We’ve all heard it: Concrete always cracks! Although that’s not 100% true, concrete does crack frequently enough that anyone working with it should understand why and how to repair cracks when necessary. Don’t let cracks ruin your good work!

Cracks form in a slab when there is a tension force in the concrete that’s greater than the concrete’s strength in tension. Concrete’s direct tensile strength is much less than its compressive strength—about 10%. So when it’s pulled too hard, it cracks. Cracks also can propagate, or continue to grow, at even lower stresses. The tension can have of a lot of causes that are external or inside the concrete itself, such as internal pressure caused by freezing water or expanding aggregates.

In many cases, cracks don’t detract from performance. Plastic shrinkage cracks in a slab’s surface often have no negative effect on performance and may be objectionable only because the owner doesn’t like the way they look. Shrinkage cracks that aren’t offset vertically or are unstable under heavy loads are really nothing more than natural contraction joints. And reinforced concrete actually needs to crack before the reinforcement begins to work, although with heavy reinforcement the cracks can be too narrow to easily see.

This article looks only at cracking in slabs, not concrete disintegration such as from freeze-thaw action or alkali-silica reaction (ASR). Those situations certainly do crack concrete, but that’s another article for another day.

Cracking of Plastic Concrete

After concrete is mixed and before it gets completely hard, it’s referred to as “plastic.” The most common cracks in plastic concrete are plastic shrinkage cracks and crazing. These are caused by the surface drying out faster than the underlying concrete—when the evaporation rate on the surface, due to dry or windy conditions, is greater than the bleeding. Very shallow plastic shrinkage cracks form as parallel lines. Crazing cracks that create a fine map or alligator pattern are similar but start after the finishing operation. Neither typically has a negative impact on slab performance but may be objectionable due to aesthetics.

Plastic shrinkage cracks often form between strike-off and finishing, and can usually be avoided by using plastic sheets or burlap, windbreaks, shading, fog sprays, and monomolecular evaporation retarders to keep the surface from drying out. Similar precautions can prevent crazing or just get the curing started as soon as possible. Don’t overwork the surface and never “bless” the concrete with water during troweling.

Settlement cracks are also common. They form directly above reinforcing bars in the slab and are due to the concrete continuing to settle below the bar. Insufficient vibration or leaking forms can be causes of these cracks, which get worse for larger bars.

Both plastic shrinkage cracks and settlement cracks can be reduced or eliminated by using synthetic microfibers in the mix.

So how can you repair these cracks? You can’t, not really. The surface could be ground deep enough to remove them or an overlay can be placed to cover them up. The best solution may be to convince the owner that crazing is a special decorative treatment you provide for free!

Drying Shrinkage Cracking

The most common cracks in slabs are random drying shrinkage cracks. After placement, as the concrete dries out, it shrinks about 0.06% but the concrete’s capacity to resist this is only about 0.015%. The cracks form because of restraint – anything that prevents the slab from moving freely, such as the subgrade, creates restraint. If we could magically suspend the slab in midair or place it on a perfectly flat slip sheet that had zero friction, much cracking could be avoided. (Someone needs to invent a way to levitate the slab until it’s done shrinking.)

The degree of cracking is controlled by the amount of shrinkage, sometimes called shrinkage potential. Shrinkage potential can be minimized by reducing the amount of water in the mix and by using larger, and low-shrinkage, aggregate. Don’t add any more water than absolutely necessary to place the slab because a little bit of extra water can result in lots of extra cracks. Think of it as liquid cracks! Shrinkage-reducing admixtures can also help. Good curing practices produce a stronger, crack-free surface but, unfortunately, don’t reduce the ultimate shrinkage and therefore don’t reduce drying shrinkage cracks.

In typical concrete placed in a strip, like a sidewalk, shrinkage cracks will form in the transverse direction about every 15 feet. This is why we put in contraction joints (sometimes called control joints), which are nothing but controlled cracks. The joint creates a thinner, and therefore weaker, cross section so that’s where the crack goes. Properly tooled or sawed contraction joints, installed at the right time and at the right depth, will force the cracks to stay in the joints. Today, floors are being placed without joints or with very widely spaced joints (see “Floors Without Joints” on page 25 of our September/October 2018 issue).

For a typical interior floor, contraction joints should be installed at the column lines, then intermediate joints can be equally spaced between. The old standard for the spacing of contraction joints in feet is the slab thickness in inches times 2 or 3—so for a 5-inch slab, the joints would be between 10 feet and 15 feet apart. Always try to keep the panels as square as possible (aspect ratio), but never have one dimension more than 1.5 times the other.

Joints can be tooled in or sawed. Depth should be at least a quarter of the slab thickness or 1 inch minimum. Install them as soon as possible without messing up the surface—about 4 hours after finishing in hot weather and up to 12 hours in cold weather. For early-entry sawing (for example, Husqvarna Construction Products’ Soff-Cut), joints should be 1 inch deep (never more than 1.25 inches) and installed as soon as the surface doesn’t ravel, which is usually 1 hour to 4 hours after finishing. Timing is critical—wait too long and the natural shrinkage cracks will beat you to the punch.

Cracking Due to Other Causes

Two other common causes of slab cracks are settlement of the soil support system and overloading. A proper subbase is essential for crack-free success (see “Subgrades and Subbases for Slabs”). Overloading often occurs during construction when the slab may not have achieved its full strength. A parking lot intended for passenger cars that ends up being driven on by heavy trucks, for example, will often suffer overloading cracks.

Repairing Cracks

In many, perhaps most, cases, random drying shrinkage cracks function just as well as contraction joints; they just look bad and the edges can become broken down so the crack looks worse than it actually is. But owners usually want some sort of repair, so start by evaluating the cause and repair options.

First measure the crack widths and determine if there’s reinforcing steel running through the crack. Use a crack comparator to measure the width. Cracks less than about 0.035-inch will transfer loads from aggregate interlock and prevent vertical offset. Wider cracks could offset vertically, requiring grinding on one side. If reinforcing steel was used in the slab, the crack will stay tight and not move but it will still be a crack.

There’s really no perfect fix. Assuming the subbase support system was properly consolidated (not always a good assumption!), the edges of the crack will stay level and the best fix may just be cosmetic. If the slab is subjected to heavier loads, the crack may move and the edges degrade, necessitating a structural repair.

If a structural repair is needed, the typical approach is to rout the crack, clean it as well as possible, and inject epoxy to bond the crack back together (see “Epoxy Injection of Cracked Slabs”). If the subbase isn’t supportive enough, that could require re-establishing the support. A good option for this is using polyurethane (see “Lifting Slabs with Polyurethane”). If more positive load transfer at a crack is needed, stitches can be placed perpendicular to the crack (see “Carbon Fiber Crack Repair” sidebar).

For nonstructural repairs, the typical approach is to rout the crack with a crack chaser and fill it with a semirigid joint filler. But you should evaluate first to determine if the crack is moving. If it is, a flexible material is best—it won’t support the edges of the cracks like a rigid filler, but it will move a little so the elastomeric material won’t crack. You’ll need to widen and deepen the crack a bit to create a reservoir for the sealant so there’s enough material to tolerate movement (see “Crack Evaluation and Repair” for more tips on selecting repair materials).

If your objective is to make the crack look good after the repair, consider not enlarging the crack and using a very-low-viscosity filler. Some of these materials can be colored to match your slab, especially appealing when working with colored concrete.

Cracks are basically an inherent characteristic of concrete, but they can be controlled and even eliminated by understanding the causes.

For more information, get American Concrete Institute (ACI) reports “Causes, Evaluation, and Repair of Cracks in Concrete Structures, ACI 224.1R-07;” “Control of Cracking in Concrete Structures, ACI 224R-01;” and “Guide to Concrete Floor and Slab Construction, ACI 302.1R-15.” Another great reference is “Concrete Repair and Maintenance Illustrated” by Peter H. Emmons.