# Construction joints Concrete, tunnel and drainage sealants — 550–615% elongation, elastic recovery up to 80%. Construction sealant family: KA340A and KA340B. [KA340A ### Construction Sealant Flexible sealant for concrete joints, tunnels and drainage.](https://karmindadhesive.com/products/ka340a-construction-sealant/)[KA340B ### Construction Sealant Low-movement, water-pressure-resistant grade for tunnels and drainage.](https://karmindadhesive.com/products/ka340b-construction-sealant/) ## Compare the models | Model | Tensile strength | Elongation at break | Skin time | Shore A | | KA340A | 2.5 | 615 | 65 | 35 | | KA340B | 2.0 | 550 | 40 | 48 | Typical values from the catalogue; the batch record carries the measured result. ## Choosing one Pick by movement first. A façade joint that opens 25% needs the softer KA340A at 615% elongation and 75% recovery; a below-grade or drainage joint that resists water pressure needs KA340B and its higher modulus. Movement class decides, then substrate condition, then primer. ## Where it goes wrong A joint that leaks at the flank was usually sealed green, primed on a damp face, or detailed deeper than it is wide. In a 20 mm joint the last one is easy to do: 25 mm of sealant over no backer rod cannot stretch, so it tears instead of following the movement. Elongation and recovery are different claims. 615% tells you how far the bead stretches before it tears; 75% tells you how much it comes back. A joint that stays open needs the first, a joint that cycles needs the second. ## Decision table | If | Then | Because | | Facade joint with ±25% movement | KA340A | 615% elongation and 75% recovery | | Tunnel or drainage joint under head | KA340B | higher modulus and 80% recovery | | Concrete is less than 28 days old | wait | mix water still leaving the slab lifts the bead | | The joint is deeper than it is wide | backer rod first | three-sided adhesion cannot stretch | ## Read before you specify - [movement class before product](https://karmindadhesive.com/resources/construction-joint-sealant-guide/) - [the number that picks the sealant](https://karmindadhesive.com/resources/joint-movement-class/) - [why 28 days is not a formality](https://karmindadhesive.com/resources/sealing-green-concrete/) Send joint width, expected movement and substrate and we will return the class and the grade. [martin@karmindadhesive.com](mailto:martin@karmindadhesive.com) ## Other families - [Automotive glass bonding](https://karmindadhesive.com/products/automotive-glass/) — Windscreen, side and rear glass adhesives — 310 ml cartridges, sausages and 200 L barrels. - [Body and sheet metal](https://karmindadhesive.com/products/automotive-body/) — Seam sealers for body panels, doors and container joints — sandable, paintable, four colours. - [Industrial and STP](https://karmindadhesive.com/products/industrial/) — Silyl-terminated polymer sealant for metal, plastic and glass — primerless and paintable. - [Wood and glulam](https://karmindadhesive.com/products/wood/) — One-component PUR for engineered timber, glulam beams and sandwich panels. - [Activators and primers](https://karmindadhesive.com/products/accessories/) — Surface preparation that decides whether the bead holds: K-901 activator, K-801 primer.