The concrete using polycarboxylate superplasticizer, compared with the concrete using traditional naphthalene series, aliphatic superplasticizers, the bleeding will be lower in most cases, and the bleeding will be larger in less cases. However, once the use of polycarboxylic acid concrete excessive bleeding situation, it is often difficult to adjust.
- Molecular mechanism design of polycarboxylate superplasticizer
High performance water reducing Polycarboxylate superplasticizeer can reduce bleeding in different ways. First of all, polycarboxylate superplasticizer water reducer can reduce the water consumption under the premise of ensuring the workability of cement-based materials. When the water consumption of the mix is reduced, the secretion will be significantly reduced. Secondly, polycarboxylate ether superplasticizer has an excellent dispersion effect, which can fully disperse cement particles into water, prevent the settlement of larger particles, and thus reduce bleeding. Third, polycarboxylic acid molecules are adsorbed on the surface of cement particles, making cement particles repel each other, reducing the effective particle size of cement particles flocculated in a mass, delaying the sedimentation rate, and reducing bleeding. Another advantage is that the molecules of polycarboxylate superplasticizer are highly customizable. When the molecular weight is small, the dispersion will increase and the bleeding will be slightly reduced, but at the same time the loss of concrete fluidity should also be considered. Similar to the effect of molecular weight on bleeding, the main chain adsorption group content, graft chain density and branch chain length of polycarboxylate superplasticizer molecules all affect the bleeding of concrete.
- Additives are added to polycarboxylate superplasticizer
The addition of excipients, such as air entraining agent, defoamer, coagulant, retarder, viscosity regulator, etc. to polycarboxylate superplasticizer water reducer 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 static charge, these tiny bubbles will be adsorbed on the surface of the cement particles, and the average density of the cement particles adsorbed by bubbles will be reduced, and the sedimentation rate will be reduced, thus reducing the bleeding. The mechanism of defoamer is similar, but the effect is opposite.
Coagulants can shorten the setting time, so the settling time of solid particles will also be shortened, and the surface bleeding will be reduced. In contrast, the retarder increases the bleeding rate because it increases the time for water to rise to the surface of the newly mixed cement slurry.
The viscosity regulator is used to improve the viscosity of the mixed water, and then adjust the settling rate of the solid particles to adjust the bleeding
- Concrete aggregate grading
Suitable aggregate gradation is helpful to reduce bleeding, which is mainly reflected in sand rate and sand modulus.
When the sand rate is too small, the amount of mortar is insufficient, and enough mortar layer can not be formed around the coarse aggregate to lubricate and fill, which will reduce the fluidity of the mixture, make the cohesion and water retention of the concrete mix worse, make the concrete mixture appear coarse, coarse aggregate segregation, cement slurry loss, and water precipitation easily. Appropriate increase of sand rate can reduce bleeding. Conversely, it can increase bleeding.
The effect of sand modulus on water secretion actually refers to the smallest particle part of the aggregate, especially the part with a particle size less than 0.315mm, which is particularly critical to the water retention performance of concrete. Appropriate reduction of sand modulus or addition of appropriate amount of silt can help reduce concrete bleeding. Conversely, it can increase bleeding.
- concrete admixture
Concrete bleeding, in fact, can be seen as a finer level of segregation, water as a liquid phase, all solid particles are solid phase, the process of bleeding is the viscous resistance of water is not enough to overcome the gravity of cement particles process. At this time, consider adding some finer mixtures, such as finer fly ash, silica fume, etc., because these materials have smaller particles, they will not be separated from water, but are evenly mixed with water to form the liquid phase, increase the liquid viscosity, and enhance the ability to prevent cement particles from settling under the action of gravity, so as to reduce water secretion.
- cement
The contribution of cement to the bleeding of concrete is mainly in two aspects, one is fineness and the other is admixture.
An increase in cement fineness has two positive effects on reducing bleeding. First, a decrease in cement particle size means a slower settling rate. In addition, the finer the cement particles, the larger the specific surface area, the faster the reaction with water, the higher the viscosity, the faster the system coagulation and hardening, which leads to a decrease in the effective time of solid particle settlement, and thus the decrease in water secretion.
Different types of cement admixtures also lead to differences in bleeding rate, which is the same as the effect of admixtures in concrete.
Post time: Jan-11-2024