Rubber waterstops come in three types according to where they sit: buried (centred in the concrete), surface-mounted (bonded to the water-facing face) and steel-edge (buried, with galvanised steel strips on both flanks). The names suggest a difference of position only. In practice pressure resistance, sequence of work and failure mode all differ. Choosing the wrong type costs far more in leak repair than the price difference in material.
What actually separates them
Buried. The waterstop sits inside the concrete with its centre bulb on the joint centreline, so half is cast into each side. The concrete grips the rubber and forms a continuous impermeable barrier.
Surface-mounted. Also called external or face-mounted. It is not embedded but bonded or fixed to the outer face of the cast structure, then overlapped with the waterproofing membrane.
Steel-edge. A buried type with galvanised steel strips vulcanised onto both flanks. The strips are perforated so concrete flows through and forms a mechanical interlock, closing the path along the rubber–concrete interface.
Selection table
| Item | Buried | Surface-mounted | Steel-edge |
|---|---|---|---|
| Position | Centred inside concrete | Bonded to water-facing face | Centred, steel strips both sides |
| Pressure resistance | Medium to high | Low to medium | High |
| Typical use | Tunnel, metro, culvert joints | Repair, secondary sealing | Critical high-pressure joints |
| Installation | Medium; must be fixed against displacement | Simple; bond in place | Demanding; steel may be tack-welded, rubber must never be welded |
| Material cost | Medium | Low | High |
| Usual failure | Displacement during casting | Bond failure, voids behind | Steel corrosion, rubber–steel separation |
Which figures matter
GB 18173.2-2014 governs hardness, tensile strength, elongation at break, compression set and tear strength. Measured values from a 450×12 buried waterstop we sent for third-party testing in 2026:
| Property | Measured | Requirement |
|---|---|---|
| Hardness (Shore A) | 61.4 | 60 ±5 |
| Tensile strength | 17.2 MPa | ≥15 MPa |
| Elongation at break | 451% | ≥380% |
| Compression set, 23 °C × 168 h | 15.5% | ≤20% |
| Compression set, 70 °C × 24 h | 24.6% | ≤35% |
| Tear strength | 36.7 kN/m | ≥35 kN/m |
| Brittleness temperature | No failure at -45 °C | ≤-45 °C |
Elongation at break and compression set are the two figures worth chasing. Low elongation means heavy filler loading and poor elasticity — the waterstop snaps as soon as the joint opens. High compression set means the rubber cannot spring back, so after a few thermal cycles it no longer seals and water finds the interface.
Hardness is not a case of higher being better. A stiff compound (above about 70 Shore A) does not follow the concrete and is more likely to debond at the interface.
Reading the designation
The usual designation is type plus width × thickness: 300×8 means 300 mm wide, 8 mm thick, the most common size in practice. Wider options include 350×8, 350×10 and 400×10.
Type 651 is a single-bulb buried section; type 652 has two bulbs. The bulbs provide room for movement and lengthen the anchorage path. Always select to the type and size stated on the design drawing rather than substituting a section.
The three installation mistakes that cause leaks
Off-centre placement. Displacement during casting is the single most common cause of failure. Once the waterstop shifts, one side is under-embedded and water tracks along the interface. Fix it with added rebar or dedicated clamps at no more than 500 mm centres.
Fixing with nails. Never. Every penetration is a leakage path. Clamps and tying wire are the only acceptable methods.
Poker vibrator against the waterstop. Vibrate thoroughly but keep the poker clear of the rubber, working symmetrically from both sides so concrete flows around it.
Jointing
Waterstops cannot be supplied in one piece for a whole project, and the splice decides whether the barrier is continuous.
- Cold splice. Grind the cut faces back 5–10 cm, apply dedicated cold-splice adhesive and press. Fast, adequate for ordinary work.
- Hot vulcanised splice. Vulcanise in a heated mould with uncured sheet at 140–150 °C on site. Splice strength approaches the parent material; mandatory on important works.
A site where splices are simply overlapped and tied can be assumed to leak.
When to specify two barriers
For high groundwater pressure, large movement, or locations where repair is extremely costly (metro station-to-running-tunnel interfaces, deep tunnels, large culverts), the standard practice is buried plus surface-mounted: the buried waterstop carries the primary seal, the surface-mounted one provides an external secondary barrier, and the cavity between them can be grout-injected. A single barrier leaves no margin for error.
Send us the drawing and we will match the section and supply the corresponding test report.