The Cause of Bubbles
1. Material Reason:
Unreasonable material gradation: According to the principle of aggregate gradation and compactness, if the gradation of coarse and fine aggregates is unreasonable, for example, the material contains too many needle-like particles, the gradation of coarse aggregate does not meet the requirements, resulting in excessive porosity. Large, or the sand rate of the material during construction is too small, so that the coarse and fine aggregates cannot reach the maximum compactness, which will cause the material to form free voids, which are more likely to generate air bubbles.
Unreasonable water-cement ratio: When designing the concrete mix ratio, the higher the water-cement ratio of the material, the more air bubbles will be on the surface of the concrete structure. This is mainly because when the concrete reaches water saturation, the excess water will dissociate to the surface of the concrete, be evaporated or be absorbed by the concrete itself during curing to form air bubbles. On the contrary, if the amount of cement is increased and water is reduced while the strength of the concrete can be guaranteed, the air bubbles will decrease. The reason is that the excess cement slurry can fill the gaps caused by unreasonable aggregate grading or other factors, and reducing the amount of water can reduce the air bubbles formed by free water.
2. Reasons for the construction process
Improper mixing time: The mixing time will also affect the generation of air bubbles in the concrete. During the mixing process of concrete, if the mixing is uneven, with the same water-cement ratio, the bubbles generated in the part with more admixture will increase, while the unmixed part will have uneven slump, large slump loss, segregation, etc. Phenomenon, but excessive mixing will cause more and more air bubbles entrained in the concrete during the mixing process, which will have a negative effect.
Insufficient vibration: Vibration is a direct means to expel air bubbles in concrete. The more fully the concrete is vibrated, the better the compactness of the aggregate particles will be. The time of vibrating is generally 3~5 minutes. If the time is too long, the tiny air bubbles inside the concrete will be broken and reorganized under mechanical action, from small to large; if the vibrating time is short and the place that is not vibrated will cause the concrete to not be dense and form natural pores Or irregular large air bubbles, resulting in air bubbles and honeycomb pockmarks on the surface of the concrete structure.
In addition, the speed at which the vibrator is inserted also affects the ejection of air bubbles. Insert it quickly to ensure that the concrete is vibrated up and down for the same amount of time. Pull it out slowly, in order to make the position of the vibrating rod aggregate and fill slowly during the vibrating process, especially dry hard concrete.
3. Precautions:
Concrete raw materials:
(1) Inspect the strength grade, variety and performance of the cement used in strict accordance with the requirements. If the precast concrete manufacturer has many cement suppliers, the cement with low alkali content and less surface foaming should be preferred. And the strength grade of the cement should be compatible with the strength grade of the precast concrete mix ratio. In addition, the use of admixtures should be strictly controlled to strictly ensure that the air content of the concrete does not exceed 4%.
(2) Within the scope permitted by the specification, aggregates with large particle size can be used to prepare concrete. This is because the increase in aggregate particle size can effectively reduce the water consumption of concrete, thereby reducing the bleeding and shrinkage of concrete. However, if the aggregate particle size is too large, it is easy to cause concrete segregation. Therefore, it is necessary to choose the aggregate base embryo reasonably to make the proportion of coarse and fine aggregates moderate. The specifications of the stones can be selected in combination with the specific construction conditions. When choosing, try to choose those stones with good gradation and large particle size, so as to reduce the amount of cement and water, reduce the wetness and shrinkage of concrete, and can also Effectively reduce the heat generated during cement hydration.
4. Process
1) Check the formwork and release agent carefully before construction. To ensure the smoothness of the formwork surface, use a high-quality release agent to avoid sticking effects on the air bubbles on the concrete surface. When applying the release agent, ensure that the whole is uniform, and it is not suitable to apply too much or too thick. When the concrete is just removed from the formwork, it is necessary to treat the pitted surface, honeycomb and other air bubbles with a mixed slurry with an appropriate viscosity, and then use the removed formwork to plaster and compact it.
(2) Pay great attention to the vibration of precast concrete. It is necessary to select the appropriate vibrating equipment, strictly control the vibrating time, reasonably select the vibrating radius and the vibrating frequency of the vibrating equipment, pay attention to the vibrating process throughout the process, ensure the compactness of the concrete after vibrating, and resolutely avoid leakage and under-vibration and the occurrence of over-vibration. Vibration should adopt the method of "quick insertion and slow removal". "Quick insertion" is conducive to obtaining uniform vibration time for the upper and lower layers of concrete, and "slow extraction" is conducive to filling the space left by the vibrator with concrete slurry and preventing For voids, especially for hard concrete, "slow pull" is particularly important. "Slow pull" is not only conducive to the aggregation and filling of concrete slurry, but also can promote the escape of air bubbles.
(3) Measures should be taken for uneven heat distribution to effectively reduce the influence of temperature changes on air bubbles on the surface of precast concrete. The transportation and pouring of precast concrete must meet the relevant construction requirements. For example, during construction in summer, sunshade or cooling measures should be taken for the coarse and fine aggregates of concrete; . In addition, certain heat preservation measures should be taken during construction in winter, especially for precast concrete molds. The forming process of concrete components is also a process of hydration reaction. The hydration reaction will release a certain amount of heat, causing the temperature difference of the structure to increase, causing problems such as cracking and air bubbles. Therefore, the maintenance of components should be strengthened, and steam curing should be used as much as possible during maintenance. For those that do not have steam curing conditions, try to keep the surface of the concrete member moist, and control the temperature of the mold to prevent the formation of a temperature gradient.
