Strengthening/retrofitting of masonry by using thin layers of Sprayed Strain-Hardening Cement-Based Composites (SSHCC)

This paper presents an overview of ongoing research at the Institute of Construction Materials of TU Dresden on the subject of strengthening of masonry by means of highly ductile cementitious materials with short polymeric fibres, so-called strain-hardening cement-based composites (SHCC). SHCC compositions with 1.8% and 2% by volume of PVA fibre were developed on the basis of appropriate rheological experiments on fresh concrete with subsequent spraying and of mechanical tests on hardened material samples produced both by spraying and casting. Afterwards, specimens built of three bricks (triplets) and reinforced with thin layers of sprayed SHCC were monotonically loaded in a deformation-controlled regime in order to investigate the strengthening effect of the applied SHCC layers. The results obtained from these experiments showed, that the static load bearing capacity of such triplets increased significantly, along with even more pronounced augmentation in energy absorption with a remaining high carrying capacity even at high deformations. In further laboratory tests, masonry walls were reinforced on both sides with sprayed SHCC layers and later subjected to cyclic shear loading. An evident increase in mechanical performance was observed in comparison to reference masonry walls with no strengthening layers. Additionally, two surface treatment methods (impregnation with water repellent agents and priming) were investigated for the effect on water absorption of the masonry surface, bond strength between masonry and applied SHCC layer, and finally, the workability of SHCC straight after its application (when finishing). The Karsten tube method was used to evaluate the surface water absorption for the chosen surface treatment methods in comparison to a reference brick face. Surface tension tests showed that impregnation with water repellent agents reduce the surface water intake, weakening the bond strength of the two materials. Priming, on the other hand, achieved a reduction in surface water absorption, an increase in bond strength and easier finishing of the sprayed composite.