It is nearly impossible to prevent cracks from developing in concrete slabs on grade, especially exterior slabs. The reasons are explained below.
First of all, whenever concrete changes from a semi-liquid or plastic (i.e. from fresh, ready-mix concrete) to a hardened state (i.e. cured-out), it undergoes a volume shrinkage. This volume shrinkage is primarily related to loss of water. Concrete generally consists of sand, portland cement, gravel and water. The water combines (chemically-reacts) with the portland cement to create the rock-like, hardened mass we call concrete. During this process, much of the water is used up. In addition, a significant amount of water is lost to either evaporation or seepage into the subgrade. To help prevent rapid loss of water in freshly placed concrete, especially during warm/hot/windy days, the finished surface of the concrete should be kept moist and covered with plastic or wet burlap for several days. (This helps to maintain moisture to slow down the moisture loss process.)
In spite of extended curing [in a moist state], the eventual loss of water in fresh concrete will cause a significant volume change (reduction) from the ready-mix to the final, hardened state. In relatively thin concrete slabs on grade, this volume change results in a significant dimension change in the surface (plane) of the slab. As the slab shrinks, frictional forces can be generated along its bottom face (due to contact with the subgrade); this can set-up tensile stresses in the concrete. Also, if the concrete loses moisture through the surface only, due to the presence of an underlying plastic vapor barrier, the top face of the slab will shrink more than the bottom. This causes the slab to curl up around the perimeter edges. Both of these conditions can lead to cracking, because both (dimension shrinkage and curling) set-up tensile stresses in the slab.
Tensile stresses are detrimental because, while strong in compression, concrete is very weak in tension; i.e., it is relatively easy to “pull-apart” plain (not reinforced) concrete. Therefore, steel (wire mesh) is embedded in concrete to provide it with the tensile strength needed to control crack formation. Effective steel reinforcement, however, is more easily attempted than accomplished. Unless the wire mesh is held up by closely-spaced chairs, it is not likely to remain properly positioned near the top-center of the hardened slab (the position at which it is most effective). The success of wire mesh in crack control depends upon whether the contractor/finisher systematically reaches down through the concrete mix with a hooked bar and pulls the steel mesh off of the ground and up into the plastic mix. Unfortunately, as is well documented, test cuts consistently show that these precautions are not taken; more often than not, the wire mesh is found lying on the ground, barely embedded in the concrete. Under these circumstances, it serves no purpose. It is also necessary to realize that larger slabs require larger steel for crack control. Engineers may be mindful of this, but few concrete finishers are.
Once an exterior concrete slab on grade is placed, finished, and cured, it is subjected to perpetual, direct weather exposure, including periodic (and sometimes drastic) changes in temperature and humidity. Because concrete tends to absorb and/or give off moisture according to the surrounding weather conditions, weather changes affect the concrete’s moisture content; and, along with any moisture content change comes a volume (dimension) change. Concrete also expands/contracts with changes in temperature. Hence, once hairline cracks form in concrete, the effects of Mother Nature usually start to take their toll: initially small, invisible shrinkage cracks later enlarge. Crack enlargement facilitates direct moisture penetration into the crack, which can lead to isolated subgrade saturation or freezing expansion of trapped water during extremely cold weather. (Everyone knows that water expands upon freezing; hence, trapped water inside concrete cracks can pry cracks apart during freezing weather.)
In addition, moisture that seeps into the subgrade beneath a slab on grade can affect the behavior of some soils–especially highly plastic or silty-clays. Highly plastic clay soils exhibit an electro-chemical affinity for water, actually incorporating water molecules into their chemical structure (crystalline lattice). Thus, these soils are typically called expansive clays. During the winter, which is the normal rainy season in North Alabama, these soils take on moisture and expand in volume. Soil expansion can lift up or heave a slab-on-grade. During the summer, when droughts and hot weather frequently occur, the soils desiccate and shrink in volume, causing slabs to sink or settle. Hence, changes in the subgrade moisture content can cause slabs on grade to “move” throughout the year. Fine-grained soils, like silts and clays, are also very weak when wet or saturated and easily yield (deform) under a load. Hence, placing concrete slabs directly on top of highly plastic clay soils is a bad idea–especially for driveways which support heavy vehicle wheel loads. There are many types of highly plastic silty and expansive clays in North Alabama. They typically occur above/around limestone bedrock deposits. Contact the county extension agent at the nearest USDA Soil Conservation Service to learn more about the locations of expansive clay soils in your area.
In addition to the adverse effects of shrink-swell soils beneath driveway slabs, consider the problems that water beneath slabs can cause. Water that penetrates slab cracks can puddle beneath slabs; then, whenever a heavy vehicle crosses the slab, it will deflect under the wheel load and force water from the underlying puddle through the cracks (this is called “subgrade pumping”). The water usually carries fine-grained soil particles from the ground with it. After a sufficient amount of erosion has occurred, a void typically forms beneath the slab; then, when a heavy wheel load crosses the slab, it could cause an isolated portion of the slab to collapse into the eroded hole, resulting in pavement failure.
Generally, closely-spaced control joints are installed in slabs to prevent/conceal unsightly cracks. Either pre-formed or saw-cut, control joints create pre-planned planes of weakness in concrete slabs, thereby predetermining straight (as opposed to random, jagged) lines in the slab surface.
Whenever an engineer or architect is left out of a concrete slab construction project, the concrete contractor is responsible for providing proper control joint spacing. A rule of thumb states that control joints should be spaced at intervals (measured in feet) equal to three times the slab thickness (measured in inches). In other words, in a 4 inch thick slab, control joints should be spaced at (4” x 3 = 12′) 12 foot intervals. Moreover, control joints should be placed in areas of abrupt slab dimension change. In order to be effective, saw-cut control joints must penetrate ¼ of the slab thickness. Hence, a 4 inch thick slab must have 1 inch deep, saw-cut control joints. In narrow pavements, like sidewalks, control joint spacings should range from what to no more than twice the width of the sidewalk. If wider control joint spacings are desired, steel reinforcement can be utilized to prevent random shrinkage cracking.
Another cause of concrete cracking is structural overload. Concrete slabs should be placed on well-compacted beds of gravel. This helps to provide uniform bearing pressure beneath the slab whenever it is exposed to surface loads. Heavy wheel loads, for example, are transferred through the slab and onto the subgrade. If the subgrade consists of weak or saturated soil, it will likely deform under the pressure. This can set up bending stresses in the concrete slab which, in turn, can generate adverse shear and tensile stresses. Under some heavy loads and poor subgrade bearing conditions, concrete slabs crack. As stated, this lets water seep into the subgrade and the application of future heavy wheel loads leads to a worsening condition (see the preceding discussion of “subgrade pumping”).
In summary, large, crack free, unreinforced concrete slabs on grade are rare. To have a long-lasting, aesthetically pleasing concrete slab on grade: utilize a high quality concrete mix with proper air-entrainment (see section on surface defects), place the slab on a well-compacted layer of gravel (subgrade), saw cut control joints at a close-spacing (throughout the slab) within 12 – 24 hours of placement (in order to provide pre-planned planes of weakness for crack control), then properly cure the slab for several days. You’ll find that, in a relatively short time, cracks will form at each of the control joints. You can seal these cracks with a commercially-available concrete crack sealer/caulking to preclude water entry. Other construction precautions include utilizing a quality concrete ready mix during placement and adding the least amount of water possible to the delivered concrete prior to placement. Proper curing entails wetting the surface of the concrete after finishing, then covering the slab with plastic or wet burlap. The concrete should be kept moist in this manner for as long as possible–at least three to seven days. If this is impossible, spray the slab with a commercial concrete sealer (according to directions of the sealer manufacturer). You’ll generally find that a very generous application of sealer is required to form a thick layer of surface sealer, i.e., one capable of preventing excessive moisture loss through the slab. Lastly, finished slabs should drain freely onto the adjacent ground or designated areas which direct runoff away from the slab.
For more information you can check out the web site called Concrete Basics from the Portland Cement Association.