In the field of modern construction, concrete, as a widely used main building material, is crucial for ensuring the safety and durability of structures. However, during actual applications, we sometimes encounter the issue of concrete surfaces, such as roadways, floors, walls, beams, and columns, developing a sandy or powdery appearance. This not only affects the aesthetic appeal of the concrete but may also have adverse effects on its structural performance. What causes this change in the concrete surface, and how should one address this situation?
Reason Analysis and Measures for Concrete Surface Dusting
The surface structure of concrete is loose and has low strength. The main reasons for the loose surface structure and low strength of concrete are twofold:
1) The water-cement ratio of the concrete surface is greater than that of the internal concrete, resulting in a less dense interlocking of hydration products and a higher porosity.
2) Improper concrete curing leads to rapid water loss in the early stages of construction, resulting in a high number of water pores. The surface cement does not receive sufficient moisture for hydration.
2. Assessing the hydration level of cement in the concrete surface can help identify the cause of "powdering."
The higher hydration degree of the surface cement is mainly due to bleeding, whereas the lower hydration degree on the surface is primarily caused by improper construction and maintenance.
3. Factors Affecting the Water-Cement Ratio on Concrete Surfaces
3.1 Concrete Mix Proportions
3.1.1 The higher the water-cement ratio in concrete, the longer the time for cement to set and harden, the more free water there is, and the longer the time for water to separate from cement, making the concrete more prone to bleeding.
3.2.2 Excessive dosage of admixtures in concrete, or an excessive amount of retarding components, can lead to significant bleeding and segregation in freshly mixed concrete. A large amount of free water is discharged from the concrete surface, affecting the setting and hardening of cement, reducing the water-retaining performance of concrete, and causing severe bleeding.
3.2 Composition of Concrete
3.2.1 When sand and gravel aggregates contain a high amount of mud, it severely affects the early hydration of cement. The clay's colloidal particles wrap around the cement grains, delaying and obstructing the hydration process and the setting of concrete, thereby exacerbating the bleeding of concrete.
3.2.2 The finer modulus of sand increases, the coarser the sand becomes, and the more likely it is to cause concrete bleeding, especially for particle contents below 0.315mm and above 2.5mm, which have a significant impact on bleeding: the fewer the fine particles, the more the coarse particles, the more likely the concrete is to bleed.
3.2.3 The particle size distribution of mineral admixtures also affects the bleeding performance of concrete. If the fine particle content of the mineral admixture is low and the coarse particle content is high, it is prone to cause bleeding in concrete. The use of finely ground slag as an admixture, due to the reduced cement content in the mix, results in a slower hydration rate of slag and a poorer water-retaining performance of the slag glass, which often increases the bleeding amount of concrete.
3.2.4 Coarse fly ash, weakened by the effect of fine aggregates, will increase the bleeding water content of concrete.
3.2.5 The setting time, fineness, specific surface area, and particle distribution of cement all affect the bleeding performance of concrete. The longer the setting time of cement, the longer the setting time of the concrete it is mixed with, and the increase in setting time is several times greater than that of cement paste. The longer the settling time of cement particles before the concrete is placed and sets, the more likely the concrete is to bleed; the rougher the fineness of cement, the smaller the specific surface area, and the less the content of fine particles (＜5um) in the particle distribution, the less the early cement hydration, and the fewer hydration products are not enough to block the capillaries in the concrete, allowing moisture to move upwards from the bottom, resulting in more severe bleeding of the concrete.
3.3 Construction and Maintenance
3.3.1 Over-vibration during the construction process does not cause the denser admixtures or aggregates to be vibrated to the surface of the concrete; instead, it exacerbates the bleeding of the concrete, resulting in an increased water-cement ratio on the concrete surface.
When water is applied for curing or the surface is washed by rain before the cement in the concrete surface has hardened, it can also cause the water-cement ratio on the concrete surface to increase.
During the construction and maintenance of concrete, when the sun is excessively hot or the weather is extremely dry, the evaporation of surface moisture exceeds the bleeding rate of concrete, leading to significant loss of surface moisture. This causes the surface cement to lack adequate hydration and fails to develop sufficient surface strength, resulting in the phenomenon of powdering.
Therefore, construction and maintenance methods should be adjusted promptly according to different climatic conditions, various strength grades of concrete, and different types of cement to ensure adequate wet curing of the concrete after construction and before it reaches sufficient strength, while avoiding severe bleeding.
How to Prevent Concrete Surface from Developing a Powdered Appearance?
4.1 Concrete must have good water retention to prevent excessive bleeding, which can lead to an overly high water-cement ratio on the concrete surface. Starting from the mix ratio and selection of constituent materials, it is important to control the water-cement ratio to avoid being too high, not to overuse admixtures, and to ensure an appropriate setting time. The aggregate of sand and stone must meet national quality requirements, with particular attention to the content of particles below 0.315mm in the sand. The setting time of cement should not be too long, the surface area should not be too small, and the particle gradation should not be overly concentrated.
4.2 Prevent excessive vibration during the construction process to avoid serious segregation and bleeding of concrete.
4.3 Post-construction, it is crucial to promptly carry out maintenance. This includes preventing the concrete surface from being washed away by rain before it hardens, which can lead to an excessive water-cement ratio on the surface. Additionally, it is important to avoid the loss of moisture in the concrete before the surface has gained strength, especially in the case of concrete blended with fly ash or slag. This is due to the lower early strength, where there isn't enough hydration products to seal the large capillary pores on the surface. Neglecting adequate wet curing in the early stages can result in rapid and excessive moisture loss from the concrete surface, insufficient hydration of the surface cement, and ultimately, lower surface concrete strength and a loose structure. Typically, as the concrete approaches final setting, it should be subjected to a second smoothing or pressing to enhance the density of the concrete surface structure.
02Analysis of the Causes of Surface Sanding
The cement mortar mixture has an excessive water-cement ratio, which has reduced the strength of the plastering layer.
2. Lack of understanding of the basic principles of cement hardening leads to premature or delayed ground smoothing.
Improper maintenance can lead to issues after the completion of concrete flooring. If the curing period is insufficient and the drying environment causes rapid evaporation of moisture, the hydration process of the cement will be affected, leading to dehydration of the mortar and impacting its strength and wear resistance. Additionally, watering the floor too early can result in widespread peeling, exposure of sand particles, and sanding issues upon use.
4. Premature use of concrete floors. Concrete floors are used before reaching sufficient strength, leading to damage and easy sanding. This issue is particularly severe during winter.
5. Freezing damage. Due to winter construction without sealed doors and windows or heating equipment, freezing damage occurred, resulting in sanding and peeling.
6. Misuse of newly laid floors in winter. Burning fires to heat up rooms with newly constructed cement floors in winter produces harmful carbon dioxide gas. When it comes into contact with the cement mortar surface, it reacts with the unhardened calcium hydroxide in the cement, forming calcium carbonate. This hinders the normal hydration process of the cement mortar, significantly reducing the strength of the floor surface, often leading to sanding of the floor.
7. Raw materials do not meet requirements.
Low cement strength or the use of expired cement, dampened and caked cement; this type of cement has low activity, severely reducing the surface strength and wear resistance.
b. High clay content in sand. The clay content in sand used for ground surfaces exceeds 10%, resulting in a decrease of 20-50% in the strength of the ground surface layer, poor adhesion, and severely causing sanding of the ground.
C. Sand is too fine. The large surface area of the sand requires more water during mixing, which increases the water-cement ratio and decreases the strength.
Reasons for Sanding in Cement Mortar Floors
1. The mortar consistency is not up to standard; it's too thin. The higher the water-cement ratio, the lower the strength of the cement mortar. Therefore, excessive water usage during construction will significantly reduce the surface mortar strength, leading to loose cement dust on the surface after walking on it.
2. Lack of understanding of the basic principles of cement hardening, improper process scheduling, and issues like the sub-layer being too dry or too wet can lead to premature or delayed ground smoothing. Early smoothing results in the hydration process of cement just beginning, with the gel not fully formed and a significant amount of free water remaining, causing surface floating water and reducing the strength of the cement mortar surface. Delayed smoothing means the cement has already set and hardened, making it impossible to eliminate the capillaries and smudges on the cement mortar surface. Additionally, it can disturb the already hardened surface, significantly reducing the surface strength and abrasion resistance.
3. Improper maintenance. As cement enters the hardening stage, the hydration process continues and penetrates deeper into the cement particles. The deeper the hydration, the higher the strength of the cement mortar. If the hydration process is affected due to a lack of water, it can slow down or even stop the hardening process, leading to dehydration of the cement mortar, which in turn affects its strength and wear resistance.
4. Inadequate protection of finished products has led to the concrete floor mortar not reaching sufficient strength before foot traffic or subsequent construction work. This has caused friction on the ground, resulting in sanding. The cement mortar flooring may also be damaged due to freezing, leading to reduced adhesion and forming loose particles.
5. Raw materials did not meet requirements. Low cement grade and excessively fine sand particle size can cause sanding on the cement mortar surface.
04Analysis of Reasons for Concrete Mortar Floor Delamination
1. Improper cleanliness and excessive smoothness of the base surface. During decorative construction, it is typically done in the order of ceiling, walls, followed by the floor. Therefore, during floor construction, the base surface often has loose mortar films and other construction debris, especially lime砂浆 from indoor painting, which sticks to the floorboards and is extremely difficult to clean thoroughly. This surface dust significantly affects the bonding strength between the base and the surface layer. Excessive smoothness of the base surface, during the casting and shaping process of the cast-in-place reinforced concrete floor, results in an insufficiently smooth and rough concrete surface treatment, leading to inadequate bonding strength between the base and the surface layer, causing the surface layer to hollow out.
2. The base surface is overly dry or there is water accumulation on the base surface. An overly dry base surface leads to the mortar losing moisture too quickly after application, resulting in low strength. Moreover, when the base surface is excessively dry, it absorbs a layer of powder, which acts as an insulating layer between the surface and the base, causing the bonding between the base and the surface to be loose, leading to hollow spots.
During the construction of concrete mortar flooring, it is common to brush a layer of plain cement mortar on the base surface. If the brushing is done too early, the brushed cement mortar has already dried and hardened, which not only fails to increase the bonding strength but also acts as an isolation layer between the base and the surface. If the dry cement is sprinkled first and then swept for construction, it can lead to uneven wetting and drying of the cement mortar, which is one of the hidden dangers causing hollowing in the cement floor.
05Reasons for Cracking in Cement Mortar Floors
Primarily, the issue lies in the expansive floor area, leading to excessive volume shrinkage of the concrete during the curing process.
1. Poor cement stability or hot cement fresh from the mill. The used cement has a significant shrinkage during setting and hardening, and the mixing of different types or grades of cement within the same floor leads to cracks on the surface due to varying setting times and shrinkage amounts.
2. The sand particle size is too fine or contains too much silt. The finer the sand particles, the larger the surface area, which results in a greater amount of cement paste adhering to the sand. Consequently, with the cement amount unchanged, the strength of the cement mortar will decrease. Moreover, an excessive amount of silt in the sand means that the soil in the cement mortar will shrink during the hardening and dehydration process, leading to cracking on the floor surface.
3. Inadequate maintenance or lack of maintenance. After the final setting of cement mortar, hydration will continue. During the hot and dry season, if not properly maintained or maintenance is delayed, the cement surface layer may develop dry shrinkage cracks.
4. If the cement mortar is too thin or not mixed evenly, it can lead to a decrease in the tensile strength of the mortar. Once the overall surface of the cement mortar is subjected to tensile stress, cracking on the cement surface may occur.
5. Poor backfill quality. Poor soil quality or inadequate compaction of the backfill can lead to uneven settlement and cracking of the ground surface after completion. Additionally, large areas of ground without construction joints and structural deformation can also cause the ground surface to crack.