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Development and application of concrete and its reinforcement

Lightweight concrete news 2021-07-29 13:41 152
Concrete is the main material of modern engineering structure. The annual consumption of concrete in China is about 1 billion m3 and the consumption of reinforcement is about 25 million T. it is large in scale and huge in cost, ranking in the forefront of the world. It can be predicted that reinforced concrete will still be an important engineering structural material in China for a long time in the future. Material is the foundation. The development of materials will play a decisive role in the design method, construction technology, test technology and maintenance management of reinforced concrete structures. This paper briefly introduces the application and development of concrete and its reinforcement, the main material of reinforced concrete, from the perspective of engineering application

1 concrete

is one of the main materials of reinforced concrete. The development direction of concrete is high strength, light weight, durability (wear resistance, freeze-thaw resistance, impermeability), disaster resistance (earthquake, wind, fire), explosion resistance, etc

1.1 high performance concrete (HPC)

HPC is an important development direction of concrete materials in recent years. The so-called high performance refers to the superior properties of concrete in many aspects, such as high strength, high durability and high fluidity. In terms of strength, concrete with compressive strength greater than C50 belongs to high-strength concrete. Improving the strength of concrete is an important measure to develop high-rise buildings, high-rise structures and long-span structures. The use of high-strength concrete can reduce the section size and self weight, so it can obtain greater economic benefits. Moreover, high-strength concrete generally has good durability. C100 concrete has been made in China. It has been reported in the literature 1) that the strength of cement paste can reach 662mpa (compressive) and 64.7mpa (tensile) under the conditions of high temperature and high pressure in the laboratory abroad. In the actual project, the 56d compressive strength of pumped concrete in Seattle Shuanglian square is 133.5mpa

the main measures adopted to improve the strength of warm setting concrete in China are [1]: (1) rational use of superplasticizer, use of high-quality aggregate, high-quality cement and high-quality admixtures, such as high-quality ground fly ash, silica fume, natural zeolite or ultra-fine slag. Using superplasticizer to reduce water cement ratio is the main technical measure to obtain high strength and high fluidity concrete( 2) 525625725 sulphoaluminate cement, ferroaluminate cement and corresponding admixtures are adopted, which is the main technical measure for the preparation of high-performance concrete by China Academy of building materials( 3) The preparation of alkali slag high strength concrete with slag, alkali components and aggregates is a technical measure for the development of high strength concrete proposed by Chongqing Architecture and Architecture University on the basis of introducing the research results of the former Soviet Union( 4) Tianjin Harbor Engineering Research Institute of the Ministry of communications adopts composite superplasticizer, with 525 cement of 320kg / m3, water cement ratio of 0.43, and 425 cement of 480KG / m3, water cement ratio of 0.32. High strength concrete with compressive strength of 68mpa and 65mpa respectively is made in the laboratory

document [2] reports that using some metal ore coarse aggregates such as hematite and ilmenite can obtain high-performance concrete with higher strength, durability and ductility than using ordinary stone as coarse aggregate

high strength concrete has excellent physical and mechanical properties and good durability. Its main disadvantage is poor ductility. The tensile, flexural and shear strength of fiber reinforced high strength concrete made by adding an appropriate amount of steel fiber into high strength concrete can be improved, and its toughness (ductility), fatigue and impact resistance can be greatly improved. In addition, in the high-strength concrete columns of high-rise buildings, structural measures such as X-shaped reinforcement, rigid reinforcement or concrete-filled steel tubular can also be used to improve the ductility and seismic performance of high-strength concrete columns [3]

1.2 reactive powder concrete (RPC) [4]

RPC is a kind of ultra-high strength concrete, with cube compressive strength of 200-800mpa, tensile strength of 25-150mpa, fracture energy of 30kj / m2 and unit volume mass of 2.5-3.0t/m3. The main measures for making this kind of concrete are: (1) reducing the maximum size of particles and improving the uniformity of concrete( 2) Use micro powder and very micro powder materials to achieve the optimal packing density( 3) Reduce the water consumption of concrete and use non hydrated cement particles as filler to increase the bulk density( 4) Adding steel fiber to improve its ductility( 5) In the hardening process, it is pressurized and heated to achieve high strength

The grading curve of

ordinary concrete is continuous, while the grading curve of RPC is discontinuous step curve, and its aggregate particle size is very small, which is close to the size of cement particles. The water cement ratio of RPC can be as low as 0.15. A large amount of superplasticizer needs to be added to improve its workability. The price of RPC is slightly higher than that of common concrete, but much lower than that of steel. It can be designed as a slender or thin-walled structure to expand the freedom of building use. In Canada, sherbrook has designed and built a B200 RPC prestressed truss bridge for pedestrian motorcycle with a span of 60m and a height of 3.47M

1.3 low strength concrete [4]

Committee 229 of American Concrete Society (AC1) proposed a low strength concrete that can be controlled in batching, transportation and pouring, and its compressive strength is 8Mpa or less. This material can be used for filling, padding and isolation of foundation and pile foundation, as subgrade or filling holes, and can also be used for underground structures. In some specific cases, it can be used to adjust the performance indexes such as relative density, workability, compressive strength and elastic modulus of concrete, and it is not easy to produce shrinkage cracks. Low strength mortar (LSM) was used in a tunnel project in the Netherlands. Its components are: Cement 150kg / m3, sand; 1080kg / m3, 570kg / m3 of water, 6kg / m3 of superplasticizer and 35kg / m3 of bentonite. The compressive strength of LSM is 3.5Mpa and the elastic modulus is lower than 500MPa. The tunnel closure block made of LSM can save 50% of the cost compared with the conventional soil stabilization method, so this kind of concrete is expected to be developed and applied in soft soil engineering

1.4 lightweight concrete [5]

lightweight concrete made of natural lightweight aggregate (such as pumice, tuff, etc.), industrial waste lightweight aggregate (such as slag, fly ash ceramsite, spontaneous combustion coal gangue, etc.) and artificial lightweight aggregate (shale ceramsite, clay ceramsite, expanded perlite, etc.) has the advantages of low density, high relative strength, thermal insulation Good frost resistance and other advantages. The preparation of lightweight concrete by using industrial waste residue, such as waste boiler cinder, coal gangue of coal mine and fly ash of thermal power station, can reduce the production cost of concrete, turn waste into use, reduce the pollution of city or plant area, and reduce the land occupied by accumulated waste, which is also beneficial to environmental protection

1.5 fiber reinforced concrete [6]

in order to improve the poor tensile performance and ductility of concrete, the research on adding fiber to concrete to improve the performance of concrete has developed quite rapidly. At present, steel fiber, alkali resistant glass fiber, carbon fiber, aramid fiber, polypropylene fiber or nylon synthetic fiber concrete are more studied

in load-bearing structures, steel fiber reinforced concrete develops rapidly and is widely used. Steel fibers mainly include carbon steel fibers used in civil engineering and stainless steel fibers used in refractory industry. There are several production methods of steel fiber used in Civil Engineering: (1) steel wire cutting method( 2) Thin plate shear method( 3) Ingot (thick plate) milling method( 4) Molten steel drawing method

when the fiber length and aspect ratio are in the common range, and the fiber content is in the range of 1% to 2% (volume fraction, which refers to volume fraction in this paper), compared with the matrix concrete, the tensile strength, flexural strength, shear strength and compressive strength of steel fiber concrete can be increased by 40% ~ 80%, 50% ~ 120%, 50% ~ 100% and 0 ~ 25%, The deformation in elastic stage is not significantly different from that of matrix concrete, but the toughness of steel fiber reinforced concrete can be greatly improved

China Engineering Construction Standardization Association approved and promulgated the code for design and construction of steel fiber reinforced concrete structures (CECs 38:92) prepared by Dalian University of technology and other units in 1992, which has played an important role in promoting the application of steel fiber reinforced concrete

steel fiber concrete adopts conventional construction technology, and its steel fiber content is generally 0.6% ~ 2.0%. The higher the content, the steel fiber will be easy to agglomerate into balls in the construction mixing process, which will affect the quality of steel fiber concrete. However, a kind of mortar infiltrated steel fiber concrete with steel fiber content of 5% ~ 27%, referred to as sifcon, is being studied at home and abroad. Its construction technology is different from the general mixing and pouring steel fiber concrete. It is to loosely fill the steel fiber in the mold, and then pour cement slurry or mortar to harden it. Compared with ordinary steel fiber reinforced concrete, sifcon is characterized by a significant increase in compressive strength, up to 100 ~ 200MPa, and its tensile, flexural, shear strength, ductility and toughness are also greater than those of ordinary steel fiber reinforced concrete [7]

another construction method called mortar infiltrated steel fiber mesh concrete (SIMCON) is basically the same as that of sifcon, except that the steel fiber mesh is not randomly distributed but pre filled in the mold. In the products, the fiber content is generally 4% ~ 6%, SIMCON can obtain the same strength and toughness as sifcon with lower steel fiber content, so as to save materials and cost compared with sifcon

although sifcon or SIMCON have excellent mechanical properties, due to their large amount of steel fiber, high one-time investment and special construction technology, they are only used for some special structures or components when necessary, such as the emergency repair of rocket launch pad and expressway

the thin-walled structure made of steel wire mesh and skeleton reinforcement between mesh and mesh (referred to as steel wire mesh cement) laid in mortar has good crack resistance and deformation capacity. It is widely used in shipbuilding, water conservancy and construction projects at home and abroad. In recent years, steel fiber is added into steel mesh cement to build highway pavement, fishing boat and agricultural boat, which has achieved better double toughening and strengthening effect

1.6 self compacting concrete

self compacting concrete does not need mechanical vibration, but depends on self weight to make the concrete dense. Although the fluidity of concrete is high, it can still prevent segregation. The methods for preparing this kind of concrete are [4]: (1) the volume of coarse aggregate is 50% of the volume of solid concrete( 2) The volume of fine aggregate is 40% of the volume of mortar( 3) The water cement ratio is 0.9-1.0(