Block, Mortar, Grout and Prism Strength Relation 1

[jike]

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.

 

[UcfSE]

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.