prestressed slab repair options

huangyhg

prestressed slab repair options
i'm writing this in hopes that people will offer their experience in dealing with prestressed concrete slab/beam repairs as it relates to my situation.  i'm weighing the options of replacement with temporary (~2-10 yr life) repairs while the client weighs their options.  
i'm dealing with a series of prestressed slabs (~4' wide and 12" thick).  the slabs span about 28', 8' above salt water and are simply supported.  they are reinforced with 16 1/2" diameter sr strands on the bottom face and 4 similar strands on the top.  #4 epoxy-coated stirrups are placed at regular intervals throughout the span.
the top surface is covered with asphalt, so only the bottoms are visible.  the slabs exhibit servere deterioration and corrosion that has manifested itself through concrete spalling, discoloration due to chloride penetration, partial strand exposure and sometimes 100% section loss through the strands.  we were able to verify through non-destructive observations that at least 8 of the 16 bottom strands were at 100% loss.  
in addition to possible overloading and other contributing factors, the slabs had a concrete cover of only 1-1/4" on the bottom face.  i'm not trying to correct the design or increase their capacity, only bring them back to their original design strength.  many of the slabs are in good condition, suggesting that the day-to-day loading is not an issue.
space is restricted and all repairs will be needed to be made from below.
i think this about covers it, your input and experience on possible methods of repair is greatly appreciated.
thanks.
have you though about frp sheet reinforcement to the underside. i would consider this repair temporary though.
keep in mind that you now have more strands in the top of the slab than the bottom inducing an additional bending moment.
chloride penetration will continue to corrode remaining tendons unless something is done about it.
personally, i think you have walked into a hornets nest.
i realized that when i performed the analysis.  actually, what happens is that the effective depth within the section shoots upward and the top reinforcement moves into the tension region.  
i've considered adding frp to the bottom surface but the slabs are so cracked/spalled in some areas that we'll have to do significant patchwork in order to have a smooth bonding surface.  this might be an option, but i'll need to consider whether i'll get the tensile strength equal to that of the corroded strands.
personally, i don't really see any viable options other than replacement but i'm just trying to be diligent and research any and all alternatives before i conclude on that.
i think you should focus on the corrosion isssue - what is to stop this corrosion continuing even after the repairs.
i would recomment that they replace, or find another engineer. if they carried out regular maintenance and inspection on this thing it would never have deteriorated this far.
the fact that these units are modular with a four foot width lends support to the replacement option.  price precast replacement slabs and the cost to switch out the existing units.  use epoxy coated reinforcement with silica fume mix for dense concrete with 40 mm cover.
if among many slabs only few of them are showing distress, then you are better of replacing them.  the size and shape of the slabs are pretty standard, so you should not have very much difficulty having them fabricated.
even if you manage to successfully patch the deteriorated areas, more than likely the neighboring strands will start to rust after repair.