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Measures for controlling concrete bleeding using polycarboxylate superplasticizer

Concrete using polycarboxylate superplasticizers generally has lower water bleeding than concrete using traditional naphthalene and aliphatic superplasticizers, and in rare cases, it may show greater bleeding. However, once excessive bleeding occurs in concrete using polycarboxylate, it is often difficult to adjust.

Molecular Structure Design of polycarboxylate superplasticizer

Polycarboxylate superplasticizers can reduce bleeding in different ways.

Firstly, polycarboxylate superplasticizer reduces water consumption while ensuring the workability of cement-based materials. When the water consumption of the mixture decreases, the bleeding will be significantly reduced.

Secondly, polycarboxylate superplasticizer has excellent dispersibility, which can fully disperse cement particles into water, prevent the settling of larger particles, and thus reduce bleeding.

Third, polycarboxylate molecules are adsorbed on the surface of cement particles, causing them to repel each other, reducing the effective particle size of cement particles that aggregate together, delaying the settling speed, and reducing bleeding.

Another advantage is that polycarboxylate superplasticizer molecules are highly customizable. When the molecular weight is small, dispersibility increases and bleeding slightly decreases, but at the same time, the loss of concrete fluidity should also be considered. Similar to the influence of molecular weight on bleeding, factors such as the content of main-chain adsorption groups, graft-chain density, and branch length in polycarboxylic acid molecules all affect concrete bleeding.

Adding auxiliary materials to polycarboxylate superplasticizer

Adding auxiliary materials such as air entraining agents, defoamers, accelerators, retarders, viscosity modifiers, etc. to polycarboxylate superplasticizers can improve concrete bleeding. Air entraining agents can reduce the surface tension of water and introduce tiny bubbles. Due to the different polarity of the electrostatic charge carried, these tiny bubbles will adsorb on the surface of cement particles, and the average density of cement particles adsorbed with bubbles will decrease, reducing the settling speed and thus reducing bleeding. The mechanism of defoamers is similar, but their effects are opposite.

Coagulants can shorten the setting time, thus reducing the settling time of solid particles and minimizing surface bleeding. On the contrary, retarders increase the bleeding rate because they prolong the time for water to rise to the surface of the freshly mixed cement slurry.

Viscosity modifiers are used to improve the viscosity of mixing water, thereby regulating the settling speed of solid particles and regulating bleeding.

Grading of concrete aggregates

Appropriate aggregate gradation can help reduce bleeding, mainly reflected in two aspects: sand ratio and sand modulus.

When the sand ratio is too low, the amount of mortar is insufficient. A sufficient mortar layer cannot be formed around the coarse aggregate to lubricate and fill, which will reduce the fluidity of the mixture, deteriorate the cohesion and water retention of the concrete mixture, make the concrete mixture appear rough, cause the coarse aggregate to segregate, cause the cement slurry to be lost, and cause moisture to easily precipitate. Properly increasing the sand ratio can reduce bleeding; conversely, it can increase bleeding.

The influence of sand modulus on bleeding actually refers to the smallest particle size in the aggregate, especially the part with a particle size less than 0.315mm, which is particularly critical for the water retention performance of concrete. Properly reducing the modulus of sand or adding an appropriate amount of fine sand can help reduce concrete bleeding; conversely, it can increase bleeding.

Concrete admixture

The bleeding of concrete can essentially be seen as a finer level of segregation. Water is in the liquid phase, and all solid particles are in the solid phase. Bleeding occurs when the viscous resistance of water is no longer sufficient to overcome the gravity of cement particles.

At this point, it is considered to add some finer mixing materials, such as finer fly ash, silica fume, etc. Because these materials have small particles, they do not separate from water. Still, they will mix evenly with water to form a liquid phase, increase the viscosity of the liquid phase, and enhance the ability to prevent cement particles from settling under gravity, thus reducing bleeding.

Cement

The contribution of cement to concrete bleeding is mainly in two aspects: fineness and admixtures.

Increased cement fineness has two positive effects on reducing bleeding. Firstly, the decrease in cement particle size means a slower settling speed. In addition, the finer the cement particles and the larger the specific surface area, the faster the reaction rate with water, the higher the viscosity, and the faster the system sets and hardens. Therefore, this reduces the effective time for solid particle settling, which in turn reduces the amount of water released.

Different varieties of cement admixtures can also lead to differences in bleeding rates, which is similar to the role of admixtures in concrete and will not be repeated here.