A metal waterstop is a steel strip cast half into the first pour and half into the next, so water cannot track along the joint between them. Its most common position is the wall-floor joint, where the slab meets the rising wall. Construction joints and shrinkage joints are sealed the same way.
The principle is simple: the plate lengthens the path water would have to take through the joint and closes it off. Many types carry a coating or a swelling layer on one or both faces that expands when it takes up moisture and presses the last gaps shut.
Types and forms
- Flat or L-shaped — an L-section sits easily on the top reinforcement; a flat strip is more flexible around corners and curves
- Galvanised steel — plate thickness in the order of half a millimetre to a millimetre
- With a coating — bitumen-butyl rubber, or a mineral, self-healing layer
- Combined with a swelling strip — the steel provides the mechanical barrier, the swelling rubber the seal against the concrete
- Horizontal and vertical use — some types are rated for water under pressure
How it is installed
- Set the position — central in the wall thickness, on the line of the future construction joint
- Fix it to the reinforcement — clip, tie or clamp it to the top reinforcement of the slab or foundation
- Keep the embedment depth — the strip has to sit deep enough in the concrete; the supplier states the figure, usually several centimetres
- Join the lengths — overlap the strips and connect them with clamping strips or clips; a continuous run is essential
- Pour and compact — the concrete has to surround the plate completely; a void alongside it makes the seal worthless
Points to watch
- Continuity — one break at a corner or a service penetration is all water needs; work the details out in advance
- Do not displace it while pouring — vibrating hard against the plate can shift it, so fix it securely
- Keep it clean — concrete residue on the protruding half stops the next pour bonding
- Under water pressure — choose a type tested for hydrostatic pressure and follow the supplier’s detailing; this is not the place to improvise
Related terms
- Wall-floor joint
- Kicker formwork
- Basement
- Formwork
- Reinforcement
- Expansion joint
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