1. Bleeding
1. Overview
Simply put, concrete is made by mixing cement, coarse aggregate, fine aggregate and water. Among these components, water has the smallest specific gravity, so during the concrete pouring process, when other components settle During this time, water continues to migrate upward. Some can reach the surface of the concrete and overflow, while some cannot reach the surface. This is bleeding. This process continues until the concrete no longer settles.
2. The impact of bleeding on concrete quality
If the bleeding speed is different from the concrete quality Surface water evaporates at the same rate and bleeding is harmless. The moisture content of concrete is very necessary to ensure workability and improve constructability. If excess water can overflow through bleeding after pouring, the water-cement ratio can be reduced and the density of concrete can be improved. Early bleeding when the concrete is still in the plastic stage does no harm to the concrete. At this time, the concrete is still in the settling and vibrating stage. If the concrete has lost its plasticity and begins to set initially, the bleeding at this time will have a negative impact on the performance of the concrete.
The total amount of bleeding is closely related to the concrete mix ratio, especially the water consumption, fine particle content (cement, fly ash and other mineral additives, fine bone materials, etc.). The greater the water consumption, the greater the amount of bleeding; the greater the content of fine particles, the smaller the amount of bleeding. The amount of bleeding is also related to the thickness of the concrete pouring. The thicker the thickness, the more bleeding.
Generally, bleeding occurs evenly across the entire concrete surface. However, in some special cases, if there are vertical bleeding channels inside the concrete, the water will migrate upward very quickly in these channels, which may bring out some fine particles, leaving “worm eyes” inside and forming on the surface. “Sand bar”. These drainage channels are usually caused by excessive bleeding. The bleeding channel leaves an open path on the surface of the concrete, allowing corrosive substances such as deicing salts to easily enter the interior of the concrete, causing the concrete to suffer from corrosion of steel bars, reduced durability and other diseases.
During the bleeding process, water may be retained by flat or flaky aggregates and steel bars, and water will form under these aggregates or steel bars, causing What is called “endocrine water” is formed. This endogenous water may also eventually exude from the surface, forming pooling water on the concrete surface. The endocrine water trapped under the aggregate or steel bars will increase the local water-cement ratio, affecting the bonding strength of the cement stone, aggregates and steel bars, forming a fragile joint surface that will be damaged by factors such as hydration heat and dry shrinkage. Under the action of the tensile stress caused, micro-cracking is prone to occur. During the service of the structure, affected by the load, these micro-cracks will gradually intensify, directly affecting the safety and durability of the concrete structure.
Surface bleeding will reduce the density and friction resistance of the concrete surface. If the bleeding water on the surface is smoothed and polished before it has completely evaporated, the bleeding water will be pressed back to the surface of the concrete, increasing the water-cement ratio of the surface layer, reducing the strength of the surface layer, easily inducing plastic cracking, dry shrinkage cracking, and affecting the strength of the concrete. Durability, and may also cause bubbles, peeling, sanding and other phenomena on the concrete surface. Therefore, the concrete surface must be roughly smoothed before bleeding occurs, and then polished after the bleeding has completely evaporated.
Bleeding will also cause a large amount of cement slurry to accumulate at the horizontal construction joints, and a low-strength bonding layer will appear at the construction joints.
3. Effect of concrete components on bleeding
1. Water-cement ratio: The larger the water-cement ratio, the more bleeding will occur. If the water consumption increases by 20%, the amount of bleeding may increase by 2 to 3 times.
2. Cement: The type, fineness and dosage of cement will affect bleeding. The finer the cement, the less the amount of bleeding; the greater the amount of cement, the less the amount of bleeding. Therefore, increasing the amount of cement and reducing the water-cement ratio can reduce bleeding.
3. Mineral additives: Mineral additives such as fly ash, slag, silica powder and natural volcanic ash can reduce bleeding, especially silica powder. The effect on reducing bleeding is very obvious. Therefore, early maintenance of silica fume concrete should be strengthened to prevent plastic shrinkage and cracking.
4. Aggregate: The higher the mud content of the aggregate, the less bleeding. However, high mud content will have a negative impact on other properties of concrete, such as increasing water demand, increasing shrinkage, etc.
5. Chemical admixtures: Air-entraining agents can reduce bleeding. Water reducers will also reduce bleeding.
4. Measures to reduce bleeding
1. Improve the mix ratio of concrete;
2. Mix the concrete thoroughly;
3. Use mineral additives;
4. Improve the gradation of aggregates;
5. Increase the amount of cement-based materials;
6, choose cement with finer particles;
7, use air-entraining agent;
8. Prevent over-vibration, which will increase bleeding;
2.Segregation
1. What is segregation?
The segregation of concrete means that the coarse aggregate in the concrete is not evenly and fully wrapped by the fine aggregate and cement slurry, and the texture of the concrete is uneven after hardening. There are two types of segregation. The first is when the coarse aggregate separates from the concrete mixture when the concrete is in a plastic state. The second is the separation of cement slurry from the concrete mix.
2. Causes of concrete segregation
Concrete is aggregate, cement, water and additives A mixture of ingredients that vary in size and specific gravity. For example, aggregates typically have a specific gravity of 2.6 to 2.7, while most cements have a specific gravity of 3.1 to 3.2. Differences in size and specific gravity give these components a tendency to separate from each other. If errors occur during production and construction, segregation will occur.
The main causes of segregation include:
1. Unreasonable aggregate gradation and water use Excessive quantity is the main cause of segregation.
2. The concrete mix ratio is improper, and the cementitious material is too small to fully wrap the coarse aggregate, causing the coarse aggregate to settle and segregate.
3. The concrete is not mixed sufficiently.
4. During the construction process, the vertical drop of concrete is too large, causing segregation.
5. When mixing concrete on site, if the mixer is of poor quality or the blades are severely worn, it may easily cause concrete to segregate.
6. High slump concrete or pumped concrete will cause segregation if there is excessive vibration or improper use of vibrators.
7. Surface smoothing and polishing operations immediately after concrete pouring may cause the coarse aggregate to settle and the cement slurry to float, leading to segregation.
8. Too dense reinforcement can easily lead to segregation.
3. Measures to prevent concrete segregation
1. Optimize the mix ratio of concrete to improve Cohesion of concrete. Such as reducing the water-cement ratio, increasing the amount of cementitious materials, adding fly ash, silica powder and other mineral additives, etc.
2. Strengthen the quality control of construction links such as transportation, pouring, and vibration.
3. The use of air-entraining agents can reduce water consumption without sacrificing workability, thereby reducing segregation and bleeding.
4. When pouring concrete, avoid free falling of concrete from high places. The shorter the vertical drop distance, the better.
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