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Development status and future of new concrete at home and abroad (concrete section I)

Superplasticizer in concrete 2021-07-29 12:03 174
Concrete durability some unreinforced concrete structures that have stood for 2000 years, such as the ancient Roman Pantheon built with slowly hardening volcanic ash lime cement and several old water transmission roads in Europe are still intact; At the same time, the reinforced concrete structure built with Portland cement in the 20th century deteriorated rapidly. When exposed to corrosive environments, such as deicing salts and seawater, serious durability problems occur in bridge decks, parking lots, subsea tunnels and other marine structures in less than 20 years. P. K. Mehta., building durable structures in the 21st century. CI. March, 2001. In the 1930s, when the concrete structure was first vigorously built, it was generally believed that its design service life was 40 ~ 50 years, and there was basically no maintenance, which was more than enough. Until the 1970s, the premature deterioration of concrete was still regarded as an exception, which was caused by problems in the specifications or improper materials and construction. P. K. Mehta. Durability - critical issues for the future. Concrete international July 1997. The durability of concrete structures caused a sensation in the United States, which was caused by the report submitted by the national material Advisory Committee in 1987. The report reports that the deck slabs of about 253000 concrete bridges, some of which have been used for less than 20 years, have been damaged to varying degrees, and 35000 will be added every year. P. K. Mehta. Durability - critical issues for the future. Concrete international July 1997. In the same year, litvan and Bickley published a test report on Canadian parking lots. They found that a large number of parking lots were damaged much earlier than the expected service life. Gerwick et al reported the premature and serious deterioration of subsea tunnels, marine pile foundations and railway sleepers in some countries. The research shows that the design, materials and construction of these structures are in line with the development level of modern technology. P. K. Mehta. Durability - critical issues for the future. Concrete international. July 1997. Due to the widespread cracking of concrete bridge decks in the United States, it seems that it is useless to turn to high-strength concrete. According to the inspection results of the national highway cooperative research program in 1995, there were penetrating cracks with an interval of 1 ~ 3m in the upper 100000 concrete bridge decks within one month after concrete pouring. In 1996, the Federal Highway Administration of the United States launched the high performance concrete (HPC) bridge deck project. Combined with the work of the project, four states with different climatic environments were selected to pave HPC bridge deck. However, by 2000, they all had obvious cracks. From the prediction of the future world and its demand and the understanding of the current situation of today's concrete industry, concrete durability can determine a series of key problems that must be solved quickly as the most popular ordinary building material. It seems that the most important and urgent problems that need to be paid attention to are the durability of ordinary concrete and the growing public's lack of confidence in the long-term service life of the material. For a long time, people have always believed that the strength of concrete is directly related to durability, which dominates the mix proportion design of durable concrete. In order to meet the higher and higher strength requirements in the 20th century, the concrete industry inevitably violated the basic law of material science, that is, the close relationship between cracking and durability. In order to achieve the goal of building a sustainable concrete structure, it is necessary to update some concepts and construction practice. P. K. Mehta., building durable structures in the 21st century. CI. March, 2001. Driven by rapid construction, today's concrete mixtures tend to use high-volume ordinary portland cement or early strength Portland cement. The poor tensile or crack resistance of this kind of concrete is due to the increase of dry shrinkage, temperature shrinkage and elastic modulus on the one hand, and the decrease of creep coefficient on the other hand. This is why high-strength concrete is easier to crack than medium or low strength concrete. Dongting Lake Bridge is an impermeable, discontinuous micro crack and porous reinforced concrete structure. Environmental action (phase I) (no visible damage) 1. Erosion (cold heat cycle, dry wet cycle) 2. Load action (cyclic load, impact load) is connected with pores due to micro cracks, Water impermeability gradually loses its environmental effect (the second stage) (the beginning and expansion of damage) the infiltration of water into O2, CO2 into acidic ions (Cl -, SO4 -) into a: the increase of hydrostatic pressure in pores and concrete expansion due to the following reasons: reinforcement corrosion, alkali aggregate reaction, water icing, sulfate erosion B: the reduction of concrete strength and stiffness, cracking Spalling and loss of integrity the causes of concrete structure cracking caused by the \
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