Joint movement class: the number that decides the sealant

A sealant is not chosen by how far it can stretch in a laboratory. It is chosen by how much the joint will move in service, expressed as a percentage of the joint width at installation.

The calculation

movement class = (expected movement / joint width) × 100

A 20 mm joint that opens 5 mm is a 25% joint. A 10 mm joint that opens 5 mm is a 50% joint — the same absolute movement, twice the demand, because the sealant has half the width to distribute it over.

This is why “we used a high-movement sealant” is not an answer. The joint width sets the class, and the class selects the product.

What the classes mean

ISO 11600 classifies sealants by movement capability and modulus — for example 25LM (25% movement, low modulus), 25HM (25%, high modulus), 20LM, 12.5E (12.5%, elastic). ASTM C920 uses a comparable class system for elastomeric joint sealants.

The practical reading: a 25LM product is designed for joints that move up to 25% and is soft enough to follow them. A 12.5E product is for joints with less movement and more elasticity requirement. Using a 12.5 product in a 25% joint is the most common specification failure in façade work.

Elongation is not movement class

A sealant with 600% elongation at break can still be rated for 25% joint movement. The test for movement class cycles the joint repeatedly and measures adhesion and recovery; the elongation test pulls a cured specimen to failure once. One is fatigue, the other is strength.

That is also why elastic recovery matters: a bead that stretches to 600% but recovers only 40% accumulates strain with every cycle.

Where buyers get caught

• Design movement not stated. If nobody calculated it, the default is “assume 25%” — which may be wrong in both directions.
• Joint width changed on site. A 25 mm joint built as 15 mm doubles the movement class without changing the drawing.
• Temperature at installation. A joint installed at maximum width has no compression reserve in summer.

Worked example across a year

Take a 6 m concrete panel, a 20 mm joint, and a 40 °C swing between the coldest installation day and the hottest service day.

movement = 6,000 mm × 40 °C × 10 × 10⁻⁶ = 2.4 mm
class    = 2.4 ÷ 20 = 12%

A 12.5E or a 20LM product covers that joint with margin. Now double the panel length to 12 m:

movement = 12,000 × 40 × 10⁻⁶ = 4.8 mm
class    = 4.8 ÷ 20 = 24%  → 25LM

Same sealant, same joint width, twice the demand — because the panel length doubled. This is why the movement class belongs on the drawing next to the panel layout, not in the sealant specification alone.

The swing that matters is not the annual average; it is between the temperature at installation and the extreme the joint will see. A joint sealed in a 35 °C July and exposed to −10 °C in January sees 45 °C of movement, not 25 °C.

Why the class test is a fatigue test

The ISO 11600 and ASTM C920 class tests cycle the joint — extend, hold, compress, hold — many times, at different temperatures, and then inspect adhesion and cohesion. That is a different question from “how far does it stretch before breaking”.

A product can pass a 600% elongation test and fail a 25% cyclic test, because the failure mechanism in service is adhesion fatigue at the bond line, not tensile rupture in the middle of the bead. When a supplier quotes elongation as evidence of movement capability, the right reply is: “show me the cyclic test at the class you are claiming.”

What this means for buyers

1. Ask for the calculated movement class in writing before the sealant is selected.
2. Match the product class to the calculated movement, not to the widest number in the catalogue.
3. Check elastic recovery as well as elongation.
4. Record the joint width and installation temperature — both change the class in service.

Standards

• ISO 11600:2002 — classification by movement capability and modulus.
• ASTM C920 — the US specification, same idea with a different class table.