I made these DIY dumbbells out of reused materials. They each weigh 10 KG (22 lbs) and I am going to show you instructions for a single dumbbell.

Supplies

Materials

Two 3 liter containers.I used 6 liter water bottles that I cut to size.
50cm (16") long metal pipe 3/4" in diameter
Two 10cm (4") bolts 10mm in diameter and 4 accompanying nuts
25 kg (50 lb) bag of cement
Pipe insulation or a pool noodle

Tools

Drill
X-acto knife or a box cutter

Step 1: Measurements

To find out the height that you need to end up with 3 liter containers, pour 3 liters of water in the jug and mark the water level with a sharpie.

Cut the jug into two pieces cutting a few inches above the water level.

Step 2: Mixing the Cement

I started pouring 500 ml of water into the container and gradually added 2.5 liters of cement. This ratio might vary for you so start with a little bit first and go by feel.

It will change the weight of the dumbbell, however, not significantly.

Step 3: The Crossbar Assembly

You need to make an anchor on each end of the crossbar as shown on the photos.

Drill a hole through the pipe on each end and secure the bolts with a single nut on either side. The holes do not need to be parallel to each other.

Step 4: Fix the Crossbar in Place

To secure the pipe in the center you need to cut out a hole in the cap, slide the top portion of the jug onto the rod and secure it with duct tape.

Let it get hard enough so that it would support the pipe in the center, after which take the top section off and let it cure overnight.

Prior to this step, cut a few vent holes into the top portion of the jug.

Step 5: Second Half

For the other half repeat the same process. Except this time you have to slid the top part of the jug onto the pipe before I inserting it into the other half of the dumbbell.

Prop it up in the corner so that the halves will stay parallel to each other and let it cure overnight.

Step 6: The Handle

For the handle cut a slit into a pool noodle (or a pipe insulation) and slide it onto the pipe. Secure it with a strip of electrical tape on either ends.

For extra security you can glue the noodle in place.

Step 7: Additional Notes

I would definitely make the handle 8-10 cm shorter than the. I didn't consider the indent on the bottom of the container which made the whole thing even longer. Now it's a bit awkward to do alternating bicep curls with it but for the other exercises that I do it's totally fine.
The walls of the bottles are thin and flimsy and the mixture warped it. Which is not a big deal, it doesn't really interfere with the balance of the whole thing but it's kind of annoying. To avoid warping taping the whole mold with duct tape might be a way out.

Thank you for attention. Stay Hydrated!