Waterproofing of a construction joint in a waterproof basement envelope

Solutions & Applications · Joint Sealing & Waterstops

A watertight basin does not leak from the concrete — it leaks from the joints. Properly designed and compacted concrete of sufficient thickness is practically impermeable. Every basement leak can be traced to one of four points: a construction joint, a penetration, a crack or a nail hole.

A joint is not a flaw — it is a design choice

No basement is concreted in a single pour. The joint between the plinth and the wall, and the joints between successive layers of concrete, are a given. The question is not whether they will exist, but where they will be located and how they will be waterproofed.

The difference from a crack is crucial: a crack is random and cannot be anticipated. A joint is known in advance — and is therefore the only potential leak point that can be addressed preventatively and with precision.

Three sealing technologies

	Water-swelling cord	Waterstop tape	Injection pipe
How it works	It swells when exposed to moisture and seals the joint by exerting pressure on its walls	Flexible PVC-P tape embedded in concrete — a mechanical barrier that extends the water’s path	It remains empty; if a leak occurs, resin is injected subsequently
Installation	On the surface of the hardened concrete, prior to the next concreting	Inside the formwork, secured to the reinforcement prior to concreting	In the joint, with ends in accessible injection boxes
Requirement for accuracy	Low — simple, quick, without interfering with the formwork	High — requires fixing to the reinforcement and welding of joints; displacement during concreting renders the system ineffective	The positioning of the boxes is of moderate to critical importance
Advantage	No welding, no joints — works even on uneven surfaces	Functions independently of the presence of water; also covers moving joints	Allows for repairs without excavation
Role	First line of defence	At the forefront of high-water-level projects	Safety measure — repair without excavation

The three are not mutually exclusive. In a project with a high water table or in a structure where future access is impossible — a basement beneath neighbouring buildings, a tank, or a lift shaft — the combination of a cord and an injection pipe is the sensible choice: the former provides a watertight seal, whilst the latter allows for rectification should anything go wrong.

Preparation of the joint

1. Removal of cement laitance: the loose layer on the surface of hardened concrete is removed by water jetting, shot blasting or wire brushing. A cord bonded to laitance has no substrate.
2. Cleaning: dust, formwork oils and residues must be completely removed.
3. Moisture control: the surface must be dry at the time of application — otherwise the cord will start to swell prematurely.
4. Positioning: in the centre of the thickness, with the required overlap on both sides.

Fitting the cord

• Continuous run: the strip is not interrupted. At joints, the ends must overlap by approximately 5 cm; they must never simply be placed side by side — a gap of one centimetre is an open passage.
• Full contact: fixing with a suitable adhesive or mechanically, at close intervals. A strip that comes away from and is dragged along by the fresh concrete does not provide any waterproofing.
• Protection against premature expansion: if rain falls before the next concreting operation, the cord expands and has exhausted its capacity. On exposed building sites, covering is required, or a type with delayed expansion should be selected.
• Concreting: thorough compaction around the joint area to ensure no voids remain — the vibrator must not come into contact with the string.

ESHAHYDROBENT is a water-swelling bentonite cord with a swelling capacity of over 400 per cent. It seals the joint from the inside, without any bonding or joints — it is activated by the very water it is designed to stop.

The remainder of the waterproof basement envelope

The joint is the critical point, but not the only one. The system is completed by three further layers:

Level	Role	Material
Repairs & drains	Levelling, filling cavities, curved floor-to-wall joints	Waterproofing repair mortar
External waterproofing	Continuous barrier on the side facing the ground	Bituminous membrane, liquid-applied coating or cementitious
Protection & drainage	Waterproofing reinforcement and relief of hydrostatic pressure	Dimpled drainage membrane with filter, perforated pipe

The third line is the one that protects the work of the first. The best joint is the one that is never put to the test: proper perimeter drainage means that no column of water forms on the wall, so the joint is not subjected to hydrostatic pressure. Drainage is not merely supplementary to the waterstop — it is the prerequisite that allows it to function effectively.

The remaining three leakage paths

• Penetrations: each pipe passing through the wall must be sealed individually — using a water-swelling ring or a specialised fitting, never with simple foam.
• Holes for dowels: these must be sealed with non-shrink mortar to their full depth, not just superficially on both sides.
• Cracks: these are controlled by means of reinforcement and the spacing of construction joints; where they do occur, they are treated by injection.

The expansion joint is another issue

A construction joint is fixed: two successive concrete pours that simply must not leak into one another. An expansion joint is the exact opposite — the only part of the structure designed to move, undergoing thousands of cycles of contraction and expansion over its lifetime.

These technologies are not interchangeable. A water-swelling cord in a moving joint will fail; an elastic sealant in a stop joint is superfluous. For movement joints, the requirement is a certified movement class in accordance with EN 15651-1 and elastic recovery after each cycle.

Two details that determine the service life

• Filling cord: expanded closed-cell polyethylene, placed at the bottom of the joint before the sealant is applied. It is not merely a cost-saving filler — it controls the depth of the sealing layer and, crucially, prevents adhesion to the substrate. Sealant bonded on all three sides cannot deform freely and will tear within the first few cycles.
• Joint priming: without the correct primer, even the best polyurethane sealant will delaminate during the first winter. It is the cheapest material in the joint and the first to be omitted.

Design series

1. Water table: seepage water or constant hydrostatic pressure — determines the type and number of defence lines.
2. Concrete placement plan: where the joints will be located, in what order and in which stress zones.
3. Wall thickness and position of reinforcement: these determine whether there is the required cover for a water-swelling cord.
4. Accessibility for future repairs: where this is not provided, an injection pipe shall be incorporated from the outset.
5. External waterproofing: selection of materials based, amongst other criteria, on the adhesion of the subsequent layer.
6. Drainage: perimeter drain and, where required, a retention trench.

Construction Joint

Expansion and Movement Joints

External Wall Waterproofing

Protection & Pressure Relief

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