For more than 50 years, concrete finishers have been warned about the dangers of using tools or methods that densify and seal the concrete surface before bleeding has stopped. Trapping bleedwater, air, or both beneath a sealed surface may cause blistering or surface delamination and accelerate surface scaling of exterior concrete in cold climates.

Experience indicates that wood bull floats are good tools for avoiding sealing concrete after strikeoff because they open the concrete surface, allowing bleedwater to evaporate. Fresnos, on the other hand, are bad tools to use as floats because they seal the surface, allowing bleedwater to collect beneath it. And magnesium bull floats are not as good as wood for keeping the surface open, but they are needed on air-entrained concrete because wood floats tear the surface.

But these assumptions are based on observations, not data. They don't measure the relative ability of different finishing tools to keep the surface open. It would be helpful to know how much more water exits the concrete when finishers use a bull float instead of a magnesium float or a fresno. To find out, the authors devised a test to measure surface water loss and then used it with different concretes and finishing-tool combinations. Some of the results were surprising.