Twin wall construction is a hybrid of precast and in-situ concrete walls and floors. Each wall unit comprises of two plates of precast concrete with a cavity between, linked by a lattice of steel reinforcement and placed on site; effectively as permanent concrete formwork. Once units and water bars are in place the cavity is filled with in-situ concrete to complete the structural wall.
Twin wall systems offer all the benefits of precast concrete but with the added benefit of continuous cast in situ concrete across the whole wall and, potentially, floor.
An example of concrete twin wall construction.