Abstract: A hollow tank is formed by placing a hollow glass fiber reinforced inner container within a hollow rigid form comprising pieces assembled around the inner container and spaced from it to leave an air gap between the inner container and the inner wall of the assembled form. A suitable concrete material is introduced into the space between the inner container and the assembled form and is allowed to cure as a jacket of concrete integrally attached to and enclosing the inner container. The inner container may be formed by coating an inflated balloon with a thin layer of polyester resin and thereafter spraying on the glass fiber reinforced material to a thickness of a few mm. The concrete may be a standard flowable concrete or a polyester concrete, which is a resin-bonded gravel mixture. The concrete may be the outer layer formed as a jacket around the glass fiber reinforced inner container, or another glass fiber reinforced layer may be formed on the outer surface of the concrete.