The fire-resistance rating of concrete masonry units composed of a combination of aggregate types shall be based on equivalent thickness values determined as follows:

Tr = T1 x V1 + T2 x V2... = Tn x Vn

where:

Tr - The minimum required equivalent thickness corresponding to the desired fire-resistance rating as listed in Table 7-B for C.M.U. manufactured with a particular combination of aggregate types.

T1, T2,...Tn - The equivalent thickness for each aggregate Type 1,2,...n, respectively, used as indicated in Table 7-B for the desired fire-resistance rating.

V1, V2,...Vn - The percentage by volume of each aggregate Type 1, 2,...n, respectively, which is used in the manufacture of the C.M.U.