In response to questions raised regarding PFI acting as both separation and expansion between the slab and the diamond block-out pours we submit the following facts:
Old fashioned wooden isolation pockets often contain a layer of compressible fibrous sheet material that is specified by architects and engineers. The reason for this is simple if concrete is cast against other concrete, the second application of concrete takes the shape of the first portion of concrete. Since the exterior faces of the first volume of concrete are rough due to the ragged nature of the wooden formwork and removal of the same, it is easy to slide in opposition (allowance for movement) to one another. A compressible fibrous “expansion joint” was seen around the perimeter of these old-fashioned isolation pockets. Please keep in mind that concrete never expands thermally greater than the amount of initial shrinkage due to hydration and drying; therefore the joint material was actually functioning more as a “slide-bearing” joint instead of a true expansion joint. When PFI is utilized, a very different situation occurs. The first major difference is that concrete bonds tenaciously with wood, making removal of wooden formwork from hardened concrete very difficult. PFI is constructed of high-density polyethylene (HDPE) and therefore does not form a permanent bond to concrete; it merely takes the shape, but as the concrete dries and shrinks it pulls slightly away from the PFI . The distance that shrinkage draws the concrete away from the PFI is affected by many variable such as: temperature, humidity, subgrade material composition, vapor barrier presence (or absence), water/cement (w/c) ratio, control joint spacing, and concrete finishing techniques. This distance only rarely exceeds one-sixteenth of an inch. The volumes of concrete on each side of the isolation pocket formwork (interior and exterior) are both subject to shrinkage, but the interior volume will often reveal an immeasurable amount of shrinkage, since shrinkage is based on a percentage of the area of the “slab”. Since in the case of PFI there is no bond tenacity, the concrete is free to move slightly away as it shrinks and moves slightly back toward PFI when it expands. (This expansion will never exceed the shrinkage dimension that the aforementioned concrete has already experienced). This is why expansion joint filler material is not required when PFI is utilized. PFI stays in place for the life of the structure and prevents the two volumes of concrete from contacting one another. Whether the slab is cast first or last, or in monolithic casting situations (possible for the first time with PFI ), PFI is the perfect answer to this age-old problem!