In order to ensure the pumpability and durability of the pumped concrete, an appropriate amount of air entraining agent is usually added (the air entraining agent can improve the workability of the fresh concrete mix and the durability of the hardened concrete), but the excessive lead The amount of gas (when there are too many harmful bubbles) has a direct effect on the strength of the concrete.

The distribution of bubbles in concrete, ie the size of the pores, the number of bubbles and the distribution of the bubbles, will have a significant effect on the workability, strength and durability of the concrete.

The type of bubble in concrete

Bubbles generated in concrete, called 100% or more, are called large bubbles, 100-50nm are called medium bubbles, 50-20nm are called low-impact or harmless, and below 20nm are called beneficial bubbles.

It should be noted that the gas content in the concrete is appropriate, and the microbubbles have a certain stability in the concrete construction process under uniform and tightly sealed conditions. Theoretically speaking, the void formed by such a small diameter bubble belongs to the range of capillary pores or harmless pores and less harmful pores, which not only does not reduce the strength, but also greatly improves the durability of the concrete.

It can usually be observed after the test block is broken:

1. The bubble spacing should be greater than 5 times the bubble diameter;

2, the amount of large bubbles should not be too much and concentrated;

3. The bubble distribution in the test block should not be connected to a straight line, and the amount of large bubbles on a single straight line should not be too much.

The harm of air bubbles to concrete structures

When the concrete contains more than 4% of gas and there are too many large bubbles, it will cause certain harm to the concrete:

(1) Reduce the strength of concrete structures

Since the bubble is large, the cross-section volume of the concrete is reduced, so that the interior of the concrete is not dense, thereby reducing the strength of the concrete. The concrete application technical specification stipulates that when the concrete gas content increases by 1%, the 28d compressive strength decreases by 5%. When the gas content is large, the compressive strength may be reduced by 4% to 6% for every 1% of the amount of bleed air. In low-strength concrete, when the gas content is 3-6%, the effect on the strength is small. When the value of the bleed air exceeds 6%, the compressive strength is bound to be greatly affected.

(2) Reduce the corrosion resistance of concrete

Due to the large number of bubbles on the concrete surface, the effective thickness of the protective layer of the steel is reduced, and the carbonization process of the concrete surface is accelerated, thereby affecting the corrosion resistance.

(3) Serious impact on the appearance of concrete Large bubbles will cause concrete to show the appearance of honeycomb pockmarks, affecting its appearance.

The cause of bubbles

The causes of bubbles in concrete are more complicated, and generally include:

1. Raw materials

(1) Bubbles have a very close relationship with cement varieties

In the cement production process, the use of grinding aids (external grinding aids, manufacturers are very many, the quality difference is very large, usually contains more surfactants), usually the occurrence of excessive bubbles, and in the cement If the alkali content is too high, the fineness of the cement is too fine and the gas content will increase.

In addition, in the low-grade concrete mixing process with less cement consumption, the water consumed by the hydration reaction is less, so that the film combines water and free water relatively more. As a result, the probability of bubble formation is significantly increased (bubbles in concrete become bubbles after evaporation). This is the reason why concrete with a large amount of water and high water ash is easy to generate bubbles.

Recommendation: Different brands of cement will produce significantly different amounts of bubbles. Preference is given to low-alkali, non-abrasive grinding agents, strong adaptability, certain brands, large scale, stable quality and less mixed foam in the test.

(2) Admixture type and dosage have a great influence on the formation of bubbles

If the concrete contains a large number of large bubbles, it may usually be related to the poorer air entraining component in the water reducing agent. In general, water reducing agents (especially polycarboxylic acid water reducing agents and lignosulfonate water reducing agents) or air pumping agents may be incorporated into the pumping agent, and the amount of water reducing agent is increased. The bubbles are also increased; in addition, when the added admixture is a rosin-based air entraining agent, the generated bubbles are slightly more than other types of admixtures.

Recommendation: Let the water reducing agent compounding plant cancel the bleed air component in the water reducing agent. When mixing water-reducing agents, it is not advisable to use raw materials that have been fermented or stored for a long time when selecting compound materials, such as waste liquid from paper mills or sugar mills (usually with obvious odor or odor).

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

According to the principle of compaction of pellets, during the construction process. The material grade is unreasonable, the coarse aggregate is too large, the size is improper, the content of needle-like particles in the crushed stone is too much, and the actual sand rate in the production process is smaller than the sand rate provided by the laboratory, so that the fine particles are insufficient. Filling the coarse-grained voids results in the pellets not being dense and forming free voids, providing conditions for the generation of bubbles.

When the particle size of the sand is in the range of 0.3-0.6 mm, the gas content of the concrete is the largest, and when it is less than 0.3 mm or more than 0.6 mm, the gas content of the concrete is significantly decreased.

Investigation plan and improvement measures for harmful bubbles on concrete surface

In view of the harmful bubbles appearing on the concrete surface, certain measures should be taken to find out the cause and improve it. The CTF synergist does not contain any bleed air components, but under the combined action of different water reducing agents or pumping agents and cement, there may be more bubbles, and the bubbles can be controlled by technical means.

1. Raw materials

(1) First check the variety, performance and label of the cement used. If there are many cement suppliers in the factory, it is preferred to use cement with less bubbles and low alkali content. The cement label should be compatible with the concrete mix ratio.

(2) The admixture used in the second inspection. At present, there are many brands in this area, which cannot be generalized. However, for the actual production, the easiest and most convenient way is to make several sets of test pieces and select the admixture with good chemical composition for production.

If the water reducing agent contains the causative ingredients, try different water reducing agents for comparison, and notify the water reducing agent manufacturer to cancel the air entraining agent with the bleed air component or control the air bleed amount. The admixture used in general concrete is cited. The gas volume is controlled within 4%, and the high-grade concrete (such as C50, C60) is controlled within 3%.

When using the air entraining agent, the air entraining agent with small air bubble and uniform distribution should be selected, and the air entraining agent containing the rosin type should be used as little as possible. The air bubbles generated by the air entraining agent are larger.

(3) Check the aggregate properties. Close the good materials, strictly control the aggregate size and the content of the needle-like particles. Carefully select and eliminate the unqualified materials when preparing the materials. Choose a reasonable grading to make the ratio of coarse aggregate to fine aggregate moderate.

(4) In the CTF concrete synergist, the defoaming agent for suppressing harmful bubbles will be incorporated, and it needs to be tested and debugged with the water reducing agent factory. Defoaming agent can improve the compactness and uniformity of concrete, improve the impermeability and increase the strength. (See the annex for the mechanism of defoamer)

2, construction

(1)     Pay attention to the mixing time. Special emphasis is placed: some commercial concrete requires a long transportation time from the factory to the construction site. At this time, due to the possibility of large damage, some manufacturers technicians will use the admixture for secondary deployment. In this case, it must be Strengthen the mixing of concrete evenly. However, it is not a long time of mixing. In the "Common Materials Testing Manual for Construction Engineering", it is clearly stated that "the air entraining agent water reducing agent concrete must be mechanically stirred, and the stirring time should not be greater than 5 min and less than 3 min." The longer the agitation time, the larger the bubbles produced.

(2) Check the template used and the release agent. The template should be kept clean and the release agent should be applied evenly but not too thick.

Professor Zhang Jiebo of Xi'an University of Architecture and Technology has studied and demonstrated that it is very effective to remove the bubbles on the concrete surface from the release agent of the template. At present, many units in the market have developed a release agent with defoaming chemical composition. After the use of the defoaming release agent, the bubbles generated by the concrete and the release agent contained in the template surface are contained. After the defoaming agent meets, the defoaming agent will immediately burst or become smaller and smaller, and the concrete surface will be extremely smooth and dense.

(3) Pay attention to the vibrating process to prevent the under-vibration, leakage and super-vibration of concrete.

During the construction of concrete, the fabric should be layered and stratified. The thickness of the layering is preferably not more than 50 cm, otherwise the bubbles are not easily discharged upward from the interior of the concrete. At the same time, we must choose the appropriate vibrating equipment, the best vibrating time, reasonable vibration radius and frequency.

(4) Factors such as low temperature, large amount of cement, and large water consumption will directly affect the amount of bleed air. It should be analyzed and adjusted according to the specific conditions on site.

Attachment: Principle of defoamer

The antifoaming agent is a substance which is more easily adsorbed than the foam stabilizer, and it can replace the adsorbed gas-liquid surface active material, thereby making it difficult to form a stable film. After the defoaming agent enters the liquid film, the viscosity of the liquid is lowered, the liquid film loses its elasticity, the liquid is exuded, and finally the liquid film is thinned and broken, thereby reducing the content of bubbles in the concrete system, especially some large bubbles.

Defoaming can be divided into two types: one is the foam breaking effect, which can break the bubbles that have formed, destroy the conditions in which the bubbles are stable, and make the stable bubbles become unstable bubbles and further enlarge them. . The second is the antifoaming effect, which is different from the foam breaking effect in that it can not only break the bubble that has already formed, but also maintain the defoaming effect for a long time after the antifoaming agent is added, and it is certain It also prevents the generation of bubbles or foam.