Preparing the mold properly for a pour is essential for many reasons:
- It needs to be watertight so that it will hold the wet concrete without leaking.
- Every detail in the mold will transfer to the countertop. Even the slight texture on the melamine will be the same texture on the countertop when you first break the mold.
- It must be level and well supported. You will be pouring hundreds of pounds of concrete into the mold, so you must make sure that it doesn't buckle or bend under the weight. Also, you're going to need to vibrate the concrete, so it needs to be screwed to its support.
- The concrete will start setting in 30 minutes. Once the concrete is mixed you'll need to work fast so you want to make sure the mold is fully prepared.
I don't have any pictures of this, but you'll need to properly seal the mold using silicone caulk. I used black caulk so you could see it against the white melamine. You want the caulk to have extra clean edges, so I used blue painters tape on either side of the seams, backed off about an 1/8". After you apply the caulk, carefully remove the tape before it dries and you'll have super clean edges that will require minimum grinding. You'll want to let the caulk dry overnight.
You also need to make sure that any cut sides of melamine that will be exposed to wet concrete are sealed so they won't swell up with moisture. To do this I sprayed spray adhesive on the exposed edges, then put down clear packing tape on it. I trimmed the excess tape with an exacto knife. To avoid getting excess spray adhesive in the mold I used a straight piece of cardboard to shield the overspray.
The tape covered up most of the screw heads in the mold, but there were some that were still exposed. Since you need to remove the screws to break the mold, make sure that any exposed screw heads are filled with some type of clay or putty (plumbers putty, plasticine clay, etc.). This will avoid getting the concrete into the screw heads, which would make it next to impossible to unscrew them.
To support the mold I used 2 very stout sawhorses with 2 sheets of 3/4" plywood stacked on top of each other and then drilled into the sawhorses. I laid the mold on top and screwed the edges of the mold into the plywood. For added support, I took some scrap pieces of wood, cut them to size, and screwed them in around the edges of the mold.
I didn't have any significant overhangs with my countertop, so I didn't use rebar to support it. I wanted more support so I used 2 flat sheets of concrete wire mesh. I cut them to size, then stacked and offset them, and tied them with wire. You'll also notice that I stacked two pieces of styrofoam to make 1" spacers off the bottom of the mold. Remember that if the wire mesh gets too close to the bottom or the sides of the mold (which is the top of your finished countertop) you will have problems with ghosting. By using spacers we're ensuring that the mesh will be halfway in the middle of the countertop. Also, make sure the mesh is 1" away from all the edges of the mold.
Now that I had the support cage made, I lifted it out for final preparation. First off, I vacuumed the mold and cleaned it thoroughly with rubbing alcohol. To add further personalization to the countertops I wanted to embed coins from places that my wife and I had traveled to. This was easily accomplished by rubbing a bit of caulk on the side of the coin I wanted exposed and arranging them face down on the mold. The caulk gives just enough adhesion to keep the coin in place during the pour and I didn't have to wait for it to dry.
Mr. P Note: "Remember, the side of the coin that is facing down will be exposed when the countertop is finished. In other words, the side you put the silicone on will be exposed."
The next steps were to put the styrofoam spacers back in the mold, lay the cage down, and then tie wire to support the cage once the spacers were removed. I put screws around the outside edges of the mold so I could tie the wire around the heads and then wrap them around the edge of the cage. Once it was all tied off, I slid the spacers out from under the cage which was now suspended off the bottom of the mold.
The other addition to my countertop was to have crushed blue glass in it which matched the blue glass mosaic tile in my kitchen. I purchased the glass from Cheng, but you could crush your own glass if you like. The directions suggested using spray adhesive on the mold so it will be tacky before putting down the glass. I did this and even though it doesn't stick to the glass very hard, it makes it tacky enough so the glass won't pool together when the concrete is poured. Also, make sure the tip on your can of spray adhesive is clean. Mine wasn't and it put little bubbles on the mold which transferred to the countertop. I then dispersed the glass as evenly as possible.
I was now finally ready to pour!