Why joints are the main weak point
A cast-in-situ basement or tank is not concreted in one go. Every break leaves a construction joint: between the foundation slab and the wall, between wall pours, between the wall and the slab above. Fresh concrete bonds poorly to hardened concrete, laitance and debris remain at the interface, and water passes along this plane much more easily than through solid concrete. Besides construction joints, there are movement joints — breaks provided for in the design, along which the structure moves due to temperature and settlement — as well as pipe penetrations, formwork ties, junctions with existing structures and cracks.
That is why joints are dealt with at the design stage and during concreting, not after the first leak. ShNK 2.03.11-24 explicitly includes among the measures for protecting concrete and reinforced concrete structures against corrosion the sealing of concrete construction joints with hydroactive profiled cords and waterstops while the concrete is being placed.
Solutions for construction joints
Swelling profiles
A profile made of hydrophilic rubber, an acrylic polymer or butyl rubber with bentonite is laid on the surface of the first pour before the next one is concreted. On contact with water, it increases in volume and seals the joint with its swelling pressure. This solution is easy to install: no cut-outs in the formwork, no welding and no changes to the reinforcement are needed. In the catalogue: SikaSwell A, Mapei Idrostop and PENEBAR SW-55.
Swelling profiles have conditions without which they do not work:
- Dense concrete around the profile. Swelling pressure develops only in concrete free of honeycombing; the profile is placed in the middle of the section, and according to the Sika and Mapei data sheets there must be at least 8 cm of concrete on each side. In thin walls and at the edge of a slab, the profile can spall the concrete.
- Limited water pressure. The manufacturer recommends SikaSwell A for pressures of up to 2 bar, and Idrostop for up to 5 atm. At a higher head, the joint is dealt with by a waterstop or a combination of solutions.
- Water composition. In saline water, swelling is weaker and slower — this is stated explicitly in the SikaSwell data sheets. For saline groundwater, versions designed for such conditions or other solutions are chosen.
- Non-moving joints only. Swelling profiles and sealants are not intended for movement joints.
Swelling sealants and pastes
One-component products in cartridges — SikaSwell S-2, Mapeproof Swell — are applied as a bead along the joint, around pipes and cast-in items, and on uneven surfaces where a profile would not lie tight. SikaSwell S-2 also serves as an adhesive for fixing profiles. Their permissible pressure is, as a rule, lower than that of profiles: up to 2 bar in the SikaSwell S-2 data sheet, while for Mapeproof Swell the watertightness is stated at 1 atm.
Waterstops
- PVC waterstops are either internal — installed in the middle of the section and tied to the reinforcement — or external, laid on the blinding or fixed to the formwork on the water side. This is the classic solution for high heads: according to the data sheet, Fosroc Supercast PVC profiles are rated for a head of up to 100 m. Their reliability depends on the welded joints: the joints are welded with a heated blade in a jig, and intersections are made with factory-made fittings.
- Bonded waterstops — strips of flexible polyolefin with a rough surface, for example Sika Waterbar FB-125. They are embedded in fresh concrete, and after hardening the strip bonds to it, so water cannot travel along the waterstop: according to the data sheet, this has been confirmed by a test at 7 bar for 7 days.
Injection hoses
A perforated hose is laid in the construction joint and fixed with clips, and its ends are brought out of the structure to an accessible point. If the joint starts to let water through after concreting, an injection resin is pumped through the hose without drilling the concrete. SikaFuko VT-1 hoses have been tested at water pressures of up to 10 bar; with acrylate and microfine cement products, injection through them can be repeated, but with polyurethane and epoxy resins only once, so if re-injection may be needed, the first injection is carried out with acrylate or microfine cement. On critical below-ground structures, the hose is often installed together with a swelling profile — as a second, “repair” barrier.
Movement joints
A movement joint must remain watertight when the structure moves. Swelling profiles, rigid mortars and penetrating compounds do not work here. The following are used:
- PVC waterstops for movement joints, cast in during concreting; for Fosroc Supercast PVC, the permissible movement according to the data sheet is up to 10 mm;
- external tapes bonded with epoxy adhesive — for example, Sikadur-Combiflex SG-10 P with an elongation of more than 650 %, tested for watertightness at 6 bar for 72 hours. Such a tape is also used in repairs where no waterstop was cast into the joint;
- elastic sealants designed for joint movement and, for tanks, for permanent immersion — see “Tanks, swimming pools and treatment plants”;
- in cementitious waterproofing — elastic tapes embedded in the layer, for example Mapeband Easy.
Pipe penetrations and cast-in items
Water supply and sewer pipes and cables pass through basement walls. Around every pipe there is a joint between the concrete and the pipe material. During concreting, a swelling profile is placed or a swelling sealant applied around the pipe; in existing walls, the penetration is cut out and sealed with a swelling paste or by injection. Formwork ties, cast-in items and anchors also create a path for water — their holes are sealed following the waterproofing manufacturer’s procedure.
Installation: what to check before concreting
- Continuity. The profile, waterstop or hose must run without breaks around the entire perimeter. Lengths of profile are joined as required by the data sheet: Idrostop butt-jointed or with a 2–3 cm overlap, PENEBAR SW-55 butt-jointed with opposing 45° cuts, without overlapping.
- Fixing. The fresh concrete must not displace or lift the profile: it is fixed with adhesive, nails, staples or clips. When Idrostop is fixed with adhesive, for example, concreting takes place after 24 hours.
- Position. The middle of the section and the cover as specified in the data sheet; for the bonded Sika Waterbar FB-125 waterstop, an embedment in fresh concrete of at least 30 mm.
- Protection against premature swelling. Rain and standing water before concreting trigger swelling too early. Idrostop must not be installed if the structure is under water at the time of installation.
- A clean joint. Laitance and debris are removed, and the concrete around the profile and the waterstop is thoroughly compacted with a vibrator.
- Documentation. The outlets of the injection hoses are labelled and marked on the as-built drawing, otherwise they will not be found a few years later.
Common mistakes
- A swelling profile right at the face of the wall — the concrete spalls when the profile swells.
- The profile got wet in the rain before concreting and swelled prematurely.
- The profile was not fixed, and the fresh concrete displaced or lifted it.
- A swelling profile or sealant in a movement joint.
- Breaks and leaky joints in PVC waterstops, “home-made” intersections instead of factory-made fittings.
- The outlets of the injection hoses have been cast over or lost.
- A swelling solution at a head above that recommended by the manufacturer, or in saline water without checking.
See also: waterproofing — overview, injection waterproofing, penetrating waterproofing, cementitious waterproofing.
Materials in the catalogue
- Sika Waterbar FB-125 — Sika — fully bonded flexible polyolefin waterstop for construction joints in concrete.
- SikaSwell A — Sika — water-swelling acrylic profile for construction joints and pipe penetrations in concrete.
- SikaSwell S-2 — Sika — water-swelling PU sealant for construction joints and bonding SikaSwell profiles.
- SikaFuko VT-1 — Sika — re-injectable injection hose for construction joints, tested up to 10 bar.
- Sikadur-Combiflex SG-10 P — Sika — FPO tape bonded with epoxy for waterproofing expansion joints and cracks.
- Idrostop — Mapei — hydrophilic swelling acrylic rubber profile for construction joints, watertight up to 5 atm.
- Mapeband Easy — Mapei — elastic rubber tape for corners and control joints in cementitious waterproofing.
- Mapeproof Swell — Mapei — water-swelling rubber paste for sealing pipe penetrations, construction joints and cracks.
- Fosroc Supercast PVC — Fosroc — PVC waterstops for expansion and construction joints, up to 100 m head.
- PENEBAR SW-55 — Penetron — bentonite-filled butyl rubber swelling waterstop for non-moving construction joints.
- Penecrete — Penetron — mortar for sealing static joints, cracks and pipe penetrations in chased grooves.
- Penebar — Penetron — swelling 17×23 mm waterstop cord for construction joints and pipe penetrations.
- PeneBand S — Penetron — elastic tape bonded with epoxy for expansion joints, elongation over 1000 %.
- PeneBand — Penetron — elastic expansion-joint tape that can be installed on damp substrates and underwater.
- PenePoxy — Penetron — one-component elastic adhesive sealant for PeneBand tape and pipe penetrations.
- Mapeband TPE — Mapei — thermoplastic elastomer tape for waterproofing expansion joints with 5–10 mm movement.
Frequently asked questions
What should be chosen for a construction joint: a swelling profile or a waterstop?
It depends on the head and on how critical the structure is. A swelling profile is easier to install, but it is limited in pressure and requires dense concrete around it. A waterstop is designed for a higher head and does not depend on swelling, but it requires precise installation and welding of the joints. On critical projects, an injection hose is added to either solution.
Can a joint be sealed if no waterstop was cast in?
Yes, but it is more difficult: a construction joint is injected with resins, while a movement joint is covered with a tape bonded with epoxy adhesive or with an elastic sealant according to the manufacturer’s system. Active leaks are stopped beforehand.
How long does a swelling profile last?
According to Mapei, the properties of Idrostop do not change after repeated cycles of swelling and shrinkage, including in sea water and wastewater. But the actual service life depends on the surrounding concrete, the composition of the water and the quality of installation, so on critical joints the profile is backed up with an injection hose.
Is joint waterproofing needed if a crystalline admixture has been added to the concrete?
Yes. The admixture densifies the concrete, but it does not seal construction joints, pipe penetrations or movement joints — these are dealt with separately.
We will select solutions for the joints and pipe penetrations of your project and calculate the supply — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.