Do you mean you are lining the planters with concrete, or that the concrete is the outer coating for the planter?

But for avoiding bowing, you need to put in reinforcements at a regular spacing along the length of the planter. There's a few ways to do it. Interior staking, exterior staking, interior framing, using bracing rods.. maybe some other options.

@beesneeds. Thanks. I updated the post with an image and explanation. I could also weld aluminum hollow square beams to the sheet and then pour the concrete on that. Then the aluminum will get stuck and can't move. But not sure how it wll warp during the heat of the summer.

Thank you for the clarification. A couple things to consider. Cement and aluminum don't tend to bond. They also have different expansion/contractions. And you need equipment to cut, bend, and wielding or bonding with aluminum sheet sturdy enough to use.

Why do you want to make a planter like this? As in concrete lined aluminum sheet?

Why not make your form with particle board only, remove it and then install your aluminum facing around the planter as a separate unit? This would solve your two different materials expanding and contracting issue.

@beesneeds. Thanks. I think the aluminum planter will look cool and be unique. I am having a fabrication shop do the cutting and bending of the metal. I enjoy doing challenging and odd projects like this. The challenge is to figure out what to do so the aluminum remains attached to the concrete.

@mojavemaria I think attaching the aluminum to dried concrete is more challenging and getting a perfect fit will be much harder than to have the aluminium be part of the mold for the concrete

I have nothing to contribute on the building of these but I'm thinking about the welfare of the plants. Is there adequate drainage? Are they to be bottomless and directly on the earth? If not you'll need holes in the bottom and/or sides.