How to prevent concrete from bleeding?

What is concrete bleeding? 

The phenomenon that coarse aggregate sinks and water floats up in the process of transportation, vibration, and pumping of concrete is called concrete bleeding. 

Bleeding is an important aspect of the workability of fresh concrete. 

Generally speaking, the indexes describing the water secretion characteristics of concrete are the amount of water secreted (that is, the average amount of water secreted per unit area of concrete mixture) and the rate of water secretion (that is, the ratio of the amount of water secreted to the water content of concrete mixture). 

According to the principle of concrete bleeding and the mechanism of various factors affecting concrete bleeding, the main methods to solve concrete bleeding are as follows. 

1. Mix the proportion of concrete. 

Properly increase the amount of cementitious material, improve the sand ratio of concrete, and make the concrete bleed air appropriately because it does not meet other properties. 

Under the premise of ensuring the construction performance, reduce the unit water consumption as much as possible. 

2. Raw materials. 

Fine cementitious materials and high-quality air-entraining agents are selected. 

3. Superplasticizer. 

The superplasticizer with low bleeding and high fluidity of concrete is selected. 

Suppose the mixture ratio is fixed when the standard and use requirements are met. In that case, the amount of water reducer with appropriate water reducing rate is selected to avoid bleeding caused by too high water reducing rate. 

When mixing admixtures, small materials such as polyether rheological agents, water retaining agents, mud inhibitors, and glue reducers must be added to control the bleeding of concrete, improve the workability of concrete, increase the cohesion, fluidity, and water retention of concrete, and improve the durability of concrete. 

4. Construction. 

Strictly control the vibration time of concrete to avoid overvibration. 

In addition, for the performance control of cast-in-place concrete, the appropriate control points are selected to make the control help reduce the bleeding of concrete. 

Suppose the maximum air content is to be controlled. In that case, the control point can be selected at the warehouse entrance to minimize the influence of air content loss on water secretion in concrete transportation. 

When the bleeding has already appeared on the warehouse surface, it must be eliminated in time. The most effective method is to absorb water by vacuum, manually drawing water from the warehouse surface, or absorbing water with highly absorbent materials such as sponges, especially when the concrete is finished; the bleeding should be absorbed in time to facilitate the concrete surface to ensure the appearance quality of the concrete. 

It is strictly forbidden to open a hole in the formwork, resulting in the loss of cementitious materials and affecting the quality of concrete. 

Especially in the concrete noodles, we should absorb the bleeding water in time to facilitate the concrete noodles. 

5. Improve the bleeding of concrete by concrete admixture. 

Concrete admixture (superplasticizer) is generally an organic polymer. 

Organic polymers' molecular weight or chain length directly affects their properties. 

If the molecular weight of the superplasticizer is larger and the molecular chain is longer, the water secretion of concrete will be reduced, but at the same time, the water-reducing rate of the superplasticizer is lower; if the molecular weight is smaller and the molecular chain is short, the water-reducing rate of concrete will be increased at the same time. 

Some superplasticizers have branched chains on the main molecular chain. No matter if the branched chains of the main chains are longer, the water-reducing rate of concrete will be reduced correspondingly. If the main chain is short and the branched chains are long, the water secretion will be reduced, and the water-reducing rate will not be affected much. 

In general, a superplasticizer is not composed of molecules with a single molecular weight but a mixture of molecules with various molecular weights. 

In the case of reducing water secretion and ensuring water reduction rate, it is necessary to optimize the molecular size of the superplasticizer to achieve the best collocation relationship between small molecules and macromolecular substances. 

When mixing admixtures, small materials such as polyether rheological agents, water retaining agents, mud inhibitors, and glue reducers must be added to control the bleeding of concrete, improve the workability of concrete, increase the cohesion, fluidity, and water retention of concrete, and improve the durability of concrete.

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