The local sand in our ready mixed concrete contains lignite. When we are doing flatwork the lignite works its way to the top and later it causes discoloration and small popouts. Would a topping made of a different sand be the best solution?
ASTM C 33, "Standard Specification for Concrete Aggregates," permits a maximum of 0.5 percent lignite in aggregate used in flatwork or other concrete where surface appearance is important. (For other concrete as much as 1.0 percent is permissible.) If your sand has more than 0.5 percent lignite, as it undoubtedly does, you should either obtain another sand or ask that this sand be beneficiated to remove lignite. If you have no other alternative to using this sand the concrete should be properly air entrained and placed at the lowest possible slump. Such concretes will bleed very little. It appears that the lignite is being brought to the surface by the bleeding process. A topping could be used to eliminate the condition but this twocourse method of construction is costly when compared with onecourse construction.