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Fire resistance of concrete masonry units is determined by the "equivalent thickness" of the block and the type of aggregate. The equivalent thickness is the theoretical thickness of the block if all the concrete was molded into a solid unit. It is calculated by multiplying the actual thickness by the percent of solids. As an example, an 8" block with 52% solids has an equivalent of 4.0 in. (7.625 x 52% = 4). The greater the equivalent thickness, the greater the fire rating.

Lightweight aggregate (expanded clay) has better fire rating than standard weight block (siliceous gravel). As an example, an 8" standard weight block has a 1 hour rating and an 8" lightweight block has a 2 hour rating. Filling the cores of the block with grout or other non-combustible material will increase the fire resistance. Applying plaster of gypsum wallboard will also increase the fire resistance.

Estimated Fire Resistive Periods of Walls and Partitions of Hollow Concrete Masonry: