BIRGER GIGLA and FRITZ WENZEL University of Karlsruhe
A research project at the University of Karlsruhe is investigating the bond strength of supplementary injection anchors as a repair method in historic masonry. Such anchors are necessary to carry tensile forces, or for new connections. They basically consist of a tensile element – usually steel -inserted into the slightly larger borehole with the annulus grouted with cement. The solid plug of injection material transfers the tensile forces to the masonry. Specimens under investigation are monolithic blocks, brick walls and blockwork of various varieties of natural stone. Because of the present lack of Codes dealing with admissible bond stresses there is uncertainty in the design of injection anchors. Complementary standards for concrete should not be applied unless they are considerably reduced. If the anchors relate to structural safety then field pull-out tests are required by the Local Authority. From the results of 350 laboratory and in situ pull-out tests the bond strengths between anchor steel, the injected mortar plug and borehole surface are examined and compared. One relevant factor is the strength of the injected mortar plug, which depends on the grouting suspension and importantly also on the water absorption capacity of the surrounding masonry. Other parameters include the presence of bed or cross joints and conditions of restraint to free transverse deformations.