Q.: We are building hoppers in a 15-foot-square grain dump pit. The floors of these hoppers slope at a 45-degree angle, and to get the concrete to stay in place on this slope we started with a 6-sack mix at 3-inch slump and the foreman has been adding lime to make the mix stickier. This works pretty well for the purpose but we now have trouble getting a hard, smooth finish. Our finishes are grainy and sandy. Is there any admixture or coating that would solve our problem?
A.: The lime is making the plastic concrete sticky but preventing the production of a hard surface. You might try eliminating the lime but add both fly ash and cement to increase the stickiness yet make it possible to get a hard surface.
If this method doesn't work you might just coat the surface with a hard epoxy. The choice of a suitable epoxy should be made in consultation with a knowledgeable supplier.