Here is an interesting write up of test results for the free fall of concrete: 
http://www.buildblock.com/pdfFiles/...%20(3).pdf

Alton, I have encountered this with an occasional specifier. This paper as well as at least one more have established the limit as a bit of an old wives tale. However, we still pour into the sides of walls sometimes in very tall walls (particularly on hot days) in order to keep the rebar clean until your ready to fill that section. Regards.

I had not seen that article, but free fall concrete has been SOP for caissons in the 38 years I have been involved with them in the upper midwest. The only time I have ever seen a tremmy used for large foundations was for concrete placed under a head of bentonite.

That does not mean, however, that you can indescriminately dump concrete into wall or column forms and not worry about proper consolidation techniques.

Bruce

TexasICF,

I agree that keeping the rebar clean is important especially when there is a chance it might set up before the form is filled.

Bruce,

I think that proper consolidation is critical as well as the proper mix.  Too stiff a mix in a tall form may never make it to the lowest level.