In actual construction, the optimal air content range of concrete should be determined during the trial mixing process according to the location of the project and the functions of the concrete, and should be effectively controlled to help ensure the construction quality of concrete.

Types of bubbles in concrete

Before concrete is vibrated, there are a large number of bubbles inside it, mainly including air bubbles introduced during concrete mixing, transportation and blanking, bubbles introduced by water-reducing agents, and tiny bubbles introduced by air-entraining agents.

(1) Air bubbles introduced during concrete mixing, transportation and blanking. This type of bubble has a large diameter, uneven distribution and is extremely unstable. It is easy to aggregate into bubbles with a larger diameter and burst easily, so it is called unstable bubble. This unstable bubble introduced by mechanical mixing will have a negative impact on the fluidity of concrete, as well as on the mechanical properties and durability of concrete after hardening.

(2) Bubbles introduced by water reducing agent. The water-reducing agent can introduce a certain amount of bubbles. Due to the same electrical repulsion, these bubbles are between the cement particles like ball bearings to disperse the cement particles, thereby increasing the sliding effect between the cement particles. However, these bubbles are uneven in size, irregular in shape and unstable. As transportation and vibration proceed, they tend to aggregate and merge with each other to form large bubbles, which eventually overflow outwards and reach the surface of the concrete to form apparent bubbles, resulting in honeycomb pitted surface defects. .

(3) Bubbles introduced by air-entraining agents. The air-entraining agent can form many fine bubbles with a size between (20-200)um and evenly distributed inside the concrete. The liquid film on the surface of this type of bubble is relatively strong. From a thermodynamic point of view, that is, the electrokinetic potential of the liquid film is high, which can prevent the bubbles from coalescing. The bubbles are relatively stable and difficult to burst. It is essentially different from the bubbles introduced by water-reducing agents, and is beneficial to the impermeability and other durability of concrete.

Defoamer help eliminate large air bubbles in concrete. On the one hand, adding defoamer can eliminate the bubbles between the concrete and the formwork to a certain extent, effectively prevent or eliminate the generation of honeycombs and pitted surfaces on the concrete surface, and make the surface of the concrete have higher smoothness and gloss. On the other hand, defoamer can eliminate a large number of bubbles inside concrete, reduce the air content and internal porosity of concrete, and improve the mechanical properties and durability of concrete. defoamer in concrete mainly eliminate bubbles introduced by water-reducing agents. Therefore, defoamer are often compounded with polycarboxylic acid-based water-reducing agents in projects to solve the problem of large air entrainment of polycarboxylic acid-based water-reducing agents. .

Compounding of Defoamer and Polycarboxylic Acid Water-reducing Agent

Since the polycarboxylic acid-based water reducing agent mother liquor itself has a large gas content, high surface activity, and good foam retention, when used directly on concrete, it will cause adverse effects such as high gas content, many apparent bubbles, and low strength of the concrete. Therefore, it needs to be rehydrated. Use an appropriate amount of defoamer to eliminate large bubbles in the concrete. The basic performance testing of the combination of defoamer and polycarboxylic acid water-reducing agent generally includes the compatibility of defoamer and water-reducing agent and the impact of defoamer on concrete properties.

(1) Compatibility of Defoamer and Water Reducing Agent

Defoamer and polycarboxylic acid-based water-reducing agents is the compatibility issue with the water-reducing agents. The compatibility of the defoamer and the water-reducing agent can be evaluated by testing the dissolution state of the defoamer in the polycarboxylic acid-based water-reducing agent. The defoamer has good solubility in the polycarboxylic acid-based water-reducing agent and is long-lasting. If there is no stratification over time, the compatibility is good and can be compounded with the water-reducing agent; while the defoamer with poor compatibility cannot be compounded with the water-reducing agent and can only be added to the concrete alone. Add defoamer and polycarboxylic acid water-reducing agent to the cement slurry. Testing the initial fluidity and fluidity loss over time of the cement slurry can also be used to evaluate the performance of the defoamer and polycarboxylic acid-based water-reducing agent. compatibility. The defoamer with good compatibility with the polycarboxylic acid-based water reducing agent should be a defoamer that has no obvious adverse effects on the initial fluidity and fluidity loss over time of the cement slurry.

(2) The impact of defoamer on concrete properties The impact of defoamer on concrete properties is reflected in two aspects: the working performance of concrete and the mechanical properties after hardening. Generally, the impact of defoamer on the performance of concrete is evaluated by testing the slump and slump loss, air content and strength of concrete. defoamer that can significantly reduce the air content of concrete, have less impact on concrete slump and slump loss, and significantly improve the strength of concrete are effective.