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What are the correct mixing steps for concrete admixtures?

Concrete admixtures, such as high-efficiency water reducers, accelerators, and retarders, are crucial for optimizing concrete performance, as they can improve workability, strength, and durability. However, improper mixing can counteract the advantages of additives, leading to segregation, uneven performance, or loss of strength. Improper timing or method of adding admixtures may render them ineffective, and even worse, may damage the integrity of the entire batch of concrete.

This guide provides detailed steps for correctly adding admixtures to concrete mixtures.

Why mixing steps is crucial?

Before delving deeper into ‘how to operate’, let’s first understand ‘why’. The order in which various ingredients are added to a concrete mixer fundamentally changes the chemical reactions that occur. Improper mixing of additives can lead to:

Reduced effectiveness: Additives may not disperse properly and fail to perform their intended functions (for example, water-reducing agents may not reduce water use).

Uneven: Different parts of a concrete batch may exhibit varying characteristics, leading to inconsistent strength, workability, and setting time.

Unpredictable solidification: Improper addition of retarders or accelerators can lead to “flash setting” (instantaneous hardening) or excessive delay, causing significant logistical issues at the construction site.

Waste of materials and funds: Admixtures are a significant cost component. Improper mixing is equivalent to wasting money.

Basic guidelines for adding concrete admixtures

There is an unbreakable rule when using most concrete admixtures:

The golden rule: Do not add additives directly to dry cement or dry aggregates.

Most additives are designed for use in aqueous environments. Adding them to dry materials can cause them to clump, become insoluble, or react inappropriately with cement, leading to “cement balls” or rapid setting.

Keeping this principle in mind, the following is a universally accepted standard mixing sequence that yields the best results.

The correct mixing steps for concrete admixtures

Follow the steps below to ensure even dispersion of the additive:

Step 1: Add aggregates and cement to the mixer

First, load the coarse aggregate, then the fine aggregate, and finally the cement.

Dry mix for 30-60 seconds to evenly distribute the solid; this can prevent the admixture from adhering to isolated cement blocks.

Step 2: Add most of the water and stir

Pour 70% to 80% of the total water into the mixer.

Stir for 1-2 minutes to form a uniform wet paste (avoiding dry lumps or excessive collapse).

Step 3: Add Additives

Liquid additive: Slowly pour into a rotating mixer (to avoid splashing). For drum mixers, please add through the inlet to ensure even distribution.

Powdered additive: evenly sprinkle onto wet slurry; do not sprinkle directly onto dry aggregate (to prevent clumping).

Key rule: Do not add admixtures directly to dry cement; this can lead to excessive local concentrations and uneven performance.

Step 4: Add the remaining water and adjust the viscosity

Gradually add the remaining 20-30% water while stirring.

Check the slump (ASTM C143) or flowability table (EN 12350-2) for adjustment: if the slump is too low, add a small amount of water (instead of admixtures); If the slump is too high, add fine aggregate.

Step 5: Mix evenly

Continue stirring for 2-3 minutes (total: 4-6 minutes). For high-performance concrete, the mixing time is extended to 3-4 minutes.

Stop mixing and inspect: The concrete should be uniform (without stripes, clumps, or residual admixtures). If using coloring additives, ensure uniform color.

Step 6: Take it out in a timely manner

The mixed concrete should be discharged within 30-60 minutes (before initial setting). Avoid excessive mixing – it can introduce bubbles and weaken the concrete matrix.

Considerations for different types of additives

Liquid additives (most commonly used): Add them together with the last part of the batch water according to standard procedures.

Powdered additives (such as PCE powder): Use extra caution when using these additives to prevent clumping. The best method is to dissolve the powder in a portion of the mixing water before adding it to the mixer. If added directly in powder form, it must be slowly added to wet, flowing concrete; do not add it to dry materials.

Air entraining agents: These agents are usually added in the early stages of ingredient preparation, sometimes together with initial water, to provide sufficient mixing time to form a stable pore system.

High-efficiency water-reducing agent (HRWR): To maximize its effect on enhancing slump, it is usually added at the end or during concrete pouring on the construction site. Their effects are significant, but may only be temporary.

Retarders and accelerators: These substances are typically added during the main mixing process with batch water to ensure uniform distribution and control the set time of the entire batch.

Avoid common mistakes for mixing concrete admixtures

Inaccurate dosage: excessive or insufficient dosage of the mixture. Be sure to use calibrated equipment when measuring.

“Pouring” mixture: Pour all doses into one point of the mixer without using water to aid dispersion.

Ignore the manufacturer’s data sheet: the characteristics of each admixture are different. The manufacturer’s technical data sheet is your most important reference, providing specific instructions for use.

Insufficient mixing time: Shortening the final mixing time will result in uneven concrete.

Adding admixtures to old or hardened concrete: Adding high-efficiency water reducers to concrete that has already started to set is tantamount to self-destruction. It will not achieve the expected effect.

FAQ

Q1: Can I add all the materials (including additives) at once?
A1: It’s not possible. Adding additives to dry materials can cause clumping and uneven dispersion. Please operate in the order of “dry material mixing → wet material mixing → addition of additives”.

Q2: How long does it take to mix concrete and admixtures?
A2: The total stirring time is 4-6 minutes. For powdered admixtures or high-performance concrete, an extension of 1-2 minutes is required.

Q3: What if the concrete is too dry after mixing?
A3: Add a small amount of water (instead of additives) to adjust the slump. Avoid adding additional additives; this can disrupt the concrete mix design.

Q4: If the concrete solidifies too quickly, can I remix it?
A4: Stirring again after initial setting will reduce the strength. The correct approach is to adjust the admixture amount (e.g., adding a retarder) or environmental conditions (e.g., cooling materials) before mixing.

Conclusion

Proper mixing of concrete admixtures is a systematic process: preparation, precise measurement, sequential addition, and thorough mixing are all crucial. Following the following steps can ensure uniform dispersion of admixtures, maximize their effectiveness, and produce concrete with stable workability, strength, and durability.

For complex concrete mixtures (such as multiple additives, high-performance concrete), it is necessary to conduct small-scale testing first to verify the mixing sequence. With the increasing demand for concrete quality in construction, mastering these mixing steps has become a key factor for project success.