First, what is the phenomenon of concrete bleeding

Refers to the phenomenon of aggregate sinking and water floating in the process of concrete transportation, vibrating and pumping. Appropriate amount of bleeding can properly improve the collapse of concrete and the surface cracking of concrete in the later stage, and can indirectly reduce the actual water consumption of concrete, thereby reducing the water-cement ratio and increasing the strength. Excessive bleeding, however, adversely affects the construction, durability and appearance of concrete.

The effect of bleeding on concrete properties
1 Due to the unreasonable concrete mix ratio, clean sand and gravel with poor gradation, and good slump-preserving effect of admixtures, bleeding is prone to occur. Severe bleeding is likely to cause poor concrete wrapping and separation of sand and gravel, resulting in blockage of concrete pumping pipes, and the construction site surface is not easy to be plastered, and the hardened concrete is prone to upper and lower layers and surface cracks.

2 Cracks on the concrete surface. Bleeding leads to the formation of a high water-cement ratio in the laitance layer. After evaporation, a porous, loose and weak surface is formed. After the floor or pavement forms a slurry layer, it is easy to peel, which seriously affects the quality of the concrete.

3 Bleeding generally reduces the water-cement ratio at the bottom of the concrete and destroys the uniformity of the interior of the concrete. The mixing water generated on the concrete surface will carry a part of the cementitious material and fine particles in the aggregate, so that a layer of water content on the concrete surface is formed. The laitance layer will cause the surface concrete to be loose and porous, honeycomb pitted surface, and even exposed stones. Affects late strength growth and carbonation rebound.

Three, the cause of bleeding
1 cement

The finer the fineness of the cement, the larger the specific surface area, and the more fine-grained particles in the particle distribution, the more the early cement hydration, and the less serious the bleeding. On the contrary, the bleeding is serious.

2 aggregates

The particle gradation, surface characteristics, particle size, acicular particle content, and porosity of aggregates have an impact on bleeding. Poor stone grading will make concrete workability worse, resulting in concrete bleeding. The larger the aggregate particle size, the more serious the bleeding phenomenon. The larger the fineness modulus, the less fine particles, and the more coarse particles, the more easily the concrete will bleed.

3 Mineral admixtures

(1) Only under the premise that the quality of the fly ash is good, the fly ash is beneficial to improve the bleeding. The poor quality of the fly ash will increase the water demand, which will increase the amount of bleeding.

(2) Adding an appropriate amount of stone powder can effectively improve the bleeding problem of fresh concrete. Stone powder can also increase the slump of concrete mixtures and improve the workability of concrete.

4 admixtures

(1) The use of water reducing agent in fresh concrete will increase the amount of bleeding.

(2) Air-entraining agent can greatly improve the performance of concrete

5 Concrete mix ratio

The amount of cementitious material, sand ratio, and water-cement ratio in the mix ratio have a great influence on the bleeding of concrete.

(1) The larger the water-cement ratio, the longer the cement hardening time, the more free water, and the easier it is to bleed.

(2) The larger the amount of additives, the more bleeding and precipitation of the fresh concrete will be caused.

6 Construction Impact

(1) The transportation distance is long, the stirring time is long, and it is easy to bleed.

(2) The higher the concrete pouring height of the same proportion, the more bleeding.

(3) Excessive vibration of concrete leads to segregation and bleeding of concrete.

(4) Bleeding caused by water spraying by construction workers.

4. Solutions
1 mix ratio

(1) Strictly control the quality of admixtures, only the fly ash with better quality can effectively improve the bleeding.

(2) Appropriately increase the amount of cementitious materials and control the water consumption of the mixing ratio.

(3) Control the sand fineness modulus at 2.3-2.8. Adjust the reasonable sand rate in time according to the mix ratio and concrete state to ensure that the configured concrete has good workability.

(4) Control the particle size of the stones to prevent the grading from breaking.

(5) Select a good water reducing agent and appropriately increase the amount of air-entraining agent.
2 Construction aspects

(1) The slump should be as small as possible.

(2) It is strictly forbidden to add water to the concrete at will, and control the height of the concrete pouring.

(3) Control the vibrating time.

(4) For closed formwork, when there is bleeding on the concrete surf