Concrete cubes failed after 28 days?

I do have 9 cubes of concrete which averaged 32 N/mm2 when crushed after 28 days. The design Compressive strength is supposed to reach 35 N/mm2. The cubes were mistakenly not cured for the first five days so I am sure that the reason of failure is the lack of proper curing. However, is there any clause in the ASTM or BSI that helps me convince the Engineer that the concrete is fine without going through things like core test...etc? He knows but needs doents!!