Job sites must provide adequate support for finished slabs on ground, each site must furnish a stable working platform for concreting, finishing, and curing. When concrete is placed, the subgrade should be moist enough not to rob moisture from the fresh concrete. However, there must be no water left standing in puddles nor should the subgrade surface by muddy or soft. A layer of polyethylene may be laid over dry soil to prevent it from taking moisture from the concrete. All screeds and preformed joints must be in good condition and installed ahead of time, set firmly at the correct elevation. The keys for keyed joints should be securely attached to the inside of the form. To prevent cracks, columns should be boxed out to provide joints around them and all walls should be isolated. Square forms around columns should be rotated 45 degrees from the position of the column so their corners meet the ends of the joints running between columns. When reinforcing steel is used to control cracking its proper placement is all important. In slabs four to six inches thick welded wire fabric should be placed in the middle of the slab. In thicker slabs it is moved to the top third but a concrete cover of at least two inches over the steel must be maintained. Steel sometimes is used in two layers, one at the top and one at the bottom. Most slabs with dual reinforcement are from six to 14 inches thick. Another method, not quite as good, is to place the concrete in the lower part of the slab, screed it roughly to the level where the fabric is to be placed, lay out the fabric, and then place the remaining concrete. In this method care must be taken to see that workmen do not force the steel down by walking on it. The use of a stiffer concrete mix may be useful.