Complaints are often made regarding the variation from batch to batch in concrete performance. Often variations occur when there has been no apparent change in mix design or jobsite practices. Although they may be hidden, there are always reasons for such variations. What can cause concrete that has been developing satisfactory strengths suddenly to drop below the specification limit? There could be several causes for this. Most often, changes in strength mean that a change in water/cement ratio or air content has taken place. Have scheduling difficulties resulted in long waits for ready mix trucks at the jobsite? Especially on hot, sunny days, this results in a loss of slump and, if the concrete is retempered by the addition of water, the result will be lowered strength and lowered air content. Other causes could be improperly made and handled strength cylinders and excessively high air contents. The reasons for variations in slump can be traced to the many factors which reduce or increase the water content of a given concrete mix. Variations in the amount of a water reducing admixture added to the mix and /or the air content can have a pronounced effect on slump. The mixing time, especially on hot days, will influence slump to a great extent. Transporting and placing techniques will also influence the slump. Variations in finishability of concrete slab work usually stem from variations in (1) water content, (2) aggregate gradation, (3) aggregate particle shape, (4) admixtures used, and (5) amount of bleeding. Other common problems discussed are variations in air content, rate of hardening, and color.