Replies

1. 
   Guest_ | Jan 01, 1999 11:39pm | #1

   *
   Erik: It's good that there was insulation on the slab for ten days - the longer the concrete is kept from freezing, the better. If it freezes before it develops enough strength, the expanding ice crystals can cause cracks and surface damage. Hopefully the insulation, heat coming up from the (unfrozen?) ground, and the initial heat generated by hydration reactions in the concrete as it cured was enough to keep it above freezing. If it hasn't got well below freezing, I wouldn't worry about the snow, it will insulate the concrete from getting colder and the concrete/snow interface is probably very close to freezing.

   But if you had a lot of weather well below freezing (and clear nights are worse, causing objects to lose another 5-10 F) damage may already have been done. Up here (Kenai, Alaska) the compacted base material is warmed before hand and the concrete is kept warm after the pour with a heater under a temporary tent of plastic sheeting. Or, one can wait for spring and avoid those hassles.

   If you're going to lose sleep over this, you could take a core sample and have a concrete lab test it for strength - talk to them about what size core sample they want. But I'd guess the concrete coring will cost a few hundred (most mob/demob charges) and the concrete testing pretty cheap if you find someone who'll run one or two samples for a homeowner.

   -David

2. 
   Fred_ | Jan 02, 1999 01:45am | #2

   *
   Leave the snow on, it will help insulate. Your concrete has reached 70% of its design strength in the first 7 days, so unless you placed the slab using a low cement content mix or had a high water/cement ratio, you are probably alright. Leaving the snow on, and assuming you don't get extreme freezing temperatures (below 10d F for extended periods), your strength should be fine, I'd be most concerned with basemant walls-they have a lot more exposed surface area, harder to insulate and are much more difficult to place in cold weather. If you need guidance on cold-weather concrete placement, consult ACI (American Concrete Institute) Publication 306.

1. 
   Erik_Kolb | Jan 02, 1999 03:33am | #3

   *The footings and walls were, thankfully, placed during some unseasonably warm weather we had here in KC in November. For the floor, the ground was not frozen (had been up around 50F the previous day), the concrete was a 4000 psi, heated water mix. Since it was cold the day of the pour, I placed the blankets on the slab as soon as I was able to walk on it.

3. 
   Erik_Kolb | Jan 02, 1999 01:51pm | #4

   *
   I have a two week old basement slab which is exposed to the elements until next week when framing starts. We are starting to receive some significant snow today - is it worth my while to shovel the snow off of the slab? Will leaving snow on new slab cause any problems? When the slab was placed, a vinyl sealer was applied after finishing and insulating blankets were used for the first 10 days.

4. 
   Guest_ | Jan 02, 1999 01:51pm | #5

   *
   Erik: Sounds like you're in good shape and I'd agree: leave the snow on as it will be insulation against the colder air. -David