suspended slab exposed bottom re bar

huangyhg

suspended slab exposed bottom re bar
hi,
we have a situation where the suspended floor slab is in a bad way, loss of concrete exposing reinforcement some places all around the bars. i am thinking that any replacement material under side of the old slab will need to be dowelled in at fairly close spacing to enable the section to act as one again otherwise the bottom mat is redundant. is that how others would approach the problem?
it depends on the depth of spalling, condition of reinforcement, depth of chloride ingress, and depth of carbonation.  if the reinforcement is still generally in good condition, it may be just a concrete repair job.  if not, some reinforcement may need to be added.  only in very deep repairs, and probably not on soffits, would dowels be used between the original concrete and the repair mortar.
well the problem i see is that the spalling has not just exposed the face of the rc it has spalled to the back of it so that you have areas where re bar is just hanging there, with quite a gap behind. would a repair mortar be good enough to re-link (for want of a better phrase) the reinforcement back into the slab in this case. it looks like its been this way a while, the re bar has a lot of rust. i haven't been up to measure if any loss of section the the bars has occured.  
concrete repairs perform best when the repair mortar is worked behind the bars.  yes, you may need some means of holding the bars in their correct position during the repairs, but not in the repaired slab.  all repairs should be done in accordance with recommendations of the manufacturer of the mortars.  this involves removal of spalled material, replacing reinforcement if required, priming reinforcement, priming the concrete, placing the repair mortar, and curing.  it requires experience and attention to detail on the part of the repair company.  i suggest you get some advice from one of the suppliers of concrete repair system materials.  parchem, which supplies fosroc mortars, is one of the best.  one other thing...don't just choose a repair mortar because it is the strongest.  the mortar characteristics should match the concrete substrate as closely as possible, except for density.  in overhead applications, lightweight mortars are preferable for constructability.
specify a non-sag repair material for overhead applications such as emaco by basf. can be spray applied or troweled on.
typically on a highway deck a full depth patch would be specified.
that won't happen i'm afraid! this is operaational area above, there is no way we could repair to full depth, especially considering the extent of the problem in plan.  
i came across the same problem 10 years ago in a university chemistry building.  they had used the area to store a brine solution and had small areas where the top of the concrete had spalled away from the rebar, exposing the top rebar.  it looked like it would be a pretty easy fix until i asked to see it from below.  directly underneath it was a very, very large ventilation fan which took a bit of time to get access to.  once i saw it, i said, "oh s#it!"  there were sheets of concrete missing from the bottom of the slab.
we opted for repair using sika's carbodur product:
about 4 years ago, we performed beam strengthening using carbon fiber sheets.  this was under a crane beam in a port pier deck over salt water.  we cleaned up the concrete between piles, chipped back behind the rebar to gain at least an inch of grout under the bars, applied epoxy and a grout, followed by adding a layer of carbon fiber below the new grout.  we used an epoxy on the carbon fiber to gain the tension reinforcing required to upgrade the beam for heavier loads than the original design.  it's been working fine, with no problems.