Under-ordering stops the site; over-ordering leaves drums that expire. Judging the cure time too early means the bead is flooded before it cures and the whole joint has to be cut out. Both problems are common on site and both are solved by one formula and three time points.
The dosage formula
Sealant consumption depends on three things only: joint width, joint depth and compound density.
kg per metre = width (mm) × depth (mm) × density (g/cm³) ÷ 1000
Measured density is 1.6 g/cm³ for polysulfide and 1.52 g/cm³ for two-component polyurethane. Use 1.5–1.6 for estimating.
Worked example: a 20 × 20 mm joint
20 × 20 × 1.6 ÷ 1000 = 0.64 kg/m
Add 5–10% for application loss: order about 0.67–0.70 kg/m.
Common joint sizes
| Width × depth | kg/m at 1.6 g/cm³ | Order with 8% loss |
|---|---|---|
| 10 × 10 mm | 0.16 | 0.17 |
| 15 × 15 mm | 0.36 | 0.39 |
| 20 × 20 mm | 0.64 | 0.69 |
| 25 × 25 mm | 1.00 | 1.08 |
| 30 × 30 mm | 1.44 | 1.56 |
| 20 × 30 mm | 0.96 | 1.04 |
Two-component products ship as separate A and B components. Calculate on the combined mixed weight, not on the A component alone — this is the most frequent cause of mis-ordered quantities.
Deeper is not stronger
A thicker bead feels safer but behaves worse. The sealant only works if it can stretch when the joint opens. Excessive depth concentrates stress at the bond line and promotes debonding.
Normal practice: depth = 0.5–1.0 × width, and not more than 20 mm. Where the design calls for more, place closed-cell PE foam board as a backer to bring the actual bead depth to the design value and to prevent three-sided adhesion.
The backer must be closed-cell. Open-cell foam absorbs water and wicks it to the bottom of the joint.
Three time points, not one
Skin formation. Surface films over and stops being tacky. Measured 10 h for polysulfide and 5 h for two-component polyurethane (both required ≤24 h). A skinned surface says nothing about the interior.
Full cure. At 25 °C, polysulfide needs 3–7 days and two-component polyurethane about 7 days to reach tensile strength and waterproofing performance. Lower temperature and higher humidity both slow it down. At around 5 °C in winter, polyurethane can take 10–14 days.
Watertight, backfillable, trafficable. Only after full cure.
| Stage | Polysulfide at 25 °C | 2K polyurethane at 25 °C |
|---|---|---|
| Skin formation | about 10 h | about 5 h |
| Handling strength | 24 h | 24 h |
| Full cure | 3–7 days | 7 days |
| Fill / traffic | after full cure | after full cure |
Basins and canals must not be filled until the sealant is fully cured. Water on an uncured bead seals the surface and the interior never reaches design strength — effectively an untreated joint.
Mixing is not negotiable
Two-component products are normally mixed A:B = 10:1 by weight, with some batches between 100:8 and 100:12. Follow the product datasheet.
- Weigh on a scale. No volume ratios, no eyeballing. Keep error within ±1–5%.
- Mix with a power stirrer for 3–5 minutes until the colour is uniform with no pale streaks.
- Mix only what can be placed within the tooling life. 8.5 h for polysulfide, 3 h for two-component polyurethane.
The consequences of a bad ratio are predictable: too little B and the bead stays soft and never cures; too much B and it cures brittle, loses elasticity, and polysulfide may suffer reverse vulcanisation with no remedy.
Two things sites overlook
The substrate must be dry and free of laitance. Laitance has almost no strength, so sealant bonded to it is bonded to dust. Grind back with a wire brush or angle grinder and blow out the dust. Damp substrates need mechanical drying, not a rush to beat the programme.
The joint must be free of oil and grease. Oil destroys the bond interface, and the metres that debond are usually the ones that were contaminated.
Get dosage, ratio and curing right and the leak rate drops by an order of magnitude. The remaining variable is grade selection: see polysulfide versus polyurethane.