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Precast tension stresses

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We are working on a precast concrete project.  One of the pieces is a planter box.  It will be suspended and vines will grow from it.  Our contract documents call for prestress and for a design that will not have any tension in the concrete.  The precaster missed the note and only has mild reinforcing in his bid.  It would be relatively easy to simply make them match the contract documents, but we would like to offer them a performance spec that could potentially be met with mild reinforcing.  Our goal is to prevent concrete cracking and potentially having a leak problem.
For those of you that design concrete water tanks, how do you insure they don't leak?  What should we use for a maximum allowable tension stress in the concrete?  IIRC, according ACI concrete cracks at 5 square roots.  We need to be sure there won't be problems long term.  Should we spec a different limit?  Of is there another method than simply specifying a limiting tension stress?
Thanks in advance for your help,
Jake Watson, P.E.
Salt Lake City, Utah