historic slab construction-is it still safe

huangyhg

historic slab construction-is it still safe??
we had to design some dunnage steel that is supported on posts that sit on existing columns on the roof of a building.  we did some props in order to locate the columns and realized that the slab construction is made with cinder concrete.  there are steel beams from column to column 16" deep however the slab is just gravel-like.  we were able to put a rod through 5'-0 down all the way to the ceiling.  at the contractor was able to pull out a steel rod 8'-0 long that had rusted.  
i suspect that the rod is sort of a tension reinforcement.  if there are more rods and they are rusted then will we have a collapse?  maybe.  so how do i start investigating this case?  i was just hired to do the dunnage.  how is the new investigation introduced?  i simply do not know if there is a real problem.  any ideas will be highly appreciated.
sounds dodgy to me.  tell your client contact what you have found and the possible implications.  i've found that photos and samples generally help in convincing clients they may have a problem.
if you don't know what to do here, it may be best if you work with the owner to hire a structural engineer who has experience in forensic building studies.  
soft concrete, rusted steel, pulling out rebar from concrete.....i agree with daveminter above - sounds dodgy.
are the steel beams in the same depth of the concrete?
if so then it sounds like what they call slab and beam construction (or something like that) in the uk. the concrete is assumed to arch between the steel beams and the rods are probably in place to stop the end beams from spreading.
csd
there were similar type structures in the us that csd mentioned were used in the uk. steel beams with segmental arches or flat arches between steel beams with concrete fill above and tie rods. the arches were sometimes made of terra cotta but i have also seen some made of common brick.
the actual construction may not be particularly relevant.  from the description it is clear that this cannot be assumed to be safe.
i don't know what dunnage is, but it seems likley that you have implicitly assumed that the slab is safe in winning this work.  the slab safety is now doubtful so you cannot continue to design on the assumption that it is...
it sounds like the owner will have to pay someone to investigate further.  until such time as that is done - it's not your problem.
i worked on rehabing an old (100+ years) concrete porch. the reinforcing was exposed and revealed itself to be an expanded metal grid, aka "hey, it looks like chicken wire." there were structural cracks all over the porch so the owner agreed with our suggestion that the porches simply be replaced. the contractor did remark, however, that the concrete was "pretty tough to crack up." rather than wait for the whole thing to fail, however, i didn't care how "strong" some of the concrete was.
there are times where the type of construction may be important. in the type of structure that i mentioned previously, the concrete is simply fill and not structural. the arches,the tie rods and beams are the structural piece.
in this case there is no slab.  the beams are 16" deep and look ok, however the slab is just ash and gravel.  there is a layer of lightweight concrete above and then ash and soil in order to build up the pitch.  then as you dig further (with hand tools) you keep going down.  we did not want to make a big hole therefore we just inserted a rod 1/2" in diameter and we went down 5'-0" to hit what we thought was the ceiling.  since this building is a school we were not allowed to investigate from below.
by the way dunnage refers to the steel on the roof to support mechanical equipment.  this steel does not bear directly on the roof but rather on the existing columns below via steel posts.
the roof under question has an area of approximatelly 6500 square feet.