Our contractor poured our basement less than 24 hours ago and we are already noticing some cracks. They are very thin cracks and some are 6-8 ft long. I have asked around and some have said that they are called spider cracks. Are these normal? How soon is too soon for these cracks to appear? Within 8 hours?

They sound like shrinkage cracks...as concrete cures, it also shrinks slightly. One of the purposes of steel reinforcing bars or wire mesh in the concrete is to control this cracking. It will still occur, but if the steel is properly placed, the cracking will be microscopic. Some concrete contains certain fibers for the same purpose. If the cracks are only "hairline cracks" they are nothing to worry about. If they open up by any measurable amount, they should be dealt with. The cracks will appear at varying times, depending on the concrete mix used, the ambient temperature, and the amount and placement of reinforcing steel.

Thank you so much for responding. I don't believe that we have wire mesh in our floor because we have radiant heat. We have insulation and tubing running through the floor. What exactly is a microscopic amount (sorry if this is a dumb question)?The cracks are very visible, thin, but visible. One crack goes the entire length of the home.

A microscopic crack is one that can only be seen through a microscope, in other words essentially invisible.

I'd like to know more about the exact pattern of cracks, if they can be decribed in the smae way as the one that runs the length of the house.

The cracks are either shrinkage cracks, or else they are due to an incorrect concrete mixture or extreme temperature while the concrete is setting. If there's no mesh, and the radiant tubing is plastic, shrinkage is a good bet.

If they stay visible but not open, they are not a big concern. If they begin to open up, then we have to think about them again. You will know in a month whether they will open up or not. Concrete continues to cure virtually forever, at an ever-decreasing rate, but it is essentially fully cured in 28 days.