It's often easier to do an underwater concrete repair than you might think because concrete doesn't have to dry out in order to set and cure. In fact, it cures harder when immersed in water than it does in air. That means setting a post in ground in which the water table is unusually high isn't much of a problem.
In fact, if you have to set a post in wet ground, the main thing you should be concerned about is waterproofing the post to prevent it from moisture deterioration. If you use a pressure-treated post rated for ground contact, it shouldn't rot, and the concrete mix should hold it securely even if there is standing water in the hole when you set the post.
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What Happens to Concrete in Water?
Concrete needs moisture to begin the curing process, which happens when the compounds in the Portland cement become hydrated. They react with the water to form calcium silicate hydrate compounds, which are of course harder than the dry cement. This process doesn't depend on air, and in fact happens more completely when the concrete is immersed.
Anyone who pours concrete knows that you get better results when you premix it, but you shouldn't use too much water because that causes the mixture to quickly form a fragile outer casing. While this casing is a problem above ground because it lacks the strength of the concrete core, it's an advantage under water. It forms quickly, and it prevents the concrete in the core from washing away before it has a chance to cure.
How to Set a Post in Standing Water
If you dig into the ground only to find the water table is almost at the surface, you can set a post just as you would in dry ground. After you've dug the hole the width and depth you need, simply drop in the post, level it, and brace it to keep it straight while the concrete sets.
Premix the concrete to a stiff consistency before you shovel it into the hole. This gives the concrete mix the bulk to displace water. The concrete mix will also set if you pour it into the hole directly from the bag, but the block it forms won't be as uniform or as strong. Let the concrete cure for at least a day before you remove the braces from the post.
How to Set a Post Under Water
If you want to set posts for a dock on a lake or river, you don't need a special type of underwater concrete. Any concrete mix like Quikrete will work, but unless you're digging holes in the lakebed or riverbed, you'll need to construct a form to place around the post and contain the concrete. You'll also need a way to deliver the concrete into the form or hole without having it disperse in the water.
Probably the easiest way to deliver the concrete in a controlled way is to order a concrete truck, which will come equipped with a delivery hose. If you're setting just one or two posts, you may want to rent a concrete pump to deliver the concrete. Remember that the concrete should be premixed to a stiff consistency before you deliver it or it will wash away.