Blog

The effect of polycarboxylate superplasticizer on the air content of concrete

The air content affects the frost resistance, workability, strength, and durability of concrete. When the gas content is too high (over 8%), for every 1% increase in gas content, the compressive strength will decrease by 5% -10%. When the air content is too low (less than 3% in cold regions), concrete is prone to cracking due to frost heave. As the core of modern concrete admixtures, polycarboxylate superplasticizer (PCE) has a complex yet controllable effect on air content.

This article will delve into how PCE affects concrete’s air content, clarify the key influencing factors, and highlight its practical significance for concrete mix design and construction.

The Importance of Air Content in Concrete

Air content refers to the volume percentage of air bubbles introduced (intentionally added) and carried (unintentionally mixed) in fresh concrete. Introducing bubbles can enhance frost resistance; carrying them will weaken the concrete matrix.

1. Air inclusions: These are large, irregularly shaped pores (usually larger than 1 millimeter) that inadvertently mix into the concrete mixture during mixing, handling, and pouring. They are usually disadvantageous because they reduce concrete strength and may increase permeability.

2. Air entraining: These are tiny, uniformly distributed bubbles (usually 10-300 microns) intentionally introduced into the concrete mixture through the use of air entraining agents (AEA).

The importance of concrete air entrainment

Freeze-thaw durability: It provides a pressure relief valve for water expansion during freeze-thaw cycles to prevent internal stress and damage.
Improve construction performance: act as a lubricant, enhance the plasticity and plasterability of concrete.
Reduce segregation and water seepage: Helps maintain the suspended state of aggregates and reduces the migration of moisture to the surface.

The goal is to minimize air entrainment while maintaining the required stable level (if necessary).

How does polycarboxylate superplasticizer affect the air content of concrete?

1.1. Direct impact: The effect of PCE molecular structure on bubble stability

PCE molecules are composed of an anionic main chain (adsorbed onto cement particles) and long polyether side chains (providing dispersibility), which directly affect the formation and retention of bubbles

Bubble stabilization (positive effect): The polyether side chains of PCE can adsorb onto bubble surfaces, forming a “steric barrier” that prevents small bubbles from merging into large ones, thereby stabilizing bubble introduction – this is particularly significant in low-water-cement-ratio concrete.

Bubble damage (adverse effect): When high molecular weight PCE or its dosage is too high, it can damage the structure of the bubble membrane. The strong dispersing effect of PCE may shatter fragile bubbles, resulting in a 1% -3% decrease in air content compared to concrete without PCE.

Standard PCE typically reduces the air content by 0.5%-2 % in concrete without air-entraining agents; in concrete containing air-entraining agents, the final effect depends on the compatibility between PCE and the air-entraining agent.

2.Indirect effects: interaction between PCE and air entraining agents (AEAs)

This may be the most crucial and complex aspect. PCEs can interact with AEAs in various ways, often reducing the effectiveness or stability of air entrainment.

Competitive adsorption: PCE and AEA are both surfactants that can adsorb onto cement particles. PCE molecules have a larger volume and typically higher concentration (relative to the surface area of cement), thus competing with AEA for adsorption sites. This will reduce the amount of AEA available for stabilizing bubbles.

Reduced bubble stability: The presence of PCE molecules can alter the surface tension and interfacial properties of mixed water, thereby reducing the stability of bubbles formed by AEA. This may lead to:

Reduced air content: Achieving the target air content may be more difficult with standard AEA doses.
Reduce bubble size and spacing: The characteristics of the pore system are crucial for freeze-thaw durability and may be negatively affected.

Over time, entrained air may become less stable and more easily lost during transportation, pumping, and placement.
Defoamer effect: The polycarboxylate superplasticizer’s structure can defoam or exhaust. They can decompose or coalesce bubbles, thereby counteracting the action of gas-phase adsorbents (AEA). This is usually a design feature of “low gas phase” or “air modified” PCE.

Factors Of polycarboxylate superplasticizer affect the air content of concrete?

1.Types of polycarboxylate superplasticizers

At present, there are different types of polycarboxylate superplasticizers on the market that can be customized.

2.PCE dosage

Higher doses of PCE typically result in more significant effects on air content, whether it increases trapped air or reduces the stability of entrained air.

3.Type and dosage of air entraining agent

The specific air entraining agent used, and its dosage, are crucial. Some air-entraining agents are more resistant to the effects of polychlorinated biphenyls (PCBs) than others.

4.Cement type and composition

Alkali content: Higher alkali content in cement sometimes reduces the effectiveness of alkali enhancers and interacts differently with polycarboxylate compounds.

Fineness: Finer cement has a larger surface area, which may lead to stronger adsorption competition.

C3A content: can affect early hydration and surface chemical properties.

5.Mix design parameters

Water-cement ratio: A lower water-cement ratio (achieved with PCE) can sometimes make air entrainment more challenging.

Aggregate characteristics: Grading, shape, and water absorption can affect the air content and stability.

The presence of other additives: Other chemical additives may also interact with PCE and AEA.

6.Mixing conditions

Mixing time and intensity: Excessive mixing can lead to air loss, while insufficient mixing can prevent proper air entrainment.

Blender type: Different mixers can generate different shear forces.

7.Temperature

Higher concrete temperatures can reduce the stability of bubbles and enhance the defoaming effect of certain polyvinyl chloride.

Conclusion: effect of polycarboxylate superplasticizer on the air content of concrete

Polycarboxylate superplasticizer are indispensable tools in modern concrete production, but their impact on concrete air content is a key factor that cannot be ignored. Although polycarboxylate superplasticizers offer significant advantages in improving concrete workability and strength, they can increase the amount of trapped air and, more importantly, compromise the effectiveness and stability of air entrainment.

By understanding its potential mechanisms, identifying influencing factors, and implementing strict testing and control measures, concrete professionals can successfully leverage the advantages of polycarboxylate superplasticizers while ensuring the concrete reaches the required air content, thereby achieving optimal performance, especially long-term durability under freeze-thaw cycles.