Fabric bags, for example made from hessian, can sometimes be used as permanent formwork. Applications include underwater work (where the primary purpose of the bag is to prevent cement and fine material being washed out of the concrete) and simple retaining walls (such as river bank reinstatement). The bags contribute nothing to the strength of the final structure.
Polypropylene and polyethylene fabrics or meshes are mainly used in geotextile applications. Those with smaller fabric sizes have been used as permanent formwork for foundations and underwater concrete. In the latter application, the holes in the fabric allow the displaced water to flow easily out of the area being concreted. The fabrics contribute nothing to the strength of the completed structure.