Concrete Mix Design and Placement Responsibility

The construction of the stairs is the engineer's responsibility. Location of vapor barrier would be included in the construction of the stairs. Allowing admixtures to an alreay approved concrete mix design should be evaluated by the engineer.

Since we're not materials engineers, how does one rationalize this? Evaluating it means we're taking responsibility for it, no? Why take more responsibility just to benefit the contractor?

Just trying to figure out who's really responsible for what in the event there's major cracking. I dont want anyone to turn around and say it's the engineer's fault because he approved it.

Ask that the contractor provide whatever documentation you require; this may include an engineering report from a third party... At the end of the day, you have to be prepared to accept this report...

Dik

While the concrete supplier may give the Engineer the mix design for "no objections" - the Engineer is not buying the concrete - the contractor who is constructing the work is doing so. In fact, I believe that the Contractor should be supplying the mix design to the Engineer - not the supplier as there is no contractual link between the two (on the vast majority of projects). The contractual arrangement is between the concrete supplier and the general contractor and the discussions between them is between them.
The contractor is requesting information from you as a variance to the project - the specs may not call for a retarder. If you wish, ask the contractor to provide details as to the revision of the mix design, confirmation that the revised mix design will meet the project specifications and you may give a "no objection" or not. If the retarder is a variance to the design, you don't have to accept it. Similarly, the vapor barrier is your call as the Design Engineer. If you are not the design engineer, then the design engineer needs to be queried.