Question:

We have a dusting problem on a new concrete floor we pumped, placed and finished last winter for our best customer. The floor surface is very porous and weak, and in some areas appears to be covered with white flour. We sweep it and scrub it but it returns the next day. Some areas are even showing the surface of the coarse aggregate. A review of our placement records shows that we did everything according to the book, but now we have this awful situation. What do you think happened and what can we do?

Answer:

Your letter further explained that the concrete mix was not too wet or high in slump. Troweling was not done too early. The surface was hard and firm when your crew completed finishing. Lastly, the surface did not crust or dry out before it was covered in some cases and cured in other cases.

Here is what I suspect went wrong. Your floor is a victim of the cold weather scourge of concrete: Carbonation. Fossil fuel heaters give off high levels of carbon dioxide gas. If not properly vented to the outdoors carbon dioxide gas from the heaters will float down to the surface of fresh concrete and react chemically with the fresh cement paste, halting its set. Carbonated concrete surfaces will fail to properly set or harden completely and instead will dry out to a weak, porous finish, which in severe cases as the one you describe evidences itself as a fine powdery dust. The depth of this weak carbonated concrete surface will vary due to the degree of exposure to carbon dioxide gases.

To prevent dusting from carbon dioxide gas use indirect fired heaters or fireboxes vented from and to the outside. The combustion air and combustion chamber need to be outside in fresh air. Do not allow the heater to combust the recycled interior air or the air will become more laden with carbon dioxide and soon destroy the integrity of the newly placed concrete surface.

To minimize or eliminate the effects of carbonation on an already placed concrete floor a number of costly steps will be required, depending upon the severity of the carbonation. For lightly carbonated surfaces, many of our customers have found the application of SEAL HARD and CHEM HARD in multiple coats has yielded satisfactory results. On more severely carbonated floors a more expensive, but necessary, treatment may be required which would involve surface grinding of the floor using a system like the one mentioned in L&M's September, 2001 Concrete News. FGS (Flat Grinding System) removes the weak, carbonated cement paste, polishes the remaining floor, which is then reinforced with a two-coat application of SEAL HARD. Finally, in areas where coarse aggregate is already showing, carbonation has done its maximum damage. The remedy for these distressed surfaces requires grinding in preparation of a properly bonded latex-modified floor topping like our DURATOP.

Carbonation can be scientifically identified on a suspicious surface. Generally a good concrete lab can determine the presence and depth of carbonation. It is necessary that concrete cores be taken from the areas in question for this test.

It is always a good idea to consult with your L&M technical representative to review the project condition and to allow us to help you. Thanks for bringing up this costly, yet easily prevented problem.