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Re: Low Concrete Quality

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lEONARDO
your problem is interesting.
First of all i would like to comment on the grade of concrete that is specified in your country. The range of concrete grades normally in practice, which is also suported by many code provisions, are:
i) Grade 21 MPA or 3000 psi, 28 days cylinder strength.
  with type-V cement; for foundations & most substructures.
ii) Grade 28 MPA or 4000 psi, 28 days cylinder strength.
  with type-I cement; for all superstructural members.

You are using a 250 kg/cm2 (implying 25 MPA) concrete, this is unusual to me, especially you will experience difficulty in finding values for Ru & P in the design handbooks against such a combination of materaial strengths. By the way it reminds me, you didn't mention the ultimate strength of reinforcing steel that you are working with.

Coming back to the actual problem of a durable concrete. Well, you must note durability of concrete is basically derived from DENSENESS of concrete. The more dense a concrete the more it is efficient in countering any sort of attack, be it from aggressive soil, aggressive environment implying sulphates & chlorides in humid air, due to proximity to sea, etc.

Now, how to make a dense concrete?

I don't want to lecture you on this, which you can find in any reference on CONCRETE. But, a balanced w/c ratio is key to a dense concrete. A dense concrete can also be achieved by using any pozzolana, especially FLY ASH. And, use of SILICA FUME is also one option which are being practised now a days, and has a comparably recent known history of use. So, my advise to you for a dense & durable concrete is:

i)  use a mix design that relies on less water; you can use any water
   reducing agent for the purpose.
ii) use type-V cement, which is sulphate resisting cement.
iii) use either FLY ASH or SILICA FUME, both will serve same purpose.
And, most importantly, take good care of curing aspect of concrete.

hope that helps, Regards.

SYED FAIZ AHMAD, MENGG, Mem ASCE
SENIOR STRUCTURAL ENGINEER
SAUDI OGER LTD
RIYADH, SAUDI ARABIA.

From: Leonardo Zapata <Lzapata(--nospam--at)otepi.com.ve>
Reply-To: seaint(--nospam--at)seaint.org
To: seaint(--nospam--at)seaint.org
Subject: Low Concrete Quality
Date: Fri, 15 Jun 2001 11:17:58 -0400

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
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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