Why stop at concrete ?
I've owned basses with bodies made from acrylic, Luthite and plywood topped with hardboard, all sounded great. Other materials I recall being used include CF, aluminium and one year a marble body guitar appeared at Musik Messe; it weighed a ton and was mounted on a stand π
Companies build bodies from wood for 2 reasons, it's relatively cheap and tradition; creative types are generally very conservative when it comes to instruments π