EXTERNALLY BONDED TEXTILES FOR SEISMIC RETROFITTING … · Reinforced Mortar (TRM) for the seismic retrofitting of architectural heritage. Two different applications are presented,

EXTERNALLY BONDED TEXTILES FOR SEISMIC RETROFITTING

OF HISTORICAL STRUCTURES

Gianmarco de Felice1

ABSTRACT

Externally bonded composites are emerging as an effective solution for structural upgrade and seismic retrofitting, since their high strength-to-weight ratio ensures significant improvement of the mechanical performance of existing structures with minimum encumbrance and mass increase. The use of mortar matrix instead of polymeric one makes this technique suitable for application to masonry structure and retrofitting of architectural heritage. This paper shows relevant applications of Textile Reinforced Mortar (TRM) for the seismic retrofitting of architectural heritage. Two different applications are presented, related to seismic retrofitting of masonry walls with respect to out-of-plane loads and extrados strengthening of vaults. The performances of these systems are discussed with reference to the results of the experimental tests carried out in the laboratory. On this basis, the paper intends to provide an overview of the potentialities and limitations of TRM for seismic retrofitting of historic masonry.

Key words: Masonry, Cultural heritage, Retrofitting, Textile Reinforced Mortar, FRCM, Composites, Arches.

1 INTRODUCTION

The performance of cultural heritage buildings under seismic action is one of the challenging research topic due to the necessity to withstand safety requirementswhile fulfilling preservation criteria.

Traditional construction techniques are clearly adequate for ensuring the accomplishment of preservation criteriaand, in most cases, are also sufficient for ensuring seismic safety. In earthquake-prone regions, ancient building masters generally have the proper knowledge and experience to size and design properly the building against the earthquake risk. In those regions, the design rules and the construction techniques have progressedin time, to provide theseismic protection. In such cases, the approach for seismic retrofitting can simply afford on the proper comprehension and reproduction of traditional construction detailstoensure the fulfillment to the “rule of the art” of masonry construction.

However, there are a number of cases, in which architectural heritage is strongly exposed to the seismic risk, for a number of reasons that can be summarized in a few categories:

i. Material deteriorationhave occurred due to the lack of proper maintenance; ii. Alterationshappened in the architectural and structural systems over the centuries that have

compromised the original structural performance; iii. The construction does not fulfilwith the “rule of the art”; iv. Traditional construction rules are still insufficient for an adequate protection against seismic

actions.

This latter item should be considered carefully since, at present time, the request for safety of buildings has increased with respect to the past, to ensure the protection of human life against natural hazard, and guarantee the conservation of heritage sites,avoiding the risk of break in historic continuity.Therefore, there is a need for the design of remedial actions and for the development of new strategies and technologies for the repair and seismic upgrade of historic constructions.

1 Roma Tre University, Department of Engineering

Uluslararası Katılımlı 6. Tarihi Yapıların Korunması ve Güçlendirilmesi Sempozyumu / 2-3-4 Kasım 2017

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For instance, therecognition of the damages suffered by historic buildings in Italy after recent earthquakes, have shown the dramatic vulnerability of stone masonry walls that have failed by separation of the two leaves and disaggregation (Figure 1). Damages have been observed also in thin brick masonry vaults. Due to their slenderness, these vaults wereparticularly vulnerable against the relative displacement of the abutments caused by theearthquake (Figure 2).

Figure 1. Failure by disaggregation of masonry experienced by historic building during the 2016 Earthquake in Amatrice, Italy.

Figure 2. Failure of in-folio vault during the L’Aquila, Italy 2009 earthquake.

Therefore, in numerous existing historic masonry structures in seismic-prone regions, the walls need to be repaired to improve their solidity and the vaults need retrofitting to ensure an adequate safety level according to current standard codes. Among the different retrofitting techniques, externally bonded


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reinforcements with composite materials have proven to be particularly advantageous [1-2] to ensure an adequate safety level, since they provide high mechanical performances with minimum thickness and mass increase.

In the last two decades, most research activities and field applications have made use of composites with polymeric matrix (Fibre Reinforced Polymers, FRP). Nevertheless, a new generation of composites has been recently proposed that consist of high strength fabrics (made out of steel, carbon, basalt, glass, PBO or natural open meshes) embedded into inorganic (mortar) matrices [3]. Mortar-based reinforcements are named Textile Reinforced Mortars (TRM) or Fabric Reinforced Cementitious Matrix (FRCM) when comprising carbon, glass, basalt, or PBO fabrics, arranged in the form of open meshes, or Steel Reinforced Grout (SRG) when making use of steel textiles. The use of mortar in place of resin ensures the fulfilment of thepreservationcriteria, such as reversibility, durability, and compatibility with original substrates and decorative settings. Therefore, itappears particularly promising for applications to masonry structures and cultural heritage.

In the recent years, increasing research efforts have been devoted to the study of TRMs, fostering a fast technological and industrial development. At the same time, several applications have been realized for safeguarding cultural heritage against earthquakes. The TRM-to brick/masonry bond behaviour has been investigated by a number of studies [3-5] that provided fundamental information on bond strength and failure modes, and highlighted the role played by the mechanical properties of the matrix, the layout of the textile, and the roughness of the surface of the substrate. Nevertheless, a better knowledge still needs to be gained, especially on issues such as durability, performance levels, installation details and, apart from the ACI 549.4R-13document, no design guidelines are yet available.

The purpose of this paper is to explore how recent advances in composite materials can be conjugated with the design of remedial actions for the seismic protection and retrofitting of architectural heritage. The aim is to investigate to what extentthese new technologies can be used to increase safety of historic building without loosing cultural value.As regards the masonry walls, the results of shake table tests on a wall restrained at the top and subjected to out-of-plane seismic loading which activates vertical bending before and after strengthening with TRM is presented. As concern the vaults, the results ofan experimental investigation carried out on brick masonry vaults before and after the application of TRM systems on the extrados, are illustrated. The tests allow to detect the change in the structural behaviour of masonry due to the application of the reinforcement and to get a deeper knowledge on the effectiveness of TRM in terms of strength and displacement capacity.

2 MECHANICAL PROPERTIES OF THE TRM SYSTEM

Among the different mortar based composite systems that are available in the market, in both applications, a unidirectional fabric consisting in very thin cords of Ultra High Tensile Strength Steel (UHTSS)externally bonded with inorganic matrix for retrofitting masonry structures. The steel textile have about 3000 N/mm2 tensile strength and 190kN/mm2 Young’s modulus (Figure 3). It is applied with a lime-based mortar,having 15 N/mm2 compressive strength and 22kN/mm2 Young’s modulus.With respect to the other fabrics available in the market, steel textiles are stiffer and stronger than glass and basalt and thicker than carbon, aramid and PBO, are isotropic (which provides better toughness), more durable in alkaline environment, and need lower cost and energy for production.


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Figure 3. Steel Reinforced Grout (SRG): details of one single steel wire and of the steel fabric; examplesof its application to masonry vaults and walls.

3 STRENGTHENING OF MASONRY WALLS

The need for strengthening masonry walls is mainly related to their weakness under earthquake action, as dramatically revealed by the damage suffered by historic masonry buildings during seismic events (Figure 2). The most dangerous condition is that of poor masonry walls, made of small units and weak mortar, without a careful arrangement of the stone units [6]. These walls may fail by disaggregation and, in such a case, the use of TRM on both the internal and external sides, with adequate transversal connectors, would provide the requested seismic upgrading.

This technique, however, requires the removal of the original plaster from both the internal and external faces, resulting in a significant loss in terms of cultural and historical values. Its use should therefore be restricted to the most critical cases of very poor masonry structures, in which other available techniques would fail in ensuring the adequate transversal connection to the wall.

Aiming at assessing the effective increase in horizontal acceleration and lateral displacement capacity of the TRM systems applied to masonry walls, a shake table experimental investigation has been carried out on two full-scale walls. The specimens were of the same size: 3.7m high and 25cm thick, one in stone masonryand the other in tuff blocks. The walls were provided with a top beam in reinforced brickwork, having high strength steel textiles in the bed joints,provided with steel connectors to the walls. Test setup was designed to prevent the horizontal displacement of the top beam, leaving free rotations and upward displacements (Figure 4). Both walls, which differ on the type of masonry the stone arrangement and the specific weight were tested simultaneously, to allow a direct comparison between their seismic response.

Natural accelerogramswere applied in both, the horizontal and the vertical directions, with increasing scaling factor, up to collapse. Based on previous experience on shaking table tests on masonry specimens [7], a set of five natural records was selected for the tests, amongst the most severe Italian earthquakes of the last 40 years.

1 2 3 54mm

1

2

3

4

5

00

10 20 30 5040mm

10

20

30

40

50

00


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Figure 4. Boundary condition of the specimens on the shaking table and expected failure mechanism.

An innovative 3D motion optical system named 3DVisionwas used in addition to accelerometers and displacement transducers, to measure displacements during the shake table tests. 3DVision system makes use of wireless passive spherical retro-reflecting markers positioned on several points of the specimen, whose spatial displacements are recorded by near-infrared digital cameras. The analyses in the time domain allowed the monitoring of the deformations of the walls and the detection of the fundamental frequencies and modal shapes.

Figure 5. 3D motion optical system: display of the markers in the monitor and detail of one of the retro-reflecting markers

Both the walls failed by the development of horizontal hinges. The peak ground accelerations that caused the collapse were 0.52g for the stone wall and 0.86g for the tuff wall. Test movies are available at the following links:

https://www.facebook.com/Repubblica/videos/10154916764456151/ http://video.repubblica.it/natura/dall-irpinia-ad-amatrice-cosi-i-terremoti-distruggono-gli-

edifici/263244/263608?ref=HREC1-25


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A further shake table test series was carried out on the same specimens, after repair and reinforcement was undertaken. The TRM systems used for retrofitting the masonry walls,comprise either a Ultra High Tensile Strength Steel unidirectional textile (for the tuff wall) or a basalt and stainless steel bidirectional fabric (for the stone wall). Both textiles were applied with lime-based mortar. The successive phases of the application of SRG reinforcement are depicted in Figure 6.

Figure 6. Installation process of vertical strips of Steel Reinforced Grout reinforcement on tuff masonry walls: a) application of a primer for enhancing the bond and then a first layer of the inorganic matrix; b) application of the UHTSS strips with careful insertion of the textile at the base od the wall; c) covering of the steel strips with a second layer of mortar and injection of the mortar into the holes at

the basement.

The same input signalsthatwere already used for the tests on the unreinforced walls,were applied with increasing scaling factor up to the collapse of the specimens. The overall test results are still under elaboration, however some preliminary outcome are available as shown in Figures 7-8, where the displacement pattern and the maximum acceleration-displacement results for L’Aquila record are depicted, respectively. Through the comparison with those obtained on the unreinforced specimens, the experimental results clearly show the gain in the seismic capacity provided by TRM reinforcements.

Figure 7. Envelope of the maximum displacement pattern (on the x axis in mm) of the tuff wall under L’Aquila 2009 earthquake record for increasing scaling factors: a) unreinforced specimen for scaling factors ranging from 0.25 to 1.25; b) specimen reinforced with SRG for scaling factor ranging from

0.50 to 2.25.


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The tests results (Figures 7-8) for L’Aquila record can be summarized as follow: the presence of the SRG reinforcement on the tuff masonry wall led to an increase of the load carrying capacity under out-of-plane vertical bending of about 200%. The failure mechanism and the displacement capacity seemed to be not strongly affected by the reinforcement. The SRG reinforcement was effective in restraining the opening of cracks up to when the bond capacity in the hinges regions was reached and the mortar matrix started damaging and detaching from the specimen.

Figure 8. Seismic performance of a tuff masonry wall before and after retrofitting under L’Aquila 2009 earthquake record scaled at increasing PGA. The points represent the maximum PGA and the

corresponding maximum displacement experienced by the specimen.

4 EXTRADOS STRENGTHENING OF BRICK MASONRY VAULTS

The second application of TRM on historic structures is related to the strengthening of a vault. Since about twenty years ago, a number of experimental and theoretical studies have been carried out on the strengthening of masonry vaults with externally bonded composites [1]. The composite, being able to carry the tensile load, avoids the opening of the cracks, thus preventing the activation of the four hinges failure mechanism. Both applications, either at the intrados or at the extrados of the vault, succeed in providing the increase in load carrying capacity. However, when applied at the extrados, the tensile load in the composite induces compressive stress on the substrate; conversely, when applied at the intrados, tensile stress are induced. In the case of FRP reinforcement, the curvature plays a minor role, since the appraisal of the tensile or compressive stress on the bond interface does not affect the behaviour of the bond, as failure occurs according to cohesive mode II. Conversely, in the case of TRM systems, the behaviour changes completely, since failure generally occurs within the matrix and is rather sensitive to the normal stress, i.e. the cohesive failure mode I and mode II are coupled [8].

Figure 9shows the installation process of the Steel TRM on the extrados of a vault. Given the one-directional properties of the steel wires that constitute the reinforcement, the application is carried out in two subsequent phases in the orthogonal directions, according to the double curvature of the vault. The width and the spacing between the strips are designed to provide the requested increase in strength as well as to prevent any local fall of bricks during seismic events. The steel wires are directly inserted into drilled holes at the abutments and injected as to provide a further restraint. Accordingly, if for any reason the bond between the reinforcement and the extrados of the vault deteriorates, the end connections would still allow the reinforcement to contribute as unbonded tendons.The presence of the steel fabric at the extrados of the vault is able to prevent the opening of cracks, thus inhibiting the four hinges collapse mechanism that would take place in unreinforced vaults. Additionally, the retrofitting of the vault with this technique succeeds in preserving decorative settings at the intrados with no changes in the thickness nor increase in the mass of the structure.


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Figure9. Phases of strengthening brick masonry vaults at the extrados: a) preparation of the extrados of the vault after the removal of the filling; b) application of a primer for enhancing the bond between the

extrados of the vault and the inorganic matrix; c) application of the Ultra High Tensile Stress Steel strips; d) end con-nections at the abutments; e) application of the covering layer of mortar matrix and of a second transversal set of strips; g) construction of brickwork spandrel walls for the backing and

filling with granular material.

The tests carried out in the laboratory were aimed at simulating as closely as possible the effective behaviour of the vault, including the contribution of the buttressesand of the filling materials. As it is well known, both these contributions are crucial in the stress transfer and cannot be neglected in the analysis. Since the activity is currently underway, only some preliminary results are presented in this note, which however are sufficient to show the effectiveness of the strengthening system. As a preliminary activity, several double lap bond tests were carried out aiming at evaluating the bond properties between the composite system and the brick masonry substrate in the presence of curvature (Figure 10).These tests provided the failure mechanisms and the increase in the bond strength related to the convex curvature, allowing to design the strengthening of the vault.

Figure 10. Bond failure between TRM and brick masonry for curved substrates.

The experimental setup was designed to simulate the worst loading conditions that bring the vault up to failure. The load was applied over the filling at about one third of the span by means of an hydraulic actuator. Tests were run under displacement control to follow the whole load-displacement post-peak response up to failure.


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Only two tests on the vaults have been carried out to date, consisting of two sections of the vaults, with2800 mm span, 55 mm thickness and 500 mm depth. The first specimen was representative of the actual conditions of the vault before retrofitting, while the second specimen was retrofitted by applying the SRG reinforcement to the extrados, consisting in UHTSS with 8 cord/inch, with 150 mm with, having an overall cross section of 25.2mm2.

Figure 11. Laboratory tests on real-scale in-folio vaults retrofitted with Textile Reinforced Mortar: a) collapse configuration and b) analysis of the displacement field with Digital Image Correlation.

The applied load was directly provided by the loading cell of the actuator, while the displacement filed of the vault and of the filling during the test was recorded by means of nonconventional contactless measurement methods, such as the Digital Image Correlation. Strain gauges were also glued to the reinforcement textile to record the stress and estimate the load carried by the steel cords.

The tests results (Figure 11-12) can be summarized as follow: the presence of the SRG reinforcement led to an increase of the load carrying capacity of the vault of 280% (from 5.7 kN to 15.4 kN. In the unreinforced specimen, a four hinge mechanism activated, while the SRG strips bonded prevented the development of the cracks at the extrados. Failure occurred by a combination of crushing in the masonry and shear sliding between the bricks under the load application point. The SRG strips detached from the substrates in some portions (detachment occurred at the textile-to-matrix interface).


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Figure 12. Laboratory tests on real-scale in-folio vaults retrofitted with SRG: comparison of the load

displacement experimental results in the unreinforced wall (blue line) and the retrofitted one (red line). The displacements on the left are upward and those on the right are downward at the load application

point.

5 CONCLUSIONS

Laboratory tests on walls and vaults were carried out to investigate the effectiveness of mortar-based composite systems for the reinforcement of architectural heritage. The tests were conducted on full-scale specimens in order to reproduce asfar as possible the actual field conditions.Innovative contactless sensors such as Digital image correlation were used to record the displacement and reproduce the crack pattern during the tests.

For both applications, the mortar-based composite limited the development of cracks and restrained the activation of the hinges mechanism that was responsible for the failure of the unreinforced structures. The displacement field recorded during the test showed a more smooth deformation pattern with lower concentration of the strainat the hinge sections. Close to the ultimate capacity, the reinforcement locally detached at the textile-to-matrix interface, and this appeared mainly related to instability in the compressed portions of the reinforcement rather than to shear stresses. The application of the TRM reinforcement proved to increase the load capacity of more than two times in both applications. No decrease in deformation capacity has been experienced with the tests carried out.

It is worth highlighting that the performance of the strengthening system strongly depends on the accuracy of the installation, on the preparation of the surface of the substrate (whose roughness should be ensured), and on the curing conditions of the mortars. A larger number of experimental results is needed to develop a deeper understanding of the mechanical behaviour of mortar based composites for the retrofitting of masonry structures. Nevertheless, the studies carried out so far have indicated that SRG can be an effective and cost efficient solution for the safeguarding of historic masonry structures against earthquakes, and, more in general, for the conservation of architectural heritage.

ACKNOWLEDGEMENT

The experimental investigation is carried out within the following partnerships: i. KerakollSpA Research Agreement “Seismic retrofitting of masonry structures with composites”

ii. Ministero degli Affari Esteri e della Cooperazione Internazionale(ItalianMinistry for Foreign Affairs), ITALY – USA Science and Technology Cooperation Project Nr. PGR00234 “Composites with inorganic matrix for sustainable strengthening of architectural heritage”

iii. Regione Lazio. Progetto COBRA “Sviluppo e diffusione di metodi, tecnologie e strumenti avanzati per la Conservazione dei Beni culturali, basati sull’applicazione di Radiazioni e di tecnologie Abilitanti”


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iv. Reluis-DPC Executive Projects 2017 “Assessment and Mitigation of Seismic Vulnerability of Existing Masonry Structures”

v. Rilem Technical Committee 250-CSM“Composites for the Sustainable Strengthening of Masonry” The technical participation of Dr. Stefano De Santis, Dr.Gerardo De Canio, Dr. Ivan Roselli, Dr.

Lorena Sguerriand Dr.Francesca Rosciniin experimental activitiesis gratefully acknowledged.

REFERENCES

Valluzzi MR, Valdemarca M, Modena C. 2001. Behaviour of brick masonry vaults strengthened with FRP laminates. J Compos Constr; 5(3):163-169.

Papanicolaou, C.G.; Trinantafillou, T.C.; Lekka, M., 2011, "Externally bonded grids as strengthening and seismic retrofitting materials of masonry panels," Construction and Building Materials, V.25, No. 2, pp. 504-515.

de Felice G, De Santis S, Garmendia L, Ghiassi B, Larrinaga P, Lourenço PB, Oliveira DV, Paolacci F, Papanicolaou CG. 2014. Mortar-based systems for externally bonded strengthening of masonry. Mater Struct; 47(12):2021-2037.

Razavizadeh A, Ghiassi B, Oliveira DV. 2014. Bond behavior of SRG-strengthened masonry units: Testing and numerical modeling. Constr Build Mater; 64:387-397.

De Santis S, de Felice G. 2015. Steel reinforced grout systems for the strengthening of masonry. Compos Struct;134:533-548.

De Felice G. 2011. Out-of-plane seismic capacity of masonry depending on wall section morphology. Int J Arch Heritage; 5:466-482.

De Santis S, Casadei P, De Canio G, de Felice G, Malena M, Mongelli M, Roselli I. 2016. Seismic performance of masonry walls retrofitted with steel reinforced grout. Earthq Engineering& Struct. Dynamics, 45(2):229-251.

Malena M, de Felice G. 2014. Debonding of composites on a curved masonry substrate: experimental results and analytical formulation. Compos Struct; 112:194-206.


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http://www.sciencedirect.com/science/article/pii/S0950061814003870

http://www.sciencedirect.com/science/article/pii/S0950061814003870

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Documents

EXTERNALLY BONDED TEXTILES FOR SEISMIC RETROFITTING … · Reinforced Mortar (TRM) for the seismic retrofitting of architectural heritage. Two different applications are presented,