Causes and measures of concrete bleeding

1. What is concrete bleeding

In layman’s terms, it means that the particle gradation in cement concrete is unreasonable , the proportion of large-diameter particles is relatively large, so that water cannot be dispersed evenly and stably into the gaps between particles. During the process of concrete transportation, vibration, and pumping, the cement and aggregate settle, causing water seepage before the concrete solidifies. The phenomenon that comes out of the concrete surface is called bleeding.

An appropriate amount of bleeding in a normal concrete mixture can reduce the actual water-cement ratio, thereby making the concrete denser. At the same time, on the surface of the concrete, an appropriate amount of Bleeding can play a certain role in modifying and plastering the surface, and can also prevent the surface of newly poured concrete from drying out and cracking. However, excessive bleeding will have a negative impact on the quality of concrete.

2. The hazards of concrete bleeding

1. Harm to concrete surface

Concrete with quicksand watermark defects, surface strength, weathering resistance and Less resistant to erosion. At the same time, the rising water leaves bleeding channels in the concrete, which creates a large number of capillary channel networks that develop from the bottom to the top. These channels weaken the anti-penetration ability of the concrete, making it easy for salt solutions, moisture, and harmful substances to enter the concrete. It is very easy to damage the concrete surface.

Bleeding causes the water-cement ratio on the concrete surface to increase, and floating slurry appears, that is, the floating water contains a large amount of cement. The particles form a reslurry layer on the surface of the concrete. After hardening, the strength is very low, and the wear resistance of the concrete decreases. This is very harmful to concrete that has wear-resistant requirements such as pavement.

2. Harm to the internal structure and performance of concrete

Water pockets are formed around concrete coarse aggregate and steel bars. As the water gradually evaporates, gaps are formed, thus affecting the density of concrete, the interface strength of aggregates, and the holding force between concrete and steel bars. , leading to a reduction in the overall strength of concrete.

Plastic shrinkage caused by concrete bleeding is an irreversible deformation. The settlement of concrete caused by bleeding will lead to plastic cracks in concrete, which will reduce the strength of cement concrete. In particular, bleeding concrete will cause overall settlement. When the pouring depth is large, the mixture near the top moves longer and the settlement is hindered. If it encounters obstacles such as steel bars, plastic settlement cracks will occur from the surface downward to the top of the steel bars. .

Concrete cast in layers is affected by the bleeding on the surface of the lower concrete layer, resulting in a reduction in the bonding strength between concrete layers and easy formation of cracks.

3. Effect on the durability of concrete

Bleeding can also destroy the corrosion resistance and frost resistance of concrete. The factors causing these problems are also related to the internal bleeding channels that appear after bleeding. Corrosive substances can reach the concrete through the bleeding channels. Inside the concrete, corrosion of the steel bars will form on the surface of the steel bars, and the hydration products will cause corrosion reactions and damage the concrete. Bleeding channels can promote water saturation inside the concrete. Highly saturated concrete will suffer freeze-thaw damage under the action of low temperature.

3. Causes of concrete bleeding

The bleeding of concrete is related to almost all aspects of concrete production, such as cementitious materials, aggregate gradation, mix ratio, air content, admixtures, vibration process, etc. Summarize the following factors that affect concrete bleeding:

1. The influence of cementitious materials on concrete bleeding

Cement, as the most important cementing material in concrete, is closely related to the bleeding performance of concrete. The setting time, fineness, specific surface area and particle distribution of cement will all affect the bleeding performance of concrete. The low content of C3A in cement is prone to bleeding; the standard consistency of cement requires less water and is prone to bleeding; slag is prone to bleeding than ordinary silica; pozzolanic Portland cement is prone to bleeding; cement mixed with non-hydrophilic mixed materials is prone to bleeding.

The longer the setting time of cement, the longer the setting time of the prepared concrete, and the extension of the setting time is greater than that of cement. The slurry grows exponentially. Before the concrete is allowed to sit and set and harden, the longer the cement particles settle, the easier it is for the concrete to bleed;

The coarser the fineness of the cement, the smaller the specific surface area, and the smaller the content of fine particles in the particle distribution, the less early hydration of the cement. The smaller hydration products are not enough to block the capillary pores in the concrete, resulting in internal Water tends to move from bottom to top, and the concrete will bleed more seriously. In addition, there are also some cements ground by large mills (especially systems with high-efficiency powder separators). Although the specific surface area is large and the fineness is fine, due to the high powder selection efficiency, the fine particles in the cement (less than 3~ 5μm) content is small, and it is also easy to cause bleeding and powdering on the concrete surface.

2. The influence of aggregates on concrete bleeding

Among the constituent materials of concrete When sand and gravel aggregates contain a lot of mud, it will seriously affect the early hydration of cement., try to reduce unit water consumption.

During the trial mixing of concrete, the concrete should have a slump loss of 20 to 30 mm under static conditions (1h ), concrete is not prone to segregation in actual production.

2. Raw materials

Strictly control the mud content of aggregates, optimize the synthetic gradation of aggregates, and avoid uneven particle composition; select finer cementitious materials and high-quality air-entraining agents.

3. Admixtures

Choose a water-reducing agent with less bleeding. If the mix ratio is fixed, select a slightly lower water-reducing rate or appropriately reduce the amount of water-reducing agent if the standards and usage requirements are met to avoid bleeding caused by excessive water-reducing rates.

A certain amount of thickening agent can be compounded into the concrete admixture; a certain amount of air-entraining agent can also be compounded into the admixture. It can enhance the cohesion of concrete and improve the segregation resistance of concrete;

The water-reducing agent must be compatible when added Experiment to avoid the side effects of water-reducing agents; when it is necessary to reduce bleeding while ensuring the water-reducing rate, it is necessary to optimize the component ratio of the water-reducing agent to achieve the best matching relationship between small molecules and macromolecules.

4. Construction technology

Improve the vibration technology and strictly control the concrete vibration time to avoid excessive vibration.

When the vertical cutting height of concrete exceeds 2 meters, string cutting is used to buffer the impact between the concrete and the contact surface. , to prevent concrete from segregating and bleeding.

When the transportation distance is slightly longer, concrete mixer trucks are generally used for transportation, and agricultural transport trucks are avoided;

In addition, for the performance control of cast-in-place concrete, appropriate control points should be selected so that the control is conducive to reducing concrete bleeding. If you want to control the maximum air content, the control point can be selected at the entrance of the warehouse to minimize the impact of air content loss on bleeding during concrete transportation.

When bleeding has occurred in the poured warehouse surface, it must be eliminated in time. The most effective method is vacuum water absorption, manual Drain water from the warehouse surface or use sponges and other highly absorbent materials to absorb water. Especially when closing the concrete, the bleeding water should be absorbed in time to facilitate the closing of the concrete and ensure the appearance quality of the concrete. It is strictly forbidden to open holes in the formwork to flow freely, which will cause the loss of cementitious materials and affect the quality of the concrete. </p