Can a mix that is more than two hours old be placed without producing a cold joint?
The prevention of a cold joint does not depend strictly on the age of the concrete that has been placed or that of the concrete about to be placed. It depends on whether the concrete in place has taken its initial set. (Of course attempting to place a completely unworkable fresh mix could also produce a cold joint.) If the concrete in place has already set, no matter whether 1 hour or 24 hours old, a cold joint will be produced. If bond is needed at that point it must be obtained by proper use of a portland cement grout or other bonding agent. If the concrete to be placed has set it should be discarded.