Effects Of Multiple Overadmixtures On Concrete Properties
The dosage of water-reducing agent exceeds the normal dosage several times, and its impact on the performance of concrete should be determined according to specific circumstances.
In the first case, in ultra-high-strength concrete, since the water-cement ratio is ≤0.3 or even as low as 0.2, it usually indicates that the state of the concrete is not sensitive to the amount of superplasticizer. To achieve ideal flow, reduce moisture. The dosage of this agent is usually 5-8 times the normal dosage, that is, the dosage of polycarboxylic acid needs to reach 5%-8%. Such high levels are incredibly high for concrete below C50. However, the test results show that the strength of concrete at all ages develops well under this dosage, and the 28d strength of concrete prepared with this strength is greater than 100MPa.
The reason is: the dispersion of water-reducing agent on cement is only physical adsorption. Water reducing agent molecules are adsorbed on the surface of cement particles. Through steric hindrance and electrostatic repulsion, the flocculation structure of cement particles is broken down and free water is released. , thereby increasing the fluidity of concrete, and because the polycarboxylate water-reducing agent has a special comb-like structure, it can prevent cement particles from re-aggregating within a certain period of time, so it has good slump retention performance. Once a certain period of time has passed, the cement hydration products will completely encapsulate the superplasticizer molecules adsorbed on the surface of the cement particles. After the water-reducing agent molecules are shielded, the dispersion completely disappears and no longer has any effect or influence on the concrete. Cement is usually water and the strength of concrete develops normally.
Of course, because the water-reducing agent content is high, the concentration of water-reducing agent molecules in the 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 overlapping rapidly. The formation of a network prolongs the setting time to a certain extent, but generally cement will not set for more than 24 hours.
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