Abstract: Formwork apparatus for casting concrete comprising a plurality of flattened tubular elongated elements each having one wall with an outer surface for contacting cast concrete and an opposite wall with a series of aligned apertures therein. The elongated elements are assembled in juxtaposed relation to constitute the form work by means of retainers which are disengageably interfittable with adjacent elongated elements. Each retainer includes two projecting flanges which penetrate into the interior of apertures of adjacent elongated elements and the flanges are provided with notches for engaging the walls of the elements at the ends of the apertures so as to lock the elements in place.