Papers by Author: Cristina Gentilini

Abstract: In the present study, a series of pull-off tests is performed to determine the bonding properties of fiber reinforced polymer (FRP) composites applied to a brittle substrate. Pull-off test is simple, fast and cheap and it is generally employed to evaluate the bond strength between the composite and the substrate. It is a procedure that can be easily performed on-site for checking the quality of the reinforcement adhesion.A steel composite made of a unidirectional steel fabric embedded in an organic matrix (bi-component epoxy resin) was applied to the surface of fired-clay bricks. A shallow core perpendicular to the surface was realized in the test specimen, leaving the core attached to the brick. A steel dolly was glued to the external surface of the core using epoxy adhesive. A loading device was employed to apply the tensile force until failure. The effects of moisture presence on the substrate and shape of the cores on the pull-off strength were investigated.

Abstract: In recent years, steel reinforced polymer (SRP) composites have emerged as a new technology for structural strengthening, and several researches have validated the effectiveness of SRP for masonry strengthening. Research has been carried out to study the bond behavior of SRP composites applied to a masonry substrate. However, how the moist and salt on masonry surface will affect bond, which is the weak link in real strengthening applications, is little known yet. This study aims at investigating the bond behavior of SRP composites applied to moist and salt-laden masonry blocks that were subjected to an artificial weathering protocol. Single-lap shear tests were conducted to determine the bond behavior, while ion chromatography provided the salts distribution of weathered specimens to interpret some results of the shear tests.

Abstract: High strength steel bars are widely used for the strengthening of masonry buildings, in particular to improve the connection between different structural elements, such as orthogonal walls and multi-leaf walls. A particular type of steel connector is the twisted bar, which due to its particular shape works as a self-threading screw, anchoring to the support material without any binder. The effectiveness of such technique mainly relies on the bond between the bar and the substrate, where adhesion, mechanical interlocking and friction play an important role. In this paper, a preliminary experimental study on pull-out behavior of twisted steel connectors inserted in brick units of different materials that can be commonly found in existing masonry buildings in Italy and in Europe is presented. Additionally, mechanical characterization of the materials is conducted to understand the influence of the compression strength and elastic modulus on the adhesion between the connector and the substrate. Preliminary results show that the pull-out response strongly depends on the mechanical properties of the substrate material.

Abstract: The paper reports the results of a numerical simulation performed to study the experimental pull-out behavior of twisted steel connectors inserted in fired-clay brick units. The experimental results obtained in a previous campaign are used to calibrate a 3D refined numerical model developed by means of the finite element program Abaqus. The numerical model is tuned to accurately reproduce the experimental results in terms of loads and bar displacements.

Abstract: In this paper, preliminary results of an experimental campaign carried out on masonry triplets subjected to weathering cycles in salt solution are presented. Weathering cycles are constituted of a wetting phase by capillary rise of a sodium chloride solution and a drying phase in oven. At the end of the last conditioning cycle, triplets are shear tested in order to quantitatively assess the effects of salts on their mechanical behaviour, in particular on the Mohr-Coulomb relationship. To this aim, three levels of pre-compression are applied during the shear tests. For comparison purposes, unconditioned triplets are also tested under the same loading conditions.

Abstract: In this paper, the preliminary results of a series of pull-out tests conducted on mortar cylinders with embedded bars are presented. The bars are made of high strength stainless steel and are of helical shape to increase mechanical interlocking with the surrounding mortar. Usually, such bars are employed in situ to realize structural repointing in the case of fair-faced masonry walls. To this aim, they are inserted in the mortar bed joints of masonry for providing tensile strength to the walls and with the function of crack stitching. The aim of the present experimental tests is to determine the bond-slip relationship for bars embedded in masonry. Firstly, pull-out tests are conducted on mortar cylinders considering different embedded lengths of the bars. Further tests are on-going on masonry specimens with bars embedded in the mortar joints. An analytical investigation is also carried out for the interpretation of the pull-out test results.

Abstract: In this study, confined masonry specimens with regular arranged openings are tested in order to study the influence of different enhancements of the columns on seismic failure modes. In particular, five brick masonry walls and three half-scale two-storey masonry structures are tested under quasi-static loads. The experimental results show that increasing column ratio improves the seismic behavior of the wall specimens to some extent, but an excessive reinforcement ratio of the columns decreases the ductility. The global failure mode of the two-storey masonry structures is modified by inserting iron wires in the mortar bed joints, improving the structural collapse resistant capacity effectively.

Abstract: In this study the results of an experimental campaign that involves masonry specimens subjected to a purposely designed splitting test are presented. The specimens are reinforced in the mortar joints by means of carbon bundles impregnated with a water based resin. Unreinforced specimens are also tested for comparison purposes. Some specimens are characterized by an initial crack obtained artificially by cutting the bricks to highlight the crack arrestor function of the carbon bundles. Results show that the failure mechanism from brittle becomes ductile and a load bearing capacity increment is registered in reinforced specimens.

Abstract: Experimental and numerical results show that the predominant failure mode of FRP strengthened masonry structures is the interfacial debonding, which occurs prior to reaching the compressive strength of the substrate and/or the tensile strength of the FRP composite. In this paper, a three-dimensional numerical model is developed to simulate the experimental response of direct shear test of FRP-masonry joints and. A damage model is adopted for both mortar and bricks characterized by a different behavior in tension and compression.

Abstract: Strengthening of masonry walls is a key aspect during building retrofitting operations having conservation goals. “Reinforced repointing technique” involves the application of materials having high tensile strength such as steel bars, steel textile sheets or composites thin pultruded laminae with cracks arrestors function, to reduce the vulnerability of masonry structures against in-plane and out-of-plane actions. In this paper, a three-dimensional numerical model is developed to evaluate the efficiency of reinforcing steel bars facing crack propagation phenomenon under increasing compressive loads.