Application
Sealants for concrete, tunnel and drainage joints
Concrete moves. A joint sealed at 20 °C is compressed at 35 °C and pulled open at −10 °C, and the bead has to follow both without losing adhesion at the flanks. Two questions decide the grade: how much movement the joint carries, and whether water is pressing against it.
KA340A or KA340B: movement first, water second
KA340A is the movement grade: 615% elongation, 75% elastic recovery and 2.5 MPa tensile, for expansion joints in concrete floors, plaza decks and bridge abutments where the gap opens and closes with temperature.
KA340B is the water grade: 550% elongation, 80% recovery and a higher modulus, for joints that see hydrostatic head — tunnel linings, drainage channels, precast panel joints and basement walls. The lower elongation is not a downgrade; a stiffer bead resists being pushed out of the joint under pressure.
The trap is specifying by elongation alone. A 615% sealant in a tunnel joint under 3 bar will fail at the flanks before it reaches its elongation, because the failure is adhesion under pressure, not cohesive tearing.
Joint geometry decides more than the data sheet
The sealant only works within the range the joint allows. A 20 mm joint with ±25% movement is a 25LM class joint under ISO 11600; the same sealant in a 5 mm joint has four times less absolute movement to absorb.
Width-to-depth matters as much: a bead deeper than it is wide is constrained on three sides and cannot stretch. The usual rule is depth between 0.5 and 0.7 of width, with a backer rod to set the depth and prevent three-sided adhesion. In a 20 mm joint that is 10–14 mm of sealant over a rod.
Concrete must be dry and mature. Sealing green concrete traps mix water and the bead lifts within weeks; 28 days is the usual minimum, and a moisture test on the joint face is cheaper than a rework.
Water pressure, waterstops and the sequence
Below about 1 bar, a correctly applied KA340B bead with good flank adhesion holds on its own. Above that, the joint needs a waterstop or an injection hose behind the sealant: the sealant is the first line, not the only one.
Sequence matters in tunnels: waterstop cast into the joint, injection hose fixed behind it, then the sealant bead after the concrete has cured. Sealing before the hose is installed means cutting the bead out later.
Drainage joints are the opposite case: they carry water rather than resist it, and the sealant there is a fillet that keeps water in the channel and stops the edges spalling. KA340A is the grade for that detail.
Models for this job
KA340A
Construction Sealant
Flexible sealant for concrete joints, tunnels and drainage.
2.5 tensile · 615 elongation
KA340B
Construction Sealant
Low-movement, water-pressure-resistant grade for tunnels and drainage.
2.0 tensile · 550 elongation
Typical values from the catalogue. Joint class, width and depth come from the project engineer, not from the sealant supplier; confirm them before ordering.
Questions that come up
Can I seal a joint in new concrete?
Not immediately. Concrete should be at least 28 days old and surface-dry, because mix water still leaving the slab will lift a fresh bead. If the programme forces an early seal, use a vapour-permeable temporary seal and return later.
Which grade for a tunnel joint under water pressure?
KA340B, and only with a waterstop or injection hose behind it. The sealant resists pressure at the flanks; it is not designed to be the only barrier in a joint that is permanently wet.
Do I need a primer on concrete?
On dry, dust-free concrete the construction grades bond without a primer. On dusty, release-agent-contaminated or damp surfaces, prime and test a coupon first; adhesion failure at the flank is the most common cause of a leaking joint.
Other applications
- Windscreen replacement — Bonded glazing is structural. The bead holds the glass in a crash, seals against water at …
- Body shop and sheet metal — A seam sealer is judged twice: by the painter, who has to sand and cover it, and by the cu…
- Glulam and timber — PUR wood adhesive cures by reacting with moisture in the wood and the air. That makes it s…