One of the keys in good concrete work is keeping the surface moist during the initial curing time. Concrete curing is not not a matter of drying but rather a chemical reaction that uses water. If the concrete dries out then the reaction stops and we would end up with weaker, dustier concrete. In the summer with higher temperatures and sunlight this is much more of an issue than it is this time of year. If we were pouring in July or August, as I intended, we would be working a lot harder to keep the concrete moist. As you can see here, Will is having a bit of fun making water waves in the air as he splashes water across the slab.
In the background are sheets of pink foam. When not wetting the concrete we have these and hay down on the surface of the slab to help retain the heat that the concrete generates. Is an exothermic reaction so the concrete is actually keeping itself warm. We’ve been fortunate that the temperatures have been quite warm since we poured. There is also pink board in all of the formwork, again, in part, to help retain the heat to keep the concrete warm as it cures. If the concrete gets too cool the curing stops. If the concrete freezes the ice crystals can damage the concrete making it spall, flake and weaker – something we definitely want to avoid.
A mentioned before, this is our first ever cold weather pour. For a control test, I poured another section of footer using the same batch of concrete completely separate from the cottage and did not insulate it. This used up the extra concrete and will provide a base for a future footer. It contains rebar and fiber so even if there is cracking or other damage it won’t be an issue. This lets me observe the properly done section, the cottage, vs the straight pour in the uninsulated footer. So far I see no difference. It did freeze Tuesday night (54째F/28째F) so that fresh concrete has been exposed to a night sky and freezing conditions as well as bright warming sunlight during the day.
Why was there extra concrete you ask??? Well, I added 10% to my estimate for the concrete needed. I figure it was far worse to be shy of concrete than to have a little too much. Turned out I was dead on in my math so I used that 10% extra for that experiment. I had this experiment picked out ahead of time just incase I was right on my cubic-yardage estimate. It isn’t wasted since it will be part of a future section and it is poured in a place that it won’t matter about the cold joint. I left the top surface very rough so it will bond with the next layer above. Additionally it is bonded directly to the ledge making it that much stronger. Think of it as a rock in just the right place.
Dotting every t and crossing every i – your government at work. -WJ
57째F/47째F Partly Sunny, 2″ rain last night