Matteo Panizza

Experimental Assessment of Bond Behaviour of Fibre-Reinforced Polymers on Brick Masonry

Existing masonry structures represent a significant amount of the architectural heritage. Many of these buildings are vulnerable to earthquakes. Consequently, they need structural improvements in order to meet the seismic requirements of recent building guidelines. In the last decade, there has been a growing interest in the application of Externally Bonded-Fibre Reinforced Polymers (EB-FRP) as strengthening and repair materials because of their high-performance mechanical characteristics, feasibility of application in civil structures, resistance to chemical attacks and other potentials. Brick masonry components are the most suitable substrates susceptible to improvements because of their more regular surface in comparison with stonework or rubble masonry. The bond behaviour of FRP, applied on a masonry substrate, is a critical issue for the effectiveness of the technique. In this paper, the results of an experimental assessment of the local behaviour of EB-FRP applied on clay bricks are presented. Experimental failure load results were compared with predictive bond strength models proposed in literature for concrete substrates. On the basis of measured strengths and local deformations, interface fracture energies were calibrated and an analytical function was proposed as bond stress-slip law. Finally, a bilinear law was calibrated for practical design applications.

Experimental Study of the Bond of FRP Applied to Natural Stones and Masonry Prisms

Compared to more traditional techniques, the application of Externally Bonded-Fibre Reinforced Polymers (EB-FRP) represents a viable alternative for the strengthening of masonry structures, also in case of Cultural Heritage buildings where strict requirements need to be met, aimed at minimizing the impact of the intervention. Since the FRP-to-masonry bond behaviour strongly affects design and effectiveness of such interventions, several investigations have been carried out in recent years to study this phenomenon, generally based on the longer experience developed for concrete substrates. Mortar joints, which are geometrical and mechanical discontinuities, distinguish and characterize masonry substrates from concrete ones, and therefore deserve a special attention as far as their role in the bond behaviour is not clarified yet. This paper, aimed at giving a contribution also from a methodological point of view, presents the main experimental results of shear tests carried out on glass composites (GFRP) applied to natural calcareous stones (pietra leccese), to lime mortar blocks and to masonry prisms made by coupling stones and lime mortar. Overall 22 shear tests were performed, keeping a bonded length of 200 mm for stones and mortar specimens while it was changed from 65 mm (corresponding to one stone and one mortar joint) to 195 mm (three stones and three mortar joints) in the case of masonry prisms. The effect of the FRP end anchorage on the test development was investigated as well, and results of the experimental tests are herein discussed in detail.

Influence of Salt Crystallization on Composites-to-Masonry Bond Evaluated on Site by Pull-Off Tests

A research, made in collaboration with Politecnico di Milano and University of Padua, has started with the aim of evaluating the influence of aggressive environmental conditions (moisture, temperature and soluble salts presence) on the bond between FRP/TRM and masonry substrates. Experimental tests were carried out on site on full-scale masonry models, built in open air in a polluted area in the hinterland of Milan, naturally subjected to thermal, R.H. and UV cycles; furthermore, they were artificially subjected to capillary rise of a solution of water and Sodium Sulphate so as to create salt crystallization cycles. On two different masonry substrates, representing a simple historic masonry structure, made either by soft-mud bricks or sandstone units with lime mortar, various FRP and TRM strips were vertically applied in order to evaluate the bond at three heights. TRM were applied with two different premixed inorganic matrices (pozzolanic lime based mortar and cement polymer modified mortar) and FRP with an organic matrix (epoxy resin). The fibres used were unidirectional carbon and glass fabrics for FRP, and a bidirectional carbon net for TRM. The bond was evaluated by means of pull-off tests and the first results, herein presented, show the influence of moisture presence and of salt crystallization cycles on the different used matrices.

On-Site Pull-Out Tests of Steel Anchor Spikes Applied to Brickwork Masonry

Externally Bonded (EB) composite materials are becoming a widespread solution for strengthening interventions on masonry buildings, even Cultural Heritage structures, due to several positive aspects mainly related to their high strength-to-weight ratio. In recent years, beside common epoxy-based Fibre-Reinforced Polymers (FRP), steel-based composites have been proposed: they are composed by unidirectional high-strength steel cords that can be coupled to either organic (Steel Reinforced Polymers, SRP) or inorganic (Steel Reinforced Grouts, SRG) matrices, in relation to their optimized spacing. The bond behaviour of all these EB composites has a strong influence over the effectiveness of interventions, since the detachment of reinforcements from the substrate generally represents the weaker failure mechanism. In order to improve this aspect, several anchorage devices have been proposed, being spikes, among them, one of the most suitable for masonry supports. Spikes are made of a bundle of fibres partly in the form of a bar, to be inserted and glued into a hole drilled in the substrate, and partly loose, to be spread and connected to reinforcement strips. Despite their importance also from a design point of view and considering the variety of shapes and materials, there are still few investigations in this field, being clear that both the spike-to-reinforcement and the spike-to-masonry connections need to be studied. Focused on the spike-to-masonry connection, this paper is aimed at investigating the performance of steel cord spikes applied to existing clay brick masonry, by means of overall 39 pull-out tests carried out taking into account the bonded length (equal to the hole depth), the type of embedding material and the number of steel cords forming the anchorage. The main results of this experimentation are herein presented and discussed.

Experimental Characterization of Solid Clay Bricks: Correlations Among Mechanical Properties

Strengthening interventions on existing structures, especially in the case of Architectural Heritage, require an in-depth knowledge of construction techniques, geometry and materials for an optimal design aiming at the minimum intervention approach ideal. Nonetheless, conservative values of material properties, often derived from codes or literature, might hinder the effectiveness of the design approach. Non-Destructive Tests (NDT) and Minor Destructive Tests (MDT) are fundamental tools for the characterization of existing materials with a minimum or no impact. The paper presents an experimental study that investigated the possibility of defining empirical correlations among the main mechanical properties of solid clay bricks, which are one of the most common unit for masonry load-bearing members. Extruded bricks, typical of modern constructions, and soft-mud bricks, resembling historical units, were tested to cover for the ample variability of solid clay bricks. The examined mechanical properties were compressive, bending, splitting and pull-off strengths. The dataset of mechanical properties allowed calibrating linear correlations expressing one property as a function of another, thus giving the possibility of estimating a set of strengths based on the results of the MDT pull-off test. An innovative aspect consisted in performing the four tests on the same unit, so that the calibrated linear correlations are based on punctual data instead of average values. A practical application can consist in the on-site execution of pull-off tests, which are minor destructive and can be easily performed on a wall surface, for estimating the compressive strength of clay units.

Moisture and Temperature Influence on Biocomposites-to-Timber Bonding

Compatibility and durability are fundamental aspects to take into account when dealing with strengthening and repair of structural components in existing buildings. In particular, timber elements in floors and roofs, like beams and joists, can be improved in their load-bearing capacity by the use of composite (fiber-reinforced) materials. Applied as textiles or laminates, those materials can provide additional tensile strength without weight increase, and are easily applied in situ. Recently, an increasing interest towards the biocomposites (flax, hemp) opened new perspectives: the advantages in comparison with traditional FRPs includes the higher mechanical compatibility and sustainability, especially if glues suitable for wood (e.g., vinyl) are used as matrix, instead of epoxy resins.In the paper, the results of an experimental campaign carried out at the University of Padua, aimed at investigating the influence of humidity and temperature on the adhesion of composite materials (carbon or glass FRP) and biocomposites (flax) reinforcing spruce timber elements are presented. Epoxy resins were used as matrix for all composites; flax was also applied with vinyl glue and considering a protective treatment (alkaline-based). Effectiveness of bond at the interface timber-reinforcement was measured by pull-off tests, and analyzed at micro-structural level by optical microscopy and infrared spectroscopy.