Frequently Asked Questions

What is the advantage of Hydrocell XL rubber waterstop over Nitofoam 40 foam waterstop for a deep basement construction joint?

Both Hydrocell XL (hydrophilic rubber) and Nitofoam 40 (hydrophilic polyurethane foam) are effective construction joint waterstop products that seal the joint by swelling on water contact. The key differences that make Hydrocell XL preferable for deep basement and high-pressure applications are: robustness and density — the rubber compound is denser and mechanically tougher than the polyurethane foam, making it less susceptible to being displaced or damaged when concrete is poured and vibrated around it during the second pour; and performance under high hydrostatic head — the denser rubber compound, when confined within the construction joint and fully swollen, provides a higher resistance to water penetration under the elevated hydrostatic pressure experienced at greater depth below the water table. For a basement at 10 metres below the water table (1 bar hydrostatic pressure) or a metro tunnel at 20 metres below water table (2 bar), the additional robustness and pressure resistance of Hydrocell XL over the foam product provides greater assurance of watertight performance.

How are construction joint waterstops like Hydrocell XL incorporated into the overall basement waterproofing system?

Construction joint waterstops are one element of a complete basement waterproofing system — they address specifically the construction joint plane, which is the most vulnerable water ingress path in a concrete structure. A complete basement waterproofing system typically combines: an external waterproofing membrane (such as Fosroc Proofex Engage or Proofex 300) applied to the outside of the below-ground walls and raft before backfilling; construction joint waterstops (Hydrocell XL or Nitofoam 40) installed at all construction cold joints; penetration seals at all service pipe and cable penetrations through the concrete; and an appropriate drainage layer between the membrane and the backfill soil to reduce hydrostatic pressure on the waterproofing. The construction joint waterstop and the external membrane together provide redundant waterproofing at the most vulnerable joint location — if water penetrates through the external membrane at the joint (for example due to a pinhole or imperfect adhesion), the hydrophilic waterstop within the joint provides a second line of defence. A complete specification for the basement waterproofing system including joint details should be prepared by a waterproofing specialist, and Space Arc Engineering can provide technical support and Fosroc product specifications for all elements of the system.

What precautions must be taken to prevent Hydrocell XL from pre-swelling before the second concrete pour is cast?

Pre-swelling of the hydrophilic waterstop strip before the second concrete pour is cast is a significant risk that reduces the effectiveness of the waterstop. If the strip swells freely in the unconfined space before the concrete is cast over it, it will swell to its maximum volume and then the concrete is cast around the already-swollen strip — there is no further swelling available when water subsequently reaches the joint, and the soft swollen foam or rubber provides less compression against the joint faces than a strip that swells within the confined joint. Precautions to prevent pre-swelling: install the Hydrocell XL strip only immediately before the second pour is cast — never more than 24 hours in advance in wet or humid conditions; protect the installed strip from rain, running water, and condensation during the period between installation and the second pour by covering with polyethylene sheeting; in high groundwater locations where the hardened concrete substrate is wet at the joint face, apply the adhesive and stick the waterstop immediately before casting and minimise the time between installation and concrete placement; and never flood the substrate with water to test bond of the adhesive before the strip is confined by the concrete.