The phenomenon of aggregate sinking and water floating will occur in the process of transportation, vibration and pumping of concrete. Slight secretion of water can improve the collapse of concrete and the surface cracking of concrete in the later stage, and indirectly reduce the actual water consumption of concrete so as to reduce the water-cement ratio and improve the strength. However, excessive bleeding has an adverse impact on the construction, durability and appearance of concrete.
What is the effect of bleeding on the performance of concrete?
Due to the unreasonable mix ratio of concrete, clean sand and gravel, poor gradation and lagging effect of admixture, the phenomenon of water bleeding is easy to occur. Serious bleeding can easily lead to poor encapsulation of concrete and separation of sand and gravel, resulting in pipe blockage of concrete pumping, construction of the site surface is not easy, and hardened concrete is easy to produce upper and lower layers and surface cracks.
It will result in cracks on the surface of concrete. Water secretion leads to the formation of high water-cement ratio in the floating slurry layer, and a loose and weak surface is formed after evaporation. The floor or pavement is easy to peel after the formation of the slurry layer, which seriously affects the quality of concrete.
Bleeding will generally reduce the water-cement ratio at the bottom of the concrete and destroy the internal uniformity of the concrete. When mixed water is generated on the surface of the concrete, it will carry part of the cementitious materials and fine particles in the aggregate, which will form a floating slurry layer with large water content on the concrete surface, resulting in loose and porous concrete, honeycomb hemp surface, and even exposed rock. It affects the growth of strength and carbonization rebound in the later stage.
Why does concrete bleed water?
The finer the fineness of cement, the larger the specific surface area, and the more the content of fine particles in the particle distribution, the more hydration of early cement, and the less serious the bleeding. On the contrary, the bleeding is serious.
The particle gradation, surface characteristics, particle size, needle-like particle content and porosity of aggregate have influence on water secretion. Poor gradation of stones will make the concrete and workability worse, resulting in the bleeding of concrete. The larger the aggregate size is, the more serious the bleeding phenomenon is. The larger the fineness modulus is, the less fine particles are, and the more sand with coarse particles is, the easier it is for concrete to bleed water.
Only under the premise of good quality of fly ash, fly ash is beneficial to improve water secretion. If the quality of fly ash is poor and the water demand increases, the amount of water secreted will increase.
The use of superplasticizer in fresh concrete will increase the amount of water bleeding.
concrete mix ratio
The amount of cementitious material, sand ratio and water-cement ratio in the mix ratio have a great influence on the water secretion of concrete.
The higher the water-cement ratio, the longer the hardening time of cement, the more free water, the easier it is to secrete water.
The more the admixture is added, the more the fresh concrete will bleed and precipitate.
If the transportation distance is long and the stirring time is long, it is easy to secrete water.
The higher the pouring height of concrete with the same proportion, the more water bleeding.
Excessive vibration of concrete will lead to segregation and bleeding of concrete.
Sprinkling water can lead to bleeding.
How to solve concrete bleeding.
Strictly control the quality of admixture, only the fly ash with better quality can effectively improve the water secretion.
Properly increase the amount of cementitious material and control the water consumption of mixture ratio.
Control the fineness modulus of sand in 2.3-2.8. Adjust the reasonable sand ratio in time according to the mix ratio and the state of concrete to ensure that the concrete has good workability.
Control the particle size of stones to prevent gradation from breaking.
Select a good water reducer and appropriately increase the amount of air entraining agent.
Try to keep the slump as small as possible.
It is strictly forbidden to add water to concrete at will and control the pouring height of concrete.
Control the vibrating time.
For the closed formwork, when there is water bleeding on the concrete surface, use a rubber tube to draw the water.
Cannot make a hole in the template, which will cause the loss of cementitious material.
Concrete self-leveling regulating water-retaining agent, also known as concrete water-retaining agent, or concrete vibration-free tamping agent, concrete self-compacting agent and other terms, can improve the surface tension of concrete colloid, so that concrete can achieve self-leveling without vibrating.
The self-leveling regulating water-retaining agent can also adjust the bulk density, improve the strength and prevent the bleeding of slurry in high fluidity concrete. When applied to large flow concrete, it can solve the problems such as unreasonable gradation of sand and stone, excessive stone powder, machine-made sand, low glue material, low-grade large flow concrete, and poor waterline and workability of pumping concrete after setting. For large flow concrete, combining with air entraining agent and defoamer can achieve good effect.
The floating of concrete paste is adjusted by air entraining agent, and the paste and aggregate are bonded and wrapped by adding self-leveling regulator. Add an appropriate amount of defoamer to eliminate the air bubbles in the coarse aggregate, so as to achieve the effect of no bottom, no exposed stone, no water, no slurry, and the effect of water retention.
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