Expert: Arshad Khan - 4/14/2009

Question
QUESTION: Dear sir, what is the relation between test results are obtained from cylindrical specimen & rect angular block in concrete compression for same grade of concrete. If certain results are taken from a rectangular block can we convert these results in the approximate value of cylindrical test? or from c test to rect. Please explain the relation in ratio.

ANSWER: Hi Vijay,

Generally a cylinder test strength is between 5% and 25% less than a cube test strength for a given concrete mix, the percentage difference decreasing with an increase in the concrete strength. See table below:

Cylinder Strength  Cube Strength
MPa Psi          MPa Psi
12 1740          15 2175
16 2320          20 2900
20 2900          25 3625
25 3625          30 4350
30 4350          37 5365
35 5075          45 6525
50 7250          60 8700


---------- FOLLOW-UP ----------

QUESTION: Again I am gratful to you, Now please illustrate that which strength should be reffered from design point of view. Generally designer says C35/20 & we as consultant receive the cubical strength & remain silent so plz clarify this also. Designer never tells about this & in IS & BS codes also separate design coiefficients are not given from this aspect of view.BS 8110-3 & IS-Sp-16. Thanks

Answer
Hi Vijay,

You need to be careful as to which codes the concrete is referring to. If it is to the old codes, including BS 8110, then C35/20 means 35N/mm2 cube strength at 28 days and that the nominal aggregate size is 20mm.

If you look at recent codes, then the C30/37 would mean 30N/mm2 cylinder strength or 37N/mm2 cube strength. The relevant new codes are:

BS EN 206-1:2000 Concrete. Specification, performance, production and conformity
BS 8500:2002, Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier