How Much Do You Know About Concrete?

Concrete is a common building material in civil engineering and is widely used in all kinds of civil engineering. It is mainly used in the structural parts of buildings, such as foundations, walls, beams and columns. Concrete has high strength, durability and good plasticity, which makes it an ideal material for building high-rise buildings, bridges, roads and other infrastructure. Mass concrete refers to mass concrete with the minimum geometric size of not less than 1 meter, which is often used in high-rise building foundation, large equipment foundation, water conservancy dam and other projects. Because of its large volume, concentrated release of hydration heat of cement and rapid internal temperature rise, it is easy to cause temperature cracks, so it is necessary to pay special attention to the hydration heat and the volume deformation caused by it. Permeable concrete is a kind of building material with good permeability, which is widely used in roads, sidewalks, squares, parking lots and other places. It can effectively drain water, prevent water accumulation and flooding, reduce the consumption of groundwater resources and improve the utilization efficiency of water resources.

I. Why should concrete be graded in strength?

The strength grade of concrete refers to the compressive strength of concrete. According to the Standard for Inspection and Evaluation of Concrete Strength (GB/T 50107—2010), the strength grade of concrete should be determined according to its cube compressive strength standard value, which is expressed by symbol C and cube compressive strength standard value (in N/mm2 or MPa). Ordinary concrete is divided into 14 strength grades, such as C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75 and C80, according to the standard value of cubic compressive strength. Because the loads on concrete in different parts of general engineering are different, some are large and some are small, so it is impossible to use a kind of strength grade concrete completely. For the parts under great pressure, high-strength grade concrete should be used, and for the parts under small pressure, low-strength grade concrete should be used. Therefore, concrete is divided into different strength grades to meet the needs of different projects and selected through design and calculation.

II. Why is the 28d strength of concrete specified as the standard strength?

Concrete is hardened gradually by the cementation of cement, and its compressive strength is improved. Because the hardening of cement is not completed at once, it is gradually completed with the increase of time. Under normal curing conditions, the compressive strength increased rapidly in the first seven days, and slightly slower between the 7th and 14th days, and even slower after the 28th day. That is to say, the compressive strength after 28d is the standard strength, which is used as the standard for quality inspection in design and construction. Obviously, if the strength less than 28d is taken as the standard strength, the performance of concrete will not be fully exerted. If the strength greater than 28d is taken as the standard strength, although the performance of concrete can be brought into full play, it takes too long to reach the standard strength, which affects the construction progress.

III. Why does concrete have requirements for water quality?

Concrete requires water quality because impure water quality will affect the hydration and condensation of cement and the strength of concrete, and may also corrode steel bars and affect the durability of concrete. Harmful substances in mixed water refer to insoluble substances, soluble substances, chlorides, sulfides and sulfates. Domestic water that meets the national standards is qualified water for mixing concrete. In addition to domestic water, other types of mixing water must be tested for harmful substances before it can be used as water for concrete preparation.

IV. Why are concrete test blocks in groups of three?

Concrete test block is the standard to measure the strength of concrete members, that is, the strength of the test block under compression is taken as the strength that the members can have. Therefore, in addition to making test blocks as components, there must be a certain number of test blocks. Because although we try our best to make the concrete of the test block the same as that of the component, there are some differences after all. If we only rely on one test block to identify the strength of the component, it will be difficult to be reliable. Therefore, in a group of three, the average strength of three test blocks is taken as the strength of this group of test blocks (in special cases, some values need to be removed), that is, it is taken as the strength of the component.

V. What does the durability of concrete mean?

The durability of concrete refers to the ability of concrete to resist various destructive factors and maintain its strength and appearance integrity for a long time under actual use conditions. In addition to proper strength, concrete should also include characteristics such as frost resistance, water penetration resistance, chloride ion penetration resistance, shrinkage, carbonation, steel corrosion resistance, sulfate resistance, compressive fatigue deformation, alkali-aggregate reaction and so on, which are collectively called durability.

VI. Why do concrete have natural curing and steam curing?

Concrete has natural curing and steam curing because: at the natural curing temperature, the strength of concrete increases very slowly, and the strength of concrete after 28 days can only be 30~70% at the age of 7 days. It takes a long time to ensure the strength of formwork removal and factory strength, which will prolong the cycle time of the whole production process, and at the same time, it requires a lot of formwork equipment and a lot of production area, which will increase the capital investment.

In order to speed up the growth of concrete strength, steam curing can be adopted, and the concrete can be heated by steam, so that the concrete can be quickly hardened under the conditions of high temperature (70~90℃) and high humidity (about 90% or more). However, it is still suitable for natural conservation in areas with bright and warm climate. This can save fuel and a corresponding set of equipment investment, and reduce costs.

VII. What is the workability of concrete?

The workability of concrete refers to whether the composition of concrete mixture can maintain symmetry and whether it is easy to be poured and vibrated in the actual operation of production and manufacturing. The workability of concrete is a comprehensive index value. It includes three aspects: fluidity, cohesiveness and water retention of concrete. The fluidity of concrete refers to the characteristics that concrete mixture is easy to flow, transport and fill concrete formwork under the self-acting force or mechanical power effect. The cohesiveness of concrete means that there is a certain cohesiveness between the raw materials that make up the concrete mixture during the processing, which will not cause the segregation of layers and concrete. The water retention of concrete means that the concrete mixture will not cause serious bleeding during processing and has a certain water retention capacity.

VIII. How to measure the workability of concrete?

To determine the workability of concrete, the following steps can be adopted:

1. The concrete mixture is packed in three layers, and each layer is rammed 25 times. After tamping, smooth it, and mention the slump cone vertically and steadily.

2. Turn the transparent disc to the top of the concrete sample and gently drop it to touch the top of the concrete wall.

3. In addition, open the vibration platform and stopwatch, and record the time required for the bottom edge of the fully transparent disc to be covered with cement mortar.