What is the reason why polycarboxylic acid superplasticizer causes lagging bleeding of concrete?
Polycarboxylic acid superplasticizer is a water-soluble compound superplasticizer mainly composed of carboxylate grafted polymer, which can effectively improve various properties of concrete, improve the strength, anti-permeability, anti-carbonation, anti-shrinkage and durability of concrete. However, due to the influence of various factors, concrete mixed with polycarboxylic acid water reducer will often appear lag, bleeding and other phenomena, which will seriously affect the construction.
The main reasons are as follows:
(1) Excessive use of polycarboxylic acid water reducer
Polycarboxylic acid water-reducing agent can effectively improve concrete slump, but the amount of mixing is too large and the water reduction rate is too high (near the limit of mixing, water reduction rate is more than 40%). The dispersion ability is so strong that it will significantly reduce the cohesion of concrete and more free water is released, thus it increases the possibility of concrete bleeding. Especially when people use slow release and slump type polycarboxylic acid water reducer, mixing too much is easy to cause lag concrete bleeding and even to cause segregation when it is serious. Therefore, before use, it is necessary to conduct an advanced time test to observe the change of concrete state.
(2) polycarboxylic acid water reducer is sensitive to environmental temperature and has slow release property
Compared with other water-reducing agents, the properties of polycarboxylic acid water-reducing agent are greatly affected by temperature. In summer, when the temperature is higher, the polycarboxylic acid water reducer with good slump performance is likely to appear slump degree "pour big" phenomenon when the temperature suddenly decreases. Therefore, when the temperature suddenly drops, the original polycarboxylic acid superplasticizer should be tested before use. During production, the stirring time should be appropriately extended to completely disperse and release the polycarboxylic acid superplasticizer, so as to avoid the effect of slow release in the later stage, resulting in poor water retention of concrete and water precipitation.
(3) Add polycarboxylic acid water reducer to adjust slump
Before concrete pouring, when it is found that the workability of concrete mixture does not meet the construction requirements, the workability of concrete is often adjusted by adding polycarboxylic acid superplasticizer to the concrete mixer. However, when the second addition is adjusted, sometimes it is impossible to properly grasp the amount of use. When the amount of superplasticizer exceeds the theoretical requirement, the excessive superplasticizer still plays a role after concrete pouring, making the middle of concrete water distribution analysis. Therefore, when using superplasticizer to adjust the workability of concrete, the concrete should be added in different times according to the quantity of concrete, and the mixing should be strengthened to ensure even mixing and controllable adding amount.
(4) When the polycarboxylic acid superplasticizer was compounded, the retarder was not used properly
In order to obtain good slump protection performance of concrete, a certain amount of retarding components are often added when water reducing agent is mixed. When the amount of retarding component is too large, or when the temperature is good but the compound polycarboxylic acid superplasticizer encounters a sudden temperature decrease, too much retarding component will slow down the hydration of cement and cause bleeding of concrete. Therefore you should pay attention to the influence of components of the retarded concrete secrete water. Especially in the big temperature gap between day and night in the spring and autumn season, you should pay more attention to the amount of retarder, so that in the concrete setting process, you can prevent water slowly floating, aggregate sinking, make concrete lag bleeding.
(5) compatibility between cement and polycarboxylic acid water reducer
The composition of concrete is complex and it is easy to produce uncoordinated chemical changes among the components. If the compatibility of cement and polycarboxylic acid superplasticizer is poor, the saturated content of superplasticizer will be increased, and the excess superplasticizer will prolong the initial setting time of concrete, and make the free water in the concrete separate out. In addition, when the soluble alkali in cement is low, the hydration rate of cement becomes slow, and the phenomenon of lagging bleeding often occurs.
In addition, in the actual production process of concrete, people often encounter the sand aggregate quality fluctuations, such as the mud content is more or less, water content is larger or smaller, aggregate gradation, etc. All of these will cause concrete bleeding or even segregation.