The man accused of murdering Sally Anne Bowman said he had sex with her semi-naked body after stumbling across her while on a trip to buy cocaine.
Taking the stand, chef Mark Dixie said he came across the teen lying in front of a house in Blenheim Crescent.
Dixie said he thought she was passed out, not dead, because he "didn't expect to see someone dead in that street".
Dixie told the court he had gone walking early in the morning because he knew a drug dealer living nearby.
When the dealer was not home, Dixie walked along Blenheim Crescent towards where he was staying when he saw Sally Anne's legs.
"All I saw was a pair of legs, genitalia, skirt up above her waist and I took advantage of her," Dixie told the court.
Asked by prosecutor Brian Altman if he meant he had sexual intercourse with Sally Anne, Dixie replied: "Yes".
Panicking afterwards, Dixie said he put concrete in Sally Anne's mouth and vagina in a bid to hid DNA evidence.
"I knew I had to cover up my DNA," Dixie said.
Mr Altman asked Dixie if he realised going to trial would mean Sally Anne's family would be exposed to the details leading to her death.
Dixie said he had wanted to spare his family and Sally Anne's family the details of the night but he had to give his side.
Dixie also told the court his comment to police when he was arrested that he would have to be "mental" to do anything to Sally Anne was related to the sexual attack, not her murder.
"It had nothing to do with murder whatsoever," Dixie said.
After having sex, Dixie said he had run back to the flat where he was staying.
"I sat in the lounge and finished off the spliff, the joint, that was in the ash tray to calm myself down," Dixie said.
The trial continues.
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