Shrinkage cracking and upward curling, which result from shrinkage differentials, are common troubles for enclosed industrial slabs on grade. The most important factor contributing to shrinkage is the amount of water per unit volume of concrete. Understanding this, designers should select materials and specify concrete to minimize shrinkage and therefore reduce curling and cracking. Enclosed slabs on grade made with portland cement concrete have worse shrinkage and curling problems today than 25 years ago for several reasons: * Shrinkage is neglected. * Compressive strength is overemphasized. * Clean, low-shrinkage aggregates are less available. * Floor slabs are being built on higher-moisture-content subgrades. Research on shrinkage shows there is a cumulative effect on shrinkage of making poor choices in the selection of material to be used. One study of eight factors that influence the water content of concrete concluded that their influence on shrinkage could total 400%.