Waterstops exist because concrete joints are the weakest point in any water-retaining or below-ground structure. The concrete itself, if properly mixed and cured, will resist water ingress. The joint between two pours, or between a slab and a wall, will not. Get the waterstop selection or installation wrong and the consequence is water tracking through the structure, whether that is a potable water reservoir, a basement carpark, a sewage treatment tank or a below-grade plant room.
AS 3735 (Concrete Structures for Retaining Liquids) sets the baseline for liquid-retaining structures in Australia. It requires that joints be designed and detailed to remain watertight under the full hydraulic head the structure will experience. The standard does not prescribe a single waterstop type; it requires that the designer select a system appropriate to the joint type and the service conditions. That is where the three main categories come in.
Joint Type Drives Waterstop Selection
Before specifying a waterstop, you need to know what the joint is doing. A construction joint is a planned pour stop where fresh concrete is cast against hardened concrete. There is no intended movement; the joint exists purely because the structure could not be poured monolithically. A movement joint (expansion or contraction joint) is designed to accommodate differential movement between adjacent sections. Those two joint types have different demands, and they require different waterstop systems.
PVC and Rubber Waterstops: Cast-In for Movement Joints
PVC and rubber centrebulb waterstops are the standard choice for movement joints. The centrebulb sits at the joint plane and accommodates movement by deforming; the flanges are cast into the concrete on each side. Typical profiles range from 150 mm to 320 mm wide, with the centrebulb diameter and wall thickness varying by the expected movement and hydrostatic head.
PVC waterstops are cost-effective and widely available. Rubber waterstops, including EPDM profiles, offer better performance at temperature extremes and in contact with some chemicals. For sewage and industrial liquid containment, confirm chemical compatibility against the manufacturer's datasheet before specifying PVC.
The installation requirement that causes the most failures is concrete cover. The flange of a PVC waterstop needs adequate cover on both faces to develop the bond that makes it watertight. A minimum of 75 mm cover to the waterstop flange edge is commonly specified, though the project engineer should confirm this against the design hydraulic head. Insufficient cover means the concrete at the flange edge can crack or delaminate, creating a direct water path alongside the waterstop rather than through it.
The second failure mode is displacement during the pour. A PVC or rubber waterstop must be held rigidly in position while concrete is placed and vibrated. Standard practice is to wire the waterstop to the reinforcement at 300 mm centres. If the waterstop is simply clipped to formwork and left unsupported through the full depth of the pour, vibration will shift it. A waterstop that has rotated 15 to 20 degrees from its design position no longer presents its full cross-section to the water path, and the centrebulb may no longer align with the joint plane.
Splicing: The Detail That Is Routinely Underspecified
PVC waterstops are supplied in rolls or straight lengths. On any real structure, joins are unavoidable. A poorly executed splice is a direct leak path. PVC waterstops must be heat-welded using a purpose-made jig that holds the two sections in alignment while the joint is fused. A cold join using adhesive alone is not adequate for hydrostatic service. Corner and T-junction pieces should be factory-fabricated where possible; site-formed corners welded with a hand iron require a skilled operator and should be tested by pulling the joint apart before the pour proceeds.
Rubber waterstops are vulcanised at splices. This requires a vulcanising press and is not a site improvisation. If the specification calls for rubber and the splice is made with contact adhesive, the joint will fail.
Hydrophilic Waterstops: Construction Joints Only
Hydrophilic waterstops are strips of bentonite-modified rubber or hydrophilic polyurethane that swell when they contact water. Swelling pressures from 0.5 MPa to over 3 MPa are typical depending on the product, which means the expanded strip exerts pressure against the concrete on both sides of the joint, blocking the water path.
They are the right choice for construction joints in basement walls, raft slabs and tank walls because they are easy to install, do not need to be threaded through reinforcement, and do not require welding. The strip is fixed to the face of the hardened concrete before the next pour is cast against it.
The installation failure that causes leaks with hydrophilic strips is pre-swelling. If the strip gets wet before the concrete is cast against it, it will swell, and once it has swelled and dried, it has consumed part of its swelling capacity. A strip that has cycled through wet and dry before installation may not generate enough pressure against the fresh concrete face to seal the joint under full hydrostatic head.
The practical consequence: hydrophilic strips must be stored dry and installed immediately before the pour. If rain is forecast and the strip will be exposed on the construction joint face for more than a few hours, cover it with plastic sheeting. Some manufacturers supply strips with a retarding coating that delays initial swelling; check the product datasheet for the delay period and whether that period suits your pour schedule.
Hydrophilic strips are not suitable for movement joints. They cannot accommodate differential movement; if the joint opens, the strip loses contact pressure and the seal fails.
Concrete Cover for Hydrophilic Strips
The same cover requirement that applies to PVC waterstops applies here. The strip needs enough concrete mass on both sides to resist the swelling pressure. If the strip is positioned too close to the concrete face, the swelling pressure will spall the cover rather than seal the joint. A minimum of 50 mm cover to the strip edge is commonly cited; again, the project engineer should confirm this for the specific hydrostatic head.
Injectable Hose Systems: Remedial Fallback
An injectable hose system consists of a perforated or slotted hose cast into the construction joint. If the joint leaks after the structure is complete, a polyurethane or acrylic resin can be injected through the hose to fill the void and seal the leak. The hose remains in the joint permanently and can be re-injected if the seal degrades.
Injectable hoses are sometimes specified as a primary system on construction joints where access for future remediation is constrained, for example in a basement wall that will be permanently backfilled. More often, they are installed alongside a hydrophilic strip as a fallback: if the strip fails to seal fully, the hose provides a repair path without breaking out concrete.
Installation requires that the hose be continuous, properly terminated at an accessible injection port, and not kinked or crushed during the pour. A hose that is pinched by a vibrator or bent sharply at a corner cannot be injected. The injection ports must be labelled and protected so they can be located after formwork is stripped and backfill is placed.
The limitation of injectable systems is that they address a leak after it has occurred. For a potable water tank or a structure where any ingress is unacceptable, relying on injection as the primary seal is not good practice. Use it as the second line of defence.
Matching the System to the Structure
A practical decision matrix for below-ground and water-retaining structures:
- Movement joints in tanks, reservoirs and basement walls: centrebulb PVC or rubber waterstop, cast in, with factory-fabricated corners and heat-welded splices. Cover to flange edge confirmed by the structural engineer.
- Construction joints in basement walls and raft slabs: hydrophilic strip, installed dry immediately before the pour, with an injectable hose as a secondary measure where future access is restricted.
- Construction joints in liquid-retaining structures under AS 3735: hydrophilic strip or flat-flange PVC waterstop depending on the design hydraulic head and the engineer's specification. The designer should confirm which system is appropriate for the specific head and chemical exposure.
- Remedial sealing of leaking construction joints: injection of polyurethane or acrylic resin through a pre-installed hose, or crack injection into the joint face if no hose was installed. Crack injection into an active leak is a specialist operation; it is not a site fix.
What Goes Wrong and Why
Most waterstop failures trace back to four causes:
- Displacement during the pour. Fix waterstops to reinforcement at close centres. Inspect position before and after each concrete truck is discharged.
- Inadequate cover. The waterstop flange or strip must have enough concrete mass around it. Confirm cover dimensions before the pour, not after.
- Pre-swelling of hydrophilic strips. Store dry, install late, and protect from rain. A strip that has swelled and dried before the pour is compromised.
- Unqualified splices. PVC waterstops must be heat-welded with a jig. Rubber waterstops must be vulcanised. Adhesive-only joins will not hold under hydrostatic pressure.
None of these failures are difficult to prevent. They happen because waterstops are installed early in the pour sequence, covered by concrete, and invisible by the time the structure is tested. The consequence of getting it wrong is water in the structure, which means either accepting the ingress or breaking out concrete to repair it.
Fosroc Waterstop Products at CSA
Construction Supplies Australia supplies the Fosroc waterstop range, including hydrophilic waterstop strips and injectable hose systems, as part of the broader Fosroc construction chemicals offer. Fosroc product datasheets specify swelling pressures, cover requirements and chemical resistance; those datasheets are the reference point for specifying the product against your project conditions.
For projects requiring PVC or rubber centrebulb profiles, quantities, splice requirements and compatible accessories, contact the CSA trade desk or visit constructionsupplies.group/au. Where the joint design requires engineering sign-off under AS 3735, the project structural engineer or a hydraulic structures specialist should confirm the specification before the pour schedule is locked in.