There is a persistent myth among amateur contractors that a heavy-duty walk-behind power trowel can fix a bad concrete pour. They assume that if they dump the mud poorly, the spinning blades will eventually iron it all out. This is a catastrophic misconception. The trowel is merely the final polish; the structural integrity and flatness of the floor are dictated hours earlier by internal consolidation.
If my crew is pouring a 150mm (approx. 6-inch) thick structural slab, the concrete must be aggressively vibrated using internal "stinger" vibrators or a heavy-duty vibratory screed before the trowel ever touches it. If the concrete is not internally vibrated, massive pockets of trapped air (honeycombing) remain hidden just below the surface.
When the concrete begins to set and I walk my 100 kg (approx. 220 lbs) gasoline trowel onto the floor, the intense weight and friction of the spinning blades will pass over these hidden air voids. The thin layer of surface cream will instantly collapse into the void, creating a deep, unfixable crater right in the middle of your perfectly smooth pass. Furthermore, if the slab is not struck off perfectly flat with a screed, the power trowel will simply follow the rolling hills and valleys of the bad pour. The wide blades of a 1200mm (approx. 46-inch) trowel will bridge across the low spots, leaving them unsealed and porous, while aggressively grinding the high spots down to the raw aggregate. A pristine, glass-like burnish from a gas trowel is the reward you get for executing a flawless pouring and vibrating phase.
