Generally, all the dry materials used in making quality concrete are heavier than water. Thus, shortly after placement, these materials will have a tendency to settle to the bottom and force any excess water to the surface. This reaction is commonly called "bleeding." This bleeding usually occurs with non-air-entrained concrete. It is of utmost importance that the first operations of placing, screeding and darbying be performed before any bleeding takes place. The concrete should not be allowed to remain in wheelbarrows, buggies, or buckets any longer than is absolutely necessary. It should be dumped and spread as soon as possible and struck off to proper grade, and then immediately screeded- followed at once by darbying. These last two operations should be performed before any free water has bled to the surface. The concrete should not be spread over a large area before screeding- nor should a large area be screeded and allowed to remain before darbying. If any operation is performed on the surface while the bled water is present, serious scaling, dusting or crazing can result. This point cannot be overemphasized and is the basic rule for successful finishing of concrete surfaces. The surface is struck off or rodded by moving a straightedge back and forth with a sawlike motion across the top of the forms or screeds. After the concrete has been struck off or rodded, and in some cases tamped, it is smoothed with a darby to level any raised spots and fill depressions. When all bled water and water sheen has left the surface and the concrete has started to stiffen, it is time for the other finishing operations. Where edging is necessary, this could be the next operation. Immediately following edging, the slab is jointed. The cutting edge or bit of the jointing tool cuts joints in the slab called control or contraction joints. These joints are used to control any cracking tendency in the concrete that may be due to shrinkage stresses caused by drying out or by temperature changes. After edging and hand jointing operations, the slab should be floated. Curing of a concrete slab is one of the most important construction operations, but is often one of the most neglected. Concrete should be protected so that little or no moisture is lost. This may be done with burlap or canvas coverings kept continuously wet. Another method of moist curing is by ponding.