There are some projects where cracking is just not acceptable. In a food warehouse, for example, sanitary considerations may require the filling of all cracks. Preventing the cracks in the first place can cut maintenance costs for the life of the building. Or an owner may simply insist on a crack-free floor. On two projects, post-tensioning produced superflat floors that were and remain wholly without cracks. The first post-tensioned superflat floor in North America if not the world was installed for Carlton Cards Limited of Ontario. Shortly thereafter, another post-tensioned floor was constructed in Kansas for Hallmark Cards. Each was a high-rise, high-density warehouse.
Floor areas were comparable: 133,000 square feet and 98,000 square feet. Both slabs were 10 inches thick. Both were laid out in parallel strips about 14 « feet by 300 feet, each strip corresponding to a warehouse aisle. Different proprietary systems were used, but both projects used unbonded tendons laid in a horizontal plane at the slab's mid-depth. Stressing took place in two stages and steps were taken to allow the slabs to shrink freely to lessen the risk of early cracking. These included careful grading of the subbase, isolation of all slab penetrations, and the use of a polyethylene slipsheet to reduce friction with the subbase.
Along with the similarities, there were some major differences between the two jobs that show how post-tensioning can work under varied conditions. Although both jobs showed that post-tensioning can eliminate cracks in superflat floors, there are drawbacks and the extra cost cannot always be justified. Two-way post-tensioning probably makes sense only when a poor subbase, such as expansive clay, make an ordinary slab on grade questionable. But when a floor simply must be crack-free, longitudinal post-tensioning in one direction only provides a way to achieve this without compromising on flatness. In some cases the cost may be at least partly offset by reductions in slab thickness and in the amount of steel reinforcement used.