Welcome to my instructable! In coming up with this idea, I needed protection for my lawn sprinkler heads, and the donuts from Home Depot etc. were small, sinking, breaking, too easily missed by mowers. I tried to buy larger ones anywhere, including some concrete shops, and was surprised that no one sold them. Using the method here, you can make your own large, hard-to-miss donuts easily and cheaply.
The emphasis here is simplicity, with readily available materials. You don't have to be a concrete expert, but it helps if you're handy.
The key is a mold (aka baking pan) for angel food cake, available at Target, Wal-Mart etc. It's tapered and has a center piece to provide the hole for the sprinkler head. (If you want to get fancy, molds for bundt cakes are about the same size and might also work, but the smooth edges of the angel food variety are less prone to breaking).
Obtain the materials described here, shown in the first photo.
1 bag of standard concrete (or other concrete suitable for free-standing outdoor item, if you already have a preference). 60 lbs. was used here to make 7 donuts
1 mold for angel food cake, about $13 at Target
Cooking spray or other light oil to coat mold
Small container to mix concrete; I used a tray for a paint roller
Water, less than 1 quart, from garden hose or pitcher.
Cement trowel, garden trowel, or other tool to mix and scoop concrete.
(optional) Bright colored paint; spray paint or house paint
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Step 1: Measure Out the Materials
Pour the desired amount of dry concrete into the mold. In my experience, half filling it works, but you can use more or less to suit your needs. Half filling the mold will give you a donut about 2" thick. The donut will be tapered, and will be about 7.5" top and 8.5" bottom, depending on the size of your mold. The taper gives strength, and also makes it easier to remove the donut from the mold.
Step 2: Prepare the Concrete
Empty the mold into your mixing container. Follow the mix instructions on the concrete package. Add a little water, mix (I use a garden trowel) and repeat. Avoid making too soupy of a mix, and be very thorough. If you're not a concrete wiz but have mixed a cake, it's a bit like that. I use a garden hose set on low to add the water; you could also use a pitcher or other container; anything that allows you to add the water very gradually. Repeat water + mix until the texture is consistent, it's all wet, and no separated powder. Part of the goal is to avoid air pockets (bubbles), which will weaken the donut.
Step 3: Lube the Mold, Then Add the Concrete
As with a cake, it'll be easier to remove the finished item if you lubricate the mold first. First clean and dry the mold to remove any concrete dust, then lubricate. I use cooking spray, but any light oil should do. You don't need a lot, but cover anywhere the concrete will be, including the mold's center piece. I did try one donut without the spray and was able to get it out, but it was much easier with the spray and less chance of cracking the thing. Keep in mind, concrete is fairly brittle, especially the cheap standard concrete.
Now use the same garden trowel (or other tool if you prefer) to transfer the concrete mix into the lubed pan. Move the concrete around in the pan until it's even. I then recommend tapping the pan lightly so any bubbles can escape.
Step 4: Now We Wait / Finished Product
Standard concrete can take 24 hours or more to cure. Be patient.
If it's at all soft, give it more time! I dumped one out too early - before it had sufficiently hardened - and it cracked. Concrete is pretty brittle compared to other materials.
When the concrete is firm, invert the mold onto a soft surface like grass so it falls out. If it hangs, a light tap should do it. Don't leave the uncured donut out in the rain or otherwise get it wet.
Once it's cured, you might want to paint it (optional).
Unpainted concrete is not very conspicuous, and the idea here is than no one mow or trip over these things. I had thought to paint them yellow, but the wife weighed in. Usually more aesthetically inclined, she surprised me by suggesting we paint these a loud "traffic cone" color (see photo). In the interest of keeping the project simple, I used spray paint, but it's a bit thin for this rough surface and took a few coats. You might want to use a thicker paint intended for concrete.
Clean up your mold and pan outdoors with a spray hose, the mix should come off easily if you rinse it right away.
Now your donut is ready to use. Not the quickest process, but relatively easy (compared to professional concrete work) and inexpensive.
Pictured is the donut in place. The lizard seems to approve.