We have a 1700 yard parking garage to pour in July. Pumping has always been an issue and we are looking at the mix designs and such to be sure this job goes goes smooth. Does anybody have a recommendation for an admixture to enhace pumping?

water is a good one...and cement.

You need a good amount of paste to lube the pipeline. The gradation of the sand has a lot to do with the pumpability. http://www.polysteel.com/manual/ps3000/ps3000m139-140.pdf Rock is merely a filler, but too much (actually not enough sand and paste to carry the rock) and it jams up. See also: http://www.durocrete.com/Mix%20Design%20Manual.pdf Page 37 discusses pumpability, it raises some concerns about shrinkage, but modern water reducers help alleviate this.

Use Sika 2100 super 'P' and it will pump as easy as a hot knife going through butter.

Flyash makes a nice fluffy pumpable mix take out some stone add some sand your good to go

If the mix is borderline pumpable, you can add slickpack to it to make it pumpable as well.  It will add to the setup time of the cement a bit but it'll get you thru the job.

use rice husk ash

Make sure there's not too much rock in it. 1720lb per cubic yard is great. i pump up to 1850 but the more rock the more scraping and friction, makes it harder to pump.

Hi Mate,I don't know much about your guys concrete but in NZ we have a couple of places that have a lack of sand in there mixes we use sika pump aid or better we use cemplus, which is used in motar (used for bricklaying) in holds the water in the concrete which helps hold the water in the concrete 

Good concrete pumps good, better concrete pumps better.  Cement is THE best pumping aid.  That being said, sands are the key to pumpability.  Rock is merely an inert filler in concrete, it takes up space, reducing the cement (paste) demand.  the paste (water and cement) is what creates shrinkage, IE: more paste more shrinkage, but paste is also what lubricates the walls of the pipe, it's a trade-off as far as the pumper is concerned.  Generally, the finishers will bitch if the concrete is too stiff, the pump will usually still pump it. 

A parking garage is supposed to last for a LONG time, concrete is not the place to get savings (cement powder) on construction.