This Instructable will show you how to make concrete boxes using cardboard and duct tape. This example is a planter, but they could also be useful as storage cubbies. This project is nice because it's cheap, quick, and easy.

You could cast concrete in a cardboard box... except for one thing - it's not water tight. The cardboard will suck the moisture out of the concrete, leaving a chalky finish at best. At worst the box will blow out or fall apart before the concrete has cured.

The solution to this problem is, of course, Duct Tape. Taping the inside of the box will make it water tight, and also give the concrete some texture. Taping cardboard edging to the corners will help strengthen the box and minimize the bowing under pressure that happens when casting.

Materials + Supplies:

Cardboard Boxes (2)
Cardboard Edge Protectors (90 degree bends)
Duct Tape (1/3 Roll)
CHENG Pro-Formula Concrete Mix (Tahoe Color Shown)
Sakrete 5000+ Concrete Mix
Weights / Rocks / Sand / Something Heavy

Concrete Mixing:

5 Gallon Bucket
Shovel / Trowel
Particle Mask
Rubber Gloves

Concrete Finishing:

Step 1: Tape the Boxes

What makes a good cardboard box for this project:

Heavy Duty
Rigid / Relatively New
Not too big (12"x12"x12" of concrete weighs about 135+ pounds, think about the pressure)

Less than Ideal:

Flimsy
Dirty
Thin cardboard

Find two boxes that fit inside each other with a 1" gap on all sides. More than 1" and the box could be quite heavy, less than 1" and it won't be as strong.

On the larger box (the main form):

1. Fold the flaps outward and tape them down.
2. Tape the inside edges of the box, especially the corners.
3. Tape the entire inside of the box, with some overlap, in any pattern you want.
4. Tape cardboard edging on the outside wherever possible to help strengthen the box.
5. Add any decoration, like leaves, rubber stamps, pieces of tile, with spray adhesive.

On the smaller box (the knockout):

1. Fold the flaps inward.
2. Tape the outside of the box completely.

Place the smaller box inside the larger box. Lift up the smaller box until there is a 1" gap at the bottom. Make a mark on the small box, even with the rim of the larger box. When casting, you'll use this line as a reference so you know how deep to push the smaller box down.

Step 2: Mix + Pour + Cure

Have a few of these boxes ready and expect to mix up 60-80 lbs. of concrete. Keep some smaller boxes ready too or some other projects in case something doesn't work out.

For more tips on mixing concrete, check out the How-To-Mix Concrete Instructable.

1. Add the dry material to a 5 gallon bucket (CHENG Pro-Formula, Sakrete 5000+ Concrete Mix).
2. Add water and mix with a trowel, scraping dry material off the walls of the bucket and gradually adding more water until the mix is a good consistency (fully mixed, somewhat stiff, and not too wet).
3. Fill the large box halfway, then press the knockout down into the concrete and hold it there.
4. Put something in the knockout that's heavy (rocks, sand, weights).
5. Continue adding concrete until it reaches the top of the box.
6. Tap on the sides of the box continually to help air bubbles escape to the surface.
7. Cover with plastic to keep humidity inside.
8. Let cure for 4 days in a comfortable environment.

Step 3: Demolding + Finishing

Demolding is easy, just be sure that the concrete has fully cured. If it hasn't cured for at least 2 days it might still be brittle and will break if you're not careful during these steps.

1. On the larger box, tear or slice the cardboard in one corner.
2. Repeat on the remaining corners until all walls are cut down.
3. Squish the smaller box away from the concrete and pull it out.
4. Sand any sharp edges with a diamond polishing pad.
5. (Optional) Seal with CHENG Concrete Countertop Sealer, visit the Instructable page for tips on sealing concrete.

Step 4: Finished Concrete

Using cardboard boxes as a forming material for concrete is great because they come in a huge range of sizes and you can get them for free. The versatility of duct tape makes it ideal for temporarily water-proofing and reinforcing the cardboard box. I like this project because it's somewhere in between rigid and fabric formwork. The concrete bulges out the cardboard walls in a subtle way, and each box you make will have it's own unique character.

Thanks to Instructables and their sponsors for the Cardboard and Duct Tape contest, and thanks for following.