Q: I'm 4000 square feet into an 11,000-square-foot stamped concrete driveway. With impending cold weather, I'm wondering if I could wait until concrete is hard enough, broadcast hay, then cover it with insulating blankets? My idea is that the hay would keep the blankets from resting on the stamped surface and prevent discoloration, but I'm concerned about hay staining. Is there any proven method to protect decorative concrete from freezing?
A: There is no foolproof method for covering colored concrete without leaving markings or staining. One way we've seen used is to cover the surface with 1-mil plastic, lay curing blankets over that, and then cover with plastic again weighing everything down so that cold air can't find its way into any part of the slab. But efflorescence was always a problem afterwards. If you sealed the slab carefully to keep outside air away from the surface of the concrete, and if you could wash and scrub the surface carefully after you took the cover off, staining due to efflorescence might be minimal.
You could also try laying down felt paper first (with no asphaltic binders in it), then add the layer of straw you mentioned to give water a chance to evaporating curing blankets and plastic cover over that. This might help keep water from collecting on the surface of the concrete. Water coming from the slab is bringing calcium hydroxide up with it. When this material reacts with carbon dioxide from the air, calcium carbonates are formed (efflorescence). This is the reaction that you are trying to control.
Senior editor Joe Nasvik says that he's had luck using winter admixtures such as Pozzutec 20 by Master Builders, or Excelguard 90 by Euclid. "By placing the right dosage of the admixture in concrete before placing it, you can provide protection for your slab down to about 20 degrees through the first 24 hours. (I've actually seen these admixtures used when the temperature went considerably lower than that, but the manufacturers recommend using it only down to a low of 20° F.) This concrete quickly builds its strength to about 1000 psi, which is enough to handle freeze-thaw damage. You don't cover the slab when you're done stamping, so that part of the problem is solved. These admixtures produce little bleed, so there isn't much efflorescence activity either. You should see your ready-mix rep for more information about this. These admixtures are expensive, so arrange your contract to include them as an extra."