Fire resistance by equivalent thickness: what does the number mean?
When a block’s fire resistance report says “4 hours,” the figure applies to a specific sample, not the entire product family.
What is equivalent thickness?
Hollow block isn’t a solid mass, so its fire resistance can’t be calculated from its apparent thickness. Instead, the “equivalent thickness” is calculated — the wall thickness if the block’s material were redistributed as a solid mass — and then converted into a resistance duration using the NCMA TEK method tables.
Figures issued for factory samples
- Volcanic 20×20×40 + blue center-slab insulation: equivalent thickness 122.5 mm → 4 hours.
- Volcanic 20×20×40 + 7.5 cm blue sandwich: 111 mm → 3.5 hours.
- Block 20×20×40: 97.6 mm → 2.8 hours.
- Standard volcanic 20×20×40: 87.5 mm → 2.3 hours.
- 15 cm wide block: 84.7 mm → 2.2 hours.
- 10 cm wide block: 67.5 mm → 1.4 hours.
- Hollow 20×20×40, four cores: 102 mm → 1.25 hours.
Why do two similar samples get different numbers?
Because equivalent thickness depends on the number and size of the cores and on whether there is an insulation slab and where it sits. That’s why standard volcanic and insulated volcanic are rated differently at the same size.
How to use these figures
If the drawings specify a fire resistance duration for the wall, three things must all match: block type, size, and insulation type and position. Then request the report that covers that exact assembly.
The final decision on whether the assembly is suitable for structural use rests with the design engineer or the consultant.
