Fully grouted rockbolts are common anchorages used in the domain of rockfalls protection devices. Their intern behaviour being not established, current design methods are conservative. The issues are due to the complex behaviour at the intern interfaces. To investigate the behaviour of the bolt-grout interface, a series of pull-out tests was performed on fully grouted rockbolts within a limestone rock cliff. Optical fiber sensors were embedded in the steel bars so that the axial deformations along the bars were monitored. Considering the elastic behaviour for steel under loads applied, the tangential stress of the bolt-grout interface was determined along the bars. The study underlines the propagation of tangential load along the grout-bolt interface from the head of grout with increasing axial load. The interface is progressivelydamaged : firstly, cracks occur and then, the contact between steel and grout is completely debonded.