The durability of concrete refers to the role of concrete in resisting the environmental boundary quality and its non-destructive performance after long-term use under the condition of meeting the design requirements. Insufficient durability will cause different degrees of damage to the structure, and the workforce and material resources invested in its repair work are often extensive. Therefore, improving the durability of concrete has essential practical and long-term significance for extending the service life of concrete buildings and saving national investment.
The causes of concrete damage and destruction include external environmental conditions, internal defects of concrete, and concrete constituent materials. The external environment exists objectively and can hardly be changed. Therefore, improving the durability of concrete must reduce the concrete’s internal defects to enhance the performance and quality of the concrete, reduce the internal defects of the concrete, and extend the service life of the concrete building.
The durability of concrete is directly related to impermeability. The penetration of concrete determines, to a large extent, that concrete is vulnerable to external adverse factors. Therefore, to improve the durability of concrete, the impermeability of concrete must first be improved.
1. Reduce the water-cement ratio
Concrete is made by mixing cement, coarse and fine aggregates, and water. According to the principle of complete hydration of cement, the water requirement is only about 25% of cement weight. Still, when mixing concrete, to obtain the necessary fluidity and meet the construction requirements, more water is often used, a larger water-cement ratio W/C. When the concrete hardens, the excess water is evaporated to form capillary pores. The greater the water consumption, the more capillary pores left after cement hydration, and the greater the permeability coefficient. 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 diminished, and the density should be increased to improve the impermeability of the concrete
2. Admixture
Admixture is the best way to improve the performance of concrete. One is the air-entrained water reducer. On the one hand, the air-entraining water-reducing agent produces uniform, stable, and incommunicable tiny bubbles inside the concrete to prevent liquid penetration; On the other hand, it can also significantly reduce the water consumption concrete and improve the density of concrete. At present, the DHG-entrained air-entraining superplasticizer, which is widely used in major projects, belongs to this performance additive. The admixture content is 0.7%. The admixture content is 0.7%, the air-entraining amount is 3%-5%, the water reduction rate is more than 20%, has good concrete workability, and its impermeability can be increased by more than double. The second is to use an impermeable agent. The impermeable agent forms a colloidal complex in the concrete, which fills and blocks the pores and gaps in the concrete, thereby improving the concrete’s compactness. The third is to add an expansion agent. A chemical reaction occurs through the addition of an expansion agent to cause the concrete to expand. Under the constraint of external force, the density of the concrete is increased.
3. Choose the right material
The cement number is generally not less than 425#; the fine aggregate requires the selection of river sand with uniform particles, smooth and hard texture, and should be medium sand with an average particle size of about 0.4mm, with a mud content of less than 3%, and an appropriate amount of silt sand. Choose the maximum particle size of coarse aggregate reasonably. Generally, except for large volumes, 5-30mm is reasonable, the maximum particle size does not exceed 40mm, and the mud content does not exceed 1% with fine structure, neat particles, and hard texture.
4. Prevent cracks
Common cracks in concrete buildings include shrinkage cracks, settlement cracks, temperature cracks, etc. Prevent shrinkage cracks and settlement cracks. In addition to the 4 items mentioned above, it is also necessary to pay attention to the concrete’s proper mixing time, do not cut the material too fast when pouring, prevent accumulation, vibrate compactly, and avoid excessive vibration. Generally, the vibrating time is 10-15s/time. Before the initial setting of the concrete, it should be smoothed and calendered. After calendering, it should be cured with a wet grass curtain or sprayed with a curing agent. In summer, the temperature is high, the wind speed is high, and the environment is dry.
Measures to prevent temperature cracks: First, slag cement and fly ash cement should be considered during construction; For mass concrete, use medium heat or low heat cement; At the same time, a certain amount of active admixtures, such as fly ash, slag powder, etc., are added under the condition of ensuring the strength index. Within a specific range, the greater the number of active admixtures used to replace cement, the better the effect of reducing concrete temperature. Besides, it can make full use of the later strength of concrete, reduce cement consumption, and reduce temperature stress. Then add retarding superplasticizer and air-entraining superplasticizer to large volume concrete to improve its workability, fluidity, cohesion, and water retention. Through water reduction, retardation and dispersion, water consumption can be reduced, the density and strength of concrete and hydration heat can be improved. The time of temperature peaks can be delayed.
5. Strengthen maintenance
With improper early maintenance, the cement cannot be hydrated naturally. It will reduce the concrete’s compactness and then affect the impermeability. Therefore, the early wetting and maintenance of concrete must be strengthened and shall not be less than 14 days.
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