Isolate the slab

All slabs settle to some extent, while foundation walls and columns are usually more stable. Slabs also shrink as they dry, and they continue to expand and contract with changes in temperature. Using isolation joints around the perimeter of the slab and around any penetrations allows it to move independently and helps prevent cracks.

Isolation joints should be made from 1/2-inch-thick materials like asphalt-impregnated fiberboard, neoprene, and even cork. Some builders use sill seal to make isolation joints, but this material is only 1/4 inch thick. Though it may serve as a bond break, it might not create a big enough gap to allow for differential movement between the slab and the foundation walls. Regardless of the material you use, the joint must be set at the proper grade and be at least as wide as the thickness of the slab, so the slab won’t partially bond to the walls.

Is wire mesh really necessary?

Most garage slabs have embedded steel reinforcement. This steel rarely serves a structural purpose; instead, it’s intended to keep small cracks in the slab from opening wider. However, with a good uniform subbase, a low-shrinkage concrete mix, and proper joint spacing, a slab is unlikely to have a lot of problems with random cracking, even without rebar or welded wire fabric or mesh reinforcement. For that reason, a growing number of contractors are pouring their garage slabs without steel.

If you do use steel use flat sheets rather than rolls of mesh, which are difficult to handle and nearly impossible to flatten. For most garage slabs, a grid of #3 rebar at 18 inches on-center in each direction provides better reinforcement than mesh and is easier to install. Use rebar with wire mesh to beef up parts of the slab that are particularly vulnerable to cracking. For instance, we recommend placing a couple of 4-foot-long #3 bars diagonally across re-entrant corners.

Reinforcing steel won’t do much good if it isn’t centered vertically in the slab. This can be done by suspending mesh or rebar on pieces of concrete brick or on small supports (chairs) during the pour. Hooking and pulling unchaired mesh isn’t effective because the mesh gets walked on again and goes back to the bottom of the concrete. Some contractors even install a double layer of mesh; the lower layer props up the second layer and keeps it higher in the slab.

Use fiber reinforcement

At about $8 per cubic yard, fiber reinforcement is a relatively inexpensive way to improve the surface of the slab and add extra impact and abrasion resistance. The fibers won’t do much to prevent shrinkage cracking (the typical random cracks that develop in the slab as it dries), but they can be very effective at reducing plastic-shrinkage cracks, or crazing (the network of finely spaced, very thin cracks in the concrete surface). Made of polypropylene or nylon, these fibers are mixed in with the concrete before placement and improve mix cohesion.

Be aware that fiber reinforcement can affect the way the concrete is placed and finished; slump is reduced a bit and it is sometimes necessary to use a vibrating screed to bury the fibers. If strikeoff is done by hand, the finished surface can sometimes look a little hairy, but that quickly wears off with traffic.

Don’t start finishing too soon

Even though air-entrained concrete won’t bleed much, you shouldn’t trowel the surface until whatever bleedwater there is has evaporated and the concrete has gained enough strength to support finishing operations. If bleedwater is still present, troweling will work it into the surface of the concrete, creating a soft top layer prone to dusting, scaling, and crazing. Use the thumbprint method; a 1/4-inch thumbprint impression means the concrete is set up enough to begin final finishing. (See “Finishing Timeline” on the left.)

Saw-cut contraction joints

As concrete dries, it shrinks and cracks. Contraction joints, or control joints, create planes of weakness in the slab that help it crack where you want it to crack and in straight lines.

In order to get shrinkage to activate the contraction joint properly, the depth of the joint should be equal to one-quarter the thickness of the slab. Contraction joint spacing should be two to three times in feet what the slab thickness is in inches. That means that a 5-inch-thick slab should have 11/4-inch-deep control joints spaced 10 to 15 feet apart (slightly closer for wetter concrete). Panels formed by these control joints should be as close to square as conditions allow.

Some contractors use plastic zip strips to make contraction joints, but it’s difficult to pull off the “zip” section of the strip without creating a rough-looking joint. Hand-tooled joints often are not deep enough to be effective, so we recommend saw-cut control joints. They’re deeper and more consistent, which reduces the likelihood of random cracking. Early-entry dry-cut saws are better than conventional water-cooled saws because they can be used within one to two hours after finishing, before shrinkage cracks have a chance to form.

Cure the surface properly

A slab will shrink and crack like a parched mud puddle if the surface is allowed to dry too quickly. Cement can’t hydrate without water, so a dry surface will create a weak layer in the very place where you want the concrete to be strongest. Poorly cured surfaces end up with crazing cracks and dusting, and the entire slab may curl upward at the edges if the top dries more rapidly than the bottom.

One way to make sure this doesn’t happen is to spray a membrane-forming curing compound (or a cure-and-seal)onto the surface. This will create a thin impermeable membrane that traps the slab’s own moisture and prevents premature drying. Some compounds can be colored with white pigment so it’s easier to see where they’ve been sprayed.

Curing compounds can interfere with adhesion, though, so if the slab will be getting some sort of coating, a better option might be to use poly sheeting or curing blankets. Poly is inexpensive but can leave mottled marks on the floor.

Curing blankets like HydraCure S16 ( produce reliable curing and a more even appearance. The S16 is a single-use product, but multiuse blankets that can be rinsed and stored are also available from HydraCure and other manufacturers. Curing blankets for cold-weather are usually insulated and sometimes even heated; they’re often available for rent. Fasten the poly or blankets at the edges so they don’t blow away.

- William D. Palmer Jr. is editor in chief of Concrete Construction. Paul Newman is a custom home builder in Colorado. This article first appeared in the Journal of Light Construction.

Crack-Free Slab Details

1. Undisturbed subsoil. Remove problem soils like expansive clay or organics and replace with compacted fill.

2. Sand and gravel backfill compacted in 4- to 6-inch lifts.

3. Minimum 4-inch-thick compacted subbase of unwashed crushed stone or gravel.

4. For radiant slab, install 1- to 2-inch rigid EPS insulation followed by vapor barrier. Reinforce slab with 6x6 W2.9xW2.9 welded wire reinforcement centered vertically in the slab and discontinued at the joints. Support the wire during concrete placement; do not hook into position.

5. 10-mil (min.) plastic vapor barrier, seams overlapped 6 inches and sealed with tape.

6. Minimum 4-inch slab (5 inches for heavy trucks), 3500-psi (min.) concrete, 4500-psi concrete for severe weathering areas.

7. Use low-water-content air-entrained concrete (5% to 7% air entrainment) in freezing climates and 11/4-inch (top size) aggregate.

8. 1/2-inch isolation joints around perimeter and at penetrations through slab (such as columns).

9. Fiber reinforcement added to mix at plant, typically 1.5 pounds per cubic yard, to reduce plastic shrinkage cracking.

10. Saw-cut contraction joints, depth equal to one-fourth the slab thickness and spaced two to three times in feet the thickness of the slab in inches (so a 4-inch slab should be cut 1-inch-deep every 8 to 12 feet. Keep panels as square as possible.

11. Reinforce inside (reentrant) corners with 4-foot lengths of #3 rebar laid diagonally to minimize cracking.

12. Spray-on curing compound or curing blankets (leave curing blankets in place for seven days after concrete placement).

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