Common problems and analysis of polycarboxylate water reducer in ready-mixed concrete
With its superior performance and pollution-free production, Polycarboxylate superplasticizer have developed rapidly abroad in recent years, especially in Japan, where the ratio of polycarboxylate to naphthalene has exceeded 7:3. Polycarboxylate superplasticizer are very different from traditional superplasticizers in terms of molecular structure, action principle and behavior in concrete. Therefore, correct understanding and rational use are important links to promote the application of Polycarboxylate superplasticizer.
Polycarboxylate superplasticizer has the characteristics of less dosage, high water reducing rate, good plastic retention performance, strong adaptability to cement, and small shrinkage of concrete. In this way, the feeling given to the first-time users is that the water reducer is more convenient, safe and efficient than previous generations of water reducers. However, in the process of engineering application, it was found that polycarboxylate-based water-reducers, like other water-reducers, also have certain limitations. If used improperly, it will also cause rapid setting, retarding, bleeding, delamination, and reduced strength.Therefore, in the process of production and use, it can still be applied after passing the test and inspection. The following lists some common problems in the engineering application process, analyzes and proposes solutions according to the actual situation.
1 Applicability to cement
The composition of cement and cementitious materials is complex and changeable. From the perspective of adsorption-dispersion mechanism, it is impossible to find a superplasticizer suitable for everything. Although Polycarboxylate superplasticizer have wider adaptability than naphthalene series, they are still It may have poor adaptability to some cements. This adaptability is mostly reflected in: the reduction of water reducing rate and the increase of slump loss. Even if it is the same cement, the effect of the water reducing agent will be different when the ball is milled to different fineness.
Phenomenon: A mixing station uses P-042.5R cement to supply C50 concrete to a construction site. It uses polycarboxylate high-efficiency water reducer. When making concrete mix ratios, it is found that the amount of water reducer used in cement is higher than that of other cements. Slightly more, but during actual mixing, the slump of the concrete mixture delivered from the factory was 210mm by visual inspection. When the concrete was unloaded into the concrete pump truck at the construction site, it was found that the concrete could not be unloaded, and the factory was notified to send a bucket of water reducer After adding and stirring, the visual slump is 160mm, which can basically meet the pumping requirements, but the phenomenon that it cannot be unloaded just after unloading, immediately return the truck concrete to the factory, add a large amount of water and a small amount of water reducer , was barely unloaded, and almost solidified in the mixer truck.
Analysis of the cause: The adaptability test of each batch of cement and admixture was not insisted on before opening.
Prevention: Do a compound test with the construction mix ratio for each batch of cement before opening. Choose the right admixture, "coal gangue" as the admixture of cement and polycarboxylate superplasticizer has poor adaptability, so avoid using it.
2 Sensitivity to water usage
Due to the use of polycarboxylate superplasticizer, the water consumption of concrete is greatly reduced, and the water consumption of single-square concrete is mostly 130-165kg; the water-binder ratio is 0.3-0.4, or even less than 0.3. In the case of low water consumption, fluctuations in the amount of water added may lead to large changes in slump, causing sudden increase in slump and bleeding of concrete mixtures.
Phenomenon: A mixing plant uses P-032.5R cement to prepare C30 concrete. The contract requires that the slump to the construction site be 150mm:t30mm. When the concrete leaves the factory, the measured slump is 180mm. 210mm, two consecutive trucks of concrete were returned, and returned to the factory to verify that the slump was still 210mm, and there were bleeding and stratification.
Analysis of the cause: The cement has good adaptability to this superplasticizer, and the amount of superplasticizer is slightly larger. The mixing time is not enough, and the slump of the concrete when the machine is discharged is not the real slump due to the short mixing time.
Prevention: For cement sensitive to the amount of polycarboxylate superplasticizer admixture, the amount of admixture should be appropriate and the measurement accuracy should be high. Appropriately extend the stirring time, even with twin-shaft forced mixer, the stirring time should not be less than 40 seconds, preferably more than 60 seconds.
3 The amount of polycarboxylate high-efficiency water reducer is too large, and there are too many air bubbles on the surface of the concrete structure
From the point of view of pumpability and durability, it is beneficial to increase the air-entraining property appropriately, and many Polycarboxylate superplasticizer have relatively large air-entraining properties. Polycarboxylate superplasticizer have the same saturation points as naphthalene superplasticizers. For different types of cement and different amounts of cement, the saturation points of the admixture in concrete are different. If the amount of the admixture is close to its saturation point, the fluidity of the concrete mixture can only be improved by adjusting the amount of slurry in the concrete or by other methods.
Phenomenon: A mixing station has been using polycarboxylate water reducer to prepare concrete for a period of time. Suddenly, one day, a construction site reported that after the shear wall was demolished, it was found that there were too many bubbles on the surface of the wall and the appearance was too bad.
Analysis of the cause: On the day when the concrete was poured, the construction site repeatedly reported that the slump was small and the fluidity was poor, and the personnel on duty in the laboratory of the concrete mixing station increased the amount of admixture. The construction site used a large formwork made of shaped steel. During pouring, too much material was fed at one time, and the vibration was uneven.
Prevention: To strengthen communication with the construction site, it is recommended that the feeding height and vibration method be operated in strict accordance with the specifications. Improve the fluidity of the concrete mixture by adjusting the amount of slurry in the concrete or by other methods.
4 Polycarboxylate superplasticizer is super mixed, and the setting time is prolonged
Phenomenon: The concrete slump is large, and the concrete will not set until 24 hours. 15 hours after the structural beam slab concrete was poured on a construction site, it was reported to the mixing station that part of the concrete had not yet solidified. The mixing station sent engineers to check, and after heating treatment, the final setting took 24 hours.
Analysis of the cause: The amount of superplasticizer is large, and the ambient temperature at night is low, so the concrete hydration reaction is slow. The unloading workers at the construction site added water to the concrete without permission, and the concrete consumed a lot of water.
Prevention: The amount of admixture should be appropriate and the measurement should be accurate. Remind workers to pay attention to heat preservation and maintenance when the temperature becomes low, and the polycarboxylate admixture is sensitive to water consumption, so do not add water at will.
Phenomenon: Case 1, the same part of the concrete prepared by polycarboxylate-based water-reducer and naphthalene-based water-reducer was poured, resulting in severe mold sticking phenomenon when the shear wall was demolished During the period, concrete was provided to the two construction sites with different mixing ratios with naphthalene-based water-reducing agents. Due to a power outage at one construction site, a truckload of concrete prepared with polycarboxylate-based admixtures was dispatched to this construction site. As a result, the construction site was demolished in the wall Severe mold sticking was found during molding. Case 2: Mixing the leftover concrete with naphthalene-based water reducer resulted in extremely poor concrete performance Concrete, as a result, the slump of the concrete was 190mm when it was delivered from the factory , and it was only 100mm after delivery to the construction site, and it had almost no fluidity. It took a long time for the hole to be bridged after the vibrating rod was pulled out during vibration.
Analysis of the cause: Concrete prepared by two different admixtures is caused by different setting time and shrinkage. Adverse reactions occur after mixing two admixtures with different properties.
Prevention: Concrete prepared with two different admixtures cannot be used in the same part at the same time. After discovering the problem, communicate with the construction site in advance and delay the demolition time appropriately. The two admixtures must be used strictly separately, and it is best to set the station, set the car, and set the construction site during production.The strength of the concrete prepared after mixing the two water reducers is extremely low. A worker at a mixing station accidentally unloaded a small amount of naphthalene-based water reducer into the polycarboxylate-based admixture when unloading the admixture. The concrete prepared with the mixed admixture, Not only the amount of admixture is large, but also the fluidity of the mixture is poor, the slump loss is fast, but the setting time is delayed, and the strength of the concrete specimen is 30%-50% lower than that of the concrete with the same amount of cement. Concrete prepared by two different admixtures is caused by different setting time and shrinkage.With the current technical conditions, the two admixtures cannot be mixed. During the production process, management should be strengthened, and the labels should be clearly marked to prevent such accidents.
6 Conclusion
Polycarboxylate superplasticizer is a high-performance superplasticizer with low dosage, high water reducing rate and low slump loss. The scope of application of the agent will continue to expand. However, the mechanism of action of polycarboxylate-based superplasticizers on many construction sites has not been fully grasped, and a deep understanding of them also requires a process, and the knowledge about concrete does not come from theory but from experiments, but these experiments are based on the most basic theoretical guidance. carried out below. There are more and more types of polycarboxylate-based water reducers on the market, but their synthesis routes are not necessarily the same, the source and quality of raw materials are also different, and the production process is also different, so the performance of the product will appear greater. difference. Our understanding of polycarboxylate-based water reducers also needs to be continuously tested and summarized.
