Concrete finish normal??

[Cannonball]

Hi all.
Just finished having my monolithic pad (18'x24') placed.
Pad is 10" at edge and 6" middle, 2" foam, poly, then rebar and Pex tubing.
Placement seem to go ok. Temp. was at 0 Celsius (32 F) in the morning.
Concrete comes with hot water this time of year here. They told me that "one shot???" of calcium was added as the day time high was only going to be 8 Celsius (46 F). They were quick placing and still the thought was it was getting stiff towards the end.
Looked good when it went down, I think.
Covered it with tarps that night as it was suppose to get to freezing again.
It ended up only getting down to 4 C (40 F). When I uncovered it the finish was wet from the tarps trapping moisture.
After it dried off that afternoon it had a non uniformed look to it.
What caused this? Will it effect the strength or finish?

 

[JTS]

Looks like the normal curing process to me. I think the color will even out and become more uniform.

 

[ConCretin]

Not a problem. The variation in color is caused by water condensing under the tarp. It will fade as the concrete dries but don't let that happen right away. Keep the concrete in a continuously moist condition for at least seven days for maximum strength gain.

 

[Cannonball]

Thanks for that.

Should it be kept moist even with the temp. being so low?
Have been giving it a light misting in AM as I have been lucky that the temp. has stayed above freezing, but leaving it to dry out throughout the day so as not to have water sitting at night incase it does freeze.

Will continue to cover in the evening for the next week.

 

[ConCretin]

Generally speaking, curing is less of an issue in cold weather conditions because there is less evaporation but it's still desirable. Keep in mind that if water on the surface freezes, water in the concrete will too so it's important to protect a new slab against freezing. ACI would recommend a minimum of 3 days at 55 degrees or above before exposing the slab to freezing. You also want to avoid shocking the concrete by exposing it to the cold all at once.

From your description, I think your slab is fine but good method for curing a slab in moderately cold conditions is to cover it with poly and insulated blankets and for a week or so. The concrete will generate enough heat to get good early strength gain and as hydration tails off, the slab will cool down slowly.