Choosing expansion joint sealants
Choosing the right expansion joint sealant depends on the joint's movement range, the substrate material and actual application conditions; this article summarizes the criteria that help engineers and contractors make fast, accurate decisions.
Expansion joints are found in almost every building: between concrete floor slabs, at panel joints, wall bases, door frames and facades. An unsuitable sealant will crack, debond at the edges or sag after just a few seasons of sun and rain, opening a path for water and dust to enter.
Movement capability: the most important parameter
Movement capability expresses how much tension and compression the sealant can withstand relative to the original joint width. A sealant rated at ±25% means a 20 mm joint can open to 25 mm and close to 15 mm while the sealant continues to perform well.
| Sealant type | Movement capability | Paintable | Typical applications |
|---|---|---|---|
| Neutral-cure silicone | ±25% to ±50% | No | Facades, glass, aluminum |
| Polyurethane | ±25% | Yes | Concrete floors, panels, traffic joints |
| MS polymer | ±20% to ±25% | Yes | Multi-material, interior and exterior |
| Acrylic | ±7.5% to ±12.5% | Yes | Interior joints, small cracks |
| Polysulfide | ±25% | Yes | Water tanks, areas exposed to fuel |
Substrate compatibility
Silicone adheres very well to glass and aluminum but struggles to bond to concrete without a primer, and it can leave stains on porous natural stone. PU and MS polymer adhere well to concrete, wood and painted metal. For floors with vehicle traffic, choose a sealant with a Shore A hardness of 35 or higher so it is not torn by wheels.
Designing joint dimensions
- Width-to-depth ratio of about 2:1 for joints narrower than 25 mm.
- Minimum sealant depth of about 6 mm.
- Use a foam backer rod to control depth and prevent three-sided adhesion.
- The sealant should bond only to the two joint faces so it can move freely.
Application conditions
Surfaces must be dry, clean and free of oil, with temperatures between 5 and 40 °C. Seal joints at the time of day when the temperature is around average so the joint is at its midpoint, and avoid sealing when the joint is fully open or fully closed.
After dispensing, tool the sealant surface with a spatula or tooling stick to press it firmly against both joint faces and create a slightly concave surface that performs better when the joint opens. Applying masking tape along both edges before sealing gives a crisp sealant line and saves cleanup time.
Contact AMS Vietnam to receive a joint sealant selection chart and samples for your project.
