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Waterstops are used to waterproof construction joints in concrete.They are generally PVC or vinyl, ribbed because concrete does not bond to plastic. Waterstops are embedded in concrete joints to create a a fluid-tight diaphragm. The rest of the gap can be filled with an acrylic mastic.
Structural concrete is waterproof. It does not require surface treatment or a liner, but the concrete must be well made and care must be taken to waterproof construction joints and movement joints. Construction joints, between separately cast slabs, should be strengthened by the formation of a key (see diagram) and by carrying the reinforcing bars across the joint. The key is made by laying a wood bar in the wet concrete, beveled to facilitate removal. A waterstop is required to prevent leakage through the potential crack and to protect the steel reinforcing bar bar from water which would cause rusting.
The structure of a small pool may be able to resist the pressure from the expansion of water when it heats up or freezes. Larger pools, even if re-inforced, require waterproofed movement joints.
All but the simplest concrete structures should be detailed by qualified engineers.