The general approach to locating joints and sealing them includes designing to minimize movement at any one location and providing positive methods of sealing. These methods utilize water stops cast in the concrete and sealants located on the water side so that the head of water tends to force them into the joint instead of out of it. The kinds of joints that are useful in pools may be classified in the following groupings: rigid joints- these are commonly called construction joints, joints between segments cast at different times but purposely joint monolithically; and nonrigid joints- there are three types-expansion joints (full movement joints), contraction joints (partial movement joints) and stress relief joints (also partial movement). Choosing the type and number of joints and their location requires consideration of the shape and size of the pool and the kind of service it must render. The outdoor pool may not only shrink and expand from temperature and moisture changes, but it may move differentially because of unstable ground. Such a pool must either be built in a number of independent sections with expansion joints for full movement or else be made extremely rigid, possibly by post-tensioning. Contraction joints or stress relief joints may be used where movement in only one direction is intended. The steel crossing stress relief joints allows less movement in them. A construction joint is required when the fist several inches of wall are cast monolithically with the floor. The construction joint bonds this first small lift to the remainder of the wall. Construction joints should be introduced at any other place that concreting is halted, as at the end of the day, but whenever possible occasions should be eliminated that will require such joints. Crack inducers may sometimes be convenient to insure the correct location of contraction or stress relief joints. This is especially true when it seems necessary to introduce stress relief joints to divide a wall panel that would otherwise be 20 feet or more in length. Such a panel would be overly long for most pools unless heavily reinforced. The crack inducer can be an inflatable tube that is inserted vertically from the top of the wall to the construction joint. The tube should be inflated to about one-quarter of the wall thickness. The tube can be removed when the formwork is removed.