Q. How do I calculate the fire rating of my concrete block wall?

I often get questions from architects, masons or suppliers who need to prove to code authorities that their walls are properly fire rated. The code guys are used to seeing U.L numbers for assemblies that have been tested at Underwriters’ Laboratory. Unfortunately, if you ask for these tested assemblies you must use concrete block units that are “classified” and have been tested by the U.L. folks. The closest source of classified CMU is Boise, Idaho. I do not recommend importing block from Idaho. Not only is it a waste of transportation money, the block can be cracked and damaged by the long trip.
I tell people to use Table 7-B of the 1997 UBC or Table 719.1 of the 2000 IBC. These two tables give essentially the same information. They tell you how many inches of masonry you need to achieve a given fire rating.
There is a slight twist to this chart. If you read the footnotes in the table you will find that the code is interested in “equivalent thickness” of the concrete block, not actual thickness. Equivalent thickness is essentially the solid thickness that would be obtained if you recast the unit using the same height, same length and same amount of concrete, but eliminated the core holes. You can contact the block supplier to get the equivalent thickness of his product or you can consult Tek Note 7-1a from the National Concrete Masonry Association. According to this Tek Note, typical equivalent thicknesses are:
4” CMU 2.7” equivalent thickness
6” CMU 3.1” equivalent thickness
8” CMU 4.0” equivalent thickness
10” CMU 5.0” equivalent thickness
12” CMU 5.7” equivalent thickness
If you fill all the core holes with a non-combustible material, you can count the actual thickness as the equivalent thickness. Acceptable fill materials are grout, sand, pea gravel, crushed stone, pumice, scoria, expanded shale, expanded fly ash or perlite.
Your last challenge is to determine which category of concrete block you are using. The aggregate used in your block will define which classification you need to use. Again, contact the manufacturer and ask which aggregate is specified for your project. Pumice and expanded shale are the two most commonly used lightweight aggregates. Calcareous or siliceous gravel aggregates are used to manufacturer normal weight CMU.
Using the chart in the Code you will see that all lightweight block mixes will give you
1-hour rating with a 4” deep block
2-hour rating with an 8” block
Normal weight block (also called sand-and-gravel block) is not as efficient at dissipating the heat of the fire. You need more thickness to achieve a given fire rating
1-hour rating requires a 6” deep block
2-hour rating requires a 10” deep block
If you need to achieve a 4-hour fire rating you can either use a 12” deep lightweight block or you can fill all the core holes with sand, grout or perlite. If you fill all the cells:
4-hour rating requires a 6” deep lightweight unit
4-hour rating requires an 8” deep normal weight unit.