Prediction of the additional compressive strength due to PBO-FRCM-confinement of brick-masonry depending on the stiffening effect caused by different discontinuous wrappings: new design-oriented perspective

Fabric Reinforced Cementitious Mortar (FRCM) is an innovative composite system able to strength or rehabilitate masonry members with an efficacy, which is affected by the mechanical and geometrical properties of its constituents: matrix and fabric. Since one of the most appreciated benefits of FRCM is the breathability, the discontinuous application is commonly not considered in the retrofitting. On the other side, the thickness of the matrix may be considerable (ranging between 10 and 30 mm) implying an over-weight and over-stiffness which may influence the seismic response of the structure in a detrimental manner; especially when dealing with column confinement. Consequently, the experimental evidence and the proper design for the partially wrapping with FRCMs is currently lacking. For this reason, the present research reports on an experimental and analytical investigation about PBO (polybenzoxazoles) FRCM-confinement of clay-brick based masonry columns by varying the confinement scheme. The aim is to evaluate the effectiveness of the confinement in terms of axial bearing load capacity and ductility and, at the same time, focusing on the parameters related to the confining scheme showing how these affect the results. Two columns were tested unconfined as lower-bound reference; six columns (i.e. three couples) were partially confined respectively with 3, 4 and 5 wraps having the same width and lastly, a further couple was fully-jacketed as upper-bound reference. The findings evidenced that an appreciable gain in axial strength and deformability was ensured. On the other side, the axial stiffness increased proportionally to the number of confining wraps. Thus, a new analytical model was herein proposed considering the geometry of the confining scheme, as well as the stiffening effect related to it. In particular, a novel effectiveness coefficient (namely KE) is evaluated affecting the theoretical confining pressure based on the axial stiffness of the FRCM confined masonry. The comparison with the own and other available experimental data confirmed the reliability of the proposal which wants to offer a new perspective for the design of the FRCM partial confinement of masonry that needs further investigations to be confirmed by including a variety of masonry and FRCM typologies.