Quality Concrete Slabs 
A checklist to help avoid cracks and maximize life


By Joe Mehaffey

Many people will tell you that there are two kinds of concrete. Concrete that WILL crack and Concrete that HAS ALREADY cracked. Yet, reality is far more complex. Properly mixed and installed concrete will normally have FEW cracks and a long life while improperly installed and/or mixed concrete will have many cracks and a short life. This checklist will help you get the former.

There are many different mixes of concrete and many methods of installing concrete. The Portland Cement Association (PCA) is the concrete industry’s standards group. The PCA has published large numbers of books and guides describing in detail the proper methods of producing and installing concrete for virtually all applications. These publications are recommended reading.

Many work crews are not aware of the subtleties of the art and are happy with less than what is achievable by just following industry standard procedures. This checklist will help you get the quality concrete slab floor you are looking for.  A good motto to remember is:  A  well INSPECTED job is necessary to get the job you EXPECT.

Note: The general methods described here may have to be adjusted to suit local requirements and conditions. However, in the majority of cases, these construction methods, with proper attention to detail, will achieve quality slabs  in residential slab applications.

Slab Floors

Concrete slabs are used in basements, garages,  porches, floors, decks, sidewalks, and driveways in homes and buildings all over America. We are all used to seeing cracks, large and small in household concrete. Some concrete installers would have us believe that whatever cracking occurs in concrete is uncontrollable and nothing can be done about it and DEFINITELY it is not their fault! It is true that cracks may occur in large concrete slabs no matter what you do. It is also very true that if you do it wrong, you may have 20 to even 100+ cracks in 1000 square feet and if you do it exactly RIGHT you will have perhaps 1 crack or less per thousand square feet. AND.. If you do it right, the cracks will be small, limited in running distance and hopefully will not pull apart.

How to minimize cracking

Like many things, the easy, quick, and cheap way of making concrete slabs is NOT going to produce high quality. Many of the methods to be described add little or nothing to the cost of a slab. However, some of the improved performance options suggested will add moderate cost above a “normal” slab.

Much household concrete is poured without expansion joints.  For most such applications, this is a good compromise. Even properly installed concrete may crack every 15 to 30 feet with a shrinkage crack. In properly poured concrete, the shrinkage crack thus formed will be small and will generally be less noticeable than a sawed or otherwise installed expansion joint. The only drawback of a “natural” expansion joint is that it will typically meander across a floor in an unpredictable manner  The following items are ESSENTIAL to a good concrete slab job and a purchaser should insist that they are “standard”.

Prepare a firm soil base

A major quality item to watch for is a well packed base. If the soil under the slab is loose dirt or just not well packed, the concrete may eventually crack as the soil under it shifts. Be sure to machine tamp or roll the subsurface. Just running a rubber tired Bobcat or similar tractor over the surface IS NOT enough, yet it is done every day.

Prepare an aggregate base

After the base is prepared, a 4 inch (minimum) layer of aggregate (gravel) should be placed and tamped. The aggregate base has as its principal function to distribute the load uniformly over the soil surface. It also lifts the vapor barrier above the soil surface and thus helps keep moisture from getting into the basement through the floor. Under no circumstances should an interior slab floor be poured directly on earth without an aggregate layer.

Put a Vapor Barrier on top of the aggregate

Most people abhor a damp basement. Some people may save a few dollars and omit a vapor barrier under the slab. This is a mistake on two fronts. First, it will result in additional dampness being let into the basement through the slab. Second, the absence of a vapor barrier will let moisture escape from the concrete during curing and allow the concrete to set faster than it would with the bather. The standard vapor bather is 6 mu Polyethylene. See~figure la, and lb.) It is tough and resists tearing but it can be damaged by walking on it during construction operations. A tougher but more expensive material such as Grifflon has a nylon mesh sandwiched between two layers of polyethylene. This material cannot be torn by ordinary working and resists punctures better than 6 mil polyethylene.

The vapor bather should ALWAYS cover the exposed footing and run up the wall to prevent bonding of the floor to the wall. In addition, there should be a layer of sand or gravel on top of the footing and under the vapor barrier film. See figure Ia and lb. Any excess vapor barrier protruding above the edge of the slab after pouring can be left or trimmed away as desired.

Put aggregate or sand on top of the lip of the footing.

Standard practice is to put about 4 inches of aggregate (gravel) under the concrete and on top of the footing lip. However, some crews may dig down 4 inches below the footing lip, and put in four inches of aggregate to bring the aggregate level back up to even with the footing lip. Then they pour the slab right on top of the edge of the footing. This is NOT  a good idea!  One of the MAJOR ‘causes of cracking in slab floors is bonding of the slab around the edges to the footing. Normal 4 inch slump concrete shrinks about 1/16 inch per 10 feet of distance as it hardens. (Wetter concrete shrinks lots more, dryer shrinks less.)  If the slab is firmly bonded to the footing or to the wall at the edges, it MUST crack when it shrinks as the footing is unlikely to move! A proper slab will FLOAT and be independent and isolated from the footing and ALL walls of the basement. Don't let anyone tell you that a good slab is bonded to the footing and/or walls though this is a common misconception!

The easiest way to be sure that the slab is floating and not attached to the footing or walls (see figure la below) is to a) use an expansion joint connection at all walls, and b) use sand or gravel as an isolating bearing surface on top of the footing with a vapor barrier on top of the sand or gravel. The sand may be as little as 1/4 inch thick, and if gravel is used, 2 to 4 inches above the footing lip is recommended. (Use 4 inches of gravel total over the main slab area.)

Many experts suggest using a designed for purpose 1/4 or 1/2 inch expansion joint material between the slab and the wall all around the slab. Others suggest just running the 6 mil vapor barrier film up the wall to prevent the floor from bonding to the wall. If the slab is poured in hot weather, either method should work. If the slab is poured in cold weather, it conceivably could expand in hot weather and buckle. On balance, the 35 cents per linear foot cost for the half inch expansion joint material is a good buy. Many termite treatment firms require that the floor to wall joint be sealed with an asphalt or silicone sealant to prevent the possibility of insect intrusion. This is essential if the joint is hidden behind a wall.

The main point is to ISOLATE the floor from the walls and the footing. the slab should float and be able to expand and contract without pulling or pushing on any other structure.

Steel Reinforcement

A heavy wire mesh is normally used to strengthen and reinforce concrete in residential applications. Half inch Rebar on 18" to 24" centers is stronger, but more expensive.  Many experts suggest that the wire mesh or rebar reinforcement  has little effect on whether or not concrete will crack. However,there is general agreement that the reinforcement  will limit the size of cracks if they start. Just as important, the reinforcement  will limit the vertical movement of the two parts of the broken slab. Vertical displacement of a cracked slab is perhaps the most objectionable kind of crack. Make sure to have your concrete slab installer support the wire mesh or rebar off the vapor barrier with a physical means such as the special wire supports made for the purpose. The optimum position for the wire mesh or other reinforcement material is about 1/3 of the way up from the bottom of the concrete slab.  Many crews will attempt to pull the wire or rebar up off the vapor barrier into the concrete with rakes or similar tools, but this is a haphazard technique at best and cannot insure uniformity of location of the reinforcement  within the concrete slab. This “lift with a rake” procedure can also puncture the vapor barrier.

In heavy duty applications, such as garages and driveways, where heavy loads are anticipated, half inch or larger steel reinforcing bar is often  used to strengthen concrete. This is usually not needed in residential applications. However, installing a few reinforcing bars at all corners of a slab, but especially interior corners (see figure 2 below) where concrete is most likely to have cracks to start is an excellent technique.  These rebars are in addition to the normal heavy wire mesh.

The Concrete

The concrete mix can make or break a slab floor. If the concrete is too stiff, it is hard to work into a smooth flat surface. If the mix is too wet, it is easy to work, but it shrinks a great deal more when it dries and cracks much more extensively. A good compromise is 3500 to 4000psi, maximum 3 inch slump mix. Note: The Portland Cement Association recommends a slump of from 1 to 3 inches for slabs. They allow UP TO A MAXIMUM SLUMP OF 4 inches for handworking IF REQUIRED. A three inch slump mix is fairly easy to work and yet normally will have a reasonable number of cracks. Keep concrete moist while it is setting up if possible. It is a good idea to use a mist spray, moist burlap, or a plastic film over the slab after initial setup and finishing to assure minimum shrinkage and increased strength.  For exterior slabs, air entrained concrete is recommended as a) it is less inclined to crack in cold/hot temperature swings and b) the air entrainment makes the concrete easier for workmen to push around so they are less inclined to water it down.

Note: Concrete mixtures can vary widely and still produce quality concrete. However, as a general rule, the less water added to concrete, the stronger it will be and the fewer cracks you will have in the finished product. Each 1% increase in water content increases concrete shrinkage by 2%. Thus, it is extremely important to use an absolute minimum of water (lowest slump) in the concrete mixture. For slab floors, concrete slump more than 4 inches maximum should be avoided to minimize shrinkage and consequent cracking of the slab. Coarse aggregate content should be maximized in the mix as this will also minimize shrinkage and cracking. Generally, the water/cement ratio by weight for 3000psi concrete is 0.58 or lower for non air-entrained concrete. For 3500psi concrete, the ratio would be .51 or lower.  Concrete should NOT be poured during freezing weather unless special precautions are taken.

If possible, have an independent expert watch the pour to insure that good quality concrete is poured and that the crew adds NO water to the specified mix which would bring the slump above 4 ABSOLUTE MAXIMUM. For best results, insure that you get nominal 3 inch slump concrete OUT OF THE HOPPER of the concrete truck. If there is a question about the slump of the mix in the concrete truck, ask the concrete provider for a slump test. Many concrete providers are prepared to perform a slump test on the site, if requested in advance. Note: Some concrete installers may try to make work easy for themselves by adding too much water to the mix or by adding water to the surface of the slab during finishing. Don’t do it! If you see loose water on top of the mix as it is being poured or loose water on the vapor barrier film, the mix is too wet and will result in excessive cracking in the slab. If water does appear on top of the slab, do not continue to finish without first removing any pooled water.

Warning:    One of the main causes of surface defects in concrete slab surfaces is finishing the surface with bleed water on the surface of the concrete. No finishing operation should be performed on any concrete surface while water is present on the surface as this will cause serious crazing, dusting or scaling of the surface. (Quoted from Portland Cement Association Handbook)

Concrete mix EXTRAS

There are a number of additives to concrete that you should know about. Some are good and some are bad depending on conditions.

1)    Calcium is an additive used to accelerate the set up of concrete. It is useful in cold weather to hasten set up if a freeze is expected. Otherwise, you should avoid using it as the faster concrete sets up, the more prone it is to cracking and crazing and other surface defects as it ages.. Note:    The addition of calcium to concrete increases the apparent slump and care should be used to maintain the recommended 3 inch slump in the delivered product. Also, calcium additive will cause discoloration and non uniform appearance of the finished concrete surface.

Generally try to avoid Calcium or other “quick set” additives. Many slab crews are in a hurry and want the concrete to set up as fast as possible. This desire is the opposite of what you as a purchaser want. Concrete that sets up fast shrinks more and over a shorter period of time. This results in a much higher probability of crack formation.

2)    Fiber Fill is an additive (often glass or plastic fibers) designed to add tensile strength to the concrete. Concrete has very little strength in tension and the fiber improves the situation substantially. If concrete does crack, the fiber fill tends to keep the crack from enlarging. The extra cost for fiber fill concrete is perhaps $6 a cubic yard and it adds nothing to the installation effort. It will result in more durable concrete for most slab applications. Note: The finished fiber filled concrete will have “hair” protruding from the surface which will wear away with traffic or it can be easily cleared with a buffing or sanding machine once the concrete is dry..

3)    Air Entrained concrete is a special mixture for use outdoors in areas subject to freeze/thaw cycles. The air entrainment additives trap millions of microscopic air bubbles in the mixture. These air bubbles enable the concrete to survive freeze/thaw cycles much better than ordinary concrete. Portland Concrete Association (PCA) outdoor samples of air entrained concrete in Wisconsin showed little deterioration after 20 years of freeze/thaw whereas the surface of normal concrete was significantly deteriorated.

4)    Expansion Joints (if used) should generally be installed in a symmetrical pattern. As a rule of thumb, joints are installed at intervals of about 3 ft per inch of slab thickness. Thus 12 ft. for a 4 inch slab.  I  personally like a sawed control joint, but others prefer the rounded pressed-in-with-a-tool loints.  Either is quite OK.

Slab Flatness

It is unfortunate but true that many (most?) building codes do not have a flatness requirement for household concrete. Many slab purchasers (even builders) do not specify any measure of concrete flatness and take what they get as long as the customer does not complain. Here are a few rules of thumb.

1)    About the very best that can be achieved in flatness over large areas using ordinary techniques is +/- 1/8 inch about a nominal. This is an expensive slab and not achievable by most slab crews.
2)    Arbitration awards have been given homeowners when the slab elevation changed by more - than 1/4 inch in an 8 foot distance. Many homeowners accept variations considerably larger than this and are not even aware of it.
3)    A standard specification for slab flatness in quality residential applications is +/- 1/4 inch about a nominal. Over a large area, most slab crews can maintain flatness to better than +/- 1/4 inch with just a little extra care. The variation over any distance of 8 feet on the slab should be less than 3/16 inch. When complete, there should be no pools of water at any point on the slab that are more than 1/4 inch deep. Any standing water should drain toward an outside portal or a floor drain as appropriate.
4)    If you are looking for the cheapest slab, specify slab flatness at +/- 3/8 of an inch about a nominal. ANY slab crew should agree to a slab with maximum flatness variation of +/- 3/8 of an inch over the entire slab area. If your crew will not agree to this MAXIMUM, you should look for another crew.

Note:    Many slab crews use metal or wood spikes in the gravel to aid in maintaining a level surface. Once the markers are covered with concrete, the crew must try to visually maintain flatness. This method will result in considerably larger variations in elevation than if a movable string or other above slab reference means is used. It is important to discuss slab flatness with your potential slab installer and specify a WRITTEN flatness requirement in your specification. If you fail to ask this question and expect a flat slab, you may be in for a surprise.

Surface Sealer

Finished concrete has a surface which tends to turn to dust as it wears. A concrete sealer is a material which is applied to the surface of the concrete about 6 to 10 hours after pouring (and sometimes much later). This sealer, if installed a few hours after the concrete surface has set up can hold moisture inside the concrete and aid in curing the concrete. The sealer will also bind the surface of the concrete and produce a smoother surface which will not produce dust.For most home applications, a sealer is a good investment. There are after cure sealers that are applied as paint.’ The proper selection of a sealer is dependent on the application and you would be well advised to consult a building supply outlet or concrete materials supplier for options.

Drainage

The soil subsurface under a slab floor should be sited and constructed to freely drain and not collect water. A foundation drainage pipe system should be laid outside the foundation to prevent moisture rising above the lower aggregate level under vapor barrier. See figures la and lb. The TOP level of the drain tube should not be higher than the BOTTOM of the concrete slab surface to prevent moisture rising to the level of the concrete slab. Four inches of aggregate is usually provided under a slab to provide a capillary break between the subsurface (ground) and the concrete. (See figures la and 1b). Caution: If the slab lays on top of the footing without the four inches of aggregate on top of the footing, then the drain MUST be put beside the footing and NOT on top of the footing. The general rule is to MAKE SURE that the bottom of the concrete slab is HIGHER than the TOP of the foundation drain pipe. If the drain pipe is installed as shown in figure (1c), the installer is asking for a damp basement.

About four inches (minimum 2 inches) of aggregate should be under the drain pipe and perhaps 12 inches (minimum, 6 inches)of aggregate above the drain pipe. This will facilitate long term drainage and help prevent dirt clogging of the drain pipe over the years. The width of the gravel about the drain pipe should be a minimum of 8 inches
.

Concrete Slab to Footing  Details
Figures 1 and 2 above


Summary

Concrete slab and wall design and installation is a mature science. Many volumes have been written covering the manufacture of concrete and the installation thereof. This short document can but scratch the surface of the technology. For further information on specific applications or problems, consult your library or the Portland Cement Association, Skokie, Illinois USA.  Much of the information from which this document was derived can be found in the following publications. Additional details and references may be found in these publications as well.

References:
Design and Control of Concrete Mixtures, Kosmatka and Panarese, Portland Cement Association publication, Skokie, Ill.
Contracting Your Home, McGuerty & Lester, Betterway Books, Cincinnati, Ohio 45207 phone 800-289-0963