Is It Normal for New Concrete to Crack? When to Worry (and When Not To)

Yes, it is normal for new concrete to crack, even within a week. A crack of about 1/8 inch or less, with both sides level and no growth, is typically ordinary shrinkage and not a structural problem. The ones to worry about are wider than that, still growing, or sitting at two different heights.

If you have just paid for a slab and found a line across it, the first reaction is usually panic. This guide explains why concrete cracks, how to tell the cosmetic kind from the serious kind, and what to do about it.

Why concrete cracks at all

Concrete is mixed with more water than the cement needs to harden. That extra water makes it workable, and then it leaves. As it leaves, the slab shrinks. The slab is stuck to the ground below it and to anything it touches, so it cannot shrink freely. The tension that builds up is more than concrete can handle, and it cracks.

Euclid Chemical, a concrete admixture maker, calls restrained drying shrinkage the most common cause of cracking in hardened concrete. It puts the amount at about 0.05 to 0.06 percent at 28 days, or roughly 5/8 inch per 100 feet.

Here is what that means for a slab you can picture.

• Shrinkage per 100 feet: about 5/8 inch
• A 20-foot slab is one fifth of 100 feet
• 5/8 inch × 20 ÷ 100 = 0.125 inch, which is 1/8 inch

So a 20-foot slab wants to get about 1/8 inch shorter as it dries. If it cannot slide on the ground, the shortening shows up as a crack. That is the reason 1/8 inch keeps coming up as the threshold in practice.

The two kinds of early cracks

The Federal Highway Administration’s pavement guidance separates two timings.

Plastic shrinkage cracks happen in the first few hours, before the concrete sets, when water evaporates from the surface faster than it rises from below. Hot, dry or windy weather makes it worse. FHWA says they are usually more cosmetic when the concrete was adequately moisture cured, except where the concrete protects reinforcing steel. They are shallow, and you may not see them until the next day.

Drying shrinkage cracks form after the concrete hardens, over days and weeks. The FHWA chapter says they can be reduced by good workmanship, proper mix proportions and “sufficient jointing performed soon after hardening”. They show up as the slab dries, which is why a crack appearing a week after the pour is common.

What a normal crack looks like

Feature	Usually normal	Worth a closer look
Width	1/8 inch or less	Wider than 1/8 inch
Height difference	Both sides level	One side higher than the other
Behavior	Stable	Growing or “active”
Pattern	Straight, or lines near a joint or corner	Closely spaced cracking, Y-shaped patterns, rust stains
Surroundings	Nothing else changing	Sticking doors or windows, cracked drywall or tile, humped floors

BuildingAdvisor, a building science site, gives the thresholds: a crack of 1/8 inch or less is typically a normal shrinkage crack, and you should involve a structural engineer if a crack is larger, growing, or has one side higher than the other. It lists other signs of settlement too: sticking doors or windows, cracks in drywall or tile, sloped or humped floors, and displaced cabinets or trim.

Control joints: telling the crack where to go

A slab will crack somewhere. Control joints choose where. They are saw cuts or tooled grooves that make a thinner, weaker section, so the crack forms in a straight line below the cut. Euclid is blunt about it: “the control joint is telling the concrete where to crack.”

Concrete Decor magazine gives the Portland Cement Association guideline for spacing:

• Space joints at about 2 times the slab thickness in feet, when the coarse aggregate is under 3/4 inch.
• A 5-inch slab gets joints every 10 feet.
• With aggregate over 3/4 inch, spacing can stretch to about 2.5 times, which is about 13 feet for 5 inches.
• Some contractors use the ACI “rule of three” instead, which gives 12 feet for a 4-inch slab, and tighten up to 8 feet in clay soil.

Timing matters. Concrete Decor lists standard saw cuts at 24 to 72 hours after the pour and early-entry saws as soon as the slab can be walked on. A contractor who cuts late may find cracks have already formed, as one finisher in the article warns. Depth varies by who you ask, from 25 percent to two thirds of the slab thickness, so ask your contractor which they use.

A worked example

A 12 by 20 foot patio slab, 5 inches thick.

• Joint spacing: 2 × 5 = 10 feet
• Along the 20-foot side: one joint at 10 feet, giving 2 sections
• Across the 12-foot side: one joint at 6 feet, giving 2 sections
• Panels: 2 × 2 = 4 panels of about 10 by 6 feet
• Volume: 12 × 20 × (5 ÷ 12) = 100 cubic feet, and 100 ÷ 27 = 3.7 cubic yards

You can check your own slab volume with the Concrete Slab Calculator and compare thickness options with the Concrete Thickness Calculator. If a slab has no joints at all, it still cracks. It just picks its own line.

Red flags that need an engineer

Euclid lists several kinds of cracking that point to something other than normal shrinkage:

• Structural cracks, caused by loads that exceed the design strength
• Corrosion of reinforcing steel, which expands as it rusts and cracks the concrete near the rebar, often with rust staining
• Sulfate attack, with closely spaced cracks and often a whitish bloom
• Alkali-silica reaction, which shows a distinctive Y-shaped pattern
• D-cracking, where water in absorptive aggregate freezes and cracks the slab from the bottom up

Euclid says suspected structural cracks should be checked by a licensed engineer, and that the underlying cause should be fixed before any repair or the repair may fail.

Add the BuildingAdvisor triggers: cracks wider than about 1/8 inch, active cracks, a vertical offset across the crack, or other signs of settlement. Structural causes it names are overloading, uneven soil settlement and water leaking under the slab.

One more practical point. FHWA warns that cracks reaching toward reinforcement reduce its protection from chlorides and carbon dioxide. A hairline crack in a patio matters less than the same crack in a slab with rebar close to the surface in a place with road salt or freeze and thaw.

What to do if your slab has cracked

1. Photograph the crack with a coin or ruler beside it, and note the date.
2. Measure the width and check whether one side sits higher than the other with a straightedge.
3. Recheck in a few weeks and after any heavy rain or frost. A crack that is growing is the one to escalate.
4. If the slab is new, tell the installer in writing. Ask what the warranty covers and whether hairline cracking is included.
5. For a stable crack under 1/8 inch, seal it. BuildingAdvisor suggests self-leveling crack filler or a flexible polyurethane sealant, and says it is worth testing how the repair looks first.
6. If the crack is wider, offset or growing, hire a licensed structural engineer. Repairs for settlement can include soil stabilization, slabjacking or piers, and a bad slab may need to be replaced.

If tile is going on the slab, use a crack-isolation membrane under it, as BuildingAdvisor recommends, so slab cracks do not reflect through.

How to prevent the next one

Most of these steps are for the pour, so they matter for your next slab.

• Use less water. BuildingAdvisor says a wet, high-slump mix is the main cause of drying shrinkage.
• Cure the slab. Keeping it damp and covered during the first weeks helps it gain strength. FHWA says plastic shrinkage cracks are mostly cosmetic when the slab was adequately moisture cured.
• Compact the base. Good soil and gravel compaction help avoid structural cracks.
• Cut joints on time and at the right spacing.

Sources

• Concrete pavement guidance, Chapter 4, Federal Highway Administration
• Concrete Cracks, Euclid Chemical
• Are Cracks in Concrete Slab Normal?, BuildingAdvisor
• Control Your Cracks, Concrete Decor