DIY CONCRETE:: Planter Box

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Introduction: DIY CONCRETE:: Planter Box

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: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.

Cardboard and Duct Tape Contest

Runner Up in the
Cardboard and Duct Tape Contest

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    42 Discussions

    Thanks for the guide, what about drainage holes?

    Kuhlmom here - You could make them with hypertufa - mixing equal parts of perlite and peatmoss with the concrete would dramatically reduce the weight and pressure - there is a mold release spay that helps get your concrete out of whatever you used to mold it :)

    Kuhlmom here - You could make them with hypertufa - mixing equal parts of perlite and peatmoss with the concrete would dramatically reduce the weight and pressure - there is a mold release spay that helps get your concrete out of whatever you used to mold it :)

    un excellent instructable qui démontre que le commerce n'ait pas seulement affaire d'argent mais peut être fait par des passionnés qui cherchent à démontrer l'utilité de leurs produits.

    The weight depends on the thickness - In this case it's right between 30 and 40 pounds. Heavy for a planter, but very stable!

    very nice!
    you could use wax to seal the cardboard, or a varnish.
    just a thought, You could wax or varnish a heavy texture fabric onto the mold as a stripe or spot.

    1 reply

    I would also suggest to anyone making this that they either make them 2 inches thick(Manufactures don't pay that extra money to still make them thick for no reason) Or, go to the hardware store and get a 3 foot tall roll of some cheap metal fencing and embed a piece into each side.

    I agree that a square shaped planter is not an original idea. However the instructable detail how to use cardboard and duct tape to create a mold. I believe votes determine contest winners, clearly many thought this was a winning entry. I suspect many who post instructables, also market what the build. They could decide to just market, and to to share

    i sent this link to my friends too coz just like they where to looking for a solution so thanx again

    awesome job iv been looking for something like this for ages thanx for the share

    This article is true self serving duplicity in the part of Cheng with a very old topic, he has plastered YouTube with his multitude of concrete countertop videos as well, selling his commercial products. This is not original and very commercial, outside the spirit of Instructables.

    3 replies

    I agree with Eric, It's completely unfair how an unoriginal idea, of a concrete box, can win a contest. So creative.

    I LOVE this project and would like to use it for my summer camp art class with my kids!

    Never touch cement powder or wet concrete mix with your skin.
    May as well set the inner box on the mix in the bigger box and start adding weights, rather than pressing it down, which will distort it and move it sideways.