Boy oh boy. Concrete furniture has come a long way since my plywood and cinderblock make-do bookshelf in grad school. Check out Francesco Passanti's Ellen bookshelf. It sets colored blocks between thin shelves made of ultra strength Ductal concrete. MocoLoco featured several of her other compact concrete creations a while back.
Many of his designs are made with "ultra high-performance, fibered concrete." He has been working in concrete for the past 15 years. "You no longer need to add steel reinforcement, so it's less thick, and its weight becomes more manageable."
A chair, which would normally weigh some 660 lbs if cast in concrete, tips the scales at a mere 110 lbs thanks to a wooden structure which has been poured with a resin-mix mortar. By adding dyes to the mix, Passaniti offers clients a range of some 110 colors also. And different techniques for working the material means the surface can also be transformed to look like everything from crocodile skin to printed stone.
