Mr. Liou,
Thrust blocks are required when the piping joints are not able to transmit axial force, e.g. unrestrained O-ring joints. The thrust blocks must be designed to withstand the pressure thrust of PxA (in the case of a tee PxA is applied perpendicular to the branch; in the case of a bend PxA is applied in each direction, which gives a resultant of 2PxAsin(angle/2)). If the joints are able to transmit axial force, e.g. welded joints, then the thrust forces are indeed handled internally as axial stresses.
Thrust blocks are required for large diameter GRP piping systems only when the piping is supplied with unrestrained joints. This requirement has nothing to do with the material. Steel piping systems with unrestrained joints would also require thrust blocks.
Hope this helps.
Regards,
Bruce Hebb