Garage slabs take a lot of abuse. They have to support heavy cars and trucks and resist ice, dirt, salty water, de-icers, gasoline, motor oil, antifreeze, and other contaminants. Plus, since few garages are heated, they have to tolerate freeze/thaw conditions. But a garage slab that has been carefully planned, installed, and cured should have no trouble providing years of crack-free performance in any climate, even under the toughest conditions. Commercial concrete contractors do this kind of work every day; adopt their attention to detail and you’ll meet their high standard of quality, without having to raise your prices.

Start with a good base

Don’t worry too much about your soil’s bearing capacity. Even poor soils like silt and soft clay have an allowable soil pressure of around 400 pounds per square foot (psf). A 6-inch-thick slab weighs only about 75 psf, and live loads — anything that is not part of the building itself, including vehicles—typically don’t exceed 50 psf in a garage. That means that the soil under a typical garage slab only has to be able to support 125 psf.

Far more important than bearing capacity is the base’s ability to provide consistent support. If one part settles more than another, the slab will bend and potentially crack. To avoid this problem, you need to know which areas have been cut and filled, and then you need to make sure that filled areas have been well-compacted. Any soil that’s been disturbed during excavation must also be compacted. Keep in mind that it’s hard to get good compaction with soil that is too dry or too wet. To test for moisture content, squeeze a handful of soil. If you can squeeze out water, it’s too wet, and if it falls apart when you open your hand, it’s too dry. If it holds its shape, it’s about right.

The safest approach is to remove the topsoil and place a minimum 4-inch layer of compactible gravel or crushed stone as a base over the undisturbed subsoil. Use unwashed material that has a top size of 11/4 inches and includes smaller sizes right down to the fines; the irregular shapes and sizes interlock nicely when compacted. Gravel or stone provides a layer for the installation of underslab conduit and pipes, allows for water and radon to escape, and helps keep the slab thickness uniform, which saves money on concrete. It also helps spread the load over the underlying soil, so that the slab is supported more evenly. Plus, it’s easy to compact and hand-grade.

Probably the most difficult soil you will have to deal with is expansive clay, which swells when it’s wet and shrinks when it dries, and can’t be compacted easily. It’s best to remove this clay and replace it with a compactible fill. If that’s impractical, you should consult with a foundation engineer. In some cases, the engineer may recommend structural slabs or post-tensioned slabs that don’t rely on the soil for structural support.

Install a vapor barrier

Most building codes say that an unheated detached garage doesn’t require a vapor barrier, but that doesn’t mean you shouldn’t install one anyway. Moisture in the ground can wick up via capillary action, and water vapor is always present beneath slabs; any air in the subbase is almost always at 100 percent relative humidity. Without a vapor barrier, moisture will move through the concrete and condense beneath anything stored on the slab surface, leaving telltale dark spots. In extreme cases, the slab will even “sweat.” If the slab is ever covered with flooring or a finish coating, the moisture could cause delamination. A vapor barrier is cheap insurance.

Although 6-mil poly will satisfy IRC requirements, 10-mil or thicker vapor barriers designed specifically for use under slabs are less likely to puncture or deteriorate. Some examples are VaporBlock (, Stego Wrap (, Griffolyn (, and Perminator (

The vapor barrier should be placed on top of the subbase, directly underneath the concrete. If you’re installing rigid foam under the slab, put the vapor barrier on top of the foam.

Don’t place a sand or gravel blotter layer on top of the vapor barrier. There was a time when that practice was recommended to reduce curling in the slab, but if you’re using the proper low-water-content concrete mix, a blotter layer is unnecessary. It can actually trap moisture, which will then keep rising up through the slab.

The vapor barrier’s seams should overlap at least 6 inches and be sealed with tape. To prevent concrete from pushing into them and tearing the material during placement, try to orient the seams so that they will be parallel to the direction of concrete placement.

Don’t add water to the mix

The IRC requires that slabs be built with concrete with compressive strengths from 2500 to 3500 psi, depending on the climate. ACI goes further and recommends 4500-psi concrete for garages in the northern half of the country. To achieve this strength, the water-cement ratio should be kept at 0.5 or less, typically about a 5-inch slump concrete. Because this mix is slightly dry and stiff, it’s tempting to add water to make the concrete easier to place. But be careful: There’s an inverse relationship between the eventual compressive strength of concrete and the amount of water used in the mix—the higher the water-cement ratio, the lower the strength. The best way to get concrete that flows well enough to consolidate in the forms and around the reinforcement is to use a high-range water reducer, or super-plasticizer.

Super-plasticizers work by pushing the cement grains apart, so that the mix flows more easily. They can be added to the concrete at the ready-mix plant, or you can buy them in bags and add them at the site. Fritz-Pak’s Supercizer 1 ( will increase the slump of 1 cubic yard of concrete by 6 inches and maintain that slump for 30 to 45 minutes.

Retarder and accelerator, for controlling set time in hot or cold weather, are also available by the bag.

Use air-entrained concrete

Most garage floors are not made with air-entrained concrete because finishers don’t like its sticky consistency. However, this type of concrete is essential in any climate that experiences freezing temperatures. Entrained air is accomplished by adding a soaplike admixture that froths to produce billions of microscopic air bubbles. These bubbles relieve internal pressure in the concrete by providing tiny chambers for water to expand into when it freezes. Without air-entrainment, concrete exposed to freeze-thaw cycles will scale, or flake off, at the surface, and may eventually disintegrate.

The amount of entrained air required depends on the maximum aggregate size; less air is needed with larger aggregate. Climate is also a consideration, though there are only a few regions across the southern U.S. that fall outside ACI’s moderate and severe weathering zones (see map, page 26). With 3/4-inch top-size aggregate, for example, ACI recommends 5 percent air entrainment in moderate regions and 7 percent in severe regions.

How do you know you’re getting concrete with the proper water-cement ratio and strength and air content? Unless you learn to do some tests or hire a testing company, you’re going to have to trust your ready-mix provider. You can check the batch ticket, but your best insurance is a good relationship with the concrete producer. Be clear about what you want and what the concrete will be used for, and the company will adjust the mix accordingly.

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