We believe we have a certain understanding of the monomers of polycarboxylate water reducers. In order to better apply them, we should also have some knowledge about their bleeding. It is important to know that improper use of polycarboxylate water reducers can cause concrete bleeding. Below is a brief explanation of the reasons why polycarboxylate water reducers cause concrete bleeding:
(1) The quality of aggregates fluctuates. In the process of concrete production, it is common to encounter fluctuations in the quality of sand and gravel aggregates, such as an increase or decrease in mud content, fluctuations in water content, changes in aggregate grading, changes in sand fineness modulus, etc., which may increase the possibility of concrete bleeding.
(2) Polycarboxylate water reducer is sensitive to environmental temperature and has slow-release properties. Compared with other water reducing agents, the water reducing and slump retaining functions of polycarboxylate water reducing agents are greatly affected by temperature. When the temperature is high in summer, polycarboxylate superplasticizer with good slump retention function may exhibit a phenomenon of "collapse" when the temperature suddenly drops.
(3) Secondary addition of water reducing agent to adjust slump. When using polycarboxylate superplasticizer for secondary adjustment, it is sometimes difficult to control the dosage reasonably. When the dosage of superplasticizer exceeds the theoretical requirement, excessive superplasticizer still plays a role after concrete pouring, causing some moisture in the concrete to separate.
(4) Improper use of retarders when compounding polycarboxylate superplasticizers. In order to achieve outstanding slump retention function in concrete, a certain amount of retarding component is often added when compounding polycarboxylate superplasticizer. Excessive dosage of retarding components, or when polycarboxylate superplasticizer is compounded at a good temperature, will slow down cement hydration and form concrete bleeding when the temperature suddenly drops.
Regarding the compatibility between polycarboxylate 5) cement and polycarboxylate superplasticizer. Inconsistent chemical changes occur between the complex components of concrete, especially the compatibility issue between cement and polycarboxylate superplasticizer, which is particularly prominent. If the compatibility between cement and polycarboxylate superplasticizer is poor, the full dosage of superplasticizer will be increased, and excess superplasticizer will prolong the initial setting time of concrete, while also causing the free water in the concrete to separate. In addition, when the soluble alkali in cement is low, the hydration rate of cement slows down and often exhibits delayed bleeding phenomenon.
The above are the reasons for the formation of concrete bleeding by polycarboxylate water reducers. It should be noted that polycarboxylate water reducers have limited impact on concrete bleeding, while their impact on concrete durability is crucial. From the mechanism of bleeding, it can be known that after water is secreted from the inside of concrete to the surface, it forms a channel from the inside to the outside in the concrete. These channels will decrease the concrete's ability to resist infiltration.
