Let your Concrete Breathe
One of the most disturbing aspects in the world of concrete is delamination.
This is caused by the concrete's inability to expel water vapor and entrapped air. This is not to be confused with the bleeding of water to the surface, which occurs soon after placement.
"If hard troweling closes the surface of the concrete, the concrete cannot breathe..."
Under normal conditions, the stage for delamination is not set until all the bleed water is gone and the later stages of finishing are well under way. It is during this time that the concrete is undergoing a condition known as subsidence, which is a slight reduction in volume.
At this time the concrete has not developed enough strength to support its own weight and is collapsing forcing heavier materials, such as cement paste and very fine aggregate particles, into the areas occupied by water vapor and entrapped air. This hydraulic pressure forces the lighter water vapor and entrapped air to the surface of the concrete.
If hard troweling closes the surface of the concrete, the concrete cannot breathe, water vapor and entrapped air cannot escape from the mass of the concrete and will become trapped just below the dense cement paste near the surface. This can produce uplift pressures that can immediately create blisters and later lift large sections of the surface away from the mass of the concrete causing delamination. Delamination can occur days and weeks after the concrete has been placed. You can prevented this by delaying closing the slab (hard troweling) until subsidence has been completed.
© 2000 L&M Construction Chemicals, Inc. | ConcreteNews September 2000.