Concrete is made by mixing cement, sand, stone and water, and admixtures and additives in a certain proportion. After solidification, hard as stone, good compressive capacity, but poor tensile capacity, easy to fracture due to tension. In order to solve this contradiction, give full play to the compressive capacity of the concrete, often in the concrete in the area of tension or the corresponding part of a certain number of reinforcement, so that the two materials bonded into a whole, together to withstand external forces. This is equipped with reinforcement concrete, called reinforced concrete. The bonded anchorage capacity of reinforced concrete can be obtained in four ways.
① The force of chemisorption on the contact surface between the reinforcement and the concrete, also known as the cementing force.
② Concrete shrinkage, which grips the reinforcement tightly and generates friction.
③ mechanical bite between the uneven surface of the reinforcement and the concrete, also known as the bite force.
④The end of the reinforcement is bent, bent or welded in the anchorage area with short bars or angles to provide anchorage capacity.
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As the tensile strength of concrete is much lower than the compressive strength, plain concrete structures cannot be used for beams and slabs subjected to tensile stresses. If the concrete beams and slabs are configured with steel reinforcement in the tensile zone, the tensile force after cracking of the concrete can be borne by the steel reinforcement, so that the advantages of higher compressive strength of concrete and higher tensile strength of steel can be brought into full play to jointly resist the action of external forces and improve the load-bearing capacity of the concrete beams and slabs.
The fact that two materials of different nature, reinforcement and concrete, can work effectively together is due to the bonding force between concrete and reinforcement that occurs after the concrete has hardened. It consists of three parts: molecular force (adhesive force), frictional force and mechanical bite force. The decisive role is played by the mechanical bite force, which accounts for more than half of the total bonding force. The bending of the ends of the bare steel bars and the welding of the bars into a steel skeleton and mesh can enhance the bond between the steel bars and the concrete. To ensure a reliable bond between the reinforcement and the concrete and to prevent the reinforcement from being corroded, the reinforcement must be surrounded by a protective layer of concrete of a certain thickness. If the structure is in an environment with aggressive media, the thickness of the protective layer should be increased.
The tensile reinforcement in bending members such as beams and slabs is configured in the longitudinal direction on the tensile side of the structural member according to the variation of the moment diagram. In structures such as columns and arches, reinforcement is also used to increase the compressive capacity of the structure. It has two types of configuration: one is to configure longitudinal reinforcement in the direction of pressure, and concrete together with the pressure; the other is to configure transverse reinforcement network and spiral hoops perpendicular to the direction of pressure, in order to stop the lateral expansion of concrete under pressure, so that the concrete is in a three-way stress state of pressure, thus enhancing the compressive strength and deformation capacity of concrete. As the reinforcement configured in this way is not directly subjected to pressure, it is also known as indirect reinforcement. In the direction perpendicular to the longitudinal stressed reinforcement in a flexural member, distribution bars and hoop bars must also be configured to better maintain the integrity of the structure, to bear the stresses caused by concrete shrinkage and temperature changes, and to withstand transverse shear forces.
Shrinkage and creep (creep) in concrete are of great importance to reinforced concrete structures. As reinforcement hinders the free shrinkage of concrete as it hardens, it causes tensile stresses in concrete and compressive stresses in reinforcement. Creep in concrete can cause redistribution of stress between reinforcement and concrete in compressive members, increased deflection in flexural members, redistribution of internal forces in super-static structures, etc. These properties of concrete must be taken into account when designing reinforced concrete structures.
The low ultimate tensile strain values of concrete (approx. 0.15 mm/m) and the shrinkage of concrete lead to cracks in the tensile zone of members under service load conditions. To avoid concrete cracking and to reduce the width of cracks, the method of pre-stressing can be used; pre-application of pressure to the concrete (see Pre-stressed concrete structures). It has been proven that under normal conditions, cracks up to 0.3 mm in width do not reduce the load-bearing capacity and durability of reinforced concrete.
In a temperature range from -40 to 60°C, the physical and mechanical properties of both concrete and reinforcement are not significantly altered. Reinforced concrete structures can therefore be used in all climatic conditions. At temperatures above 60°C, the internal structure of the concrete material is damaged and its strength is significantly reduced. When the temperature reaches 200°C, the strength of the concrete is reduced by 30-40%. Reinforced concrete structures should therefore not be applied at temperatures above 200°C: when temperatures exceed 200°C, heat-resistant concrete must be used.
1、Steel frame structure is a structure made mainly of steel and is one of the main types of building structures. It has the following characteristics: lighter self-weight, higher reliability of work, good vibration (shock) resistance and impact resistance, higher industrialisation, easy to make into a sealed structure, easy corrosion, poor fire resistance, etc.
2, reinforced concrete structure is a structure built with steel and concrete, reinforcing steel to bear tension, concrete to bear pressure. It has the advantages of sturdiness, durability, good fire resistance, saving steel and low cost than steel structure.
As steel has good plasticity and toughness, it can have large deformation and can withstand dynamic loads well. Secondly, steel has good homogeneity and isotropy and is an ideal elastomer, which is most in line with the basic assumptions of general engineering mechanics, therefore, the seismic performance of steel structures is better than that of reinforced concrete structures.
Is a reinforced concrete structure a frame structure? The difference between a reinforced concrete structure and a frame structure is the way in which the structure carries the load. The main manifestations are.
1. A reinforced concrete structure is a reinforced concrete structure with beams, slabs, columns and walls that carry the load together.
2. Frame structure means that the building is load-bearing by a frame consisting of beams and columns. The walls are filled with materials that are not load-bearing, and the walls are built with materials such as hollow blocks, which play the role of enclosure and sound insulation.
Nowadays, most high-rise buildings use frame construction. Frame construction can be either reinforced concrete or steel.
The first is classified by material and the second by force structure.
Frame structures can be made of reinforced concrete, wood, steel or a combination of these.
Reinforced concrete structures can be used in frame structures, in shear wall structures, in framed shear wall structures, in bridge structures.
1、Locally sourced materials.
2、Good durability and fire resistance (compared with steel structure).
5、Saving steel than steel structure.
2、Low tensile strength of concrete, easy to crack.
3、Long cycle time for labour and formwork.
4、Construction is affected by the season.
5. Difficult to repair by reinforcement.
Reinforced concrete is of course mainly related to its materials, i.e. reinforcement and concrete, of which the tensile strength of the reinforcement and the compressive strength of the concrete are the most important. In addition, the construction is also related to the temperature and humidity of the weather, etc., as it affects the setting speed of the concrete.
It depends on the specific situation, first of all, the design standard, the general civil building is 50 years, large or more important buildings for 80 years or more, of course, its service life will certainly be greater than the design life, if we talk about the natural life, and concrete material characteristics, structural design, and the impact of natural conditions are closely related, its life is relatively speaking not very long, mainly because the building will appear after a long time The life span of the building is not very long, mainly because the building will be defective after a long time, such as concrete cracking to reduce the protection of the reinforcement, resulting in the acceleration of damage, thus greatly reducing the life span, and natural erosion and weathering, but its life span is certainly greater than the design life, if there are later maintenance, those defects can be made up, its life span will be greatly improved, the building will be regularly inspected, found hidden problems must be certain technical treatment, early detection early The useful life of a home is the number of years that a home can be maintained in normal use under physical wear and tear. The depreciable life of a dwelling is the number of years in which the value of the dwelling is transferred and is the socially necessary average useful life determined by the socio-economic conditions during use, also known as the economic life. The useful life of a dwelling is generally greater than its depreciable life. The national limits for depreciation for different building structures are: 60 years for reinforced concrete structures; 50 years for brick and concrete structures.
Reinforced concrete structures have a wide range of applications in civil engineering, and all kinds of engineering structures can be built using reinforced concrete.
Reinforced concrete structures are used very effectively in some special situations in atomic energy engineering, marine engineering and machinery manufacturing, such as reactor pressure vessels, marine platforms, giant oil tankers, large tonnage hydraulic press frames, etc., solving technical problems that are difficult to solve with steel structures.
1. When continuing to pour concrete at construction joints, if the interval exceeds the stipulated time, the concrete will be treated as a construction joint and allowed to continue only when the compressive strength of the concrete is not less than 1.2Mpa.
2. Before continuing to pour concrete on the hardened concrete surface, remove the cement film and loose debris or weak concrete layer from the surface, wet it well and rinse it off, and remove any water remaining on the concrete surface.
3. Before pouring, the construction joints should be paved with a layer of cement paste.
Treatment: When the surface gap is fine, the crack can be rinsed off with clean water and wiped with cement paste after it has been sufficiently moistened. Handle the interlayer with care. Before reinforcement, build temporary support to reinforce before chiselling. Remove debris and loose concrete from the interlayer, rinse it clean with clean water, moisten it sufficiently and then pour it in, using fine stone concrete of a higher strength grade to pound it in and maintain it carefully.
The design and construction of reinforced concrete structures revolves around industrial standards and practical considerations, both of which have slowly evolved in response to the experience and research accumulated during industrialisation. While new design methods, production processes and construction techniques were being developed, construction materials were also steadily evolving. In some ways, the standards of industrialisation generally reflect accepted ideas and practices based on building codes. However, the codes are usually only about certain minimum requirements, not maximum requirements. If you expect more than the minimum requirements, then meeting the minimum requirements is not your ideal goal.
Because designing and building concrete structures is a practical matter, many designers focus more on more effective industry standards than on printed codified specifications. As a result, industrial production standards influence several aspects of the design and construction of structures as follows.
(1) The methods and guidelines for design.
(2) The production and construction process.
(3) The testing and certification required.
(4) General specification requirements affecting construction plans and details.
(5) Special code requirements (e.g. fire protection).
Designers are not normally directly involved in construction work, but they must consider some of the following issues that will be encountered in practice.
(1) The maximum quantity to be poured at one time
The size of the pour is influenced by the time (e.g. 8 hours working time), the size of the work, the conditions of the site, the number of vehicles used to transport the concrete, the method of pouring and the form of the structure (e.g. in practice for multi-storey buildings only one floor can be poured at a time).
For large structures the maximum pouring volume is usually a fraction of the total volume of the structure. When the pouring has stopped for a period of time, the concrete that has been poured will harden before the next pour. The joint between the old and new concrete is called a cold joint or construction joint. The designer must consider this issue in advance – for example, as a cast-in-place structure is considered to be a single continuous structure, the designer must carefully consider the effects of such construction joints.
2. Design strength of concrete (fc)
In the early stages of the design process, the designer must first determine the design strength of the concrete. There is no doubt that this critical value is related to the performance of the structure. The designer must also consider the technology applied today, the capabilities of the contractor and the budget of the project. As a result, some designs will continually exceed the limits of today’s construction technology by requiring the best possible concrete (e.g. designing a high-rise building), while others will only require low strength concrete.
3. Accuracy of construction
Pouring on site is a very rough job and it is rarely possible to achieve precise geometry or a smooth finish. Experience has taught designers what errors are allowed and what improvements can be made – they have learned to write some careful design notes, deliberately select certain materials or do some supervision on site.
Generally speaking, however, the quality of factory-precast concrete is higher than that of cast-in-place concrete. These components are dimensionally precise and can be modified. Although precise dimensional requirements and smooth finishes are not critical to the formation of the basic structure, they can make the building superior in terms of surface finish and other constructional workmanship. Of course, if the concrete is covered or encased by something else after completion, then this disadvantage is of little consequence. However, the designer must be aware of the accuracy required for the finer connecting elements of the structure and recognise the minimum requirements for accuracy in building concrete structures.
4. Minimum dimensions of concrete elements
For practical reasons, some reinforced concrete elements must have specific dimensions in order to meet the different requirements of the plant in terms of protective cover and reinforcement spacing.
When a slab, wall or beam is equipped with bending reinforcement, its size is mainly determined by the distance between the tensile reinforcement and the outer edge of the concrete under pressure. In very thin beams, slabs and walls, therefore, the bending reinforcement plays no role.
In general, reinforcement is arranged in both directions in slabs and walls. Even if the bending action only occurs in one direction, the code requires a certain amount of reinforcement in the other direction to control cracks due to shrinkage and temperature changes. Even with a minimum protective layer thickness and minimum cross-sectional area of reinforcement, the minimum thickness of the slab should be roughly 2 inches, but with the exception of joist or shaft construction, the slab is usually thicker thus increasing the bending capacity of the plant. The general arrangement of reinforcement, whether at the top or at the bottom, is largely determined by the positive or negative bending moment.
Building codes often require additional protective cover thicknesses, specifying a minimum thickness of 4″ or greater for slabs, thus ensuring a high fire resistance rating.
Tip: The thickness of the slab is determined by the size of the aggregate used.
Walls 10 inches thick or thicker often have two layers of reinforcement. Each layer is as close to the outer surface of the wall as is permitted. Walls with criss-crossing reinforcement (for example, with horizontal and vertical reinforcement) are generally rarely less than 6 inches thick.