Elastomeric sealants are classed under ASTM C920 by movement capability, such as Class 12.5, 25, 35, 50 or 100/50, meaning they handle that percentage of extension and compression of the installed joint width. A 1-inch joint with a Class 25 sealant can open or close a quarter inch. Designers size joints by dividing expected movement by the sealant class and adding tolerance.
Calcs show 3/8 movement on a 3/4 joint, that's 50 percent, so a class 25 won't cut it.
Movement class is rated on the installed width, so a joint built narrower than drawn loses capacity in proportion.
Also heard as
- joint movement capability
- movement class
- plus/minus rating
Related terms
Expansion joint
A designed gap between building elements that lets them move with temperature, settlement and load without cracking.
Modulus
How stiff a cured sealant is, meaning how much force it pulls on its bond faces when stretched.
Silicone
A sealant with a silicon-oxygen backbone, prized for UV resistance and high movement capability.
Width-to-depth ratio
The proportion between a sealant joint's width and the sealant's depth, usually targeted at about 2 to 1.