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Fire Rating

Fire Rating 2017-05-18T19:47:56+00:00

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:

 

Type of Coarse Aggregate

Minimum Equivalent Thickness for Ratings of:

  4 hour 3 hour 2 hour 1 hour

Expanded Clay (Lightweight)

4.7″ 4.9″ 3.9″ 2.7″

Siliceous Gravel (Standard weight)

6.2″ 5.3″ 4.2″ 2.8″

(Based on Table 7-B – Uniform Building Code…as adopted by the Minnesota State Building Code.)

 
 

Wall
Thickness
Block
Number
Block
Size
(inches)
Face
Shell
Thickness
Web
Thickness
%
Solid
Equivalent
Thickness
(inches)
Fire
Resistive Rating
(hrs)
Std. Wat.
Fire
Resistive Rating
(hrs)
LW
4″ 4816 4x8x16 1″ 1″ 75 2.7 1
4″ solid 4816s 4x8x16 solid n/a 100 3.6 1 1
6″ 6186 6x8x16 1″ 1″ 57 3.2 1 1
6″ fire 6186f 6x8x16 2″ 1″ 87 4.9 2 3
8″ 8816 8x8x16 1 1/4″ 1″ 52 4.0 1 2
8″ fire2 8816f 8x8x16 1 5/8″ 1″ 64 4.9 2 3
8″ fire3 8816f 8x8x16 2 1/8″ 1 1/2″ 77 5.9 3 4
8″ solid 8816s 8x8x16 solid n/a 100 7.6 4 4
10″ 10816 10x8x16 1 1/4″ 1 1/8″ 51 4.9 2 3
12″ 12816 12x8x16 1 1/4″ 1 1/8″ 45 5.2 2 3
12″ heavy 12816h 12x8x16 1 1/2″ 1 1/8″ 47 5.5 3 3
12″ fire3 12816f 12x8x16 1 5/8″ 1 1/4″ 47 5.5 3 3
12″ fire4 12816f 12x8x16 2 1/8″ 1 1/2″ 57 6.6 4 4
14″ 14816 14x8x16 1 5/8″ 1 1/4″ 46 6.3 4 4
16″ 16816 16x8x16 1 1/2″ 1 1/4″ 44 6.9 4 4