What would cause bubbles to appear shortly after a surface has been power-troweled? Could it be entrapped air escaping through the rather coarse sand matrix? We understand the fineness modulus is high and this could possibly allow the air to channel upwards. Is there a solution for this?
The trouble is basically too early finishing. With modern power trowels the operator has a tendency to trowel with the back edge and tilt up the front edge to keep from picking up concrete on the blade. The surface is therefore actually finished with the edge of the trowel. The result is too early sealing of the surface, with air trapped beneath, resulting in the appearance of bubbles. If the concrete is still soft enough, you can re-open the surface with a wood float or a flat blade, allowing the entrapped air to escape. As always, the best remedy is prevention--be careful not to trowel too soon, and not to handle the power trowel in such a way that you trowel with the edge of the blade.