additive for concrete waterproofing

huangyhg

additive for concrete waterproofing
my concept design for a future parking deck over an existing roof is to use a system of structural steel beams with permanent metal formwork (sold here in south africa under the brandname bond-dek).  in this way the columns for the future deck can pierce the roof and the beams and concrete can placed without having to backprop tho the below tennant's space.
the bond-dek slab uses the profiled metal sheeting formwork as the bottom flexural reinf and so the slabs are very thin (the min conc could be on 3inches)  while the entire slab was to be cast at a slight fall to allow water to drain off, xypex was to be added to the conc for waterproofing.  does anyone have experience as to what the min dim would be for the xypex concrete to still be effective
sorry, i am not familiar with the waterproof additive you mentioned. just want to add a piece of advice to your very sound concept - i image a void space will be created between the old roof and the new deck, and it might not be easily accessible upon completion. if this is the case,   waterproofing and drainage to the old roof; proper ventilation for the void space, and corrosion control on deck bottom side steel could be quite crucial for long term performance.
it is unwise to depend on xypex to provide a waterproof membrane.  xypex acts in the presence of water to seal small cracks, so there must be some water penetration for the crack sealing to work.  a membrane on the top surface, specifically designed for parking structures, should be used.  you should also make sure of the flashing of penetrations through the existing roof.
if you do decide to use xypex as a "belt and braces" approach in addition to a membrane, you should carefully study the product literature and insist on supplier involvement.  it is not just a matter of adding an additive to the concrete.  the xypex specification requires a high percentage of reinforcement, for which the deck will not qualify, and a stringent curing regime, which many contractors will resist.
i advise against using composite steel forms without concrete reinforcing steel in an exterior application. i've inspected several pedestrian bridges that utilize this system. even with galvanized forms, they had all severely corroded (completely gone in some areas) within 25 years of exposure to s.e. usa weather. the decks now require replacement. some reinforcing now will be much cheaper than complete replacement in the near future.
jorton:
in the midwest, the main reason for bridge deck to corrode is the application of de-ice (salt) year after year with the help of melting snow and water runoffs. what is the main cause in the se - humidity, salt in the air? just curious.
composite deck manufacturers generally advise against using it to resist concentrated wheel loads. laboratory tests have proven that the composite action will be lost after repeated loadings of concentrated loads. in addition to this, deck is very poor at spreading out concentrated loads laterally. you need distribution steel (rebar) for this.
metal deck may be ok for forms only (depending upon the exposure) but you should have a top and bottom layer of reinforcing bars in each direction instead of using the deck as bottom reinforcing steel.
klsee1000,
i don't know that i can speak for most bridges in the se, but i think the predominant cause is leaky deck joints and unmaintained drainage systems. i'm not sure how much the humidity impacts the rate of corrosion, but i'm sure it doesn't help.
the decks of the bridges i mentioned above were regularly cleaned so they probably had more water exposure than typical bridges in the area. the owner claims solvents were not used to clean the decks, but i have my suspicions.  
agree with all suggestions.
jorton, thanks.
whether this product, hycrete, is applicable to your conditions you might want to look at the literature to see how it works.
in addition to my comments about xypex above, i agree with others about the inadvisability of depending on the bondek for reinforcing in any exterior application.
thanks to all for your invaluable advice.  wildehond