Vinyl Composition Tile: Getting to the bottom of bumpy floor tile

The vinyl composition floor tile (VCT) had been polished and looked great when the new building first opened, but a few weeks of use changed its appearance (and also reduced the level of owner satisfaction) as little, pimple-like bumps started to appear on the surface of the tile. Of course, there was speculation from numerous parties about the possible causes, from faulty concrete to unusual chemical and astrological phenomena. But the actual cause turned out to be pretty simple. The tile installer had a practice of using the same mastic container day after day, and little bits of the previous days' hardened mastic from the rim of the container found their way into the fresh mastic every day. This was not obvious in the finished product at first, as the tile was held up slightly by the ridges formed by the notched trowel that was used to spread the mastic. However, days of heavy foot traffic and passes by the owner's floor polishing equipment pressed the tile further into the mastic around the harder bits of old mastic that resisted being squashed, and they telegraphed through the tile as bumps on the surface. The flooring subcontractor had to make several return visits to the project to correct the problems as they appeared.