A. JLC assistant editor Dave Crosby, a builder in Santa Fe, N.M., responds: You can use a concrete additive called Anti-Hydro (Anti-Hydro International, 45 River Road, Flemington, NJ 08822; 800/777-1773; www.anti-hydro.com). It’s available in one-gallon jugs and five-gallon cans at most builder supply stores for about $13 per gallon. Mix one part Anti-Hydro to three parts clean water, and add enough fresh (not old, lumpy, or partially hydrated) Type I Portland cement to make a thick, rich slurry, or "slush coat." Apply this mixture liberally with a stiff brush to the clean cold joint immediately before you pour. You want this coating to be unreacted when the redimix hits it. If you’re on a big pour and the slurry has started to set up by the time you get to it, just brush on more. This will result in a strong, continuous bond.
Anti-Hydro increases the percentage of hydration in your concrete without affecting its chemical composition, resulting in a denser, harder, stronger, and more waterproof material. It also accelerates the rate of hydration, so it has many uses with all Portland cement-based products.
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