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Re: Spec needed for 3000 psi concrete

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I think your taking the more difficult route in trying to determine what should be in the concrete mix design.  Concrete mix designs can be very complex, depending upon the additive you use (accelerators, inhibitors, fly ash substitution, aggregate size and type, etc).  Some design offices come up with there own mix designs, others farm the mix design out to others...specialist in concrete mix design who work with concrete on a day to day basis and know all about the complexity of these mixes.... 
Our office requires the concrete mix design to be stamped by a licensed civil/structural engineer in the state that the concrete work is occurring that will take responsibility for the adequacy of the concrete mix design to come up to your required design strength in 28 days.  Some testing laboratories will often provide this service for free to the concrete batcher/contractor since they will charge them to do the concrete test cylinders.  The batch plant technition may be very good at concrete mix designs, but he is not a registered professional, I think you want the security of having a registered professional take some responsibility for the mix design, and it doesn't have to be just you.
Our office will typically reject concrete mix designs unless they are signed by licensed civil engineer responsible for the mix design.  The concrete batch plant usually will submit the first round of mix designs without the civil engineering stamp since they want to save a couple hundred us its simply not worth the risk, let them spend the money, which is a small amount when you consider that a single yard of concrete is probably in the range of $60 dollars to $80 (maybe more in todays wild market place)  to purchase, and probably several hundred dollars when you include labor for physical placement of concrete, and how many yards are going to be on the job.  The risk out weighs the monetary reward you get for approving the mix design. This hasn't been a problem on small jobs, including residential.
We will review the concrete mix for required strength, location on job, type and size of aggregate used (required expanded shale instead of pumice for lightweight concrete).  In many cases the concrete mix design already has a history or previous breaks and is used repeatedly by the concrete batch plant.  We will generally accept the recommendations of the licensed civil engineer signing the concrete mix design and stamp the mix design as approved for use on this particular project.
Hope that helps.
Mike Cochran.