Concrete refers to the mixing of cement, sand, stone and water according to a certain proportion, it is also known as ordinary concrete, widely used in civil engineering. The followings are the types and strength grade of concrete for construction.
Sand fills the spaces between stones.
Cement slurry mixed from Cement and water to fill the gaps of sand and stone, and warp in the surface of sand and stone. The cement slurry plays the role of filling, lubricating and cementing.
Sand is called fine aggregate, stone is called coarse aggregate. The coarse and fine aggregates generally do not react with cement, mainly play the role of skeleton and inhibit the volume deformation of cement stone.
Types of Concrete
Based on strength grade:
1. Ordinary concrete: according to national standards, concrete with 28-days compressive strength less than C30 is called the ordinary concrete.
2. High-strength concrete: according to national standards, concrete with 28-days compressive strength in the range of C30 to C80 is called high-strength concrete.
3. Ultra-high strength concrete: According to national regulations, concrete with 28-days compressive strength greater than C80 is called ultra-high strength concrete.
Based on cementing materials
1. Norganic cementing material concrete: including lime siliceous cementing material concrete, Portland cement system concrete (such as Portland cement, ordinary cement, slag cement, fly ash cement, volcanic ash cement, early strength cement concrete, etc.). Calcium aluminum cement system concrete (such as high aluminum cement, pure aluminate cement, jet cement, super hard cement concrete, etc.), gypsum concrete, magnesium cement concrete, sulfur concrete, sodium silicate silicate concrete, metal concrete (metal instead of cement as bonding material), etc.
2. Organic cementing concrete: organic cementing concrete mainly includes asphalt concrete and polymer cement concrete, resin concrete, polymer impregnated concrete and so on. In addition, inorganic and organic composite colloidal material concrete can also be divided into polymer cement concrete and polymer indigo concrete.
Asphalt concrete is a mixture of artificially selected mineral materials (crushed stone or crushed gravel, stone chips or sand, mineral powder, etc.) with a certain proportion of asphalt materials, mixed under strict control conditions. Asphalt concrete is a type of concrete commonly used to build roads, parking lots and other pavement types.
Based on the apparent density
1. Heavy concrete is the high-density concrete with an apparent density greater than 2500 kg/m3, it is made of exceptionally dense and heavy aggregates. Such as barite concrete, steel chip concrete, etc., they have the performance of X-ray and gamma rays; It is often made of barite and iron ore.
2. Ordinary concrete is commonly used in construction, with the apparent density of 1950 ~ 2500 kg/m3, its main aggregates are stand and stone.
3. Lightweight concrete is the apparent density of less than 1952m/m3. It can be divided into three categories:
A. Light aggregate concrete is apparent density of 800 to 1950 kg/m3, light aggregate including pumice, cinder, ceramic particles, expanded perlite, expanded slag, slag and so on.
B. Porous concrete (foam concrete, aerated concrete) is apparent density of 300 ~ 1000 kg/m3. Foamed concrete is made of cement slurry or cement mortar with stabilized foam. Aerated concrete is made of cement, water and aerating agent.
C. Large hole concrete (ordinary large hole concrete, light aggregate large hole concrete), its composition without fine aggregate. The apparent density range of ordinary large-hole concrete is 1500 to 1900kg/m3, which is prepared with crushed stone, soft stone and heavy slag as aggregate. The apparent density of lightweight aggregate large-hole concrete is 500 to 1500 kg/m3, and it is prepared with ceramic particles, pumice stone, broken brick, slag, etc., as aggregate.
Based on function
It can be divided into structural concrete, thermal insulation concrete, decorative concrete, waterproof concrete, refractory concrete, hydraulic concrete, Marine concrete, road concrete, radiation proof concrete, etc
Based on construction technology
It can be divided into centrifugal concrete, vacuum concrete, grouting concrete, shotcrete, roller compacted concrete, extruded concrete, pumped concrete, etc.
Based on reinforcement type
It can be divided into plain concrete (no reinforcement concrete), reinforced concrete, fiber concrete, prestressed concrete, etc.
1. Plain concrete is free of reinforcement. It can be used to internal structures of buildings.
2. Reinforced concrete is a composite material composed of steel mesh, steel plate or fiber to increase the tensile pressure of concrete. It is often used in the construction of high-rise buildings, Bridges, DAMS or internal structures that need to carry extremely heavy loads.
3. Fiber concrete is a composite material composed of fiber and cement base material (cement stone, mortar or concrete). The main function of fiber is to limit the expansion of cracks in the cement base under the action of external forces.
4. Prestressed concrete is the concrete that applies compressive stress during the production process, which combines the advantages of high tensile pressure of steel and high compressive strength of concrete. The pre-compressive stress is used to reduce or offset the tensile stress of the concrete caused by the load, so that the tensile stress of the structural members is controlled in a small range, or even in a state of compression, in order to delay the appearance and development of concrete cracks, so as to improve the crack resistance and stiffness of the members. Bridges, roofs, water tanks and floor beams are generally made of prestressed concrete.
Based on the mixture
It is divided into dry hard concrete, semi-dry hard concrete, plastic concrete, flow concrete, high flow concrete, etc.
1. Dry hard concrete refers to the concrete with less cement or mortar content, more coarse aggregate, low water-cement ratio, and Wiber consistency of 20 to11s. It is often used for precast concrete products and components with high strength requirements.
2. Semi-dry hard concrete is a material with high plasticity and high strength, which is suitable for environments requiring high strength, high durability and high plasticity, such as large Bridges, tunnels, highways and other engineering projects.
3. Plastic concrete is a kind of large flow concrete with low cement content and more bentonite, clay and other materials. It has the characteristics of low strength, low elastic modulus, large strain, and good anti-seepage performance, so it is widely used in the anti-seepage of water conservancy projects.
4. Liquid concrete refers to concrete with slump of 100 to 150mm, and concrete with slump of 160mm or more is called large liquid concrete. In road and bridge engineering, liquid concrete mainly includes pumped concrete, shotcrete and underwater concrete.
Based on admixture
It is divided into fly ash concrete, silica fume concrete, slag concrete, fiber concrete, etc.
1. Fly ash concrete refers the concrete added with fly ash. Fly ash is a kind of volcanic ash material, which fills the internal pores of concrete and improves the strength and impermeability of concrete.
2. Wollastonite concrete: concrete added with wollastonite can improve the strength, impermeability and durability of concrete.
3. Slag concrete: It is made of cement, slag, sand, stone and other materials mixed in accordance with a certain proportion. It can improve the durability of concrete.
Based on use
1. Ordinary concrete: mainly used in general buildings, roads, Bridges, tunnels and other projects.
2. Special concrete: according to different uses, it can be divided into self-compacting concrete, fiber concrete, self-leveling concrete, expansion concrete and so on.
Self-compacting concrete refers to high performance concrete that can be compacted and formed without any vibration under its own gravity.
Strength Grade of Concrete for Construction
What is Concrete Strength Grade?
The concrete strength grade for construction refers to the strength of concrete under 28 days specified curing conditions. At present, concrete strength grades include 15 grades such as C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80. the C represents the concrete, the following number represents the standard compressive strength of concrete, the unit is MPa.
Why are there Strength Grades for Concrete?
Because the load of concrete in different parts of the engineering is not the same, so the strength grade of concrete can not be completely used. For the parts that bear the high pressure, it is necessary to use high-strength grade concrete; for the parts under low pressure, it is necessary to use low-strength grade concrete, so the concrete is divided into different strength grades to meet the needs of different projects.
Application of Concrete Strength Grade
1. C15, C20, C25
Concrete with C15, C20 or C25 are suitable for simpler buildings, such as civil engineering, underground drainage pipes, road foundations, cement mortar and so on.
2. C30, C35
Concrete with C30 or C35 are suitable for some moderately difficult buildings, such as water conservancy projects, underground parking lots, bridge foundations, general workshops, cement mortar and so on.
3. C40, C45, C50
Concrete with C40, C45 or C50 are suitable for some large industrial and civil buildings, such as large water conservancy projects, highways, high-rise buildings, airport runways, docks and so on.
4. C55, C60, C65, C70
Concrete with C55, C60, C65 or C70 are suitable for some special building, such as nuclear power plants, large bridges, high-speed railways, offshore platforms and so on.
Selection of Concrete Strength Grade
1. Application Requirements of the Project
According to the requirements of the project, you should choose the appropriate concrete strength grade to ensure the durability and safety of the project.
2. Construction Methods and Materials
According to the construction method and characteristics of the materials, you should choose the appropriate concrete strength to ensure the quality and efficiency of construction project.
3. Technical and Economic Factors
According to the technical and economic factors, you should choose the appropriate concrete strength not only meets the requirements of the project, but also control the cost.
Testing of Strength Grade of Concrete
Inspection of concrete strength grade is one of the important means to evaluate the performance of concrete. There are two common testing methods: non-destructive testing and destructive testing.
1. Non-destructive Testing
Non-destructive testing is to asses the strength grade of concrete by inspecting the surface. The common non-destructive testing methods include ultrasonic testing, resistivity testing, shock wave testing and so on.
2. Destructive Testing
Destructive testing is to asses the strength grade of concrete by conducting destructive test on concrete block. The common methods are pressure test, bending test, tensile test and so on.
Maintenance of Concrete Strength Grade
The maintenance of concrete strength grade is one the important measures to ensure the performance of concrete. It includes maintenance, heat preservation, as well as moisture-proof, etc.
Maintenance refers to water curing or wet curing for a certain time after the concrete is poured to ensure the full hardening and strength of the concrete.
2. Thermal Insulation
Thermal insulation means concrete poured should take the certain thermal insulation to ensure the early strength and durability of concrete.
Moisture-proof means that before concrete is poured, certain moisture-proof measures are carried out on the construction site to ensure the quality and performance of concrete.
Why the Standard Strength of Concrete is 28 days?
The enhancing of compressive strength is relying on the hardening of concrete based on the bonding action of cement. Because the hardening of concrete is not completed at once, it is gradually completed with the increase of time. Under normal maintenance conditions, the compressive strength increase rapidly in the first seven days, then the growth is slightly slower between 7 days to 14 days, and the strength growth is more slow after 28 days.
If the strength of less than 28d is used as the standard strength, the performance of concrete will not be fully developed. If the strength is more than 28d as the standard strength, although the performance of the concrete can be fully played, but there is too long, which will affect the construction progress. So the strength on 28 days is the standard strength of concrete.
Advantages of Concrete
Concrete has good plasticity and can be made into different forms of buildings components by changing the size and shape of the template.
Compared with other materials, the raw materials price of concrete is low, the source is wide, and the maintenance cost is also lower.
Concrete have one to two hours of fire proofing.
Hardened concrete has a high compressive strength, and has a strong bonding force with steel bar, so that the structural safety is fully guaranteed.
Concrete is suitable for a variety of structural forms in civil engineering. To meet various construction requirements, concrete with different strength grades can be prepared according to different requirements.
Concrete is a very durable material, ancient Roman buildings is still standing after thousands of years.
Low energy consumption
Low energy consumption: the production of concrete and its products uses less energy than other building materials.
Disadvantages of Concrete
Low tensile strength
Low tensile strength: the tensile strength of concrete is one tenth of the compressive strength and one hundredth of the tensile strength of steel bars.
It is brittle material with poor deformation ability, and it can only withstand a small amount of tension deformation. The impact resistance is poor, and brittle fracture is easy to occur under the impact load.
With the change of temperature and humidity environmental medium, its volume is easy to change, resulting in cracks and other internal defects.
The recycling of demolition wast is poor.
The recycling of demolition waste is poor.
Affected by season
Construction of concrete is affected by the season: it is easily constrained by rainy days.
Difficult repair: once a problem occurs, it is difficult to repair.