the curvature of the circular formwork correspondingly the cantilevered ends 32 and 32 39 of the formwork elements are adapted to the curvature of the circular formwork by adjusting the adjusting spindles 30 and 30 39 it will be seen in the respresentation of fig3 that due to the mounting lugs 11 12 and 13 on the one hand and 11 39 12 39 and 13 39 on the other hand arranged in the two different planes 14 and 15 the mounting bolts 29 34 and 29 39 34 39 respectively of the adjusting spindles 30 30 39 and 35 are arranged in such a manner that the adjusting spindles 30 30 39 and 35 do not interfere with each other in particular do not cross each other in addition it will be easily recognized that in the extreme case the adjusting spindle 30 or 30 39 can be arranged to extend substantially in parallel to the web 2 as the bearing piece 8 extends only over part of the length of the plates 6 and 7 in the case of the embodiment illustrated diagrammatically in fig4 a plurality of formwork beams 24 are arranged at equal spacings along the formwork lining 33 of the circular formwork 22 with every pair of formwork beams 24 embracing one device 1 in such a manner that the web 2 of the device.