Air entrainment enhances concrete by achieving the following: improved resistance to freeze-thaw cycles and de-icing salts; increased workability; improvement in cohesiveness and reduction in likelihood of segregation; lessened permeability; minimization of bleeding; slightly better resistance to certain chemicals; and increase in resistance to sulfate attack. However, if the air content is allowed to rise appreciably higher than that specified, there can be a decided reduction in the overall quality of the concrete. Because of this, close watch should be kept on the air content. If possible, it is advisable to check the air content of each load of concrete delivered. The best way to control air contents when they vary excessively is to determine the cause and take appropriate corrective measures. Not only will this correct the air-content variation, but it will often prevent problems with other concrete properties which might be affected by the source of the air-content variation. For example, of a deterioration in aggregate gradation causes a change in air content, a workability and finish ability might also be symptomatic of the gradation change. Among the factors which can cause the variations in air content are the following: quality and proportioning of materials; method and duration of mixing; length of haul; type and condition of ready mix truck used; and the transporting, placing and finishing methods used on the jobsite. In the last mentioned factor, for example, the amount of vibration and the floating and troweling given a slab will have an important bearing on the air content of the hardened concrete.