Masonry Magazine January 1966 Page.26
Fire resistance of concrete masonry walls is a function of aggregate type and equivalent thickness of the masonry units
The fire resistance ratings of most concrete masonry walls are determined by heat transmission measured by temperature rise on the cold side. Few walls fail due to load during the fire test or during cooling under the fire hose. Fire endurance can be calculated as a function of the aggregate type used in the block unit, and the solid thickness of the wall, or the equivalent solid thickness of the wall when working with hollow units. Results of research relating fire resistance in hours to the equivalent thickness of hollow concrete masonry of various aggregate types is shown below.
Equivalent thickness of hollow units is calculated from actual thickness and the percentage of solid materials (see calculation of equivalent thickness on next page). Both needed items of information are normally reported by the testing laboratory using standard ASTM procedures, such as, ASTM C 140 "Methods of sampling and testing concrete masonry units." When walls are plastered or otherwise faced with fire-resistant materials, the thickness of these materials is included in calculating the equivalent thickness effective for fire resistance.
Estimated fire resistance ratings shown in the table on the opposite page are for fully protected construction in which all structural members are of incombustible materials. Where combustible members are framed into walls, equivalent solid thickness protecting each such member should be not less than 93 percent of the thicknesses shown. Plaster is effective in increasing fire resistance when combustible members are framed in a concrete masonry wall if it is applied on the fire side of the wall, opposite the combustible member. Fire resistance of hollow unit concrete masonry walls can be increased also by filling core spaces with various fire resistant materials.