In the process of concrete mixing and production, there are 4 primary methods for adding admixtures. The dosage is generally no more than 5% of the cement weight. Different methods of incorporation affect the performance of the admixture differently:
1) Pre-mixing Method
This involves mixing the admixture with the cement first, then adding aggregates and water. This method ensures the admixture is evenly and thoroughly dispersed. However, under prolonged mixing, the concrete admixture molecules adsorbed on the surface of cement particles may detach, increasing the required dosage.
2) Simultaneous Mixing Method
Here, the cement, aggregates, and pre-prepared admixture are added together into the mixer. This allows the admixture to fully participate in the heat of hydration process of the cement, ensuring stronger adsorption of the admixture molecules on the surface of the cement particles.
3) Post-mixing Method (most ideal and commonly used in practice)
In this method, the cement, aggregates, and water are mixed first. Once thoroughly blended, the admixture is added, typically 1–2 minutes after water is added. This ensures a peak concentration of the admixture in the liquid phase for a short period, minimizing loss.
4) Incremental Addition Method
The concrete admixture is added at fixed intervals during the mixing and transportation stages. This helps maintain a stable concentration of the admixture throughout the process.
Comparison of 4 Methods
Under the same conditions, the 3rd method – post-mixing and the 4th method – incremental addition are highly effective in reducing slump loss of the mixture and can lower the required dosage of water-reducing agents.
However, the incremental addition method is often impractical for on-site construction due to construction constraints. Therefore, the post-mixing method is considered the most ideal and is the most commonly used approach in practice.
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