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FW: Low Concrete Quality

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To clarify any problem with the units:

> 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 foundations are over-dimensioned. 
> A value of compressive strength at 28 days of 250 kg/cm2 (3570 psi) 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 (4070-4370 psi). Five days later,
> all pedestals were
> poured and the average strength obtained for them at 28 days was between
> 149-166 kg/cm2 (2128-2370 psi).
> Venezuelan codes applicable in Oil Industry specify minimum 210 kg/cm2
> (3000 psi) for
> foundations, 250 kg/cm2 (3570 psi) for aggressive environment and 140
> kg/cm2 (2000 psi) for lean
> concrete.
> 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
> durability.
> Does anybody have experience about it?
> Any help is really appreciated.  

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