Experimental investigation on the structural behaviour and strengthening of three-leaf stone masonry walls

A large part of historical structures, currently in the European urban centres, is built with stone masonry walls, frequently constituted by multiple leaves. A common typology encountered is the three-leaf stone masonry wall, which is characterized by a substantial presence of voids in the inner leaf and prone to brittle collapse mechanisms. Nevertheless, the knowledge of the mechanical behaviour of three-leaf masonry walls and guidelines for proper design and control of the interventions are limited. As support for the rehabilitation design phase, some analytical approaches are available in literature. A comprehensive experimental study on the structural behaviour of three-leaf stone masonry walls has been planned at University of Minho, considering different strengthening techniques using GFRP (glass fibre reinforced polymer) materials, pozzolana-lime based mortar and lime-based grout. In the paper the experimental plan and the first experimental results on materials and three-leaf stone masonry walls are presented and discussed. joint structural repointing with GFRP rods and combination of the two previous techniques. Further developments will consist in the application of the injection technique with lime-based grout applied both individually or combined with the previous strengthening techniques. The combination of these techniques can provide useful information in order to evaluate the different strengthening configurations that can be applied to restore the masonry deficiencies noticeable on site. The subsequent repair of the tested wall specimens is also an approach under consideration whenever possible. The aims of the planned research are to characterize the behaviour of the stone masonry three-leaf walls under different strengthening configurations and developing a suitable contribution for analytical models and design guidelines. In the paper the experimental work plan and the first experimental results on materials and three-leaf stone masonry walls are presented and discussed in detail. A preliminary bibliographic study was also performed, both on the geometrical characteristics of the three-leaf walls and on the constitutive materials used (Valluzzi et al. 2004, Valluzzi 2000, Toumbakari 2002, Binda et al. 1999, Rodrigues et al. 2003, Bartos et al., 1999) in order to produce masonry specimens representative enough of the existing three-leaf stone masonry walls. For that, an experimental investigation has been done in order to select adequate materials that correctly simulate the historical three-leaf stone masonry in laboratory conditions. Granite stone and pozzolanic mortars, for bed joints and repointing purposes, were selected to build the walls. Aiming at representing as much as possible the traditional construction techniques still in use, a professional mason was hired to build all the specimens. Furthermore, experimental tests on specimens reproducing the external and the internal leaves have also been performed. 2 STRENGTHENING TECHNIQUES A brief description regarding the three main different reinforcing techniques adopted in the project is hereafter given. These techniques aim to solve specific structural deficiencies of threeleaf stone masonry walls, as follows: lacking of the connection among the leaves; reduction of the horizontal dilation due to creep damage; weakness of the internal core. The transversal tying through the thickness of multi-leaf walls is aimed to improve the connection among the leaves, in particular between the external leaves, in order to reduce the transverse deformation. For this purpose, stainless steel bars or FRP bars can be used. The bars can be easily inserted into drilled holes through the thickness of the walls and then anchored. In case of steel bars, the anchoring phase is achieved by bending the bar from the outside into a mortar joint previously grooved and then refilled with new mortar, whereas the anchoring of FRP bars is slightly more complicate because usually these bars can not be bent without their failure. In this last case, the anchorage can be achieved by using special anchoring elements (like angle bars or connector developed on purpose) or relying on the bond behaviour between the FRP bar and the mortar, developed along the thickness of the external leaf. In order to improve this last anchoring mechanism, a local grout injection around the tie can be applied instead. The bed joint structural repointing has been recently considered for the strengthening and repair of historic brick structures exhibiting horizontal dilation due to creep damage (Valluzzi et al. 2005). When stone masonry walls show a regular bond arrangement of the units with aligned horizontal bed joints, this technique can also be applied on such walls. The technique is performed by removing an external layer of the horizontal joints (up to about 6-8 cm), and placing into the groove one or two small diameter reinforcing bars (stainless steel or FRP bars can be adopted). In the case of multi-leaf walls, transversal short links can be inserted into drilled holes successively sealed to improve the confining action of the bars and to tie the external leaf of the wall. The injection is aimed to improve the weakness of the internal core, filling the voids in the inner core, and to improve its adherence to the external leaves. Several studies have been performed in the last years concerning the feasibility of this technique and its mechanically, physically and chemically compatibility (Valluzzi 2000, Toumbakari 2002, Binda et al. 1994). 818 Structural Analysis of Historical Constructions D.V. Oliveira, P.B. Lourenço, E. Garbin, M.R. Valluzzi and C. Modena Nowadays the trend is using grout mainly based on lime, in particular when the restoration works deal with historical constructions. The injection is typically performed injecting the grout starting from the bottom of the wall and reaching progressively the top. Usually, for three-leaf walls the used pressure is very low and not exceeding 50-100 kPa to avoid the undesired detachment of the external leaves. 3 CHARACTERIZATION OF THE WALL COMPONENTS