China's economy has grown rapidly, and great achievements have been made in various constructions, but at the same time, it has also paid a huge price in resources and environment. The contradiction between economic development and resources and environment is increasingly acute, and the masses have a strong reaction to environmental pollution. This situation is directly related to the unreasonable economic structure and extensive growth mode. If we don't speed up the adjustment of economic structure and change the growth mode, resources can't support it, the environment can't accommodate it, the society can't afford it, and economic development can't be sustainable. Therefore, only by adhering to economical development, clean development and safe development can we achieve sound and rapid economic development.
Similarly, driven by rapid economic growth, China's construction market is unprecedentedly prosperous. However, during the construction and demolition of old buildings, a large amount of construction waste will be produced, of which waste concrete accounts for a large proportion. According to estimates, the construction waste generated in the process of old city reconstruction is mainly reinforced concrete, concrete and muck, and its total amount reaches 65% of construction waste; The garbage generated in the construction is mainly muck, gravel and wood, accounting for 55% in total. Waste concrete is not pure garbage, it has high utilization value. At present, most of the construction waste is transported to suburbs or villages by construction units without any treatment, and it is disposed by open-air stacking or landfill, which consumes a lot of construction funds such as land acquisition fees and garbage removal. At the same time, the problems of scattering, dust and sand blowing in the process of cleaning up and stacking have caused serious environmental pollution. How to use waste concrete scientifically is a subject worthy of study.
2 Foreign waste concrete recycling status
Recycling of waste concrete is an effective way to save resources and protect ecology. In this regard, Japan, the United States, Germany and other developed countries developed earlier and provided us with many advanced experiences and treatment methods.
Generally, building materials, such as stones, are cheaper than recycled materials. Due to the small land area and relative lack of resources, the price of structural raw materials in Japan is higher than that in Europe. Therefore, the Japanese regard waste concrete and other construction wastes as "building by-products" and attach great importance to their redevelopment and utilization as renewable resources. For example, the reconstruction project of port facilities and other infrastructure accessories can use recycled stone instead of a considerable amount of natural quarry gravel materials.
1977, the Japanese government formulated the "Code for the Use of Recycled Aggregates and Recycled Concrete", and successively set up recycling processing plants in various places to produce recycled cement and recycled aggregates, with the maximum processing capacity of100 t/h ..1991year. The Japanese government formulated the "Promotion of Resource Reuse" Japan's leading policy for construction waste is: try not to discharge construction waste from the construction site; Construction waste should be reused as much as possible; If it is difficult to reuse, it should be properly handled.
The U.S. government has enacted the Superfund Law, which stipulates: "Any enterprise that produces industrial waste must dispose of it properly by itself and shall not dump it at will." Therefore, the quantity of construction waste is limited from the source, and enterprises are urged to consciously seek ways to recycle construction waste. The American Building Builders Association is promoting a kind of "resource-protected house". The walls of this house are made of recycled tires and aluminum alloy waste. Most of the steel used in the roof truss is recycled from the construction site. The board used is made of sawdust and broken wood plus 20% polyethylene. The main raw materials of the roof are old newspapers and cartons. This kind of residence not only makes active use of scrap metal, wood and cardboard, but also solves the contradiction between housing shortage and environmental protection.
France CSTB is the leading "waste and construction industry" group in Europe, which specializes in coordinating the "waste and construction industry" business in Europe. The overall plan of waste management put forward by the company has two major goals: first, to control the generation of site waste from the source by studying the environmental protection characteristics of newly designed building products; Second, in the construction, improvement and demolition projects, predict and evaluate the generation and collection of on-site waste to determine the relevant recycling application procedures, so as to improve the level of waste management. Based on a powerful database, the company uses software tools to analyze and control the whole process of construction waste from generation to treatment, thus helping decision-making at different stages of building service life. For example, it can evaluate the overall environmental protection of building products; According to the relevant implementation process, maintenance category and different types of building demolition, it can be evaluated to reduce the amount of waste generated by a product; Be able to provide consultants, general contractors and contractors (customers) with relevant overview information about the impact of a product or product series on environmental protection and health; Be able to predict the procedures and materials needed for waste management; Transportation scheme can be made according to the final use or quality of waste; Evaluate the technical, economic and environmental feasibility of any new process using "recycled" raw materials, and estimate the performance of products.
Some low-lying countries, such as Denmark and the Netherlands, are particularly short of sand and gravel resources, so they also attach great importance to the recycling of construction waste, including concrete. 1976, the then Netherlands, Belgium and the Federal Republic of Germany jointly established the "Concrete Disassembly and Reuse Committee" and began to study waste concrete as coarse and fine aggregate of concrete.
Germany divides construction waste into land excavation, broken old building materials, road excavation and construction site waste. The headquarters building of the German Federal Environment Foundation is built of old concrete aggregate. The waste treatment process developed by Siemens in Germany can separate all kinds of renewable substances from waste very cleanly and recycle them. The gas generated in the treatment process is used for power generation, and only 2 ~ 3 kg/t of harmful heavy metal substances remain in the garbage after dry distillation combustion treatment, which effectively solves the problem that the garbage occupies a large amount of cultivated land.
3 Domestic status of comprehensive utilization of construction waste
For a long time, the reuse of construction waste, especially waste concrete, has not attracted enough attention in China. Generally, it is transported to the suburbs or rural areas without any treatment, and it is piled up in the open air or buried. With the vigorous development of urban construction in China, the amount of construction waste is increasing day by day. At present, China's annual construction waste accounts for a large proportion of the total urban waste, about 30-40%, and solid waste such as concrete accounts for 55-65% of construction waste, which has become a difficult problem in waste management. Some construction companies in Shanghai, Beijing and other cities have made some attempts in the recycling of construction waste.
4 direction of recycling waste concrete
4. 1 can be used as coarse and fine aggregate of concrete (mortar).
The waste concrete blocks are crushed into particles with a diameter of about 40 mm, and heated, stirred and rubbed at a high temperature of 300℃, so that the aggregate and cement components adhered to the periphery of the aggregate are completely separated into powder, and the generated cement components are used for foundation improvement. The separated aggregate can be used in the structure similar to natural aggregate, and the recycling rate reaches 65,438+000%. The separated aggregate can be used as aggregate in cast-in-place concrete or prefabricated components, and can also be used in non-load-bearing parts of buildings. After fine grinding, waste concrete blocks can be mixed with standard sand 1: 1 to form mortar fine aggregate, which can be used for plastering walls and floors, leveling mortar layers, building mortar and making floor tiles. 1July, 1990, during the construction of seven high-rise buildings in Hua Ting and Holland in the city center, Shanghai Second Construction Engineering Company sorted out, eliminated and crushed useful waste residue fragments and used them as fine aggregates for plastering mortar and masonry mortar. * * * Recover 480 tons of construction waste residue, save sand 1.44 million yuan, and remove garbage by 3360 yuan. After deducting the purchase fee of crushing equipment, the net income is 654.38+0.24 million yuan. From June 65438 to June 0992, a company of Beijing Urban Construction Group recovered more than 840 tons of various construction wastes during the construction of 90,000 square meters of multi-storey and high-rise buildings with different structural types, and used them for masonry mortar, plastering interior walls and ceilings, concrete floors and concrete cushions, with a usable area of more than 30,000 square meters, saving more than 35,000 yuan.
4.2 can be used for foundation piling
When dealing with foundation piling, waste concrete after simple treatment is continuously filled into the pile, and then compacted, and the density of foundation pile is 3 ~ 5 times higher than that of traditional methods. A few days ago, Xinxing Science and Technology Service Company of Hebei Polytechnic University successfully developed the construction technology of rammed and expanded ultra-short special-shaped piles of construction waste, and made a breakthrough in the comprehensive utilization of construction waste. In this technology, brick, slag, gravel and other construction wastes generated during the renovation and demolition of old houses are used as fillers, and reinforced concrete enlarged head short piles are formed by heavy hammer compaction, and the supporting vibration and isolation technology is adopted, which has the functions of expanding the pile end area and compacting the foundation. The design value of vertical bearing capacity of single pile can reach 500 ~ 700 kn. It is estimated that this technology can save about 20% capital investment compared with other commonly used technologies.
4.3 can be used for road construction
Judging from the compressive strength of recycled concrete, it can completely meet the requirements of engineering construction. At present, it is widely used in subgrade, pavement, pavement brick and Maya brick in road construction. Recycled aggregate concrete is used in the double-layer concrete highway project in Lower Saxony, Germany. The total thickness of the concrete pavement is 260mm, and the bottom layer 190mm thick concrete is recycled concrete. The concrete with a surface thickness of 70 mm is made of natural aggregate. The composition and particle size of bottom recycled aggregate are as follows: 0 ~ 2mm accounts for 30%, 2 ~ 8mm accounts for 14%,/kloc-0 ~ 20mm accounts for 30%, 20 ~ 36mm accounts for 6%, cement dosage is 350kg/m3, and concrete bulk density is 23 10kg/m3. Then it is applied to a single-layer concrete highway pavement. The particle size of recycled aggregate is as follows: 0 ~ 2mm accounts for 27%, 2 ~ 8mm accounts for 10%, 8 ~ 16~22mm accounts for 28%, 16~22mm accounts for 35%, cement dosage is 350kg/m3, and concrete bulk density is 2373kg/ m3.
5 Conclusions and suggestions
The recycling of waste concrete has an important contribution to energy saving and emission reduction, and has excellent technical and economic indicators. At present, it has been widely used in Japan, Europe and America, and our technicians have also done some research work. It can be predicted that with the deepening of the practice and research of recycled concrete materials, recycled concrete will become a commonly used building material, which is applied to more and more structures and has great economic, environmental and social benefits. In order to promote the application of concrete recycling, the following measures should also be taken:
5. 1 Determine the management department. It is difficult to count the amount of waste concrete produced in China every year. If recycling is significant, it is necessary to determine a department to undertake management functions. In view of the present situation of the management function of ready-mixed concrete in bulk cement office in most parts of the country, we can consider adding this function to this institution.
5.2 Strengthen scientific research. Although the recycling of waste concrete can learn from foreign experience, specific technical problems need to be solved one by one, such as recycling production technology, technical equipment, etc., which need to be studied and digested first to provide technical support for industrial development.
5.3 Introduce preferential policies. According to the newly revised Administrative Measures for the Identification of Comprehensive Utilization of Resources Encouraged by the State jointly issued by the National Development and Reform Commission, the Ministry of Finance and State Taxation Administration of The People's Republic of China, local governments can determine that enterprises that produce and use recycled waste concrete resources can enjoy preferential tax policies that the state encourages enterprises to carry out comprehensive utilization of resources.
5.4 Develop technical standards. Of course, the recycling of waste concrete cannot be at the expense of building quality. The management department shall organize relevant units and experts to formulate technical standards, guide production and standardize the market.