Abstract: A device for the performance of a supporting function in a passage includes a resilient plug which is expandible to the cross-sectional area of the passage and contractible to a lesser cross-sectional area. Contraction of the plug after its supporting function has been completed permits the plug to be readily withdrawn from the passage. The device is particularly useful for forming a passage in an element composed of a flowable and hardenable material such as concrete. The material is poured around the plug and the plug is contracted and withdrawn from the material once this has hardened.