Vrije Universiteit Brussel

Design, casting and fracture analysis of textile reinforced cementitious shellsTsangouri, Eleni; Van Driessche, Aron Jef M; Livitsanos, Georgios; Angelis, Dimitrios

Published in:Developments in the built environment

DOI:10.1016/j.dibe.2020.100013

Publication date:2020

License:CC BY-NC-ND

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Citation for published version (APA):Tsangouri, E., Van Driessche, A. J. M., Livitsanos, G., & Angelis, D. (2020). Design, casting and fractureanalysis of textile reinforced cementitious shells. Developments in the built environment, 3, [100013].https://doi.org/10.1016/j.dibe.2020.100013

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Developments in the Built Environment 3 (2020) 100013

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Developments in the Built Environment

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Design, casting and fracture analysis of textile reinforced cementitious shells

Eleni Tsangouri *, Aron Van Driessche, Georgios Livitsanos, Dimitrios G. Aggelis

Dept. Mechanics of Materials & Constructions (MeMC), Vrije Universiteit Brussel (VUB), Belgium

A R T I C L E I N F O

Keywords:Textile reinforced cementitiousShellDuctilityCurvatureSlendernessFractureAcoustic emissionDigital image correlationFull-field structural health monitoring

* Corresponding author.E-mail address: eleni.tsangouri@vub.be (E. Tsan

https://doi.org/10.1016/j.dibe.2020.100013Received 3 May 2020; Received in revised form 28Available online 5 June 20202666-1659/© 2020 The Author(s). Published by Elsnc-nd/4.0/).

A B S T R A C T

The high potentials of thin, light-weight and slender textile reinforced cementitious (TRC) composites permitsmulti-layered, effective, highly-aesthetic and elaborated designs as launched nowadays: domes, vaults, free-formstructures. Their casting and installation appear challenging requiring textiles secure mounting and in-timemortar casting. In this study, TRC roof shells are cast into a reusable formwork that provides controlled thick-ness and smooth surfaces. Casting is limited to a few hours with the minimum labor. TRC shells are tested underdistributed loading. Digital Image Correlation measures online, in full-field the deflection and cracks opening.Acoustic Emission tracks cracks onset. It is demonstrated that at the early loading, TRC shells behave as acompressive element. Beyond cement strength, the textiles elongate to bear loading. In higher curvature shells,the stiffness rises and several wider cracks form. The catastrophic collapse is evaded, and ductility is achieved.TRCs demonstrate optimal performance with non-confined supports, offering new perspectives on free-formdesigns.

1. Introduction

1.1. Historic review

There is an enormous gap in the history and evolution of concreteshell structures. In the decade of 1950, Felix Candela provided a glimpseon future lightweight and load-bearing efficient shell elements. TheXochimilco double-curved shells in Mexico were cast by shotcrete andreinforced with steel, an outcome based on pure experimentation (Burgerand Billington, 2006) (Fig. 1a). In 1968, Jo~ao da Gama Filgueiras Lima, aBrazilian architect, designed the roof of Taguatinga hospital inthin-walled shell forms made of ferrocement (Vilela Júnior, 2012). Thestructure faced durability issues soon afterwards due to extensivecorrosion effect. In 1977, the idea of Candela had awakened once againin Germany, where a pavilion for National Garden Festival in Stuttgart isbuilt in a similar hypar-units design principle, but this time glass fibrereinforcement is used (Holgate and Menges, 1997) (Fig. 1b). The tran-sition to this new reinforcement reduced the shell thickness from 80 toonly 12 mm. The design still carried a vital drawback: the shells instal-lation required labor-intensive fabrication and complex in-situ formworksolutions. For these reasons, the concept remained silent for several de-cades until in early 2000, when the design principles of sustainable andeco-friendly design called for thin, lightweight and slender structures. Atthat moment, research focused again on shells since curved plates

gouri).

May 2020; Accepted 30 May 20

evier Ltd. This is an open access a

ingeniously embody load-bearing efficiency and lightness, better thanany other structural geometry. And it is only recently that research ac-tivity resurges since today modelling tools of great computational powerare developed to effectively design shells, as revolutionarily done byBlock Research Group (Adriaenssens et al., 2014; Van Mele and Block,2011), analytical models exist to predict the shell response to loading(Hawkins et al., 2017, 2018), materials are made with the utmost per-formance (Hegger et al., 2018) and the formwork digital design is opti-mized (West, 2016).

The shells design nowadays appears revolutionary from differentperspectives:

Elaborated forms. Using the principle of ‘form follows force’ (Li,2018), shells are formed in single-curve or more complex shapes, up tofreeform complex geometries, in a way that the element not only opti-mally withstands its self-weight, but also service loads. The shells formand complexity vary significantly based on the design approach, indic-atively: funicular geometries are introduced following the fabric dynamicrelaxation form; hypars are designed based on mathematical forms;parametric optimization leads to designs of NURBS surfaces. In all cases,the geometrical complexity is considered as a tool to optimally designbased on loads distribution. With an improved load-bearing capacity,shells are designed resistant to dead loads, but also and most importantlyto wind, snow and other weathering loads being suitable as outdoor roofelements.

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rticle under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-

Fig. 1. a) The Xochimilco double-curved shells in Mexico (1958 (Burger and Billington, 2006),); b) The hypar-units pavilion for the National Garden Festival inStuttgart (1977 (Holgate and Menges, 1997),).

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

Materials optimization. The replacement of steel rebars with alkali-resistant (commonly glass or carbon) fibre textiles structured in anorthogonal design gives the option for thin shells design. The reinforce-ment is not suffering from corrosion anymore; therefore the section canbe thinner and fine-grained cement be used. The latter improves thestructure's carbon footprint since cement-related CO2 emissions arereduced, the matrix more effectively flows and impregnates the textilesproviding enhanced cohesion at both material and composite level.Additionally, both reinforcement and cover are more durable and morereliable today: the tensile resistance of textiles is enhanced by impreg-nating the fibres into polymer coating, the concrete tensile strength hasbeen increased and the strain hardening response has been improved. Asa result, TRC composites appear more ductile with higher energy ab-sorption capacity (Scholzen et al., 2015).

Formworks optimization. Prefabrication based on in-situ castinginto formworks is introduced to eliminate the laborious and time-consuming single-sided laminating method (Veenendaal et al., 2011).Nowadays, formworks are pre-cast, modular and flexible providing

Fig. 2. a) Pavilion TRC roof elements (Scholzen et al., 2015); b) Bicycles roof TRC stinstallation after casting (bft-international, 2013); d) IBETON-EPFL design of a TRCcompared to traditional flat steel-reinforced concrete element (Hawkins et al., 2020

2

enhanced workability. Manufacturing based on hollow casting approachis effectively done in a few hours and the result is repeatable. Shotcreteuse is limited today; instead, the fine-grained concrete can be easilypoured in formworks into which the textiles are mounted. In a recentstudy, fibre-reinforced cementitious shells were manufactured in woodenformwork and later assembled and disassembled in an optimized timeframe (Scheerer et al., 2017).

Curved TRCs are implemented on the design of infrastructure ele-ments at RWTH Aachen Univ. Campus, an impressive demonstration ofthe newmaterial design capacities: a 49 square-meter large pavilion withroof elements made of hyperbolically shaped TRC shells (Fig. 2a(Scholzen et al., 2015),) and a prototype bicycles roof stand made ofsingly-curved TRC shells (Fig. 2b and c). In parallel, at IBeton-EPFL lab.,light-weight large-span TRC shells were effectively manufactured(Fig. 2d (Valeri et al., 2019),). Vaulted floor slabs are cast from TRC byHawkins et al., 2017, 2018, 2020 and demonstrated significant reductionon the embodied carbon in comparison to conventional flat slab solutions(Fig. 2e).

and at RWTH Aachen University campus (Sharei et al., 2017a); c) The roof standshell roof element (Valeri et al., 2019); e) Floor slab made of thin TRC shell

).

Fig. 3. Fragments of cylindrical shells designed with a fixed distance betweensupports equal to 1 m and varying curvature.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

1.2. The effect of the geometry and the mechanical response analysis

The simplest shell structure is a single-curved plate with typical ge-ometry of a cylindrical shell. One-way cylindrical shells are designed asroof elements and in service are continuously supported over their edges(Fig. 2b–d). A TRC shell is a load-bearing structure since apart from itsself-weight, they are designed to carry external active loads such as windand snow. Thin shells have the advantage to optimally resist to loadthrough efficient membrane actions. Also, stresses are effectively trans-mitted through the contact edges to the supporting structure underneath(Veenendaal et al., 2011).

TRC is an ideal composite for the design of highly curved shells fortwo reasons. First, and compared to traditional steel reinforcement,textiles are flexible, therefore they can be easily bent and shaped incomplex creative forms. Secondly, although shells are designed to actprimarily in compression, matrix reinforcement is required to providerobustness and effective response to tensile actions built under intricatepatterns of live loading (Hawkins et al., 2018). It should be highlightedthat although shells can be optimally designed in theory approaching thefunicular geometry to resist pure compression, in reality, variations inlive loads, differential settlement or movement of the supports andmanufacturing errors lead to introduction of bending actions. To over-come this complex loading patterns, the use of TRC appears as aneffective solution since TRCs are composed of a strong cementmatrix thatcan effectively resist to compressive actions and of fibre textiles that cansignificantly deform providing sufficient durability, but also withstandhigh tensile stresses.

Considering the challenging design and application principles, a se-ries of questions arise and are tackled in this research paper. To beginwith, the effect of curvature on the developed stress field and theconsequent damage should be investigated since there are limited rele-vant studies in the literature, mainly based on theoretical models. In thepast, it was reported that shells responding as compressive forms cancarry great membrane actions, significantly bigger than the bending ac-tions of a flat plate (Taylor et al., 2001). However, there is no experi-mental evidence of this in the case of TRC shells of different curvatureand slenderness. In a step further, the state-of-the-art lacks a comparativestudy on the effect of shells geometry and reinforcement configuration onthe damage progress and effective textiles activation. This study providesa preliminary report in this direction.

In this direction, cylindrical shells of two different radii (512;725 mm) are considered. The shells are fragments of a thin cylindricalelement with stable support distance equal to 1 m, as illustrated in Fig. 3.The way the shell is supported, confined or free to move, was alsoinvestigated further. In the past, shells were made of concrete in formsthat can carry only compressive forces, therefore the shell had to belaterally confined. Today and by using TRCs, textiles are embedded intothe material allowing significant ductility and building up of plastichinge mechanisms. The latter can have a beneficial result on materialdamage response since brittle failure is avoided, as long as the service-ability limits are respected. In any case, the composite response of TRCsin curved form should be validated. In this respect, the study compara-tively evaluates the damage development of TRC shells with reinforce-ment volume fraction above and lower to the critical value since in thesecond case, the performance of the textiles can be restricted. To effec-tively track the damage progress and detect cracks onset and propaga-tion, but also internal deformation of the textiles, acoustic emission (AE)and digital image correlation (DIC) are used building an integrated full-field monitoring methodology that innovatively provides an extensiveand comprehensive insight of shells fracture.

2. Materials and method

2.1. Materials

The cementitious matrix is cast using ready-mixed Sika-grout 217

3

powder (cement, siliceous sand and admixtures) with particles size up to1.6 mm, selected due to excellent workability and flowability. Thepowder is mixed with water in a w/c ratio set at 0.15. The fine-grainedcement paste is characterized by high compressive and flexural (σm)strength equal to 70 and 12 MPa (28 days) respectively, and stiffness(Em) up to 9 GPa (Tsangouri et al., 2019). The shell is cast with 22 mmthickness (the minimum size that ensures effective force transmissionbetween the textile yarns and the cement (El Kadi et al., 2018)) andconsidering 2 layers of textile set in 10 mm distance (schematicallyillustrated in Fig. 4b).

The textile is a 2D arranged alkali-resistant (AR) glass (Sitgrid 200,Fig. 4a). The textile yarns are coated with styrene-butadiene. Theorthogonal grid size is equal to 17.5 mm and the textile density is62.6 kg/m2. The textile strength (σf) was measured up 526 MPa in ten-sion and its stiffness (Ef) was equal to 67 GPa. Plastic spacers provided bydistTEX (Fig. 4c) were knit between the grids to hold together the textilelayers (detail view in Fig. 4d). Another series of spacers made of PVCcylinders with a diameter of 20 mm was used to set well the distance ofthe textiles from the formwork ensuring this way that enough cementcover is provided. A picture of the attached textiles is given in Fig. 4e(PVC spaces depicted in dark grey color). The use of 3D textiles wasassessed in a preliminary test setup, but it was concluded that building acurved TRC shell was not feasible since the rigid 3D textiles cannot bebent enough to obtain the high curvature design.

2.2. Geometry

In total four shells were cast as depicted in Fig. 5 and presented inTable 1. The study considers shells of two different curvatures with arespective radius of 512 and 725 mm. The cylindrical shape is selectedconsidering potential applications in the constructions field. Testing twodifferent curvatures aims to investigate the effect of curvature on thedeveloped stress field and the TRC response under service loads. One ofthe shells (400_22_Con_1) is fixed at the supports and the others havesupports that allow horizontal displacement. The case at which anoma-lies and errors are introduced during casting leading to thicker section orirregular textiles placement is also investigated by designing a shell(200_28_Free_4) at which the cement thickness is not adequately

Fig. 4. a) AR-glass textile; b) drawing of cross-section; c) plastic spacer (@distTEX); d) two layers of glass textiles are fixed in constant distance using the plastic spacer,close-up view of the spacer attached at both sides to the textiles; e) the two layers of textiles are bent accordingly and bonded together. The reinforcement layers areready to be placed into the formwork.

Fig. 5. Drawing with the geometry features of each test series. The code names are reported in Table 1.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

controlled and inevitably the fibre volume fraction drops below thecritical value (Vf ¼ 0.8% < Vf

crit ¼ 0.99%) as derived from the equation(Heidi Cuypers, 2002):

Vcritf ¼ σm

σf � σmð1� ðEf =EmÞÞ

2.3. Casting and curing

An effective casting of TRC shell appears a challenging task in practicesince the textiles should remain bent and in place, the cementitiousmatrix should smoothly flow and cover the textiles forming constantthickness composite and finally the outer surface should remain smoothand featureless. The formwork (provided by Twinplast n.v.), into whichthe shell is cast, consists of two complementing pieces (upper and lower)that fit together leaving a gap as thick as the TRC shell. The formwork is

Table 1Code names and design features for each testing sample.

Code name Curvature radius (mm) Rise from the bottom line (mm)

400_22_Con_1 512 400400_22_Free_2 512 400200_22_Free_3 725 200200_28_Free_4 725 200

4

made by Expanded Polystyrene (EPS) and is shown in Fig. 6a. A finecover of impact-resistant polystyrene film is used to establish good con-tact with the cement. The EPS formwork surfaces are covered withgreasing oil to ensure smooth and effective demolding. The formwork isset at vertical position (Fig. 6b) and the textiles are mounted in the gap.Then the formwork is sealed, and the two parts are confined using timbersupports as depicted in Fig. 7a.

The opening of the two parts is restricted with the use of rebar-boltsystems (screw jacks), positioned at different heights of the formwork.The use of screw jacks ensures that cement leakage is eliminated, but alsosets a constant thickness despite the differential hydrostatic forces intothe TRC at different heights. The formwork is then placed on top of avibration table that vibrates on low mode to ensure effective air voidsrelease from the cement during casting. The cement is progressivelypoured into the formwork from the top side with the use of small buckets(Fig. 7b). During filling up, the level of cement is assessed with naked

Slenderness (diameter/thickness) Thickness (mm) Supports

46.5 22 Confined46.5 22 Free65.9 22 Free51.8 Range:24–32/Average: 28 Free

Fig. 6. a) The two-pieces formwork; b) Casting setup.

Fig. 7. a) The formwork with the rebar-bolts system that ensures constant TRC thickness and eliminates cement leakage; b) The TRC shell casting into theEPS formwork.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

eye. Shell 200_28_Free_4 is cast into a formwork with limited restrictionson widening and therefore shell thickness varies at different sections andheights of the sample, a case imitating manufacturing errors.

The time needed for the manufacturing process of a single shell is 3 hwith a crew of four workers. After casting, the shell is sealed off and curedin ambient environment conditions for 28 days. After demolding, nocracking is noticed, and the cement surface is smooth without substantialsurface voids.

2.4. Testing

Loading. A 3D coordinate system is set and based on that, the shellsides are respectively marked in Fig. 8a. The support lines run along the Yaxis. The side covered with digital image correlation (DIC) specklepattern (Y ¼ 0 along it) is marked as the front side. The EPS formworkupper part is used as a shell cover that forms a horizontal plane on whichload could be safely placed (Fig. 8b). Cement bags of 25 kg are used asloads since they could be easily moved by a crane and effectively andprogressively placed in layers and in a distributed way at the top hori-zontal plane. The loading process aims to simulate service-life loadingconditions. In a quasi-static mode, a bag of cement is added at the topplane every 1 min (loaded shells illustrated in Fig. 9a and b). The bags areuniformly distributed at the bottom layers close to the EPS cover plane,however at higher layers, less bags are placed at the edges of the plane forsafety reasons, progressively forming this way a bell distribution ofcement bags. The test ends if the shell collapses or in case that no extrabags can be added for safety reasons (catastrophic failure of setup). Theload is expressed relative to the area under loading, in kN/m. A shellloaded with 1600 kg (15.7 N/m) is shown in Fig. 9a.

Digital Image Correlation. The middle zone at the TRC front side(Fig. 8a, section B) is painted in white and on top of it a layer of randomly

5

distributed black speckles with an average diameter of 1 mm is applied(Fig. 8d). A pair of high-definition 5MP cameras are fixed in a stabletripod and positioned in an angle facing with a stereoscopic vision thefront side of the shell (Fig. 9a and b). A pair of lenses of 8 mm focal lengthare mounted on the cameras. The total area of interest is 22 mm high andup to 500 mm long, as a result the resolution is approximately 5 pixels/mm in both directions. A subset of 27 pixels and a step of 7 pixels areused. A DIC image is taken each time a cement bag is added. Vic-Snap andVic-3D softwares provided by Correlated Solutions are used to captureand respectively post-process the images (https://www.correlatedsol).Full-field horizontal (X) and vertical (Y) displacements and the respectivestrains (Exx, Eyy, Exy calculated by the Lagrangian finite strain tensor)are obtained by setting a strain window equal to 11 pixels.

Acoustic Emission. Eight AE transducers with resonant frequency at150 kHz are applied. The transducers aremountedwith the use ofmagneticholders at the bottom side of the shell (Fig. 8c, section A) on positions givenin Fig. 8a, in a configuration that covers themiddle zone of the shell, whereextensive AE activity is attended. The AEwin software (https://www.phy-sicalacousa) is used to capture the emitted hits and the amplitude thresholdis set at 35 dB. Pre-amplification at 40 dB is considered and the data arecollected with a sampling rate of 2 MHz. AE hits are stored and furtherpost-processed using Noesis software (https://www.physicalacousb). Thetransducers position (projected in-plane) and the wave velocity (3000m/s,measured by pencil lead-breakage test) are given as input on the planarlocalization algorithm provided by AEwin.

3. Results

3.1. Damage onset and crack analysis

The shells are spatial membranes that withstand the external loadings

Fig. 8. a) Bottom view of the shell with the AE transducers and actuator positions' marked on the drawing; b) Load setup: the load applied in the form of bags and theEPS cover are illustrated; c) Section A (see Fig. 8a) with three AE transducers mounted at the bottom side of the sample; d) Section B (see Fig. 8a) shows the DICspeckle pattern area at the middle zone at the front side. Detail view of the speckle with an indicative representation of the subset size.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

by developing internal normal membrane forces (on the shell's plane)and bending moments (perpendicular to the shell's plane). As analyticallypresented in (Ishakov and Ribakov, 2015), shells under service loadsdevelop the highest displacement in the middle section. Deflectiongradually drops towards the edge supports, where deflection is zero. Onlyif the support is free to move and due to thrust, vertical displacement maychange sign (edges movement outwards). A meso-level damage progresssimilar to the one reported on flat TRCs under mode-I fracture (Avestonand Kelly, 1973; Cuypers and Wastiels, 2011) is tracked. Initially, thecement elastically deforms, and the composite remains intact. Reachingthe ultimate strength of the matrix, cracks nucleate and propagatethrough the cross-section and local debonding from the textiles occurs.Beyond that point, textiles elongate and deform in an attempt to restrictthe widening of the cracks. Few studies in literature report on how acompressive structure, such as shells, mechanically responds at thatdamage state (Sharei et al., 2017b; Hawkins et al., 2019). For this reason,

Fig. 9. a) Free shell 400_22_Free_2 under loading; b) Confined shell 400_22_Con_1 fuof the middle zone deformed at the end of the test.

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the composite response should be extensively investigated and in-depthassessed by continuous online monitoring methods. In this directionand to begin with, the TRC progressive failure is visualized by DIC strainmaps in Fig. 10a representatively for 400_22_Free_2.

As shown in the load-deflection curve, the shell is significantlydeflected (1.7 mm in the middle section) when the first crack forms in themiddle of the shell (Fig. 10b). Beyond that point, more cracks form,marking a jump on crack opening (measured by subtracting the defor-mation between two points standing at the crack sides at the bottom line,see Fig. 10B) curves, and the deflection increases (Fig. 10 c-d). The testends as no more cracks nucleate, therefore the cracks widening stage isreached. Due to the effective composite action, multiple cracking isachieved, hinge mechanisms are formed and stress redistribution isreached.

AE analysis, complementary to DIC that monitors only surfacedisplacement, provides continuous information on the internal damage

lly loaded; c) The AE transducers mounted at the bottom plane; d) Closeup view

Fig. 10. a) DIC strain maps visualizing the cracks progress at testing moments A-E; b) Load-deflection as derived by DIC at the middle section; c) Load-sum of cracksopening; d) Load-opening for each crack individually with color codes added on DIC strain maps (400_22_Free_2). (For interpretation of the references to color in thisfigure legend, the reader is referred to the Web version of this article.)

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

evolution. Continuous AE activity is recorded as early as the beginning ofthe test. The AE hits and energy distribution during testing are depictedin Fig. 11a. AE energy is presented since this wave feature can beindicative of cracks’ intensity (Carpinteri et al., 2013). AE activity equalto 10.9% of AE total hits and with energy up to 20% of the final cumu-lative energy, are recorded at early pre-damage stage proving thatconsiderable micro-cracking takes place much earlier than the visuallyobserved macro-crack (stage B, Fig. 10a). Furthermore, a great number ofevents are localized at this early loading stage, as depicted in red-coloredpopulation in Fig. 11b. In the same scatter plot, the events localized as thefirst crack forms are marked in black and it is shown that are standing atthe zone where pre-cracking events were reported. The rest of the eventslocalized later marked in lighter grey color and are more widelydistributed around the middle zone of the shell where multiple saggingcracks gradually formed.

In extension to previous studies (Blom et al., 2014), the analysisproves that AE can predict and localize with sufficient engineering ac-curacy the damage nucleation even in cases that complex geometry andcomposite materials are tested. Indicatively, the average horizontal co-ordinate of the localized events during the 1st crack formation is at505 mm, only 6 mm away from the actual crack. By correlating the DICmaps with the AE energy plot, it is shown that AE activity instantly in-creases and AE energy is emitted in a greater scale every time a new cracknucleates (see arrows in Fig. 11a). In other words, AE energy as emittedby fracture and measured by the transducers can provide an indirecttoughness assessment and pinpoint the critical stages on damage progress

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at which significant fracture energy is absorbed (Carpinteri et al., 2016).Additionally, in this study at which upscaling of previous findings isattempted (Cuypers andWastiels, 2011; Sharei et al., 2017b), it is proventhat the applied optical-acoustic sensing system can effectively trackcracks on curved TRCs along with their service life, while the previousstudies focused on small coupons or flat geometries (Blom et al., 2014).

3.2. Effect of geometry and test conditions

The fracture progress is controlled by the shell curvature and thesupports configuration and as a result the initial stiffness, the cracks size,density and distribution should be comparatively and extensivelyinvestigated. In this direction, the load-deflection and cumulative cracksopening plots of all shells are given in Fig. 12a and b. Also, in Fig. 13, thebottom line deflection of each shell is plotted at load intervals as visu-alized by DIC. In parallel, an overview of early stiffness, deflection,number of cracks, AE hits and events trends are summarized in Table 2. Itshould be noted that the stiffness is calculated as the slope of load-deflection curve at early elastic region, earlier to any matrix crack for-mation, approximately at 10% of ultimate load (~1.5 kN/m).

Effect of support conditions. A first interesting observation is that,the laterally confined shell (400_22_Con_1) develops the highest stiffnessamong others. As long as the shell is restricted, the deflection remainslimited, however no evidence excludes potential internal damage builtup even at early low load level. It is shown that 12.7% of hits occur at thepre-cracking stage, proof of significant internal damage at micro-level

Fig. 11. a) AE hits and energy (*10�2 μVs) plots in time (400_22_Free_2). The load is projected at the secondary horizontal axis; b) AE localized events plane-projected:red-colored events occur at the pre-cracking stage; black-colored events correspond to the 1st crack formation and the rest AE events marked in grey color occur afterthe first crack. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 12. a) Load-deflection curves for each shell; b) Sum of cracks opening evolution for each shell.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

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Fig. 13. Movement of the shells at discrete loading stages. The position of each shell at the moment that the first crack forms and the moment beyond the cracksaturation are highlighted with red and black color respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Webversion of this article.)

Table 2Overview of the test outcome.

Code name Initial tangent stiffness(load/deflection)

Cracksnumber

Deflection before1st crack

Hits before 1stcrack/Total hits

Load at 1stcrack

Events during 1stcrack/Total events

Finaldeflection

Number ofhits

(kN/m2) – (mm) (%) (kN/m) (%) (mm) –

400_22_Con_1 426 1 1.53 12.7 12.1 99.0 6.1 15750400_22_Free_2 370 6 1.66 10.9 5.5 11.6 28.7 32080200_22_Free_3 173 2 0.7 4.0 3.0 35.0 9.8 25000200_28_Free_4 359 1 1.9 29.5 9.6 3.3 3.4 41580

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

(Table 2), however limited AE events are respectively detected. When theedges are released, the shell significantly deforms and a unique crackforms. As the crack instantly nucleates, the shell deflects up to 5 timesmore that earlier stage (Figs. 12a–13a).

Curvature effect. As expected, the initial stiffness increases as thecurvature rises and the slenderness drops (Tables 1 and 2). Similar trendsare reported in masonry and concrete shells that respond as purecompressive structures, however the use of textile reinforcement differ-entiates the response to damage compared to traditional monolithicshells. It is shown that the first crack forms earlier in low-curvature shell(Fig. 12a, Table 2), on the other side the high-curvature shell carriestextiles in an orientation that enhances the composite bending stiffness,therefore delays the damage onset. As a sequence, more AE hits accu-mulate at this early pre-cracking stage, as reported in Table 2. Up to10.9% of total hits are emitted earlier to damage onset for the highercurvature shell, the latter indicating that important fracture process zoneis built by microcracks that surround the section where the first crack willoccur.

As the first crack forms, matrix relaxation occurs at the vicinity of thecrack and the textile fibres provide the load-bearing capacity that is lostby the cement matrix. Once again, the higher curvature shell respondswith higher bending stiffness; therefore fibres bear larger stresses,

9

consequently undergo larger strains (Blom et al., 2017). In Fig. 12, theinstant deflection and crack opening jump at the first crack is greater forthe higher curvature shell. Similarly, each time a new crack nucleates,both deflection and crack opening are greater for the higher curvatureTRC shell. It is of great importance that composite action is present sincein both cases at least one crack is formed beyond the matrix ultimatestrength, compared to confined series at which stress redistribution is notreached.

Despite the composite action that imposes multiple cracking, a sig-nificant difference in the number of cracks is monitored by DIC. Only twosagging cracks form at the lower curvature shell (200_22_Free_3)compared to six cracks that form at 400_22_Free_2. Multiple cracking isevidence of effective composite action between the matrix and the textilereinforcement. Indirectly, this study proves that by increasing the cur-vature and the geometrical complexity of the TRC, the textile rein-forcement is optimally activated and the TRC fracture toughness isenhanced.

Indicatively, at crack saturation stage, at the end of the multiplecracking zone, the sum of cracks opening and the deflection are five andthree times greater for the higher curvature shell (compared to200_22_Free_3, see Figs. 12–13 and Table 2). It is concluded that thehigher curvature TRC deflects more and its cracks widen further in an

Fig. 14. AE distribution as projected along X-axis for each test case and images of bottom plane taken after testing.

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

attempt to build multiple cracks and form hinge mechanisms that appearonly beneficial for the global mechanical response. The latter observationhighlights the new potentials that TRC complex geometry design offers.

Effect of textiles volume fraction. The effect of irregular thicknessthroughout the shell is examined in this section, considering the scenarioof thickness increase and consequently drop of reinforcement volumefraction lower to the critical one. In this case, thefibres are not able to carrythe load capacity beyond the cement matrix ultimate strength and as aresult, cracks redistribution is not achieved and catastrophic failure mayoccur (Cuypers and Wastiels, 2006). In detail, due to thickness increase,the shell slenderness decreases leading to enhanced early matrix stiffness,as reported in Table 2, but also the first crack occurs at higher load level(Fig. 12). The latter justifies the vast number of AE hits detected earlier tocracking, attributed to micro-cracking that extensively occurs at thethickermatrix section (Table 2). However, only one crack nucleates due tothe limited contribution of the textiles. The unique crack opening islimited and no rise on deflection curve is noted as the crack is released(Fig. 12). All the above observations clearly indicate that there is limitedcomposite action between the textiles and the matrix; consequently,

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limited toughness and ductility is built. Beyond the first crack, the textilesslip along thematrix at the debonded zone. The latter damagemechanismis the source of several AE hits emitted at the post-cracking stage.

In Fig. 14, the population of the events is presented projected alongthe horizontal axis and the respective images of the bottom plane aftertesting. In all cases, events are restricted in the middle section of the shellin an area that, in agreement to existed literature on other concretestudies (Carpinteri et al., 2011), marks the micro-level fractured zone.The events analysis aims to summarize the main differences in thedamage progress in each test series:

- A limited number of events are detected as the supports of thelaterally confined TRC shell (400_22_Con_1) are released and theunique crack forms in the middle zone. The events are not extensivelydistributed along the horizontal axis, a result that indicates a limitedtextiles debonded area.

- The presence of several cracks at the higher curvature shell(400_22_Free_2) leads to an almost normal events distribution at themiddle zone. Each time a new crack forms, a high number of AE

E. Tsangouri et al. Developments in the Built Environment 3 (2020) 100013

events, approximately 10% of the total events (Table 2), are detected.This proves progressive and controlled damage accumulation. Theimage taken at the bottom plane of the shell highlights the numerousbranches built among cracks along the width of the shell and validatesthat stress redistribution was locally achieved throughout the middlezone of the shell.

- The two cracks that form along the width of the TRC shell of lowercurvature (200_22_Free_3) can be depicted in AE events distributionplot and the respective image (Fig. 14c). This result demonstrates theeffective (but limited -due to only two cracks formation-compared togreater curvature case) stress redistribution. Up to 35% of events aredetected as the first crack forms (Table 2).

- A high number of events are widely spread across the middle sectionand evenly distributed throughout testing for the thicker TRC shellwith insufficient reinforcement (200_28_Free_4). The latter pinpointsthe extended debonded zone that leads to dominant textile slippage.

The extensive AE analysis validates the mechanical tests outcomeeven in the case of complex material and loading configurations. Futurework should incorporate the integrated monitoring system on larger andmore complex TRC elements (i.e. (Tysmans et al., 2009)) since the effectof propagation distance on the wave shape needs to be clarified.

4. Conclusions

The study presents an open dialogue between architecture and frac-ture damage engineering using non-destructive methodology and thishybrid original approach on this innovative material project worthfurther investigation. Using textile composites, new failure sequences areintroduced in the response of shells, which traditionally are designed justas compressive structural elements. In essence, at intact stage the shellwithstands compression (confined case) and bending (freely moved case)under service load, but when the cement matrix cracks the textiles areactivated to withstand further loading and they actually elongate intro-ducing further cracks opening and mode-I fracture. The damage progressalternates since the TRC is not continuous anymore and performs non-rigidly since hinge mechanisms are built. The composite performance,in other words the interaction between the matrix and the textiles, de-pends on the reinforcement volume fraction, the supports setup and theshell slenderness. This is the first time in literature that the complex TRCcurved shell's response to damage is reported.

A series of only four TRC shell samples are considered, providinglimited preliminary, but robust observations regarding their damageprogress. A preliminary proof-of-concept analysis proves the efficiency ofboth novel manufacturing and testing/monitoring methodologies. Infuture, more TRC shells should be tested considering different materials,geometries and loading configurations proving.

The main contributions of this study to the state-of-the-art are sum-marized as follows:

Optimization of casting and manufacturing using a reusableformwork. Thin TRC shell elements are cast with constant cross-sectionthroughout the plate and textiles fixed position by designing a time-effective casting and installation procedure using a reusable EPS form-work is applied that reduces the necessary labor.

Validation of composite action. TRC composite shells do not breakin a brittle mode, on the other hand composite action provides extendedstrain hardening. Eventually, none of the shells collapsed even beyondthe cracks saturation point demonstrating the design effectiveness. Thedensity and distribution of cracks proofs the composite action in differentgeometrical configurations. Indicatively, in high curvature and subse-quently low slenderness shell, the early stiffness is enhanced, the AEactivity is restricted and cracking occurs at higher load levels. A networkof complex crack paths is formed at the bottom zone of the higher cur-vature shell indicating that at crack saturation stage, the sagging area isfully covered by cracks that effectively provide load transfer to the textilereinforcement.

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Transition towards free-form and free-support shells. Historically,shells are designed to be fixed without any displacement freedom of thesupport edges. This is done in practice, in order to eliminate extendeddeflection that arises critical serviceability issues. Indicatively, Eurocode2 limits the deflections under serviceability load combination to span/250 (CEN, 2004), equal to 4 mm in our confined configuration. The studyhas shown that this is the deflection level at which the crack onset occurs.The confined high curvature shell can effectively withstand deflection upto 6 mm, a result that approves the design and use of TRC curved plates.On the other side, the possibility to design freeform and free-to-moveTRC structures are assessed and appear feasible since highly curvedshells can withstand up to 28 mm of deflection at crack saturation stage.

An integrated monitoring protocol. DIC effectively tracks theevolution of the sagging cracks, but in current configuration, DIC pro-vides no information on potential thrust phenomena at the edges. Due tothe thin-plate curved shape of the shell, a compensation is required be-tween the size of the area of interest and the resolution to track motionalong the whole TRC length. AE detects online and in time early micro-scale damage that leads to cracks formation even in the case that thesupports movement is confined. The continuous inspection provides anaccurate view of internal damage by detecting the onset of the cracks andby localizing the zones where TRC composite breaks.

TRC shells can be a great load-bearing structural component thatstands slender, lightweight and aesthetic. Reinforced with textiles, TRCproves that concrete can be organically shaped in free-form geometries toexpress movement. The introduction of curves on TRC design is thefuture of architectural engineering targeting to sustainable solutions,however their complex response to damage still needs to be well un-derstood. This study contributes to this end, by proving the effectivenessof the reinforcement but also by forming a proof-of-concept testing andmonitoring platform.

Acknowledgements

Financial support of FWO (Fonds Wetenschappelijk Onderzoek-Vlaanderen, projects number G.0C38.15 & 12J7720 N) is gratefullyacknowledged.

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