Q.: Last fall we placed a shop floor in an enclosed but unheated farm building here in North Dakota, using heaters during construction and the remainder of the first 24 hours because the outdoor temperatures were in the 40s and 50s Fahrenheit. The concrete was a 5 1/2-bag cement-and-fly ash mix with a 4- to 5-inch slump (estimated, not measured), air-entrained, but with no other admixtures. Within 7 to 10 days we began having a few popouts and in the next 2 or 3 weeks we had many. We attribute the trouble to shale particles. Our state limits the shale particles to 0.7 percent of the aggregate in concrete used for highway work. Is there a way to determine whether the aggregate in our concrete exceeded 0.7 percent shale particles?
A.: Assuming that there is some way in which the state specifications govern your job, there is a way to determine the shale content, though it may be considered a little expensive. It involves taking at least one 4- or 6-inch core containing a popout, plus some chips from around some of the other popouts, and sending them to a qualified petrographer. It would also be desirable to diagram a typical portion of the floor, showing the locations of popouts. For about $500 to $700 the petrographer could determine the percentage of offending shale in the floor.
It should also be said, however, that even if the shale content were below the state requirement of 0.7 percent there could still be an objectionable number of popouts in concrete used for a floor. The 0.7 percent limitation was probably established for highways, where small popouts may not be particularly objectionable.