The use of polycarboxylate superplasticizer concrete, compared to the use of traditional naphthalene series, aliphatic superplasticizer concrete, most of the time bleeding will be lower, less bleeding will be larger. But once the use of carboxylic acid concrete excessive bleeding, often difficult to adjust up.
1.Molecular mechanism design of polycarboxylate superplasticizer
Polycarboxylate superplasticizer can reduce bleeding in different ways. Firstly, polycarboxylate superplasticizer reduces water consumption on the premise of guaranteeing the workability of cement-based materials. When the water consumption of mixing materials decreases, bleeding water will be significantly reduced. Secondly, the polycarboxylate water-reducing agent has an excellent dispersion effect, which can fully disperse cement particles into the water to prevent the sedimentation of larger particles, thereby reducing bleeding. Thirdly, polycarboxylate molecules were adsorbed on the surface of cement particles, making cement particles repel each other, reducing the effective cement particle size of flocculation in a mass, delaying the sedimentation rate and reducing bleeding water. Another advantage is that the polycarboxylate superplasticizer molecules are highly customizable. Smaller molecular weight, dispersion will increase, bleeding slightly reduced, but at the same time brought concrete liquidity loss should be considered. Similar to the effect of molecular weight on bleeding, the content of the main chain adsorption group in polycarboxylate molecules, the density of graft chain, length of branch chain and other factors have an effect on concrete bleeding.
2 Polycarboxylate superplasticizer added excipients
Adding auxiliary materials in polycarboxylate superplasticizer, such as air-conditioner, defoamer, coagulant promoter, retarder, viscosity regulator, etc., can improve concrete bleeding.
Air entrainer can reduce the surface tension of water and introduce tiny bubbles. Due to the different polarity of the static charge, these tiny bubbles will adsorb on the surface of cement particles, and the average density of the cement particles adsorbing the bubbles will decrease, which will reduce the sedimentation speed and thus reduce bleeding. Antifoaming agents have a similar mechanism but the opposite effect. Coagulant can shorten the coagulation time, so the solid particle deposition time will be shortened, surface bleeding will be reduced. Conversely, retarders increase bleeding rates because they extend the time it takes for water to rise to the surface of the freshly mixed slurry. Viscosity regulator is used to improving the viscosity of mixing water, and then adjust the sedimentation rate of solid particles, to achieve the regulation of bleeding.
3.Gradation of concrete aggregate
Suitable aggregate gradation, help to reduce bleeding, mainly reflected in the sand rate, sand modulus two aspects.
When the sand rate is too small, the amount of mortar is insufficient, and enough mortar layer can not be formed around coarse aggregate for lubrication and filling, which will reduce the fluidity of the mixture, make the cohesion and water retention of the concrete mixture become poor, making the concrete mixture appear rough, coarse aggregate segregation, cement slurry loss, water is easy to precipitate out. An appropriate increase in sand rate can reduce bleeding; Otherwise, bleeding can be increased. The influence of sand modulus on bleeding water actually refers to the smallest particle part of aggregate, especially the part whose particle size is less than 0.315mm, which is especially critical to the water retention performance of concrete. Reduce the modulus of sand appropriately, or add the right amount of silt, help to reduce concrete bleeding; Otherwise, bleeding can be increased.
4 Concrete admixture
Concrete bleeding essentially can be seen as a finer level of segregation, water as a liquid phase, all solid particles are a solid phase, the process of bleeding is water viscous resistance is not enough to overcome the gravity of cement particles process. Then consider adding some fine mixing materials, such as a fine fly ash, silicon ash, because these material particles is small, not be separated from water, and water evenly mixed together constitute the liquid phase, increasing the liquid viscosity, to enhance ability to prevent cement particles under the effect of gravity settlement, thus can reduce bleeding.
The contribution of cement to concrete bleeding is mainly two aspects, one is fineness, the other is admixture.
The increase in cement fineness has two positive effects on the reduction of bleeding. First, the decrease of cement particle size means that the sedimentation speed is slowed down. In addition, the finer the cement particles, the larger the specific surface area, the faster the reaction rate with water, the higher the viscosity, the faster the condensation and hardening of the system will be, thus leading to a decrease in the effective time of solid particles deposition, and thus the reduction of bleeding water.
Different types of cement admixtures also lead to differences in bleeding rate, which is the same as the role of concrete admixtures.
Concrete bleeding is a double-edged sword, only control the appropriate bleeding rate, in order to ensure the smooth concrete pouring and hardening of the indicators. The index of bleeding water is affected by many factors such as water-reducing agent, admixture, cement, aggregate, air temperature, mixing ratio, and construction method. When it is found that the concrete bleeding rate is too high or too low, the relevant units in the upstream and downstream, including construction parties, concrete companies, raw material suppliers, especially admixture manufacturers, should take timely measures to select one or more schemes to control the bleeding rate in a reasonable range, which is conducive to eliminating hidden dangers and ensuring the quality of the project.
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