During construction, the amount of water-reducing agent affects the workability, bleeding rate, setting time and compressive strength of concrete. It is necessary to prepare fresh concrete with good flow properties, disassemble and reduce the adhesion between cement particles that hinders the flow. The stagnant structure enables the cement particles to be fully dispersed in the water medium. Concrete is the main raw material for modern building construction projects. It is popular among practitioners because of its advantages such as high strength and long service life. During the concrete mixing process, appropriate amounts of admixtures need to be added to improve the performance of the concrete. Combining many years of work experience with relevant theoretical knowledge. There are many properties that affect the melting of cement, such as the mineral composition of cement, the shape and size of cement particles, the completeness of mineral crystallization, operating conditions and environmental factors. The various factors mentioned above directly or indirectly control the stability of cement particles in the slurry. Different medium conditions may change the charge value of cement particles in the slurry, that is, change the electrostatic repulsion between particles.
When an appropriate amount of water-reducing agent is added to fresh concrete, the potential carried by the cement particles increases, and the electrical repulsion between the cement particles greatly increases, resulting in a decrease in the viscosity of the fresh concrete, which promotes The stability of the entire dispersion system is improved and the fluidity is improved.
In addition, there is an intermediate state between the dilution and agglomeration state of the cement slurry, that is, the thixotropic state. This is caused by the redispersion of the cohesive structure in the cement paste when the shear rate increases. The specific performance is that when the shear rate increases, the resistance decreases and the viscosity decreases. That is, the slurry does not become agglomerated when it is still. If the slurry that has been agglomerated is stirred or shaken, it regains fluidity. Generally, adding an appropriate amount of water-reducing agent to the cement paste can cause the fresh concrete to show strong thixotropy. This is due to the formation of a solvation film layer on the surface of the cement particles adsorbing the water-reducing agent and the increase in potential. If it is vibrated slightly, it will show better fluidity. The thixotropy of fresh concrete without superplasticizer is much weaker.
01 Affects the slump of concrete
Under normal circumstances, practitioners will mix a certain amount of water-reducing agent into concrete to increase the slump of concrete. This can effectively improve the workability of fresh concrete.
Experience shows that concrete without water-reducing agent loses less slump within 1 hour than concrete with water-reducing agent. The main reason for this “strange” phenomenon is that the concrete After adding the water reducing agent, it still needs to go through the processes of mixing, transportation and pouring.
Therefore, at the concrete construction site, it is necessary to add water to restore the slump of concrete that has been mixed with water-reducing agents. However, this method will significantly reduce the concrete properties such as strength, which may lead to concrete Cracking, abnormal hardening and other phenomena appear.
In order to effectively solve the problem of slump loss of concrete mixed with water-reducing admixtures, it is recommended that practitioners adopt methods such as adding water-reducing admixtures in batches and post-mixing methods.
Among the above methods, the batch mixing method can not only effectively avoid the problem of concrete slump loss, but also control production costs to a certain extent and maximize the benefits of concrete construction.
02 The impact of too much water-reducing agent on concrete
Due to the influence of various adverse factors, the quality of the water-reducing agent added to concrete during the actual construction process often Exceeding the standard, and the amount of water-reducing admixture exceeding the standard, especially seriously exceeding the standard, has very serious consequences.
Test data shows that when the mass of the water-reducing agent mixed into concrete exceeds the recommended mass by more than 1 times, the hardening and setting time of the concrete will be greatly extended, and the early strength of the concrete will not reach the design Requirements, in some cases the concrete will fail to solidify.
Practitioners must strictly abide by the relevant national concrete admixture dosage regulations during actual operations. Before officially starting the admixture addition work, the construction unit must arrange for professionals to conduct concrete trial mixing work. , thereby determining the optimal dosage of admixtures and ensuring reliable quality of concrete.
03 Gas introduction
After adding water-reducing agent to concrete, the gas content inside the concrete will increase significantly. Normally, if the concrete contains a certain amount of uniform bubbles, the amount of bleeding in the concrete will be significantly reduced, and the workability can be effectively improved.
And it can improve the impermeability and freeze-thaw resistance of concrete to a certain extent, so the concrete water-reducing agent mixing method is used in underground waterproofing projects and other projects that require high concrete impermeability performance. appears often.
However, in recent years, practitioners have discovered that adding water-reducing agents to concrete in some cases not only fails to improve the performance of the concrete, but will also significantly reduce the flexural strength and compressive strength of the concrete. .
By conducting a large number of controlled variable experiments, technicians found that the air content of concrete has a very subtle impact on the performance of concrete.
The experimental conclusion points out that under the premise that the water-cement ratio of the concrete remains unchanged, for every one percentage point increase in the air content in the concrete, the flexural strength of the concrete will decrease by 2% to 3%, and the compressive strength will decrease by 4%. %~5%.
In addition, when the air entrainment amount of concrete exceeds the critical point of 6%, the durability, strength and other properties of the concrete will be affected.will be significantly reduced, and the concrete at this time can no longer meet the requirements of the project.
Therefore, air-entraining water-reducing agents have strong instability. Of course, this does not mean that air-entraining water-reducing agents cannot be used, as long as practitioners use air-entraining water-reducing agents. During the process, pay attention to controlling the air content of the concrete within a reasonable range.
04 The adaptability issue between cement and water-reducing admixture
During a certain concrete mixing operation, it was found that after a small amount of water-reducing admixture was added to the cement, the hardening state of the cement became abnormal. Change, after a few minutes, the cement solidification speed suddenly becomes faster, and the so-called rapid setting phenomenon occurs.
Through analysis, the cause of the abnormal phenomenon was finally determined. Substances such as gypsum, mixed materials and mineral raw materials in cement are not suitable for superplasticizers.
In view of the above situation, when practitioners find that the fluidity of concrete becomes poor or rapid setting occurs after mixing with qualified water-reducing agent, they cannot use the construction method of forcibly adding water, but should start from the cement and water-reducing agent. Start looking for solutions in terms of aqueous agent adaptability.
Only in this way can the quality of finished concrete products be guaranteed to the greatest extent. When using different types of admixtures in combination, great attention must be paid to their compatibility to prevent the performance of concrete from being adversely affected.
For example, concrete has excessive retardation, false setting or rapid setting, slump that does not meet the requirements, and poor cohesion, water retention, fluidity, etc.
Therefore, before formal use, corresponding admixture compatibility tests must be carried out to ensure that they meet the corresponding requirements before they can be officially put into use.
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