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.
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.
There are three different ways I know of to waterproof the concrete pond, all starting with the solid foundation just described.
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).
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.
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 firstname.lastname@example.org with questions; I’d be delighted to help.