Molding concrete is made easy for do-it-yourselfers with wood forms that create the perfect shape. Instead of setting the concrete in place, you can cast the concrete in a convenient location like a patio or garage. You don't even have to worry about troweling the top smooth. The form does most of the work for you. The concrete is cast upside-down, so the mold's smooth bottom becomes the concrete's top.
Making a concrete mold from wood is simple and straightforward. If you get this step right, a large part of the casting process is already done for you.
Basics of Making Concrete Molds From Wood
Start With a Coated Surface
Uncoated wood is routinely used to make large molds for curbs and house footings. After curing, the concrete will unstick from the wood relatively easily. But using wood coated with a slick surface will make the release even easier. For this, a 3/4-inch thick MDF board topped with melamine is ideal.
Use an Inexpensive Disposable Wood
After being used for molding concrete, the melamine board is often too crusted with concrete to be reused. Melamine-faced board is inexpensive and can be discarded after use.
Use a Release Agent
To aid the release of the concrete from the mold, coat the surface with a release agent. Cooking spray, motor oil, vegetable oil, and dishwashing detergent mixed with water (in a 1:10 ratio) are a few of the release agents available around the home. Or you can use a product designed solely for this purpose, such as a spray or brush-on silicone-based, semi-permanent sealer and release agent.
Use Exposed Removable Fasteners
When building the mold, construct it with fasteners that are easy to remove. Screws are perfect for making a mold because they can be turned out. The heads should be exposed, too. If a fastener's head is embedded in the concrete, you will not be able to access it to remove it. Face-screw the fasteners—no need to sink them into pocket holes.
Equipment / Tools
- Circular saw
- Caulking gun
- Cordless drill
- Drill bits and drivers
- 2 Bar clamps
- 1 Marble or ball bearing
- Latex gloves
- 1 Melamine white MDF panel, common: 3/4-inch by 4 feet by 8 feet
- 22 1-inch drywall screws
- Silicone caulk
Cut the Melamine Board to Size
With the circular saw, cut the 4-foot by 8-foot melamine board into the following pieces:
1 piece of 24 inches by 48 inches
2 pieces of 3 inches by 48 inches
2 pieces of 3 inches by 25 1/2 inches
To minimize chipping the melamine surface, use a fine-toothed saw blade and set the saw depth to just below the depth of the saw cut (about 7/8-inch).
Clamp the Long Side Walls to the Mold Base
Lay the large melamine board on a work surface with the melamine side facing upward. Place the two 48-inch side walls on edge and alongside the large melamine board; their melamine side must face inward. Hold them in place with the bar clamps.
Add the Long Side Walls to the Mold Base
Drill six equally spaced pilot holes per side wall. Follow with the 1-inch drywall screws. Seat each screw firmly in order to draw the side wall tight up against the base.
Add the Short Side Walls to the Mold
Place the two 25-1/2-inch side walls on edge and alongside the large melamine board. As with the long walls, their melamine sides must face inward. Start with pilot holes, then add five screws per side wall.
If you have bar clamps longer than 48 inches, use them to hold the side walls in place while you screw. Otherwise, use a wall as a stop to hold the mold in place.
Bevel the Corners
While wearing gloves, squirt a 1/4- to 1/2-inch bead of silicone caulk into every inside corner of the mold (for a total of eight corners). Dip the marble or ball bearing in water. Draw it down each corner to create a smooth curve.
Remove the Excess Caulk
After the silicone caulk has cured, remove the two strips of excess silicone on each corner. Pick at the end of a strip with your fingernail. Pull the excess bead away and dispose of it.
Making Wood Grain Concrete
To cast concrete that has a wood grain texture, you'll need to take the intermediary step of casting a polyurethane rubber mold of the wood. That's because concrete casting produces the negative of the mold material. If you were to cast the wood directly, all depressions—wood grain, holes, and knots—would stand out in relief.