Power Trowel Burn on a Polished Retail Slab
A 4,000 ft² retail floor is specified as a burnished, power-troweled slab with a densifier and a stain guard. The mix is a 4,500 psi mix with a retarder for a 6:00 a.m. placement in a 65°F building. The finisher begins power troweling with a ride-on machine as soon as the slab can bear the machine's weight, then runs multiple passes with the blades pitched progressively flatter. By mid-afternoon the surface shows dark, glossy patches and fine crazing in the troweled areas, and a straightedge reveals slight dishing in the same zones. The owner wants a uniform sheen.
1. Explain the cause of the dark glossy patches, the crazing, and the dishing.
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The finisher troweled too early and too aggressively. Starting the ride-on as soon as it bears weight means the concrete is still plastic beneath a set surface; the trowel's weight and blade pressure push paste and water to the surface, creating the dark, glossy, overworked patches (burnishing of a rich paste layer) and trapping water that later causes crazing as that layer shrinks.
The dishing is from the trowel's pressure on concrete that had not developed enough strength to resist the machine, so the surface was displaced rather than compacted. Progressive flattening of the blades on an under-set slab concentrates pressure and worsens both the burnish and the dishing. The result is a non-uniform sheen, exactly what the owner is seeing.
2. What is the correct troweling sequence and timing, and how do you recover a uniform sheen now?
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Correct sequence: wait until bleed water has disappeared and the concrete supports the trowel with minimal depression. Start with a float or a trowel with blades pitched up (low contact) to open and level the surface, then make successive passes with progressively flatter blades, each pass after the concrete has stiffened enough that the previous pass's marks are gone and the machine does not dig in. Do not trowel while bleed water is present; do not over-trowel to chase gloss. A densifier is applied after the surface is sound and cured per the manufacturer, not as a fix for overworking.
Recovery for the dished, crazed areas: the dishing and crazing are surface defects that will not be hidden by more troweling. Grind or hone the affected zones to a uniform plane and remove the crazed layer, then apply the densifier and stain guard per the system. If the dishing is deeper than the topping tolerance, a bonded micro-topping may be needed. Set a written troweling sequence and a field set-point check (machine does not depress the surface) for the crew before the next pour.