Cracking garage slab in 6 month old house

[claudiod]

Hi.
6 months ago I took possesion on my new house. The garage slab is 23' wide by 22' long. It was poured last October.
Recently, I have noticed a lot of cracks forming in it. They all run along the width(perpendicular to the direction that the cars are parked). Some are right at the edge and very minor. They are visible but the gap in them is neglibile. However, they do run through the entire 4" thickness of the slab. I noticed one of these on each side and they are about 7' long.
The one that concerns me is in the middle of the slab. It is about 14' long. I have not measured the width of the crack becasue I just noticed it this morning. The builder has not yet done the final grade on the lot and I am worried about the sand base used for the slab washing away. I have noticed that it is eroding at the edges.
Should I be concerned about this, or is it fairly normal.
Also, I am building a garage this summer. Once that is complete, how can I repair the cracks so that they aren't very noticeable?

[mjpliv]

Wish I could help you there but there basically two rules when it comes to non-structural concrete slabs.

Rule # 1 - Non-structural concrete slabs will, in most cases, crack to some extent.

Rule # 2 - If you are looking for a non-structural concrete slab that will not crack, refer to rule #1.

All you can really do is make sure the slab is poured on a base that was installed to your local specifications (I am surprised to see sand was placed underneath it) and even if not required by your local building authorities I would suggest at least one layer of 6"x6" (6/6 or 4/4 gauge) welded wire mesh in the concrete. The mesh will not stop cracks but should prevent one area rising above another.

If you want a crack proof floor, have it engineered which involves a fair amount of rebar installed.

One way to minimize cracking is to add 3-5% "air" (a chemical addmixture that creates tiny air bubbles in the concrete) which allows the concrete to expand and contract without creating stress cracks. This is very important if the concrete is exposed to feeze and thaw cycles. It will also allow the concrete to flex more than it can without the "air"

On larger slabs you can request that the contractor return after a few days and cutting some control joints into the slab. These control cuts can be filled /leveled with a mastic product to keep them clean and waterproof.

Another important thing to watch is moisture. Concrete does not dry - it actually hydrates. The concrete curing process breaks down the water into the two components (oxygen and hydrogen). If I remember correctly the concrete uses the hydrogen and releases the oxygen. This chemical process is what causes the heat build up in all concrete products as they cure. You concrete arrives on site with a specific water/cement ratio to give you the compression strength in the slab specifications (3000 PSI, 4000 PSI, etc.) If the water evaporates from the concrete faster than the concrete can break it down and use it, then your concrete will become brittle and not have the compressive strength that was expected. Talk to anyone "in the know" and they will tell you that cool, damp, overcast days are best for pouring slabs. If exposed to sun light or hot dry weather, cover the slabs with tarps or straw to slow down the evaporation. This also reduce the amount of shrinkage on the slab.