Several Repair Options

Click here to read about options for repairing dust-covered concrete.

{Question} We placed a slab in a warehouse in the cooler months and now it’s dusty. The contractor blames us, the producer. Why does this happen and what can be done?

{Answer} Dusting of slabs has several causes but always results when a layer of weak paste sits at concrete’s surface. That paste has little strength and can be readily removed from the surface, resulting in a dusty appearance. Ironically, removing the dust, such as through sweeping, generates more dust.

Leaving the slab in this condition results in premature wear and accelerated wear in the high-traffic areas.

One reason this occurs is the working of bleed water or other water from blessing the slab surfaces into the surface of the concrete. This increases the water-cement ratio at the slab surface, defeating the purpose of steel troweling and causing a weak layer to occur. This is also a good way to install blisters, so avoid this finishing practice.

Occasionally this type of finishing occurs when the concrete is placed on a cool subgrade or when the air above the fresh concrete is much warmer or cooler than the concrete and subgrade. As a result, the concrete will set differentially throughout its thickness, resulting in a slab that is difficult to finish as it sets from the top down, or that feels rubbery as it’s being finished as it sets from the bottom up. This problem can initiate excessive finishing.

In floated slabs and floors the material that sits at the surface due to bleeding can also be dusty. This layer, or laitance, occurs when the slab is lightly floated. It can normally be removed by light abrasion and won’t continue to dust. Sometimes aggressive sweeping or treatment with acetic acid may be required.

Premature drying

Sometimes, the problem is caused by prematurely exposing the concrete to drying. This is best identified by placing phenolphthalein in water or alcohol on the surface. When scratched, the concrete should turn a dark purple to pink where the indicator has been placed. If not, then curing was not effective. In cold weather this can be made much worse by exposure to the combustion products from salamanders or other direct-fired heaters. Carbon dioxide from combustion combines with drying concrete to accelerate carbonation, reducing the pH and exacerbating dusting.

However, don’t confuse these gases with carbon monoxide. Many contractors have carbon monoxide alarms, but these don’t check for carbon dioxide, and the absence of the alarm doesn’t indicate that everything is fine. ACI 306 requires vented, indirect heaters to avoid this issue.

Another cause is light freezing at the surface. This can be detected by observing the frost crystal impressions in the surface, however they can be hidden by troweling.

Another useful diagnostic test is to place a drop of water onto the surface. The speed at which the water is absorbed into the surface tells you about the size and amount of porosity at the surface. Try this on a good slab as well so you can see the difference. Placing water can also show a network of fine cracking, called crazing. This is a sign of premature drying related to poor curing or excessive carbonation. The depth of the carbonation is the distinguishing factor.

Curing can be a problem. Don’t have the concrete, air temperature, and subgrade temperature far apart, as this increases the risk. Discuss heating arrangements with the contractor.

There are many causes of dusty floors, however it’s rarely a cement or pozzolan issue.