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Dusting, also known as chalking, is the formation of fine, loose powdery material on the surface of hardened concrete that can be swept away easily. Though dusting is observed inside as well as outside a building structure, it is more problematic when it develops indoors.
Dusting occurs due to a thin, weak layer of a concrete surface, termed laitance, which is composed of water, cement, and fine particles. So, any element that reduces concrete strength, especially the top layer, increases the risk of dusting. For instance, conducting finishing works while concrete still bleeds, placing concrete on non-absorptive surfaces, inadequate curing, etc.
Concrete surface dusting can be prevented by producing strong, wear-resistant concrete surface. This can be achieved through proper curing, avoiding premature finishing, using a low water-cement ratio, and avoiding the addition of dry cement and water to facilitate finishing.
Causes of Dusting
1. Premature Finishing
Carry out the final finishing operation before the bleeding of the concrete ends. This will increase the water-cement ratio of the top layer of concrete, which will reduce the strength.
Similarly, troweling following condensation of moisture from warm, humid air on cold concrete will decrease the strength of the top layer of concrete.
Water will condense on freshly placed concrete when the concrete is poured in cold weather with fairly high humidity, such as cold concrete in basement floors.
2. Placing Concrete on Non-Absorptive Surface
Pouring concrete over polyethylene vapor retarder or non-absorptive subgrade reduces normal absorption by the subgrade, increases bleeding, and consequently increases the likelihood of concrete surface dusting.
3. Exposure of Concrete to Rain and Freezing
Exposure of freshly placed concrete to rain, snow, drying wind, and freezing may weaken its surface and increase the risk of dusting.
4. Inadequate Ventilation in Enclosed Areas
Carbon dioxide from construction machinery in enclosed spaces like gasoline engines and mixer engines may lead to carbonation, decreasing the strength of concrete and increasing dusting possibilities.
5. Insufficient Curing
Inadequate curing creates soft surface skin which easily disintegrates under foot traffic and results in dusting.
6. Low Cement Content
Low cement content mix tends to experience dusting.
7. Adding Excessive Water to Mixture
Adding water more that what is required by the mix design may lead to increased bleed water, resulting in excessive water and fines on a concrete surface. Ultimately, a weak, permeable surface layer with low wear resistance is produced.
Preventive Measures
1. Use Low Water-cement Ratio Concrete
Consider using concrete with low water-cement ratio and adequate workability to produce a strong, durable, and wear-resistant surface. The concrete mixture should be designed to provide designated strength without excessive bleeding.
In general, use a maximum slump value of 125 mm. A higher value is allowed if it does not lead to unacceptable bleeding. Water-reducing admixtures can increase concrete workability while maintaining a low water-cement ratio.
2. Proper Finishing Procedure
Avoid sprinkling dry cement into the surface of plastic concrete to absorb bleed water. Reduce excessive bleeding of concrete by using air-entrained concrete, modifying mix proportions, or accelerating setting time.
Do not carry out finishing while water is present on the concrete surface or when the concrete continues to bleed. Initial screeding should be followed by bull floating immediately; otherwise, delayed bull-floating causes bleed water to be worked into concrete.
Avoid adding water to the concrete surface to facilitate finishing operation, and prevent using a jitterbug of tamper because it tends to bring excess mortar to the surface.
3. Avoid Pouring Concrete in Cold Weather Conditions
Do not place concrete in cold weather unless the temperature exceeds 10°C, and add accelerating admixture to the concrete mixture.
4. Proper Curing
Cure concrete properly using a suitable curing regime like liquid membrane curing compound, covering the concrete surface with water, wet burlap, or other curing material.
5. Do not Place Concrete on Dry Surface
Do not place concrete directly on polyethylene vapor retarder or non-absorptive subgrade. If such a condition arises, place 75 mm to 100 mm of a trimmable, compactible fill over vapor retarder or non-absorptive subgrade before concrete placement.
If a high evaporation rate exists, lightly dampen the absorptive subgrade before concrete placement. Make sure the water does not collect on the surface of the subgrade.
FAQs
Dusting, also known as chalking, is the formation of fine loose powdery fine material on the surface of hardened concrete that can be swept up easily.
Dusting occurs due to a thin, weak layer of a concrete surface, termed laitance, which is composed of water, cement, and fine particles. So, any factor that reduces concrete strength, especially the top layer, increases the risk of dusting.
Concrete surface dusting can be prevented by producing strong, wear-resistant concrete surface. This can be achieved through proper curing, avoiding premature finishing, using a low water-cement ratio, and avoiding the addition of dry cement and water to facilitate finishing.
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