It appears that the cause of the cracks has not yet been established i.e. design or workmanship / materials. You should therefore use your powers to instruct the Contractor to search for a Defect under clause 43.1 explaining your reasons.
If a Defect is found then as Supervisor you notify this to the Contractor under clause 43.2 and the Contractor is liable for correcting it under clause 44.1. Failure to do so may lead to the PM not being able to issue the Completion certificate if the Defect prevents the Employer from using the works and Others from doing their work.
If no Defect is found then the Contractor is compensated cost and time under clause 60.1(10).
If it was the Contractor's design then there would be no need to establish liability first, you would just notify notify under clause 43.2.
If a Defect is found then as Supervisor you notify this to the Contractor under clause 43.2 and the Contractor is liable for correcting it under clause 44.1. Failure to do so may lead to the PM not being able to issue the Completion certificate if the Defect prevents the Employer from using the works and Others from doing their work.
If no Defect is found then the Contractor is compensated cost and time under clause 60.1(10).
If it was the Contractor's design then there would be no need to establish liability first, you would just notify notify under clause 43.2.