Abstract: Introduces the problems of modern concrete in terms of durability, analyzes the changes in concrete properties brought about by the development of cement production technology and concrete construction technology, analyzes the impact of concrete properties on its durability, and then proposes solutions to concrete durability The effective way of the problem.
Keywords: modern concrete; durability; building materials
Concrete is the largest building material in the world today. China's concrete usage ranks first in the world, with an annual consumption of 2 billion tons, which plays an extremely important role in adapting to rapid economic development. Many problems are exposed during the application of concrete, among which the durability problem is particularly prominent. For example, many projects need to be repaired after 10 to 20 years of use, and some even after several years of use. Most of the concrete works are permanent, with a large amount of construction and a lot of cost. If the durability is poor, it will cause a very heavy burden on the future society. Therefore, from the comprehensive consideration of capital saving, effective use of resources and environmental protection, the durability of concrete must be studied in depth.
1 Rethinking concrete materials
1.1 Historical inspiration
First of all, let â€™s focus on two facts: â‘ In the 1950s and 1960s, due to the limitations of domestic production technology at that time, the cement produced had low activity and low grades. In order to meet the lower strength and construction requirements and maximize the saving of cement, prepare concrete The amount of cement used and water consumption are small, the fluidity of the mixture is small, the stability is good, the early strength development is slow, the cracks after hardening are less, the later strength development is larger, the durability is generally better, and some are still a few decades later. use. â‘¡The â€œconcreteâ€ buildings built by ancient Rome's construction craftsmen using volcanic ash and lime as cementing materials, such as the famous Pantheon, arena, harbour, diversion aqueduct and bathroom, have experienced more than 2000 years of running water, rain, snow, sea water and other natural The effect of factors is still in use today, which is amazing; the study found that the amount of ancient "concrete" cement material is very small, the water-cement ratio is very small (by tamping), and the strength growth is extremely slow, almost not due to drying and temperature Changes produce stress and cracks.
1.2 Development status of concrete technology
Today, with highly developed science and technology, people are increasingly demanding concrete structures from construction to performance, striving for fast construction speed and high strength. Improvements in cement production technology and advancements in concrete construction technology have made it possible. The result is that the cement used has high label, high activity, high dosage, fast hydration speed, early strength and high strength of concrete, large elastic modulus, and poor deformability; in order to facilitate transportation and pouring, the slump has increased from 0 to 20mm in the past To 180mm or even larger. However, on the other hand, changes in concrete properties brought about by the development of cement production technology and concrete construction technology directly affect the durability of concrete. Therefore, from the perspective of improving the durability of concrete, we must re-understand the artificial material of concrete.
2 The influence of the performance of concrete on its durability
2.1 Strength and durability
Concrete strength (compression) is the most important mechanical performance index of concrete, which directly affects its application. Improving the strength of concrete has always been the goal people have pursued. In theory, the higher the strength of concrete, the denser the structure, the stronger the ability to resist the external environment, and the better the durability. But this is not the case. Because to achieve high strength, it is necessary to increase the amount of cement and increase the grade of cement, which causes a violent hydration reaction, hydration heat is more and faster, and the self-generated shrinkage, drying shrinkage and temperature shrinkage of the concrete are strong, and the resulting tensile stress is enough to cause Concrete cracks. Once cracks (textures) appear in the concrete structure, it opens a convenient door for degradation effects such as freeze-thaw, chemical erosion, and alkali-aggregate reactions, and durability is inevitable. It can be seen that too low concrete strength is certainly not good for durability, but too high will also bring risks to durability . This is also one of the reasons why experts recommend the development of C25-C30 HPC  in China.
2.2 Mobility and durability
The fluidity of the concrete mixture has developed from 70 to 80 mm 10 years ago to a large number of commercial concrete 180 to 200 mm, which has achieved concrete pumping and high throwing, greatly improved the construction efficiency, and ensured the quality of vibration. This is undoubtedly the concrete construction technology Great progress. However, large fluidity requires a large amount of ash and water consumption, and this is an important reason for shrinkage cracks in concrete; although the use of water-reducing agent can ensure that the water-cement ratio does not change or reduce the premise of the premise The performance is improved, but the homogeneity and stability of the mixture are significantly deteriorated. Segregation, sedimentation, and bleeding are prone to occur during transportation, pouring and molding, and thus at the interface between aggregate and cement slurry, and The interface between steel and concrete forms a weak transition zone. After the concrete hardens, a large number of pores and micro cracks are formed. This is the root cause of the decrease in durability of concrete structures. Therefore, from the perspective of improving the durability of concrete, it is not appropriate to increase the fluidity of the mixture excessively. It should be comprehensively considered according to the characteristics of the project, focusing on the workability of the mixture. The fluidity should be subject to volume stability and homogeneity  .
2.3 Extensibility and durability
The extensibility of concrete is closely related to creep, deformation modulus and tensile strength, see Figure 1. The concrete with large creep, small deformation modulus and high tensile strength has good extensibility, the development of cracks can be delayed and reduced, and it has certain self-healing ability, and the durability of concrete is good. As the strength of concrete increases, the creep relaxation effect decreases sharply and the deformation modulus increases. Therefore, although the tensile strength of concrete has been improved with the increase of compressive strength, the elongation is greatly reduced, and the cracking time is advanced . The width of cracks increases and the durability is deteriorated.
In summary, the improvement of cement production technology and the development of concrete construction technology have a significant impact on the performance of concrete, which objectively causes the volume stability of concrete mixtures to decrease, the homogeneity deteriorates, and the deformability after hardening decreases; There are hidden dangers in the structure itself. The deformation during the hardening of the concrete is restricted and cannot be relieved by the creep relaxation, resulting in a much larger tensile stress than before. The combined effect of the two causes the continuous expansion and connection of concrete cracks, plus The pore defects of the structure itself become inevitable due to the premature deterioration, that is, the decline in durability under the action of the external environment.
3 Effective ways to improve the durability of concrete
Although the durability of concrete is diverse, the causes of poor durability can be attributed to two aspects, one is the external environment, and the other is the internal defects of the concrete and the characteristics of the constituent materials. Therefore, improving the durability of concrete must start from improving the structure's ability to resist environmental degradation, reducing internal defects in concrete, and improving the performance of its constituent materials.
3.1 Strengthen the pertinence of the environment
Durability is a general concept, and it must be related to the environment in which the structure is located to have a clear meaning. Environmental conditions vary greatly, and it is obviously unscientific or even impossible to design an concrete with high durability. Therefore, the premise of concrete durability design is to investigate the environment in which the structure is located, and to determine which one or which is the main damage factor, and then to design the durability for the damage factor.
3.2 Optimize design, select materials, and strengthen construction management
Update design concepts, correctly understand the relationship between concrete strength and durability, change the long-held concept of "the higher the concrete strength, the more durable", choose the appropriate strength, and achieve both durability and strength; strictly control the water-cement ratio With the amount of cement, change the concept of "the more the amount of cement is, the more durable", and meet the fluidity requirements on the premise of ensuring good workability of the mixture. In terms of material selection, cement varieties should be reasonably selected according to the environment in which the project is located, and sand and stone aggregates with good quality and qualified technical conditions should be selected to improve their grain shape and grading quality parameters. Construction management should be strengthened, the concrete should be mixed evenly, poured and vibrated and compacted, and the maintenance should be strengthened.
3.3 Incorporation of highly effective active mineral admixtures and additives
Practice has proved that mixing silica, fly ash, ground slag and other mineral admixtures in cement can reduce the heat of hydration and shrinkage stress and improve the crack resistance of concrete while ensuring strength; secondary hydration products It can block the pores in the cement stone, block the penetration path, improve the impermeability, frost resistance and erosion resistance of the concrete, and avoid the alkali aggregate reaction. Some superfine mineral admixtures can also improve the interface structure and interface area performance of aggregate and cement stone. This move not only makes use of waste, reduces costs, but also reduces the cracks and porosity of concrete, and has an essential contribution to improving the durability of concrete. As the fifth component of concrete, admixtures, some of which (high-efficiency air-entraining agents, waterproofing agents, rust inhibitors, etc.) have an obvious effect on improving the durability of concrete, but the application of admixtures in China is still far from Some levels. Therefore, the use of admixtures should be promoted scientifically, reasonably and extensively, so as to better serve the improvement of the durability of concrete.
3.4 Establish a complete concrete durability testing and evaluation system
Develop a set of enhanced rapid test system, so that it can objectively reflect the main damage factors and actual operating conditions of concrete in the use environment, and can be mutually confirmed with the actual structure detection, and make a scientific evaluation of the deterioration state of concrete, and Accordingly, effective and effective maintenance measures are taken to extend the service life of the concrete structure. In this regard, the research results of European and American countries and Japan are worthy of reference.
Due to the complexity of the raw materials of concrete, the fluctuation of construction conditions, the complexity and continuous development of the concrete structure itself, the diversity and composite effects of environmental conditions, etc., it has caused a high degree of complexity in the study of concrete durability. The author believes that changes in concrete properties caused by the development of cement production technology and concrete construction technology are important reasons for poor durability of concrete. To eliminate some misunderstandings in traditional knowledge, and adopt comprehensive measures in terms of mix design, material selection, construction management, inspection and prediction, etc., in order to fundamentally improve the durability of concrete.
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