By Joseph De Avila, Andrew Grossman and Eliot Brown

Hundreds of unionized concrete workers walked off the job at the World Trade Center on Monday, setting up for a possible strike in the coming days.

The contract covering the workers expired on July 1. Since then, the concrete workers have been operating without a new contract.

Officials with the Cement and Concrete Workers District Council representing the concrete workers declined to comment. Officials with the Cement League, a contractors' association that represents management, couldn't be reached for comment Monday. However, a source familiar with the matter said the two parties are currently negotiating.

Without concrete workers, work cannot proceed on part of the World Trade Center's transit hub, according to a person familiar with the matter. Other construction tradesmen continued work at the site on Monday, but a prolonged work stoppage could affect their work if they need concrete to proceed, according to that same person.