Waterproofing of Basement Floors (Radier) with a Rising Water Table

Solutions & Applications · Basement Waterproofing

When the water table is close to the foundation level, the basement floor is exposed to water from the one side we cannot see: from below. The solution depends first and foremost on one question — has the raft foundation already been concreted or not? In a new build, the water is stopped outside the concrete before the concrete is poured. In an existing basement, the problem is tackled from the inside, using materials that can withstand negative pressure and work within the concrete itself.

New or existing ground slab?

Status	Where does waterproofing come in?	Basic rationale
New project — prior to concreting	Beneath the slab and around the perimeter of the walls (positive side)	Continuous membrane waterproofing, concrete with mass waterproofing, joint sealing with waterstop
Existing basement — damp or water ingress into the floor	On the upper surface of the slab (reverse side)	Sealing of penetrations, crystalline waterproofing, damp-proof course and subsequent finishing

An external waterproofing membrane is always the safest solution, as the water presses the waterproofing against the concrete. However, once the slab has been concreted, its underside is no longer accessible — and any work must be carried out from the inside, against the pressure of the water.

Positive and negative pressure

Side	What’s happening?	Suitable materials
Positive	The waterproofing is on the water side — the pressure presses it firmly against the concrete	Geomembranes, bentonite membranes, bituminous membranes, liquid-applied coatings
Negative	The waterproofing is on the opposite side — the pressure is trying to peel it away	Only materials with a declared resistance to negative pressure: crystalline and special cementitious materials

Liquid-applied coatings and paints that merely adhere to the surface do not constitute negative-side waterproofing. The first selection criterion is always the same: does the manufacturer specify resistance to negative pressure?

New radier: waterproofing before the concrete is poured

In new construction with a high water table, the waterproofing is designed as a continuous membrane: the membrane is laid on the concrete, turned up around the perimeter towards the walls and secured before the reinforcement is installed. The construction detail is analysed in the section on the waterproof basement envelope; the options are summarised here.

Item	Material	What to bear in mind
Waterproofing beneath the slab	MACLINE HDPE geomembrane	Virgin HDPE free from plasticisers, resistant to chemicals, salts and roots. The seams are heat-sealed and inspected
Alternative barrier	MACLINE GCL bentonite membrane	It self-heals in drill holes, but only when encased; it requires an overlying load to develop expansion pressure
Membrane protection	ESHAPROTECT or protective geotextile	Reinforcement, spacers and the contractor crew traffic are the primary causes of perforation
Concrete slab	SINTOMIX SM N mass waterproofing	Dense-pored concrete for ground slabs and basement walls; chloride-free, suitable for reinforced concrete with reinforcement
Slab-to-wall joint	ESHAHYDROBENT or PVC Waterstop	The construction joint is the weakest point of the formwork — it is sealed before the second concrete pour

Existing ground slab: internal waterproofing in five stages

Stage 1 — Sealing active ingress points

Before any general application, identify the active leakage points: cracks, cavities, penetration holes and the floor-to-wall joint. VANDEX PLUG is a fast-setting cementitious compound that sets in approximately 30 seconds after mixing and stops the flow of water, even on wet surfaces. The perimeter joint is reinforced with a channel of VANDEX UNIMORTAR, a waterproofing repair mortar for positive and negative hydrostatic pressure.

Stage 2 — Crystalline waterproofing

Crystalline products do not form a film on the concrete — they work within it. Their active compounds react with the free calcium and moisture in the pores to form insoluble crystals that seal pores and small cracks, whilst the concrete continues to ‘breathe’.

Product	What the manufacturer states
VANDEX SUPER	Crystalline-action penetrating waterproofing · negative pressure up to 14 bar · protection against seawater and chemically aggressive groundwater · approved for drinking water
CERESIT CR 90	Crystalline slurry; positive side up to 15 m, negative side up to 5 m water column; seals capillary cracks up to 0.4 mm; vapour-permeable

They are applied to sound concrete, fresh but not waterlogged, and are kept damp for as long as specified by the manufacturer. Where an additional cementitious layer is required, select a material with a specified strength on the negative side — VANDEX BB 75 (negative pressure 7 bar) or Ceresit CR 65 — from the same product family as the crystalline coating.

Stage 3 — Moisture measurement and primer for the barrier

The residual moisture content of the concrete is measured prior to the first organic coat. The CERESIT R 755 epoxy primer seals concrete floors with moisture content up to 7 per cent CM and cement screeds up to 6 per cent CM. For rising damp, two coats are applied in a criss-cross pattern, with the second coat applied no later than 48 hours after the first has dried.

Stage 4 — Levelling

An epoxy finish requires a smooth and level substrate. Ceresit DH Maxi is a high-strength, self-levelling compound for layers of 3–30 mm in interior applications. CERESIT CN 76 covers thicknesses of 4–50 mm, both indoors and outdoors, where greater thicknesses are required.

Stage 5 — Epoxy finish

Priming with SINMAST S2 WB, a water-soluble, solvent-free epoxy resin, virtually odourless — important in basements with limited ventilation. Top coat with a two-component, solvent-free epoxy paint, SINMAST RM 32 or SINMAST RM 22, in a RAL colour of your choice: impervious to liquids, abrasion-resistant, easy to clean.

Mistakes that cost

Incorrect	Consistency
Liquid-applied coating on the negative side	Delamination and blistering — the water pushes it from behind
Crystalline coating over old paint or cement screed	It does not reach the pores; the reaction does not take place
Epoxy without moisture measurement	Bubbles and delamination during the first winter
Omission of the floor-to-wall joint	Water bypasses the waterproofing along the perimeter
New slab without membrane protection	Holes caused by reinforcement and spacers, which are not repaired after concreting

The sequence of works on an existing raft foundation

1. Diagnosis: points of ingress, water level, condition of concrete.
2. Cleaning of sound concrete — removal of coatings, loose parts and cement screed.
3. Sealing active ingress points with quick-setting mortar and a drip channel at the floor-to-wall joint.
4. Crystalline waterproofing on a damp substrate, with the required wet curing.
5. Cementitious layer on the negative side, where required.
6. Moisture measurement and primer in two cross-layers.
7. Self-levelling compound to the required thickness.
8. Epoxy primer and paint.

New Radier · Trough & Joints

Existing Radier · Negative Side

Levelling & Epoxy Finish

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