If its short and fairly straight you could fill it with expanding foam and a layer of duct tape then chip the foam out later. Other options, if the sleeve is long or curved, fill it with styrofoam "popcorn" packing material, or broken up styrofoam chunks and duct tape the hell out of each end. If you can swing a thin plug of expanding foam on each end in front of the loose stryrofoam chunks then duct tape it that would be good too. I am assuming you want to keep wet concrete out of the thing. As far as fastening goes, lots of tie wire and two small short rebar pieces at each end, tied in to the concrete guy's rebar..
Edited by trollog (06/01/08 12:36 PM)
Edit Reason: typing errors