If the officer thought the vehicle was truly unsafe he could have charged you with Operate unsafe vehicle under S. 84 of the HTA, at which point your car would have been ordered off the road until it was repaired and you would have been given a Summons to court. Careless Driving seems like an odd charge to pick for driving with a damaged bumper. For now do what Decatur suggested and request disclosure so you can see the officer's notes. Once you see the notes you will have a better idea of what the prosecution is going to try and argue and then you can start building a defense.
Thanks for your information, Daggx. If cop wants to charge me S.84. how's the demerit point? I cannot find corresponding offense. Any ideas?