Characteristics of superplasticizer
As far as commercial concrete is concerned, the superplasticizer has the following features :(1) the ideal water-reducing rate can be achieved at a lower content;(2) excellent strength growth effect;(3) with good fluidity and slump protection, it has little influence on the setting time of concrete, and can meet the concrete pumping requirements of long-distance and super high-rise buildings;(4) good adaptability to cement;(5) the volume stability of concrete is enjoyable, which has a good control effect on the crack of the in-situ slab.
Common problems and solutions in the application of superplasticizer in commercial concrete
1. Stability is poor
As a significant advantage of the superplasticizer, it has better adaptability to cement than other superplasticizer. However, due to the variety of cement and the complexity of admixture, it is easy to have poor stability.
The poor stability of concrete referred to in this paper is mainly reflected in the fact that the concrete is in good state after being discharged, but bleeding occurs after resting for a period of time, or the concrete is in good state after being removed, and the resting state is in good state, but severe bleeding or even segregation occurs after the concrete transport truck continues to stir. The emergence of such problems makes it more difficult to control the quality of concrete. Through testing and summarizing the specific raw materials with such issues and tracking and observing the quality of concrete, except the poor stability caused by poor adaptability of cement, it is mainly due to the following two reasons:
(1) Insufficient stirring. In the railway concrete mixing station, the concrete mixing time has strict requirements, and even if the work task is enormous, the concrete mixing time must not be less than 2 minutes.
(2) Unsatisfactory grading of coarse aggregate. Commercial concrete mixing station generally USES 5-31.5mm coarse aggregate; coarse aggregate gradation is not proper, the maximum grain size proportion increases, also easy to cause the concrete stability.
Solution :(1) strictly control mixing time in the actual production process;(2) ensure coarse aggregate gradation and control the maximum grain size. Or give up continuous gradation and adopt double gradation for compounding.
2. After pumping, the concrete appears stratification, segregation or reduced mobility
With the increase of the layer number of the main body, the extension of pumping pipe, need greater fluidity, but in the process of production and construction, will often appear before injecting concrete workability is good, but after tapping to operational areas appear satisfied, segregation or unfortunate liquidity situation, primary performance for mortar in structure, net buoyancy, coarse aggregate sinking, severe when even slurry erosion. The main reason for this problem is that when the concrete mixed with polycarboxylate superplasticizer has high fluidity, and it's slurry viscosity drops sharply and cannot maintain its original state under the action of higher pressure.
Solution: the actual sand rate should be appropriately increased, and the maximum particle size of coarse aggregate should be controlled when construction is carried out on higher floors or when the distance between pumping pipes is longer.
3. Cracks appear on the surface of the cast plate
One of the significant advantages of superplasticizer is that the concrete mixed with it has excellent volume stability and can better control the cracks on the slab surface of cast-in-place concrete. However, cracks on the slab surface still exist in the actual use process. Through a period of time tracking, at the same site, the same construction conditions and construction technology of casting of the concrete board face, in the case of concrete workability is right, even if the real slump larger when into the mold, also won't appear crack after surface hardening, so not because after solidification of the surface cracks caused by slurry thickness. Crystal research found that when the aggregate particle size of coarse concrete aggregate overall slant great or beautiful aggregate stone content increased resulting in poor aggregate gradation, it is straightforward to cause cracks in the plate surface, the reason is mainly because of poor aggregate gradation, more accessible to bleeding, segregation, and other conditions.
Solution: Superplasticizer is very sensitive to aggregate gradation, as far as possible to control the aggregate gradation, under the condition that conditions allow, coarse aggregate as far as possible to use double gradation.
4. Honeycomb and pitted surface are natural to appear in vertical components
In the application of the polycarboxylate superplasticizer to concrete vertical members, especially for the vertical members with relatively high height, it is occasionally found that after the removal of the formwork, the surface roughness of individual parts of the vertical members is not enough, resulting in honeycomb and pitted surface.
Solution: because the polycarboxylate superplasticizer has a certain amount of air intake, so the use of superplasticizer mixed concrete after entering the mold due to timely, full vibration, avoid the appearance of honeycomb surface.
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