Learn how to create your own concrete rocks for building rock walls, fire pits, decorative piece and more!
Step 1: Mold Preparation
The mold box was built by using plywood plastic and cardboard. The rocks are laid out so that their height are as even as possible. It is a good idea to spray or brush mold release onto the rocks. I have not come across a mold release that won’t work with ComposiMold just don’t over do it. Using too much mold release will cause undesirable lines and marks in the mold. Some examples of mold release that can be used are vegetable oil, WD40, Vaseline, and silicone mold release.
Once the mold release has been applied it is recommended but not required for a medium coat of bubble buster to be sprayed. Bubble buster is an easy method to reduce bubbles. It works by breaking the surface tension and allowing bubbles to rise rather than sticking to objects.
Step 2: Creating the Mold
Once the rocks are wet with bubble buster it is a good time to pour the ComposiMold.
To melt the ComposiMold use a double boiler, microwave or any controlled heat source that will not expose the ComposiMold to temperatures above 200F.
Pour in increments if there isn't enough ComposiMold to cover the rocks in one pour. When pouring in layers the melted composimold will melt and fuse into the layer that has already been poured.
Be carful when handling ComposiMold because it will be hot, gloves should be worn.
Once all the ComposiMold has been poured it is time to wait for it to solidify. For this mold I waited overnight before removing the rocks from the mold. ComposiMolds cure time will vary depending on the mold size and room temperature. This mold was 75lbs of ComposiMold and the room temperature was 50-60 degrees overnight.
Step 3: Removing the Rocks
If the mold box is designed well and sprayed with mold release it should be easy to remove. Just make sure not to use a mold box that can’t be taken apart.
The nice thing about composimold is that it is flexible enough to remove objects without too much struggle but still strong enough to create concrete rocks.
While removing these rocks I did a lot of trimming to reduce the undercuts, ComposiMold can be easily trimmed with a knife or scissors. Because ComposiMold is re-usable the pieces that were cut off should be saved and re- used when needed.
Step 4: Prepping and Pouring the Concrete
Now that the rocks are removed the mold can be prepared for the concrete.
Vegetable oil mold release works great with cement. Apply a very thin and even layer. Other mold releases that work well with cement include WD40 silicone mold release and others.
Place rebar or a metal mesh in the mold. When molding large parts like this one it is a good idea to use Rebar or a metal mesh to increase the strength of the piece.
The concrete I used was ProFinish 5000 by quickrete because it works great with ComposiMold and cures fast. Other concrete and cement mixes will work as well. Ideally you want a concrete mix that needs very little water and has a fast cure time. just make sure it doesn’t create too much heat. If your cement mix creates a lot of heat keep your mold cold!
mix ¾ gallons of water with 80lbs of cement. The mixture will seem very thick but that is normal, just make sure your drill can handle mixing it. Also It is a lot easier to have help, if possible have one person mixing wile the other is pouring the concrete, but it is possible to do it alone if no one is available.
Now smooth out the concrete. The easiest way to even out the concrete to remove voids and air pockets is to pat the concrete with your hands, kick The mold and shake the mold. For even better results set the mold on a vibrating table for a few minutes.
The next step after the concrete has been poured and smoothed out is to wait. This piece was removed after 8hours and looks great! For a smoother texture use a concrete mix with less rocks. Or you can add sand to a cement mix. A cement mix that works well with ComposiMold is Quikrete Masonry Cement Type N.
Step 5: Thank You!
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