Concrete Mold Construction

We love concrete because it perfectly reproduces the form you put it in. However, that means the mold needs to be solid and smooth because grinding hardened concrete is much more difficult than fixing a poorly fitting mold. Below is a concrete design created by raised sections attached to the forms at the Portland Cement Association (PCA) headquarters.

Things to think about when putting together a mold.

1. Mold material should be rigid - When the mold (and whatever you have the mold sitting on) is constructed it needs to support the weight of the fresh concrete. If the selected mold material is not rigid enough there is a risk it may deflect when full of concrete and create a wavy surface. At standard weight concrete weight of 150 lbs/cubic foot and a 1 foot by 1 foot section that is 2 inches thick will weigh 25 lbs.

2. Mold surface should be smooth - Since the concrete will reproduce any imperfections the inside of the mold should be as smooth as possible. This is best done by using a laminated particle board or rigid plastic that have uniform surface texture and smoothness.

3. Mold materials should be easy to work with - Ideally special tools should not be required to create the mold. Particle board or plastic molds can be easily built using a circular saw and drill.

4. Material should be impermeable - Concrete contains water, when wood absorbs water it swells. Egyptians would place a small wood wedge in a stone crack then pour water over the wood. The small piece would swell and crack the large stone pieces. Concrete shrinks when it cures and wood swells when wet so to prevent unwanted cracks, make sure any wood in the mold is sealed. This is especially important if you have a wooden section inside your concrete such as a sink opening.

5. Mold should be designed so it can be disassembled when full of concrete - Remember, the mold will be filled with concrete. Construction of the mold should be done in reverse to allow deconstruction once the concrete is hard.

Commonly used materials include:

- Melamine (laminated particle board available at Home Depot or any other home improvement store),

- Plywood or particle board painted with exterior paint to prevent moisture penetration, or

- Extruded rigid plastic sections fixed to a rigid surface with double-stick tape or screws(available through the Cheng concrete on-line store)

Unless you are building a free-standing piece such as a table, a template will have to be made so the sink or countertop fits in the space. Walls are never straight and this prevents unwanted gaps between the wall and concrete piece. The template can be pretty much anything, plywood, particle board, cardboard, or thick plastic. The template below for the sink was 1/2-inch plywood strips glued together. Be sure to label the front, back, sides, and locations of any knockouts needed for faucets or sinks. The template will be inverted for mold construction, keep track of which direction is the front to prevent misplacing the openings. In this picture the bath vanity will have two sets of faucets with handles and one central drain. Later the drain was moved from the center to the right side.

A little note on design, start with something easy first THEN get more complicated. For this sink I tried incorporating many complicated aspects at once. It turned out well and will provide good learning experience for future project.

This mold construction is a pretty standard technique used by many concrete countertop makers. The Cheng concrete countertop book is a good reference for mold design and construction.

Here is a beginning of mold construction using 3/4-inch melamine particle board. The dimensions are roughly 42-inches long, 23-inches wide, and 4-inches deep. The sides are screwed to the bottom piece and into the adjacent side pieces. This sink bottom will be flat and slope to the single drain now located on the right side. So in this picture the far side will be the front and the near side the back. The piece on the right will be the low point of the sink where the drain is located.
The nearly completed mold. The sink bottom slopes to the right side. Wooden dowels wrapped in clear tape are screwed from the underside to make the space for the faucet and handles. The larger area for the faucet connections was made by wrapping 1/4-inch foam around the dowel and covering with clear tape. Home improvement stores have a wide variety of foam that work well for concrete projects.

Any exposed wood or screw heads get covered with clear tape. Joints and any wood to wood connections get sealed with silicone caulk to prevent moisture soaking into the wood. A dark colored silicone is easy to see on the white melamine.

Notice there is no rebar or wire reinforcement in this particular piece. Concrete is it's biggest volume when fresh and shrinks when it cures. Therefore concrete cracks. Cracking can be controlled by using a good concrete recipe and limiting the size of our pieces. The main purpose of reinforcement in concrete is to provide strength should the concrete crack. So for large concrete pieces or those with cantilevers reinforcement is good, for little pieces with good support reinforcement is not needed.