Lime-based mortar reinforced with randomly oriented polyvinyl-alcohol (PVA) fibers for strengthening historical masonry structures

Abstract

The seismic vulnerability of unreinforced masonries underlines the importance of developing new methods and materials for the retrofitting of historical structures. Nowadays, two are the most widely diffused strengthening systems for masonry structures, i.e. fiber reinforced polymers (FRP) and fabric reinforced cementitious matrix (FRCM) composites. Despite some degree of effectiveness, FRPs show poor chemical compatibility with masonry supports, that makes them unsuitable for intervention on historical heritage structures. FRCMs frequently consists of a cement-based mortar and, because of this, the cementitious nature of the inorganic matrix can not ensure the chemical compatibility with the historical masonry substrate; also, being FRCMs composites made out of fiber fabrics embedded in an inorganic matrix, the mono- or bi-directionality characterizing the fiber fabric makes this type of reinforcement ineffective out of the fiber axial direction. This paper aims to overcome the listed drawbacks presenting a systematic study on a newly developed lime-based mortar, reinforced with randomly oriented polyvinyl alcohol (PVA) fibers: the lime-based nature of the mortar meets the requirement of chemical compatibility with the historical substrate, that is fundamental for the restoration of heritage buildings, and the random orientation of the PVA fibers makes them globally effective, being the stress state induced by seismic action directionally unknown. Specimens measuring 160 mm × 40 mm x 40 mm characterized by six different fiber contents and two different fiber lengths, namely 6 mm and 12 mm, in addition with plain mortar samples, are tested in three-point-bending configuration to compute both flexural strength and fracture energy. Then, the two broken pieces resulting from the flexural tests, each one measuring 80 mm × 40 mm x 40 mm, are tested in splitting configuration and in compression, and the resulting tensile strength and compressive strength are computed. An analytical formulation is finally proposed by fitting the experimental data with linear, parabolic and exponential regression models. The results related to averaged values of mechanical properties show that, for both short and long fiber lengths, the flexural strength and the fracture energy linearly increase with the fiber content and that the tensile and the compressive strengths parabolically increase with the fiber content. Overall, the newly proposed mortar stands as a valid system for the strengthening of masonry structures, being especially suitable for intervention on cultural heritage buildings.