What is the reason for air bubbles after concrete pouring and formwork removal?

The causes of bubbles in concrete are very complicated, generally including:

1, raw materials

(1) Foam is closely related to cement varieties.

In the process of cement production, under the action of grinding AIDS (there are many manufacturers mixed with special grinding AIDS, which vary greatly in quality and usually contain more surfactants), there are usually too many bubbles, too high alkali content in cement and too fine cement, and the gas content will also increase.

In addition, the hydration reaction of low-grade concrete with less cement consumption consumes less water during mixing, which makes the membrane bound water and free water relatively more. In this way, the probability of bubble formation is obviously increased (bubbles in concrete evaporate and become bubbles). This is the reason why concrete with large water consumption and high water cement ratio is easy to produce bubbles.

Suggestion: Different brands of cement will produce obviously different bubbles. Give priority to cement varieties with low alkali, helpless grinding agent, strong adaptability, certain brand, large scale, stable quality and few bubbles in trial mixing.

(2) The type and content of additives have great influence on the formation of bubbles.

If concrete contains a large number of large bubbles, it may usually be related to the poor air-entraining composition in the water reducer. In general water reducers (especially polycarboxylic acid water reducers and lignosulfonate water reducers) or pumping agents, air entraining agents with some air entraining components can be mixed. With the increase of water reducing agent content, bubbles will also increase. In addition, when the additive is rosin air entraining agent, the bubbles produced are slightly more than other types of additives.

Suggestion: let the water reducer compound and cancel the air-entraining component in the water reducer. When preparing water reducer, it is not advisable to use raw materials that have been fermented or stored for a long time, such as waste liquid from paper mills or sugar mills (usually with obvious odor or peculiar smell).

(3) The coarse and fine aggregates also have a certain influence on the generation of bubbles.

According to the principle of aggregate gradation and compactness, in the construction process, the material gradation is unreasonable, the coarse aggregate is too much and the size is not suitable, the needle-like particles in the gravel are too much, and the sand ratio actually used in the production process is less than that provided by the laboratory, which makes the fine aggregate not enough to fill the gap of the coarse aggregate, resulting in loose aggregate and free gap, which provides conditions for the generation of bubbles.

When the particle size of sand is in the range of 0.3-0.6mm, the air content of concrete is the largest, while when it is less than 0.3mm or more than 0.6mm, the air content of concrete will be significantly reduced.

2. Architecture

(1) Mixing time will have different effects on bubbles produced in concrete.

If the mixing is uneven, there will be more bubbles in the parts with more additives (the water-cement ratio is the same). However, excessive mixing will lead to the formation of more and more bubbles in concrete, which will have a negative impact.

(2) Improper use of release agent will affect the generation of bubbles.

Because some construction units have extended the service life of the old release agent, they often use the waste engine oil recovered from the machinery factory. This kind of waste engine oil has strong adsorption to bubbles. Once the bubbles in concrete contact with it, they will be adsorbed on the formwork to form the surface of concrete structure. There is also a release agent, even a water-based release agent, which still has an adsorption effect on the bubbles generated in concrete, so that the bubbles in concrete cannot rise gradually with the mechanical vibration and the contact surface of the template, thus making it impossible to discharge the bubbles generated in concrete. The release agent should be carefully selected.

(3) Vibration affects the generation of bubbles.

Because of the different vibrating environment in construction, the operation of vibrating hand has a fundamental difference on the number of bubbles on the concrete surface. As a concrete structure, the better the vibration, the denser the internal structure of concrete will be. From two aspects: the vibrating height and vibrating time, it is shown that the higher the layering height, that is, the higher the blanking height each time, the more difficult it is for bubbles in concrete to be discharged upward. However, the longer the vibrating time (over-vibration) or the shorter the vibrating time (under-vibration) and the place where it is not vibrated (vibration leakage) will cause more and more bubble defects on the concrete surface. Supervision will make the tiny bubbles inside the concrete rupture and recombine under the mechanical action, from small to large. Under-vibration and leakage vibration will make concrete not compact, which will lead to natural cavities or air-type irregular bubbles in concrete.