Run a search at this site. http://www2.worldofconcrete.com/HomePage/WOCHomePage/1,1156,'article_archive~search_article_archive',00.asp
there are lots of articles you can look through.
Steve hit some of the problems and there are others such as the salt, improper floating, rusting rebar, etc.
You may be able to patch over it with a special patching compound. Quikcrete and others makes it.
It's a real bummer. Good luck. Raise cain with the contractor, maybe you can get him to do something although it may not entirely be his fault. Same for the concrete company but maybe you can get both of them to cover all or most of replacement. Read all the articles at the site above and you will know more than both of them put together.