Composite materials for masonry structures: the adhesion issue

Masonry is often a non-engineered material used for building constructions during the past decades; those kind of structures have been rarely designed against horizontal actions induced by earthquakes and usually have been simply realized according to rules of common practice. Consequently, they are often in need for strengthening in order to fit the safety standards assumed by the modern codes. Strengthening interventions on ordinary structures can be particularly addressed to improve in-plane shear strength of masonry walls that are, on the contrary, rather fit to sustain axial loads. Ferrocement-based techniques have been often utilized though resulting in non-negligible increase of structural masses and quite irreversible intervention. More reversible and less invasive interventions can be carried out by using fiber-based materials such as Fiber Reinforced Plastics (FRP) or Fiber Reinforced Cement Matrices (FRCM) even made of carbon or glass fibers. Adhesion between the strengthening materials and masonry often controls the effectiveness of the intervention as a whole, resulting in an upper limit for shear strength enhancement of masonry walls. On the bases of a wide collection of experimental results, the present paper is devoted to calibrate a general relation for determining the key mechanical properties characterizing the interface behaviour between the composite strengthening and the masonry support. The behaviour of both polymer- and cement-based matrix composites will be examined. Such parameters (namely the value of fracture energy or other relevant measures of the adhesion strength) can be suitably utilized for evaluating shear strength of masonry walls strengthened through composite materials.