Abstract: A method and a device to cast elongate concrete structural members formed internally with channel means in horizontal moulds. The channel means is formed by a double-wall inflatable hose of an elastic and flexible material which is introduced into the mould and is provided with support walls extending between the inner and outer hose walls so as to divide the space therebetween into longitudinal chambers. After inflation of these chambers, concrete is poured about the hose while at the same time weights displaceable in the interior of and in the longitudinal direction of said hose, exert a load on the hose, thus preventing it from being lifted from its predetermined position inside the mould by the concrete.