waterproof a tiled dome

huangyhg

waterproof a tiled dome?
we have a potential project dealing with a number of historical buildings.  one building includes a dome approximately 20 feet in diameter on the roof and is constructed of:
1.  a bottom concrete ring support.
2.  vertical steel strap channels which run from the concrete base ring upward on an arc to a top concrete compression ring which is approximately 5 feet in diameter.
3.  horizontal steel strap channels running around the perimeter of the dome, wired to the vertical straps and spaced about 2 feet o.c.
4.  metal lath wired to the horizontal straps.
5.  a concrete base about 2 to 3 inches thick which is applied to the lath and bonded well.
6.  an exterior finish made up of small ceramic tiles about 1/2" square and set in a mortar bed on the concrete base.
the issue is that the dome leaks.  the metal straps all appear to be only slightly rusted (surface scale) and do not appear to be a big issue.  however, some verical cracks up the dome face are apparently in line with the vertical straps.
a lot of micro cracking around the tile is also visible.  the metal lath and concrete base appear to be in good condition.  there is no visual deformation of the dome, just the nuisance of water getting in.
the tile cannot be removed as it is of historical significance.  the owners (who have a lot of maintenance knowlege) had mentioned a product that they used on a glass/concrete pit cover.  it was a very low viscosity rubber-based waterproofing sealant that worked great and apparently doesn't discolor.
question:
would a sealant over the tile make sense?  i've learned over the years that its not always wise to seal masonry, but ceramic tile?  the inside of the dome can be vented or air conditioned if necessary.  the rubber product is said to be able to bridge 1/16" cracks, depending on the thickness of application.  it also isn't supposed to discolor.
if a sealant isn't a good idea, what else can be done? the fear is that water leaking through the substrate of the dome would rust the straps, deteriorate the concrete base, and weaken the dome.  the interior of the dome is not a finished/accessible space so we could waterproof the floor inside and drain, but you still weaken the dome.
the building is in a mild environment (i.e. no severe winters) but does get some freezing.
any ideas/thoughts?  
jae...i have not seen any coating that would not ultimately discolor.  for clear coatings, the result is usually either hazing or yellowing.  for pigmented coatings, usually fading and/or chalking.
can the tile be carefully removed then placed back?  if so, there are numerous waterproofing options available from fluid applied "membranes" ( personal opinion is that it is a stretch to call them membranes), to true membranes or even a carbon fiber overlay to provide both strength and waterproofing.
if the tile cannot be removed, then the option is only one of using a coating that will require some periodic maintenance, no matter which coating is selected.  while penetrating sealers (methacrylate or epoxy) could be used with some effect, they do not bridge cracks well.
if the microcracking occurs in a grout, it would be possible to remove the grout and replace it with either a polymer modified or epoxy grout, both of which offer greater water-resistance than conventional grout as well as offering greater flexibility to reduce future cracking.
jae - just a few thoughts.
i recall that fosroc  manufacture a product for sealing concrete decks and the like.  it is is meant to be flexible too. perhaps you could check it out.
there may be problems sticking tiles back onto the liquid "membrane" and this will need investigation.
nb

my suggestion is to avoid (as far as possible) any treatment that cannot be easily removed: in 20 or 50 years someone with the problem of restoring again the historical monument will blame you for having used something that deteriorates with time and cannot be replaced.
prex
due to your criteria that the tile couldn't be removed. so we have less alternative.
if we concern about water ingress into concrete that make the reinforce bar corrode we shold stop the ingress of water.
pu foam injection will be the best alternative in my openion but this must be performed by an expert otherwise the tile will be explode from the substrate.
there is a little known waterbased pu product on the market that is used for sealing marble and ceramic tiles it comes in clear and coloured.  the coloured version has a ten year guarantee against uv fading.
i have used it on floors and roof tiles and it works well.
send me you contact info and i will get some info sent to you.
that should be