Pond Construction with Concrete

4´ deep koi pond with 4˝ bottom drain, 2˝ fiber reinforced concrete sprayed over 45mil EPDM liner.

Concrete and mortar have been favored by pond and fountain builders since the Roman Empire, and the formula has changed little in the past 2000 years. Limestone and clay are mixed, heated and ground to the silky powder we call cement. Concrete is made by adding water, sand and gravel to the cement, while mortar refers to a finer sand and cement mixture used for bonding brick and stone.

Our modern formula was first cooked up on a stove in 1824 by British inventor Joseph Aspdin, a real ‘kitchen chemist.’ He named it Portland cement for its similarity to stone from the nearby Isle of Portland, but it’s basically the same stuff that built Ancient Rome.

Properly mixed and applied, under the right conditions concrete can last a very long time – the intact, magnificent roof of the Pantheon is still the world’s largest unreinforced concrete dome almost two thousand years after it was built. Roman aqueducts and fountains built two millennia ago are still in daily use today. That said, concrete is NOT a “set it and forget it” material. There are a number of requirements to be met if it is to last.

Cement and Water

Portland is hydraulic cement, meaning it hardens and cures upon contact with water, but the exact process is so complicated that we don’t fully understand it even today. The proper amount of water is critical in mixing concrete – too little won’t fully hydrate the mix, leading to an uneven cure and too much will weaken it. The curing process doesn’t stop with mixing, either. Concrete needs to be kept evenly moist for as long as feasible after pouring to develop its full strength, at least 3 days, but it will continue to strengthen for a year or more if kept moist.

Untreated concrete is usually quite porous, absorbing and allowing water to slowly seep through microscopic spaces, but it can be made highly resistant to the passage of water by using fine aggregates and waterproofing additives to close the pores. This makes it an ideal material for ponds and waterfalls in warm weather climates, where prolonged freezes aren’t an issue. In colder areas water trapped inside the concrete can freeze, expanding from within and causing cracking and spalling. Ice sheets on the surface of the water can force walls apart, causing structural damage and leaks. Damage from frost is usually progressive if left unattended, so temperatures below 32°F are a cause for concern and require different strategies than warm weather water features.

Foundations for Concrete Structures

Concrete has tremendous compressive strength, capable of supporting thousands of pounds per square inch, but doesn’t bend or stretch very well, so the most critical requirement of any concrete structure is a proper foundation, set on undisturbed soil that will not move or settle. In colder climes footings must be set below the frost line to avoid movement during freeze-thaw cycles, usually 3 to 4 feet below grade.

That may sound very deep, but if we’re considering the expense of concrete to start with, then we’re generally talking about a more elaborate pond, perhaps a koi pond where the deeper the water is, the better. Most koi ponds are deeper than the frost line, into undisturbed soil, so the depth of the footing is often of no great concern. We’ll usually dig out the pond as carefully as possible first, leaving the walls vertical, then we’ll excavate the trench for the footing at least 8˝ deeper than the rest of the pond, always below the frost line and at least 12˝ wide, to provide a solid, stable base that’s a little wider than the walls.

We pour the footing level and set some kind of ‘key’ to lock the walls to the footings. This can be as simple as regularly spaced rebar rods set into the wet concrete, or a groove in the top of the footing that the wall can lock into. An 8˝ x12˝ footing may sound like overkill, but it ensures the walls will stay straight and solid and support any load likely to be set on them, and with 8˝ walls the 12˝ wide footing provides a 4˝ shelf that the floor will key into later.

Waterproofing

There are three different ways I know of to waterproof the concrete pond, all starting with the solid foundation just described.

psi poured concrete pond design

In my first pond, I used 11 pounds of 4,800-psi poured concrete. It was overkill!

The first method is the conventional way to pour a fully waterproof concrete shell. The pond is excavated, a footing poured, the appropriate reinforcement rods and wire set in place and forms built. Latex or acrylic waterproofing solutions are added to the mixing water, to close the pores in the concrete and eliminate seepage through the walls. The concrete must be carefully mixed and vibrated into place to eliminate air bubbles, and the additives can be costly, but in warm climates where frost isn’t an issue this method works very well. Although the entire shell of a good-sized pond can be poured all at once by an experienced crew, we prefer doing the footing first, then pouring the walls, then the floor, sealing the seams between the three elements with waterstop gasketing. The flexible gasket is set on the footing before the wall is poured on top of it, sealing the joint from the inside as the gasket swells on contact with water. We’ll place an expansion joint vertically on the inside of the wall and a second gasket on that 4˝ shelf at the base of the wall before the floor is poured, to form an elastic seal that allows for expansion and contraction of the floor. The coping is set on top of the walls with a simple, strong, waterproof mortar made by mixing one 45lb. bag of thinset to two 70lb. bags of Type S mortar, a great mix I first heard about online from Doug Hoover of Aquamedia (many thanks for giving this great formula away for free!) This method of pond construction is effective, permanent and fully waterproof as long as there are no cracks, so it’s ideal for the Southern States and the West Coast. It isn’t optimal where freeze-thaw cycles are a concern.

Burying a flexible liner inside a concrete wall offers advantages over either method alone. In contrast to straight concrete, seams and small cracks cannot leak, so integrated gaskets, waterproofing additives and coatings are unnecessary, and freeze-thaw cycles are no longer a concern.

The second method dispenses with the cost and additional labor of integral concrete waterproofing by applying a waterproofing coating on the inner surfaces of the pond after the shell is constructed, and it works with either poured shells or with cinder block construction. There are many types of coatings, ranging from liquid EPDM rubber compounds to two-part epoxies to cement-based slurries to simple paints, so there’s a waterproofing compound for every job. The more elastic preparations bridge small cracks and even tolerate a small amount of movement, so they can be very forgiving and are often used to waterproof leaking existing concrete ponds. The key to these applications is proper surface preparation, so the manufacturer’s instructions must be strictly followed. The more stable the base, the better the coating will perform, so this method of pond construction also works best where winters are mild.

Both of these methods are well known and the steps involved in their construction well documented, so I won’t go into further detail, but unless you’re planning on draining the water feature for the winter, we’ve found neither is ideal in harsh winter country. Where we build, in the mildest area of New York, a hundred freeze-thaw cycles is a gentle winter, temps regularly visit the 20’s and we can stay below freezing for weeks. We needed a way to permanently waterproof concrete regardless of weather, so we developed a simple way to construct a concrete pond so it will always stay completely sealed under all conditions. We combine flexible and concrete liners.

Burying a flexible liner inside a concrete wall offers advantages over either method alone. In contrast to straight concrete, seams and small cracks cannot leak, so integrated gaskets, waterproofing additives and coatings are unnecessary, and freeze-thaw cycles are no longer a concern. In contrast to liner ponds, the waterproof EPDM or PVC membrane is fully encapsulated between geotextile layers surrounded by concrete, permanently protected from sunlight, weather, wear and vandalism, so it’s ideal for harsh conditions or public water feature construction sites.

Although liner and geotextile is an additional expense over plain concrete or cinderblock, it is usually comparable to the cost of acrylic additives or two-part coatings, and typically less expensive than sprayed polyurea foam, and it involves little additional technical expertise. This isn’t brain surgery. On the contrary, this simple, obvious method is easier and more tolerant of adverse conditions or less-than-ideal preparation, so we’ve found it adapts well to any jobsite.

Hybrid Liner/Concrete Pond Construction

We tried, with some success, to simply skim-coat EPDM liner with a couple of inches of cement, but found the simple way wasn’t so simple – the liner was vulnerable to penetration from sharps in the ground, tree roots and even shifting soils that settled and exposed the membrane, not to mention those hideous destructive juggernaut, rodents (hint: Chip and Dale do bite).

Nowadays, we’ll build outer walls on a solid footing just like the first two methods, either pouring or mortaring cinder block in place; the type and thickness depend on the application. If we’re going to pour the walls, the soil can serve as the outer form for the pour if we’re careful and cut the walls vertical.

If we’re going to use block we make the excavation a little wider all the way around so there’s a little room to work: we’ll backfill after the walls are set. We always use galvanized wall reinforcement, like Durawall, between our courses of cinderblock and we fill each course with concrete – the small additional expense adds tremendous strength and resistance to displacement. The walls don’t come all the way to the level of the water surface; we stop 8-10˝ below the intended water level to create a Rock Shelf for the natural rock coping to come. Once the walls are in, we backfill and level the soil behind them to create a broad shelf for the coping (and any Perimeter Bogs we might install behind the coping).

set stones portland surry

While we set stones, we smeared the rubber liner with a thin Portland slurry. When dry, it will give the next layer of sand and cement something to adhere to.

We’ll be pouring the floor last, but we need to grade the floor out now so we can cover the entire excavation – floor, walls and Rock Shelf – with a non-woven 6oz. geotextile, leaving plenty of extra to pull up behind coping and bogs to above water level. Our waterproof liner, usually 45mil EPDM, goes over the geotextile, again leaving enough above the walls to cover the shelf then come up another foot to well above the water line. We could cover the liner directly with concrete at this point, but we’ve found it’s both safer and much easier to cover the liner with another layer of geotextile, not just for protection, but because cement sticks to it like crazy, even vertically.

The final step is to cover the liner/geotextile ‘sandwich’ completely. Depending on the job, we may build both inner and outer walls of 4˝ block, or spray cement stucco over the geotextile, 3/8˝ at a time, with our little Tirolessa sprayer that we absolutely love for smaller jobs. Finally, we dump out a rich, fiber-reinforced mix on the floor a wheelbarrow at a time and trowel the sides and bottom smooth, working our way out as we go. We leave at least 3˝ on the floor and 2˝ on the walls, and the ‘gorilla hair’ type poly fibers help keep the cement in place even if it cracks or crazes on the surface.

Consider creating a design or covering the floor with pebbles if the job warrants a special touch – it’s always appreciated, even if it’s rarely seen after the pond grows in. The coping goes in last, with the largest stones mortared in with that 140lb mortar/40lb thinset mix.

For koi ponds, we lay in smaller stones dry in front of Perimeter Bogs, simple gravel beds 6 to 8˝ deep on 2 to 3-foot wide areas of the Rock shelf with the liner pulled up at the outer edge, so water can filter in and get filtered by the roots. Look up Active Bog Filtration for some really cool ideas to keep koi ponds algae-free.

Winding up

There isn’t enough room on these pages to go into greater detail, but I hope I’ve given you the idea that concrete can be a great option.

In warmer areas concrete ponds:

  • can be built and waterproofed many different ways, so they’re easily adaptable to most sites;
  • provide strong, lasting, virtually limitless structure regardless of soil conditions;
  • can be shaped and smoothed to make cleaning easier and safer than liner ponds;
  • properly constructed and waterproofed, are very low maintenance;
  • offer resistance to damage and vandalism that bare liner, or even gravel-bottom ponds cannot match.

In colder climates, using a membrane buried in the concrete shell to waterproof the pond offers all of the previous advantages, and is impervious to leaking from cracking and crazing that’s almost inevitable where winter holds an icy grip. Maintenance is even lower than in the warm weather ponds, since there’s no coating to scratch or wear off and settling cracks do no harm, and the liners can last virtually forever protected by their stony armor.

There’s no reason to shy away; concrete is far easier to use than ever before, thanks to advances in additive technology and delivery systems, and adding an impermeable liner makes ponds that are literally bulletproof. We’ve used this technique for ponds from 250 to 10000 gallons, and I’m pretty sure it’s adaptable to much more than I’ve run into. Give concrete construction a try next time you need a long-lasting, low maintenance, virtually indestructible pond.

Contact me at demi@atlanticwatergardens.com with questions; I’d be delighted to help.

16 Responses to Pond Construction with Concrete

  1. Shaun April 26, 2015 at 3:11 AM #

    Very informative

  2. Mark j June 22, 2015 at 5:39 PM #

    Thanks dude
    Great advice
    Good luck in future
    Bless mj

  3. certified pool builders August 22, 2015 at 6:55 PM #

    Thanks for finally writing about pond construction with concrete POND
    Trade Magazine. Really iked it!

  4. Lisa Pannier September 14, 2015 at 4:34 PM #

    Great post about pond construction. You just need to make sure that you use a sealer as concrete is very susceptible to cracking and can easily make the water alkaline. A pond liner over the concrete is the best solution to insure it doesn’t leak.

  5. Douglas Scott September 17, 2015 at 1:21 PM #

    Is there an easy way to take the old liquid epdm off and put on epoxy back on

  6. Marg Hefner March 9, 2016 at 9:31 AM #

    I had no idea. THIS is what I needed to read.

    I may have to change a way I don’t want to, for details in my “Frog Pond.” Having made a point of living in warmer climates than where I’ve decided to stay; there are big differences I’ve only guessed at.

    You’ve done me a huge service to write and make this article available.

    Thank you,
    Marg Hefner

  7. Steve Rice May 10, 2016 at 9:34 AM #

    Great information. Thank you, that is exactly what I needed to know. I build swimming pools and was trying to figure out a way to incorporate concrete and a liner into my koi pond I’m building. Thanks again.

  8. Cynthia June 14, 2016 at 4:26 PM #

    Thanks so much for the information. It’s nice to know there are still caring people willing to help others. Trying to pour my own 10 foot diamter above ground fountain basin in St. Louis and didn’t know how to handle the freeze thaw cycles to prevent cracking of the basin. This is the perfect information. Please know your a real Godsend. (Called everyone in the area asociated with fountains but none really knew how to do this. They normally just put a small basin on 6″ of gravel and pour a pad.)

  9. Ralph July 10, 2016 at 4:21 PM #

    Thank you! I’ve recently come across an issue whereas hawks are trying to swoop down and grab the koi fish therefore ripping the liner.

  10. Koi pond construction January 24, 2018 at 4:47 AM #

    Was a really good read, thanks for the advice!

  11. Joe March 7, 2018 at 11:40 AM #

    Very informative and helpful article. I have had two ponds with just the liner and maintenance has been a bear. As well as it constantly springing leaks. We have a huge husky in our neighborhood who likes to play in the pond on a hot day and leaves punctures in the lining.

    Anyhow, I am reshaping and changing aspect of the pond and decided that this tine we are going to pour concrete. Thanks for the helpful article.

  12. Kimberly McGinn April 14, 2018 at 8:50 PM #

    It must of been meant to be. So glad I accidentally ended up on this page reading your article. It was super helpful. I almost gave up searching for an article on concrete ponds not plain liner ones. These girls thank you!

    • Demi John Fortuna April 17, 2018 at 9:40 AM #

      Glad you liked it, Kimberly. Hope you and the girls have fun with your project! Be Well, Demi

  13. Lynn in Atlanta May 19, 2018 at 8:01 AM #

    Wow! This was incredibly helpful! We purchased a property with TWO concrete koi ponds, non-functioning. Have gotten the pump systems up and working, but were struggling with the concrete ponds. No one in our area will touch them; having to do all the work ourselves. Your article is just what we needed to read. Thank you!

  14. Richard Murphy September 22, 2018 at 3:36 PM #

    Yesterday I had a man who says he builds ponds tell my contractor and myself, that we DO NOT need a Footer. He said, in a monolithic pour (all at once sprayed Shotcrete) it is all self supporting. My previous “contractor” (actually a scam artist), was installing the Rebar by driving the verticals into the ground (amongst other not to code work). This consultant/new man says that is OK, is good to be in the soil, That is how he has done it for years.
    My counties Engineers say the Rebar is not allowed to touch the soil, must be UP in the concrete Footer. That is FL and County Code.
    I worked on building a Coast Guard Base in north Florida in the late 1960s and all our rebar was set OFF the Ground, the slab’s Rebars were on ‘Chairs’, (small metal props to lift the rebar 3″ off the soil) which will thus position the rebar INSIDE the concrete. I have not worked in the concrete aspect of construction for decades, but firmly believe this supposed Pond contractor is ill advising my current Contractor and myself.
    I believe, regardless of whether the cement is poured into forms or shot (as in Shotcrete) a minimum 8″ thick by 12″ wide footer is mandatory.
    Since the mid-1960s, the American Shotcrete Association refers to both forms of pneumatically applied concrete as shotcrete, with one called wet-mix and the other dry-mix. However, the pool and spa industry continues to refer to them colloquially as Gunite (dry mix) = dry until mixed at the nozzle and Shotcrete (wet mix) mixed wet before entering Shotcrete hose.
    ANYONE OUT THERE HAVE EXPERIENCE IN BUILDING A LARGE CONCRETE POND (mine will be 15,000 gallons). Your responses will be greatly appreciated (via this site or to surf1939@gmail.com).
    Thank you to all wet (and dry) animal lovers.

  15. Ralph V. November 30, 2018 at 4:44 PM #

    Thanks for the informative article. I have just built a small pond with interior dimensions of 2.5′ by 5′ with a waterfall. The concrete is very smoothly finished. I wonder why I have to seal it all? I like the concrete surface work that my crew did and don’t want a heavy epoxy or slurry sealer covering it up. It rained a week ago and water filled the pond an inch and didn’t go anywhere for days. I don’t plan to have fish or plants in the pond either. The only problem I can think of is that the rebar could corrode, expand and crack the concrete. If I do need to seal it, is there a lighweight penetrating or no sheen sealer I could use?

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