Police have released this shocking footage of a man having a concrete slab smashed over his head in a vicious city centre attack in a bid to trace the victim.Shortly after 4.35am on Wednesday, CCTV operators in Manchester captured two men embracing briefly before the offender removes a slab of concrete that was concealed at the back of his trousers. He then lifts the weapon above his head when the victim’s back is turned and smashes it over his head.