I made this 10 kg (23 lbs) dumbbell out of really cheap and recycled materials. It does the job really well, except it's very large compared to a regular dumbbell.

I hope you'll find it useful, enjoy.

Step 1: Tools & Materials

For a single dumbbell you are going to need

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 2: Measuring

To learn how tall 3 liters would up to be I poured 3 liters of water in the bottle and marked the water level with a sharpie.

Step 3: Mixing Concrete

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.

Step 4: Assembling the Pipes

You need it to act as an anchor so that the pipe won't slip out and smash your skull while you're doing triceps extensions.

Step 5: Fixing the Rod

To secure the rod in the center I cut out the hole in the cap of the bottle and slid the top portion of the bottle onto the rod. Then I secured it with a sticky tape. I let it get hard enough so that it would support the rod in the center, after which I took the top section off and let it cure overnight.

Step 6: Vent Holes

I had cut out the vent holes in the top portion of the bottle beforehand.

Step 7: The Other Half

For the other half I did the same process. Except this time i had to slid the top part of the bottle onto a rod before I would insert it into the other half of the dumbbell. I propped it up in the corner so that the halves would stay parallel to each other.

Step 8: Handle

For the handle I wrapped the rod with a pipe insulation and secured it with a strip of electrical tape on either ends.

Step 9: What Would I Do Differently

I would definitely make the handle 8-10 cm shorter. 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.