In this article, you will learn what is concrete dusting, causes, effects, and how to stop concrete dusting. So, read till the last.
What is concrete dusting?
The white powdery substance found on the surface of concrete due to the abrasion of the surface is called concrete dusting. This dust is the fine particles of crystalline silica that are abraded from the surface as a concrete block is made up of sand and gravel that contain free silica or crystalline silica.
This can be easily recognized by scratching nails or even by sweeping and are powder under any kind of traffic.
Causes of concrete dusting
Dusting is the result of wearing of weak surface and improper bonding of matrix with fine aggregates. The reasons are:
Addition of more water
Addition of water more than the requirement in the mix that increases the bleeding resulting in the water and fines on the surface. And this makes the surface low wear resistance.
By doing the finishing operation while bleed water is on the surface, makes the water come back to the surface of the concrete. It produces a very high water-cement ratio and the outcome will be a low strength surface layer.
If proper curing is done and allowed concrete surface for drying rapidly mainly in hot, dry, and windy season, then it will result in the weaker surface layer. They may contain dust even under foot traffic.
Reduction on abrasion resistance is the result of improper compaction of surface and bad finishing.
Use of Dry cement
For allowing the earlier finishing, dry cement is used for soaking the surface water. This increases the quantity of fine material at the surface, resulting in the weak surface layer.
Freezing the surface
Surface freezes earlier for gaining the strength and the concrete mixture are disrupted by the expansion of the water producing a weak layer.
No proper ventilation in enclosed place
Carbonation reaction can be caused by carbon dioxide released from salamanders, gasoline engines, or generator or mixer engines which can greatly reduce the strength and hardness of the concrete surface.
If rainwater falls into the surface before concrete reaches its final set, will cause effects same as premature finishing.
Initial precaution to prevent concrete dusting?
Concrete having the lowest water content with enough slump should be used for placing and finishing to get a strong, durable, and water-resistant surface.
Concrete has a low moderate slump not exceeding 5 inches [125mm] should be used. And higher slump can be used for providing required strength without excessive bleeding and segregation.
Water-reducing admixtures should be used to increase the slump for maintaining low water content in the mixture which is necessary for cold weather when delayed set produces prolonged bleeding.
Never sprinkle or towel dry cement on the surface of plastic concrete for absorbing bleeding water or rainwater. Bleed water should be removed by dragging the garden hose across the surface or rubber squeegee should be used.
No finishing operation should be performed with water present on the surface or while concrete continues to bleed. The method of initial screening must be followed by full floating, delaying it can cause water to work in the surface layer and water should not be added.
Concrete should not be directly placed on polyethylene vapor retarders or on non-absorptive subgrades because this can create a problem like dusting, scaling, and cracking.
Trimmable and compactable materials should be placed over vapor retarders or non-absorptive subgrades for 3 to 4 inches [75 to 100mm] for concrete placement.
The lightly damped absorptive subgrade should be placed just prior to concrete placement so that water doesn’t pond or collect on the surface of the subgrade when high evaporation rates exist.
Proper curing should be provided by using liquid membrane curing compound or cover the surface with water, wet burlap, or other curing materials immediately after finishing to retain the moisture in the slab.
If the concrete is being done in cold weather, it requires a temperature exceeding 50°F [ 10°] as well as accelerating admixtures.
Overwork should not be done in concrete initially, it will seal the surface and trap bleed water under the surface layer and fine materials will be brought to the surface. Water should not be added more than the requirement in the concrete mix.
Now, let us move toward how to stop concrete dusting.
How to stop concrete dusting / Repair?
It may be very difficult for repairing the concrete dust surface, so it will be favorable if we use the ways to prevent the dusting. But in the case for repairing of concrete dust surface there are some ways like:
Application of chemical surface
Using surface hardeners like sodium or fluorosilicates can help to minimize dusting and should be applied on concrete that should be 28 days older in minimum.
Calcium hydroxide in the concrete reacts with it to give additional cementitious compounds but cannot improve the appearances of surface loss by wearing.
Grinding of surfaces
A concrete grinder is used for complete removal of the weak surfaces but can alter the appearances making aggregates visible. Grinding of concrete having adequate strength can be an effective way.
Topping is used for removing of the weak surface layer by scabbling and replacing. Topping products that can bond to existing concrete is placed at a thickness of about 10mm for maintaining the levels. About 70 to 80mm thick unbonded topping is used on the existing floor if levels are flexible.
Covering the floor with carpet or tiles are a cost-effective solution. To bond the floor covering, the concrete surface should be checked if it has adequate strength.
Things to be remembered during curing for preventing from dusting
- Never let the concrete dry too fast.
- Cover newly cast concrete with polythene or damp hessian and make sure that end of the framework is also covered to protect it from the wind.
- Use aliphatic alcohol and also use concrete curing agents on the fresh concrete to seal the surface for stopping the moisture leaving concrete early.
- After de-molding, the concrete elements should be kept out of direct sunlight and wind.
- To keep the surface moist, spray a fine water mist when the initial setting and hardening of concrete has taken place.
Rules to be followed for preventing concrete dusting
- Should use moderate slump concrete that doesn’t exceed 125mm.
- Should not do finishing operation while concrete is bleeding.
- Should not sprinkle water before or during the finishing operation.
- There should be adequate venting to exhaust gas in enclosed space.
- Curing should be done properly so that concrete retain moisture for the first 3 to 7 days.
Effects of concrete dusting
- Can create a serious health issue in human
- Changes the property of the concrete structure
- Hamper the appearances of the concrete surface
- Makes concrete wear to bear the load.
- Can create cracks in concrete
Health risks of concrete dusting
Many types of cement contain substances like lime, nickel, cobalt, and chromium compound and alkaline substance are destructive to human tissue and chromium are allergic.
Concrete dust can cause coughing, sneezing, and shortness of breath by irritating the nose, throat, and respiratory tract due to mechanical abrasion. This will cause chronic bronchitis and aggravate pre-existing lung diseases like emphysema, bronchitis, asthma.
Long time exposure with concrete dust can cause damage to the human lungs and also crystalline silica can cause silicosis that is incurable. In addition, silica dust can form scar tissue in the lungs by reducing the ability to take oxygen.
I hope this article on “How to stop concrete dusting” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Shreya Parajuli