Concrete Project - All Laboratories Including Calculus

Strength of concrete in compression

Main reasons :

• Quality control

• Design of structural memebers

Both of them are based on characteristic strength in compression of concrete -

In order to find these main properties we have to test cubes specimen, cylindres specimen, cores specimen.

This laboratory consits in analyzing the way the concrete acts under compressive stresses. Different samples will be used and we will experimentally determine the values of the compressive strength.

Compressive strength determined on cubes

Compressive strength determined on cube has served for defining the mark of the concrete for a long time. The mark is the value from the standard marks scale, just inferior to the average compressive strength, determined on cubes, with a side of 200 mm, that had been manufactured, kept and tried after 28 days.

Compressive strength is computed with the relation:

–is the force at failure [N]

A- is the concrete area [mm2]

For determining the compressive strength, there are two criteria :

• It is applied when there are considered the results of 6 or more samples noted X1, X2, ….Xn.

The compressive strength must accomplish the following conditions:

- the mean value of strengths

- the minimum value of strengths

– is the characteristic strength to compression at 28 days (function of the concrete grade) determined on cylinders or cubes;

and k – constants that depends on the number of samples and security degree (Table 1.3)

n – number of control samples

• It is applied in the case when conformity is appreciated considering the results of 3 samples X1, X2, X3.

The compressive strength must accomplish the following conditions:

- is considered the average value of results.

Concrete failure in compression of cubic samples with

friction/ without friction.

Cylindrical strength

The cylindrical strength is determined on samples with 150 mm diameter and 300 mm height.

The value of the compressive strength is determined with relation:

Pr –is the force at failure [N]

Ab- is the contact concrete area, Ab = [mm2]

The relation between the cylindrical and cubical strength:

fcyl = 0,83 fcube

Prismatic strength of the concrete

The prismatic strength is given by the unit breaking stress of prismatic test samples, with ,(100x100x500mm, 100x100x550 mm, 200x200x700 mm, 200x200x800 mm), subjected to axial compression.

The determination of the prismatic strength cannot be made by practical tests, but with the help of empirical relations. Gvozdev proposed the relations:

fc - is the cubic strength.

Crushing strength of the concrete

The crushing strength is given by the unit breaking stress of cubic test samples, of 200x200x200 mm, subjected to axial compression. On the middle of superior surface of the cube there is placed a metallic plate, 50x50 mm (Amp=2500 mm2). The axial compression force is transmitted by the metallic plate to the cube.

fcrush=