The South-to-North Water Diversion Project has large investment, long routes and many individual projects, including channels, bridges, culverts, pipelines, inverted siphons, and aqueducts built with concrete panels. The engineering structure is complex, the forms are various, the exposed area is large, and it is easily eroded by the bad external environment, so the concrete panels and various concrete buildings are durable.
Concrete durability refers to the ability of concrete to resist the effects of environmental factors and not be destroyed after many years of use under the condition that it meets the design requirements. If the durability is insufficient, the structure will be damaged to varying degrees. Once damaged, the manpower and material resources invested in the repair work are often large. 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.
The causes of concrete damage and destruction are external environmental conditions, internal defects of concrete and concrete constituent materials. External environmental conditions such as wind and rain, sun exposure, cold and heat, dry and wet weather, etc., extreme temperature effects, abrasion, erosion of chemical media, etc.; internal defects such as penetration of concrete, alkali-aggregate reaction, etc.; composition materials such as aggregate and cement Volume changes caused by different stone thermal properties, etc. The external environment exists objectively and can hardly be changed, so improving the durability of concrete starts from reducing the internal defects of concrete and improving its constituent materials, so as to improve the performance of concrete, improve the quality of concrete, reduce or reduce the internal defects of concrete, and prolong the durability of concrete buildings. Use period.
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 external unfavorable factors. Therefore, in order to improve the durability of concrete, the impermeability of concrete should be improved first.
1.1 Reduce the water-cement ratio
Concrete is made up of cement, coarse and fine aggregates and water. According to the principle of complete hydration of cement, the water demand is only about 25% of the weight of cement, but 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 capillary pores are formed. For example, when the water-cement ratio increases from 0.4 to 0.7, the permeability coefficient increases by 100 times, or even more; from 0.65 to 0.55, the permeability coefficient decreases to 1/3 of the original. In addition, due to the increase in water consumption and other reasons, the freshly mixed concrete is also prone to segregation, bleeding, and floating water that stays under the stones or horizontal steel bars or rises around the stones and steel bars, forming Connected channels, voids are formed after water evaporates. On the one hand, it affects the adhesion between aggregate and cement paste, steel bar and concrete, and reduces the density and strength of concrete. On the other hand, these connected channels also become external liquids, harmful A shortcut for gas intrusion into concrete. Therefore, when mixing concrete, in order to meet 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 the concrete.
1.2 Mixing with admixtures
The use of admixtures is the best way to improve concrete performance. One is the air-entraining water reducing agent. Air-entraining water-reducing admixtures, on the one hand, produce uniform, stable, and non-communicating tiny air bubbles inside the concrete, preventing liquid penetration; on the other hand, it can greatly reduce the water consumption of concrete and improve the compactness of concrete. At present, the DHG air-entraining superplasticizer, which is widely used in major projects, belongs to this performance admixture. Using this admixture, the dosage is 0.7%. The amount of entrained air is 3% to 5%, and the water reduction rate is above 20%. And the concrete has good workability, and its impermeability is more than doubled. The second is the use of impermeable agents. The impermeable agent forms a colloidal complex in the concrete, which fills and blocks the capillary cracks in the concrete, thereby improving the compactness of the concrete. The third is mixing expansion agent. The chemical reaction occurs by adding an expansion agent, so that the concrete expands, and under the constraint of external force, the density of the concrete is increased.
1.3 Choose the right material
The cement grade is generally not less than 425#; the fine aggregate requires the selection of medium sand with an average particle size of about 0.4mm in river sand with uniform particles, smooth and hard texture, with a mud content of less than 3% and an appropriate amount of silt; The maximum particle size of coarse aggregate should be reasonable. Except for large volume, generally 5-30mm is suitable, the maximum particle size should not exceed 40mm, 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.
If the early maintenance is not good, the cement will not be properly hydrated, which will reduce the compactness of the concrete, and then affect the impermeability. Therefore, it is necessary to strengthen the early wet curing of concrete, and the curing time shall not be less than 14d.
Post time: Jan-14-2022