Q.: About 5 years ago I built my first pond. It measured 5x8 feet and was about 2 feet deep. I just dug a hole with outward sloping sides and used bagged posthole-type cement from Home Depot. I made it 2 1/2 inches thick, with a little chicken wire, and have never had any problems or cracks.
Now I am building a little bigger pond 10x12 feet and maybe 3 feet deep. I want to build a wall for the sides using 6x8x16-inch blocks, four high. I dug a 12x24-inch footing all around and figure on a bottom about 6 inches thick. The pallets of bagged cement I bought have seen too much rain since I bought them about 5 months ago, so I called the local ready-mix company and asked for a quote on the concrete. They weren't sure if standard concrete was sufficient, saying I might need denser concrete than that used for driveways. Are they right? Do I need a high-strength concrete for my pond?
A.: It’s amazing that your first pond held up so well! The thickness of the concrete is minimal, as is the use of chicken wire as a reinforcing material.
You didn't mention whether you live in a freeze/thaw climate. If you do, a 4000-psi concrete mix design with air entrainment will be necessary. If you are in a climate where the soil doesn't freeze in the winter, then a 3000-psi mix design is possible. Using ready-mixed concrete would save you some work, but you can buy 5000-psi cement in bags at the Home Depot.
My suggestion is that you reinforce your new pond with 3/8-inch rebar (referred to as #3 bar). Place it in both directions on 12-inch or 18-inch centers. Tie the bars to each other with wire so that they stay in place. You can either use special "chairs" or "dobes" (small concrete pieces of the right thickness) to lift the rebar to the center of the concrete.
For the floor of your pond, plan on 4-inch-thick concrete. If your pond floor is 10x12 feet, that means you will need about 1.5 cubic yards--or about 6000 pounds--of concrete. You should consider pouring concrete for the sides of the pond as well. If they are vertical, you will have to form them with wood first. The reinforcement should extend from the floor into the wall. The thickness of the wall should probably be about 6 inches. You can place the floor and wall together, or place the wall as a separate operation.
If the sides of the pond are sloped, you can place this without forms and at the same time as the floor, forming the shape with a magnesium hand float.
I would suggest that you use a good moisture barrier under the concrete just in case cracks develop some day. Some materials will react with the high alkalinity of the concrete over time and degrade. Be sure to use a compatible material. Good luck with your project.