Size Effect of Concrete Why is the smaller the size of the cube specimen, the greater the compressiv

Hoop effect, with relatively few internal defects at volume.

When carrying out the concrete strength test, the size, shape, surface state, and moisture content of the specimen.

Experimental factors such as experimental loading speed will affect the test results of concrete strength experiments. a.Specimen shape and size Determination of concrete cubes.

Compressive strength of the specimen.

It is also possible to choose different specimen sizes according to the size of the maximum particle size of the coarse aggregate. However, the different sizes and shapes of the specimens will affect the results of the compressive strength measurement of the specimens. 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 pressing plate is smaller than that of concrete, so the frictional force is formed on the compression surface of the pressing plate and the concrete specimen, which plays a restraining role on the transverse expansion of the specimen, and this restraining effect is called"Hoop effect"。"Hoop effect"It has the effect of improving the compressive strength of concrete.

The farther away from the platen,"Hoop effect"small, this effect disappears outside the range of about the length of the specimen side from the compression surface of the specimen. Concrete Compression Failure When performing a strength test, the larger the size of the specimen, the lower the measured strength value. There are two reasons for this:

One is"Hoop effect"；Second, due to the high probability of defects such as pores, cracks and local poor defects in large specimens, the strength of the material is reduced. National standard GBJ107-87 "Concrete Strength Inspection and Evaluation Standard".

The cube specimen with a side length of 150mm is specified as the 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 conversion factor shall be as follows.

The minimum allowable size of the concrete compressive strength test block is the maximum particle diameter of the aggregate (mm) Conversion factor The size of the test block (mm) 100 100 100 (non-standard test block) 40 150 150 150 (standard test block) 60 200 200 200 (non-standard test block) BSurface state When the compression surface of the concrete is very smooth (if there is grease), due to the reduction of the friction between the pressing plate and the surface of the specimen, the hoop effect will be reduced, and the specimen will appear vertical cracks and fail, and the measured concrete strength value is low. c.

Degree of moisture content The higher the moisture content of the concrete specimen, the lower its strength. d.Loading speed When the compressive test of the concrete specimen is carried out, the loading speed is too fast, and the speed of material crack propagation is slower than the load increase rate, so the measured strength value is high.

When carrying out the compressive strength test of concrete cubes, it should be carried out according to the specified loading speed.

To sum up, through the analysis of the influencing factors of concrete strength, the measures to improve the strength of concrete are: the use of cement with high strength grade; Low water-cement ratio is adopted.

Adopt aggregates with less harmful impurities, good gradation, appropriate particles and reasonable sand rate; Adopt reasonable mechanical stirring and vibration process; Maintain a reasonable curing temperature and a certain humidity, and use moist heat if possible.

Maintenance; Incorporation with suitable concrete admixtures.

and admixtures.

The size effect of a material is generally considered to be related to the mesostructure of the material, and the strength of the material is inversely proportional to the internal defects of the material, and the smaller the size, the fewer the internal defects, the higher the strength. If it refers to the conversion factor of the concrete cube strength test, it is related to the hoop effect caused by the friction between the specimen and the press surface.

Hoop effect: The friction force between the pressing plate and the compression surface of the concrete specimen is formed, and this friction force plays a restraining role on the lateral expansion of the specimen.

When subjected to the same force, the larger the size of the concrete, the larger the expansion range, and the binding force of the pressing plate on the concrete is also larger.

Conversely, the smaller the size of the concrete, the less binding it is.

You can understand it by drawing a force diagram.

Due to the fact that the angle angle ; The large concrete waist is very thin and is broken first, so it is less strong (the shaded area is the volume of destruction, you should have a similar diagram in your book).

In addition to the hoop effect, there is another reason: the probability of pores, cracks and other defects in large-sized concrete is relatively large and numerous, which reduces the strength of the material, so the strength is also small.

Force analysis as shown in the figure below.

The smaller the size of the specimen, the higher the measured strength, which is due to the increased probability of voids, cracks or local defects in the large specimen, which reduces the strength.

It has nothing to do with the size, the compressive strength of concrete is only related to its strength grade, you are miscalculated, non-standard sizes are to be converted.