Natural FRCM and Heritage Buildings: Experimental Approach to Innovative Interventions on “Wall Beams”

Abstract

In this paper, an innovative strengthening system for masonry walls made of externally bonded Fabric-Reinforced Cementitious Matrix (FRCM) is presented. Due to the good mechanical properties and the compatibility with the architectural heritage, the FRCM is an adequate alternative to the use of Fiber-Reinforced Polymer (FRP) composites and other traditional techniques. The proposed system is applied to the strengthening of a classical architectural typology in cultural heritage architecture, which is the “in falso” masonry: a load-bearing wall built over a masonry vault, and hence without a direct load path to the ground. A research program, characterized by an experimental campaign, has been started in order to devise and verify an optimal strengthening system that assures for the masonry wall a structural behavior similar to a “wall beam”, so to prevent progressive collapses when the underlying masonry vault loses its carrier function. In particular, rather than the canonical application, consisting in widespread application to the whole surface of the masonry wall, an innovative intervention made of “Green Tape” of composites has been designed and verified by a specifically designed experimental set-up. The main objective of the research is to propose a reinforcement strategy not detrimental to unmovable artistic assets and tied to the safety and robustness of the architectural heritage.

1. Introduction

The Italian architectural heritage has a central and iconic role, whose protection is closely linked to a territory that faces risks related to natural hazards, such as seismic events. The knowledge and conservation of buildings through synergies between universities, companies and institutions therefore represent a challenge for society, strategic for cultural and economic growth. These are the premises for being able to better understand this ongoing research, inspired by a previous design and modeling experience [1,2] concerning a heritage architecture site designed by the prominent architect Luigi Vanvitelli: the Murena Palace in Perugia. Such a building, as with many other historic ones, is characterized by peculiar features and additions that must be analyzed before the beginning of any design process [3,4,5,6]. In particular, there is an intrinsic structural asymmetry with clusters of rooms with masonry vaults, combining different heights, where load-bearing walls are standing on top of the vaults, that are called “in falso” walls in the jargon, and have to be analyzed as a sub-structures with regard to the seismic vulnerability. The results presented are part of a research project aimed at the protection of the architectural heritage thorough critical methodological reflections and careful analysis, particularly linked to site-specific choices about techniques and materials in contexts of high cultural value [7,8,9]. The research aims to develop an innovative application criterion for the FRCM composites—based on natural materials—with particular reference to the issue of progressive collapse in heritage buildings: an element failure results in the failure of adjoining elements, which, in turn, causes further elements’ failure. Buildings which are sensitive to progressive collapse are those with “in falso” masonries, which can be defined as load-bearing walls that rely for their support and load transfer on masonry vaults and slabs, as shown in Figure 1. Specifically, for such peculiarities, rather than the usual FRCM application, where the fibers are applied on the entire surface of the wall, an innovative intervention named “Green Tape” is proposed. The originality of this approach consists in limiting the application of the fibers, grouped to form unidirectional tapes, only to the most significant areas, such as those where traction is expected, thus guaranteeing structural improvement in the face of a reduced quantity of composites. The FRCM application is thus characterized by unprecedented sustainability, both from an environmental and architectural point of view. Furthermore, to reinterpret the traditional construction techniques (e.g., the reinforced plaster) within new increasingly circular processes of construction, materials with natural origin are chosen for the two phases of the composite under investigation: basaltic fibers that are embedded in the matrix phase made of lime inorganic mortar. The project was preceded by an in-depth analytical study conducted with respect to the state of the art and taking into account industry procedures, regulations, guidelines and scientific findings produced by numerous authors and research institutions [1,10,11,12,13,14,15]. In particular, several authors have dealt with the complex issue of composite materials, their structural behavior and their consequent modeling. In this sense, reviews on the state of the art concerning experimental tests aimed at characterizing FRCM and historical masonries under different profiles have been presented [16,17,18,19,20,21,22,23,24]. This phase was followed by the analysis of the current modeling issues for the topic in question [25,26,27,28]. Furthermore, only a few authors have addressed the topic of “in falso” walls [29,30] and, for this reason, an innovative experimental setup is proposed in this paper, aimed at investigating their structural behavior and possible intervention strategies for their reinforcement. In addition, considering that the interventions proposed in the paper are designed for architectural heritage, different approaches and data acquisition techniques are also briefly analyzed that are both respectful of the environment and the architectural value of historical architecture [31,32,33,34,35,36]. Finally, it is then foreseen, as a future development of such ongoing research, that the experimental campaign and its evolution could be the basis for the creation of a proper numerical modeling strategy. In the first part of the paper, the design of the innovative experimental setup is described and then the tests’ results are discussed in order to achieve a comparative assessment of the “Green Tape” strategy’s influence in the prevention of progressive collapse related to the aforementioned structural behavior.

2. Innovative Experimental Campaign

2.1. Non-Canonical Application Criteria for FRCM on Wall Beams

The FRCM system represents the next generation of fiber-reinforced composites and is becoming broadly used in heritage restoration. The key feature of this emergent technology is the replacement of the classical polymeric matrix with an inorganic one, making it particularly effective in historical masonry reinforcement given also the chemical–physical compatibility with ancient substrates. Some of the FRCM benefits are the following:

• good mechanical properties in the face of low thickness and little weight;
• easy installation modalities ensuring the continuation of the buildings’ activities;
• use of inorganic mortar (less aggressive than epoxy resins) that permits better transpiration to masonry;
• chance to recycle, considering the opportunity to use natural origin nets and matrices.

Furthermore, among the qualities of the system, it is necessary to emphasize the topic of durability. Such an aspect is not secondary to the issue of mechanical performance and is checked in different contexts by means of severe accelerated aging tests designed to simulate some of the typical conditions of chemical–physical degradation concerning historical masonries (e.g., presence of humidity, alkaline environment, saline environment, freeze–thaw cycles) [37,38,39]. In the specific case of the materials making up the proposed system, good performance regarding the aforementioned phenomena is guaranteed by the mechanical characteristics and porosity of the mortar and by the mineral origin of the basaltic fibers, which have a better response to aging if compared to vegetable ones. On this, although not plant-based, the basaltic fibres are not synthetic and can certainly be defined as natural: their production cycle consists only in the fragmentation, melting and subsequent spinning of this type of volcanic rock. Moreover, they offer multiple chemical and physical characteristics that are interesting from an environmental point of view, e.g., production with reduced energy impact, high thermal inertia, etc. The “Green Tape”—through a process of conception, development, prototype construction and experimental tests—takes advantage of this potential, exploiting the know-how already at the disposal of installers specialized in the application of these systems. In particular, the “Green Tape” intervention consists in FRCM tapes, each one 1 cm thick and 20 cm wide (a measure that represents a multiple of the dimension with which the meshes in basaltic fiber are produced). The tapes are used only on the weakest walls’ areas, using therefore a smaller amount of composite, since generally this type of system is applied evenly on the wall’s surfaces. Moreover, a strict installation order is devised which requires at first to apply the tapes on the diagonals subject to traction and then to the lower band of the masonry panel and, moreover, to the vertical borders useful for maintaining adhered to the wall the ends of the other strips of composite material; see Figure 2. The samples’ preparation, concerning the application of each tape of composite, requires:

• the cleaning of the surface of the masonry substrate and its saturation and wetting (condition s.s.a., in technical jargon);
• application of a first layer of lime inorganic mortar made by a fine grain, with a maximum diameter of the aggregate equal to 1.2 mm;
• positioning of the basaltic fibers that are a bi-directional net with 20 mm square mesh;
• application of a second layer of mortar with the same features as the previous one.

It is worth noting that, according to the configuration of Figure 2, the wall is simply supported at the two lower ends. Therefore, the principal stress directions are those shown in Figure 3, obtained using a finite element model developed in SAP2000 [40] using four-node homogeneous shell elements with thickness 0.12 m. Indeed, as only in-plane analysis was done, the shell element employs only a membrane formulation with drilling degrees of freedom. The overall length is 2.4 m and the overall width is 1.2 m, and 12 elements were used in the horizontal direction and 6 elements in the vertical direction, giving a total of 72 elements used. The coarse mesh used to highlight in the figures the principal directions allows anyway to maintain the qualitative correctness of the results. The material is homogeneous and has Young’s modulus $E=24.8\times {10}^6$ kN m${}^{-2}$ and Poisson’s coefficient $\nu =0.2$. The upper nodes are subjected to a force downward of 2.4 kN (the internal ones) and 1.2 kN (the two at the ends), corresponding to a distributed load of 100 kN m${}^{-2}$. The weight was assumed to be zero, in order to highlight the principal stress direction due to load only. A linear elastic analysis was performed. As can be appreciated, the diagonal strips of FRCM have been placed where the principal stress directions are inclined by approximately 45° with respect to the horizontal direction, and the direction of the strips corresponds to the principal stress in traction. This layout is different from the one usually employed in earthquake engineering, where the masonry beams above the openings are reinforced by placing the fiber strips in the shape of a cross, since the stress distribution is different.

2.2. Experimental Setup and Prototypes’ Mechanical Properties

In the scientific literature, there are several references to experimental tests for the FRCM materials, both codified and innovative ones, but there is none similar to the one described below [41,42,43]. In particular, the authors are interested in evaluating the bearing capacity of an “in falso” wall when the slab over which it relies collapses. In this configuration, the wall acts as a beam hinged at the ends and subjected to a vertical load coming from the slabs at the upper level, as shown in Figure 3, and it must transfer the load to the supports horizontally. It is worth noting that the configuration of the diagonal and horizontal “Green Tapes” is such that they correspond to the principal traction directions.

An experimental setup was designed to load full-size masonry walls (ordinary and FRCM-reinforced) with actions similar to those described for the case of horizontal slab collapse: masonry panels suspended at the bottom, with the exception of lateral supports of reduced length. With the aim of loading the prototypes until failure occurred, a steel reaction frame was created to be a closed self-balanced system. Moreover, the specimens were built directly on top of steel temporary supports, designed with a dual function: to facilitate their placing inside the steel frame and to support the walls themselves until the beginning of the test—in both cases, without causing damages or deformations to the samples. Thanks to a forklift, it was possible to remove the support and begin the test with the walls, already equipped with sensors, resting only on the remaining permanent lateral supports; see Figure 4.

The load was applied gradually by means of a hydraulic jack placed on the top of the wall; between the wall and the jack, a load cell was placed to measure the applied force. Force distribution steel beams were used to create a load concentrated in two points, by means of two steel plates, so that the applied load was somewhat similar to an equivalent distributed load. A total of 8 linear variable displacement transducer (LVDT) sensors were installed on the wall, 4 for each side, using extension rods to cover the whole length of the diagonal of the half-wall, equal to around 1 m. These, sensors along with the load cell, aimed at measuring the applied load, were connected to an acquisition system, specifically a IMQ CRONOS PL, with a sampling rate of 1000 Hz. Several experimental activities have been carried out so far, also during previous experiences, both concerning the certification of the composite system and the mechanical characterization of the materials used for the campaign in question; for further details, refer to such data as [44,45,46]. In particular, the masonry specimens consist of walls of 2.4 m width and 1.2 m height, made of a single type of solid brick and with similar dimensions to the ones provided by the Italian Building Code for the verification of masonry to shear load [47]. Concerning the materials used for their construction, solid bricks (

Table 1. Mechanical properties of the solid bricks: W weight, $\rho$ gross density, $f_{bm}$ compression strength in the vertical load direction and $f_{bm}^{\prime }$ compression strength orthogonal to vertical loads.

W [kg]	$\mathit{\rho }$ [kg m${}^{-3}$]	${\mathit{f}}_{\mathit{bm}}$ [MPa]	${\mathit{f}}_{\mathit{bm}}^{\prime }$ [MPa]
2.7	1700	35	12

) having dimensions 12 cm × 24 cm × 5.5 cm and lime inorganic M2.5 mortar with a particle size distribution of a maximum of 3 mm were employed; in particular, a mortar with poor mechanical properties was created to simulate the mortar usually found in historical masonries (

Table 2. Mechanical properties of the mortar M2.5: ${\sigma }_C$ compression strength, ${\sigma }_B$ bending strength, $\tau$ shear strength ([47]) and E Young’s modulus.

${\mathit{\sigma }}_{\mathit{C}}$ [MPa]	${\mathit{\sigma }}_{\mathit{B}}$ [MPa]	$\mathit{\tau }$ [MPa]	E [MPa]
>2.5	>1.2	0.15	>5000

).

The mechanical properties of the composite material are reported in

Table 3. Mechanical properties of the basaltic fiber net: T equivalent thickness, Q tensile breaking load of the warp, E Young’s modulus, $\epsilon$ strain, G grammage, ${\tau }_u$ ultimate tensile strength.

T [mm]	Q [N mm${}^{-1}$]	E [GPa]	$\mathit{\epsilon }$ [−]	G [kg m${}^{-2}$]	${\mathit{\tau }}_{\mathit{u}}$ [MPa]
0.035	78	$89\pm 2$	$<8$	0.24	3100

and

Table 4. Mechanical properties of the matrix: ${\sigma }_C$ compression strength, ${\sigma }_B$ bending strength, $\tau$ shear strength [47] and E Young’s modulus.

${\mathit{\sigma }}_{\mathit{C}}$ [MPa]	${\mathit{\sigma }}_{\mathit{B}}$ [MPa]	$\mathit{\tau }$ [MPa]	E [MPa]
>15	>4	0.15	9600

. In particular, a bidirectional basaltic fiber net characterized by strands 4 mm wide placed with an interaxis of 20 mm has been used, while the matrix was a lime inorganic M15 mortar with maximum 1.20 mm particle size distribution. The reinforcement intervention was applied with the same criteria on both sides of the walls (see Figure 5). After the curing period exceeding 56 days, 28 days for the masonries plus the same duration for the FRCM, the experimental tests were performed.

3. Experimental Results

In this section, the results of the experimental tests are reported, analyzing them from different points of view, while in the next section, they will be also compared in terms of the intervention’s performances. Thus far, three walls have been examined: an ordinary wall and two masonries reinforced on both faces with different patterns of application for the “Green Tape”. The prototypes were characterized, except from the composites’ presence, by the same characteristics in terms of geometry features, masonry texture and materials’ mechanical properties. Moreover, considering that the present application is an innovative and non-codified type of test, the outcomes of the first test on the unreinforced masonry have been used as a basis for the subsequent tests, which, except for the application of the composite materials, in every part were analogous to the first one, also in terms of points of view for the photos and videos and sensor’s application; see Figure 6. Moreover, the scheduled steps of the campaign were postponed as a result of the pandemic outbreak and therefore more tests are planned for the near future (e.g., on the wall retrofitted according to the canonical criteria).

Test #1

Experimental test on the unreinforce wall

The failure during Test #1 occurred due to the separation and failure of the mortar joints, as can be seen in Figure 7, with the damage starting at a load of approximately 40 kN. Due to the structural softening, even before the removal of the sensor’s support rods, the progression of the collapse and of the damage pattern occurred and an arched-type stress redistribution mechanism was recognizable; see Figure 7.

Moreover, the time histories of the displacements recorded by the LVDTs are shown in Figure 8 (sg1 and sg5 are not shown since they went out-of-range due to a failure in the connection).

Test #2

Experimental test on the reinforced wall

In this case, it was possible to witness a peculiar failure phenomenon, which manifested itself first with partial lesions on the composites and then with a collapse (Figure 9) due mainly to the bricks breaking (and not by the separation of the mortar joints, as seen in Test #1). Moreover, a load of around 140 kN was necessary to cause the failure of this wall, compared to the 40 kN needed for the unreinforced one; see Figure 10. Even if the beginning of a fracture in the lower left reinforcement band has been observed during the test, the wall collapsed because of a large fracture located on its right side (from the photos/videos’ point of view) due to the breakage of bricks, mortar joints and composites, with reference to both the basaltic net and matrix, as can be seen in Figure 9.

Test #3

Experimental test on the reinforced wall with “Green Tape”

The innovative feature of the “Green Tape” project consists in the idea of using a reduced quantity of FRCM, with the purpose of promoting even more their use in heritage buildings’ restoration, often marked by the presence of decorations, stuccoes, frescoes, etc. To achieve this, unlike in the previous Test #2, the masonry prototype of Test #3 was reinforced without the midspan vertical bands of composite and the two smaller diagonal strips from both the sides of the wall (a reduction in the quantity of the composites greater than 60 percent if compared to the classic application where the FRCM covers the whole surface of the wall). Even in this case, the failure mode was quite brittle and mainly located at one side (Figure 11). A load of around 160 kN, the greatest so far, as can be seen in Figure 12), was necessary to cause failure. It is worth noting that the test was interrupted at the moment in which the load cell registered a pressure drop: different lesions involving bricks, mortar and composite matrix manifested and evolved, with a vertical principal lesion covering the whole height of the wall.

Later, the outermost layers of the matrix in two of the main areas where the lesions occurred were removed and the state of the fiber net was assessed, and it was found that the fibers were not broken, denoting that, likely, when the masonry and the mortar cracked, a sliding mechanism involving the fiber net occurred.

4. Critical Analysis of Outcomes

In the following, the term displacement is used for brevity; however, elongations/relative displacements are the quantities at play. Moreover, the presented outcomes are comparable to the results obtained by other researchers [29,30]. In a future work, all the recorded data will be further elaborated and then compared with digital video/optical flow analysis thanks to the application before the beginning of the tests of QR code markers. In the next activities, the elastic behavior of the prototypes with loading–unloading cycles will be used to analyze the variations in the masonry stiffness due to the “Green Tape” application.

The total collapse of the reinforced walls occurred for a load of around 160 kN in Test #3 and 140 kN in Test 2, compared to the 40kN required in Test #1. For the purpose of comparison, the average values recorded for each matching sensor pair on opposite faces of the wall are plotted in relation to the applied load in Figure 13. From a comparison with the video frames, recorded for each of the tests, in relation to the load histories, it emerges that where the curves show a change in behavior, partial damage in the fiber-reinforced materials likely occurred. On both the reinforced prototypes, the evolution of damage in the masonry manifested at first with lesions on the lower composite band and then engaged the inclined bands.

Finally, the average values deriving from the sensors labelled—in all tests—sg3 and sg7 and in matching positions on the opposite faces of the wall were plotted in relation to the applied load, which, in the authors’ opinion, represent reliable measurements in all the experimental tests; see Figure 14. It is observable not only how the reinforced walls were considerably more resistant than the unreinforced one while preserving adequate ductility, but that the presence of the “Green Tape” has not only contributed to increasing the in-plane resistance but has also prevented, despite the damages, the total disintegration of the walls. This result is of primary importance in terms of the global robustness of historic buildings and protection of human life, thus configuring the proposed strategy as a tool useful during seismic events and the catastrophic collapse of “in falso” walls. Indeed, the composites act as a passive defense in reducing earthquake risks and enhancing security. It is also observable, in sustainability terms, that in Test #3, which concerned a panel reinforced with small quantities of FRCM, the “Green Tape” application seemed to ensure an adequate reinforcement equal in this configuration to approximately four times the one manifested by the unreinforced wall—using only 35 percent of the FRCM adopted in canonical applications. Considering the methods of intervention and the compatibility characteristics of the materials, one of the tests scheduled for the future will concern the restoration of the wall damaged during Test #3. As a matter of fact, with great interest for study and maintenance reasons, and as proof of the intervention’s reversibility, it was possible to replace the still not totally disintegrated wall on the temporary support, and proceed with the removal of the previous FRCM intervention, without causing any damage to the masonry, and to reapply the material again according to a “Green Tape” layout.

In addition to new experimental tests, the ongoing research will also include, in the future, the adequate numerical modeling of the reinforced walls, both through homogenization (in elastic and plastic field [48,49,50]) and finite element analysis.

5. Conclusions

A pioneering and non-standardized experimental setup was designed and realized to evaluate the influence of an innovative FRCM application through the “Green Tape” system: a design approach promoting interventions on “in falso” walls with reduced quantities of composites and improving even more their use in architectural heritage, often marked by the presence of valuable elements of art and architecture, such as windows, stuccoes, pavings, frescoes and painted vaults. Different experimental tests have been performed, involving an unreinforced masonry panel and two reinforced walls, and permitted us to analyze the influence of such innovative applications for the reinforcement of the “in falso” walls, acting, in the event of damage and seismic action, as “wall beams”. The tests highlighted promising results: the innovative application criteria, using smaller amounts of composites, are more compatible with heritage buildings and the environment, ensuring at the same time an adequate reinforcement and limiting the risks of progressive collapses in masonry buildings. The tests have shown how the proposed intervention is effective in increasing the resistance of the wall panels subjected to the conditions described, acting as a “passive defence”, which comes into action in the event of an earthquake or the collapse of underlying masonry vaults. The system also provides great benefits in terms of human life and safety, by ensuring the non-immediate collapse of the walls and also preventing their total disintegration in the case of severe damage. Consequently, always in terms of the prevention of progressive collapses in heritage buildings, new tests are scheduled for the future concerning other application variants (e.g., X configuration, FRM at only one side, different amounts of textile layers). Indeed, the experimental campaign is still in progress and the small number of specimens tested so far has also been affected by the advent of the pandemic. Once a sufficient number of specimens have been tested, a numerical model will be created to replicate the experimental tests and assess the influence of innovative natural composite materials.