1 Overview
Today into the 21st century, concrete is still one of the most widely used materials in civil engineering projects. Since waterproofing projects generally use concrete structures as the base layer, and after certain technical measures are taken, concrete can also be formulated into waterproof concrete with certain impermeability grades. Therefore, it is important to ensure the quality of concrete materials and their structures. Regrettably, a great deal of engineering practice over the past decade has shown that accidents caused by defects in concrete structures (mainly cracks) are prominent in various natural and man-made disasters, and the resulting economic losses are also alarming.
Because civil engineering is a systematic project, the defects caused by concrete structures will inevitably affect the quality of waterproofing projects. Therefore, popularizing this knowledge will not only help ensure the quality of waterproofing works including concrete.
2 The relationship between concrete cracks and durability
Concrete durability means that the concrete that meets the design specifications will not be destroyed after years of use, it will continue to serve in the engineering environment, and it will have certain resistance to eroding media and maintain its adaptability.
For a long time, people have always believed that there is a corresponding relationship between strength and durability, and thus the concrete mix ratio design method that meets the durability requirements is determined. In fact, the corrosion resistance of concrete to chemical media is not necessarily related to mechanical strength. Because high-strength concrete does not necessarily have high corrosion resistance, general-strength concrete prepared from materials that are resistant to chemical erosion has a relatively high level of durability.
In recent years, studies have shown that concrete cracks and micro cracks have the greatest impact on structural durability. For example, through long-term observation, the China National Building Materials Research Institute believes that the cracking time of concrete exposed to φ12 mm steel rust specimens outdoors is 7 to 10 years, with an average of 8.5 years, and the crack width is less than 0.1 mm. The average weight loss rate is calculated to be 0.779%. However, the steel weight loss rate of φ12 mm steel rust specimens exceeds 0.4% in indoor natural conditions. If the indoor relative humidity is greater than 90%, some of them exceed 0.6%. , but the protective layer did not crack. After the concrete structure cracks, the corrosion of the steel bar will be promoted under the alternation of wet and dry cycles, and the bond between the steel bar and the concrete interface will be affected. Under the influence of the freeze-thaw cycle, the internal structure of the concrete will be destroyed, and the ability of the concrete to resist the expansion pressure generated by the steel rust will be reduced. In addition, due to the difference in linear expansion coefficient between steel and concrete, when the temperature difference varies greatly, if the concrete protective layer loses sufficient alkalinity and the steel bar is rusted, the interface between the concrete and the steel bar will be damaged, thus affecting the bearing capacity of the structure. .
It is worth pointing out. At present, due to the widespread use of early-strength, high-strength commercial concrete, cracking of concrete structures is increasingly serious. This issue is not unique to our country but it is world-wide. Judging from the feedback from engineering construction units in recent years, the phenomenon of cracking of cast-in-place reinforced concrete slabs and continuous walls of underground works in China's housing construction is very serious, and its consequences cannot be ignored. In addition, the current urban pollution is even worse, and there are erosive media in the atmosphere and rain. The impregnation of these harmful media will accelerate the corrosion of the steel bars, destroy the internal structure of the concrete, and make the cracking time of the concrete earlier. Failure to take effective preventive measures will inevitably affect the durability of the structure. Therefore, cracks in concrete structures are often the main factor in controlling the service life of buildings.
Through a large number of engineering practices and theoretical analysis, cracks with widths below 0.2-0.3 mm (commonly believed to be harmless) occur in reinforced concrete structures (including new types of expansion agents, water repellents, and composite-type compensated shrinkage concrete). It is unavoidable and should be considered acceptable. However, problems such as corrosion of steel bars, spalling of concrete, reduction of structural bearing capacity and durability cannot be underestimated. Relevant literature points out that leakage water accounts for 60% of various adverse consequences caused by concrete cracks. In terms of physics, water molecules have a diameter of about 0.3 gm and can pass through any cracked field that is visible to the naked eye. Therefore, theoretically speaking, any cracks produced in concrete structures should be controlled. This is the basic requirement for waterproofing.
3 The problem of frozen concrete damage is getting worse
The freeze-thaw damage of hydraulic concrete accounts for 100% of the projects in the Three North Regions (ie Northeast, Northwest, and North China). These large-scale concrete projects generally operate for about 30 years, and some even less than 20 years. In particular, the project of exposure to seawater is even more serious due to its frost damage.
Concretes, such as hydropower stations, industrial plants, railway bridges and culverts, concrete roads, bridges, and municipal engineering in the transportation sector, which are located in cold areas, are exposed to the effects of freeze-thaw damage due to exposure to rainwater, steam, and drainage systems. For example, in the cooling tower of the Tongliao Power Plant, the seepage of water causes the concrete to be frozen and the phenomenon of skin shedding and empty drums occurs.
With the continuous expansion of the construction scale, the problem of concrete frost damage in industrial and civil buildings has become increasingly serious. Because during the construction of large-volume concrete in winter, the ice layer of the base cannot be completely removed, and under the negative temperature environment, the temperature stress between the newly-concreted concrete and the permafrost layer cannot be ignored. With the increasing number of concrete pouring and the gradual increase of the internal temperature of the concrete, the huge temperature difference will increase the chance of cracks in the unhardened concrete (including the short-lived concrete) and become the leakage of water. The hidden danger. We have verified a lot of temperature measurement data of mass concrete in winter construction, which shows that the temperature curve of concrete under construction is very different from that under normal conditions. In other words, the risk of cracking in mass concrete during winter construction is greater. Therefore, in this case, it is only one-sided to evaluate the construction quality of winter concrete with the strength and impermeability rating of the concrete test block.
In addition, it is important to prevent initial freezing during the pouring of concrete in winter. However, since we have accumulated rich experience in this area and are clearly stipulated in the current technical specifications, there are generally no major problems as long as we strictly observe it. On the contrary, for concrete structures that have been completed over a span of years, such as long-term exposure and no overwintering protection measures, the severity of adverse consequences is poorly understood. The author observed observations of a three-storey underground building (with a depth of about 15 m) in the north for 15 months (over two winter months). The result showed that the underground three-story concrete floor (thickness: 1.4 m) In the winter construction period, there was no leakage of water in more than one year. In the second winter, the stress released during the icing expansion process caused not only the fresh penetrating cracks in the concrete structure but also the construction due to the water contained in the concrete structure (water accumulated during the construction period). Some surface cracks and deep cracks that appeared at the initial stage were also continuously expanded and penetrated to form leaking water channels, which caused water leakage in the basement. It is worth mentioning that we also observed in the basement that, in the presence of seepage water on the floor, due to the pressure of the groundwater through the capillary action, the horizontal construction joints of the floor and the shear wall, the vertical cracks of the shear wall The ice edges of different size and length appear at the general spacing of about 3 to 4 m. Therefore, how to prevent construction water, rain, snow, etc. from immersing into the concrete structure and causing freezing damage remains to be studied. In the past reading notes, the author also found that the European CEB technical specification in the antifreeze safety regulations for concrete has the requirement of “eliminating the structure of accumulated waterâ€. It can be seen that the understanding of this issue at home and abroad is similar. .
4 Conclusions and Suggestions
From the above-mentioned several concrete structure defects, the cracks are mainly the problem of concrete, and other issues such as frost damage are also related to cracks. After cracks in the concrete structure should be carefully analyzed, belonging to the harmful cracks affecting the safety of the structure, reinforcement measures should be taken. Other general cracks should also be closed or plugged so as not to cause leakage of water. With cracks that do not heal for a long time, they can cause corrosion of steel bars and peeling of concrete, and affect structural strength, stiffness, and stability. In order to reduce the consumption of natural resources and realize the sustainable development of human civilization, we hereby make the following recommendations:
1) Conduct research on structural defects in underground engineering. The relevant departments should be organized to conduct in-depth studies on structural cracks, corrosion of steel bars, physical and chemical properties of underground concrete, and maintenance and safety of underground engineering structures. This should include the contents of the concrete freeze damage and the protection of basement wintering in the Three North Zones. .
2) It is not advisable to use concrete structures for waterproofing alone in major projects. At present, most of the underground projects that only use concrete structures for self-waterproofing have different levels of leakage water, which causes long-term humidity in the basement, which not only affects the use of functions, but also is harmful to human health. In addition, concrete structures, such as long-term exposure to groundwater and various corrosive media, will reduce the useful life of buildings. Therefore, proceeding from environmental protection and improving the use of functions, it is once again calling for the use of structural self-water-repellent constructions in important underground waterproofing projects.
3) The promotion of new products is inseparable from the cooperation of intermediate tests and construction processes. With the development of building a conservation-minded society, new multifunctional materials such as roof insulation, heat insulation, waterproofing, and energy saving have begun to be applied. However, there are few experimental studies on construction technology. The quality problems after construction have already appeared and deserve attention. For a long time, one of the errors in the development of the waterproof industry in China has been the research of heavy product development, light application technology, and construction technology (including machine tools and spare parts). As a result, although the proportion of various advanced waterproofing products has been continuously expanding, it is difficult to change the reality of severe leakage in today's projects. In addition, due to the diversification of building functions, construction technology cooperation between civil and waterproof construction is also crucial. For example, in an external wall thermal insulation system project, it is necessary to solve the problem of fixing between the thermal insulation board and the wall, the plaster layer and the thermal insulation board, the adhesion between the waterproof coating and the plaster layer, and the prevention of cracking. Multi-unit and multi-scientific and technical personnel carry out experimental research. At the same time, a long-term follow-up observation of a group of demonstration projects is required before a scientific conclusion can be drawn. The construction method thus formulated has credibility and operability. History has proven that the promotion of new products cannot be separated from the cooperation of intermediate tests and construction processes. If this process is abandoned or reckless, the resulting losses will be unpredictable.
4) The general contractor project manager should grasp the waterproof quality. Construction conditions, construction procedures and product protection are recognized as the three major factors affecting the quality of waterproofing projects. In practice, how to implement measures, positions in place, and clear penalties are a process that requires constant deepening and continuous improvement. Since this work involves the cooperation of multiple units and multiple types of work related to waterproofing, it must be organized and implemented by the project manager of the construction contractor. If this is not possible, then all correct design and construction plans and even the best quality assurance measures will be neglected and neglected in practical work, ensuring that the waterproof function and project quality will become empty words.
Today into the 21st century, concrete is still one of the most widely used materials in civil engineering projects. Since waterproofing projects generally use concrete structures as the base layer, and after certain technical measures are taken, concrete can also be formulated into waterproof concrete with certain impermeability grades. Therefore, it is important to ensure the quality of concrete materials and their structures. Regrettably, a great deal of engineering practice over the past decade has shown that accidents caused by defects in concrete structures (mainly cracks) are prominent in various natural and man-made disasters, and the resulting economic losses are also alarming.
Because civil engineering is a systematic project, the defects caused by concrete structures will inevitably affect the quality of waterproofing projects. Therefore, popularizing this knowledge will not only help ensure the quality of waterproofing works including concrete.
2 The relationship between concrete cracks and durability
Concrete durability means that the concrete that meets the design specifications will not be destroyed after years of use, it will continue to serve in the engineering environment, and it will have certain resistance to eroding media and maintain its adaptability.
For a long time, people have always believed that there is a corresponding relationship between strength and durability, and thus the concrete mix ratio design method that meets the durability requirements is determined. In fact, the corrosion resistance of concrete to chemical media is not necessarily related to mechanical strength. Because high-strength concrete does not necessarily have high corrosion resistance, general-strength concrete prepared from materials that are resistant to chemical erosion has a relatively high level of durability.
In recent years, studies have shown that concrete cracks and micro cracks have the greatest impact on structural durability. For example, through long-term observation, the China National Building Materials Research Institute believes that the cracking time of concrete exposed to φ12 mm steel rust specimens outdoors is 7 to 10 years, with an average of 8.5 years, and the crack width is less than 0.1 mm. The average weight loss rate is calculated to be 0.779%. However, the steel weight loss rate of φ12 mm steel rust specimens exceeds 0.4% in indoor natural conditions. If the indoor relative humidity is greater than 90%, some of them exceed 0.6%. , but the protective layer did not crack. After the concrete structure cracks, the corrosion of the steel bar will be promoted under the alternation of wet and dry cycles, and the bond between the steel bar and the concrete interface will be affected. Under the influence of the freeze-thaw cycle, the internal structure of the concrete will be destroyed, and the ability of the concrete to resist the expansion pressure generated by the steel rust will be reduced. In addition, due to the difference in linear expansion coefficient between steel and concrete, when the temperature difference varies greatly, if the concrete protective layer loses sufficient alkalinity and the steel bar is rusted, the interface between the concrete and the steel bar will be damaged, thus affecting the bearing capacity of the structure. .
It is worth pointing out. At present, due to the widespread use of early-strength, high-strength commercial concrete, cracking of concrete structures is increasingly serious. This issue is not unique to our country but it is world-wide. Judging from the feedback from engineering construction units in recent years, the phenomenon of cracking of cast-in-place reinforced concrete slabs and continuous walls of underground works in China's housing construction is very serious, and its consequences cannot be ignored. In addition, the current urban pollution is even worse, and there are erosive media in the atmosphere and rain. The impregnation of these harmful media will accelerate the corrosion of the steel bars, destroy the internal structure of the concrete, and make the cracking time of the concrete earlier. Failure to take effective preventive measures will inevitably affect the durability of the structure. Therefore, cracks in concrete structures are often the main factor in controlling the service life of buildings.
Through a large number of engineering practices and theoretical analysis, cracks with widths below 0.2-0.3 mm (commonly believed to be harmless) occur in reinforced concrete structures (including new types of expansion agents, water repellents, and composite-type compensated shrinkage concrete). It is unavoidable and should be considered acceptable. However, problems such as corrosion of steel bars, spalling of concrete, reduction of structural bearing capacity and durability cannot be underestimated. Relevant literature points out that leakage water accounts for 60% of various adverse consequences caused by concrete cracks. In terms of physics, water molecules have a diameter of about 0.3 gm and can pass through any cracked field that is visible to the naked eye. Therefore, theoretically speaking, any cracks produced in concrete structures should be controlled. This is the basic requirement for waterproofing.
3 The problem of frozen concrete damage is getting worse
The freeze-thaw damage of hydraulic concrete accounts for 100% of the projects in the Three North Regions (ie Northeast, Northwest, and North China). These large-scale concrete projects generally operate for about 30 years, and some even less than 20 years. In particular, the project of exposure to seawater is even more serious due to its frost damage.
Concretes, such as hydropower stations, industrial plants, railway bridges and culverts, concrete roads, bridges, and municipal engineering in the transportation sector, which are located in cold areas, are exposed to the effects of freeze-thaw damage due to exposure to rainwater, steam, and drainage systems. For example, in the cooling tower of the Tongliao Power Plant, the seepage of water causes the concrete to be frozen and the phenomenon of skin shedding and empty drums occurs.
With the continuous expansion of the construction scale, the problem of concrete frost damage in industrial and civil buildings has become increasingly serious. Because during the construction of large-volume concrete in winter, the ice layer of the base cannot be completely removed, and under the negative temperature environment, the temperature stress between the newly-concreted concrete and the permafrost layer cannot be ignored. With the increasing number of concrete pouring and the gradual increase of the internal temperature of the concrete, the huge temperature difference will increase the chance of cracks in the unhardened concrete (including the short-lived concrete) and become the leakage of water. The hidden danger. We have verified a lot of temperature measurement data of mass concrete in winter construction, which shows that the temperature curve of concrete under construction is very different from that under normal conditions. In other words, the risk of cracking in mass concrete during winter construction is greater. Therefore, in this case, it is only one-sided to evaluate the construction quality of winter concrete with the strength and impermeability rating of the concrete test block.
In addition, it is important to prevent initial freezing during the pouring of concrete in winter. However, since we have accumulated rich experience in this area and are clearly stipulated in the current technical specifications, there are generally no major problems as long as we strictly observe it. On the contrary, for concrete structures that have been completed over a span of years, such as long-term exposure and no overwintering protection measures, the severity of adverse consequences is poorly understood. The author observed observations of a three-storey underground building (with a depth of about 15 m) in the north for 15 months (over two winter months). The result showed that the underground three-story concrete floor (thickness: 1.4 m) In the winter construction period, there was no leakage of water in more than one year. In the second winter, the stress released during the icing expansion process caused not only the fresh penetrating cracks in the concrete structure but also the construction due to the water contained in the concrete structure (water accumulated during the construction period). Some surface cracks and deep cracks that appeared at the initial stage were also continuously expanded and penetrated to form leaking water channels, which caused water leakage in the basement. It is worth mentioning that we also observed in the basement that, in the presence of seepage water on the floor, due to the pressure of the groundwater through the capillary action, the horizontal construction joints of the floor and the shear wall, the vertical cracks of the shear wall The ice edges of different size and length appear at the general spacing of about 3 to 4 m. Therefore, how to prevent construction water, rain, snow, etc. from immersing into the concrete structure and causing freezing damage remains to be studied. In the past reading notes, the author also found that the European CEB technical specification in the antifreeze safety regulations for concrete has the requirement of “eliminating the structure of accumulated waterâ€. It can be seen that the understanding of this issue at home and abroad is similar. .
4 Conclusions and Suggestions
From the above-mentioned several concrete structure defects, the cracks are mainly the problem of concrete, and other issues such as frost damage are also related to cracks. After cracks in the concrete structure should be carefully analyzed, belonging to the harmful cracks affecting the safety of the structure, reinforcement measures should be taken. Other general cracks should also be closed or plugged so as not to cause leakage of water. With cracks that do not heal for a long time, they can cause corrosion of steel bars and peeling of concrete, and affect structural strength, stiffness, and stability. In order to reduce the consumption of natural resources and realize the sustainable development of human civilization, we hereby make the following recommendations:
1) Conduct research on structural defects in underground engineering. The relevant departments should be organized to conduct in-depth studies on structural cracks, corrosion of steel bars, physical and chemical properties of underground concrete, and maintenance and safety of underground engineering structures. This should include the contents of the concrete freeze damage and the protection of basement wintering in the Three North Zones. .
2) It is not advisable to use concrete structures for waterproofing alone in major projects. At present, most of the underground projects that only use concrete structures for self-waterproofing have different levels of leakage water, which causes long-term humidity in the basement, which not only affects the use of functions, but also is harmful to human health. In addition, concrete structures, such as long-term exposure to groundwater and various corrosive media, will reduce the useful life of buildings. Therefore, proceeding from environmental protection and improving the use of functions, it is once again calling for the use of structural self-water-repellent constructions in important underground waterproofing projects.
3) The promotion of new products is inseparable from the cooperation of intermediate tests and construction processes. With the development of building a conservation-minded society, new multifunctional materials such as roof insulation, heat insulation, waterproofing, and energy saving have begun to be applied. However, there are few experimental studies on construction technology. The quality problems after construction have already appeared and deserve attention. For a long time, one of the errors in the development of the waterproof industry in China has been the research of heavy product development, light application technology, and construction technology (including machine tools and spare parts). As a result, although the proportion of various advanced waterproofing products has been continuously expanding, it is difficult to change the reality of severe leakage in today's projects. In addition, due to the diversification of building functions, construction technology cooperation between civil and waterproof construction is also crucial. For example, in an external wall thermal insulation system project, it is necessary to solve the problem of fixing between the thermal insulation board and the wall, the plaster layer and the thermal insulation board, the adhesion between the waterproof coating and the plaster layer, and the prevention of cracking. Multi-unit and multi-scientific and technical personnel carry out experimental research. At the same time, a long-term follow-up observation of a group of demonstration projects is required before a scientific conclusion can be drawn. The construction method thus formulated has credibility and operability. History has proven that the promotion of new products cannot be separated from the cooperation of intermediate tests and construction processes. If this process is abandoned or reckless, the resulting losses will be unpredictable.
4) The general contractor project manager should grasp the waterproof quality. Construction conditions, construction procedures and product protection are recognized as the three major factors affecting the quality of waterproofing projects. In practice, how to implement measures, positions in place, and clear penalties are a process that requires constant deepening and continuous improvement. Since this work involves the cooperation of multiple units and multiple types of work related to waterproofing, it must be organized and implemented by the project manager of the construction contractor. If this is not possible, then all correct design and construction plans and even the best quality assurance measures will be neglected and neglected in practical work, ensuring that the waterproof function and project quality will become empty words.