Concrete Compressive Strength, Cube Test, Procedure, Results

The compressive strength of concrete provides an idea of ​​all the characteristics of concrete. With this individual test, someone can decide whether or not the concrete was properly worked out. The compressive strength of concrete for general construction ranges from 15 MPa (2200 psi) to 30 MPa (4400 psi) and above in commercial and industrial structures

The compressive strength of concrete depends on many factors such as the ratio of water to cement, cement strength, quality of concrete materials, quality control during concrete production, etc.

The compressive strength is tested on either the cube or the cylinder. Various standard codes recommend the use of a concrete cylinder or concrete cube as a standard sample for testing. The American Society for Material Test ASTM C39 / C39M provides a standard test method for the compressive strength of cylindrical concrete samples.

Define the compressive strength of the concrete

Compressive strength is the ability of a material or structure to carry loads on its surface without any breakage or deviation. The material under pressure tends to reduce volume, while in tension, the volume lengthens.

The compressive strength of concrete

The compressive strength formula of concrete

The compressive strength formula of any material is the load applied at the point of failure on the cross-sectional area of ​​the face to which the load was applied.

Procedure: Test the compressive strength of concrete cubes

For cube testing, two types of samples are used, either cubes at a size of 15 cm x 15 cm x 15 cm or 10 cm x 10 cm x 10 cm depending on the volume of the aggregate. For most businesses, cubic molds of 15 cm x 15 cm x 15 cm are used.

This concrete is poured into the mold and diluted properly so that there are no voids. After 24 hours these molds are removed and test samples are put in the water for treatment. The top surface of this sample should be equal and smooth. This is done by applying cement paste and spreading smoothly over the entire sample area.

These samples are tested by a pressure test machine 7 days after processing or 28 days after processing. The load should be applied gradually at a rate of 140 kg / cm2 per minute until the samples fail. The load upon failure divided by the sample area gives the compressive strength of the concrete.

Below is a test of the compressive strength of the concrete cubes

  • A device for testing a concrete cube
  • Pressure testing machine
  • Prepare the concrete cube sample
  • The proportions of the materials needed to make these samples are of the same concrete used in this field.

Example: A sample of 6 cubes with a size of 15 cm and a height of 15 m or higher

  • Concrete is mixed to test the cube, mixing concrete either by hand or in a laboratory batch mixer

Hand Mixing: Mix the cement and fine aggregate on a highly absorbent platform without water until the mixture mixes well and has a uniform color.

Coarse aggregate is added and mixed with cement and fine aggregate until coarse aggregates are uniformly distributed throughout the batch.

Add and mix the water until the concrete looks homogeneous and the desired consistency

  • Sampling of cubes for testing: by filling concrete in molds in layers with a thickness of about 5 cm compressed each layer with a minimum of 35 limits per layer using a tamping rod (steel strip diameter 16 mm and length 60 cm, a pointed bullet at the bottom end)

The test samples are stored in moist air for 24 hours and after this period the samples are marked and removed from the molds and remain immersed in clear fresh water until they are removed before the test.

Precautions when testing:

The water for treatment should be tested every 7 days, and the water temperature should be at + 27 ° -2.

Concrete cube test procedures

Remove the sample from the water after the specified processing time and wipe off the excess water from the surface.

  • Take the sample dimension to the nearest 0.2 m
  • Surface cleaning of the bearing of the test machine
  • Place the sample in the machine so that the load is applied to the opposite sides of the cast cube.
  • Centrally aligning the sample on the machine base plate.
  • Gently rotate the moving part by hand so that it touches the top surface of the sample.
  • Carry out the load gradually without shock, and continuously at a rate of 140 kg / cm2 / min until the sample fails
  • Record the maximum load and note any unusual features of the failure type.

Note: At least three samples must be tested for each specific age. If the strength of any sample varies by more than 15 percent of the mean strength, the results of this sample should be rejected. An average of three samples gives concrete cracking strength. And the requirements of tangible power.

Compressive strength calculations: Have the following steps:

Cube size = 15 cm x 15 cm x 15 cm

Sample area (calculated from the average sample size) = 225 cm2

Distinguishing compressive strength (f ck) in 7 days =

Expected maximum load = fck x area x f.s

A similar calculation must be made for 28 days of compression strength

Cube test reports

The report contains the following information: identification mark – test date – sample age – treatment of conditions, including the date the sample was made

  • Broken faces of concrete and fracture type appear, if unusual

Concrete cube test results

Average pressure strength of the concrete cube = ………. n / mm2 (in 7 days)

Average compressive strength of a concrete cube = ………. N / mm2 in 28 days

القوة الضاغطة للخرسانة

The compressive strength of concrete in different eras

The strength of concrete increases with age. The table shows the strength of concrete at different ages compared to the strength in 28 days after casting.

Duration (age)Power ratio
day16%
3 day40%
7 day65%
14 day90%
28 day99%

 

Compressive strength of different grades of concrete in 7 and 28 days

Concrete rateLowest compressive strength in 7 days N/mm2Compressive strength properties in 28 days N/mm2
15 m1015
20 m13.520
25 m1725
30 m2030
35 m23.535
40 m2740
45 m3045

Read also: Smart Building Materials – Applications in Civil Engineering

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