Air bubbles in concrete: Why and How to deal with it?

The bubbles in concrete above 100μm are called large harmful bubbles, 100 ~ 50μm are called harmful bubbles, 50 ~ 20μm are called low harmful bubbles or harmless bubbles, and those below 20μm are called beneficial bubbles.

When the air content in concrete is appropriate, micro-bubbles have a certain stability in the process of concrete construction under the condition of uniform distribution and airtight independence. In terms of concrete structure theory, the voids formed by these tiny bubbles belong to the range of capillary pores or harmless pores or less harmful pores, which not only will not reduce the strength, but also greatly improve the durability of concrete.

Causes of bubbles:

1. Raw materials

Effect of cement on Bubble formation

When grinding aids are used in cement production, there are usually too many bubbles, and the alkali content in cement is too high, the fineness of cement is too fine, and the air content will increase. The amount of cement and water consumption is also an important cause of bubbles. In the trial blending of concrete, the amount of cement is mainly considered for the strength of concrete. if the amount of cement is increased on the premise of meeting the strength of concrete, thus the water-cement ratio is reduced, the amount of air bubbles will be greatly reduced. Its function is to use excess mortar to fill the remaining space, so as to block the bubble formation space.

Effect of coarse and fine aggregate on bubble formation

According to the grain gradation compaction principle, in the process of construction, the material gradation is unreasonable, the coarse aggregate is too much and the size is inappropriate, the content of needle-like particles is too much, and the actual sand rate used in the production process is smaller than that provided by the laboratory. Such fine particles are not enough to fill the coarse material voids, resulting in uncompacted particles and forming free voids, which provides conditions for the generation of air bubbles.

2. Construction technology

Effect of stirring time on Bubble formation

The mixing time also has different effects on the bubbles produced in the concrete. In the process of concrete mixing, if the concrete is mixed unevenly, the same water-cement ratio will make the admixture not be uniformly distributed, and there will be more air bubbles in the surplus part of the admixture, while the part that has not been mixed to the admixture will appear the phenomena of uneven slump, large slump, segregation and so on, but excessive mixing will make more and more air bubbles formed in the mixing process of concrete, thus producing negative effects.

Effect of release Agent on Bubble formation

The pure oil release agent will still be used by some construction units, often using the waste oil recovered from the machinery factory, which has a strong adsorption to air bubbles. Once there are air bubbles in the concrete, it will be adsorbed on the formwork and formed on the surface of the concrete structure. There is to pay attention to the selection of release agent should be combined with the type of template for adaptive selection, so that the use of combination can reduce the amount of bubbles.

Effect of Vibration on Bubble formation

Due to the different vibration environment in the construction, the operation of the vibrator has different effects on the number of air bubbles on the concrete surface. As a concrete structure, the better the vibration, the denser the internal structure of the concrete. from the two aspects of the height and time of the layered vibration, the higher the height of the layered vibration, the less likely the air bubbles inside the concrete will be discharged. The insertion speed of the vibrator will also affect the discharge of air bubbles, requiring "quick insertion and slow pull-out", that is, the insertion speed should be fast, so that the upper and lower concrete will be vibrated almost at the same time, and it will be slow when pulled out, otherwise the position of the vibrator is not easy to be filled and form a gap, which is not conducive to the discharge of air bubbles. However, with the emergence of overvibration, undervibration and leakage vibration, there will be more and more air bubble defects on the surface of concrete. Supervibration will cause the tiny air bubbles in the concrete to break up and recombine under the mechanical action, from small to large. Both undervibration and leakage vibration will lead to uncompacted concrete natural voids or air-type irregular bubbles.

How to deal with the bubbles?

1. It is suggested that priority should be given to the use of cement with few bubbles and low alkali content.

2. For aggregates, the aggregate size gradation and needle-like particle content should be strictly controlled, and the unqualified materials should be carefully screened when preparing materials. Choose reasonable aggregate gradation and sand ratio.

3. Pay attention to the mixing time, so that the admixture can be fully and evenly mixed with other raw materials.

4 It is recommended to use high quality mold release agent, and check whether the template and release agent match. The template should be kept smooth and clean, and the release agent should be applied evenly but not too much or too thick.

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