2/kloc-0 Since the beginning of the century, with the continuous acceleration of urbanization, the demand for concrete, as the most important material basis of urbanization, has also increased rapidly. At present, the annual output of concrete in the world is about 28× 108m3, and the annual output of concrete in China accounts for 45% of the world total, reaching13×108 ~14×108m3. In these concrete raw materials, coarse and fine aggregates account for about three-quarters of the total concrete. According to this calculation, the global demand for coarse and fine aggregates is about 2 1× 108 m3 every year, while China's construction industry is booming, and there is a great demand for coarse and fine aggregates. China's demand for coarse and fine aggregates accounts for about half of the world's demand, and will increase with the development. With such a large consumption, the earth's natural primary coarse and fine aggregates will be exhausted, so it is very important to find substitutes for primary aggregates from the perspective of rational development and sustainable development of resources.
Corresponding to the huge demand for coarse and fine aggregate of concrete, there are a lot of waste concrete. The amount of waste concrete demolished every year in the world, the amount of waste concrete produced by new buildings, and the amount of waste concrete produced by concrete factories and precast component factories are amazing. According to the latest information from the seminar on comprehensive utilization of construction waste and promotion of new technologies held in Xiamen in April, 2006, the solid waste generated by demolition of buildings in China is more than 200 million tons each year, and the solid waste generated by new buildings is about 654.38 billion tons, totaling about 300 million tons. However, at present, there are obviously not many treatment methods for these waste concrete. The traditional treatment methods are mainly transported to the suburbs for open storage or landfill. The huge treatment cost and environmental problems brought by this method are very prominent. Waste concrete contains a lot of sand and gravel aggregate. If they can be recycled reasonably to produce recycled concrete for new buildings, it can not only reduce the cost, save natural resources, alleviate the contradiction between supply and demand of aggregate, but also reduce the pollution of waste concrete to the environment, which is an important part of the sustainable development strategy. Therefore, how to make full, efficient and economical use of construction waste, especially waste concrete, has become a research topic in many countries.
Recycled aggregate is the aggregate obtained by crushing, cleaning, grading and proportioning waste concrete. The concrete prepared with recycled aggregate as part or all of the aggregate is called recycled aggregate concrete, which is called recycled concrete for short. Recycled aggregate can be divided into recycled coarse aggregate and recycled fine aggregate according to its size; According to the source, it can be divided into road recycled aggregate and building recycled aggregate; According to the purpose, it can be divided into recycled aggregate of concrete, recycled aggregate of mortar and recycled aggregate of block. Using recycled aggregate concrete technology, waste concrete can be reprocessed, its original performance can be restored, new building materials products can be formed, limited resources can be reused, and some environmental protection problems can be solved. This is one of the main measures to develop green concrete and realize the sustainable development of building resources and environment, leaving precious wealth for future generations.
First, the use of recycled concrete in developed capitalist countries.
Developed countries such as the United States, Japan and Europe have studied the reuse of waste concrete earlier. After World War II, Germany, Japan and other countries studied and recycled waste concrete, and held three international conferences on recycling waste concrete, proposing that concrete must be green. The use of concrete has become a research topic in developed countries, and some countries have adopted legislation to ensure the development of special research and application. Some developed countries have been widely used in practical projects.
(1) Japan
Due to the small land area and relatively scarce resources, Japan regards construction waste as a by-product and attaches great importance to the redevelopment and utilization of waste concrete as a renewable resource. As early as 1977, the Japanese government formulated the Code for the Use of Recycled Aggregate and Recycled Concrete, and successively established recycling treatment plants to treat concrete waste in various places, and formulated a number of laws and regulations to ensure the development of recycled concrete. In addition, Japan has also systematically studied the water absorption, strength, mix proportion, shrinkage and frost resistance of recycled concrete.
(2) the United States
The Superfund Act promulgated by the American government provides a legal guarantee for the development of recycled concrete. The United States not only encourages the application of recycled concrete, but also studies its performance. For example, according to two roads paved with recycled concrete in Michigan, USA, the dry shrinkage performance of recycled aggregate concrete is studied. The test shows that the shrinkage rate of recycled aggregate concrete is greater than that of natural aggregate concrete. American companies use microwave technology to produce recycled asphalt concrete pavement, which has the same quality as the new asphalt concrete pavement fabric, but the cost is reduced by 1/3, and the cost of garbage removal and treatment is saved, which greatly reduces the environmental pollution of the city.
(3) European countries
At present, Germany and other European countries mainly use recycled concrete for highway pavement. 1In August, 1998, the German Reinforcement Committee put forward the "Application Guide for Using Recycled Aggregate in Concrete", which requires that the concrete prepared with recycled aggregate must fully meet the national standard of natural aggregate concrete; Relevant tests in Austria show that the strength value of concrete prepared with 50% recycled aggregate can reach the Austrian standard, and it is found that the elastic modulus of recycled aggregate concrete decreases. France also produced concrete blocks from broken concrete and broken bricks, and the obtained concrete blocks were determined to meet the standards related to concrete block materials.
Second, the use of recycled concrete in China
Although there will be no sudden crisis of concrete raw materials in China in the short term, China will definitely face the shortage of raw materials in the future. Moreover, the development of China's construction industry far exceeds that of some developed countries, and the research on the development of recycled concrete is later than that of industrialized countries. Therefore, our government also encourages the research and application of waste, and some domestic experts and scholars have also stepped up the research and utilization of recycled concrete. For example, Shanghai Building Component Products Co., Ltd. uses waste concrete such as foundation pit support demolished by blasting in construction site to make concrete hollow blocks, and its product technical indicators fully meet the engineering specifications of small concrete hollow blocks in Shanghai. Broken or crushed fragments of waste concrete are also used as aggregates of fresh concrete in some reconstruction or reconstruction projects.
Recycled concrete in China is not only used in the construction industry, but also in the transportation industry. When the concrete pavement of concrete road reaches its service life, or it is damaged due to the rolling of heavy objects, it needs to be repaired or rebuilt. Now the general practice is to dispose of the old cement concrete surface, discard it, repair the base, and then re-pave it. At present, the main application of cement concrete pavement recycling technology in China is in-situ recycling technology, that is, the existing pavement is broken or crushed, and then the broken or crushed pavement is used as the base or subbase in the new pavement structure, which is widely used in highway maintenance in China. For example, the expressway from Hefei to Nanjing uses recycled concrete aggregate as the aggregate of fresh concrete to pour concrete pavement. Hefei-Nanjing Expressway, cement concrete pavement, 199 1 completed and opened to traffic. With the increase of traffic volume and the extension of service life, various diseases have appeared on the pavement, and the amount of pavement maintenance is large every year, resulting in a large number of old concrete. Therefore, in the process of maintenance, according to the characteristics of expressway rapid passage, recycled concrete aggregate and early strength agent are used to achieve the purpose of rapid passage. Before construction, the apparent density, water absorption, crushing value, firmness and impact value of recycled concrete aggregate are tested, and the maximum particle size and gradation of aggregate are fully paid attention to. Using recycled concrete aggregate instead of natural aggregate, the utilization rate of recycled concrete aggregate can reach 80%, which can save a lot of aggregate transportation costs every year. At the same time, the land cost occupied by waste concrete is saved. This not only saves a lot of maintenance funds, but also is beneficial to environmental protection and has achieved good social and economic benefits.
Generally speaking, although some properties of recycled aggregate are not as good as natural aggregate, the properties of concrete components developed and produced with recycled aggregate are also worse than natural aggregate. However, if additives are added, the performance of recycled concrete can be greatly improved. Recycled concrete can be widely used as long as appropriate additives are selected. Moreover, using waste concrete as aggregate to produce recycled concrete is of great significance for recycling resources, purifying the environment and benefiting future generations. Therefore, this requires the government to strengthen publicity, introduce some compulsory measures to limit the discharge of waste concrete, and establish corresponding garbage treatment plants. At the same time, the government should provide financial and policy support, formulate the industrial standard of recycled concrete, promote the development of recycled concrete, a new building material, and promote China's economic development.
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