Crumbling Tires??, brand new car, tire place

I don't know if you can help me, but I'll try anyways.
I purchased a brand new car about 8 months ago, with 4 brand new tires.  Recently we had a big snowstorm.  It was the first time I had actually driven my car in the snow.  The car felt a little funny at first, but I thought it was because of the weather, so I kept driving.  There also wasn't anywhere for me to turn off.  I drove about 1 to 2 miles before I was able to pull off.  I noticed that I had a flat tire.  I had roadside assistance come and change my tire for me.  Me and him both thought that the tire looke really good.  Until he flipped it over.  The back of it was all shredded.  I guess because I drove on it.  We couldn't find any punctures in the good parts of the tire.  So I took it in to a tire place to get a new tire.  I tried to get money off the tire, since I'm not sure it was all my fault.  The guy grabbed the tire out of the car, and the back of it just crumbled.  As he carried it into the store there were chunks falling off of it.  He had it in the back for at least 20 minutes before coming back to ask me if he could send it off to a specialist to see what happened.  
Since he was doing that I assumed that it probably looked to him that it wasn't my fault.  And I'm wondering if I just had a deffective tire.  They are making me pay for half of the tire.  But if it's a deffective tire I dont' think i should have to pay for any of it.  
Does this sound to you like a deffective tire?  I know that driving on it could damage the tire, but would the tire actually crumble?
Please help!

Thanks,
Kristine

Answer
sounds to me like you got a flat tire and by driveing on it for a couple of miles you put a lot of heat into it. it could have even been rubbing on the inside of the fender well or a suspension componet. it really doesn't sound like a problem with the tire. i would be happy with the half priced replacement.
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