If light and strong is what you're looking for in a chair, concrete is the last material you'd want to use. It seems like those designs would work well in some kind of foam, which can be cast in a mold. If you want to use 3D printing in this project, use it to print the mold in sections that can be bolted together. Here's an example of a chair made that way: http://www.jorislaarman.com/work/arm-chair/