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July 27, 2020
The process of cracks gradually destroying the concrete dam of a hydropower station
As concrete is widely used in various types of hydropower dams, the construction height and durability of hydropower dams have been improved.
However, the construction of concrete dams of hydropower stations has brought fundamental changes due to the development and widespread use of superplasticizers, but at the same time it also increases the risk of modern concrete cracking. The design, construction, supervision and other links have insufficient understanding of modern concrete, and the construction technology of pumping concrete The development of concrete dam also greatly increases the technical difficulty of crack control in concrete dams.
With the change of engineering construction mode, excessive pursuit of engineering progress will not only lead to insufficient curing of concrete, but also inevitably lead to the pursuit of high and early strength of concrete.
So, how did the crack gradually destroy the concrete dam of the hydropower station?
Interpretation of cracks
1. Concrete cracks are inevitable
Cracks are not only a symptom of poor construction quality, but also the most unfavorable factor that accelerates the deterioration of concrete durability. Concrete cracks are inevitable, but the degree is controllable. In most cases of poor concrete durability, it is affected by the surrounding environment or reacts with the surrounding environment media, accompanied by swelling, spalling and collapse or other performance failures. phenomenon.
2. The nature of concrete cracks
Cement concrete is prone to cracking due to the nature of its brittle material. There are only weak physical bond interactions between particles and the tensile strength is an order of magnitude smaller than the compressive strength.
3. Material factors
In recent years, the cracking of concrete dams in my country is mainly due to changes in cement, pumping concrete (additives, grout-to-bone ratio), increasing concrete strength levels, and increasing mass concrete consumption.
Early cracks that dominate the life of concrete
1. Cracks are the channels for all aggressive media to invade
Cracks are the intrusion channels of all corrosive media. The expansion of existing cracks is easier than newly generated cracks; the development of existing cracks is easier than newly generated cracks. Therefore, the early cracking will inevitably crack in the later stage, and the early non-cracking may not be cracked in the later stage, but try to It is necessary to reduce the shrinkage stress formed early. Controlling early cracks can reduce the width and number of later cracks.
2. The causes of modern concrete cracking are more complicated
In the early stage, shrinkage cracking was the main cause of cracking in concrete structures, and the temperature stress caused by the excessive temperature difference between inside and outside was sufficient to cause concrete cracking. The cracking stress is also related to the elastic modulus of concrete, whether cracking is related to the size of the cracking stress and the tensile strength of concrete.
The water-binder ratio of modern concrete becomes lower, the bleeding rate is reduced, and the concrete surface is more likely to dry; the initial setting time is significantly earlier, the curing needs to be advanced, and the curing method needs to be improved. The traditional curing methods can no longer meet the development requirements of modern concrete.
3. High strength may not be durable, and low strength may not be impervious
At present, the Ministry of Housing and Urban-Rural Development of China has a strong desire to rapidly promote the application of high-strength concrete, hoping to greatly increase its proportion in concrete, but high-strength may not be durable. People may think that high-strength concrete is dense and dense concrete is durable. Cracked.
We need a series of measures to control the formation of early cracks:
1. The causes of modern concrete cracking are more complicated
First, control the temperature and humidity; secondly, reduce constraints; thirdly, reduce the cracking sensitivity of concrete, reduce the strength as much as possible, reduce water consumption, use crack-sensitive cement, optimize aggregates, and use minerals with good crack resistance. And the material; fourth, the construction of the molding process is the final key link.
2. Pay attention to early maintenance
The key point of early concrete crack control is to pay attention to early curing. In actual construction, there are more technologies and materials available in various regions, such as curing agents, covering curing, and monomolecular film curing agents.
3. The mix ratio is "three lows"
In order to obtain good crack resistance, the concrete mix design must also undergo a major change. That is to follow the "three low" concrete mix technical route of low cement content, low water-binder ratio, and low water consumption per unit volume under the premise of ensuring workability. At the same time, it is necessary to change the concept. The so-called concrete durability can be controlled by controlling the water-binder ratio. Compared with water glue ratio, water consumption is more important to control cracking.
4. Vibration mixing technology
Vibration stirring is an economical and effective method of mechanically strengthening the stirring process. When the concrete is vibrating, the particles inside it will vibrate continuously around a certain unstable and balanced intermediate position, and the adjacent particles will be displaced, and the relative movement frequency between the material particles will increase, thereby causing the friction between the particles. The force and adhesion force decrease sharply. Detong's vibration mixing technology uses the characteristics of concrete to vibrate while mixing to make the cement particles vibrate, thereby destroying the cement agglomerates and evenly distributing the cement particles, so as to achieve the uniformity of the concrete on the macro and micro levels. Therefore, vibrating mixing improves the solid-liquid-gas three-phase structure of the mixture in a microscopic way, fundamentally reduces the occurrence of cracks, and improves the durability of concrete.
Although the concrete dam of a hydropower station can work normally with cracks, the existence of cracks will cause corrosion of steel bars and reduce the service life of the structure; it not only affects the beauty of the building, but also causes unsafe mental pressure and psychological burden to people. Therefore, the use of new technologies and technologies to improve the durability of concrete dams and rational use of resources can not only create huge economic and social value for the country, but also bring more security to the people.