If you look in the back of my trailer, you won't just see one type of trowel blade. The steel touching the concrete dictates the aesthetic of the final floor, and a seasoned operator tailors their blade metallurgy to the specific architectural requirements of the slab. The standard workhorse for a walk-behind gas trowel is the high-carbon "blue steel" finishing blade.
Blue steel is highly flexible. When I pitch the blades steeply for the final burnish, the gasoline engine's weight bows the blue steel slightly, creating an incredibly tight, high-pressure contact patch that glazes standard grey concrete into a mirror finish. However, blue steel has a massive flaw when working with decorative, color-hardened, or pure white architectural concrete. The extreme friction at 130 RPM causes the carbon steel to literally shed microscopic metal particles onto the floor. On standard concrete, this just makes the floor look darker. On white concrete, it leaves dark, ugly, permanent burn marks.
To solve this, we swap the steel for specialized UHMW (Ultra-High-Molecular-Weight) polyethylene plastic blades. Plastic blades don't leave burn marks. They allow you to run the heavy gas-powered finisher over delicate, light-colored slabs without staining them. But plastic blades require a completely different operating technique. They are thicker and do not flex like spring steel. You cannot pitch them as steeply, and they wear out significantly faster on abrasive aggregate. Knowing when to drop the steel and bolt on the plastic is the hallmark of a high-end decorative concrete contractor.
