A removable use of FRP for the confinement of heritage masonry columns

For masonry structures in historical heritage with architecturally valuable features, such as frescoed surfaces, the application of structural reinforcement techniques appears to be very complex due to the requirements of removability and limited invasiveness. This is valid with reference to both traditional techniques and modern techniques, such as the external reinforcement with fibre-reinforced composite materials. In this scenario, the use of fibersreinforcedpolymers (FRPs) is drastically forbidden due to the use of epoxy-based matrix, which does not allow the removal of the intervention without damage of the substrate, even if the mechanical effectiveness of this system has been largely tested and proved. In fact, the reversibility is one of the most relevant aspects in the field of Heritage engineering. Thus, many efforts need to be spent in order to meet possible solutions, able to mitigate the risk, especially against seismic forces and other natural risks, while ensuring the conservation of the built heritage. This experimental research, which follows a first study on a smaller scale, aims to answer the question: how could a masonry column with frescoes and valuable surfaces be strengthened or repaired in a completely reversible manner?. Two strengthening methods were studied and are proposed herein by assuring the removability of the FRP-confinement of masonry columns. The first technique consists of a liquid adhesion inhibitor applied by brush before the hand lay-up installation of the FRP. The second is set by the interposition of a Mylar™ layer between the substrate and the FRP jacket. Uniaxial compression tests were performed in order to demonstrate the efficacy of the new strengthening techniques in increasing the axial strength (+ 39% and + 27% on average for the tuff- and limestone-based masonry, respectively) and displacement capacity (+ 32% and + 171% on average for the tuff- and limestone-based masonry, respectively) with respect to un-confined columns. Masonry columns FRP-confined with traditional wet lay-up were also tested for direct comparison. At a later moment, the FRP-jacket was removed to observe the substrate, which has been found effectively preserved from the adhesive, without any discoloration. The experimental results are extensively shown and discussed in the paper.