An inner form assembly for forming concrete building shafts has a quadrant of boards and corner posts therebetween. Each corner post has angled interface surfaces converging towards exposed orthogonal skin surfaces. The boards have respective angled interface surfaces meeting respective angled interface surfaces of the corner posts so that the boards are held orthogonally by the corner posts. The angled interface surfaces of the corner posts and the boards slidably bear against each other and mechanically interlock with catches sliding along slots so that the boards move in together to form a compacted cross section configuration when the corner posts rise with respect to the boards and the boards move out to form an expanded cross section configuration when the corner posts fall with respect to the boards and wherein the orthogonal skin surfaces come into alignment with respective skin surfaces of adjacent boards.

A shrinkable core (100) for inserting in a mold (200) for forming a precast load bearing wall panel having a cavity, the shrinkable core (100) comprises a first wall (110) and a second wall (120), a first side element (112) and a second side element (122), and a spacing element (130). The first wall (110) and second wall (120) are spaced from each other by a first distance (d1) to define an internal region (115) in-between. The first side element (112) and the second side element (122) are arranged to close opposite edge portions of the spaced first wall (110) and second wall (120) such that fluid concrete cannot pass the opposite edge portions to get into the internal region (115), the first side element (112) and second side element (122) being spaced by a second distance (d2). The spacing element (130) is configured to vary at least one of the first distance (d1) and the second distance (d2) such that a circumference along the first and second walls (110, 120) and the first and second side elements (112, 122) shrinks monotonically with lowering said at least one distance.

An inner form assembly for forming concrete building shafts has a quadrant of boards and corner posts therebetween. Each corner post has angled interface surfaces converging towards exposed orthogonal skin surfaces. The boards have respective angled interface surfaces meeting respective angled interface surfaces of the corner posts so that the boards are held orthogonally by the corner posts. The angled interface surfaces of the corner posts and the boards slidably bear against each other and mechanically interlock with catches sliding along slots so that the boards move in together to form a compacted cross section configuration when the corner posts rise with respect to the boards and the boards move out to form an expanded cross section configuration when the corner posts fall with respect to the boards and wherein the orthogonal skin surfaces come into alignment with respective skin surfaces of adjacent boards.