Why does new concrete crack?

New concrete cracks because it shrinks. As concrete sets and dries it contracts, and when the slab is held back by the ground, a wall or its own weight, the shrinkage pulls it apart at the weakest point. Joints are simply pre-planned cracks, cut so the slab breaks along a straight line instead of across its face.

The reassuring part: many early cracks are hairline and cosmetic. The useful part is working out which kind you have.

The usual causes

The usual suspects are missing or badly placed joints, a poorly prepared base, too much water in the mix, the surface drying too fast, and too little curing. Here is how each one shows up.

Cracks that appear while the concrete is still wet

Short, shallow cracks that show up in the first hours, often roughly parallel to each other and stopping short of the edges, are usually plastic shrinkage: the surface is drying faster than the concrete beneath it can replace the moisture. A hot, breezy summer afternoon is enough. VDOT's specification says care shall be taken in hot, dry, or windy weather to protect the concrete from shrinkage cracking by applying the curing medium at the earliest possible time after finishing operations and after the sheen has disappeared from the surface.^[1] Such cracks are usually shallow, but they can be weak points where later cracks start.

Joints too far apart, too shallow or cut too late

When a crack runs straight across a panel in the first weeks, the joints are often the reason. Quikrete's instructions cut 1-inch control joints into the slab every 6 to 8 feet with a grooving tool.^[4] VDOT's sidewalk specification divides the slab into sections approximately 5 feet long with control joints that extend into the concrete at least 1/4 of its depth, and where slabs are more than 7 feet wide it forms joints lengthwise to obtain uniform blocks that are approximately square.^[1] Long, narrow or L-shaped panels crack at the inside corner or across the middle.

Timing matters too: joints have to go in while the concrete is still workable, or soon after with a saw, before the slab picks its own crack. A joint that did its job has a crack you cannot see, at the bottom of the groove. Quikrete explains that control joints induce cracking at the base of the joint rather than across the face of the slab.^[4]

Missing isolation joints

A slab poured tight against the house, a post, steps or another slab cannot move on its own. VDOT requires preformed joint filler between its concrete sidewalk and any adjacent fixed structure.^[1] Without it at your slab, look for cracks running out from corners and posts.

The base

The ground carries the slab. VDOT's specification has its contractor excavate, shape and compact the earthen foundation to a firm, even surface before concrete goes down.^[1] A crack where one side has dropped, often over a filled trench or a soft spot, is a base problem, not a surface one.

Mesh and fibers do not make up for a poor base. Quikrete says it may be desirable to add wire mesh when using its Concrete Mix, and that its Fiber-Reinforced Concrete Mix eliminates the need for wire mesh,^[4] but joints, the base and curing do more to decide where cracks form.

Too much water, too little curing

Wet concrete is easier to place, which is why water gets added on site, but extra water weakens the mix and makes cracking more likely. Quikrete's data sheet starts with approximately 6 pints of clean water for each 80 lb bag.^[3] Curing works the other way. Quikrete calls it one of the most important steps in concrete construction, says a poor curing job can ruin an otherwise well-done project, and says curing should continue for 5 days in warm weather at 70 F or higher, or 7 days in colder weather at 50 F to 70 F.^[3]

Weather

Cold is a cracking risk too. VDOT's specification has its contractor prevent the temperature at the surface of the concrete from falling below 40 degrees F during the first 72 hours after placement, and says concrete damaged by frost or by freezing shall be removed and replaced at the contractor's expense.^[1] A slab poured late in the year without protection can crack, scale or crumble.

Cracks that show up later

Most early cracks are cosmetic, but a few later ones matter. The Federal Highway Administration lists several concrete deterioration types that start as cracking.^[2] D-cracking, from freezing and thawing of susceptible coarse aggregates, shows as cracking parallel to joints and cracks and later spalling, typically 10 to 15 years in. Alkali-silica reactivity shows as map cracking over the entire slab, from a reaction between alkalis in the pore solution and reactive silica in the aggregate. Corrosion of embedded steel shows as spalling and cracking over or around the steel, and adequate concrete cover reduces it.^[2] Any of those calls for a professional look, as do rust stains along a crack.

What to do about it

A stable hairline crack in a new slab usually needs nothing more than watching. Mark the ends, date it, and photograph it next to a ruler, then look again after a few weeks and after the first hard freeze. If the slab is under a contractor's warranty, DPOR's advice is to address problems to the contractor in writing, and to file a written complaint with the Board for Contractors if the contractor refuses to make corrections.^[5] Before you hire anyone for a new slab, the questions to ask a concrete contractor cover joints, base and curing.

If you are pouring your own, how to pour a small concrete slab covers joint layout and curing, and how long to wait before driving on new concrete covers the first weeks.

Stop and call a pro if

• One side of the crack is higher than the other, or the slab has sunk or tilted.
• The crack keeps getting wider between checks.
• Cracks run toward the house, the foundation or the steps, or the slab is pulling away from them.
• Water is getting into a basement or crawlspace near the crack.
• You see rust stains or flaking that exposes stone in the surface.

Getting help in Virginia

Loclis lists DPOR-licensed concrete businesses with their licence number and class. Look up concrete contractors in Blacksburg, browse concrete repair, or see all concrete and hardscaping answers.

Describe the crack once and licensed local concrete businesses quote the repair.