Concrete is a general term for engineering composite materials made by binding aggregates together with cementing materials, and it is widely used in civil engineering. Because it consists of solid, liquid, and gaseous phases, air bubbles are inevitably generated after concrete is poured. However, the presence of these bubbles not only affects the appearance of the project, but more importantly, it poses some hidden risks. However, as long as relevant personnel pay sufficient attention and use effective technical means, combating these concrete bubbles is not a problem.
I. Causes of air bubbles in concrete
1. Equipment type and time for concrete mixing and transportation
Uneven concrete vibration and insufficient vibration time can easily cause air to get in during the vibration process. This prevents air bubbles in the concrete mix from escaping, and the weight of the mixture itself cannot expel these air bubbles. In addition, improper operating methods can also lead to air bubbles in the concrete.
2. Improper use of release agent
Release agent is a substance applied to the formwork before concrete pouring to prevent the formwork from sticking to the concrete surface after pouring, thus preventing difficulty in demolding and ensuring a smooth concrete surface. Its main function is to isolate the formwork from the concrete surface through a film formed on the formwork. However, in actual construction, uneven application of the release agent or improper use can cause air bubbles to rise along the formwork.
3. Concrete pouring is too thick
In actual construction, if the concrete layers are too thick, the effective compaction depth of the vibrator will not meet the construction requirements. Furthermore, some workers determine the concrete pouring height solely based on experience or intuition, making it difficult for air bubbles inside the concrete to escape.
4. Unreasonable aggregate gradation
Excessive coarse aggregate, insufficient fine aggregate, poor particle shape, or low sand ratio may cause the fine aggregate to fail to fill the pores of the coarse aggregate, resulting in air bubbles.
5. Admixtures, cement, and binders
The amount of cement is reduced, the amount of water used and the amount of free water are increased, but if too much admixture is used, the viscosity of the concrete will be increased, making it difficult for air bubbles to escape.
II. Measures to Control Air Bubbles in Concrete
1. Select appropriate vibration equipment
Determine the appropriate vibration time, radius, and frequency. Vibrate using the "quick insertion, slow withdrawal, up-and-down pulling" method, ensuring the vibrator moves straight up and down , inserting quickly and withdrawing slowly to avoid missed areas. During vibration, move the vibrator up and down, maintaining the duration of each vibration point until a surface layer of slurry appears, allowing air bubbles to escape. Strictly prevent under-vibration, missed vibration, and over-vibration of the concrete.
2. Strengthen the cleanup of templates.
Choose a release agent carefully and strictly control its quality; oil-based release agents must not be used. Apply the release agent evenly during application, but do not apply too much to avoid accumulation and dripping.
3. Concrete should be poured in layers, with each layer having a specific thickness.
The maximum paving thickness should be determined based on factors such as mixing and transportation capacity, pouring speed, vibration capacity, and structural characteristics. The maximum paving thickness for pumped concrete should not exceed 600 mm, and for other types of concrete, it should not exceed 400 mm.
4. Strictly select aggregates
Control the aggregate particle size and the content of needle-shaped and flaky particles. Carefully screen the aggregate during preparation and discard any unqualified aggregates. Determine a reasonable gradation to ensure a proper ratio of coarse to fine aggregates.
5. Select admixtures and cement with uniform and stable distribution and excellent chemical composition.
In actual production, compatibility tests between cement and admixtures should be conducted, and materials with better compatibility should be selected for production. Furthermore, care should be taken not to blindly pursue high workability of concrete, and to ensure that admixtures are not over-added.
These methods not only ensure the visual quality of the concrete, but also effectively control air bubbles in the concrete.
