What Is a Concrete Shell?

Ordinary concrete refers to artificial stone with cement as the main cementitious material, water, sand, stones, if necessary, chemical admixtures and mineral admixtures, mixed in an appropriate proportion, and uniformly stirred, compacted and cured. . Concrete is mainly divided into two stages and states: the plastic state before setting and hardening, that is, freshly mixed concrete or concrete mixture; and the hard state after hardening, that is, hardened concrete or concrete. The concrete strength grade is divided by the cubic compressive strength standard value. China's ordinary concrete strength grade is divided into 14 grades: C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75 and C80. .

Ordinary concrete (referred to as concrete) is composed of cement, sand, stone and water. For improvement

In concrete, sand and stone play a skeleton role, called aggregate; cement and water form a cement slurry, cement

The technical properties of concrete are largely determined by the nature of the raw materials and their relative content. It is also related to the construction process (mixing, forming, curing). Therefore, we must understand the nature, function and quality requirements of its raw materials, and choose raw materials reasonably in order to ensure the quality of concrete.

1. The concept of concrete mixture:

Each component of concrete is mixed according to a certain proportion. After being stirred evenly, before being hardened, it is called a concrete mixture;

2. Concept of workability:

Workability means that the concrete mixture is easy to operate (mixing, transporting, pouring, tamping) and can obtain the properties of uniform and compact concrete. Workability is a comprehensive technical property, including fluidity The meaning of cohesiveness and water retention.

Flowability: refers to the performance of fresh concrete that can flow under the weight or mechanical vibration, and fills all corners of the formwork uniformly and densely;

Cohesiveness: It is the property that the fresh mixed concrete has a certain cohesive force between its constituent materials during construction, so that the concrete will not delaminate and segregate;

Water retention: The ability of fresh concrete to keep water from leaching during construction.

3. Workability determination method:

The workability is usually evaluated by measuring the fluidity of the mixture, and the cohesiveness and water retention are mainly evaluated by observation methods.

4. The main factors affecting the workability of concrete:

(1) Composition materials: including cement properties, water consumption, water-cement ratio, and aggregate properties;

(2) Environmental conditions: including temperature, humidity, wind speed, etc .;

(3) Time: Over time, part of the water evaporates or is absorbed by the aggregate, the mixture becomes dry and dense, and the fluidity decreases.

5. Strength of concrete

Compressive strength of concrete cubes:

According to the national standard "Test Methods for Mechanical Properties of Common Concrete", the concrete mixture was made into a cube test piece with a side length of 150mm. Under standard conditions (temperature 20 ± 2 , relative humidity above 95%), it was cured to 28d Age, the measured compressive strength value is the compressive strength of the concrete cube specimen (cubic compressive strength for short), expressed as cu.

6. Factors affecting the strength of concrete:

A. Cement strength and water-cement ratio: The higher the cement strength, the smaller the water-cement ratio, the higher the strength of the prepared concrete; on the contrary, the lower the concrete strength.

B. Aggregate effect: The strength of concrete is also related to the surface condition of the aggregate (especially coarse aggregate).

Impermeability

Impermeability refers to the ability of concrete to resist the penetration of pressure water (or oil).

Freezing resistance

Frost resistance of concrete refers to the ability of concrete to maintain strength and appearance integrity in the environment of use, which has been subjected to multiple freeze-thaw cycles.

Anti-erosion

The environmental media's erosion of concrete is mainly the erosion of cement stones, usually soft water erosion, acid, alkali, salt erosion.

1. Technical requirements for concrete design:

(1) Meet

1. High-strength concrete: The average strength of concrete used in western developed countries has exceeded 30 MPa. In China, high-strength concrete refers to concrete with a strength level of C60 or higher.

2. Light concrete

Lightweight aggregate concrete: Lightweight concrete made up of light coarse aggregate, light fine aggregate (or ordinary sand), cement and water is called light aggregate concrete.

Porous concrete: It is a kind of concrete with fine pores evenly distributed inside, without aggregate. It is divided into aerated concrete and foam concrete. Aerated concrete is generally used for roofing materials and wall materials. Foam concrete is mainly used for roofing insulation .

Macroporous concrete: Concrete made of coarse aggregate, cement, water with similar particle sizes, and sometimes with additives, mainly suitable for wall materials.

3. Impervious concrete

Impervious concrete refers to concrete with an impermeability level equal to or greater than P6.

4. Polymer concrete:

Polymer concrete is a new type of concrete composed of organic polymers, inorganic cementitious materials and aggregates. Polymer concrete embodies the advantages of organic polymers and inorganic cementitious materials, and overcomes some of the shortcomings of cement concrete. Polymerization Concrete can be divided into three types:

polymer cement concrete polymer impregnated concrete polymer cement concrete.

5. Antifreeze concrete:

Concrete with a frost resistance level equal to or greater than F50 is called antifreeze concrete;

6. Pumping concrete:

Pumped concrete refers to concrete whose slump is not less than 100mm, and is pumped for construction. In addition to the strength required for the project, the pumped concrete also needs to meet the fluidity, non-segregation and less bleeding. Pumping process requirements.

The code stipulates that pumped concrete should be selected from portland cement, ordinary cement, slag cement and fly ash cement, and pozzolan cement should not be used.

7. Mass Concrete

The minimum size of the concrete structure entity is equal to or greater than 1m, or the concrete that is expected to cause cracks due to the excessive temperature difference between the inside and outside of the concrete due to the heat of hydration of the cement is called mass concrete.

8. Fiber concrete:

Fiber-reinforced concrete is based on concrete. The purpose of mixing fibers is to increase the tensile strength and reduce the brittleness of the concrete. Common fiber materials are: glass fiber, mineral wool, steel fiber, carbon fiber and various organic fibers. Fiber concrete has gradually It is widely used in aircraft runways, bridge decks, light structures with thin end faces, and pressure pipelines, etc. ^[1] .