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I have a cabin next to the salt water and want to build an 8' x 8' extension. The water is 16' away from the cabin at high tide. The cabin is actually on a Tickle that leads out to the ocean...well protected.

The cabin has a concrete foundation built on exposed bedrock. Does anyone know how to build the concrete foundation on the bedrock and connect it to the existing foundation?

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What I have done in a similar situation (not bedrock, but a huge boulder) is to make (rather tediously) form boards that closely fit the rock, and drill holes into the rock to insert steel reinforcing bars into, which are tied into the slab reinforcing steel. Similar holes would go into the existing foundation.

Neither may strictly be needed - bedrock is about as firm a foundation as you will get, so there should be no relative motion, and no tie-in whatever may be just fine. But it might be reassuring to at least have the bolts anchoring the walls drilled and grouted well into the bedrock, when the wind gets howling....

A local concrete company would be a better source of information about details relating to the satwater exposure and any specific details related to that (hasn't been one I've had to deal with, but I vaguely recall that there are things one does to seaside concrete to increase the longevity in that hostile-due-to-salt environment.) Presumably at least one part would be to wash down the area where the new slab will go with fresh water to remove any salts, before pouring concrete.

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There are 3 types of concrete placement in relationship to salt water: 1) submerged, 2) splash, and 3) atmospheric.

As you can imagine, the severity of design against the salt environment is critical for each situation. I believe you fall in category 2) splash, because you indicated your foundation will be on “exposed bedrock”.

There are many considerations for such an installation, like concrete design, additives, cover from face of rebar to edge of footing (top, sides and bottom), control joints, tie into existing foundation, etc.

Here is a great article to help you:

https://www.concreteconstruction.net/_view-object?id=00000153-8bb0-dbf3-a177-9fb9d6200000

Remember, concrete is porous. “Cover” will be one of the most important decisions you’ll make. The article recommends a 2” cover. I live in a very corrosive atmosphere and we do not go below 3”..I highly recommend 3” minimum.

BTW, I would not “tie” your new foundation to the bedrock, unless you’re positive the existing foundation is. I’d just rest it on bedrock, because in case of ANY seismic activity, the rock will go one way and your house will go another (due to weight, depth, etc.).

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