A waterstop is usually a section of flexible, waterproof material placed at any joint in concrete to prevent the passage of water. There are several kinds of waterstops. Rubber waterstops are resilient, stay firmly locked in the concrete and movement due to settlement, and do not reduce joint efficiency. Polyvinlchloride was the first plastic to be developed as a waterstop and is still the most popular. For all normal jobs, plastics and rubber waterstops offer a roughly equal performance. At least one manufacture markets a combination metal-neoprene waterstop- a product that has all the advantages and disadvantages of both materials. There are several essential requirements for proper installation of waterstops: (1) position the waterstop correctly. The waterstop must be located accurately and must be braced or lashed firmly to prevent movement during placing of the concrete. For many jobs, temporary wedged timber bracing can be used until placing actually reaches the waterstop. (2) The waterstop must be clean. It is impossible to achieve a good seal with the concrete if the waterstop is greasy or dirty. (3) The concrete must be properly compacted. Waterstops are designed to provide an effective waterproof barrier across construction, contraction, and expansion joints. They are not intended as a remedy for porous concrete. Finally, (4) all joints must be sealed properly. Although a waterstop is a complete seal in itself, it is also advisable to seal all joints with an appropriate filler. The filler will prevent the entry of dirt and foreign material and provide a preliminary line of defense against the penetration of water.