A formwork (10) having a hollow region defined therein for use embedded in an elongate concrete member during casting of a concrete material. The formwork (10) is arranged to be lightweight, environmentally friendly and inexpensive and to have sufficient strength. The formwork (10) includes a tubular member (11) formed by bending an oblong plate of corrugated cardboard (5). The corrugated cardboard plate includes a plurality of corrugated cardboards laminated together and each formed by bonding liner sheets to surfaces of a corrugated sheet. The corrugated cardboards are laminated together with flutes of the corrugated cardboards oriented in a direction across a thickness of the corrugated cardboard plate, and a sheet is bonded to front and rear major surfaces thereof.
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