"A couple of months ago, we had a 5 - 8' cement slab poured for our garage foundation. We don't want to build the walls and roof of the garage until spring. To protect the slab from the elements (snow, freezing temps) over the winter, we are thinking of putting bails of hay on top of it and then placing a tarp over that and securing tarp around the edges with stones. Would that be sufficient to prevent any cracking from occurring, or is it necessary to build the garage now - before the snow?"

Did you have the concrete properly scored every so many feet or is this just one continuous slab? Also how cold does it get where you live? If I did this in Northeast Ohio where I live the well below zero conditions in the winter would crack it good.

A concrete sidewalk or driveway would not get covered so why would you think you have to protect a garage pad slab? Does the slab have any drains in it?

No drains in the slab. No scoring, either.

Concrete is not scored. The contracter thought it wouldn't be necessary. Is it too late to have this done? Maybe not necessary if we put in bails of hay and a tarp..

We will likely get temps well below zero, and may have lots of snow.

You are not going to like this answer. I'm a professional engineer and asked that very same question to several seasoned structural engineers about concrete slabs in waste water plant buildings.

The answer: Concrete will always be subject to cracking and the only resort is to design around this inherent flaw by using large amounts of reinforcement steel and expansion joints and other theoretical formulation strategies. Mostly it's the expansion joint that helps us to endure concretes' obvious inadequacies. The expansion joint is that evenly spaced crease you see in sidewalks. It's there so that when the concrete expands and decides to crack, it happens at the joint instead of the middle of the slab.

A well designed garage floor slab will have a nice quad style expansion joint dividing it into four sections, and hopefully when the slab was poured the ground was tamped to a very firm industry spec compaction level, some rebar was put in and the concrete mixture had the appropriate amount of limestone and aggregate but who knows if that done correctly or not. Improper mix is the plague of the concrete business.

So to answer your question, what should be done? I'd say it's mostly out of your control. You can build the garage over the slab before the severe winter sets in, and the slab may or may not crack anyway. Hopefully if it does decide to crack from settlement (which could be a result of temperature change), it would occur at the expansion joint, if it has them. Some builders do not include them. That's a design mistake.

Unfortunately concrete is very strong in compression, and weak in tensile strength. The material is basically a compromise of construction technology. Until someone can find anything better, it's all we got to do the job at this time. It's kind of disappointing.

Thank you, Ivanh for your helpful response. Given this, do you think that my alternate plan, namely, to place bales of hay on top of the slab and on top of that, a tarp, secured firmly to the ground, will help prevent cracking?

I have an additional question for you: although the slab has been installed and has hardened, can these "expansion joints" you refer to still be scored? (Or is it too late?) Thank you kindly for your help.

The tarp may help protect the slab from surface spalding somwhat. Do it. Just don't put salt on it! Unsure about adding EJ on existing slab. Perhaps not. Even if it does crack, it's still robust and will last many years.

Again, you would not do this for a concrete sidewalk or driveway and you do not need to do it for a slab. I am very surprised your contractor thought it was not necessary to score your slab, quite unusal for a contractor to not know his business. Concrete will always get cracks because it continues to get harder and harder for centuries. Most cracks will be hairline and noy an issue with structural integrity.

Just wondering if the bales of hay would insulate it sufficiently?

Marilyn, insulation only serves to keep heat in something, your slab has no heat and you do not have to do anything as long as there are no drains in the floor

The tarp can keep water from freezing and thawing in small cracks, and the bales of hay may insulate it a bit more, but freezing will still occur deeper than the thickness of the concrete in many areas of the country. Keeping the snow and ice off it should not allow more damage to occur than actually having an unheated building atop it. Avoiding salt is critical to keeping the surface in good shape. Keeping a tarp over it will eliminate the need for salt, so that may be a good way to go.

Once concrete has cured (as ballenger said that goes on just about forever but 28 days is sufficient in most cases) temperature changes don't have much effect on the cocrete proper.
I don't see why control joints (1/2 inch or so depth) can't be sawed to help control cracking if they were not installed during the pour.
If the slab is reinforced, attempts to install expansion joints would probably do more harm than good since the reinf. Would be interrupted and if the slab is not confined between immovable objects, I don't think expansion is a problem.
Keeping concrete warm (and saturated) during the first week or so during initial cure is very important but not after the seven or so days.
Frost heave can always cause damage if the base was not properly designed, installed and drained. Insulation might help but not much and should only be considered if the garage will eventually be heated.
Concrete is not "disappointing". When properly designed and installed it is the most durable, economical building product ever in my opinion. It is almost always abused during installation (too much water, inadequate curing etc) but in 90% of the situations it still performs above and beyond the call of duty. If you leave the house frequently, concrete has your life in its "hands" thousands of times per day. In bridges, buildings, subways and many other critical situations. And many of those structures may be 100 years old. Concrete was once described as "the casting on stone in the most undesirable conditions possible".