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compressive strength testing on cored co

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Steve,

In the 1950's, 2,000 psi and 2,500 psi concrete was the commonly specified 
concrete strengths.  3,000 psi concrete was *strong* concrete.  *Very strong* 
concrete was 3,750 psi  (Yes, 3,750 psi; not 3,500 or 4,000, but 3,750 psi.  
CRSI had a whole handbook based on 3,750 psi concrete.)  Design working 
stress was .45f'(c) in flexure.

You might want to get a couple of more cores from around where the 1260 psi 
core came from to see if there was a batch of real low strength concrete 
placed or whether this was a bad test.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Steve Bian wrote:

. > The compressive test, strangely shows really low compressive strength
. > of the three cores: 2520 & 2410 psi respectively on two cores from one
. > cell and only 1260 psi from another cell!!!!!

. > For pour in place concrete, these strength is really low although I did
. > not find the design strength on record drawings. I did expect minimum
. > 3,500 psi - 4000 psi for this structural concrete.