Dear Muhammad Husain!

Cubes are based on BS code and its strength is higher than cylinders because of the platen effect of cubes.Generly,cube strength is 20 % higher than cylinder strength.But cylinder strength is precise and nearly equal to actual strength (field concrete) so it is recommended to use cylinder instead of cubes.

Cylinder of course is better because:

- its shape proportion is same as stress distribution under axial load

- Direction of loading is normal to concrete layer (like site condition).

I agree with Abbas Cylinder of course is better

Actually the cylinder sample gives more accurate results because there is no interference between load transferring zones at top and bottom of the samples

Dear Mr. Abdur Rasheed ….. you mention actual strength (field concrete), what do you mean ?

I think our design calculations assume it.

Thank you Dr. Abbas…

Dear Mohammad Husain!

I mean the cylinder strength is closer to the actual strength of concrete.For example,if we design concrete for 5000 psi at 28 days then cylider strength will be closer to 5000 psi compared to cubes strength which may be much higher than 5000 psi.That is why when we test cubes at site,at the end we multipy that strength by 0.8 factor to get cylinder strength.

Thanks!

Dear Mr. Abdur Rasheed. Thanks first

How you can get the actual strength of concrete ? 

I think its only assumption.

''EN'' standard defines concrete class with both, cylinder and cube strength. First number is cylinder, second is cube strength: C 16/20, 20/25, 25/30, 30/37,... So both strengths are acceptable in quality control.

Cylinder strength is also used in structure design, so it is closer to truth. However, at cylinder sample you have to polish upper surface, or flatten it in different way, which is not the case with cubes.

Dear Mr. Tomislav Feric

First thank you...

Cylinder strength is not closer to truth. Codes and specification design procedures assume that.

Both test methods are acceptable but beware if you did a cube and a cylinder of the same mix you will get different results. cylinders typically give 0.8 of the cylinder. in the british standards concrete is specified as C16/20, C20/25, C25/30, C30/37, C35/45, C40/50, C45/55, C50/60 which means if the concrete specified is  C25/30 then the teast result should be 25MPa with a cylinder or 30MPa with a cube.

cylinder is better.

Thank you Mr. Des Vlietstra.

Thank you Dr. Abbas.

the compressive strength tested by the cube(150mm or 100mm), and the ratio between the strength test on cubic sample is 1.0/0.95。 According to some experiments , the strength of cube is about less 10MPa than that of cylindrical sample. Some researchers found the cube/cylinder strength ratio decreases by the increase in concrete strength. A ratio of 1.15 to 1.25 is expected for normal concrete (25 to 50MPa cube strength), while this value reaches 1.1 for high strength concrete (55 to 85MPa cube strength).

European standards, in strength classification, use both cylinder and cube strength: C 25/30, 30/37, 35/45 and so on. In structure design, cylinder strength is used.

Cylinder strength is lower because height to diameter ratio is 2 and at cube height to side ratio is 1. Also, friction at upper and lower plane of specimen with plates is different so that influences too.

Further Reading

To Determine The Compressive Strength of Concrete Cubes And Cylinders.

To determine the compressive strength of hardened concrete by using the Schmidt hammer

Dear Dr.Mohannad,

Totally agreed with Dr.Abbas,

Cylindrical samples simulates site loading

Mostly cubes in Iraq.

cylinder is better.it has limited effect of platen restraint.

Depends on the one that is available in your lab.

CYLINDER SAMPLES ARE SUPERIOR TO CUBES CONSIDERING THE DIRECTION OF LOADING, ASPECT RATIO

First: * resistance in the cylinder = (0.8) the resistance in the cube (one-tenth of the cube) and from this we conclude that the cubes are the largest resistance of the cylinder, so about designing concrete, we perform the calculations on the basis that the concrete will take the shape of the cylinder, but in most cases, it takes the shape of the cube... This gives greater resistance upon examination. In the sense that through many experiments, it was found that cubic samples have more resistance than cylindrical ones by a percentage of (% to %) due to the increase in the sample size in the cylinder than in the cube, which increases the possibility of weak areas in it, which leads to The weakness of the resistance and also the increase in the volume increases the distance between the two ends of the sample, which makes the sample in the middle exposed to a greater force (strain) and therefore a decrease in resistance. Second: * Consider the result successful if the following two conditions are met: 1- The average fracture resistance of the sample is not less than the design resistance after a day.2- That the fracture resistance of any of the specimens shall not be less than % of the design resistance.

Dear Mohannad,

The cylinder test gives you the best results as compared to the cube because in cube platen effect comes due to which cube gives greater strength as compared to cylinder. Due to friction between plate and cube surface triaxial compression is high in the cube as compared to the cube. Cube strength = 1.25 cylinder strength.@Mohannad

Regards

Fazal Hussain

Cylindrical cube is better because its shape proportion is the same as stress distribution under axial loadand no platen effects.