Characteristic Strength of Concrete

Concrete uses different constituents like coarse aggregate, fine aggregate, water, plasticisers, etc. All of the components come in different variety, and thus the concrete prepared from the same mixture and taken from the same tray can have significantly varying compressive strengths. This variation can be controlled by assuring quality control at the site.
Some ways to have reasonable quality control on the concrete mix can be the following:
  • Controlling the size of aggregates by proper sieve analysis. 
  • For coarse aggregates, there are tests like flakiness test and elongation test.
  • For fine aggregates, bulking of sand should be in a reasonable range.
  • Water content should be proper.
  • Mixing of components should be uniform.





The graph becomes more and more flatter when quality control is more miserable at the site. If quality control is better, then a sharp peak can be observed. One thing can be concluded that if more specimens are near average compressive strength, your mix is better quality-wise.
The compressive strength of concrete is studied in terms of standard deviation and coefficient of variation. 
                                             Coefficient of variation = Standard Deviation / Mean strength

Experimentally it is found that the coefficient of variation of concrete varies between 0.01 to 
0.02. Here, we can notice that if the coefficient of variation is near 0.01, then it will infer to reasonable quality control on the concrete mix.
Followings are some essential points regarding testing of concrete block:
  • 150 mm cube specimen is tested under uniaxial compression after 28 days of curing.
  • As per IS 456: 2000 there should be at least 3 specimens tested, and the final result should be average of three.
  • The individual strength of blocks should not be more or less than 15% of the average compressive strength. If it happens, the test should be discarded.
Now the question arises why do we need characteristic strength?

From the above statements, we can now understand the uncertainty of the compressive strength values of concrete. So, we need a certain minimum value from a mix. "The characteristic strength is that strength of material below which not more than 5% of tests results are expected to fail".
So, one final conclusion we can make is that the mean strength of concrete is always higher than the characteristic compressive strength of the concrete. 

If you want to read about types of concrete, then follow this link:




 

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