DIY Manufactured Flagstone

During the concept phase of remodeling my backyard I came up with the idea of bordering a stream i was building with flagstones to make a more appealing transition from river rock to gravel. Sort of like a carpet to tile transition in a home. Flagstone is pretty expensive, so if you've looked through any of my other 'ibles you'll know I'm too cheap for that. Since my remodel was going to be over the course of a year or so I figured I would have time to work on faux flagstones and spread the cost out.

Tools:

Drill and various drill bits

Saw

Bucket(s)

Shovel/Trowel for mixing concrete

Latex/Rubber Gloves

Materials:

2x3

Plywood

Screws

Clingwrap

Craft Foam

Cardboard

Portland Cement

Medium Sand

Concrete Colorant

The molds are pretty straight forward. I started out with only one to prove my concept. I had some plywood and MDF scraps from other projects and used this as the base of the mold which would eventually be the "top" of the flagstones. Next I cut up random lengths of 2x3 for the mold walls and predrilled some screw holes for securing them to the wood base.

While arranging the mold walls I wanted to come out with a flagstone that was elongated to maximize the length without using too much concrete but also make it look natural and aesthetically pleasing. After my first "rock" was complete I made three different molds. I have a total of 5 different shapes at the end by changing the position of the 2x3 on two of the molds after I had made several of the original.

I didn't want stones that looked flat or slick like curated slate. Some fancy flagstone for sale looks almost like tile because they take extra care while separating the layers so they get a smooth surface. I didn't want this look. Instead I wanted the surface to look more like a stone you would find naturally with grooves, pits, and a coarse surface. The first attempt was done by layering random cutouts of 1/8" craft foam. This came out really well but still looked a little flat, so for later attempts I started adding random cut outs of cardboard from whatever spare boxes I had laying around. The cardboard mixed with the foam added another level of depth.

The last detail was lining the molds with clingwrap. This served two functions. First, I wanted to reuse the molds possibly dozens of times so I needed to protect them from the moisture of the concrete mix. Second, the tiny folds and wrinkles of the layered clingwrap added more character to the final surface.

So for the concrete mix I went basic with Portland cement, medium sand, water, and colorant. I did this so I could play with the mixtures and give the rocks more sandy textures. They aren't going to be supporting any weight other than the occasional person stepping on them. Since the final strength wasn't as important as the look then I was free to get loose with the ratios until I got something that looked good. My rocks ae supposed to be imitating desert sandstone so I tried various mixtures of colorant. In the end I found that the terra cotta color with a little bit of red gave a good result but it varied depending on the sand/cement mix. Generally I went with 3 parts sand to 2 parts cement with a little more water than needed. This resulted in a pretty grainy mixture.

After putting the cardboard and foam down I lined the mold with the clingwrap leaving enough extra to fold over the concrete once it was poured. This will help keep the moisture in while curing. The very first rock I made i just began pouring the concrete on top of the clingwrap which worked fine but the result was way too smooth almost to the point of being glossy. The next time I experimented with putting a thin layer of sand down first. This gave the finished product a sandy texture. The rocks were poured in layers to give it a striated look from the sides. I mixed small batches of concrete in a 5 gallon bucket so the variations in color between batches made the different layers stand out but subtly. One thing I also tried was putting a layer of sand between pouring layers of concrete in the hopes that if I wanted to break the rocks they would break in a layers but that didn't really work out. While pouring each layer I also built little sand dams along the sides to give the edges of the rocks more depth instead of just being flat from the forms. The rocks vary in thickness from 1.5" to 2.5". After pouring all the concrete I let them sit without covering for a few hours while the excess moisture puddles evaporate off the top. When they were just barely damp still I folded the extra clingwrap over to help keep them damp while curing.

After about a day I was able to demold the rocks. They were cured enough to be solid but still damp enough to work with a little more. After taking them out I unwrapped them and laid them face up. I then put a glove on and rubbed the sand off the top. I did this pretty rough so that I would disturb some of the soft concrete with it. Because the concrete mix was sandy this made the final surface very course. I saved the sand and concrete dust that came off while doing this to use as the top layer for the next casting. Because the loose sand absorbed some of the concrete color it added more color variations to the next rocks. Some of the rocks cured dark, some cured light. I think I got a good mix.