GFRC Fireplace Mantle Project

For this project, we needed to construct a non-combustible mantle, to go above a large, hot, wood-burning Fireplace Xtrordinair.  Since a wood mantle wouldn't be permitted, we turned to concrete, and fabricating one was a much cheaper solution to some of the pre-fab cement based mantles out there, which are also... ordinary and lame!

The size of the mantle needed to be between 8-9 feet long, and thick enough to not look dwarfed on a fairly large wall space.  Going with a rock or slate texture, we decided to use with our 8" Slate Liner for face of the mantle. We added a block-out section in the middle of the mantle form, and created a custom crest logo from one of our Logo Stamp products.  The mantle form was made from basic melamine, and took about 40 minutes to put together and caulk the corners.  We used scrap melamine left over from other projects, but typically a full 4'x8' sheet of melamine is sold at any hardware store for $30-$40.  Once the mantle form was created in our shop, we then took the form on-site for the job to create the mantle and eliminate the need to transport the heavier concrete mantle later.  We also added a few foam pieces to the bottom of the mantle that had stainless steel trivet bars pushed through the center of.  When the mantle is finished, and the foam removed, it will leave a metal rod cemented in place that can be used to hang items from, under the mantle (such as Christmas stockings!)

We have a recipe page here on our website for mixing concrete and GFRC.  Using our GFRC recipe for this project, we came up with our calculations for how much material needed to spray a face coat over the whole mantle surface, and then place fiber-rich backer material over it, to build the overall thickness to 3/4".  We then pre-batched this into 5 gallon buckets.  Each batch had a bucket with the correct amount of water, and the polymer-curing aid (Polyplex) and SP-7000 Plasticizer.  We also had other buckets with the pre-weighed sand, and another with the cement for each batch.  A good electric scale is key to consistent quality control, so each batch is the same.  Once everything was laid out, it was a simple matter to take a bucket with the water liquid, and add the sand, mixing with a high-torque mixing drill and helix-style mixing paddle.  Then mixing in the cement.  And for the backer layers, then mixing in the fibers.

First step to GFRC, spray the Face Coat. 

We removed the metal straps from the form, so we could spray the cement evenly in the form.  Using the Sharpshooter 2.0 to spray the face coat mix makes it easy to angle the hopper down and into a form, since it has an angled adapter between the hopper and sprayer. It also has an adjustable orifice dial, for quick changes of the hole the spray needs to pass through.  A stiffer cement mix can spray easier through a larger hole, while a thinner mix can benefit from a smaller hole size.  For this piece, we used the 1/4" orifice size on the sprayer, and around 40 psi on the compressor. 

A tip for spraying GFRC face coat is to start on the far side of a form and work towards you (or in a mold shape such as this mantle, start at the bottom and work up).  The reason is, as you spray, there will be some overspray and loose sand that will get blown around, and if you start close to you, the sand will all blow into the rest of the form and pile up in the corners.  This sand is missing most of the cement and bonding agents from the mix, so will be a weak area, and usually not solid or smooth on the finished piece.  If you start far (or bottom) then as this looser material gets blown around, it will get left on top of the face coat layer, and not affect the finish of the piece, or the strength of those areas.
The finished face coat layer should be under 1/4" thick.  Sometimes 'just thick enough' to cover the form material is fine, but 1/8" to 1/4" is normal.