Tile and exo-skeleton tile structure

To improve structural integrity of the exo-skeleton tile structure, each tile (18) is formed with at least one rib (40) extending circumferentially across the outer surface of the tile. An end (41) of each rib projects beyond one edge of the tile, like tiles (18) being linked at overlapping edges (30) by the inter-engagement of a projecting rib of one tile with the rib of an adjacent tile. The inter-engaging ends of the ribs (40) are relatively slideable circumferentially to allow thermal expansion and contraction of the exo-skeleton structure, but sockets (42/43/44) are provided where the ribs engage so as to resist relative bending of the adjacent tiles about their linked edges (30) and impart rigidity to the structure.