Experimental and analytical study of TRM strengthened brickwork walls under eccentric compressive loading
Introduction
Masonry has been one of the most used building materials in history. Its employment has evolved from dry stone piling up to the load-bearing brick walls commonly used for residential construction until the first half of 20th century in many European countries, most of it still in service. Because of the large number of existing buildings composed of structural load bearing masonry walls, there is an increasing need to upgrade them. In particular, the increase of overload due to changes of use may require some kind of strengthening on the walls to avoid possible mechanism formation failure. As an attempt to identify possible solutions satisfying both efficiency and sustainability requirements, the strengthening of brickwork walls against mechanism formation failure by means of Textile Reinforced Mortar (TRM) is here presented. Retrofitting systems based on TRM have been recently presented for the seismic strengthening of masonry walls, especially against in-plane shear forces; however, its performance against out-of-plane second order deformation has not yet been widely tested. In this area it is worth mentioning the work of García [1] which included the study of multi-leafs masonry walls under eccentric compressive loads. The present research is intended to spread the current knowledge by presenting the experimental work on TRM strengthened brickwork wall under eccentric compressive loads.
TRM is a composite material used to strengthen masonry or concrete structures. This technological solution consists of bonding a high performance fibre grid to the structure’s surface with a plaster based on a inorganic matrix. The research carried out by Papanicolaou et al. [2] is particularly interesting for the description of TRM and its comparison with other strengthening systems like FRP (Fibre Reinforced Polymer) or NSM (Near Surface Mounted) FRP laminates. Hamed and Rabinovitch’s study [3] also provides a good experimental comparison between TRM and FRP. Studies on FRP as strengthening material were first oriented to the repair and upgrading of concrete structures. The satisfactory results obtained motivated its later application to masonry. Several researches have been carried out on the application of FRP to masonry structures since early 1990s (see for instance [4], [5], [6], [7]). However, some of these researches have detected possible problems in the application of FRP on masonry elements. These problems are mostly due to the use of organic resins as part of the FRP or as bonding material with the masonry substrate [4]. Some of these drawbacks are due to the almost null permeability of the resins (which can strongly alter the hygrometric equilibrium of masonry with its environment), the very large stiffness difference between masonry and FRP, the loss of strength of epoxy resins at temperatures above 150–200 °C (glass transition temperature) and some practical application problems (dangerous manipulation of toxic products and non-applicability on wet surfaces or at low temperatures). Because of these problems, there has been significant research on alternative reinforcing methods and materials. For example, Sathipara et al. [8] used plastic bands to improve masonry tensile strength. Engineered Cementitious Composites (ECCs) [9] showed promising results against blast and impact loads. Blanksvärd [10] compared TRM performance with Fibre Reinforced Concrete (FRC) and Mineral Based Composites (MBCs). Among these systems, TRM is advantageous because of its better chemical and mechanical compatibility with the masonry substrate. For instance, in Blanksvärd’s [10] experiments, no TRM debonding problems were observed, in contrast with the FRP debonding failures obtained in some previous investigations [4].
TRM was initially developed to be applied on strengthening reinforced concrete structures but has gained popularity at overcoming the problems of FRP in masonry application. Hence, an important part of the carried out research has been oriented to study the use of TRM as a suitable strengthening material for masonry. It was widely applied to masonry arches [11] and walls [12], [13] and its performance as strengthening technique for concrete structures was evaluated too [12], [14], [15]. Papanicolaou et al. [16] presented TRM as the most suitable solution for the strengthening of masonry members subjected to shear or out-of-plane loading conditions. According to [17], these conditions are the most critical for the structural safety.
The present research has focussed on the strengthening capabilities of TRM against second-order bending effects and mechanism formation failure of load bearing masonry walls. These effects may appear mainly in slender walls or normal walls with eccentrically applied compressive load. The research has analysed the influence of the amount of fibres, their nature and the type of binding mortar on the behaviour of the strengthened walls. In addition, the possibility of using anchors to improve the connection of the TRM strengthening to the walls has been also analysed.
Upgrading the existing masonry structures, rather than demolishing them, is advantageous from both the economic and environmental points of view. Making innovative strengthening systems available and characterising their performance against mechanism formation failure is of interest in order to foster efficient and sustainable upgrading processes. Studying the possibilities of TRM against mechanism formation failure has been the main goal of the present research. In addition to the description of the experimental investigation carried out, the present paper also provides information for the practical design of TRM reinforcement.
Section snippets
Scope and methods
The investigation presented herein is part of larger research program which involved 29 full-scale walls (20 unreinforced brickwork walls – URMW – and 9 TRM strengthened brickwork walls – TRMW). In order to study the structural performance of the TRM strengthening technique when applied on brickwork walls under compressive loads, two unreinforced walls and the nine TRM-strengthened walls were tested and compared. The two unreinforced walls, called W#13 and W#15, were chosen among the
Experimental study
In this section the experimental procedure is detailed from the characterisation of the materials to the testing procedure. Finally, the results of the experimental campaign are presented.
Calculation procedure
A simplified method based on equilibrium and strain compatibility equations is proposed. It assumes that walls failed when the stresses/strain reach the strength/maximum elongation of any of the component materials. In addition, it is supposed that the strengthening system prevents the mechanism formation failure and limits second order effects turning the problem into a section resistance analysis. Thus, the failure can be determined by the compressive strength of the masonry or the tensile
Analytical results
One N–M interaction curve for each strengthening type was obtained. The resulting values are compared with the experimental ones in Table 8. The measured axial force–bending moment interaction at the collapse instant is close to the analytic result in all cases. The dimensionless analytic bending moment shown in Table 8 is calculated with the previously presented procedure and using the experimental maximum axial load (whose dimensionless measure is shown is Table 8) as the fixed reference
Comparison and discussion
First of all, it has to be said that the results presented above might be influenced by the scattering of the materials’ properties, that there are not a large number of specimens and repeated tests and that the manual application might affect the results. Thus, the analysis presented in this section has to be considered carefully. Nevertheless, the scattering is similar to other researches involving masonry (see [21], [23], [24]) and the manual application is considered as a representative
Conclusions
An experimental campaign on real-size TRM strengthened masonry walls under eccentric compressive load has been presented. The results showed a notable increase (100%) of the load-bearing capacity of masonry walls when applying different types of TRM. Three different mortars and two different fibre types were tested. In all cases the improvement was noticeable. Moreover, all mortars reached the necessary bonding strength to assure the adherence of the TRM to the masonry. Therefore, no connectors
Acknowledgements
The authors wish to acknowledge Mr. Francesc Puigvert and Dr. Marco Antonio Pérez Martínez for providing essential support in the experimental campaign.
This study has been partially carried out within the project “Uso de nuevos materiales composites para el refuerzo y rehabilitación de estructuras de edificación y obra civil con criterios de sostenibilidad TERREME”, funded by the Spanish Ministry of Economics and Competitiveness, and Project BIA 2009-13233, funded by the DGE of the Spanish
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