Concrete water reducer technology is more and more used in engineering construction, and it plays an important role in concrete quality control. However, due to the excessive use of water reducer in the production process of commercial concrete mixing plants , this paper focuses on reducing Research and analyze the influence of water agent overdosage on the performance of concrete, and propose a treatment method.
1. The effect of a small amount of water reducing agent overuse on the performance of concrete
If the amount of superplasticizer increases and the water consumption remains unchanged, the fluidity may exceed the expected range, and the workability of the concrete will deteriorate. If the concrete itself is not in good condition, it may lead to increased bleeding, which will cause adverse effects on pumping construction . impact, and the increased bleeding may lead to a decrease in the strength and appearance of the concrete surface. If the superplasticizer is over-mixed, but the water consumption is reduced accordingly, and the workability of the concrete is good, the concrete strength will increase correspondingly due to the decrease of the water-binder ratio in this case, and there will be no adverse effects on other properties.
For superplasticizers with retarding effect, over -mixing may prolong the setting time, and the early strength is lower than that of normal-dosage concrete. Generally, it sets within 48 hours, and usually has no adverse effect on the later strength. However, it is required to strengthen the early maintenance of the concrete to avoid excessive water loss of the concrete in the plastic state, which will cause cracks on the concrete surface and affect the strength and structural durability of the concrete.
For the water reducing agent compounded with air-entraining components, the air content increases after over-mixing , and the strength of the concrete at the early and later stages may decrease. Generally, the air content below 4.5% has little effect on the strength of the concrete, and the air content An appropriate increase in the anti-freezing and anti-harmful medium erosion can play a positive and beneficial role; if the air content exceeds 5%, the strength of concrete at all ages will be seriously reduced, which may cause quality accidents.
Generally speaking, if the amount of Polycarboxylate Superplasticizer is within 2 times of the normal amount, the concrete is in good condition, the air content is less than 4.5%, and the surface maintenance is properly strengthened after pouring, it will generally not have an adverse effect on the performance of the concrete.
2. The effect of water reducing agent overuse several times on the performance of concrete
The amount of Polycarboxylate Superplasticizer is several times higher than the normal amount, and the impact on concrete performance should depend on the specific situation. The following aspects need to be paid attention to.
(1) For ultra-high-strength concrete, an appropriate amount of over-mixing can be accepted
In ultra-high-strength concrete, because the water-binder ratio is <0.3 or even as low as 0.2, it usually shows that the concrete state is not sensitive to the dosage of superplasticizer in this case. In order to achieve the ideal fluidity state, the dosage of superplasticizer is usually Generally, the normal dosage is 5-8 times, that is, the dosage of polycarboxylic acid needs to reach 5%-8%. For concrete below C50, such a high content is inconceivable. However, the test results show that the strength of concrete at each age develops well under this dosage, and the concrete with 28d strength greater than 100MPa is equipped with this.
The reason is that the dispersion of the water reducing agent on the cement is only physical adsorption, and the molecules of the water reducing agent are adsorbed on the surface of the cement particles. Through the effect of steric hindrance and electrostatic repulsion, the flocculation structure of the cement particles is disintegrated and the wrapped free water is released. , so as to increase the fluidity of concrete, and because of its special comb-like structure, the Polycarboxylate Superplasticizer can prevent cement particles from agglomerating again within a certain period of time, so it has good slump retention performance. Once a certain period of time has elapsed, the cement hydration product will completely wrap the water-reducing agent molecules adsorbed on the surface of the cement particles, and the dispersion effect will disappear completely after the water-reducer agent molecules are shielded, and then it will no longer have any effect and influence on the concrete. The strength of concrete develops normally.
Of course, due to the high content of superplasticizers, the concentration of superplasticizer molecules in concrete is high. After some molecules are covered by cement hydration products, new molecules are adsorbed on the surface of cement hydration products, preventing cement particles from quickly overlapping. The formation of a network prolongs the setting time to a certain extent, but generally the setting of cement will not exceed 24 hours.
(2) Several times overdosing of air-entraining and retarding superplasticizers may have a greater adverse effect on concrete performance.
Generally speaking, the dosage of the retarding component is determined according to the temperature environment, engineering requirements and the normal dosage of the Polycarboxylate Superplasticizer. Adsorption will affect the normal hydration of the cementitious material, and the setting time will be significantly prolonged if it is mild, and the concrete will not set for several days or permanently if it is serious .
Generally, for concrete that sets for 2 days or even longer, because the hydration process is too delayed, the type and quantity of hydration products change, resulting in a permanent reduction in concrete strength. Of course, for the construction of large-volume concrete such as subway occlusal piles (generally 72-90h initial setting) and pile foundations, caps, dams, etc., it takes a long setting time. Generally, the strength level is appropriately increased in the mix design to ensure Ensure that the 28d strength meets the design requirements.
The air-entraining water reducer is over-mixed by several times. When the air content of the concrete is appropriate at the normal dosage, the air content will be greatly increased after the over-mixing by several times. The concrete slurry is abnormally rich. When the concrete is loose and porous like a bread, the strength of the concrete is seriously reduced.
(3) Adjustment of water consumption after doubling over-mixing
Even if the superplasticizer itself has no air-entraining and retarding type, if you do not pay attention to adjusting the water consumption in time after doubling the superplasticizer, the workability of the freshly mixed concrete may be seriously deteriorated, resulting in serious bleeding , segregation, and scratching. Bottom, hardening and other phenomena, and the uniform stability after pouring is poor, and internal delamination occurs, resulting in an increase in the water-binder ratio of the concrete around the steel bar, a decrease in strength, and a serious drop in the grip of the steel bar .
A large amount of bleeding caused by severe over- mixing will also appear on the surface of the concrete and the parts in contact with the formwork, resulting in a decrease in the strength of these parts, and a large number of defects such as cracks, honeycombs, and pockmarks are prone to appear when the formwork is removed, which improves the ability of the concrete to resist external erosion Greatly reduced, seriously affecting the durability of concrete.
