We have received a specification that refers to Grade A and Grade B concrete to be used for tiltup. We have never heard of such grades and haven't found anyone who has. What do they mean ?
These terms came into use in the 1920's and are still being used in building codes. They refer to the aggregates used in the concrete. These have been divided into grades according to their fire resistance . Grade A (also known in some codes as Group I) aggregates include blast-furnace slag, limestone, calcareous gravel, traprock, expanded clay, expanded shale and expanded slate, as well as cinders containing not more than 25 percent of combustible material and not more than 5 percent of volatile material, and other materials meeting the requirements of the code specifications and containing not more than 30 percent quartz, chert, flint and similar materials. Grade B (also known as Group II) aggregates include granite, quartzite, siliceous gravel, sandstone, gneiss, cinders containing more than 25 percent but not more than 40 percent of combustible material, and other materials meeting the requirements of the code specifications and containing more than 30 percent of quartz, chert, flint and similar materials. Concretes made with Grade B aggregates are given a lower fire resistance rating. The Grade A and Grade B classifications now appear to be unrealistic, based on the findings of fire research investigations of the past twelve years or more.