There are two main reasons for loose structure and low strength of concrete surface:

1) The water-cement ratio of the concrete surface is larger than that of the concrete interior, and the bonding between the hydration products of the surface is not dense, and the void ratio is large;

2) The water loss in the early stage of construction is too fast due to improper concrete curing, forming a large number of water holes, and the cement on the surface can not get enough water for hydration.

Testing the hydration degree of cement on the concrete surface can help to identify the cause of “Dusting”.

The high hydration degree of surface cement is mainly caused by bleeding, and the low surface hydration is mainly caused by improper construction and maintenance.

Factors influencing water-cement ratio of concrete surface

The mix proportion of concrete

1. The greater the water-cement ratio of concrete is, the longer the setting and hardening time of cement is. The more free water is, and the longer the separation time between water and cement is, the more easily the concrete will bleed.
2. Excessive admixture or retarder in concrete will cause a large amount of bleeding and sedimentation of fresh concrete, a large amount of free water secreting out of the concrete surface, affecting the setting and hardening of cement, and reducing the water retention performance of concrete, resulting in serious  water bleeding.

 The component materials of concrete

1. When the sand and gravel aggregate contains large amounts of mud, it will seriously affect the early hydration of the cement, and the sticky particles in the clay will wrap the cement particles, delaying and hindering the hydration of the cement and the setting of the concrete, thus intensifying the water secretion of the concrete.
2. The larger the fineness modulus of sand is, the coarser the sand is, which is more likely to cause concrete bleeding. Especially, the content of particles below 0.315mm and above 2.5mm has a greater impact on bleeding: the less fine particles are, the more coarse particles are, and the coagulation is more likely to bleed.
3. The particle distribution of mineral admixtures also affects the bleeding performance of concrete. If the mineral admixtures contain less fine particles and more coarse particles, it is easy to cause bleeding of concrete. When ground slag is used as admixture, the hydration speed of slag is slow due to the reduction of cement content in the mix proportion, and the water retention performance of slag glass is poor, which often increases the bleeding of concrete.
4. If the fly ash is too coarse, the fine aggregate effect will be weakened, which will increase the concrete bleeding.
5. The setting time, fineness, specific surface area and particle distribution of cement will affect the bleeding performance of concrete. The longer the setting time of cement, the longer the setting time of concrete prepared, and the longer the setting time is than that of cement paste. Before concrete standing and setting and hardening, the longer the settling time of cement particles, the easier the concrete bleeding; The coarser the fineness of cement is, the smaller the specific surface area is, the less the content of fine particles (<5um) in the particle distribution is, the less the hydration amount of early cement is, and the less hydration products are not enough to block the capillary pores in concrete, resulting in the easy movement of internal moisture from bottom to top and the more serious concrete bleeding.

Construction and maintenance

1. The excessive vibration during construction does not vibrate the admixture or mixture with smaller density in the concrete to the concrete surface, but intensifies the bleeding of the concrete and increases the water-cement ratio of the concrete surface.
2. The water-cement ratio of the concrete surface layer will also increase when the cement on the concrete surface layer is not hardened before being watered for curing or the surface is scoured by rainwater.
3. In the process of construction and maintenance of concrete, when the concrete is exposed to the sun or dry weather, the evaporation of surface moisture is greater than the bleeding rate of concrete, which will lead to a large amount of volatilization of surface moisture, insufficient hydration of surface cement, and insufficient surface strength, resulting in Dusting.

Therefore, the construction and maintenance methods should be adjusted according to different climatic conditions, different strength grades of concrete and different varieties of cement in a timely manner to ensure that the concrete has sufficient wet maintenance without severe water retention after construction until sufficient strength is established.

How to avoid dusting on concrete surface?

1. The concrete itself shall have good water retention to prevent excessive water-cement ratio of the concrete surface layer caused by serious bleeding. Starting from the selection of mix proportion and composition materials, attention shall be paid to controlling the water-cement ratio not to be too large, the admixture not to be too mixed and the setting time to be appropriate. The aggregate shall meet the national quality requirements, especially the content of particles below 0.315mm in the sand. The setting time of cement should not be too long, the specific surface area should not be too small, and the particle gradation should not be too concentrated.
2. Serious segregation and bleeding of concrete caused by excessive vibration shall be prevented during construction.
3. Attention shall be paid to timely curing after construction, not only to prevent excessive water-cement ratio of concrete surface caused by rain wash before the concrete surface hardens, but also to prevent loss of water in concrete before the strength of surface layer is established. Especially for concrete mixed with fly ash or slag, due to its low early strength, there is not enough hydration products on the surface layer to block large pores on the surface layer, If sufficient wet curing is not paid attention to in the early stage, the water loss of the concrete surface layer is relatively fast, and the surface cement is not fully hydrated, which will also lead to the low strength and loose structure of the surface concrete. Generally, when the concrete is close to final set, the concrete shall be plastered or pressed again to make the concrete surface structure more compact.