Shortly after its construction and early use, some of the characteristics of a floor will become apparent. It will be known whether the floor is free of dusting, whether there has been any plastic shrinkage cracking, whether the floor is plane and whether it has proper slope for drainage if such a slope was specified. Any discoloration problems will also appear early. Changes in materials, procedures, and workmanship may be necessary to remedy problems in floor construction. Common troubles and possible causes are listed: rapid wear (mix of too low strength for traffic anticipated; surface frozen at early age), cracking (low strength mix; expansion of reinforcement rusted because of inadequate concrete cover), curling (slab too thin; improper curing), plastic shrinkage cracking (dry subgrade; too rapid evaporation before setting), dusting (too much clay in the aggregates; construction debris ground into surface), light or dark spots (variations in water-cement ratio; polyethylene curing), scaling (troweling dry cement into surfaces to dry it; freezing at early age), crazing (too much clay in the aggregates; intermittent wet curing and drying), and blisters (subgrade cooler than the concrete; sealing the surface while the underlying concrete is still plastic). Every economical crack repair method leaves some evidence of the crack. One method or repair is to saw out a section of concrete on each side of the crack, clean the edges, brush a neat cement grout into the sawed surfaces and reconcrete the intervening area. Epoxy concrete also can be used. If the patch borders or straddles a joint a full-depth joint should be built into the new concrete to allow for inevitable future movement. However, once plastic shrinkage cracks have hardened in the surface little can be done to eliminate them. Chemical or liquid floor hardeners frequently are used to correct dusting floors. Details are given in the ACI 302. Discoloration may result from trowel burns, admixtures, stains from organic materials or admixtures. Sodium hydroxide may remove trowel burns. Organic stains may succumb to applications of poultice materials such as clay or fly ash.