Question: Why do manufacturers limit the content of air in concrete in floors that are going to receive a dry-shake metallic floor hardener?

Answer: Master Builders Inc., a manufacturer of dry-shake metallic floor hardeners, recommends a 3% limit on entrained air in floors that will receive its product. When applying a dry-shake metallic floor hardener, it is necessary to have some moisture at the surface of the slab so the shake can be thoroughly worked in. This is more difficult with air-entrained concrete because of the air content's effect on bleeding. If thorough incorporation of the dry shake is not achieved, scaling can result. Excessive air contents also can lead to other surface defects, including blistering. Blisters are caused by air and water pockets trapped beneath the dense, troweled surface of the slab. Because of the prolonged bleeding associated with high air content, finishers usually begin floating and troweling too soon, thinking that the bleeding has stopped. If the surface is sealed too early, further bleeding can lead to blisters.