Glass Struct. Eng. (2018) 3:87–108https://doi.org/10.1007/s40940-017-0039-4

RESEARCH PAPER

The construction of the Crystal Houses façade: challengesand innovations

F. Oikonomopoulou · T. Bristogianni ·F. A. Veer · R. Nijsse

Received: 28 September 2016 / Accepted: 9 March 2017 / Published online: 11 April 2017© The Author(s) 2017. This article is an open access publication

Abstract A novel glass brick façade has been des-igned and engineered to reproduce the original brickfaçade of a former townhouse in Amsterdam. Based onthe original design the resulting façade comprises morethan 6500 solid glass bricks, reinterpreting the tradi-tional brick pattern, and elaborated cast glass elementsfor the replication of the window and door frames.To achieve unhindered transparency, the 10 by 12 mglass block façade has to be self-supporting. Previ-ous experimental work by Oikonomopoulou et al. (JFacade Design Eng 2(3–4):201–222, 2015b. doi:10.3233/fde-150021) concluded that it was necessary touse a clear, UV-curing adhesive of high stiffness asbondingmaterial. Experimental work on prototype ele-ments indicated that the desired monolithic structuralperformance of the glass masonry system, as well asa homogeneous visual result, are only achieved whenthe selected adhesive is applied in a 0.2–0.3 mm thicklayer. Thenearly zero thickness of the adhesive togetherwith the request for unimpeded transparency intro-duced numerous engineering challenges. These include

F. Oikonomopoulou (B) · F. A. Veer · R. NijsseDepartment of Architectural Engineering and Technology,Faculty of Architecture and the Built Environment,Delft University of Technology, Delft 2628 BL,The Netherlandse-mail: f.oikonomopoulou@tudelft.nl

T. Bristogianni · R. NijsseDepartment of Structural Engineering, Faculty of CivilEngineering and Geosciences, Delft University of Technology,Delft 2628 CN, The Netherlands

the production of highly accurate glass bricks and thehomogeneous application of the adhesive to achievethe construction of the entire façade with remark-ably tight allowable tolerances. This paper presents themain challenges confronted during the construction ofthe novel façade and records the innovative solutionsimplemented, from the casting of the glass units to thecompletion of the façade. Based on the conclusionsof the research and the technical experience gainedby the realization of the project, recommendations aremade on the further improvement of the presented glassmasonry system towards future applications.

Keywords Structural glass · Solid glass bricks ·Adhesive glass connections · Glass masonry · CrystalHouses façade · Cast glass

1 Introduction

A novel glass masonry façade has been designed andengineered to replace the brick façade of a formertownhouse in Amsterdam, aiming to preserve the city’straditional architectural style and historical ensemble.Designed by the MVRDV architectural studio (www.mvrdv.nl), the innovative façade follows the originalnineteenth century elevation down to the layering ofthe bricks and the details of the window frames, butis stretched vertically to comply with updated zoninglaws and allow for increased interior space (MVRDVArchitects 2016). Based on the brick modules of the

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Fig. 1 Illustration by MVRDV of the concept behind the Crystal Houses façade

original masonry façade, the 10 by 12 m transparentelevation employs more than 6500 solid glass bricks,each 210(±1) mm thick by 65(±0.25) mm high, rein-terpreting the traditional brickwork and the character-istic architraves above the openings; whilemassive castglass elements reproduce the classic timber door andwindow frames. As it ascends, terracotta bricks inter-mingle with glass ones, gradually transforming theglass elevation to the traditional brick façade of theupper floor (see Figs. 1, 2). The architects’ desire forunimpeded transparency excluded the use of a metalsubstructure, rendering the choice for an entirely self-supporting glass brick system as a necessary and so farunique solution.

2 Principles of the adhesively bonded, solid glassblock wall

An illustration of the structural scheme followed formaximizing transparency is shown in Fig. 3. Based onprevious realized examples of solid glass block façades

Oikonomopoulou et al. (2015b), lists three intertwinedfactors that define the structural performance and thetransparency level of a self-supporting glass blockfaçade: (1) the choice between hollow or solid glassbricks, (2) the choice between structural adhesive bond-ing or supporting substructure and (3) the façade’soverall geometry. In principle, a bearing wall of theaforementioned size employing exclusively solid glassbricks is feasible owing to the compressive strength ofglass (stated between 400–600 MPa for uniaxial load-ing by Fink (2000) and 300–420MPa byGrantaDesignLimited (2015) and the considerable cross-section ofthe solid glass bricks (210 mm) that allow the façade tocarry its dead load and have an enhanced buckling resis-tance. In comparison, a wall of the same dimensionscomprising hollow glass blocks would require a sup-porting sub-structure. Their reduced thickness resultsin internal buckling or stress concentrations that in turnlead to a relatively low stated resistance in compressiveload [defined as low as 6 MPa in ISO 21690:2006 byInternationalOrganization for Standardization (2006)].The lateral stability of the façade is guaranteed by

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Fig. 2 Left 3-Dvisualization of the façadeby MVRDV. Right Therealized façade

Fig. 3 Principle of the proposed structural glass system

four buttresses erected towards the interior by inter-laced glass bricks, resulting in a continuous envelopeof increased rigidity.

An entirely transparent structural system is obtainedby bonding the glass bricks together with a clear adhe-sive. In the developed system, the mechanical prop-erties of this adhesive are equally critical to the onesof the glass blocks; it is their interaction as one struc-tural unit that defines the system’s structural capacity

and behaviour. The most favourable structural perfor-mance is when adhesive and glass bricks fully coop-erate and the masonry wall behaves as a single rigidunit under loading, resulting in a homogeneous loaddistribution. Extended research and testing of variousadhesive types by Oikonomopoulou et al. (2015b) leadto the eventual selectionofDeloPhotobond4468; a col-orless, UV-curing, one-component acrylate of theDeloPhotobond family, designed for high strength bond-ing between glass components. Adhesives of the DeloPhotobond family have already been applied for thebonding of all-glass structures, i.e. in the frames of theglass shell of the Leibniz Institute for Solid State andMaterials Research (Delo Industrial Adhesives 2011).

The selected adhesive is optimized for high forcetransduction in glass/glass and glass/metal bonds andpresents high shear stiffness, good short and long termcompressive behavior and high humidity resistance(Delo Industrial Adhesives 2014). Visually, besidesbeing colorless, it has a similar refractive index toglass (1.5) and does not discolor when exposed to sun-light. Another important feature is its photo-catalyticcuring, allowing for fast construction: The adhesivecan be fully cured in a minimum of 40 s using60mW/cm2 UVA intensity (Delo Industrial Adhesives2014).After curing, it obtains its full structural capacityand becomes moisture- and water- resistant.

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Fig. 4 Schematic illustration of the relation between a stiff adhe-sive’s strength and thickness by Riewoldt (2014), den Ouden(2009), Wurm (2007)

The medium viscosity of the selected acrylate[7000 mPa s at 23 ◦C, measured by Brookfield vis-cosimeter (Delo Industrial Adhesives 2014)] suggestedthat only the horizontal surfaces of the glass bricks arebonded; the vertical ones are left dry, allowing as wellfor thermal expansion.

There are no clear guidelines from the adhesiveman-ufacturer on the recommended application thickness ofDeloPhotobond4468. To the knowledge of the authors,there is not yet a generally approved theory concerningthe effect of adhesive thickness in the strength of thebond. Although the classical elastic analyses predictthat the strength increases with the adhesive thickness,experimental results show the opposite (da Silva et al.2006). Research by Grant et al. (2009), da Silva et al.(2006), and Crocombe (1989) suggests different rea-sons1 why a thicker bondlayer provides a decreasedjoint strength. Based on experimental work by denOuden (2009) and Riewoldt (2014), Fig. 4 exhibitshow a comparatively thicker layer can negatively influ-ence a rigid (i.e. epoxy or acrylate) adhesive’s bondstrength and subsequently the structural performanceof the entire system. In practice Wurm (2007) men-tions that acrylates present their highest strength in anapplication thickness between 0.1 and 0.5mm,whereasPuller and Sobek (2008) suggest an optimum thicknessof 0.2 mm for a glass to metal bond with Delo Photo-bond 4468.

1 Crocombe (1989) suggests that thicker single-lap joints have alower strength considering the plasticity of the adhesive, whereasda Silva et al. (2006) found that interface stresses are higher forthicker bondlines.Grant et al. (2009) suggests that as the bondlinethickness of a T joint increases, there is an increase in the bendingstress since the bending moment increases, reducing the strengthof the joint.

In our case, structural and visual experiments indi-cated an optimum joint thickness for the selected adhe-sive between 0.2 and 0.3 mm. Small wall prototypesby Oikonomopoulou et al. (2015b), comprising glasselements of different tolerance range, pointed out thata homogeneous bond thicker than 0.3 mm cannot beobtained due to the adhesive’s flow properties andmedium viscosity. Besides compromising the visualresult, inconsistent bonding introduces weaker struc-tural zones. Especially voids against the glass substratein stiff adhesives can cause major stress concentra-tions (O’ Regan 2014). Series of four-point bendingtests in a Zwick Z100 displacement controlled univer-sal testing machine, with a speed of 2 mm/min and fail-ure at a nominal flexural stress between 4.79 and 7.01MPa demonstrated that the chosen adhesive enables theglass brick wall to behave monolithically under load-ing, when the adhesive is applied in a uniform layer ofthe optimum thickness (Oikonomopoulou et al. 2015a).The characteristic failure pattern can be seen in Fig. 5:Failure occurs with a straight cut, parallel to the load-ing path, splitting the specimen in two as if it was onesolid component. Specifically, the glass block of themiddle horizontal layer, spanning the vertical joint ofthe top and bottom layers, is split in half. No significantdelamination is observed.

Based on the adhesive’s optimum application thick-ness, it was determined that the glass blocks’ top andbottom surfaces should be flat within 0.25mm for guar-anteeing an even adhesive layer of the highest strength.

The adhesive’smediumviscosity and ideal thicknessof a quarter of a millimeter together with the elasticnature of glass require exceptionally strict tolerancesfor a homogeneous application. The presented systemis fundamentally different from a conventional mor-tar masonry, where the mortar can accommodate pos-sible discrepancies in size and surface quality of thebricks. In this case, any accumulated deviation largerthan the required 0.2–0.3 mm thickness of the adhesivecould lead to uneven and improper bonding. There-fore, not only the size of each brick, but also the thick-ness of each construction layer have to be confinedwithin a tight dimensional precision of a quarter ofa millimeter. The demand of this unprecedented highlevel of accuracy and transparency, introduced vari-ous challenges in the engineering and construction ofthe Crystal Houses façade, calling for innovative solu-tions. Such challenges and their solutions are presentedbelow.

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Fig. 5 Failure pattern of the glass beams

3 Manufacturing and quality control of the glassblocks

The required ±0.25 mm tolerance influenced thechoice of glass recipe and mould used. Solid glassbricks of comparable dimensions (200 mm × 300mm × 70 mm), used in the Atocha Memorial, theonly other adhesively bonded glass block envelope, uti-lized borosilicate glass and precision press moulds forobtaining highly accurate units (Schober et al. 2007).Borosilicate glass was favoured over soda-lime glassowing to its comparably lower thermal expansion coef-ficient [3.2–4 ×10−6/K] over soda-lime glass [9.1–9.5 ×10−6/K] (Granta Design Limited 2015). Thisresults in considerably less natural shrinkage duringcooling and accordingly to a cast element of higherdimensional accuracy. A high precision press mouldfurther confines the cast element to the desired dimen-sions, by pressing the molten glass during the ini-tial, rapid cooling stage. The dimensional toleranceachievedwith this method for the aforementioned glassbricks was ±1.0 mm (Paech and Goppert 2008) with-out any machine processing. However, in the case ofthe Crystal Houses, the required ±0.25 mm tolerancenecessitates the mechanical post- processing of theblocks’ horizontal (bonding) surfaces, even for borosil-icate glass. Therefore, soda-lime glass and open pre-cision moulds were opted for the final brick fabrica-tion to avoid an unnecessary increase in manufacturing

costs. Soda-lime is the least expensive form of glass(Corning Museum of Glass 2011) and requires a sig-nificantly lower working temperature than borosilicate[melting temperature is approximately 1200–1400 ◦Ccompared to 1400–1600 ◦C (Shand 1968)]. As a draw-back, the higher thermal expansion coefficient of soda-lime requires a considerably longer annealing time andthus manufacturing time of the components. For exam-ple, the borosilicate glass blocks of 70 mm × 200 mm× 300 mm in dimensions and 8.4 kg weight (shown inFig. 6), used in the Atocha Memorial, required a totalannealing time of circa 20h each (Paech and Goppert2008). Whereas, the comparatively smaller soda-limeglass bricks of 65 mm × 210 mm × 210 mm in dimen-sions and 7.2 kg weight used in this project, required36–38 h of annealing time respectively. High precisionopen moulds were preferred over press moulds, sincethe use of the latter was considered an expensive andunnecessary solution in view of the inevitable post-processing.

To ensure that the higher expansion coefficient ofsoda-lime glass will not result in excessive thermalstresses on the façade, a simulation of the expected ther-mal loads in a yearly cycle was performed by an exter-nal company specializing in building physics. Basedon the optical transmittance data provided by TU Delftfor the solar gain (see Fig. 7), the orientation of thespecific location, the height of the surrounding build-ings and the assumption of a constant heating load in

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Fig. 6 Left: the 300 × 200 × 70 mm borosilicate glass block ofthe AtochaMemorialmade by press mould.Center: a 210× 105× 65 mm soda-lime glass block of the Crystal Houses prior to

post-processing. Right: a 210 × 105 × 65 mm soda-lime glassblock of the Crystal Houses after post-processing

Fig. 7 Opticaltransmittance data of astandard Poesia brick byTijssen (2014)

winter and cooling load in summer from the indoors air-conditioning, heat and light transmittance of the wallwere simulated. The results indicated acceptable ther-mal strains (less than 14.3 × 10−3) for the soda-limecast glass even under the most extreme weather condi-tions for Amsterdam.

The fabrication of the approximately 7500 solidglass bricks was assigned to the Italian company Poe-sia (http://www.spaziopoesia.it). Each brick is man-ually cast by pouring molten glass in high preci-sion, open steel moulds with a removable bottom sur-face (see Fig. 8). A low-iron glass recipe is used forhigh optical quality. The final chemical compositionof the glass, as measured by a X-ray fluorescence(XRF) spectrometer, is shown in Table 1. To attainthe desired smooth external texture the steel mouldsare preheated to a constant temperature of approxi-mately 650–750 ◦C. If the mould’s temperature falls

below this range, then the hot glass coming into con-tact with the metal surface freezes instantly, creating arough, wavy surface. On the other hand, if the mouldis heated to a higher temperature, the glass tends toadhere to the walls of the mould. A release coating onthe moulds further prevents the adhesion of the moltenglass to the working surface and the development ofmicro-cracks.

After the glass is poured into the mould, it is leftat ambient temperature to rapidly cool until ∼700 ◦C.This rapid cooling through the critical crystallizationzone is essential to avoid the molecular arrangementof the melt in crystals instead of an amorphous struc-ture, which would result in a cloudy glass of reducedtransparency (Shelby 2005). During this initial coolingphase, the glass has still low viscosity that can allowany induced thermal stress to relax out to a negligi-ble amount immediately (Shelby 2005). After the glass

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Fig. 8 Left: The high precision, open steel moulds. Right: Molten glass bricks during the rapid cooling phase from 1200 to ∼ 700 ◦Cdegrees

Table 1 Composition of the applied cast glass based on XRF chemical analysis

Compound Wt% Absoluteerror (wt%)

Compound Wt% Absoluteerror (wt%)

Compound Wt% Absoluteerror (wt%)

SiO2 75.606 0.1 Sb2O3 0.821 0.03 ZrO2 0.043 0.006

Na2O 15.833 0.1 CuO 0.402 0.02 SO3 0.034 0.006

CaO 5.142 0.07 Al2O3 0.165 0.01 TiO2 0.02 0.004

K2O 1.836 0.04 MgO 0.072 0.008 Fe2O3 0.01 0.003

temperature drops to its softening point (∼720 ◦C),2the viscosity of the glass is sufficient for it to retainits shape and not deform under its own weight (Shand1968). At∼ 700 ◦C, the glass element is removed fromthe mould by suction at the top surface, and moved intothe annealing oven.

There, a long andmeticulously controlled annealingprocess eliminates any possible differential strain builtup between casting and demoulding, as well as pre-vents the generation of internal residual stresses duringfurther cooling. Upon this point, key reference tem-peratures are the annealing point (∼545 ◦C) and the

2 The temperatures given here for the softening, annealing andstrain points are indicative for soda-lime glass and are based onresearch by Napolitano and Hawkins (1964). The values maydiffer according to the exact composition of the glass. The exacttemperatures referring to the soda-lime recipe of the glass blockshave not been disclosed to the authors by Poesia.

strain point (∼505 ◦C) of soda-lime glass. The anneal-ing point is defined as the temperature at which the vis-cosity of glass will allow any induced stress to relax outsubstantially in just a few minutes (Shelby 2005). Thestrain point is the temperature where the same stress isreduced to acceptable values in 4h (Shand and Armis-tead 1958; Shand 1968). The cast glass should bemain-tained for adequate time at the annealing point to relieveany existing strains and then cooled at a rate sufficientlyslow so that residual stresseswill not reappearwhen theglass temperature has reached equilibrium (Shand andArmistead 1958). Effectively, below the strain point,stress cannot relax in time and is considered perma-nent (Watson 1999).When the temperature of the entireglass component has droppedbelow the strain point, thecomponent can cool at a faster pace until ambient tem-perature, yet still sufficiently slow to prevent breakagedue to thermal shock (Shand and Armistead 1958).

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Fig. 9 Qualitative analysis of strain concentration by polariza-tion test. Bricks such as the ones shown on the left image, with aclear indication of residual stresses, were discarded. Specimens

such as the one on the right, with no visible considerable strainconcentration, were employed in the façade

Accordingly, during the annealing range, the mag-nitude of the resulting internal stresses, is largelydetermined by the temperature difference between thewarmest and coolest parts of the glass, its coefficient ofexpansion and the thickness of the section (Shand andArmistead 1958). However, the heat transfer neededfor accomplishing the desired temperature differencein practice is influenced by various parameters that arechallenging to accurately simulate. These include theelement’s shape andmass distribution, its sides exposedto cooling, the existence and amount of other thermalmasses in the furnace, as well as the geometry andcharacteristics of the furnace itself. There are severalguides on the annealing cycle of cast objects in thescientific and industrial literature, but they are oftentailored to very specific circumstances and includeunclear assumptions (Watson 1999).

Hence, even though the heat transfer needed for pre-venting considerable stresses in a cast glass elementcan be calculated, due to all the above reasons, in prac-tice, the annealing schedule of large 3-dimensional castunits is often based on practical experience. Based onthe experience and furnace facilities ofPoesia, the com-pany concluded that each Crystal Houses brick with65 mm × 105 mm × 210 mm dimensions requiresapproximately 8 h of annealing, whereas bricks ofdouble the volume (65 mm × 210 mm × 210 mm)

require 36–38 h respectively to prevent the genera-tion of attributable permanent residual stresses. The 65mm thickness of the components hinders an accuratethrough-the-thickness stress measurement by a Scat-tered Light Polariscope (SCALP) stress-meter (usingthe current hardware/software). Instead a qualitativeanalysis of strain concentration was performed usinga polarized white light source and a crossed polarizedfilm that blocks the transmission of light. If glass issubjected to stress, it exhibits optical anisotropy. Thiscorresponds to two refractive indices, which result inthe presence of isochromatic fringes (coloured pat-terns) when polarized light passes through the com-ponent (see Fig. 9, left) (McKenzie and Hand 2011).Glass without any stress will appear completely dark(Schott AG 2004). If the specimen presents besidesblack only grey-scale spectral composition,3 it has low

3 Shribak (2015) provides an extended analysis of the interfer-ence colours seen through polarization. For small retardancethe brightness of the region increases, first with a white spec-tral composition at 200 nm. As the retardance increases, coloursstart to appear beginning with yellow, then red, blue and green.The colour changes in this sequence three more times until theretardance reaches 2000 nm. Then the interference colours turnwhite again and the retardance can no longer be reliably deter-mined using the region’s spectral composition. A continuouspresence of only black and white subsequently signifies lowresidual stresses.

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Fig. 10 Glass bricks of 210mm × 210 mm × 65 mmcoming out of the annealingoven after circa 36–38 h.The natural shrinkage isvisible on the top surface

Fig. 11 Glass bricksrapidly cooled (prior toannealing) in verticalorientation. The naturalshrinkage is evident besidesthe top surface, also alongthe larger surfaces

residual stresses. When the colour spectrum appearsthe amount of stress is higher but cannot be quanti-fied. This method was used by Poesia to control allproduced bricks. Bricks such as the one on the left ofFig. 9, with a clear indication of internal stresses by acoloured spectrum, were discarded. Only bricks suchas the one on the right of Fig. 9, with dark and whiteareas were used in the façade. The fracture pattern oftested specimens also suggested low residual stresses—there was no excessive fragmentation observed in thecomponents.

During the initial rapid cooling, natural, inevitableshrinkage occurs to the glass volume during the mate-rial’s transition from liquid to solid state. The shrinkagecauses different dimensions between units, uneven sur-faces and is larger on the top, open surface of the castingcomponent owing to the additional gravity force (seeFigs. 6, 10).

Different casting orientations were tested for min-imizing the resulting shrinkage in the larger, bonding

surfaces of the bricks. Figure 11 demonstrates that evenwhen molten glass is rapidly cooled in a vertical ori-entation, there is still visible shrinkage on the longer,bonding faces of the components.

As, regardless of orientation of the mould, thebricks’ bonding surfaces required further processing,the horizontal position was favoured in terms of aes-thetics, where the non-bonding sides are not visiblydistorted. Thus, to achieve the desired ±0.25 mm pre-cision, the blocks are cast slightly higher. After theannealing process, a CNC machine mills the top layerof each block to remove the natural convex and obtainthe precise height. Finally, both top and bottom facesof each block, i.e. the bonding surfaces, are polished toa smooth flat surface of the desired dimensional accu-racy. The four vertical surfaces remain unprocessed asthey do not influence the structural system. Mechani-cal testing on bothCNCpolished and unpolished bricksshowed no deterioration of the mechanical propertiesof the former.

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Fig. 12 a The jig used in the first dimensional control. b The set-up of the second control for checking the height and flatness of thecomponents

The processed glass bricks are then subjected to twoseparate dimensional controls to verify their confor-mity. Both controls were performed first at Poesia andthen again at TU Delft for verification. The first controlis accomplished by a cut-out metal plate jig that con-trols the total length and width of the brick in 1.00 mmaccuracy (Fig. 12a). The second control employs a cus-tomized electromechanical measurement bench withfive Linear Variable Differential Transformer (LVDT)sensors of 1µm accuracy (Fig. 12b) attached to an alu-minium frame. By taking point measurements close tothe four edges and at the center of each unit, the sen-sors check if the bricks meet the required 0.25 mmtolerance in both height and flatness from the nominalheight of 65.00mm. It should be clarified that the rangeof acceptable height varies between 64.75 and 65.25mm but within each particular brick the height devi-ation cannot exceed 0.25 mm. Accordingly, throughthis measuring control, the acceptable bricks are sortedin two groups based on the point with the maximumheight: Group A comprises bricks between 64.75 and65.00 mm high and Group B comprises bricks between65.00 and 65.25 mm high respectively. Only bricks ofthe same group were used per row of construction tomaintain the 0.2–0.3 mm requirement for the adhesivethickness.

Besides the dimensional controls, a visual inspec-tion of the bricks is performed as well at the construc-tion site. Flaws at the bricks’ bonding surfaces, usuallyin the form of minute cracks or scratches even lessthan 1 mm deep, commonly caused during the han-dling and transportation, can trigger the propagationof visible cracks after the adhesive’s curing process.In specific, during curing the adhesive shrinks by 9%

Fig. 13 Left: Aminor crack on a brick’s bonding surface. Right:The propagation of such a minor crack after the curing of theadhesive

vol at ambient temperature (Delo Industrial Adhesives2014) because of polymerization triggered by UV-light(Delo Industrial Adhesives 2007), introducing a con-siderable amount of tension to the minute cracks thatcan start to propagate, eventually resulting in visiblecracking (see Fig. 13). Only the glass components thatpass both the measuring and visual controls were usedin the construction of the façade.

4 Construction of the glass brick wall

4.1 Construction site set-up

The upper conventional masonry façade of the topresidential floor, based on a steel beam spanning itslength, was completed six months prior to the con-struction of the glass elevation (see Fig. 14). The levelof complexity of the manual bonding process of theglass façade called for a highly skilled building crew

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Fig. 14 Left: The masonry wall was already constructed prior to the glass elevation. Centre: the installed aluminium place holders ofthe opening. Right: The mast climbing working platform and one of the three mobile elevated platforms

and a strictly controlled construction. A 12 h work-ing schedule was established, 5days per week. Sevento nine highly skilled workers bonded and sealed onaverage 80–100 bricks per day under the supervisionof two quality control engineers and the constructionsite supervisor.

The special characteristics of the adhesive requiredthe construction of the façade inside a UV-filtering tentfor protection against solar radiation, adverse weatherconditions and dust. To ensure a controlled level of tem-perature and humidity, heating equipment was installedinside the tent so that the bricks and the adhesive bemaintained within workable temperatures during win-ter. During the summer, when the ambient tempera-ture exceeded 30 ◦C the construction would temporar-ily stop. Due to limited space in the construction site,the glass blocks were stored in pallets in a separatewarehouse and were gradually transported to the siteupon demand.

A scaffolding with a mast climbing working plat-form was installed for the construction of the glassbrick wall (Fig. 14). Simultaneously, three mobile ele-vated working platforms were placed at the inner sideof the wall for the construction of the buttresses. Bricksfor one full row of construction were loaded and liftedeach time on themast climbingworking platform, fromwhere they were distributed for bonding. An elabo-rate network of horizontal aluminum guides was uti-lized to prevent any misalignment during the erec-tion of the wall. Customized vertical aluminium frames

were temporarily installed as place holders of the wallopenings.

4.2 Levelling the starting bonding surface

The erection of the glass masonry wall started on topof a 0.60 m high by 0.20 m wide reinforced concreteplinth, essential for the protection of the lower part ofthe façade against hard body impact; it has been cal-culated to resist a vehicle collision travelling with avelocity of 50 km/h. To match the texture and color ofthe glass wall, the vertical faces of the concrete baseare coated with a laminate of a stainless steel sheetand annealed patterned glass, laminated together bySentryGlas® foil. A 30 mm thick stainless steel platefixed by bolts on top of the plinth, forms the base forthe glass masonry wall (see Fig. 15).

The prerequisite for extreme accuracy of the devel-oped glass block system necessitates a reference build-ing surface of corresponding flatness. Accordingly, thestainless steel plate had to be levelled to an accuracyof 0.25 mm for the 12 m length of the façade. Suchhigh measuring accuracy called for the development ofan innovative measuring and levelling system. Specifi-cally, the bolts, set 275mm apart (see Fig. 15) allow forthe levelling of the stainless steel plate in consecutivesteps. By employing standard levelling equipment theplate is initially levelled to an accuracy of 3 mm overthe 12 m length. Figure 16 demonstrates the principle

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Fig. 15 On the left the bolts used for levelling the stainless steel plate, seen on the right

Fig. 16 Principle of the developed levelling system

of the measuring system developed to further level thestainless steel plate to the desired precision: A contin-uous open metal conduit with both ends sealed, sup-ported directly on the concrete surface, is filled with anon-transparent liquid.When still, a liquid will achievenearly absolute horizontal flatness, establishing the ref-erence level for calibrating the plate. A laser scannerwith a sensor of 1µmprecision, fixed on an aluminiumframe with three legs is then moved over a set of con-secutive points on the stainless steel plate, taking mea-surements in reference to the liquid’s surface, map-ping the plate along its entire length. The use of anopaque reflective liquid (e.g. full fat milk) is essentialfor ensuring that the laser beam will take all measure-ments exactly at the same reference level. After theentire surface of the plate is mapped, by tightening orreleasing the nuts and counternuts of the bolts the platewas successfully levelled with amaximum height devi-

ation of 0.24 mm per total 12 m length. The resultinggap between the concrete base and the plate was filledwith non-shrinkage concrete and left to cure.

4.3 Bonding

The 0.2–0.3 mm optimum thickness of the adhesivelayer demanded extreme precision in each construc-tion layer. In traditional terracotta brickwork themortarplays the dual role of bonding and accommodating tol-erances in the size of the bricks. However, the selectedadhesive’s inability to compensate for any dimensionaldiscrepancies in the construction can result to an accu-mulated offset of a few centimeters in the total heightof the façade, even when the allowable tolerance perglass component is only ±0.25 mm. To eliminate thedevelopment of fluctuations in the height of the con-

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Fig. 17 Bricks of a new row laid down prior to bonding (left). Then a feeler gauge is used for checking the thickness of the resultingseam (right). The blade of the feeler gauge is sufficiently flexible and round not to induce any surface damage on glass

Fig. 18 Common flaws occurring in the adhesive layer: air gaps, capillary action and dendroid patterns

struction, all the glass bricks of a new row are laid downprior to bonding. The thickness of the resulting hori-zontal joint between the laid bricks and the bonded onesbelow is then checked by a feeler gauge (see Fig. 17).When the seam is larger than the suggested 0.25 mm,the corresponding brick is replaced with another onethat accomplishes better contact in the specific loca-tion. The final selection of bricks is then numbered toguarantee their correct bonding sequence.

Previous structural and visual tests byOikonomopoulou et al. (2015b) suggested the bondingof the complete contact surface between blocks. Theuniform application of the adhesive besides ensuringa homogeneous load distribution, is also essential formaximizing transparency. Indeed, the façade’s visualresult is deeply affected by any form of air gaps andbubbles in the adhesive layer, as well as from stainscaused by the adhesive’s overflow or capillary action(see Fig. 18). To eliminate such defects a customizedbonding procedure was applied.

Initially, the bricks are visually inspected on site forany defects, as explained in chapter 3. Then, the sur-

faces to be bonded are cleaned with 2-propanol. Spe-cially designed self-reinforced polypropylene formsout of PURE® (DIT 2016) are placed for the distri-bution of the adhesive in an X pattern, controlling theflow, spread and amount of the adhesive (see Fig. 19).To prevent any capillary effect along the vertical facesof the glass bricks, a special, UV beam light is used tolocally harden the liquid adhesive in case it arises onthe vertical seams.

Once the adhesive is evenly spread, it is initiallyexposed to low intensityUV-light for 5 swhile the brickis kept in position and under pressure. This pre-curingstepwas introduced for practical reasons4 as this partialcuring stabilizes the glass brick while still allowing thewiping-off of any adhesive overflow. After cleaning,the adhesive is further cured by low and medium inten-sity UV-radiation in the range of 20–60 mW/cm2 andfor a period of 60–120 s, according to brick size. Once

4 This pre-curing time was set experimentally. Testing of speci-mens cured in this way did not reveal any differences with spec-imens cured once off.

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Fig. 19 Bonding steps from left to the right: 1. Application ofthe adhesive with the aid of the PURE®form. 2. Resulting× pat-tern. 3. Local hardening of the adhesive by a UV beam light for

preventing capillary action. 4. UV-lamp used to cure the adhesivefor 60–120 s

a complete brick layer is bonded, all joints are sealedin order to guarantee the dust, water- and moisture-tightness of the façade. For the sealing, Delo Photo-bond 4497 (Delo Industrial Adhesives 2016b), a moreflexible and viscous, clear UV-curing Delo-Photobondacrylate, specially designed for outdoor applications,is selected due to its good visual performance, com-patibility with Delo-Photobond 4468 as well as for itseasy and quick application (see Fig. 20). This adhe-sive requires only 10 s of UV-curing to be completelyhardened.

The first row of glass blocks was directly bondedonto the stainless steel base by Delo-Photobond 4468.As previously mentioned,Delo Photobond 4468 is rec-ommendedby themanufacturer for glass tometal bond-ing as well. Previous research on such a bond hasbeen conducted by Puller and Sobek (2008). The estab-lished rigid connection was considered imperative bythe structural engineers in order to eliminate any hor-izontal movements of the free-standing façade. Any

movements due to temperature strains in the structureare compensated by the flexible connections at the sidesand top of the façade (see Chap. 4.6).

Every 2 m of elevation, the levelling along the totallength of the façade is recorded using a high accuracytotal station. Bricks with a 0.5 or 1.0 mm reduction inheight were specially manufactured for the levelling ofthe wall in case of height deviations. Such bricks wererequired to level the wall segments when reaching thelevel of the architraves of the ground and first floor.At the top of the elevation, the glass wall is connectedto a steel beam by a 22 mm thick structural modifiedsilane (MS-) polymer bond. This flexible connectioncan accommodate displacements due to the differentthermal expansion and stiffness between the upper con-struction and the glass wall. A flexible waterproof tubefilled the gap towards the interior of the wall, to fur-ther prevent water leakage. As ceramic strips cover theentire top row, the connection details are fully hidden(Fig. 21).

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The construction of the Crystal Houses façade: challenges and innovations 101

Fig. 20 Left: Sealing of the already bonded bricks. Right: the final, visual result achieved by the novel bonding method

Fig. 21 The top connection is completely masked by the ceramic strips

4.4 Construction and installation of the architraves

The architraves above the window and door openingsof the original nineteenth century elevation are alsoreinterpreted into glass components by special taperedglass bricks bonded together by the same adhesivealong their vertical surfaces. Due to the medium vis-cosity of theDelo Photobond 4468 each architrave hadto be pre-assembled into one single component in acustom made rotating steel fixture. The rotating fixtureensures the horizontal application of the adhesive, aswell as the desired arch geometry, with a straight topline in accordance to the maximum 0.25 mm devia-tion rule. The finished components, fabricated in theTU Delft Glass & Transparency Lab, were then trans-ported on site and installed one by one in situ withthe aid of a jib fixture on the fork lift, as shown inFig. 22.

4.5 Transition layer between standard and glassmasonry

To obtain a smooth, gradual connection to the standardbrickwork of the final, residential floor of the building,the initial intention of the architects was to realize atransition zone of intermixing glass and normal terra-cotta bricks towards the top of the façade. Nonetheless,the structural blend of the two materials presented var-ious practical implications, as can be seen in Fig. 23.Besides having differentmechanical properties, the twotypes of bricks vary appreciably in acceptable toler-ances. While in the glass bricks the required precisionin height is ±0.25 mm, for the terracotta bricks is atleast±1.0 mm.Most importantly, the bonding betweenthe two brick types necessitates the application of dif-ferent adhesives, involving the risk of their intermixing.Lastly, the strongly alkaline character of most mortars

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Fig. 22 Left: The special rotating fixture for the preassembly of the architraves. Right: The installation of the bonded architraves onsite

Fig. 23 Practical implications encounteredwhen combining ter-racotta and glass blocks, such as differences in acceptable toler-ances and in use of adhesives

used for the bonding of standard ceramic bricks attacksthe glass surface andmust be avoided. It should bemen-tioned that, as the upper conventional masonry façadewas completed six months prior to the construction ofthe glass elevation and themortar was fully cured, therewas no hazard of alkaline reaction between the mortarand glass.

Due to all the aforementioned reasons the option ofcombining terracotta and glass was discarded. Insteadthe following solution was applied: Glass bricks, 40mm shorter in width, clad with an 18mm thick ceramicstrip at each external side, replace the traditional bricksin the intermixing zone. The ceramic strips are bondedon the façade after all glass blocks have been bondedin place, preventing the occurrence of adhesive stainson their exterior surface. Tec 7 (Novatech 2016), abrown coloured modified silane polymer is applied for

bonding the strips to the glass units. With an applica-tion thickness of circa 3 mm the adhesive compensatesfor any difference in thermal strains between the twomaterials. Once all the ceramic strips are bonded tothe façade, the seams around the strips and the glassare sealed by Zwaluw Joint Fix 310 ml lichtgrijs (DenBraven 2017), an acrylic basedmortar of similar textureand color to the mortar used for building the wall above(see Fig. 24). The selected mortar is less brittle thannormal mortar due to its acrylic content and featuresconsiderably less volume shrinkage (5%) (Den Braven2017) after hardening in comparison to standardmortartypes, preventing thus its delamination from the glassblocks. The completed intermixing, gradient zone canbe seen in Fig. 25.

4.6 Boundary connections of the façade

The façade forms a free standing wall firmly connectedto the concrete plinth. To allow for displacements due tothe different thermal expansion and stiffness betweenthe glasswall and its boundaries, the façade is joinedviaflexible connections to the top metal beam, supportingthe residential level above, and to the stainless steelcolumns on the vertical sides. The top connection of thetwo structures is realized by a modified silane (MS-)polymer adhesive bond as analyzed in Chap. 4.3.

Regarding the connection along the vertical sides,this varies between the right and left (as seen from thestreet) side of the wall at the ground floor, since the leftside is self-supported by a buttress. On the right side at

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The construction of the Crystal Houses façade: challenges and innovations 103

Fig. 24 Left: Bonding of the ceramic strips to the shorter bricks. Right: The final visual result after the ceramic strips have been sealed

Fig. 25 End result of the intermixing, gradient zone

the groundfloor, aswell as onboth sides at thefirst floor,the glass masonry wall is connected by a 10 mm thicklayer of a clear silyl-terminated semi-elastic polymer tothe stainless steel L-shaped columns to compensate forthermal displacements of the wall. Since for the curingof the specific MS-polymer adhesive the contact withatmospheric conditions is essential, the adhesive was

applied gradually with a glue-kit dispenser using com-pressed air row by row, so that each glue layer can setuntil the next row of bricks is completed (see Fig. 26).The bricks at the right side of the ground floor are eachclad with two 1 mm thick stainless steel strips at theiradjacent to the L-shaped column sides, to mask therough detailing of the welded stainless steel structural

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Fig. 26 From left to right: Bonding of the steel plate to the corner brick. Positioning of the brick by a suction cup holder. Applicationof the semi-elastic polymer

Fig. 27 The graphitemoulds used for thefabrication of the frames

components (see Fig. 26). The cladding is applied tothe bricks prior their bonding to the façade. For such aconnection, DELO Photobond 4497 is used, to ensureimpact resistance.

4.7 Installation and bonding of the cast glass windowand door frames

The reproduction of the previous, historic elevation’swooden openings in cast glass was an extra challengeadded to the engineering and constructionof theCrystalHouses as it included the manufacturing and bondingof massive cast glass elements. The glass frames werecast by Poesia in open graphite moulds (see Fig. 27),ground along their open surface to remove the mate-

rial shrinkage layer and polished with a rotational bandmanually. As such pieces present larger tolerance prob-lems, DELO Photobond 4494 (Delo Industrial Adhe-sives 2016a) was chosen to bond the frame elementstogether due to its higher viscosity and applicationthickness that allow for easier tolerances, while main-taining a clear optical result. This adhesive has a com-paratively lower mechanical resistance to Delo Pho-tobond 4468, yet sufficient for integrating the glassframes into the construction.

The window and door frames were placed after thecompletion of the glass wall. During the bonding pro-cess, aluminum place holders were used to secure tem-porarily the location of the openings. First each framewas assembled in place by DELO Photobond 4494.Based on the UV-measurements per m2 done by Siko

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The construction of the Crystal Houses façade: challenges and innovations 105

Fig. 28 Left: The simultaneous application of the polymer from both sides.Centre and right: The final result of the bonded glass frames

b.v. the sill of each window frame, of 1145 mm ×143 mm footprint, required 4 minutes of total curingby two UV-lamps travelling back and forth along itslength.

Once the frame was in place, the side and top con-nection to the glass masonry wall was established. Thethickness of this connection was designed to be 8mm,to compensate for horizontal and vertical deviations inthe glass masonry wall, and was achieved by the sameclear silyl-terminated semi-elastic polymer used alsoat the top and side connections of the wall. To avoidthe entrapment of air, the polymer was injected at bothsides simultaneously from bottom to top (see Fig. 28).After a few days, when the polymer had reached a satis-factory strength, the aluminium frames were removed.Then the cast glass mullions were bonded to the glassframes via the same polymer applied in a 2 mm thicklayer. Finally, the glass panes were bonded to the mul-lions by a standard transparent silicone, completing thefaçade. The end result can be seen in Fig. 28.

5 Conclusions

A novel, completely transparent self-supporting glassmasonry wall system has been developed and real-ized through pioneering research in the Crystal Housesfaçade (Fig. 29). With the exclusive use of solid castglass elements bonded together by a clear, high stiff-ness, adhesive andwith the aid of geometry for enhanc-ing the lateral stability, the 10m by 12m façade com-bines the desired structural performance with puretransparency. The façade is capable of carrying its ownweight and withstanding wind loads without any addi-tional substructure when the adhesive-glass assemblyfunctions as one rigid unit under loading. Previous

experimental work by Oikonomopoulou et al. (2015b)indicated Delo Photobond 4468, a one-component,UV-curing acrylate for attaining both the desiredmono-lithic structural performance and high transparencylevel. The experiments also demonstrated that thedesired structural and visual performance is only guar-anteed when the adhesive is applied in a uniform layerof a mere 0.2–0.3 mm thickness. This in turn leads toan allowable dimensional tolerance of a quarter of amillimetre in the height and flatness of the cast glasscomponents. This demand of extreme dimensional pre-cision introduced new challenges in the engineering ofthe façade from the manufacturing of the bricks to theirbonding method, calling for pioneering solutions.

Due to the inevitable natural shrinkage of moltenglass, suchdimensional accuracy could only be attainedby CNC-cutting and polishing of the bricks’ horizontalfaces to the desired height. Soda-lime glass and open,high precision moulds were preferred over borosilicateglass and press moulds to reduce the manufacturingcost. Special measuring equipment was developed forcontrolling the dimensional accuracy of the compo-nents.

Nonetheless, even blocks of such high dimen-sional accuracy can still lead to a significant offset inthe façade’s total height. The fundamental differencebetween a conventional brickwork and the developedglass masonry system is that a standard mortar layercompensates for deviations in the size of the bricks,while the selected adhesive cannot. This manifests thelevel of complexity deriving from the manual bondingand the significance of constantly controlling the entireconstruction with high precision methods.

A completely transparent façade is moreover linkedwith the inability to hide any possible flaws in the con-struction. The development of a novel bonding method

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Fig. 29 The completed Crystal Houses façade

for the homogeneous and flawless application of theadhesive resulted in imperceptible connections in theconstructed façade.

6 Discussion and further research

Overall, the innovative glass masonry system devel-oped for the Crystal Houses façade illustrates the greatpotential of adhesively bonded cast glass blocks as ananswer to the quest of structural transparency and canform the basis for novel architecture applications. Thesystem can be further engineered in order to simplifyand accelerate its application, minimize the interlinkedchallenges and decrease the cost.

Most of the engineering puzzles elaborated in thispaper can be solvedwith the use of a thicker transparentadhesive of equal structural performance that in turncan allow for larger tolerances.

In this direction, different envelope geometries canenhance the rigidity of the structure, allowing forthicker, more elastic adhesives and correspondingly forlarger tolerances in the brick units.

Moreover, the development of a casting method ofglass units of higher accuracy without the need of post-processing would significantly facilitate the entire pro-duction and building process as well as enhance thestructural and architectural result.

Likewise, the choice of glass recipe plays a crucialrole in the total annealing time and in the scale of result-ing natural shrinkage. Although a faster andmore accu-rate casting process can be achieved with borosilicateglass instead of soda-lime, the total manufacturing costand dimensional precision prerequisites should be con-sidered prior to the glass recipe choice.

Lastly, the casting of glass units can provide thedesigner with a great freedom in the shapes and sizes

Fig. 30 Interlocking,dry-assembly glass bricksdeveloped byOikonomopoulou,Bristogianni and Barou forthe 3TU.bouw Lighthouseproject: Restorative Glass

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The construction of the Crystal Houses façade: challenges and innovations 107

of the masonry module. A promising solution for alltransparent, load-bearing glass structures is the devel-opment of interlocking cast units. Currently, researchis being conducted by the authors on the developmentof such a system that circumvents the use of adhesivesby employing dry connections instead (see Fig. 30). Inthis case, the overall stability is attained by the totalweight of the construction in combination with theunit’s interlocking geometry that provides the neces-sary constraints against lateral movement. To preventstress concentrations due to the high contact pressurebetween the glass elements, a transparent foil is placedas an intermediate layer between the units, allowing aswell for dimensional tolerances in the cast components’size.

Acknowledgements The research work was conducted bythe TU Delft Glass & Transparency Lab for Ashendene-Leeuwenstein BV whose permission to publish the results isgratefully acknowledged. MVRDV and Gietermans & Van Dijkare responsible for the architectural design, ABT b.v. for thestructural simulations, Wessels Zeist b.v. for the construction ofthe Crystal House façade and Poesia Ltd. for the manufacturingof the glass bricks. We thank Ashendene-Leeuwenstein BV andMVRDV for the 3-D impressions of the case study.We especiallywant to thank Rob Janssen from Siko BV for his valuable adviceand assistance. Ruud Hendrikx at the Department of MaterialsScience and Engineering of the Delft University of Technologyis acknowledged for the X-Ray analysis.The authors gratefullyacknowledge Kees Baardolf and Kees Van Beek for their invalu-able technical assistance and insight throughout the project.

Compliance with ethical standards

Conflict of interest On behalf of all authors, the correspondingauthor states that there is no conflict of interest.

Open Access This article is distributed under the terms ofthe Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permitsunrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) andthe source, provide a link to the Creative Commons license, andindicate if changes were made.

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