Concrete Rain — a Concrete Cloud That Rains

Concrete Rain is a recirculating water sculpture built around a simple contradiction: a cloud made out of concrete. Water falls from the concrete cloud into the basin below, where a hidden pump collects it and sends it back up to the rain head, creating a continuous loop of rainfall.

The idea started with the cloud itself. A cloud and concrete are complete opposites and should normally have nothing to do with each other. I wanted to combine those opposites by sculpting a cloud from concrete, and then take the idea one step further by making the cloud actually rain.

This was my first time working with concrete in this way, and my first attempt at making a concrete sculpture. I learned a lot of the process as I built it, so there were definitely a few mistakes, broken castings, and improvised fixes along the way.

I built this project for the Stone, Concrete & Cement Contest and was working under a pretty short deadline, which made the whole thing a bit of an experiment as I went. I've included both what worked and what I would do differently if I built it again.

1. Pre-mixed cement repair/leveling mortar (3700)
2. Water
3. PLA filament
4. Silicone Tubing
5. 5/16" steel reinforcing bar
6. 600L/h submersible water pump
7. Rubber bands
8. One big plastic bowl ( You will have to break this one later )
9. One plastic bowl smaller than the big one

Download the attached STL and STEP files and 3D print them. Body 1 and Body 2 were made specifically for the pump I used. They form a simple box with a large opening at the top for water intake and a serviceable bottom cover, allowing the pump to be removed or replaced without having to break the concrete structure apart. It's a Nubios MY-038 600L/h pump with adjustable flow control.

The remaining files were designed specifically for casting the upper rain head and pump enclosure for the bottom reservoir. PLA works fine because the molds only need to survive the casting and demolding process.

After printing, remove any supports or brims and test-fit the pieces. Make sure all mating surfaces fit together and that none of the small openings are blocked by leftover filament.

For the upper rain head, assemble the printed pieces using screws, then seal the seams. The screws are useful for holding everything together, but they are not enough on their own to make the chamber watertight or withstand the internal water pressure. Apply hot glue, silicone, or another waterproof sealant along the seams before assembly.

I used tape to seal mine because I was working under a very short deadline, and it worked surprisingly well for testing. However, I wouldn't recommend tape for a permanent build, since repeated exposure to water will eventually weaken the adhesive. A proper waterproof sealant is the better option if you want the sculpture to last.

Cut and weld two separate steel structures: one for the upper rain head and one for the bottom reservoir. Use 5/16 in (approximately 8 mm) steel reinforcing bar for both.

Keep the two structures separate for now. This allows you to cast the upper and lower concrete pieces at the same time, with their respective steel structures embedded directly into the mortar.

Position the steel structures inside their molds before casting, making sure that the ends that will eventually connect remain exposed.

After both pieces have been cast, demolded, and given enough time to gain strength, align the exposed steel sections and weld the upper and lower structures together. This way, the steel frame carries the weight of the suspended rain head rather than relying on the mortar itself.

The exact dimensions of the frame will depend on the size of your molds. The important part is that the upper structure is centered over the basin and that enough steel remains exposed after casting to weld the two halves together later. Before casting, dry-fit everything and check the height and alignment of the sculpture.

There are two ways to approach this depending on how well your molds printed.

1: Using a plastic separation layer

Before adding the mortar, cover the 3D-printed internal parts with thin plastic bags. Push the plastic closely around the shape and remove as many large wrinkles and air pockets as possible.

The plastic acts as a separation layer between the mortar and the printed components. This is especially useful around the pump enclosure, since that part needs to remain accessible instead of becoming permanently bonded to the mortar.

However, I would only recommend this as a quick solution. It worked for my build, but wrinkles from the bags transferred into the concrete and some pieces of plastic became stuck to the finished casting. If I built this again, I would use a properly sealed mold and an appropriate mold-release method instead.

Don't stretch the plastic so tightly that it tears. Fold or gather excess material toward areas that won't affect important surfaces.

2: Using the printed mold directly

If your mold printed cleanly, the sections fit tightly together, and there are no gaps where the mortar can escape, you can skip the plastic layer and cast directly into the mold using an appropriate release method.

Once everything is prepared, position the printed components and their respective steel structures inside the molds. Double-check that the pump enclosure, water openings, tubing paths, and exposed steel sections remain accessible before pouring anything.

Prepare both molds and place their respective steel structures inside them before mixing the mortar. Make sure the steel is positioned roughly in the center of the casting and does not touch the outer walls of the mold. Leave the ends that will later be welded together exposed.

For this project I used a pre-mixed cementitious leveling and repair mortar, so no additional sand was necessary. If you use a similar product, follow the water ratio recommended by its manufacturer.

Add the dry mortar to a mixing container and gradually add water while mixing. Avoid adding too much water at once. You want the mixture fluid enough to work into the molds and around the reinforcement, but not excessively runny. Peanut butter-y.

Fill both molds gradually. Every few centimeters, tap and shake the molds to help the mixture settle around the steel and release trapped air. Pay particular attention to corners and narrow sections where air pockets can form.

Fill both molds completely and level the exposed surfaces. Check one final time that the steel structures haven't shifted while filling.

Leave both pieces undisturbed in their molds for at least 24 hours before even considering demolding them, or longer if your material manufacturer recommends it. The mortar will still be far from fully cured at this point.

If you're not working under a deadline, I strongly recommend giving the pieces additional time before demolding. One of my earlier castings broke during this project, and I learned very quickly that a casting that feels hard on the surface can still be surprisingly fragile.

Once the mortar has gained enough strength, carefully begin removing the molds.

For the bottom basin, I treated the outer plastic bowl as disposable. Instead of trying to force the concrete out and risking another broken casting, I carefully cut through the bowl with a Dremel and removed it piece by piece.

Work gradually around the circumference rather than trying to release everything from one point. Avoid prying directly against thin concrete edges, since the casting is still relatively young and can chip or crack.

For the upper rain head, remove the rubber bands holding the mold sections together and carefully separate each section from the concrete.

If you used plastic bags as a separation layer, peel away as much of the plastic as possible by hand. Some of mine became trapped in wrinkles and surface imperfections, which I cleaned up afterward.

Take your time with this step. I broke an earlier casting during this project, and sacrificing a cheap mold is much better than sacrificing the concrete part you just spent a day waiting for.

If you used plastic bags during casting, small pieces may remain trapped in the surface after demolding.

Remove as much as possible mechanically by peeling, trimming, scraping, or carefully sanding it away.

Because I was working under a very short deadline, I also carefully used heat to remove some stubborn pieces from my casting. However, this can produce unpleasant or potentially harmful fumes depending on the plastic, so I would not recommend burning the plastic as the normal removal method.

A cleaner mold and proper release method should eliminate most of this problem entirely.

Once both concrete pieces have been demolded and given enough time to gain strength, position the upper structure over the basin and align the exposed steel frames.

Check that the rain head is centered over the basin before welding. I recommend making a few tack welds first, stepping back to check the alignment, and only completing the welds once you're happy with the position.

Weld the upper and lower steel structures together in short intervals to avoid putting unnecessary heat into the surrounding concrete.

Keep the pump, silicone tubing, electrical wiring, and any 3D-printed components protected from welding heat and sparks. Also make sure your welding current path remains through the steel structure rather than through any electrical components.

Before finishing everything, test the complete water system.

Fill the basin with enough water to completely submerge the pump, then briefly switch it on. Check that water reaches the upper rain head and begins flowing through all of the openings.

Watch where the streams land. Ideally, the entire curtain of rain should fall back inside the basin without splashing outside it.

My pump has adjustable flow control, so I used this test to find a setting that produced a visible curtain of rain without sending excessive water outside the sculpture.

Also check the tubing, pump enclosure, and rain head for leaks before moving on. It's much easier to fix one now than after everything has been cleaned and finished.

After demolding, the basin had a lot of excess concrete around the rim, leaving it much thicker and rougher than I wanted. I used an angle grinder with a masonry/concrete grinding disc to gradually remove the high spots and smooth the surface.

This creates a lot of concrete dust, so wear eye protection, a proper dust mask/respirator, and work outdoors or in a well-ventilated area.

Simply connect the silicone tubing to the rain head and use silicone adhesive or waterproof sealant to secure the connection and prevent leaks. Make sure the tubing is pushed firmly onto the inlet before applying the sealant, then allow it to fully cure before running water through the system.

In my case, I didn't have silicone adhesive available and was running out of time before the contest deadline, so I temporarily secured the connection with tape for the final photos. I wouldn't recommend this as a permanent solution.

Route the tubing along the steel frame where possible so that it stays out of the main viewing area. This keeps the sculpture looking cleaner and prevents the tubing from interfering with the falling water.

After grinding, wipe down every surface to remove any remaining concrete dust or debris. Give the basin a final rinse, fill it with clean water, and switch on the pump.

And that's it — Concrete Rain is finished.

This was my first concrete sculpture and definitely a learning process. I broke a casting, fought with the molds, got plastic stuck in the concrete, and had to improvise several parts of the build along the way.

But in the end, the original idea worked: a heavy piece of concrete seemingly producing a continuous curtain of rain.

If I built it again, I would spend more time designing the molds, use a better release method, and give the castings considerably more time before demolding them. Those changes would make the finished surfaces much cleaner and make the entire process easier.

For a first attempt at working with concrete like this, though, I'm pretty happy that I managed to make concrete rain.