ASTM C 31–03a
Standard Practice for Making and Curing Concrete Test Specimens in the Field
6. Testing Requirements
6.1 Blah blah blah...The cylinder diameter shall be at least 3 times the nominal maximum size of the coarse aggregate. Blah blah blah ....For acceptance testing for specified compressive strength, cylinders shall be 6 by 12 in. [150 by 300 mm] or when specified 4 by 8 in. [100 3 200 mm] (Note 2).
astm c31-06 changes that to remove "when specified"...HOWEVER, ibc has not yet adopted the new method so be sure to choose the appropriate method. i'm 99% certain that aci has adopted the astm c31-06 with the latest version out a few weeks ago and they also bumped up the number of specimens required at 28-days. i haven't received my copy so i haven't personally layed eyes on the pages but i did see that the noted changes would take place on this latest revision.
My understanding is that ACI frowns upon the 4x8 cylinders because they give slightly higher breaks (about 4% - 8%), and the standard deviation of the breaks is also higher.
the current 318-08 allows the 4x8's now. i concur that my understading is that the deviation is higher so they account for that by requiring an additional cylinder at 28-days. i don't know where the justification for the extra cylinder came from whether it was from real data or theoretical studies. i suspect real trial data...that's my hunch.
No real data...just hunches. There was a paper a long time ago that stated that the 4x8 size generated higher breaks but that was probably due to the fact that the tested 4x8s and 6x12s equally during casting, i.e. they did both in 3 lifts, not 3 lifts for 6x12s and 2 lifts for 4x8s as is current practice. You won't see a major difference now. In fact the Canadiens had a correction factor of 95% in their first implementation of the smaller size, but that went away because the results were correlating well. Several papers are in the work to establish a statistical document to show correlation for us Americans, but until then, three 4x8s or two 6x12s. ACI is just being conservative until the document is made.
