What is the basic strength index of concrete and what is the standard value of concrete strength?

The main indicator is compressive strength.

There are three basic strength indexes for concrete, which are: concrete cube compressive strength, concrete axial compressive strength (or prismatic strength) and concrete tensile strength.

Concrete cube compressive strength: the strength of concrete cube specimens is relatively stable, and this value is used as the basic index of concrete strength in China. According to the value of the standard value of the strength resistance of the concrete cube, the Code for the Design of Concrete Structures GB50010-2001 (hereinafter referred to as the "Code") stipulates that the concrete strength grade is divided into 14 grades

c15，c20，c25，c30，c35，c40，c45，c50，c55，c60，c65，c70，c75，c80。where the symbol c represents concrete, and the following number indicates the standard value of the compressive strength of the cube in n mm2.

Concrete axial compressive strength (or prismatic strength): Prismatic (height greater than side length) specimens can better reflect the actual compressive capacity of concrete components than cubic specimens. In the project, the axial compression components of reinforced concrete, such as columns, roof truss compression chords, etc., are much larger than the cross-sectional size, and the concrete strength of the components is close to the axial compressive strength of the concrete prism.

In the design of components, the strength of concrete mostly adopts axial compressive strength.

Concrete tensile strength: Usually the tensile strength of concrete is very low, only 1 181 8 of the compressive strength, and does not increase in proportion to the compressive strength. The crack resistance, shear resistance, and torsional bearing capacity of reinforced concrete are all related to the tensile strength of concrete.

In the theory of concrete strength under multiaxial stress states, the tensile strength of concrete is a very important parameter.

"Concrete" is a synonym for "concrete". The most important mechanical properties of concrete after hardening refer to the ability of concrete to resist stresses such as compression, tension, bending, and shear. The water-cement ratio, cement variety and dosage, aggregate variety and dosage, as well as mixing, forming and curing all directly affect the strength of concrete.

The strength grade of concrete according to the standard compressive strength (the cube with a side length of 150mm is used as the standard specimen, cured for 28 days under standard curing conditions, and the compressive strength of the cube with a 95% guarantee rate measured according to the standard test method) is called the label, which is divided into C10, C15, C20, C25, etc. The tensile strength of concrete is only 1 13 1 8 of its compressive strength. Improving the ratio of tensile and compressive strength of concrete is an important aspect of concrete modification.

Compressive strength of concrete specimen MPa = specimen failure load (n) Specimen bearing area (mm*mm).

Concrete cube specimen specifications 150*150*150mm and 100*100*100mm":

Its value is: for 200mm 200mm 200mm specimen;

For 100mm 100mm 100mm specimens.

When the concrete strength grade is C60, it is advisable to use the standard specimen 150mm 150mm 150mm specimen. The conversion factor of 150mm 150mm 150mm specimen is.

The standard value of concrete compressive strength is the strength value measured by making concrete into a test block with a side length of 150 mm and then curing it in a standard curing room for 28 days. The standard curing room here refers to an environment where the temperature is 20 degrees Celsius, the temperature difference between the upper and lower temperatures cannot exceed 2 degrees Celsius, and the humidity is more than 95%.

1. C15 grade: In the standard maintenance environment, the compressive strength needs to reach more than 15MPa, or more than 15N per square millimeter.

2. C20 grade: In the standard maintenance environment, the compressive strength needs to reach more than 20MPa, or more than 20N per square millimeter.

3. C25 grade: In the standard maintenance environment, the compressive strength needs to reach more than 25MPa, or more than 25N per square millimeter.

4. C30 grade: In the standard maintenance environment, the compressive strength needs to reach more than 30MPa, or more than 30N per square millimeter.

5. C35 grade: In the standard maintenance environment, the compressive strength needs to reach more than 35MPa, or more than 35N per square millimeter.

6. C40 grade: In the standard maintenance environment, the compressive strength needs to reach more than 40MPa, or more than 40N per square millimeter.

7. C45 grade: In the standard maintenance environment, the compressive strength should reach more than 45MPa, or more than 45N per square millimeter.

8. C50 grade: In the standard maintenance environment, the compressive strength needs to reach more than 50MPa, or more than 50N per square millimeter.

9. C55 grade: In the standard maintenance environment, the compressive strength needs to reach more than 55MPa, or more than 55N per square millimeter.

10. C60 grade: In the standard maintenance environment, the compressive strength needs to reach more than 60MPa, or more than 60N per square millimeter.

The strength grade of concrete refers to the compressive strength of the concrete. The strength grade of concrete should be divided by the standard value of the compressive strength of the concrete cube. The symbol c is used with the standard value of the compressive strength of the cube (in n mm 2;

or MPA meter).

The compressive strength of concrete is obtained by testing, and the cube specimen with a side length of 100mm is used as the standard size specimen of concrete compressive strength below the latest standard C60 strength. In accordance with the "Standard for Test Methods for Mechanical Properties of Ordinary Concrete" GB T50081-2002, the cube with a side length of 150mm is cured to 28 days old under the conditions of standard curing (temperature 20 2, relative humidity above 95%), and the ultimate compressive strength measured by the standard test method is called the compressive strength of the concrete standard cube, which is expressed as FCU. According to GB50010-2010 "Code for Design of Concrete Structures", in the overall distribution of the ultimate compressive strength of the cube, the compressive strength of the cube specimen with 95% strength guarantee rate is called the standard value of the compressive strength of the concrete cube (calculated as MPA), which is expressed by FCU.

The compressive strength with a 95% guarantee rate measured according to the standard experimental method is used as the concrete strength grade.

According to GB50010-2010 "Code for Design of Concrete Structures", ordinary concrete is divided into 14 grades, namely: C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80. For example, concrete with a strength class of C30 refers to 30MPa FCU<35MPa

The main factors affecting the strength grade of concrete are cement grade and water-cement ratio, aggregate, age, etc

Curing temperature and humidity are related.

There are three basic strength indexes of concrete, namely: concrete cube compressive strength, concrete axial compressive strength (or prismatic strength) and concrete tensile strength.

The strength grade of concrete.

Refers to the compressive strength of concrete.

The strength grade of ordinary concrete is determined by the standard value of the compressive strength of the cube in 28d with a 95% guarantee rate. It is the representative value of multiple groups of test blocks of the same level.

According to the concrete strength evaluation standard.

GB50107-2010 specifies the method while satisfying the average value.

The minimum value is determined by two conditions.

Conformity assessment methods.

Conformity assessment conditions.

Statistical Methods (1).

mfcu≥fcu,k

fcu,min

FCU, and at intensity class C20, FCU, min

K When the intensity level is C20, FCU, min

k. MFCU—the average value of the compressive strength of the three groups of specimens in the same batch (N mm2)Fcu,min—the minimum value of the compressive strength of the three groups of specimens in the same batch (N mm2);

fcu,k—standard value of compressive strength of concrete cube.

n/mm2）

0—the concrete strength standard of the acceptance batch.

The difference (n mm2) can be determined according to the strength data of the same type of concrete specimen in the previous inspection period.

The standard deviation of the concrete strength of the acceptance batch is calculated as follows:

0=∑δfcu,i

i from 1 — m).

Wherein: FCU,I—the difference between the maximum value and the minimum value in the concrete strength of the ith batch of specimens;

m—Used to determine the inspection batch.

Total number of batches of data with standard deviation of concrete strength 0.

Note]: The standard deviation of the concrete strength is determined to be 0

The inspection period shall not exceed three months, and the total number of inspection batches shall not be less than 15 batches during this period.

Statistical Methods (ii).

mfcu－λ1sfcu≥,k

fcu,min

2fcu,k

where: MFCU—the average value of the strength of the concrete specimens of group n (n mm2)fcu,min—the minimum value of the strength of the concrete specimens of group n (n mm2);

1, 2 - qualified judgment coefficient.

Press the table on the right to take it.

Standard deviation of strength of SFCU—N group concrete specimens (N mm2); When calculating the value sfcu

1. Concrete cube compressive strength (FCU): make a cube specimen with a side length of 150mm, and measure the compressive strength after curing to 28 days under standard conditions (temperature 20 2, relative humidity above 95).

2. Concrete cube compressive strength standard strength (fcu, k): refers to the cube specimen with a side length of 150mm made and cured according to the standard method, and the compressive strength value of not less than 95 guarantee rate is not less than 95 in the overall distribution of compressive strength measured by the standard test method after 28 days (taking C30 concrete as an example, 95 out of 100 are greater than or equal to 30MPa). This value is commonly used, and its strength grade is divided into 14 grades, C50 means that the standard value of the compressive strength of the concrete cube is 50MPa FCU, K 55MPa.

That is to say, we often say C30, 30 stands for 30MPa.

3. Axial compressive strength (FC) of concrete: It is the compressive strength measured by using 150mmm 150mmm 300mm prism as the standard specimen.

Now that we've looked at the concepts, let's think about the main differences:

1. Concrete is a commonly used material, in the FCU and FCU, K we use a 150mm cube for testing, in order to measure the compressive strength of this material, after all, we usually want to show the pure compressive performance of this material.

2. Commercial concrete and on-site concrete are preserved in liquid state before pouring, and in the construction environment, regardless of temperature, humidity, or the aggregate size of the concrete test block, may be different, so the standard strength of compressive strength must go to a probability guarantee, this number is 95%, if 100% the cost is too large, there is no need.

3. In the actual engineering, most of the reinforced concrete structure forms are rectangular, prismatic or cylindrical, in order to make the measured concrete strength close to the actual situation of the concrete structure, so the axial compressive strength is used in the design of the reinforced concrete structure, rather than the compressive strength. And measured.

The standard value of C30 axial compressive strength is also less than 30MPa. Therefore, the axial compressive strength is adopted, which is more in line with the actual situation of construction projects. Of course, in other projects, such as bridge design, the axial compressive strength of concrete is different from that in architectural design.

The basic indicator of concrete strength is the standard value of cubic compressive strength.

Concrete, abbreviated as:"Concrete (tóng).": refers to the general term of engineering composite materials that cement aggregates into a whole by cementitious materials.

The word concrete usually refers to the use of cement as cementitious material, sand and stone as aggregates; With water (can contain admixtures and admixtures) according to a certain stool carrying example, the cement concrete obtained by mixing, also known as ordinary concrete, is widely used in civil engineering.

Concrete is one of the most important materials for civil engineering today. It is an artificial stone made of cementitious materials, granular aggregates (also known as aggregates), water, and admixtures and admixtures added if necessary in a certain proportion, evenly stirred, densely molded, cured and hardened.

Concrete has the characteristics of abundant raw materials, low cost and simple production process, so that the amount of concrete is getting larger and larger. At the same time, concrete also has the characteristics of high compressive strength, good durability and wide range of strength grades.

These characteristics make it widely used, not only in a variety of civil engineering, that is, shipbuilding, machinery industry, marine development, geothermal engineering, etc., mixed jujube trapped concrete is also an important material.

Archaeologists found that 5,000 years ago, the ancestors of Lingjiatan were not only able to make exquisite jade tools, but also began rice farming, raising or hunting pigs, deer, birds and poultry and other animals. In addition, in the construction of houses, they have learned similar to reinforced concrete: "digging grooves to fill the soil, wood bones to support the mud wall" construction technology.

5,000 years ago, the Lingjiatan people did not simply build houses, it has been proved that the Lingjiatan people at that time had already understood the construction process of "digging grooves and filling in the soil, and supporting the mud walls with wooden bones", which is very similar to today's reinforced concrete.

The working bench carried the staff to say that the primitive ancestors used the fire-burned soil as the filling material for the foundation groove and the wall, and the jujube was trapped in the foundation groove and used wooden sticks as the support column of the wall, and then filled the red-burned soil blocks, and covered with thick clay on both sides of the wall, and even some of them may be reinforced with reed rods.