Concrete – How to repair thin concrete deck cover

You didn't say where you are located, but i figured in a cold area like me in Maine. First of all, 12 inch joist spacing a a waste of time and money. 16 inches will be fine especially if you plan to use 5/4 decking of any type. If you are in a non ground freeze area, just use simple blocking to support the additional 4 foot off the concrete. Assuming you are in the frozen north, consider using 3 foot porch posts. they are pre-poured concrete pyramids with a threaded insert dead center on the top. Often easier that sonotubes and mixing concrete. If your existing slab is not underfooted with a frost wall, and has not heaved or cracked over the years, then most likely a simple 8 inch concrete block in the ground the same depth or slightly deeper than the slab will probably do just fine. Remember you are only supporting 2 foot of deck (2 ft supported by slab and 2 ft supported my new foundation) and the decking itself will be spanning 8 to 12 feet onto the existing slab. Over kill is fine, always safe, but not always a good use of time and money. Also if you new deck is 12 inches off the ground, please consider some safe egress steps and side barriers. The rise of the step should not exceed 8 1/2 inches. A 12 inch step down can be a hazard, especially at night and for younger and older folks. Check your local code or the new IRC guide lines.

Google Hydraulic cement (HC), you should be able to purchase 5 lbs or so for less than $20.

Tools needed: 2Q bucket (dont expect to be able to clean it 100%), margin trowel (for mixing and scooping), 3" putty knife, rubber gloves, paint scraping implements (goggles, if you use a wire wheel)

1. Remove as much of the paint as possible, especially in the bowl area.
2. Dampen the area. Residual water is not a problem for HC
3. Following package direction, mix a small amount(1lb) of HC and working quickly (you only have 3-5 min for most versions), push a handful into the suspected bowl area. Smooth and taper the "plug".

Test the bowl immediately. The nice thing about HC is it can even be placed underwater and will cure. This treatment will work below the bowl, also. Be sure to carefully remove any loose cement, as well as paint

If the patch is not working well enough, a masonry waterproofer (to be distinguished from a masonry sealer) would be the next step. I can personally recommend the Drylock brand (no financial connection). The powered version is nice because you only makeup what you need. It is paintable, so the whole fountain can be redone.

The company's recommendation for the paintable waterproofing is to etch the surface if efflorescence is present. Efflorescence is a whiteish deposit on the surface of masonry that are a combination of minerals. The minerals will interfere with the waterproofing binding to the cement. You can try to dissolve it with 100% vinegar. If that doesn't work, you can use either their etcher or muriatic acid. This is a highly caustic, acidic product. You need hand, eye, lung, hair and clothing protection from the concentrated liquid (treat the diluted form with great respect, also). US Government data sheet on Hydrocloric acid

Outdoors is a given, a box fan is helpful, acid rated, charcoal activated respirator, water hose, box of baking soda for any spills. Thoroughly wet the fountain and the rinse off area.

It must be diluted by the following method:

Add the acid to the water in a bucket (NOT the water to the acid) This will minimize the reaction and safely dilute the acid without boiling. Most etching recommendations suggest a 10:1 dilution (10 parts water to 1 part acid) This would be starting with as 30% HCL Muratic acid. Increase the starting water as necessary if your Muratic acid is more concentrated.

Allow the acid to work for the recommended time (The salts should disappear in 5min or less.) Then you can neutralize with a gallon of water with a box of baking soda (you might just have this on hand ahead of time). Follow with a water rinse. Allow to dry overnight and then coat with the Drylok.