Time Required: 1-2 hours + 2 days to cure
Weight: 4-6 pounds
Plastic Popcorn Form ($1 section at Target)
Sacked Concrete Mix - NON-Aerated, 5000 psi is ideal (one 60lb bag will produce about 12 of these forms)
CHENG Pro-Formula Mix (1 cubic foot box per two 60lb bags)
Styrofoam (small scrap pieces)
Wood (.5" thick)
2 Part Epoxy
Clear Packing Tape
Hand Saw (Japanese style with fine / medium teeth)
Razor Blade / Utility Knife
Sand Paper / Handheld Diamond Polishing Pad
Slotted Screw Driver / Phillips Screw Driver
1/16" drill bit
Silicone (100% silicone, window and door sealant)
Buckets for Mixing
Step 1: Measure, Mark + Cut the Concrete Form
1.2 To help support the plastic container while cutting, cut a styrofoam block ( about 3¾" x 3¾" ) and push it in the opening of the form.
1.3 Starting from one corner, gently saw back and forth to start a straight cut. Go slowly at first, alternating back and forth between corners until you can connect the slices and cut through. Repeat on the remaining sides until the cut is complete.
1.4 Clean up the edges of the plastic with some rough sand paper ( 80-100 grit ) and be careful not to scratch the inside of the form (any scratches WILL be transferred to the concrete).
Step 2: Make, Tape, Silicone + Install the Concrete Knockout
2.1 Rough cut a small piece of styrofoam with a saw or utility knife into a rectangle and sand down to (1½” wide x ½" tall x ¼" - ½” deep).
2.2 Wrap the styrofoam neatly with clear packing tape. This will keep it from bonding to the concrete and will give us a smooth surface.
2.3 It might be helpful to make a small cardboard template to use as a guide to center the knockout in the form. Apply a thin coat of silicone to one of the ½" faces and press it down into the form. Wipe away any excess silicone that squeezes out and wait a few hours for it to cure.
2.4 Drill or cut a small hole (1/16") behind the knockout and install a short 1" screw so the knockout doesn't float away during the pour. (The silicone is going to keep concrete from seeping between the knockout and the form wall. If that happens, the knockout will move around and could be lost completely. The silicone is also keeping the foam from floating to the surface — foam floats in water and concrete contains water — the screw will hold the foam just in case the silicone does not).
2.5 After the silicone has completely cured (at least 3-6 hours and up to 24), remove the excess with denatured alcohol and blow any debris out of the form.
Adhesive Backed Felt Knockout:
2.1b Find some heavy duty felt with an adhesive back, cut down to the size of the knockout, stack two or three pieces together.
2.2b Wrap the stack of felt pieces in clear tape, peel off adhesive backing and fix to the inside of the form. The advantage of this method is that you don't have to wait for the silicone to dry. The disadvantage is finding and paying for the felt!
Step 3: Mix, Pour, Vibrate, Cover + Cure the Concrete
Weighing out dry materials should be done somewhere with proper ventilation and it's a good idea to wear a particle mask if you're doing it often. Small projects like this can be mixed by hand. It's best to wear rubber gloves and work in a space that is easy to clean up. It's also a good habit to start with clean tools, buckets, and especially a clean workspace so we don't accidentally get pieces of dirt or styrofoam in our batch of freshly mixed concrete.
Normal sacked concrete will be cured enough to demold in about 5 days. Smaller projects like this can be taken out of the form after 2 or 3 days.
3.1 Start with a gallon-bucket (or an old mixing bowl). Measure out 1½ popcorn containers (about 6 pounds) of sacked concrete and dump it into the bucket. Add Pro-Formula Concrete Mix (1/24th of 1cu. foot box). Mix thoroughly and break up any big clumps of material. Add about 6 ounces of water to the mix, keep stirring by hand or with a spatula or trowel, always scraping the sides and bottom of the bucket. Gradually add about 6 more ounces of water until the consistency is like runny oatmeal.
THE IDEAL CONCRETE MIX
When mixing concrete, it is important to note that the least amount of water used relative to the cement produces the least amount of shrinkage and yields the strongest, most durable concrete. At the same time, the mix needs to be fluid enough to fill all the gaps in the mold. A mix that is too dry is hard to pour and vibrate; a mix that is too wet is likely to shrink excessively and may crack.
When the concrete is fully mixed, you have about 20 - 30 minutes before it will harden and become unworkable. You can agitate the concrete by squeezing it in your hands to regain some fluidity if it gets too hard.
3.2 Pour concrete into the form, but be careful not to dislodge the knockout. While pouring, smear concrete around the inside walls of the form and under the knockout to coat the surface with concrete. This will help minimize trapped air.
3.3 To help the air bubbles rise to the surface, vibrate the concrete by tapping the base of the form on the tabletop for a few minutes. Your final piece will always have some air bubbles if you're not using a commercial concrete vibrator. If you aren't getting results you like vibrating by hand, you can try using an orbital hand sander or a sawzall without the blade and vibrating it against the form walls.
3.4 After the form is vibrated, wedge a piece of ¼" foam under one side of the container so the top surface is level.
3.5 Smooth / Trowel the surface with a plastic scraper. Take some time to remove excess concrete from the sides of the container, and get that top surface as smooth as you can (this will minimize sanding later).
3.6 Move the form carefully to a place where it can cure for up to 2 days. Cover with a plastic bag or painter's plastic. This helps hold the heat and moisture in the concrete so it doesn't cure too quickly and crack. For this reason, do NOT place the concrete to cure in direct sunlight, keep it indoors or in the shade.
Step 4: De-molding the Concrete Form
4.1 Wearing rubber gloves and safety glasses, prepare to de-mold. Scrape excess concrete off the plastic form with a razor blade.
4.2 Start at the opening of the container, peeling back the plastic by hand and slicing down into it with a sharp razor blade.
4.3 Break plastic away by hand or with pliers. Remove a little bit at a time. Peel back, slice, break away. Do this as carefully as possible, still taking care not to scratch or chip the concrete.
4.4 Be very careful when removing the bottom of the container. On each side of the form there is a tab. A small screwdriver or chisel and a quick tap from a hammer will help free those tabs.
4.5 After the plastic form has been removed, pry out the knockout with a flat screwdriver.
Step 5: Sand, Grind + Make the Concrete IPad Ledge
5.2 Using a hand saw, band saw, or tile saw, cut a ledge that fits in the groove created by the knockout. The easiest thing to do would be to start with a piece of wood that is the same thickness as the knockout (½" in this example). Softer woods will be easier to cut with hand tools if you don't have access to a band-saw. You don't really need power tools for something like this. Check out the episode Sawing Secrets from the Woodwright's Shop on PBS for some tips on working wood with hand tools.
5.3 Finish the concrete with CHENG sealer and CHENG wax before gluing the ledge in place with two-part epoxy.
Step 6: Concrete IPad Stand, Finished Images
We’re sure you’re able to come up with a ton of your own ideas for this nifty container—we’d love to hear from you and see what you’ve created—please send us your version of the Concrete iPad Stand or any other project versions at firstname.lastname@example.org. GET IN THE MIX, HAVE FUN WITH CONCRETE!