The time to eliminate cracks in concrete is before they happen. This places the contractor in a position to practice "preventive medicine." It is up to him to execute the precautionary measures that ensure a structure that satisfies his own as well as his customer's standards for quality. In restrained concrete, shrinkage and decrease in temperature produce tensile stresses and when these exceed the tensile strength of concrete, cracks will occur. Drying shrinkage can be controlled by attention to the many factors that contribute to it, as outlined below: mineral composition, shape surface texture and grading of aggregate; these affect the concrete's thermal coefficient, rate of shrinkage, stiffness, creep and flow; rates and conditions of placing; excessively rich cement content; uneven settlement of slab subgrades; high temperatures and adverse weather conditions; water loss through sedimentation of bleeding; water loss through chemical reaction to the hardening process; and improper aggregate size. There are several ways the contractor can prevent cracks from happening. In the design of the structure, the degree of restraint to which the concrete will be subjected during drying or cooling must be considered. Contraction, expansion and isolation joints should be provided at reasonable intervals. Whenever economically feasible, it is desirable to prestress the concrete. If reinforced concrete is to be used, cracks can best be controlled by using a large number of small reinforcing bars and by increasing the reinforcement. Mix materials that are known to have a good service record should be used. These include cement, aggregates and admixtures. Foreign materials, such as clay or dirt, are especially harmful. Water content is the greatest factor affecting shrinkage. For every 1 percent of increase in the quantity of mixing water, shrinkage is increased about 2 percent. Consequently, water content should be held to the minimum necessary for workability. Excessively wet mixes must never be permitted. Dampened subgrades are also recommended to reduce the possibility of cracks. In addition, subgrades must also be properly compacted. Form movement, and consequent cracking, often is caused by an inadequately compacted subgrade that subsides during or after placement. Final floating can be delayed until internal settlement and consequent cracking, if any, have taken place. The concrete can be revibrated if it can no longer be successfully worked. These same preventive measures can stop shrinkage around aggregate particles.