Q.: Your recent tilt-up articles (September 1987) mentioned several ways to produce exposed aggregate surfaces, without commenting on the importance of mix design. We can't afford the fancy seeded aggregates, but what about the aggregate in our regular mix?

A.: Space limitations kept us from covering this point. Both aggregate color and mix design are important, as well as the degree of aggregate exposure desired. Larger amounts of coarse aggregate may be needed for deep exposure, and deep exposure is often preferred because it can be obtained with greater uniformity. If bushhammering is used to expose the aggregate, well-graded mixes with softer aggregates are often recommended. Try to get the advice of a concrete mix consultant or buy the concrete from a ready mix supplier whose mixes have been not necessary. They seem to have enough field experience to support their claims. However, designers of parking structures, particularly in municipal work where maintenance budgets are limited, frequently take a "belt and suspenders" approach. At times they also specify corrosion-inhibiting admixtures in the concrete surrounding epoxy coated bars.

Remember, for an effective repair job the overlay must be well bonded to the old concrete. This means that its surface must be absolutely clean and free of foreign matter, including particles of epoxy. Carefully clean up any spills or dripping of the epoxy from the rebars onto the old concrete before placing the overlay.