The union is angry that a clause in the contract includes holiday pay in the hourly rate, which it claims undermines European law.
London UCATT official Mick Dooley said European law has given building workers the right to paid holidays. He said Raven seem to be prepared to climb a "legal Mount Everest" in its attempts to get around the law, rather than ensure workers have paid holiday.
He said: "UCATT will fight this contract on moral, legal, political and industrial fronts until right prevails."
Raven refused to comment.
In early October, Raven was the the target of another UCATT protest, when officials demonstrated against poor safety standards on site on a Bouygues site in Chelsea, west London.