Any move will have its own gains and losses: when a tendency appears, another tendency is implied. It is the correct attitude to scientifically analyze the problems arising in the development of the industry and find out the reasons and add to the solution. There are indeed some problems with the current ready-mixed concrete.
Has the quality of concrete engineering improved or declined?
With the development of the electromechanical and electronic industries, the processes of metering, mixing, and transportation that are suitable for the development of ready-mixed concrete have greatly advanced. The general use and development of high-efficiency water-reducing agents have changed everything in concrete: high-strength, high-fluidity, and elevation pumps. Delivery, pouring in narrow spaces, etc., have become possible and realistic; the homogeneity of the mixture is improved, the construction is convenient, the defects caused by improper vibration are avoided, and the construction speed is greatly accelerated.
On the other hand, on the whole, the quality of employees is low, and the quality management and control level are poor, so that the quality accidents and cracks of concrete structures have appeared more than in the past, and there are more disputes between the supply and demand sides due to the quality. Even if there is no problem in front of us, there are still many hidden dangers: the report of the concrete mix ratio is mostly distorted; after the concrete mix is shipped to the construction site, water is added; the aggregate of the concrete was washed with water 10 years ago, and now some sand contains mud The amount is as high as 7% and so on. It seems that it is not a question of progress but not progress, but gains and losses!
Is the strength of cement and concrete increasing or decreasing?
1. In the 1970s, the highest grade of cement was 500#, which is equivalent to 32.5 of the current strength grade; now 52.5 cement is equivalent to 625# cement 20 years ago in terms of strength. However, in the past, the storage period of cement was specified as three months; now the storage period is not mentioned. In addition to the necessity, the important thing is that after a storage period of less than one month, the strength of 52.5 cement will be almost the same as the strength of 42.5 cement. , And users report "not easy to use."
2. With high-efficiency water-reducing agent, it is not difficult to achieve high strength of concrete, and concrete can be used in high-rise and large-span structures. People think that high-strength concrete is not enough for fun. What is fashionable is to worship and pursue "super-high-strength" and "super-high-strength". 7 days or even 3 days for concrete, the strength reaches 100% of the 28-day design strength value, which can generally be achieved, but the strength does not increase in the later period, the cracking sensitivity increases, the self-healing ability of defects decreases, and the concrete structure How will durability be guaranteed?
3. The normal development law of traditional concrete strength: relative to 100% in 28 days, about 30% in 3 days; about 60% in 7 days. The strength of concrete above C50 can reach about 70% in 3 days and 90% in 14 days; if the strength of concrete reaches 100% in 7 days, there will be little or no growth after 28 days; if it is in 3 days If it reaches 100%, 7 days will be longer, but 28 days will not be long, and it will even shrink.
4. Withy of the United States started a 50-year experimental experiment of cement mortar, mortar and concrete at the University of Wisconsin in 1910, pouring indoor and outdoor concrete. More than 5,000 test pieces were formed at three different times in 1910, 1923 and 1937 respectively. Washa and Wendt published the results of 50 years of observation of these specimens in 1975. The result is: In 1923, the concrete prepared with Blaine fineness of 5775px2/kg had a 28-day strength of 21MPa and a 25-year strength of 52MPa; in 1937, the concrete prepared with the fast-hardening cement at that time had a 28-day strength of 35MPa and a 5-year strength. At 53 MPa, the strength begins to shrink after 10 years, and the strength shrinks to 45 MPa in 25 years, which is lower than the 25-year strength of hydrated concrete. The composition and fineness of the fast-hardening cement at that time were comparable to those of Type I cement and Type II cement in the United States today (C3S was 57%, Brinell specific surface area was 380m2/kg, and so was the common cement).
5. All measures to improve the early strength will damage the later performance of the concrete, which has been the consensus of experts. The former RILEM chairman and German concrete expert Rupert Springenschmid proved that controlling the concrete's 12-hour compressive strength to not exceed 6MPa can avoid early cracking of the concrete; after testing, it was concluded that the concrete's 24-hour compressive strength could not exceed 12MPa to control the early stage of concrete Cracked.
6. High strength is to reduce the cross-section of the component, such as a concrete column of a certain height. The higher the strength, the thinner the column can be made. However, the ratio of the height of the column to its minimum cross-sectional size is also limited by the problem of "stable bar stability". For a column of a certain height, there is a minimum cross-section requirement, that is, the strength requirement is limited. Some people say, "The current ultra-high-strength concrete used in house construction reaches 180MPa". Is this necessary?
7. Cement is now "not easy to use", it should be said that it is mainly the result of misleading users. Due to the requirement of improving the strength of concrete, the development goal of cement for more than 100 years is to pursue strength in addition to reducing energy consumption. Increasing the strength of clinker from the calcination process has achieved results, but after all it is limited. The current improvement of cement strength is inseparable from the excessively fine grinding and various grinding aids and enhancers. Therefore, while increasing the strength of the concrete, it also brings the hidden danger of damaging the durability. The trouble should end it! The key is to change the traditional concept from the user.
8. In any case, since there are advantages, disadvantages, and gains and losses, we should always see and try to avoid or reduce disadvantages and losses, make greater progress, and try to avoid taking two further steps back. The law of conservation of energy explains this phenomenon well!
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