Why does the size of the concrete test block affect the compressive strength value of concrete?

The shape and size of the concrete test block are mainly due to the following two aspects of the compressive strength of the test block.

Values have an impact:

1、"Hoop effect"。Because when the concrete specimen is compressed on the press, it also follows the Poisson's ratio while the longitudinal deformation occurs along the loading direction.

The effect produces lateral expansion. The transverse expansion of the steel pressure plate is smaller than that of concrete, so the friction force is formed on the compression surface of the pressure plate and the concrete specimen, which plays a restraining role on the transverse expansion of the specimen, and this constraint is called"Hoop effect"。Typically, the smaller the size of the test block"Hoop effect"The effect is obvious, and the compressive strength of concrete will be improved.

2. Due to the large probability of defects such as pores, cracks and local poor defects in the large specimen, the strength of the material is reduced.

Therefore, the national standard GBJ107-87 "Concrete Strength Inspection and Evaluation Standard".

A cube with a specified side length of 150 mm.

The specimen is a standard specimen. When a non-standard size specimen is used, its compressive strength should be converted into the compressive strength of the standard specimen.

The guy upstairs said it right. Let me give you a popular example, is a section of steel bars of the same diameter but of different lengths shorter and less likely to be damaged (maybe this example is not very appropriate). Because there is friction between the machine and the test block when doing the compressive strength test, the smaller the test block, the greater the restraining effect on the test block, just like the hoop effect of the steel bar.

Another small aspect I personally think is that the test blocks are all defective, the larger the test block, assuming that the probability of my defects is the same, then is the larger the easier it is to destroy? Personal opinion ha, discreet, thank you!

What is the compressive strength?

Specimen compressive strength mpa = specimen failure load (n).

Bearing area of the specimen (mm?).）。Calculations should be accurate to:

When the concrete strength class is C60, the strength values measured with non-standard specimens should be multiplied by the size conversion factor. Its value is: for 100mm 100mm 100mm specimen, when the concrete strength grade c60, it is advisable to use the standard specimen 150mm 150mm 150mm specimen.

The conversion factor of 150mm 150mm 150mm specimen is.

For example, the failure load of three test blocks of 100*100*100mm is 335kn, 352kn and 318kn, and the compressive strength is equal to (335000+352000+318000) 3 (100*100).

Because 100*100*100 is a non-standard specimen, its compressive strength coefficient ).

The failure load of the three test blocks of 150*150*150mm is 424kn, 401kn, and 396kn, and the compressive strength is equal to (424000+401000+396000) 3 (150*150), because 150*150*150 is the standard specimen, so its compressive strength coefficient is ().

Fabrication and maintenance of test blocks.

The test block participating in the concrete strength assessment is divided into standard test block and homotrophic test block, standard test block refers to the test block cured in the standard maintenance room, and the standard test block is in the temperature 20 2 range, humidity is greater than 95%, and the curing is 28 days; The isotrophic test block refers to the test block made of concrete randomly selected at the pouring site, and the isotrophic test block is a test block randomly selected at the construction site and accumulated to 600 degrees Celsius at the daily average temperature according to the on-site maintenance conditions.

At the same time, the "Specification" also stipulates that the equivalent maintenance period should not be less than 14d. In the case of high-rise building construction, usually we also have to leave the demoulding test block, in winter, the temperature is lower, the strength of concrete develops slowly, which requires the age of demoulding is longer, in summer, the temperature is high, the strength of concrete develops faster, generally under the on-site curing conditions of 7d, the concrete strength can reach more than 90%, and the age of demoulding can be appropriately shortened.

Compressive strength, failure load, compressive area.

Compressive strength MPA

The failure load is the data you press out, and the unit is kn

The compression area is 150*150 1000

i.e. mpa=kn

Kiss! First of all, the density of general concrete is about 2400 kilograms per cubic meter, and 1400 dare not say no, anyway, I have never seen it.

The volume of a test block is cubic meters = cubic decimeters = liters.

The strength values of the concrete test block are as follows.

When the pressure test is carried out on the coagulation upper test block, the side elevation at the time of sampling should be used as the compression surface.

1. When the difference between the maximum value and the minimum value and the intermediate value of the three specimens does not exceed the centrosity of the intermediate value, the arithmetic average value of the strengths of the three specimens is converted into the compressive strength of 150mm cube as the compressive strength value of the group of specimens.

2. When the difference between one of the maximum or minimum values and the intermediate value of the strength of the three specimens exceeds the clever % of the intermediate value, the intermediate value is taken and converted into the compressive strength of a 150mm cube.

3. When the difference between the maximum and minimum values and the intermediate value of the strength of the three specimens exceeds the clever % of the intermediate value, the group of specimens should not be used as the basis for strength evaluation. This phenomenon is called excessive discreteness of stick concrete.

First of all, the size of the test block should be determined whether the side length is 150mm or 100mm, so that the compression area can be known, according to the failure load and strength level provided, it is speculated that it is a standard specimen with a side length of 100mm, then the compression = 100 100 = 10000mm

Secondly, the average of the three values should be taken in general, but the data, so the unit should be kn).

Final average cubic compressive strength = . Equivalent to standard 150mm cube standard specimen compressive strength =.

%=.

Concrete. Concrete, referred to 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 aggregate; Cement concrete, also known as ordinary concrete, is widely used in civil engineering engineering when it is mixed with water (which can contain admixtures and admixtures) in a certain proportion and is mixed to obtain cement concrete.

How to calculate the strength of concrete test blocks? Kiss! Hello, I am glad to answer for you<> personally take the dust to talk about the arithmetic mean of the strength of the three specimens as the strength of each group of specimens representative value:

The 150*150mm specimen is ( ) 3= The specimen is ( ) 3=. When the difference between the maximum or minimum value of the intensity and the median value in a set of specimens exceeds 15% of the median pie collision, the median value is taken. The strength of this group of specimens represents the value.

Hope mine can help you <>

Do you have any other questions?

If it is a single group of strength judgment, the strength generally needs its strength grade, such as the C20 test block, its representative value should be 20MPa, if it is evaluated, the strength value should be 20MPa, that is to say, its strength should be 23MPa, to be qualified (C60 concrete is so).

If the number of samples (specimens) groups is more than 2 groups, less than 10 groups, then, their average value also has to be the multiple of this strength grade, and the minimum value should be greater than or equal to 95% of this strength grade (take C20 as an example: the minimum value requires 20 95% = 19MPa), meet these two conditions, the strength of this batch of concrete is assessed as qualified, otherwise it is unqualified.

If the number of sample groups is greater than 10 groups, it is necessary to carry out mathematical statistics, calculate the standard deviation of the batch of concrete, and then substitute the conformity assessment coefficient, and evaluate it after calculation. For the specific evaluation method, please refer to GB T 50107-2010 "Concrete Strength Inspection and Evaluation Standard".