Contrary to general opinion it is possible to control cracking in concrete floors. Success depends on an understanding of the causes of cracking and a readiness to do what is necessary to prevent these causes, control the location of cracks, or distribute them. Often only a little extra planning and effort are required. Basically, cracking is controlled in slabs on ground by providing carefully placed joints that are free to open and close. In structural slabs it is done by using enough steel to distribute cracks uniformly so that, though more numerous, they are small and acceptable in appearance. Concrete is notably low in tensile strength and all cracking is caused by forces that result in stresses that exceed the tensile strength of the slab. What are these forces? Highest on the list are drying shrinkage and large temperature changes that occur early in the life of the floor. Since cracking can't be prevented entirely, the designer undertakes to determine where cracks are likely to occur and then provides joints at these locations. A joint must satisfy two conditions: it should serve to relieve stresses that develop in the concrete and it should permit the concrete to move freely without decreasing the utility of the floor. To meet these requirements the joint must be constructed in such a way that it does not fill with debris and cause spalling; it should maintain a flat, even surface from one section to the next; it must no fail to transfer loads across from one side to another; and finally, it must no allow liquid to pass through to the subgrade. Welded wire fabric or steel bars are used in slabs on grade and in structural slabs to control cracking, not to prevent it. If a reinforced slab cracks, the steel will hold the cracks together so that loads can be transferred across the face of a crack by the interlocked aggregate. No reinforcing steel is allowed to continue across a slip-doweled joint because it inhibits the function of the joint which is to allow the slab to contract and expand. In this case, the dowels provide the desired load transfer function. Details on reinforcing steel and its placement in slabs are provided in the article "Preparing the site."