Q.: For a recent large prestressed double-tee project the specifications called for the concrete to be designed by an independent laboratory selected and paid for by the owner. The specifications required the double-tees to be designed by the plant producing them. The plant engineering staff determined that the lightweight concrete would have to be a minimum of 6000 psi in 28 days, and 4200 psi at time of release.

After being furnished a mix design by the laboratory on behalf of the owner, it was evident to the plant that the mix design furnished would not achieve either of the desired strengths. This opinion was based on past experience of its own staff in designing over 300,000 cubic yards of similar lightweight concrete. Several other factors in the mix design also were wrong: ratio of coarse to fine aggregate was off; yield was off; and the prescribed unit weight was over the specification. Nevertheless the plant was ordered by the owner to proceed with the owner's mix in spite of the written objections of the prestress plant. The results were as expected and the tees were rejected. How can the producer be held responsible?

A.: This question appears to be a legal problem rather than a technical problem, and we are not legal experts, but it does not seem reasonable to require the contractor to do something that his experience shows to be impossible when he has clearly stated that fact to the owner in writing.