If I was the PM on this contract, the first question I'd seek to answer would be " Did the Contractor give an early warning for the incomplete mix design as soon as he was aware of the matter ?". The Contractor can only be held liable if he did not notify an early warning as soon as he was aware. I don't think the z clause has any effect if the Contractor was not aware of the issue.
With regard to a PM assessment, I would investigate whether the delay is solely attributable to the issue relating to incomplete mix design, or did the Contractor introduce a delay himself by pursuing the alternative mix? Any contributory Contractor delay can be taken into account in the PM assessment by reference to the following part clauses :
CL63.3-"A delay to the Completion Date is assessed as the length of time that, DUE TO THE COMPENSATION EVENT, planned Completion is later.........."
CL63.7 " Assessments are based upon the assumptions that......any Defined Cost and time due to the event are reasonably incurred."
With regard to a PM assessment, I would investigate whether the delay is solely attributable to the issue relating to incomplete mix design, or did the Contractor introduce a delay himself by pursuing the alternative mix? Any contributory Contractor delay can be taken into account in the PM assessment by reference to the following part clauses :
CL63.3-"A delay to the Completion Date is assessed as the length of time that, DUE TO THE COMPENSATION EVENT, planned Completion is later.........."
CL63.7 " Assessments are based upon the assumptions that......any Defined Cost and time due to the event are reasonably incurred."