Why study the strength growth of concrete at the age of 3 days, 7 days, and 28 days

Because in the time intensity curve of cement curing process, the three time periods of 3 days, 7 days and 28 days are obviously representative.

Within 3 days, the strength of cement is growing rapidly. Generally, it can reach more than 75% of the maximum strength, and can be used basically. Within 3 to 7 days, the strength growth of cement slows down, and generally reaches more than 95% of the maximum strength, which can be used completely.

After 7 days, the cement strength increases more slowly, and by 28 days, the cement strength reaches its maximum. After 28 days, there will be no further addition. Therefore, the national standard regards these three days as the three basic time points to measure the strength of cement.

The strength growth of concrete stems from the hydration reaction of its cement, and since the hydration reaction of cement in concrete is a relatively long-term process, the strength of concrete is within a certain time range.

Grows with age. It is generally believed that there is an approximate relationship between the strength r of concrete and the age t of concrete:

ra/rb=k*lgt1/lgt2

In the above equation, k is the coefficient, and the value is different for different raw materials (mainly cement varieties).

In fact, the formula is just a conceptual formula, not an exact calculation formula, that is, if the concrete strength at a certain age is known, the error is quite large if the concrete strength at different ages is directly estimated by the above formula, and when the concrete reaches a certain age, because the cement hydration is basically completed, the concrete strength basically no longer increases with the age.

3 days can be combined with the strength of cement (because the strength of cement is evaluated by 3 days, 28 days of strength) to judge whether the strength of the concrete is normal, generally required to go to 75% of the design strength, 7 days is mainly used in the project for the removal of the formwork, the general site control in about 7 days to dismantle the formwork, at this time to judge whether the safety of the formwork is achieved through the strength of the 7-day concrete, 28 angels as to assess the quality of the project, That is to say, to judge whether the concrete used in the project meets the design requirements (the national standard and design are generally based on the 28-day strength).

The 7-day intensity is presumed to be 28-day intensity... The cut-off time is convenient for the development of the mix ratio.

As long as the concrete test block is made and cured according to the specification, the strength can basically reach about 50% of the design value in 3 days, 70-80% in 7 days, and about 100% in 28 days, or even higher.

Artificially create certain humidity and temperature conditions, so that the newly poured concrete can be normal or accelerate its hardening and strength growth. The fact that concrete can gradually harden and increase in strength is the result of cement hydration, which requires certain temperature and humidity conditions. If the surrounding environment does not exist for such conditions, the concrete needs to be cured manually.

The curing time depends on the cement type, such as ordinary Portland cement concrete, not less than 7 days and nights. The latter is suitable for high-rise structures and large-area concrete structures that are not easy to be cured by sprinkling, and the perchloroethylene resin plastic solution is sprayed on the concrete surface with a spray gun.

After the solution volatilizes, a thin film is formed on the surface of the concrete, which isolates the concrete from the air, prevents the evaporation of water in the concrete, ensures the normal hydration of the cement, and the film can age and fall off by itself after the curing is completed.

The strength of concrete is expressed by its compressive strength, and the commonly used leadless strength standard refers to the compressive strength macro value of concrete at 3 days, 7 days, 14 days and 28 days after the end of the curing period.

The specific strength value varies depending on the concrete mix ratio, aggregate type, cement variety, curing conditions and other factors. The following are some common concrete compressive strength value ranges:

3-day strength: generally 60-70% of the design strength of the concrete 7-day strength: generally 70-80% of the design strength of the concrete 14-day strength: generally 85% of the design strength of the concrete

28-day strength: generally 100% of the design strength of concrete, it should be noted that these strength values are only used as a reference, and the specific strength should be tested and evaluated according to the actual situation of concrete.

As long as the concrete test block is made and cured according to the specification, the strength of the concrete can basically reach about 50% of the design value in 3 days, 58% in 7 days, 79% in 14 days, and about 100% in 28 days, or even higher.

Concrete can reach strength after 28 days, during which one is the solidification phase and the other is the growth phase. The setting time is within 8 hours after pouring, and the strength growth period does not reach the expected strength until 28 days after pouring, and will slowly grow over the next 6 months.

Instructions for concrete curing times

1. The standard curing period of concrete specimens is 28 days, and its compressive strength is tested.

2. After the concrete is poured, it should be covered and moisturized within 12 hours. For the concrete mixed with silicate cement, silica cement and slag cement, the curing time should not be less than 7 days; For concrete mixed with coagulant admixtures or impermeability requirements, the curing time should be more than 14 days.

3. After the construction of the overall surface layer project is completed, the maintenance time should be more than 7 days. After the compressive strength reaches 5MPa, it can be walked on concrete.

In a humid environment at room temperature, concrete strength.

It is a continuous development process, but the intensity development rate is high in the early stage (14 days ago), and the intensity growth rate gradually decreases in the later stage. This consists of the mineral composition of cement, cementitious material.

Composition, water addition, environmental humidity and other factors are determined. In the spring of the laboratory environment, there are tests that prove that the strength of concrete will increase slightly after 10 years. Therefore, it is normal to have a large increase in intensity after 28 days.

General rule: use pure silicate.

Fast-hardening cement, mixed with silica fume.

or ultra-fine mineral powder, the early strength of concrete is developed into a block, and the strength growth potential in the later stage is small. The use of mineral admixtures such as fly ash and mineral powder will have a slower strength development in the early stage, but there will be a greater strength increase in the later stage, because the hydration process of fly ash and ordinary fineness mineral powder is slower. Therefore, some projects set the standard strength test age of fly ash concrete as 56 days, and even set the test age as 90 days for large amount of fly ash concrete.

The aim is to make the most of the actual strength of the fly ash concrete, as the structure is not put into use after 28 days, and it often takes three months or more for the load to actually bear the load.

In addition, the strength of concrete may also shrink, mostly in concrete structures that are not scattered. For example, internal destruction such as alkali-aggregate reactions, delayed aluminite formation, etc., has occurred. For example, by the action of damaging factors in the environment, high temperature dehydration (above 100 degrees), freeze-thaw, chemical corrosion, sulfate.

erosion, etc.

Summary. Dear Glad to answer for you. Concrete strength refers to the degree of pressure that the concrete is subjected to, and is usually measured with a "standard cube", that is, a concrete test block with a size of 150mm 150mm 150mm.

The detection of concrete strength is generally carried out for 3 days, 7 days, 14 days and 28 days, etc., and the following is the standard strength value of concrete in these periods (in mpa): -3 days: generally tested on the 3rd day after the concrete is poured, the strength of the concrete is about 10-40mpa at this time; - 7 days:

On the 7th day after the concrete is poured, the strength of the concrete is about 20-50MPa; - 14 days: Tested on the 14th day after the concrete is poured, the strength of the concrete is about 30-60MPa at this time; - 28 days: Tested on the 28th day after the concrete is poured, the strength of the concrete at this time is about 40-90MPa.

Dear Glad to answer for you. Concrete strength refers to the degree of pressure on the concrete, which is usually measured by a "standard cube", that is, a concrete test block with a cavity size of 150mm 150mm 150mm. The test of concrete strength is generally carried out for 3 days, 7 days, 14 days and 28 days, etc., and the following is the standard strength value of concrete in these periods (in MPa):

3 days: Generally, the test is carried out on the 3rd day after the concrete is poured, and the strength of the concrete is about 10-40MPa at this time; - 7 days: Tested on the 7th day after the concrete is poured, the strength of the concrete at this time is about 20-50MPa; - 14 days:

Tested on the 14th day after the concrete is poured, the strength of the concrete at this time is about 30-60MPa; - 28 days: Tested on the 28th day after the concrete is poured, the strength of the concrete is about 40-90MPa at this time.

It should be noted that the detection value of concrete strength is related to many factors such as the pouring method of concrete concrete mixed date, concrete mix ratio, and the quality of materials.

Summary. Calculation method: LG7 7 = LG28 28, where LG7 and LG28 are 7 days and 28 days of concrete strength, respectively.

1. The strength grade of concrete refers to the compressive strength of 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).

2. The compressive strength of concrete is obtained through the test, 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.

Knowing how to calculate the 7-day strength of concrete, how much is the strength of 28 days.

Hello, I have seen your question and am sorting out the answer, please wait a while

Calculation method: LG7 7 = LG28 28, where LG7 and LG28 are 7 days and 28 days of concrete strength, respectively. 1. The strength grade of concrete refers to the compressive strength of 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). 2. The compressive strength of concrete is obtained through the test, 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.

AbstractThe intensity can reach about 50% of the design value in 3 days, 70-80% in 7 days, and 100% in 28 days, or even higher.

What is the strength of concrete for 3 days, 7 days, 14 days, 28 days.

The intensity can reach about 50% of the design value in 3 days, 70-80% in 7 days, and about 100% in 28 days, or even higher.

3 days, 7 days, and 14 days are the reference intensity and cannot be used as the final evaluation basis, and 28 days are the standard intensity and cannot be used as the final evaluation basis.

Concrete has the characteristics of abundant raw materials and simple production process, so that the amount of concrete is getting larger and larger, and concrete has the characteristics of high compressive strength, good durability and wide range of strength grades. These characteristics make it widely used, not only in various civil engineering projects, but also in shipbuilding, machinery industry, marine development, geothermal engineering, etc., concrete is also an important material.

Hope it can help you, thank you.