block, mortar, grout and prism strength relation

huangyhg

block, mortar, grout and prism strength relation
can someone give me a quick explanation as to why when using a cmu strength with 1900 psi (average of 3 units) and a type "s" mortar of 1800 psi and grout of 2500 psi that we only use an assumed prism strength of f'm = 1500 psi?
i have gone thru this before but my old age is catching up with me and i cannot seem to remember the reasoning.
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i use 1500 psi because it is the minimum f'm required by the aci 530.  i know that must be supplied and it helps to keep me from running into problems if the cmu that is actually laid doesn't meet, say, f'm = 1900 psi.  sometimes i think it's overconservative but since that's the standard way to go where i work i don't rock the boat.  it seems especially overconservative when most of the grout that is placed is 3000 psi regardless of 2000 psi (another aci minimum) being spec'd.