Understanding waterstops: To ensure success, one must specify for performance, install with care

Applications for hydrophilic waterstops are not limited to cast-in-place concrete construction joints. Due to their flexibility and conformability, hydrophilic waterstops can be easily installed around pipe penetrations, I-beams, concrete pilings, and irregular-shaped surfaces. Hydrophilic waterstops are also used to seal new concrete placed against existing concrete. Additionally, they are lightweight and flexible, and do not require any special order prefabricated Ts, Ls, or crosses. The installation efficiency of this type of waterstop makes them
a favorite amongst contractors.

For hydrophilic waterstops, manufacturers typically require a minimum 50 to 75 mm (2 to 3 in.) of concrete coverage depth, depending on the profile size and material type. These waterstops must be installed in strict accordance with the manufacturer’s minimum concrete coverage guidelines to eliminate the potential of concrete spalling due to insufficient concrete coverage. They need to be installed without overlapping adjoining strip ends; the strip ends must be tightly abutted to form a continuous system.

Since these products expand in the presence of water, they should not be prematurely wetted. This requires the second concrete placement take place promptly after the waterstop placement; otherwise, the waterstop might expand if exposed to rain. The ‘free water’ in a concrete mix is not enough to physically swell a hydrophilic waterstop. After the concrete has cured, a sufficient external source of water that migrates into the joint and comes in contact with the waterstop is required for it to actually swell. If no external water source comes in contact with the hydrophilic waterstop, it simply remains dormant.

Not all hydrophilic waterstops use bentonite clay as their hydrophilic agent. Hydrophilic rubber waterstops expand upon contact with water and maintain a solid material integrity that will not deteriorate due to uncontrolled expansion. However, since they are produced with different types of hydrophilic agents, they may have different expansive properties (in terms of rate and volume) when exposed to water. Also, some are produced with portions of the profile made of non-swelling rubber—this means the entire profile does not activate and swell.

This class of hydrophilic waterstop has the same limitations as less-expensive bentonite-based waterstop products—namely, the inability to swell in fluids other than water, and the lack of expansion joint functionality.

In-situ performance of hydrophilics
Hydrophilic waterstops are effective in providing protection against water infiltration through concrete construction joints, including those under high-pressure and intermittent hydrostatic conditions. Due to their product design, hydrophilic waterstops exhibit a high degree of expansion when hydrated in a non-confined, free-swell condition, such as in 
a bowl of water.

In a non-confined condition, the product may exhibit some fracturing as it approaches maximum expansion. Although this may appear to be a negative aspect of the material, the fracturing is an indicator of positive performance for in-situ concrete joint conditions. It means the waterstop compound expands and easily conforms to the interior surfaces surrounding it within a concrete joint, no matter how irregular.

Further, the plasticity and expansion properties provide it the ability to extrude into cracks and advance around angle transitions that may be present in the joint. Bentonite compounded strips have been observed expanding through narrow cracks to fill larger void areas beyond the poorly consolidated concrete interface with the waterstop strip.

This dynamic sealing ability is possible because of the expansive properties; the same capabilities would be impossible with a high-tensile-strength material such as a hydrophilic rubber waterstop. It has been observed through lab testing that hydrophilic rubber waterstops have limited expansion into cracks, gaps, and joints that surround them due to their more rigid material properties. The hardness of the rubber may allow the waterstop to expand only a swallow convex amount across the opening of the crack and not extrude into its depth.

The maximum expansion a hydrophilic material exhibits in non-confined, free-swell conditions will not materialize in properly consolidated concrete. In-situ, hydrophilic waterstops only have to expand slightly to form the positive seal against the concrete interface around it. This limited swell affords the product a ‘reserve swell potential’ to seal poorly consolidated concrete or for concrete cracking that may occur later during building settlement or seismic activity.

Once hydrated, hydrophilic waterstops rarely undergo wet-dry cycling. When confined in concrete and below-grade, the material maintains its hydrated equilibrium and does not shrink during typical water-table fluctuations or intermittent water conditions. Interestingly, bentonite can be hydrated and dried an infinite number of times without losing its original, natural swelling capacity. Similarity it can 
be frozen and thawed repeatedly without losing swell performance. Finally, even if in-situ drying were to occur (an extremely remote possibility), bentonite has been proven to rehydrate to its original level 
of performance.

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