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Low Concrete Quality

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We have a problem in the foundations of the most important building of the
Main Process Station: Central Control Room.
It was initially a concrete structure supported on direct foundations but
after foundations construction was already started, the structure was
changed to Steel, therefore the foundation are over-dimensioned. 
A value of compressive strength at 28 days of 250 kg/cm2 was specified for
all structural concrete in the project. During the construction, all
footings were poured the same day and the average strength obtained for them
at 28 days was between 285-306 kg/cm2. Five days later, all pedestals were
poured and the average strength obtained for them at 28 days was between
149-166 kg/cm2.
Venezuelan codes applicable in Oil Industry specify minimum 210 kg/cm2 for
foundations, 250 kg/cm2 for aggressive environment and 140 kg/cm2 for lean
The facilities are located in a no-aggressive environment and in a seismic
zone with ground acceleration of 0.24g. 
Due to a bad inspection, the building is already totally built. We have
performed the structural calculation with the low strength and apparently,
there is no problem because the loads are lower than those used in the
initial foundations design. As the timelife at the facilities is stated in
25 years, our main concern is about durability of the concrete. Contractor
is arguing the use of Silica Fume in his concrete which increase the
Does anybody have experience about it?
Any help is really appreciated.  

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