A composite for constructing concrete floors is completely factory assembled and comprises a body of expanded plastic having at least two parallel channels, open on the upper surface of the panel, extending the length of the article, adapted to be filled with concrete. A steel reinforcement bar fabric is installed into each channel, which may comprise at least two parallel steel bars connected by cross bars. At least some cross bars extend beyond a lowest reinforcement fabric bar to provide spikes long enough to pass through the plastic bottom and protrude therefrom. One or more sheet metal shrouds with at least one row of aligned holes matching those of the protruding spike ends are applied to the underside surface of the plastic body hung from the protruding ends of the spike by fasteners