Have you ever walked on a concrete slab and noticed that it has a dusty surface? Not the normal kind of dust one might expect to accumulate over time, but a dust that just never seems to go away. It’s almost like it the dust seems to come from the concrete itself. Well that very well could be the case. Today I’ve decided that that will be the topic of my discussion. I’ll touch on what concrete dusting is, why does it happen and what you can do to prevent it.
What is Dusting?
Dusting on concrete is the formation of a loose powder that is the result of the disintegration of the surface of hardened concrete. It is also often referred to as chalking. This name would come from the color and consistency of the dust that is formed on the surface of the concrete. It’s chalk like. Concrete floors/slabs that are dusting will produce dust under any kind of traffic. Dusting concrete can also be easily scratched with a nail or even just by sweeping it with a broom.
Why do concrete floors dust?
So in the most basic sense a concrete floor is dusting because the surface of the concrete is weak. So what causes the surface of the concrete to be weak? Well that’s what I’ll be getting into next. There are a lot of different reasons and things that will cause dusting, as with anything involving concrete it’s probably just not one thing in particular that will cause it. One contributing factor to a weak surface and therefore dusting is performing any finishing of the concrete before the concrete is finished bleeding. What happens is the bleed water from the concrete is being worked back into the surface of the concrete. So in this way at the surface of the concrete you’ve altered the water to cement ratio and in turn weakened the surface. As I touched in a previous post all the materials that go into the production of concrete are proportioned out, so changing the proportion of any of them affects the strength. Working bleed water back into the surface increases the ratio of water to cement, and a higher ratio of water to cement means less strength in the concrete. In this case just the surface. Generally just the top quarter of an inch. In the same way performing any finishing operations when condensation has formed on the concrete. This can happen when in colder weather if the concrete is cold and there is increased humidity in the air. Condensation will form on the colder surface of the concrete. Working this water into the surface will, as before with the bleed water, increase the water to cement ratio at the surface therefore weakening the surface. Insufficient curing time will cause dusting, as the concrete hasn’t had enough time to reach its ultimate strength. When this happens there will be a soft surface skin on the concrete. Insufficient protection form the elements when freshly placed. Things like rain, snow or strong drying winds. One thing that isn’t thought about is improper ventilation when placing in an enclosed space and using a gas powered machine to assist you. First, you should make sure there is proper ventilation for your own health, but the gasses in the exhaust of a machine creates a chemical reaction known as carbonation. This also reduces the strength of the concrete at the surface.
How to prevent dusting
First, make sure you’re placing concrete with the lowest water you can to get an adequate slump that will allow for finishing. This will produce the strongest most durable surface on your concrete. Concrete should only be placed with a moderate slump that doesn’t exceed five inches. You may use water reducers to achieve a higher slump with less water. This goes back to the water to cement ratio. More cement to water means stronger concrete, both at the surface and overall throughout the entire slab thickness. Do not sprinkle toss dry cement onto the surface of the concrete to absorb excess bleed water. If you wish to remove excess bleed water the right method would be to drag a garden hose across the surface. Other ways of reducing the amount of bleed water involve making changes to the mix design or adding an accelatant that will reduce the setting time. As stated before do not perform any finishing operations to the concrete while bleed water is present on the surface. This includes bull floating. After your initial screeding you should promptly follow this up with bull floating the surface. Even delaying bull floating results in bleed water being worked back in the concrete surface. Also, do not add any water to the surface. This is known as “blessing” the concrete. Again this increase the ratio of water to cement at the surface of the concrete. Another thing you can do is protect your concrete from the elements while it is fresh. Covering concrete with a liquid membrane curing compound or even just covering the surface with wet burlap to retain moisture. This must be done after finishing though. Doing this allows for extra curing time so the concrete can gain more strength for a longer amount of time. This will protect the concrete from the elements as well.
Concrete in Practice, CIP1 “What, Why & How? Dusting Concrete Surfaces”
Slabs on Grade, Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, MI.
Guide for Concrete Floor and Slab Construction, ACI 302.RI. American Concrete Institue, Farmington Hills, MI
Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL
The Effect of Various Surface Treatments, Using Zinc and Magnesium Fluosilicate Crystals on Abrasion Resistance of Concrete Surfaces, Concrete Laboratory Report No. C-819, U.S. Bureau of Reclamation
Residential Concrete, National Association of Home Builders, Washington, DC.
Trouble Shooting Guide for Concrete Dusting, Concrete Construction, April 1996