Concrete dusting is also called as concrete Chalking. Concrete dusting is the buildup of fine, loose, powdery material on the surface of hardened concrete that can be easily removed. Dust generation is observed both inside and outside buildings, but is more problematic when it occurs indoors.
Dust is formed by a thin, weak layer on the concrete surface called slime, which is composed of water, cement, and fine particles. Therefore, elements that reduce the strength of concrete, especially the top layer, increase the risk of dust generation. For example, redoing concrete that is still oozing, concrete on non-absorbent substrates, and poor curing. Concrete surface dust can be prevented by creating a tough, wear-resistant concrete surface. This can be achieved by curing properly, avoiding premature completion, using low water-to-cement ratios, and avoiding the addition of dry cement and water to accelerate completion.
Table of Contents
Causes of Concrete Dusting
1. Early Completion
Finish before degassing the concrete edge. This increases the water-to-cement ratio of the top layer of concrete and reduces its strength.
Similarly, moisture from warm, moist air condenses on cold concrete and then smoothes out, reducing the strength of the top layer of concrete.
Water condenses on freshly poured concrete when concrete is poured during relatively humid and cold weather, such as: B. Cold concrete in the basement.
2. Concrete placement on non-absorbent surfaces
Pouring concrete over a polyethylene vapor barrier or non-absorbent substrate reduces normal absorption by the substrate, increases bleeding, and increases the likelihood of dust formation on the concrete surface.
3. Concrete loading due to rain or frost
Exposure of freshly poured concrete to rain, snow, dry wind and frost weakens the surface and increases the risk of dust formation.
4. Poor ventilation in closed rooms
Carbon dioxide from confined space construction equipment, such as gasoline engines and mixer motors, causes charring, reduces concrete strength, and increases the chances of dust formation.
5. Insufficient curing
Inadequate hardening creates a soft surface skin that easily collapses under foot load and leads to dust formation.
6. Low cement content
Mixtures with low cement content tend to generate dust.
7. Extra water in the mixture
Adding more water than required for the mix design can increase the amount of tap water and cause excess water and fines on the concrete surface. The end result is a weakly permeable surface layer with poor wear resistance.
How to Prevent Concrete Dusting
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 improve the workability of concrete while maintaining a low water-to-cement ratio.
2. Correct finishing procedure
Avoid sprinkling dry cement on plastic concrete surfaces to absorb blood water. Reduce excessive concrete bleed by using air-entrained concrete, changing mix ratios, or shortening curing time.
Do not finish if water remains on the concrete surface or if 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
Properly harden the concrete using a suitable hardening method such as liquid membrane curing compound, covering the concrete surface with water, wet burlap, or other curing material.
5. Do not put concrete on dry surface
Do not apply concrete directly to polyethylene vapor barriers or non-absorbent substrates. If this occurs, place a 75mm to 100mm size cut compressible filling on top of a vapor check or non-absorbent subbase prior to pouring.
If the evaporation rate is high, moisten the absorbent slightly before pouring the concrete. Do not allow water to pool on the surface of the substrate.