What is a swellable waterstop?

A swellable waterstop, also called a hydrophilic strip or swelling tape (in Dutch zwelstrip or zwelband), is a strip that swells when it comes into contact with water and thereby seals a joint in concrete. It is mainly used in construction joints: the joints that form when a concrete structure is poured in stages, for example between a basement slab and a basement wall. When water tries to get through the joint, the strip swells and presses the joint closed.

Types

• Bentonite waterstop: bentonite (a type of clay), sometimes combined with rubber. Bentonite can absorb a lot of water and expand strongly; manufacturers quote swelling capacities of several hundred per cent.
• Hydrophilic rubber or thermoplastic strip: a polymer that absorbs water and swells as a result.
• Swelling mastic: a gun-applied version from a cartridge, useful on rough or uneven joints.

Installation

1. Clean the concrete surface of the first pour and remove loose material.
2. Place the strip in the middle of the joint so it is surrounded by enough concrete on both sides. Keep to the minimum concrete cover specified by the manufacturer.
3. Fix the strip, for example with nails or a mesh. One manufacturer of bentonite strips recommends fixing every 30 to 40 cm.
4. Butt the ends tightly together without gaps, or overlap them if the manufacturer allows.
5. Pour the second stage. Do not discharge concrete directly onto the strip with force.

Common mistakes

• Premature swelling: a strip left in the rain or standing in water before the pour will already swell. Heavily pre-swollen strips are unsuitable and must be removed.
• Too close to the edge: with insufficient cover, the swelling pressure can spall the concrete at the edge.
• Loose strip that moves during pouring.

Not to be confused with expanding foam tape

In window installation, “swelling tape” often refers to pre-compressed expanding foam tape: an impregnated foam tape that slowly expands to close the gap between frame and masonry. That is a completely different product, designed against draughts and driving rain, not against groundwater pressure.

Related terms

• Bentonite
• Bentonite mat
• Metal waterstop
• Expanding grout
• Shrinkage crack

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