Abstract: reinforced concrete structure is often used in modern housing construction engineering. In fact, reinforced concrete structure refers to the structure made of reinforced concrete.
The main load-bearing members are made of reinforced concrete. Including thin slab structure, large formwork cast-in-situ structure and reinforced concrete structure built with Wei formwork and riser. A structure made of steel and concrete.
Reinforcement bears tension and concrete bears pressure. The utility model has the advantages of firmness, durability, good fire resistance, steel saving and low cost compared with the steel structure.
Concrete is composed of cementitious materials, cement, sand, stone and water, as well as admixtures and admixtures in a certain proportion. After solidification, it is as hard as stone, with good compression capacity, but poor tensile capacity, which is easy to fracture due to tension.
In order to solve this contradiction and give full play to the compression capacity of concrete, a certain amount of reinforcement is often added in the tensile area or corresponding parts of concrete to make the two materials bond as a whole and bear external forces together. This kind of reinforced concrete is called reinforced concrete.
① chemical adsorption force on the contact surface between reinforcement and concrete, also known as cementation force;
② concrete shrinkage, which produces friction by holding the reinforcement tightly;
③ mechanical bite effect between the uneven surface of reinforcement and concrete, also known as bite force.
④ The end of the reinforcement shall be hooked, bent or welded with short reinforcement and angle steel in the anchorage zone to provide anchorage capacity
Because the tensile strength of concrete is much lower than the compressive strength, plain concrete structure can not be used in beams and slabs with tensile stress.
If the reinforcement is arranged in the tensile area of the concrete beam and slab, the tensile force of the concrete after cracking can be borne by the reinforcement, which can give full play to the advantages of high compressive strength of the concrete and high tensile strength of the reinforcement, jointly resist the external force and improve the bearing capacity of the concrete beam and slab.
Reinforcement and concrete can work together effectively because of the bond between concrete and reinforcement after concrete hardening.
It consists of molecular force (gluing force), friction and mechanical biting force. Among them, the mechanical biting force plays a decisive role, accounting for more than half of the total bonding force. The end of the smooth reinforcement is made into a hook, and the reinforcement is welded into a reinforcement skeleton and mesh, which can enhance the adhesion between the reinforcement and the concrete.
In order to ensure the reliable bonding between reinforcement and concrete and prevent reinforcement from being corroded, there must be a certain thickness of concrete protective layer around the reinforcement. If the structure is in an environment with corrosive medium, the thickness of the protective layer should be increased.
The tensile reinforcement in flexural members such as beams and plates is longitudinally arranged on the tensile side of structural members according to the change of bending moment diagram. In structures such as columns and arches, reinforcement is also used to enhance the compressive capacity of the structure.
It has two configuration modes: one is to configure longitudinal reinforcement along the pressure direction to bear the pressure together with the concrete;
The other is to configure transverse reinforcement mesh and spiral stirrup perpendicular to the pressure direction to prevent the lateral expansion of concrete under pressure and make the concrete in the stress state of three-dimensional compression, so as to enhance the compressive strength and deformation capacity of concrete.Because the reinforcement configured in this way does not directly bear pressure, it is also called indirect reinforcement.
In the direction perpendicular to the longitudinal stressed reinforcement in the flexural member, distribution reinforcement and stirrup shall also be configured to better maintain the integrity of the structure, bear the stress caused by concrete shrinkage and temperature change, and bear the transverse shear force.
The shrinkage and creep of concrete are of great significance to the structure of reinforced concrete. Because the reinforcement will hinder the free shrinkage of concrete during hardening, it will cause tensile stress in concrete and compressive stress in reinforcement.
The creep of concrete will cause the redistribution of stress between reinforcement and concrete in compression members, the increase of deflection in flexural members, and the redistribution of internal force in statically indeterminate structures.
These characteristics of concrete must be considered in the design of reinforced concrete structures.
Due to the low ultimate tensile strain of concrete (about 0.15mm / M) and the shrinkage of concrete, cracks are easy to appear in the tensile area of members under service load. In order to avoid concrete cracking and reduce crack width, pre stressing method can be used to pre apply pressure to concrete (see prestressed concrete structure).
Practice has proved that under normal conditions, cracks with a width of less than 0.3mm will not reduce the bearing capacity and durability of reinforced concrete.
In the temperature range from – 40 ° C to 60 ° C, the physical and mechanical properties of concrete and reinforcement will not change significantly. Therefore, reinforced concrete structure can be applied under various climatic conditions.
When the temperature is higher than 60 ° C, the internal structure of concrete materials will be damaged and its strength will be significantly reduced.
When the temperature reaches 200 ° C, the concrete strength decreases by 30 ~ 40%. Therefore, reinforced concrete structure should not be applied when the temperature is higher than 200 ° C: when the temperature exceeds 200 ° C, heat-resistant concrete must be used.
1). Steel frame structure is mainly made of steel, which is one of the main types of building structure. It has the following characteristics: light self weight, high working reliability, good vibration (earthquake) resistance and impact resistance, high degree of industrialization, easy to be made into sealing structure, easy to corrode, poor fire resistance, etc.
2). Reinforced concrete structure is a kind of structure made of reinforcement and concrete. Reinforcement bears tension and concrete bears pressure. The utility model has the advantages of firmness, durability, good fire resistance, steel saving and low cost compared with the steel structure.
Because the steel has good plasticity and toughness, can have large deformation, and can well bear dynamic load. Secondly, the steel has good homogeneity and recyclability. It is an ideal elastomer, which is most in line with the basic assumptions of general engineering mechanics. Therefore, the seismic performance of steel structure is better than that of reinforced concrete structure.
The difference between reinforced concrete structure and frame structure is that the load-bearing mode of the structure is different. It is mainly reflected in:
1. Reinforced concrete structure refers to the joint bearing of beams, plates, columns and walls of reinforced concrete structure.
2. Frame structure refers to the load-bearing frame of a building composed of beams and columns. The wall is filled with non load-bearing materials.
The wall is built with hollow blocks and other materials, which plays the role of enclosure and sound insulation. Now high-rise buildings mostly use frame structure. The frame structure can be reinforced concrete structure or steel structure.
The front is divided according to the material, and the back is divided according to the stress structure.
The frame structure can be reinforced concrete structure, wood structure, steel structure and the combination of the above structures. Reinforced concrete structure can be used in frame structure, shear wall structure, frame shear wall structure and bridge structure.
1. Local materials.
2. Good durability and fire resistance (compared with steel structure).
3. Good integrity
4. Good modularity. 5. Save steel than steel structure
1. Self significant.
2. The tensile strength of concrete is low and easy to crack.
3. It is labor-consuming and time-consuming, and the template cycle is long.
4. Construction is affected by seasons.
5. Reinforcement and repair are difficult. Of course, reinforced concrete is mainly related to its materials, that is, reinforcement and concrete, in which the tensile strength of reinforcement and the compressive strength of concrete are the most important. In addition, the construction is also related to the temperature and humidity of the weather, because it will affect the setting speed of concrete.
1) . when pouring concrete at the construction joint, if the interval time exceeds the regulations, it shall be treated as the construction joint. It is allowed to continue pouring only when the compressive strength of the concrete is not less than 1.2MPa
2) Before pouring concrete on the hardened concrete surface, remove the surface cement film, loose gravel or soft concrete layer, fully wet and wash dry, and remove the water remaining on the concrete surface.
3) Before pouring, a layer of cement slurry should be laid on the construction joint. Treatment method: when the surface gap is fine, the crack can be washed with clean water and plastered with water slurry after being fully wet.
Handle the interlayer carefully. Before reinforcement, temporary support shall be erected for reinforcement before chiseling. The sundries and soft concrete in the interlayer shall be removed, washed with clean water, fully wetted, re poured, tamped with fine aggregate concrete with improved strength grade – and carefully maintained.
The design and construction of reinforced concrete structures revolve around the standards of industrialization and practical considerations, which develop slowly with the experience and research accumulated in industrialization.
While new methods, manufacturing processes and construction technologies are constantly emerging, building materials are also developing steadily. In some ways, the standard of industrialization generally reflects the thought accepted by everyone and the practice carried out according to the construction norms.
However, specifications usually only talk about some minimum requirements, not the highest requirements. If you expect more than the minimum requirements, meeting the most basic requirements is not your ideal goal.
Because the design and construction of concrete structures is a very practical thing, many designers pay more attention to more effective industrial standards rather than printed specifications.
Therefore, industrial production standards affect the following aspects of structural design and construction:
(1) Design methods and guidelines.
(2) Production and construction process.
(3) Required tests and certificates.
(4) General specification requirements affecting construction plans and details. 5) Special specification requirements (e.g. fire protection).
1) . maximum quantity of one pouring
The size of pouring is affected by time (e.g. 8-hour working time), workload, site conditions, number of vehicles transporting concrete, pouring method and structural form (for example, in actual pouring, only one layer can be poured at a time for multi-storey buildings)
For large structures, the maximum pouring volume is usually a part of the pouring volume of the whole structure. When the pouring stops for a period of time, the poured concrete will harden before the next pouring. The joint between new and old concrete is called cold joint or construction joint. The designer must consider this problem in advance – for example, since the cast-in-situ structure is considered to be a single continuous structure, the designer must carefully consider the impact of this construction joint
2) Design strength of concrete (FC) in the early stage of the design process, the designer must first determine the design strength of concrete. There is no doubt that this key value is related to the performance of the structure. Designers must also consider the technology currently applied, the capabilities of contractors and the budget of the project. Therefore, some designs will continue to exceed the limits of today’s construction technology and require the use of as good concrete as possible (such as designing high-rise buildings), while others only require the use of low-strength concrete.
3). Accuracy of construction cast-in-situ is a very rough work, and it is rarely possible to achieve accurate geometric dimensions or smooth surface course.
Experience tells designers what kind of error is allowed and what kind of error can be improved – they learn to write some design instructions carefully, choose certain materials or do some supervision on site.
However, generally speaking, the quality of factory prefabricated concrete is higher than that of cast-in-situ concrete. These components are accurate in size and can be modified. Although precise dimensional requirements and smooth surface are not key to the formation of basic structure, they can make buildings superior in surface treatment and other architectural processes.
Of course, if the concrete is covered or wrapped by other things after completion, the impact of this disadvantage will be small. However, designers must understand the accuracy required by the finer connecting members in the structure and recognize the minimum requirements for the accuracy of building concrete structures.
4) The minimum size of concrete components due to actual construction, in order to meet the different requirements of protective layer and reinforcement spacing, some reinforced concrete structures must have specific dimensions.
When the plate, wall and beam are equipped with flexural reinforcement, its size is mainly determined by the distance between the tensile reinforcement and the outer edge of the compression concrete. Therefore, in very thin beams, thin plates and walls, flexural reinforcement plays little role.
Generally, reinforcement in two directions shall be arranged in slab and wall. Even if the bending action only occurs in one direction, the code also requires that a certain number of reinforcement must be provided in the other direction to control the cracks caused by shrinkage and temperature changes.
Even if the reinforcement with the minimum cover thickness and minimum cross-sectional area is adopted, the minimum thickness of the plate should be approximately 2 inches. However, except for joist or well structure, the thickness of the plate is usually large, so as to improve the bending capacity of the plant.
In general, whether the reinforcement is arranged at the top or bottom is mainly determined by the positive and negative bending moments.
Building codes often require additional cover thickness, specifying a minimum thickness of 4 inches or more to ensure a higher fire resistance rating
Walls 10 inches thick or thicker often have two layers of reinforcement. Each layer is close to the outer surface of the wall if allowed. Walls with cross reinforcement (e.g., horizontal and vertical reinforcement) are rarely less than 6 inches thick.