Size of specimen and compressive strength of concrete

[ichabodgrant]

Hello everyone.
I have a question about compressive strength of concrete. It is said that (in my country) 150 mm x 150 mm x 150 mm concrete cube is used for concrete testing. Someone asks me why 100 mm x 100 mm x 100 mm is not used. In fact, I know that sometimes it is allowed or even preferred to use 100 mm one instead of 150 mm one. I have looked up some research papers and they mention that it depends of the designed compressive strength of concrete.

One explanation says the applied load of a compression machine is often a definite value, let's say 2000 kN. If the concrete produced is of a designed compressive strength 60 MPa, then 150 mm cube is sufficient for the test. If a higher strength concrete is produced, then a 100 mm cube is more suitable. (As stress = Force/Area, same force smaller area can give a higher stress)

My question is that what will happen if I make a 100 mm cube from the same concrete mixture and put it onto the compression machine? Is the resulting compressive strength higher or lower than that of a 150 mm cube? Some people say it should be slightly lower. What is the reasoning?

 

[PhanthomJay]

I've always specified 6 inch diameter by 12 inch cylinders(150 mm x 300 mm) although the smaller size 100 mm x 200 mm cylinders are allowed by Standards as long as aggregate size is smaller than about 3 cm. There isn't much difference in results as long as more than one cylinder is tested to allow for variability. The smaller size as you mentioned requires less test load and is more suitable for the higher strength concretes I suppose, but being old school I like the larger ones.

 

[dipstik]

here is some info on correction factors for different geometries:

http://www.buildingresearch.com.np/services/mt/mt1.php