Q. Occasionally my foundation contractor is not able to pour all of the concrete for a foundation at once, leaving vertical and horizontal cold joints. How do you prep the surface of the concrete after the first pour to make sure such joints are structurally sound?

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.

If you have rebar that extends out of the cold joint 30 to 40 bar diameters (at least 15 inches for #4 bar, 19 inches for #5 bar), you can tie in the rest of your rebar and continue the pour. If not, you can pin the joint. If your foundation was designed for unusual circumstances, such as high backfill pressures, high wind loads, tall walls, or long spans, then I would suggest getting the opinion of a structural engineer for the location, size, strength, depth, and placement of the pins before you continue. There are also many different epoxy bonding agents available from a variety of manufacturers (including Anti-Hydro). The better quality epoxies are expensive, but are supposed to provide a joint at least as strong as the concrete.

Nobody likes a cold joint, but they happen. If you know you’ve got another one coming, here are several things you can do.

  1. Try to avoid cold joints in the middle of the wall, where the loads are high.
  2. Let the rebar run 2 to 3 feet out of the concrete at the joint so you can tie into it when you continue the pour.
  3. If there isn’t already rebar in place where you stop the pour, put some in before the concrete begins to set. Don’t wait until the concrete has "gone off" to do this, since you won’t get a secure attachment for the pin.
  4. If possible, form a keyway in the concrete for the subsequent pour to lock into. This would be especially useful at the vertical joint you described. If this isn’t practical, like on diagonal or horizontal joints in places where you can’t reach, leave a rough, uneven surface on the initial pour.
  5. With cold joint preparation, the watchword is "clean." If the surfaces are dirty, you will not get the results you’re looking for. Cover your forms to prevent dirt from falling in. This will save you a lot of effort, since it’s very difficult to get fill dirt off of rough concrete down inside the forms.

Whatever you decide to do, check with your concrete contractor first.