“ASTM C920, Class 50. That’s what the engineer wrote.”
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. astm c920 class 25 vs class 50
The Sealant That Saved the Tower
“Marcus, listen to me. You’re on the west face of that tower, right? Direct solar gain. In July, that aluminum frame will hit 160°F. At 2 AM, it might be 40°F. That joint is screaming —moving 40, 50 percent easy. Class 25 on a west-facing curtain wall? You’ll see cracks by year two. Then water gets in. Then the insulation rots. Then the lawyers come. Don’t be cheap.” “ASTM C920, Class 50