Void forming core formwork for concrete beam

A formwork for use in an elongated concrete member, which is embedded within a hollow of the elongated concrete member during casting of a concrete material, which form comprises:

• a tubular member having a polygonal sectional shape formed by bending a corrugated cardboard plate made of paper,
• wherein the corrugated cardboard plate including a plurality of corrugated cardboards laminated together, each of the corrugated cardboards including a liner sheet bonded to surfaces of a corrugated sheet, the corrugated cardboards being laminated together with flutes of the corrugated cardboards oriented in a direction across a thickness of the corrugated cardboard plate, and a sheet bonded to front and rear major surfaces thereof;
• a partition wall made of a corrugated cardboard and positioned inside the tubular member for dividing a hollow of the tubular member into cells;
• and
• a connecting passage, formed in a lower portion of the partition wall, for communicating between the neighboring cells separated from each other by the partition wall, the connecting passage being adapted for draining water pooling in the hollow through a drain port in the elongated concrete member.