Eco-Friendly Mold for Small Concrete Slab

Concrete slabs are great for garden spaces, patios, or backyards. You can even use it as a planter pot base on your balcony or garden. Mold for concrete slabs at the home improvement stores is usually made from ABS plastics, it's cheap, lightweight but not eco-friendly, and sometimes looks fragile.

What if you need a re-usable, custom-sized mold with different lengths, thicknesses, and something that doesn't easily break/rupture?

I needed a small 20 x 60 cm slab with 2.5 cm depth and that's how I got into making the mold myself.

One thing to note is that this slab depth is only 2.5 cm. I intend for light use as a base for my planters and as a garden curb. Can be used for walkway footing but it's best to increase the wall height for more depth and thicker slabs.

It's a very simple, yet reliable mold made with scrap woods and hex fasteners. I advocate for a zero-waste community, so this DIY cast was built mostly from salvaged items. You may buy some cheap softwood for this mold, but if you can get scrap hardwood, you should use that (will explain this later). Some items like wire mesh & pre-mixed concrete were bought to make the slab but 100% of the materials used to build the cast were made with salvaged parts.

This DIY guide serves as an example of how you can actually make one for yourself and why using wood with fasteners is a better choice than a concrete mold made from plastics. It's not perfectly designed, but it's reliable, sturdier, and best of all; a zero-waste method! Isn't that a good way to sustain our dying environment?

A perfect dimension is not needed, it's simple and straightforward. You can adjust the tolerance to build it. Slabs made are going to be laid outside, so it's not a precision tool to be fitted or used with another item. If a different size is needed it can be modified accordingly.

Disclaimer:

Always take precautions and use appropriate safety gear while handling any tools, chemical powder/liquids. Learn how to use a tool, or chemical powder/liquid before using them. If there is a datasheet for certain chemicals, and you are unsure about it, read them first before handling them. Create a safe work area before you start any project.

Materials (Mold)

I suggest two different kinds of wood for the mold.

a) 1 x MDF or Plywood Panel for the base. (Bottom part)

b) 4 x Hardwood for mold wall (For all 4 sides of the mold)

c) 2 x Softwood for wire mesh/mold grip (Optional)

d) 1 x thin Aluminum or Zinc Sheet (Optional)

For (a)** I plan to re-use the mold many times whenever I need more slabs.

So, for the base, I used scrap B-grade Plywood. But you can also use an MDF board (coated). Both have smooth surfaces and this helps to unstick from the cured concrete easily. MDF boards are inexpensive but don't last long because they can easily get crusted if come in contact with moisture.

For (b)** For the sides, I used scrap Meranti/Shorea (untreated and unplanned). They are quite durable and withstand moisture and harsh environments. Alternatively, you may use a softwood like pine wood.

For (c)** This is optional, I use 2 Softwood sticks (2.1cm x 2.1 x 2.1 cm) on the mold, at the edge (wall) of each hardwood on both sides. I use it to position the wire mesh at the center of the wall as well as a grip to flip/lift the mold once the concrete is cured.

For (d)** This is optional and to use it with (c). A thin Aluminum/Zinc sheet is reshaped to cover the hole & provide entry for the softwood stick inside the mold area.

If not using (c) & (d), use longer hardwood for the bottom and assemble it as a closed wall for the mold.

Dimensions:

1 x MDF or Plywood Panel - (72 x 32 x 1.5 cm)

*The plywood used has a depth of 1.5cm, the higher the depth the better. The bolt can be securely tightened between the plywood and the hardwood.

4 x Hardwood - (4.3 x 4.3 x 4.3 cm)

*With different lengths

• 2 pieces ( L = 27 cm) - Sides
• 1 piece ( L = 72 cm) - Top
• 1 piece ( L = 54 cm) - Bottom

2 x Softwood for Wire mesh base - (2.1cm x 2.1 x 2.1 cm)

Length : 65 cm

Materials (Assembly)

Fasteners

a) M6 joint connector bolt (JCB) x 6

**I used an M6 joint connector bolt (JCB) with Hex drive to fasten all hardwood timber to the plywood base that I salvaged from an old cabinet. It's a strong fastener used in furniture making. Using a fastener is the best way to secure the mold on all sides. It's also easier to remove the slab as I can dismantle any side of the mold once the concrete is cured.

b) M6/M8 Long/Short Philips Screws

**for Aluminum/Zinc Insert

Insert for the Bottom wall (Optional)

1 x thin Aluminum or Zinc Sheet - (approximately 20 x 15 cm)

• Cut & shaped into an insert

Hand Tools

• Measuring Tape
• Triangle Square Ruler
• Digital Caliper
• Wood Clamp
• Hand Drill + Drill Bit (M5)
• Philips Screwdriver
• Hex Driver / Allen Wrench (M6)
• Scraper
• Tin snips (for Aluminum/Zinc sheet)
• Safety Gloves/Glasses/Mask

The first step is to build the mold wall starting from the top, left, and right sides of the board.

For the bottom hardwood, I measured and cut out an opening between the bottom and the right/left wall. This is to insert a 2.1 x 2.1 x 2.1 cm softwood stick for the wire mesh/grip for the mold.

I'll be only using one softwood stick for the wire mesh/grip, but for the purpose of measuring the opening between the wall, I used two sticks as in the picture. For this mold, it's 54 cm for the bottom wall with an opening on both ends.

Mark all the sides, clamp them in place using a wood clamp & install an M6 joint connector bolt (JCB) x 6 on the other side of the panel. I was only able to salvage four M6 JCB bolts, so for the other two, I used Philips screw instead.

Next, cut and shape the aluminum/zink sheet and create an opening for the softwood stick near the edge of the wall on both sides (optional method for the bottom wall). A simple shape will do. A barrier for the opening that has no gap except for the softwood insert. This will keep the concrete within the mold when it's poured in. Screw them in place.

This is an extra method to show you how I use the mold to create the slab from my previous work.

I use a 20 x 60 cm wire mesh with 1.5 mm thickness to place inside the mold for extra bonding with the concrete.

I use pre-mixed concrete to fill the area of the mold and let them cure. Once cured, I flip them over using the handle-like softwood stick and remove the bottom hardwood wall.

Unscrew and remove the piece. Once done, I flip them back. I slowly push a scraper at the bottom of the slab and lift them along the edges until the entire slab unsticks from the mold.

Simply clean the residue from the mold and assemble it back to re-use it another time.

That's it, you have an eco-friendly, custom-sized mold that can be used many times.