Key points of paving technology for permeable bricks, development structure, etc.
Pervious brick paving is an important skill in the construction of sponge cities, which can complete the healthy cycle of water resources and improve the ecological environment. This article outlines the development process of permeable brick pavement skills, introduces its type, structure and function, especially the technical effects of three types of permeable pavement of prefabricated bricks, cement concrete and asphalt mixtures, and discusses the primary problems at the time. It is of great significance to strengthen the engineering application and promotion of permeable paving. In recent years, problems such as the "heat island effect" of domestic cities, waterlogging caused by severe rainstorms, and the drying up of groundwater resources have seriously affected urban ecological construction and sustainable development. The construction of the "sponge city" is an important strategic approach to deal with the above problems. Sponge city construction is a systematic project that integrates planning, planning, construction, and operation. An important skill in the implementation process is the use of permeable paving. Permeable pavement uses large empty ground structure layer or drainage infiltration equipment to allow rainwater to infiltrate underground through the pavement structure, eliminate surface runoff, restore groundwater, and absorb ground dust and noise, which is useful to improve the urban ecological environment . This article introduces the current status of the development of permeable pavement technology from the perspective of the general structure of permeable pavement and its effects on the purification of rainwater runoff pollution. 1 Permeable pavement began as early as 1852, when the British announced that it did not contain fine aggregate concrete. In 1950, Britain applied porous asphalt pavement to airport runways . The United States has begun to discuss the planning of permeable concrete since the 1960s. The Netherlands began in 1972 to study drainage asphalt mixtures. In addition, the United States, Japan, Germany and other countries have also studied and applied porous asphalt pavement . In 1986, the United Kingdom began to discuss runoff suppression and pollution prevention for water-permeable paving. China began discussions on permeable concrete in 1993 and successfully developed it in 1995 . At present, only developed countries in Australia and Europe and the United States that pay more attention to the development of urban ecological environment have great success in the research of permeable paving. Domestic research on permeable paving started late, but it has also begun. 2 Type, structure and function of water-permeable pavement The water-permeable effect of water-permeable pavement depends primarily on the permeability of the surface layer, and also includes the paving information and method of the underlying layer. Permeable paving now includes permeable floor tile paving, permeable concrete paving, and permeable asphalt mixture paving. The permeable pavement is generally composed of four parts, as shown in Figure 1. The differences are: soil foundation, bottom layer, screed layer and surface layer. The surface layer selects water-permeable prefabricated brick, cement concrete or asphalt mixture according to different requirements. The screed is a layer of lime mortar or grit placed to stabilize the water-permeable and air-permeable bricks. The bottom layer generally uses natural sand and gravel material with better stability and strength, perhaps a combination of graded gravel and sandless concrete. Generally, a water collection pipe is also set up in the bottom layer as needed. 2.1 Pervious floor tile paving Pervious floor tiles are mainly divided into two categories. One is a permeable brick made of inorganic non-metal materials as the main raw material and formed by processes such as forming and curing; the other is waste industrial materials and construction waste. Etc. as the main raw material, ceramic permeable bricks made by crushing, sieving, forming and high temperature firing. The aggregate of the permeable brick is squeezed to form a skeleton. A uniform cement slurry film is wrapped around the aggregate, and a porous structure is formed by a thin layer of hardened cement slurry, and a lot of connected pores are formed inside. Ceramic permeable bricks are ceramic materials with reasonable grading under high pressure and high temperature, and form continuous opening pores through a pore-forming process. The researchers discussed the technical functions of permeable bricks . Zhang Yanjuan and others used cement and fly ash to replace cement. The planned porosity of permeable bricks was 15.8% to 20.2%, the permeability coefficient reached 1.18 to 1.60mm / s, and the average compressive strength exceeded 30 MPa . MXie and others take water plant sludge, fly ash and general sand as the main raw materials, and discuss the effects of sludge content, sintering temperature, and molding pressure on the function of bricks in water-permeable 73 low-temperature construction skills. The product has a compressive strength of 36.53 MPa and a water permeability coefficient of 1.08 mm / s . The technical indicators of commonly used permeable floor tiles should generally meet the requirements of Table 1. Table 1 Common Permeable Brick Skills Index Skill Function Porosity /% Compressive Strength / MPa Flexural Strength / MPa Water Permeability Coefficient / mm · s－1 Split Tensile Strength / MPa Requirements 15 ～ 20≥30≥3.0≥1.0≥3.02 .2 Permeable Concrete Pavement Permeable concrete is classified as fully permeable. It is a mixture of cement, special additives, aggregates and water in a certain proportion. It has good water permeability, water retention and air permeability. The permeable concrete pavement is a connected porous structure pavement that is directly paved and compacted. Permeable concrete uses single-grain coarse aggregate as a skeleton. A thin layer of cement paste or a mortar that participates in a few fine aggregates is wrapped around the surface of the coarse aggregate particles as a cementation layer between the aggregate particles. The researchers discussed the skills and functions of permeable concrete. Chen Hanchang manufactured water-permeable cement concrete with outstanding mechanical and water-permeable functions. The compressive strength reached 30 MPa, the flexural strength reached 4 MPa, and the water permeability coefficient was not less than 1.0 mm / s . Wang Ruiyan and others discussed the effects of aggregate gradation, aggregate volume ratio, and water-binder ratio on the function of permeable concrete and permeable function . In the experiment, the porosity of the permeable concrete was 13.3%, the water permeability coefficient was 3.62 mm / s, the flexural strength was 5.4 MPa, and the compressive strength was 27.6 MPa. The technical indicators of commonly used permeable concrete should generally meet the requirements of Table 2. Table 2 Common water-permeable concrete skill indicators Skill function Porosity /% Compressive strength / MPa Flexural strength / MPa Permeability coefficient / mm / s Flexural strength / MPa Requirements 10 ～ 25≥20≥3.0≥1.0≥2.52.3 Permeability Permeable asphalt pavement permeable asphalt is classified as semi-permeable. Its pavement structure is the same as that of ordinary asphalt pavement, and only permeable asphalt is used on the outer surface of the road. The paving needs to set up gravel drainage ditches on both sides of the bottom surface layer, so that seepage water flows into the ditch on both sides through the pavement bottom surface layer. The permeable asphalt pavement is an asphalt pavement structure with a large open area ratio as the pavement structural layer, allowing surface water to enter the pavement or subgrade. Pervious asphalt pavement surface materials generally use porous asphalt mixture, which is a typical skeleton-open space structure. The amount of coarse aggregate is large, accounting for about 85% of the total mass of the aggregate, and the touch area between the aggregates is reduced by about 25%, high stress on the touch point. The researchers discussed the technical functions of the permeable asphalt mixture. Qian Zhendong and other planned permeable colored asphalt mixtures have an open space ratio of 19.8%, a water permeability of 1480mL / 15s, a dynamic stability of 4682 times / mm, and a Marshall stability of 5.34kN . Xu Shifa and others improved the water-resistant and low-temperature functions of ROGFC by adding 0.3% of lignin fiber to ROGFC . The free space ratio of the obtained recycled asphalt mixture was 19.5%, the freeze-thaw splitting strength ratio was 80.6%, and the Marshall stability and dynamic stability reached 7.52kN and 12,300 times / mm, respectively. The technical indicators of commonly used water-permeable asphalt mixtures should generally meet the requirements of Table 3. Table 3 Common Permeable Asphalt Mixtures Skills Index Skill Function Vacancy rate /% Dynamic stability / time · mm-1 Marshall stability / kN Permeability coefficient / mL · 15s-1 Freeze-thaw split strength ratio /% Requirement 18 ～ 25 ≥3500≥5800≥853 Permeable pavement effects 3.1 Reducing surface runoff and making up for groundwater permeable pavements can accelerate rainwater infiltration, slow down rainwater confluence, and then reduce the total amount of urban rainfall runoff, so that the flood process line changes from peak high slope to peak low The slope is gentle, which is useful to avoid urban waterlogging . Most of the rainwater in cities can only be discharged through urban drainage pipelines, which then causes urban groundwater funnels, ground subsidence and groundwater pollution. The permeable pavement enables the groundwater to be replenished in a timely manner and is useful for improving the urban groundwater environment. 3.2 Purification of rainwater runoff Surface rainwater runoff contains many pollutants and can cause pollution to natural water bodies. The results of the research in  indicate that the removal rate of contaminated pollutants from pervious concrete paving reaches more than 80%. Rainwater in many cities has a low PH value, which can dissolve more metal elements, and the general bottom material has a lower removal rate of metal elements. The removal rate of the metal elements Cu, Pb, Zn, and FCa in the permeable road facing the runoff rainwater is 20% to 99%, 74% to 99%, 73% to 99%, and 98 to 99% . Researchers are developing some low-level materials with excellent adsorption functions to improve the removal of metal elements on permeable paving floors, such as the use of ferric nitrate-soaked quartz sand . The experimental results indicate that the pervious paving ground can stably adsorb metal elements . Road runoff carries a lot of organic pollutants, including greases, COD and PAHs . Permeable pavement can effectively remove organic pollutants. In addition to most of the organic matter in the rainwater runoff being filtered and adsorbed, some will be degraded by microorganisms . Researchers determine the degree of degradation through the degradation of oil and other organic substances during the degradation of oxygen content, the increase of carbon dioxide content, and the reduction of CH2 and CH3 groups in water. Experiments have shown that permeable brick pavement still retains more than 98% of oil pollution at 4 years . 83 Zhou Lirui, etc .: The current development of permeable pavement skills 3.3 Alleviate the heat island effect and absorb sound and reduce noise. The interior of the permeable pavement is a continuous capillary path. The water in the capillary tube is continuously transpiration at high temperatures to absorb a lot of heat, and then it has a cooling effect Mitigate the urban heat island effect. In addition, the multi-void structure of the permeable pavement has a significant sound absorption and noise reduction effect. Part of the sound wave energy enters the inside of the data and causes the air molecules in the pores to oscillate, which is converted into thermal energy by the effects of viscous resistance and friction of air movement, and then weakened. 4 Problems in Permeable Pavement (1) The maintenance skills of pervious pavement are not sophisticated, and the main purpose is to wash with high pressure. In water-permeable paving, the porosity of the surface layer is relatively large, and foreign matter such as dust and sand are easy to accumulate on the surface layer and collect in the pores, which will affect the permeability, and some even completely block and lose the permeability. (2) Compared with the conventional road, many pores in the surface layer of the permeable pavement make the strength of the pavement decrease. The low strength affects the scope of application of permeable pavements, making it only applicable to pavements such as pedestrian paths, garden paths and parking lots. The key to enhancing the strength of the permeability data is to add the number and area of cementing points, which is also the main point of discussion in the future. 5 Conclusion The rapid expansion of the city has severely affected the ecological environment of the city. Waterlogging, heat island effects, and noise pollution are all issues that people are facing. In order to improve the urban ecological environment, the construction of a "sponge city" is gradually being carried out in China. The study of permeable pavement data is the main point now. However, there are still a few problems with water-permeable pavements, such as easy clogging and poor durability. This is also what researchers need to pay attention to in the future. Permeable pavement is conducive to improving the urban ecological environment, so the permeable pavement has a great development prospect.