In recent years, with the development of the national economy, China's urbanization process has been significantly improved, and the rapid development of urbanization has led to an increasing ratio of the impervious surface distribution of the ground. The increase of impervious area in urban areas has triggered a series of changes in urban ecological environment. The precipitation in China is greatly affected by the monsoon, especially in the coastal areas of southeast China. The rainfall intensity is high, the plum rain period is long, and it is prone to heavy rain and flood. One of the most serious problems that has emerged in recent years is the increase in the amount of rainwater production and flow in urban pavements. The flood flow coefficient is obviously too large. The ground flood caused by rainfall cannot form a flood in the urban area because it cannot penetrate into the ground. For example, it occurred in Beijing in 2012. The induced disaster caused by the 7.21 torrential rain caused heavy casualties and economic losses. In July and August of 2013 and July and August of 2016, different parts of China were hit by heavy rains of different intensity. The city is already full of water. Seriously affect the normal life of the people. Heavy rain, flooding, small flooding, and poor drainage have become the city's ills, which has sounded the alarm for urban builders. As far as the current underground pipe network layout of major cities is concerned, it costs a lot of building materials and manpower to handle drainage. However, it is still unable to cope with the elastic changes in rainfall, and it is deeply mired in the cycle of “storms must be” and “seeing the sea”. Only relying on the traditional engineering-dependent water control ideas will separate the water supply and drainage. It is just a soup that stops boiling. It can't solve problems from the source. It will only bring about incalculable sequelae and make the urban ecosystem overwhelmed. How to effectively and effectively balance the harmonious relationship between man and nature is essential for the development of "green economy", which is also necessary to build a sponge city. Concrete manufacturer

1 Characteristics of water permeable concrete

Permeable concrete is a very good material, and its various characteristics meet the requirements of sponge city construction. The permeable concrete has the characteristics of water permeability, light transmission, high load bearing, freeze-thaw resistance, beautiful decoration and easy maintenance. Compared with traditional concrete, this kind of concrete has different material composition. Pervious concrete contains aggregates, high-grade cement, admixtures, stabilizers, water and other raw materials. It is more voids and lighter in weight. Concrete, which is coated with a thin slurry on the surface of the coarse aggregate, and the two materials are bonded to each other to form large and small cavities, thereby achieving good water permeability.

Since the permeable concrete mainly relies on the partial bonding of the encapsulating slurry on the surface of the coarse aggregate, the aggregates form uniform pores communicating with each other. In order to ensure the smooth and beautiful surface of the concrete after paving, the pores are small and uniform. At the same time, the aggregate particles are firmly cemented. Therefore, the coarse aggregate generally adopts a single-stage stone, and the particle size is generally 4 to 7 mm or 10 to 16 mm, and the appearance is required to be regular and has no angularity. The content of the needle-like particles should be less than 1%, the mud content should be no more than 0.5%, and the crush value should be less than 15%. In order to ensure the porosity (20% or more) and compressive strength (C20~C30) of permeable concrete, it is generally mixed with cement with P·O42.5 or above, and the three-day compressive strength of cement should be greater than 27.0 MPa. The 28-day compressive strength reached 50.0 MPa or more [2]. In order to improve the flexural strength of the permeable concrete road, it is necessary to add a cementation enhancer with a cement quality of about 2%. Tests show that after adding cementation reinforcing agent, the compressive strength of water-permeable concrete can generally be increased by about 20%, the flexural strength can be increased by more than 30%, and the late efflorescence phenomenon of water-permeable concrete can be effectively improved. Kunshan Jianguo Concrete

2 Development status of water permeable concrete

2.1 Status of foreign research

In the mid-19th century, the UK first used a sand-free macroporous concrete without fine aggregates. After 1900, the concrete was transferred to places such as South Africa and Russia. Later, a lot of research on the material was carried out, and many sand-free large-hole concrete buildings were built. In the 1960s, the idea of watering the shrubs on the boulevard was initiated and the idea of permeable pavement began to be proposed. In 1970, the United Kingdom used sand-free macroporous concrete for municipal pavements for the first time [3]. Since then, the material has been called Pervious Concrete, and it is gaining popularity as it is an ecological, green pavement material.

Since 1973, Japan has used permeable concrete. In 1973-1995, a total of 2.2 million m2 of permeable concrete pavement was built in Tokyo. At the beginning of the 21st century, the cumulative area of the country was 10 million m2 [4]. In Japan, in the maintenance of permeable concrete pavement, a small pressure washer with a pressure of 4 to 7 MPa is used to clean the road surface, so that the water permeability of the road surface can reach about 80% of the initial stage.

2.2 Status of domestic research

Domestic research on permeable concrete started late. Since 1950, the research on sand-free macroporous concrete has been carried out. It is used as a non-load-bearing material for walls. However, it is not widely used as a pavement material. After 1970, it was only It is applied as a floor material [5]. After 1990, sand-free macroporous concrete began to be studied as a green, ecological material and gradually applied to pavement engineering. During this period, the Academy of Sciences began research on permeable concrete and permeable concrete pavers. With the deepening of research, a series of results were achieved. In 2008, a large area of permeable pavement was used in the Olympic Forest Park. More than 9,700 m2 of permeable concrete was laid on the lakeside west road road project of the National Stadium “Bird's Nest” [6]. Chang'an University has carried out a series of studies on the physical and mechanical properties and road performance of permeable concrete, and has achieved certain rich results. In April 2015, the company announced the first batch of 16 “sponge city construction” demonstration cities nationwide, including Qian’an, Baicheng, Zhenjiang, Jiaxing, Chizhou, Xiamen, Pingxiang, Jinan, Hebi, Wuhan, Changde, Nanning, Chongqing, Suining, and Guizhou. Anxin District and Xixian New District.

The importance of 3 permeable concrete in sponge city

The traditional sidewalk structure construction adopts the base material, and the main component is the crushed grade gravel. Under the help of medium and coarse sand, or under the hard cement mortar, the permeable brick surface is fixed. The use of such a sidewalk design can make the sidewalk more permeable and can meet the construction requirements of the sponge city. This kind of sidewalk design requires high quality building materials, but due to the current ability of construction workers is not high. Therefore, it is easy to cause the strength of the sidewalk to be insufficient and the flatness is insufficient. When it is put into use, it will be washed away by rainwater, which will eventually lead to local settlement on the road surface. The sidewalk will be undulating and wave-like. Once there is heavy rain, it will easily accumulate water, and the water can not achieve natural penetration.

Permeable concrete is an environmentally friendly material. It can replace the non-permeable ground in traditional urban construction with the following advantages: (1) Strong in-situ permeable function, three-in-one. Not only can the road area water be drained in time, the groundwater resources in the local area can be replenished, and the groundwater pressure can be stabilized, thereby achieving the purpose of preventing land subsidence. It also prevents tires from slipping and rain splashing, reducing safety hazards. At the same time, it also relieves the pressure of water discharge in urban underground pipe network. It is indispensable for improving the living environment of street shops and houses. (2) Water-permeable concrete can also make a fuss about color. In combination with architectural aesthetics, inorganic pigments can be mixed with concrete and mixed with concrete to produce colored translucent concrete of different colors, which complements the surrounding landscape, making it a design, applicability and ornamental function. It is a functional complex. . (3) Compared with the traditional impervious road surface, it is easy to cause reflection, glare and mirage in the evening after the rain or at night. Its unique honeycomb structure and a large number of open pores are not easy to diffuse, making pedestrians and drivers safer and more secure. At the same time, it has the functions of sound absorption and noise reduction, adsorption of dust, moisture absorption and temperature regulation, and can effectively transfer heat. In the construction of sponge city, the permeable concrete can play the role of absorption, storage and slow release of rainwater, effectively alleviate the urban sputum and heat island effect, and reduce the urban runoff pollution load and save water resources.

4 problems in the application of permeable concrete in sponge city

As a new concept and technology, permeable concrete has been researched and explored by experts and scholars at home and abroad. Its emergence has changed the circulation pattern of urban rainwater runoff, greatly enhanced the controllability and coordination of new urbanization construction, and brought new ideas to the city's rainwater management and sewage prevention and control work. The inevitable development direction of the field of architectural innovation. However, in the application process, the following problems also exist: (1) Due to the need of structural design, the water-permeable concrete bottom layer has a low degree of compaction and is porous and lightweight, and is a semi-brittle material. Although the interconnected large pores make the pavement of the permeable concrete have a strong speed drainage and high permeability. However, the porosity currently common in permeable concrete is more than 18%, and may even exceed 20%, resulting in problems such as corrosion resistance, bending deformation resistance, and rutting resistance. There are also a large number of extensions applied to urban roads. Due to the large number of road vehicles and the complicated surrounding environment, impurities such as dust, sand, dirt and oil stains are easily embedded in the pores and block the pores. (2) Compared with countries such as Europe and the United States, they have mature and perfect laws and regulations and special subsidy funds for the development of permeable concrete. However, due to the late start of China, objective factors such as software equipment and simulation laboratories have restricted the development of the industry, lacking first-hand information suitable for China's national conditions and available for analysis and research, so the performance is inferior. (3) China has a vast territory, uneven distribution of rainfall distribution, and large differences in hydrogeological conditions. The unreasonable allocation of gray infrastructure has brought a test to the comprehensive construction of sponge cities.

5 Conclusion

In summary, the sponge city is the development direction of urban construction. In the process of sponge city construction, the quality of various materials should be controlled. Permeable concrete is a new type of concrete material that is very helpful for urban drainage and water storage. Especially in the construction process of the sponge city, it is necessary to strengthen the upgrading of the traditional construction materials, and the application of the permeable concrete can meet the construction requirements of the sponge city.

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