repair to small garage structure - anything hinky

huangyhg

repair to small garage structure - anything hinky?
i hope this is appropriate, i have some questions regarding a current repair job to a central two story garage in my townhouse complex.  i am stepping outside my knowledge base, please excuse any abuse of terminology.  i am based in ontario.
we were told we needed to replace the waterproof membrane on the top of the parking structure (no roof) and repair some delaminated concrete.  we hired a consulting engineer to write up the specs and oversee the project.  original quote was 200k.  three weeks into the job they have now said the entire slab requires replacement, and the project is now closer to 500k.  the slab is ~10600ft2.
1)  it was discovered that an original 8" slab was topped with a 4" slab, likely about 20 years ago.  it was recommended the slab should be removed and we did so.  it was at least a couple of days between the first concrete removal and us being alerted about the issue.
2)  areas were mark for concrete removal.  bridging was installed and they have been going at it with jackhammers.  no forming plywood was put underneath intially which some unit owner found suspicious, but the project engineer stated this goes up later.  does this sound right?
3)  i would think an initial core sample would have pointed out the double layer of concrete.  the engineer stated they don't always deem it necessary, is this the case?  core samples were taken after the top layer was removed, tested at 36 mpa, but they feel the concrete had deteriorated beyond reuse.  when trying to remove the bad spots, they can't get to good concrete to bond with.

it's hard to critique without seeing the conditions and the site;  depending on who you're dealing with they simply may not have extensive experience in existing structures.  that said:
1.  additional dead load of this magnitude is a seriously bad thing, particularly in seismic regions.  you should have been alerted well before any work took place, however the call is likely the correct one.
2.  it does go up later;  often weak spots will extend right through the concrete and require a full-through patch.  if this is the case, as it sounds it might be particularly given that they are not finding sound material, the plywood would only get damaged if placed prior to chipping operations.
3.  initial cores are not always taken, and this is up to the engineer involved.  it is purely a judgement call, but i normally favour them.  but you are correct, they would very likely have uncovered the double layer.
weather exposed concrete which has also been exposed to de-icing salts and freeze-thaw action, particularly in the nastier parts of canada, often are so badly salinated over the years that you simply would not want to keep them.
to answer your primary question:  it doesn't sound like anything hinky to me;  just the routine unexpected of a restoration job.
good luck,
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
i agree with youngstructural that unpleasant surprises can sometimes occur in the construction phase of rehabilitation projects, however considering the potential financial impact of the worst case scenario, any engineer experienced with this kind of rehabilitation, would always want to recommend strength and chloride testing prior to developing plans and specifications.  pretty standard stuff.
a 12" total thickness of slab (8" original and 4 inch topping) exceeds even most bridge deck thicknesses (7"-8" typ).  this alone should have given the design engineer a clue regarding the potential reduction of live load capacity.
there is risk with any rehabilitation project that unforseen conditions will be discovered, however a change order that increases the construction cost by 2.5 times seems pretty excessive even for a rehab project.  this is why many engineers perform what is considered reasonable due diligence testing to mitigate the potential for financial difficulty for their clients and liability expoure for the profesisonal.

ys & le,
thank you both for your quick replies.  what i am reading into it is nothing was necessarily done incorrectly, but could have been done better.  my main concern has been whether this really should have been such a surprise, if we had known the cost we may have definitely explored other options (this cost is only split among 56 units).  we may have no choice but to proceed now, the structure is unusable.   
the engineer probably thought they were doing you a favor by saving you the money that it would have cost for this investigation. it may be just a judgement call that didnt work out, they may have made the same call on nine other projects and been correct but not on this one.
anyway, this forum is not really the appropriate place to discuss these concerns.