Pat laughed. “Because I don’t trust the installation crew to keep the joint width perfect. Class 50 forgives a ½-inch joint that’s actually ⅝ inch. Class 25 demands precision. Also—check the building’s wind-sway calculation. At the 30th floor, the deflection is 2 inches. That joint is moving ±45% on the corners. Class 25 would be at 180% of its limit. It’s not a matter of if it fails—it’s when .”
But for the tower that had to survive Seattle’s wind and sun? Only Class 50 would do. astm c920 class 25 vs class 50
Marcus knew the standard by heart. ASTM C920 was the bible for elastomeric joint sealants. It defined performance for movement—the ability of a sealant to stretch and compress as the building breathed, swayed, and thermally cycled. Pat laughed
But then Elena added the poison pill: “The supplier can’t get Class 50 in the required color for three weeks. They can ship Class 25 by tomorrow.” Class 25 demands precision
The Sealant That Saved the Tower