or low humidity.
Drying shrinkage cracks: Develop over weeks and months as the slab dries. Control joints give them a planned place to form.
Crazing: A fine network of shallow surface cracks, like a map. Caused by a paste-rich surface drying too quickly, often after over-finishing or late curing. Cosmetic only.
Settlement and load cracks: Caused by a soft or poorly compacted base, or loads the slab wasn't designed for. These can be structural and should be assessed.
Basement wall cracks: Usually vertical shrinkage cracks, narrow at the base and top. Wide, horizontal or leaking cracks should be checked by a professional.
Curling.
Curling is when slab edges and corners lift as the top dries and shrinks faster than the bottom. It shows up as rocking panels or hollow-sounding corners. Lower water content, good curing and correct joint spacing all reduce it.
How to Reduce Cracking.
Start with a firm, evenly compacted base. Order the right mix and don't add water on site. Cut control joints on time and to the right depth. Protect the surface from drying in the first hours, then cure for at least 7 days.
Fibres can help control early plastic shrinkage cracking. They don't replace joints or structural reinforcing.
Frequently Asked Questions
Is a crack in my new driveway a defect?
Why did cracks appear the same day we poured?
Most likely plastic shrinkage from fast surface drying. Wind breaks, sunshades, an evaporation retarder and prompt curing help prevent it.
Why didn't the crack follow the control joint?
Joints may have been cut too late, too shallow or too far apart. Cut them as soon as the saw won't ravel the edges, to a quarter of the slab depth.
Does reinforcing stop cracking?
No. Reinforcing holds cracks tight and keeps them narrow. It doesn't prevent them.
Sealing keeps water and de-icing salts out of exterior cracks. Structural or leaking cracks need professional repair.
Disclaimer: These guides are general information. Always follow your engineer's specification and local building requirements.
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