Polyurea: spray-applied waterproofing for concrete and steel

What is polyurea?

Polyurea is an elastic membrane formed when two components are mixed: an isocyanate and an amine blend. The reaction takes seconds: the gel time of the materials in the catalogue is 5 to 15 seconds, the surface can be walked on after minutes, and full cure of Sikalastic-841 ST, for example, is reached within a day at +20 °C. That is why polyurea is not applied by roller: the components are heated and delivered under high pressure by a plural-component hot-spray rig, and they are mixed in the gun at the moment of spraying.

Hence the strengths of the material: a thick seamless layer in one or two passes, including on vertical surfaces and soffits, without reinforcement or solvents; rapid return to service; high strength, elasticity and abrasion resistance. And its weaknesses: the result depends almost entirely on substrate preparation, the condition of the equipment and the skill of the operator, and a mistake made during spraying can be corrected only by removing the layer.

Pure and hybrid polyurea

Pure polyurea — an amine component without polyols. It has higher resistance to water and chemicals, and it is pure polyurea that is used for permanent immersion where the data sheet permits it. In the catalogue: Sikalastic-841 ST, Mapei Purtop 1000, Fosroc Polyurea WPE, TEKNOPUR 320-800, and Fosroc Polyurea WPW for drinking water tanks.
Hybrid (polyurethane-polyurea) — some of the amines are replaced by polyols. It is more flexible and easier to work with, but has lower chemical resistance; many hybrids are not intended for immersion — this is stated explicitly, for example, in the Mapei Purtop 400 M data sheet. Hybrids are used on bridges, car parks, roofs and below-ground structures: Purtop 400 M, Sikalastic-851.

Most spray-applied membranes are aromatic: in the sun they yellow and change shade, and some grades also chalk, although this usually does not affect their physical properties. If the colour matters or the surface is exposed, a light-stable aliphatic polyurethane topcoat is applied over the membrane within the time stated in the data sheet — for Mapei, no later than 24 hours after spraying. The Sikalastic-851 hybrid is not UV-resistant at all, and exposed areas are covered with asphalt or a protective layer.

Where it is used

bridges, overpasses and ramps — under asphalt, with systems whose compatibility with hot asphalt has been confirmed by the tests stated in the data sheet;
car park decks, accessible and flat roofs — with a wear-resistant or light-stable topcoat;
tanks, channels, flumes, treatment plants, fire water and stormwater tanks — only pure polyurea approved for immersion; for drinking water, materials with hygiene documents (see “Tanks, swimming pools and treatment plants”);
foundations, tunnels, below-ground structures — on the positive side;
steel structures and bunds, where waterproofing, corrosion protection and abrasion resistance are all needed at once — using the manufacturer’s system for steel.

How to choose the material

Immersion or atmospheric exposure

For permanent contact with water, a pure polyurea is chosen whose data sheet explicitly permits immersion. A hybrid not intended for immersion, used in a tank or channel, is a direct risk of blistering and delamination.

Chemicals and wastewater

The good chemical resistance of polyurea does not mean resistance to everything. Manufacturers assess it case by case and explicitly prohibit certain conditions: Sikalastic-841 ST, for example, must not be used in enclosed tanks where attack by biogenic sulphuric acid is possible. Resistance to the specific medium is confirmed by our questionnaire: substance, concentration, temperature and regime. For strong acids and oxidisers, solutions from the “Chemical protection” section are also considered.

Mechanical loads

Pedestrians, vehicles, abrasive flows of water carrying sand, impacts — for each load, the strength, tear resistance and abrasion resistance in the data sheet are checked, and a decision is made on whether a wear-resistant topcoat is needed. For comparison, the tensile strength of the materials in the catalogue ranges from more than 15 N/mm² for Sikalastic-841 ST to more than 20 N/mm² for Purtop 1000.

Light, colour and temperature

Exposed surfaces require a topcoat or a light-stable system. The application temperature also matters: according to the data sheets, the reaction proceeds even in the cold — Sikalastic-841 ST allows air temperatures from −20 °C, and TEKNOPUR 320-800 cures even at −20 °C — but the substrate must still be dry and at least 3 °C warmer than the dew point.

Substrate and primer

Concrete — sound, dry and clean. For Sikalastic-841 ST, for example, the data sheet sets minimum values for the substrate: 25 N/mm² in compression and 1.5 N/mm² in the pull-off adhesion test. The surface is shot blasted, sandblasted or scarified to an open porous texture, blowholes and pores are closed with the system filler, and the dust is removed with a vacuum cleaner.
Primer — epoxy or polyurethane, often with a light broadcast of quartz sand. It seals the pores from which air escapes during spraying and provides adhesion. Each primer has an overcoating window — from a few hours to several days, depending on the material and the temperature; if it is missed, the surface is sanded or a bonding coat is applied as instructed by the manufacturer.
Steel — abrasive blast cleaning and the system primer. According to the Sikalastic-841 ST data sheet, this means grade Sa 2½ and a roughness Rz of at least 50 µm, followed by a zinc-containing primer.
Bond — the main indicator of the quality of preparation. ShNK 2.03.11-24 requires the bond strength of protective coatings to concrete to be at least 1.0 MPa; the actual value is checked by pull-off testing on control areas.

Equipment and applicator

The hot-spray rig must maintain the set pressure, the component and hose temperatures and the mixing ratio — usually 1 : 1 by volume for the materials in the catalogue — along the entire length of the hose. Any deviation produces a soft, tacky or brittle layer that may look normal. That is why a trial spray is carried out before work, a log of the parameters is kept, and when the material is changed, it is checked that the isocyanate component is connected to its own line. Manufacturers state explicitly in their data sheets that the material is intended for professional use and that the applicator must have suitable equipment and experience.

Quality control

substrate moisture and temperature, dew point — before and during the works;
thickness — from the material consumption per bay and by measurements on control samples or with a thickness gauge;
bond strength — by pull-off testing;
continuity — inspection for bubbles, pinholes and holidays, especially at edges, corners and junctions;
membrane terminations — taken into a chase or under a termination bar according to the manufacturer’s details, so that the layer does not peel away from the edge.

Common mistakes

Spraying onto damp concrete or at a substrate temperature close to the dew point — blistering and delamination.
No primer, or its overcoating window has been missed.
Open pores have not been sealed, and spraying is done while the substrate is warming up — pinholes from escaping air.
The wrong component ratio or temperature — a soft or brittle layer.
A hybrid used in a tank or channel permanently filled with water.
An aromatic membrane left exposed to the sun where the colour matters.
The layer is too thin at edges, at corners and around cast-in items — that is where leaks and peeling start.

Materials in the catalogue

Sikalastic-841 ST — Sika — hot-spray pure polyurea membrane for waterproofing civil structures and tanks.
Sikalastic-851 — Sika — hot-spray polyurethane-polyurea hybrid membrane for bridge decks, car parks and buried structures.
Purtop 1000 — Mapei — hot-spray pure polyurea membrane for hydraulic works, roofs, bridge decks and tanks.
Purtop 400 M — Mapei — hot-spray hybrid polyurea membrane for bridge decks under asphalt and large roofs.
Fosroc Polyurea WPE — Fosroc — hot-spray pure polyurea waterproofing and protective coating for concrete and steel.
TEKNOPUR 320-800 — Teknos — hot-spray pure polyurea coating for waterproofing bitumen roofs and concrete.

Frequently asked questions

How does polyurea differ from a polyurethane membrane?

A cold-applied polyurethane membrane is applied by roller or brush and cures over several hours. Polyurea is sprayed with a rig, cures within seconds, builds a thick layer in one pass and is usually stronger and more abrasion-resistant. On the other hand, it requires equipment, a trained crew and especially thorough substrate preparation.

Can polyurea be applied in cold weather?

The reaction also proceeds at low temperatures, but the substrate must be dry, free of ice and at least 3 °C warmer than the dew point. The minimum air and substrate temperatures differ from material to material and are given in the data sheet.

Is a topcoat needed?

On exposed surfaces — if the colour matters or the material is not UV-resistant. On trafficked surfaces — a wear-resistant layer or asphalt according to the manufacturer’s system. In tanks, usually not, if the data sheet permits immersion of the membrane itself.

Can polyurea be sprayed over an old coating?

As a rule, no: old coatings are removed down to a sound substrate. Exceptions can be allowed only by the manufacturer — after a trial area with pull-off testing.

We will select a polyurea system, primer and topcoat for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.