The waterstop is a long, thin barrier against water leakage. Like the links in a chain, every part of its length must do the job. There are many different waterstops and there are many different situations that require waterstops. Waterstops are used in walls, in floors and in roofs of underground structures. They are used in basements, tanks, swimming pools, dams, tunnels, canal linings, bridges, locks, sewage treatment plants, water reservoirs, mine shafts, retaining walls, aqueducts, underground vaults and parking structures.

In concrete with rebars, the size and shape of a waterstop would depend on the type of joint--any of three basic kinds: 1) construction joint, which has practically no movement; 2) control joint, which has limited movement; 3) expansion joint, which has a greater movement.

Waterstops are usually made of rubber, plastics, or moldable plastic strips. The rubber chosen for a waterstop should be alkali-resistant and should not become brittle or crack when exposed to sunlight and continuous flexing. Synthetic materials were first used for waterstops about 50 years ago. Polyvinylchloride (PVC), the first plastic to be developed for the purpose, is still very popular. Plastic strips that are moldable at ordinary temperatures are available. These are inserted into keyways of construction joints.

There are several important requirements for achieving watertightness: 1) position the waterstop correctly; 2) the formwork must be tight-fitting; 3) the waterstop must be clean; 4) concrete must be carefully consolidated; 5) splices must be correctly made.