If you want to start an argument among a crew of veteran concrete finishers, ask them exactly when to step onto a fresh slab with a commercial-grade walk-behind gasoline power trowel. In my decades on the slab, I’ve learned that the timing is not governed by the clock; it is governed entirely by the physical chemistry of the concrete. A standard 900 mm [approx. 36-inch] gasoline power trowel weighs roughly 80 kg to 100 kg [approx. 176 lbs to 220 lbs]. If you put that much heavy iron onto the mud too early, the machine will dig trenches into the wet concrete, permanently burying the aggregate and destroying the level of the floor. If you wait too long, the concrete will "pull tight" and the machine will just bounce uselessly across a rock-hard surface, leaving you with an unsealable, porous finish.
The absolute golden rule I teach every greenhorn is the "footprint test." After the slab has been poured, screeded, and bull-floated, the concrete begins to bleed. The heavier aggregates settle, forcing excess mix water to the surface. You absolutely must wait for this "bleed water" to evaporate. Once the surface loses its wet sheen and looks dull, you step onto the slab with standard flat-soled work boots. If your boot sinks in more than 5 mm to 6 mm [approx. 1/4 inch], stay off the floor. If your boot barely leaves a dusty imprint, you are too late. The perfect window is when your boot leaves a crisp indentation of about 3 mm to 4 mm [approx. 1/8 inch] deep, and no water seeps into the footprint.
This is the exact moment you fire up the 4-stroke gasoline engine. At this stage, the concrete is firm enough to support the weight of the machine without rutting, but the surface paste (the "cream") is still pliable enough to be mechanically manipulated. The heavy gasoline engine provides the raw, sustained torque necessary to push the wide float blades (or a float pan) across this sticky, high-friction surface. Understanding this precise window of hydration and evaporation is what separates a decent finisher from a master concrete artisan.
