The South-to-North Water Diversion Project has a large investment, a long route, and many individual projects, including channels, bridges, culverts, pipelines, inverted siphons, aqueducts, etc. made of concrete panels. The engineering structure is complex, with various forms, and the exposed area is large, making it susceptible to erosion by the adverse external environment. Therefore, concrete panels and various concrete buildings are durable.
Concrete durability refers to the ability of concrete to withstand the effects of environmental media and survive long-term use without being damaged if it meets the design requirements. If the durability is insufficient, the structure will be damaged to varying degrees. Once damaged, repair work often requires a lot of manpower and material resources. Therefore, improving the durability of concrete has important practical and long-term significance for extending the service life of concrete buildings and saving national investment.
Causes of concrete damage and destruction include external environmental conditions, internal defects in the concrete, and the materials that make up the concrete. External environmental conditions such as wind, rain, sun, cold and heat, dryness and humidity, extreme temperature effects, abrasion, chemical media erosion, etc.; internal defects such as concrete penetration, alkali-aggregate reaction, etc.; constituent materials such as aggregate and cement Volume changes caused by different thermal properties of stone, etc. The external environment exists objectively and can hardly be changed. Therefore, improving the durability of concrete starts from reducing the internal defects of concrete and improving its component materials to improve the performance of concrete, improve the quality of concrete, reduce or reduce the internal defects of concrete, and extend the life of concrete buildings. Years of use.
1. Improve the impermeability of concrete
The durability of concrete is directly related to its impermeability. The penetration of concrete determines to a large extent that concrete is susceptible to erosion by adverse external factors. Therefore, in order to improve the durability of concrete, the impermeability of concrete must first be improved.
1.1 Reduce water-cement ratio
Concrete is made from cement, coarse and fine aggregates and water. According to the principle of complete hydration of cement, the water requirement is only about 25% of the weight of cement. However, when mixing concrete, in order to obtain the necessary fluidity and meet the construction requirements. More water is commonly used, that is, a larger water-cement ratio W/C. When the concrete hardens, the excess water is evaporated and pores are formed. The greater the water consumption, the more pores are left after the cement is hydrated, and the greater the permeability coefficient. For example: when the water-cement ratio increases from 0.4 to 0.7, the permeability coefficient increases by 100 times or even more; when it decreases from 0.65 to 0.55, the permeability coefficient drops to 1/3 of the original value. In addition, freshly mixed concrete has poor workability due to increased water consumption and other reasons, and is prone to segregation, bleeding, and floating water that stays under the stones or horizontal steel bars or rises around the stones or steel bars, forming a Connected pores form voids after water evaporates. On the one hand, it affects the bonding force between aggregate and cement slurry, steel bars and concrete, and reduces the compactness and strength of concrete. On the other hand, these connected pores also become external liquids and harmful substances. A shortcut for gases to penetrate inside concrete. Therefore, when mixing concrete, while meeting the technical and construction requirements, the water-cement ratio should be reduced as much as possible, the water consumption should be reduced, and the density should be increased to improve the impermeability of concrete.
1.2 Adding additives
Adding admixtures is the best way to improve the properties of concrete. One is the air-entraining type water-reducing agent. On the one hand, the air-entraining water-reducing agent produces uniform, stable, and interconnected micro-bubbles inside the concrete to prevent liquid penetration; on the other hand, it can also greatly reduce the water consumption of concrete and improve the density of concrete. The DHG air-entraining high-efficiency water-reducing admixture that is currently widely used in major projects belongs to this type of performance admixture. Use this additive at a dosage of 0.7%. The air entrainment amount reaches 3% to 5%, and the water reduction rate is above 20%. Moreover, concrete has good workability and its anti-seepage capacity is more than doubled. The second is to mix anti-penetration agent. The impermeable agent forms a colloidal complex in the concrete, filling and blocking the capillary pores inside the concrete, thereby improving the compactness of the concrete. The third is to mix expansion agent. Chemical reactions occur through the addition of expansion agents, causing the concrete to expand and increase the compactness of the concrete under the constraints of external forces.
1.3 Choose the right material
The cement grade is generally no less than 425#; the fine aggregate is required to be medium sand with an average particle size of about 0.4mm from river sand with uniform, smooth and hard texture, a mud content of less than 3%, and an appropriate amount of silt; The maximum particle size of coarse aggregate should be selected reasonably. Except for large volumes, 5 to 30 mm is generally suitable. The maximum particle size should not exceed 40 mm, and the mud content should not exceed 1%. It requires fine organization, neat particles, and hard texture. In addition, the gradation should be excellent to improve the workability of concrete.
Strengthen maintenance
If the early curing is not done well, the cement will not be hydrated normally, which will reduce the compactness of the concrete and then affect the impermeability. Therefore, the early wet curing of concrete must be strengthened, and the curing time shall not be less than 14 days.