Bubble category
The bubbles generated in concrete are called large harmful bubbles above 100μm, medium harmful bubbles with 100-50μm, low-harm bubbles or harmless bubbles with 50-20μm, and beneficial bubbles below 20μm.
It should be noted that the air content in the concrete is appropriate, and the tiny air bubbles have a certain stability in the concrete construction process under the condition of uniform distribution and airtight independence. From the theory of concrete structure, the voids formed by such tiny bubbles belong to the range of capillary pores or harmless pores and less harmful pores, which not only will not reduce the strength, but also greatly improve the durability of concrete.
Cause of bubbles
raw materials
The effect of cement on the formation of air bubbles
The use of grinding aids in the cement production process usually results in excessive air bubbles, and the alkali content in the cement is too high, the fineness of the cement is too fine, and the gas content will also increase. The amount of cement and water consumption is also an important reason for the generation of air bubbles. The cement dosage is mainly considered for the strength in the concrete trial mix. If the cement dosage is increased under the premise of satisfying the concrete strength, the water-cement ratio will be reduced, and the amount of air bubbles will be greatly reduced. Its function is to fill the remaining voids with excess mortar, thereby blocking the formation of air bubbles.
Effects of admixtures on bubble formation
If the concrete contains a lot of large bubbles, it is usually related to the poor air-entraining composition in the admixture. Generally, air-entraining agents with certain air-entraining components may be mixed in water-reducing agents or composite pumping agents. As the amount of water-reducing agent increases, air bubbles will also increase.
The effect of coarse and fine aggregates on the formation of bubbles
According to the principle of granular material gradation and compaction, in the construction process, the material gradation is unreasonable, the coarse aggregate is too large, the size is inappropriate, the content of needle-like particles is too much, and the actual sand ratio used in the production process is higher than the sand provided by the laboratory. The rate is too small, so that the fine particles are not enough to fill the voids of the coarse particles, resulting in the incompactness of the particles and the formation of free voids, which provides conditions for the generation of air bubbles.
construction technology
The effect of stirring time on bubble generation
The mixing time will also have different effects on the air bubbles generated inside the concrete. During the mixing process of concrete, if the mixing is not uniform, the same water-cement ratio will make the admixture not evenly distributed, and there will be more bubbles in the part where the admixture is abundant, while the part that is not mixed with the admixture will not be evenly distributed. There will be uneven slump, large slump, segregation and other phenomena, but excessive mixing will cause more and more bubbles to be formed in the concrete during the mixing process, which will have a negative effect.
The effect of release agent selection on bubble generation
Pure oil mold release agents are still used by some construction units, and waste oil recovered from machinery factories is often used. This waste oil has strong adsorption to air bubbles. Once the air bubbles in the concrete come into contact with it, It will be adsorbed on the formwork and formed on the surface of the concrete structure. In addition, it is necessary to pay attention to the selection of mold release agents, which should be adapted according to the type of template, so that the amount of air bubbles can be reduced when used together.
Effect of Vibration on Bubble Generation
Due to the different vibrating environments during construction, the operation of the vibrator has different effects on the amount of air bubbles on the concrete surface. As a concrete structure, the better the vibration, the denser the internal structure of the concrete. In terms of the height of the layered vibration and the vibration time, the higher the layered height, the less likely the air bubbles inside the concrete will be discharged. . The speed of inserting and pulling out the vibrator will also affect the discharge of air bubbles. It is required to “plug fast and pull out slowly”, that is, the insertion speed should be fast, so that the upper and lower concrete are vibrated almost at the same time, and it should be slow when pulling out, otherwise the vibrator should be vibrated at the same time. The position of the air bubble is not easy to be filled to form a gap, which is not conducive to the discharge of air bubbles. However, the appearance of over-vibration, under-vibration and leakage vibration will cause more and more bubble defects on the surface of concrete. Ultra-vibration will cause the tiny bubbles inside the concrete to collapse and recombine under mechanical action, from small to large. Under-vibration and leakage vibration will cause the concrete to appear natural voids or air-type irregular large air bubbles caused by the lack of compaction.
Air bubble control and prevention and improvement measures
Raw materials
(1) It is recommended to use cement with less bubbles and low alkali content.
(2) The user should advise the admixture manufacturer to strictly control the proportion and dosage of the air-entraining components incorporated in the preparation of the admixture.
(3) For the aggregate, it is necessary to close the good material, strictly control the particle size gradation of the aggregate and the content of needle-like particles, and carefully screen the material when preparing to remove the unqualified materials. Select reasonable aggregate gradation and sand rate.
construction technology
(1) Pay attention to the stirring time, so that the admixture can be fully and uniformly mixed with other raw materials. There are also standard requirements for the mixing time. According to the provisions of "7.5.2 of "Ready Mixed Concrete" GB/T14902, the mixing time of each plate (calculated from the completion of all materials) during mixing production shall not be less than 30s. When preparing strength grades above C50 The mixing time should be increased when the concrete is used, or when air-entraining agents, expansion agents, and waterproofing agents are used.”
(2) It is recommended that the construction unit choose a high-quality release agent, and check whether the template used and the release agent match. The template should be kept clean and the release agent should be applied evenly but not too much or too thickly.
(3) It is recommended that the construction unit pay attention to the vibrating process. During the construction of concrete, the materials should be distributed in layers and vibrated in layers. The thickness of the layer should be no more than 50cm, otherwise the air bubbles will not be easily discharged from the inside of the concrete. The vibrating operation shall be carried out in strict accordance with the operation method of "quick plugging and slow pulling" to strictly prevent the phenomenon of under-vibration, leakage and over-vibration of concrete.
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