First things first.
Before you and your Home Inspector, and KB's team, and "another contractor" rely upon untrained speculation, hire a Structural Engineer with a track record in evaluating the dimensional stability of different types of concrete. Spend another $500 and be duly diligent.
That being said, no matter what, KB has a reputation to uphold and will not let a negative engineer's report stand, I'm pretty sure. If you think differently, then so be it. If KB is acting up, then you have recourse through their corporate channels, the Georgia Home Builder's Association, the BBB, etc.
You need a case based on facts and not uncertified speculation. You are probably playing in the greyer areas of "concrete performance" that are specifically defined in your Binding Agreement.
You better have your paperwork tight, if you want this house, or else...
You could just let the $500 go.
Or, do you love, love, love the house? Is it a dreamhome? Are you crestfallen and melancholy over the whole sordid affair? Do you just want the big mean builder to satisfy specific structural concerns and then call it a happy day?
If so, then get your due diligence on.
If not, then come on, $500? If you can let 'em have it, and suffer no penalties, then lose the money and move on.
If you do not understand the consequences of that walkaway move, then consult with a real estate litigator or else hope that you don't get sued for specific performance under a Binding Agreement.
Jenesa - I'm shocked that KB only has $500 Earnest Money on this contract.
KB - if you are reading this, step up your game. Real estate is a serious business.