Some users call it the shrink form--a reasonable enough name for a hydraulically-powered machine with a core that shrinks. The shrinking core is essentially a void-forming machine that shrinks to strip itself so it can be removed from the hardened concrete. Equally important, the shrinking core can be immediately expanded to its original shape so it's ready to be used in the next pour.
HOW THE FORM WORKS
The shrinking form is positioned first, either at the jobsite to form shafts or tunnels or at a casting yard to form precast units. Reinforcing steel is tied in place and then outer forms are set. After the concrete is cast and has hardened, hydraulic pistons in the shrinking form are actuated so that they pull in the corners of the form. And as the corners pull in, they pull in the sides of the inner form. Result: the inner form is now several inches smaller than it was and it can be removed from the concrete structure. Because the shrinking form automatically re-assumes its original shape when it is expanded, structures cast with the form are uniform in size. This is an advantage over conventional forms that have to be taken apart and carefully re-assembled to meet tolerances.