Superplasticizer News and Tech > Superplasticizer in concrete > Development status and future of new type concrete at home and abroad (concrete section 1)

Development status and future of new type concrete at home and abroad (concrete section 1)

Superplasticizer in concrete 2021-07-21 15:02 170
Durability of concrete some unreinforced concrete structures that have been standing for 2000 years, such as the pantheon of ancient Rome built with slowly hardening pozzolanic lime cement and several old water conveyance canals in Europe, are still intact; At the same time, reinforced concrete structures built with Portland cement in the 20th century deteriorated rapidly. When exposed to erosive environments, such as deicing salts and seawater, serious durability problems of bridge decks, parking lots, subsea tunnels and other marine structures occur 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 constructed, it was generally believed that its design service life was 40-50 years, and there was basically no need for maintenance, which was more than enough. Until the 1970s, the phenomenon of premature deterioration of concrete is still regarded as an exception, which is caused by the problems in the specifications or improper materials and construction. P. K. Mehta. Durability - critical issues for the future. Concrete international July 1997. The problem of durability of concrete structures caused a sensation in the United States, which was caused by the report submitted by the National Advisory Committee on materials in 1987. The report reports: about 253000 concrete bridge decks, some of which have been used for less than 20 years, have been damaged to varying degrees, and 35000 more 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 the inspection report on Canadian parking lots. They found that a large number of parking lots were damaged far ahead of the expected service life. Gerwick et al. Reported the premature 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. According to the national highway cooperative research plan, the inspection results in 1995 show that: in one month after concrete pouring, there are penetrating cracks with an interval of 1-3 meters in 100000 concrete bridge decks. In 1996, the Federal Highway Administration of the United States started the project of high performance concrete (HPC) bridge deck. Combined with the work of the project, four states with different climates and environments were selected to build HPC bridge deck. But by 2000, all of them had obvious cracking. From the prediction of the future world and its demand, as well as the understanding of the current situation of the concrete industry, we can determine a series of key problems that must be solved quickly as it continues to be the most popular common building material. It seems that the most important and urgent issues 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 always think that the strength of concrete is directly related to durability, which dominates the mix design of durable concrete. In the 20th century, in order to meet the increasing strength requirements, 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. The tensile properties or crack resistance of this kind of concrete is poor, on the one hand, it is due to the increase of dry shrinkage, temperature shrinkage and elastic modulus, on the other hand, it is due to the decrease of creep coefficient. This is why high strength concrete is more likely to crack than medium strength or low strength concrete. The Dongting Lake Bridge is impermeable, but there are discontinuous micro cracks and porous reinforced concrete structure. Environmental action (first stage) (no visible damage) 1. Erosion (cold and hot cycle, dry and wet cycle) 2. Load action (cyclic load, impact load) is connected by micro cracks and pores, Impermeability gradually loses environmental effect (the second stage) (the beginning and extension of damage) water infiltration O2, CO2 infiltration acid ions (Cl -, SO4 -) infiltration A: the following reasons make pore hydrostatic pressure increase, concrete expansion: steel corrosion, alkali aggregate reaction, water freezing, sulfate erosion B: concrete strength and stiffness decrease, cracking Spalling and loss of integrity the \
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