The invention provides that lignocellulosic materials, in particular natural fibres such as jute, are used with, preferably dispersed throughout, curable synthetic resin in sheet structures, especially tubular structures. These structures are used for making pipes, or for lining existing pipes and passageways, especially undergroung passageways, namely sewers, by inflating the structure whilst it is still flexible (i.e. resin still uncured) into a pipe or onto the surface of the pipe to be lined, followed by the curing of the resin to form a rigid pipe or lining. The fibres used are acetylated (pre-treated with acetic anhydride) as this provides that the fibres become degradation-resistant, but more particularly, they bond to the resin with great tenacity which enhances the resulting cured resin strength at a reduced cost, compared to conventional tubular structures for lining operations (used in great numbers), which heretofore have used synthetic fibres in the resin.