I am going to show you how I made a concrete block.  This block is my prototype.  You can see the broken down mold in the background.

This block is about 18 inches long x 10 inches at the base x 11 inches tall.  It has a foam core inside, the walls are about 2 inches thick.  It used one 80 pound bag of redi-mix concrete.  So it doesn't weigh that much.  

This block was a lot of work,  I had to cut a bunch of foam into triangles and then tape them together to form the core.  It also has a wire cage inside of it too.  That was such a pain to form out of 6 x 6 wire grid.

The next block will be more like a concrete flower pot, and will sit 'upside down' with the opening on the bottom.  I plan on stacking these on top of each other, with two on the bottom and a third one on top.
 This was made out an old real estate sign that was destroyed by vandels, so don't just go stealing signs. I had permission to use this sign.

Step 1: Make Ends

I started with a flat plywood panel.  

Then I added one end and braced it with a 2 x 4. 
I wanted a ledge on the top so I added a piece of wood.  notice that I tapered the edge of the board, to create some draft on that part of the mold.  I'm not sure if that was needed to aid in the removal of the block from the mold, but I worked in the plastic injection molding industry, so I know that molds need to have a draft.

I then added the other end.

If you spray your form with a bond breaker before pouring concrete it should slip out... that's what carpenters who build concrete forms for foundations do
ok i have a question. we are building a fence ontop of a concrete slab... do you have any idea how to make a slight angle on one side of that slab so our fence will be level? i can not think of anything and this is my first time working with concrete like this.
whats it look like when set in place
hey, using the electric saw's vibrations to settle the concrete was a clever touch. Great stuff! Thank you
I'm thinking I could make a scaled down version of these as mower edges, right?
Great article. Sorry for asking if it is a stupid question, but what is a draft? I am thinking of making a mold for concrete landscaping stones and am curious. Thank you.
I'm sorry, I used to work in injection molding, and the draft was the slight angle in the mold that made it possible to remove the part from the mold. If you don't have enough of a draft, then the part would have a difficult time coming out. I'm not sure how important this is in concrete molds. I think that lubing the mold with oil or using plastic sheets would be more important.
this is good project as people are always building steps decks and poarches this block would make a good suport if fibers were added to the concrete you might be able to skip the wire also making them more recycable in the future. I have heard of papercrete,hempcrete and adding rubber glass ceramic pieces to the cement mixture. for lightweight pumpic is sometimes added. also there are videos of making concrete countertops that explain how to make forms add strength to the concrete and about making re-usable forms and how to add color to the concrete add mixtures that even allow concrete to flex belive it or not.
Thanks for the comments, congrats, you are the very first to comment on any of my instructables, I only have two, but thanks anyways.
if you had used plastic sheets or oiled the forms it may have helped it slip out. Are you sure the hollow shapes will support whatever you use to backfill? I can't speak for your area but around here the army corp of engineers has the final say over seawalls/bulkheads, stepping out of their stiringent parameters can result in heavy fines. Thank you for the instructable on your project, its nice to know that others projects don't always go as predicted
Thanks for the comments. I feel so important. I didn't think about lubing the inside core. That probably would have made a huge difference. The blocks are just going to sit in the lake, with small (softball size) rocks on both sides, and fill gravel and dirt on the shore side. The blocks are pretty sturdy and the later blocks are filled with foam, so they are pretty 'rigid'. The lake they are going in is just a small 300 acre lake, the water is only 2 feet deep at the shore, and we are allowed to build up the shore with rocks, to prevent erosion. So, I figured that I would make my own 'rocks' in an efficient shape.
I'm sorry it did have fibers good idea

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