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Mechanical characterisation of a low-thickness ceramic tile cladding subject to ageing phenomena
Journal Pre-proof Mechanical characterisation of a low-thickness ceramic tile cladding subject to ageing phenomena Frida Bazzocchi, Stefano Bertagni, Cecilia Ciacci, Emiliano Colonna, Vincenzo Di Naso PII:
S2352-7102(19)31352-X
DOI:
https://doi.org/10.1016/j.jobe.2019.101105
Reference:
JOBE 101105
To appear in:
Journal of Building Engineering
Received Date: 26 July 2019 Revised Date:
4 November 2019
Accepted Date: 2 December 2019
Please cite this article as: F. Bazzocchi, S. Bertagni, C. Ciacci, E. Colonna, V. Di Naso, Mechanical characterisation of a low-thickness ceramic tile cladding subject to ageing phenomena, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2019.101105. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Mechanical characterisation of a low-thickness ceramic tile cladding
2
subject to ageing phenomena
3
Corresponding Author:
4
Frida Bazzocchia
5
Co-authors:
6
Stefano Bertagnib, Cecilia Ciaccia, Emiliano Colonnab, Vincenzo Di Nasoa
7
a
8
[email protected]
9
b
DICEA, University of Florence, Italy, email: [email protected], [email protected],
School
of
Engineering,
University
of
Florence,
Italy,
email:
[email protected],
10
[email protected]
11
Abstract
12
In recent years, an innovative system for a rainscreen façade system with low thickness and large tiles of
13
porcelain stoneware has been patented. This paper presents the results of a large experimental campaign
14
investigating mechanical performance of a ceramic tile in 3different configurations: tile without net, with
15
fiberglass net glued on the back, and the entire cladding system (slab/net/frame). The main objective is to
16
determine material mechanical characteristics (e.g. modulus of rupture and Young’s modulus)
17
considering a protocol of artificial ageing (fatigue test, freeze-thaw, heat-rain, H2O Saturation, heat
18
treatment, accelerated corrosion SO2 and NaCl, irradiation UVB and condensation) in order to understand
19
the possible decay during the service life and consequently to design the technological solution of a
20
rainscreen façade system. The results showed that on the modulus of rupture the most influences ageing
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effects are fatigue test and freeze-thaw while the influence on young’s modulus is negligible.
22
Furthermore, it is necessary to consider second-order effects for the evaluation of the resistance and,
23
finally, that the fiberglass net does not lead to a significant increase in resistance.
24
Keywords: porcelain stoneware, low-thickness tiles, external cladding, mechanical behaviour
25
Highlights
26
•
Test protocol for mechanical features of low-thickness ceramic tile are defined
27
•
Studies and experimental tests with artificial ageing were performed
1
28
•
Fatigue and freeze-thaw reduce the modulus of rupture of about 18% and 15%
29
•
The benefit of the reinforcement net on the back is related to ductility and safety
30
•
Accelerated ageing combinations decrease the strength of the sealant of about 15%
31
1. Introduction and literature background
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Porcelain stoneware rainscreen façade systems are widely used in current style of architecture for many
33
reasons especially because they offer extensive compositional variety to the architectural designer [1] [2]
34
[3]. Ceramic tile could be easily used in opaque facing with exterior insulation (advanced facades,
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ventilated facades, Vêture and ETICS -External Thermal Insulation Composite Systems- with ceramic
36
covering) to significantly improve the thermophysical performance of the envelope [4][5]. Moreover, this
37
has undoubted benefits from a life cycle cost point of view, as evidenced in the technical literature [6] [7]
38
[8]. Recently, large porcelain stoneware tiles (sizes up to 3240x1620 mm) and low thickness (minimum 3
39
mm) have been released on the market, thanks to new production technologies [9] [10] [11] [12] [13], by
40
the patented production lines of System S.p.A. in Fiorano Modenese (Italy) [14]. On the market for
41
cladding systems, there are both "simple" slabs and ceramic tiles reinforced with fibreglass net on the
42
back connected to the slab with epoxy adhesives [9] [14]. The material shows excellent eco-compatibility
43
if applied as a cladding compared to other cladding materials such as stone, glass, and aluminium. Some
44
studies confirm that the 9-10 mm traditional cladding thickness presents an excellent performance in the
45
life cycle assessment [15] [16]. Thickness and weight reduction results in increased efficiency in raw
46
material consumption, firing energy [17], packaging and energy for transporting material from gate to site
47
[16]. Some research point out that the microstructural characteristics of porcelain stoneware with low
48
thicknesses by varying the production conditions (e.g., firing temperatures) and physical characteristics
49
(density, porosity) and deriving its mechanical characteristics, mostly in terms of the modulus of rupture
50
[17] [18] [19] [20] [21] [22]. Nowadays, the current laws concern with traditional porcelain stoneware
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could be still used, in particular the UNI EN ISO 10545 [23] and UNI 11018:2003 [24].These laws and
52
references do not exhaustively address the theme of mechanical decay over time. However, there is a lack
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of experimental data, useful for the designer, to model the structural behaviour of the slabs when applied
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to the substrate structures (generally bonded with sealant products). There are also no experiments that
55
can clarify the structural role of fibreglass net that may be present on the back side of the slab and any
2
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performance modifications that the system can sustain over time. Since traditional tiles are often anchored
57
only with mechanical systems [3], in this case the ageing effects on the advanced-screen facade system
58
are not particularly felt.. All those facts have raised questions about the behaviour of the systems in terms
59
of ageing and performance decay when the envelope is naturally subjected to severe mechanical and
60
chemical stress (e.g., thermal, radiative, chemical, wind actions).
61
This paper shows the results of a large experimental campaign (carried out a few years ago) that, first,
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performed mechanical characterization of large format slabs with low thickness, both in the version with
63
net glued to the back and in the non-net version, and then the entire cladding system consisting of the
64
slab/net/frame assembly. To verify the possible time performance decay both for slabs and for the entire
65
system, a severe protocol of artificial ageing has been conceived and implemented to reproduce the real in
66
situ conditions as realistically as possible. The purpose is providing the mechanical performance of the
67
material that the designer of the cladding system can use during the design phase in relation to the service
68
conditions.
69
2. Material and Methods
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The tested slabs in porcelain stoneware come from a production batch of "fumo" colour lamina from the
71
series "collection", a kind of greyish tile without decorations on it. The slab with a reinforcement net
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takes the commercial name of "Lam's – Collection Fumo" (further called slab “S”), while the one without
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a net is called “Lam'Slab – Collection Fumo” (further called slab “N”). The manufacturer used to supply
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the fibreglass net from the company Teximpianti (TRS300, 300 g/m2). The adhesives for the fibreglass
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net (two-component adhesive based on polyurethane) are from the company Henkel S.p.A. (Macroplast
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UK 8119 B3 - Mass Glue; Macroplast Hardener UK5401 - catalyst). The sealant adhesive for fixing the
77
slab to the metal structure of anchorage and support (a cold-formed galvanized sheet according UNI EN
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10346:2015) is from Dow Corning (with trade name 993 Structural Glazing Sealant). To characterize the
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slabs and the fixing system it was considered necessary to identify a test protocol that would establish the
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behaviour at both the time of installation and during the period corresponding to the useful life cycle of a
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façade system (20 years). Since the material was different from the one required by the regulations, due to
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its small thickness, it was necessary to identify within the regulatory system those norms that best adhere
83
to the characteristics of the material.
84
The experimental campaign was organized as below:
3
85
•
Type 0 tests related to ageing processes. A varied number of ceramic tiles and assembled slabs fixed
86
to the metal support samples (named “minikit”) (Figure 1) were previously subjected to various
87
accelerated ageing tests, and then their mechanical behaviour was evaluated (named Type 1 and 2
88
tests).
89
In detail the “minikit” is assembled as follow:
90
-
ceramic tile LAM’S – Colour “Collection Fumo”;
91
-
fiberglass net: TRS 300 manufactured by the company TEXIMPIANTI;
92
-
adhesive for fiberglass net: MACROPLAST UK 5400 (catalyst); MACROPLAST UK 8119
93
(mass glue); MACROPLAST Hartvertmittler (primer) all manufactured by the company Henkel
94
S.p.A.;
95
-
cold galvanized steel plate low carbon content (UNI EN 10346-15);
96
-
sealant adhesive: 993 Structural glazing sealant manufactured by the company Dow Corning
97
Corporation (at present Dow). Type 1 tests. Tests on slab samples aiming at defining basic
98
mechanical properties (modulus of rupture, elastic modulus).
99
•
100 101 102
Type 2 tests. Tests on assembled samples (minikit) to evaluate bonding adherence (tangential and normal tensile stresses on the slabs);
•
Type 3 tests. Tests on real size prototypes of slabs assembled by bonding them on metal profiles subjected to perpendicular loads to the slab plane (Figure 5).
103
Slabs of 3 mm thickness of porcelain stoneware with glass fibre net, glued with polyurethane adhesive,
104
and slabs without this reinforcement were submitted to the tests. This procedure allowed us to verify the
105
benefits of the net reinforcement and evaluate any eventual performance decline because of accelerated
106
ageing processes.
4
107 108
Figure 1: “Minikit” scheme, and picture, of the sample used for the tests.
109
2.1 Type 0 tests (ageing)
110
To better investigate the behaviour of the ceramic slab with reinforce net (in addition to the silicon
111
adhesive), nine types of accelerated ageing treatments have been simulated:
112
•
C1- fatigue test: The intention was to evaluate stress effects on large cladding slabs, likely to be
113
applied to tall buildings, where wind action can produce "buffeting" effects due to vortex
114
separation at the downwind slabs. A study using the standard CNR-DT 207/2008 [25] (it is not
115
shown here) was preliminarily carried out over the number of cycles that simulated the conditions
116
in use (20 years of nominal life equal to approximately 105 cycles). In the laboratory, pneumatic
117
equipment specifically created for this purpose was used. In case of slabs exposed to fatigue by
118
flexion, a load with tension equal to 40% of the load required for rupture has been produced on the
119
middle. For the minikits, two different tests with 105 cycles were performed: pull-out tests and
120
traction tests with force variation from -200 to +200 N (equivalent to approximately 10% of the
121
rupture strength).
122
•
C2 – freeze-thaw: The conditioning procedure has been applied in accordance with regulation UNI
123
EN 12467:2016 [26] (category A, suitable for slabs laid on roof and subjected to severe actions)
124
with 100 cycles [27] over a temperature range from -20-23 °C.
125 126
•
C3-heat-rain: The conditioning procedure contained in UNI EN 12467:2016 [26] (category A) has been followed, as it allows the evaluation of the irradiation. The samples were subjected to
5
127
irradiation of approximately 600 W/m2 for 170 minutes alternating with water mist (1 l/min x m2)
128
for an equal period, interrupted by 10-minute pauses. The 360 minute cycle was repeated 50 times.
129
•
130 131
C4-H2O Saturation: The conditioning method does not refer to any regulation and was achieved by immersion of the samples in water at 50°C for a period of 7 days.
•
C5-Heat treatment: For this conditioning, the instructions contained in EN 1903:2015 [28] and
132
UNI EN 1296:2002 [29] have been followed. In this case, the tests were performed with a more
133
severe temperature, 150 °C, and a shorter time, 7 days, in a climatic chamber.
134
•
C6-accelerated corrosion SO2: For this conditioning, the instructions given in UNI EN ISO
135
6988:1998 [30] (which generally is applied to metal claddings) have been followed to test the
136
ageing behaviour of metal parts. The conditioning consisted of 21 cycles of 24 hours of exposure
137
in a static wet room at 45 °C with 200 ml of SO2 (exchanged at each cycle).
138
•
C7-accelerated corrosion NaCl: For this conditioning, the instructions in UNI EN ISO 9227:2012
139
[31]have been complied with in order to verify, in particular, the ageing of metal parts and
140
interfaces between the metal and sealant. The samples were inserted for 500 hours (approximately
141
21 days) in rooms at 35 °C with solution of 5% NaCl in sprayed H2O.
142
•
C8- irradiation UVB and condensation: Conditioning was inspired by the methodology of UNI EN
143
ISO 16474-3:2014 [32], in particular, the C-4 (UVB-313) method. It references an exposure of 500
144
hours at UVB (0.35 W/m2 at 340 nm), U.R. of 65% and black panel temperature of 60 °C, with
145
alternate condensation spraying. The conditioning intends to cause accelerated ageing of the
146
organic parts (glues) of the samples.
147
In addition to the single ageing conditions, a total conditioning test, defined as C1-8 (in the C1-C2-C4-
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C5-C6-C7-C8-C3 sequence), was performed to evaluate the overall effect on accelerated ageing samples.
149
2.2 Type 1 tests (samples)
150
To determine the modulus of rupture, (MoR), tests were carried out in accordance with UNI EN ISO
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10545-4:2014 [23], with the following numbers: 20 samples (250x100 mm) for S slabs [24], 20 samples
152
for the N slabs (250x100 mm), 7 samples [23] for each of the 9 anticipated ageing treatments (C1-C2-C3-
153
C4-C5-C6-C7-C8). The instrument (Figure 2 – Figure3) used for the tests was defined in line with UNI
154
EN ISO 1055-4:2014: each sample was placed on 2 cylindrical support rollers after placing a rubber
6
155
layer, then an increasing concentrated load was applied on the upper side in a central position until the
156
failure of the samples (Figure 4).
157
The main conditions for the performed test are the following ones:
158
•
distance between the 2 cylindrical support rollers (L):
230 mm
159
•
roller diameter (d):
20 mm
160
•
rubber thickness (t):
4.5 mm
161
•
distance between the support roller and the end of the sample (i):
10 mm
162
•
concentrated load (F):
1± 0,2 N/mm2·s
163 164
Figure 2 Section of the instrument used for the definition of MoR
165 166
Figure 3: Total view of the instrument used for the definition of MoR
7
167 168
Figure 4: Modulus of rupture determination test.
169
The MoR was tested on the N slabs by applying the load only on one side (considering symmetrical plate
170
behaviour), while on the S slabs it was evaluated by applying the load on both faces (on the outer face
171
named “F” and on the rear face named “R” with reinforcement net) in order to check if the S slabs have
172
different behaviour depending on the load direction applied (simulating, for example, the pressure and
173
depression actions on the slab due to the wind). In total, 249 samples were tested.
174
To identify characteristic values useful for the design, referring to p. 4.6.2.2.d of UNI 11018:2003 [24],
175
the minimum expected value (MEV) was obtained. The regulation [24] suggests that for MEV a
176
confidence level of 75% for a breaking value is acceptable, corresponding to the 5th lowest percentile.
177
Unfortunately, the regulation does not provide information on the probability distribution to be
178
considered, so we deferred to norm UNI EN 13161:2008 [33], which suggests a normal logarithmic
179
distribution. This statistical treatment has also been extended for other test results, unless otherwise
180
indicated.
181
For the MEV calculation, the values of ks (quantile factor) were used (Table 1), which were subjected to
182
the number of samples n tested: n ks
3 3.15
4 2.68
5 2.46
6 2.34
7 2.25
8 2.19
9 2.14
10 2.10
15 1.99
20 1.93
30 1.87
40 1.83
50 1.81
183
Table 1: Quantile factor (ks) according the number of samples (n).
184
To determine the value of the Young's modulus, the tests were conducted in accordance with UNI EN
185
843-2:2007 [34], (which is certainly the most relevant for the case), with the following numbers:
186 •
7 Samples (250x100 mm) of S slabs;
187 •
7 samples (250x100 mm) of N slabs;
8
188 •
14 samples (250x100 mm) only for the single cumulative ageing treatment (C1-8) that includes all
189
various treatments on the two types of slabs: S (7 samples) and N (7 samples).
190
The test was performed by applying a linear variable load starting at 5 N, reaching 35 N and returning to
191
5 N. During the test, the sample deformation value was measured with the help of a strain gauge at both
192
the loading and unloading stages. The Young's modulus was tested on the N slabs by applying the load
193
only on one side, while on the slabs S, it was evaluated only with the application of the load on the R face
194
(without net) due to measurement problems with the strain gauge in presence of the net reinforcement. In
195
total 28 samples were tested.
196
2.3 Type 2 tests (minikits)
197
Type 2 tests were done on composite samples that had to simulate the assembly of the slabs (both S type
198
and N type) with a galvanized cold formed steel profile through bonding with a structural glazing sealant
199
(Figure 1). For these kinds of samples, the silicon bonding joint had predetermined dimensions. The
200
bonding dimensions were defined either by the contact surface or by the thickness. Additional
201
reinforcement slabs were intended to facilitate the test with the MTS machine to avoid the occurrence of
202
parasitic flexions.
203
With such sample sizes, we intended to measure the strength of the structural glazing sealant joint with
204
respect to two prevailing actions applied: pull-out (with application of load of constant speed) and shear.
205
There are no regulations for these kinds of tests, so they have been executed in agreement with the
206
laboratory. Tests were done only with the S type slabs with bonding on the R side (thus with sealant
207
bonding on the reinforcement net), as we supposed that the best solution for the applications on facades
208
was the one with the reinforced slab, at least for security problems in case of partial detachment. Tests
209
were carried out on 10 non-conditioning samples, as well as 10 samples previously submitted to C1-8
210
treatment, for a total of 10 tested samples for pull-out tests and 10 for shear.
211
2.4 Type 3 tests (real-size prototypes)
212
To understand the mechanical behaviour of the entire system (slab/silicone/metal frame), using N type
213
and S type slabs, tests were performed on real-size prototypes with the purpose of understanding the
214
behaviour in system use. All type 3 tests were performed on non-conditioned materials. The
215
perpendicular load on the slab was simulated by applying fine sandbags or bituminous sheath sheets with
216
steps of uniform load. The slabs were collapsed after loading and unloading cycles (to also evaluate the
9
217
plasticity limit of the system). The deformations were measured with centesimal digital comparators
218
(fleximeters), with continuous monitoring of ambient temperature and humidity.
219
The tests types performed were the following:
220
•
Type 3.1: Tests on N and S slabs of 1200x1000 mm dimensions with a simple roller support on two
221
out of four sides. The constraint was made with 2 rollers placed 500 mm away from the sample axis.
222
In total, 2 samples with load on the front and 2 samples with load on the back were tested (Figure 5).
223
•
Type 3.2: Tests on both N and S slabs of 800x800 mm and 1000x1000 mm sizes, with a fixed
224
support on all 4 sides made by mechanical fixing (Figure 5). In total, 4 samples were tested for each
225
of the two sizes, applying the load both on the front and on the back side of the plate.
226
•
Type 3.3: Tests on S type cladding panels with dimensions of 3000x1000 mm fixed to frames
227
composed by profiles in cold formed galvanized steel (Figure 5), where longitudinal profiles have a
228
C-section, and transverse profiles have an omega section. The frame was then fixed on an
229
nondeformable support structure. The loads were applied both on the front (4 samples) and on the
230
back of the plate (4 samples).
10
231 232
Figure 5: From left to right: 5.1, 5.2, 5.3 schemes type test and fleximeter position. Measurements are expressed in mm.
233
3 Results
234
3.1 Type 0 tests
235
The ageing conditioning results have shown a visible and consistent non-alteration of the samples (except
236
for some limited corrosion effects of metal elements after conditionings C6 and C7, as well as C1-8). We
237
found a special issue regarding conditioning C1. The tension level limits during fatigue cycles had been
238
been calibrated before laboratory tests. With a tension level of 70% of the rupture average, samples broke
239
after 5000 cycles. Instead, with tension levels of 60% and 50%, samples were broken after 10000 and
240
20000 cycles. Only with a maximum value of 40% of the rupture average, did samples exceed the
241
predetermined reference value of 100000 (105) cycles.
242
3.2 Type 1 tests
243
The outcome of modulus of rupture tests results on non-conditioning samples, in particular, on N type
244
slabs, are in accordance with other studies [18]. The norm ISO 13006:2012 [35] provides an indicative
245
value for more traditional slabs applications, equal to 35 MPa, largely exceeded both in the mean and in
11
246
the minimum expected value (MEV) calculated per item as 4.6.1.2.d. from UNI 11018:2003 [24]. The
247
behaviour of slabs equipped with reinforcement net (S type) changed according to the direction of
248
application of the load. On S type samples loaded on the front side (indicated in the table 2 with the “SF”
249
symbol), MEV values were reduced by 12% compared to N type slabs. In the case of S type samples
250
loaded at the back side (in the table 2 indicated by "SR"), the MEV reduction was 15% lower than N type
251
slabs. Sample Type
SR
SF
N
Ageing
Mean
Std Deviation
None C1 C2 C3 C4 C5 C6 C7 C8 C1-8 None C1 C2 C3 C4 C5 C6 C7 C8 C1-8 None C1 C2 C3 C4 C5 C6 C7 C8 C1-8
[MPa] 42.94 42.26 42.95 45.86 43.46 41.96 51.51 47.76 46.12 33.43 49.91 48.25 50.32 53.43 47.62 50.06 55.80 53.85 56.46 44.57 51.67 50.02 47.20 55.00 51.01 52.09 56.72 57.00 57.69 44.71
1.73 1.58 4.02 3.13 1.28 1.71 5.46 2.33 3.03 11.24 4.99 3.98 3.16 4.50 1.84 2.74 6.04 2.63 6.29 8.16 2.71 5.87 3.74 4.20 2.79 3.83 4.41 4.39 4.04 3.99
Minimum Expected Value [MPa] 39.70 38.85 34.51 39.09 40.69 38.26 40.78 42.73 39.76 11.78 40.98 40.00 43.29 43.68 43.67 44.26 43.12 48.24 43.64 28.45 46.59 38.17 39.51 46.12 45.16 43.98 47.54 47.54 49.07 36.58
MEV variation -2.1% -13.1% -1.5% 2.5% -3.6% 2.7% 7.6% 0.2% -70.3% -2.4% 5.6% 6.6% 6.6% 8.0% 5.2% 17.7% 6.5% -30.6% -18.1% -15.2% -1.0% -3.1% -5.6% 2.0% 2.0% 5.3% -21.5%
252
Table 2: Experimental results for modulus of rupture. Minimum expected value and its variation for different ageing conditionings.
253
The conditionings may produce alternate sign variations with respect to non-conditioned samples. The
254
most significant reductions were produced by conditioning C1 (which seems to primarily afflict N type
255
samples) and C2 (which primarily afflicts N type and SF samples). Cumulative C1-8 produces sensitive
256
reductions of MEV in any case: from -21% in N type to -70% in SR samples. The verification of the
12
257
effects of performance reduction as a consequence of the cumulative classifications (C1-8 in our case)
258
was not provided either in the literature or in legislation [36] [37].
259
From the tests on non-conditioned samples, the role of the reinforcement net is clear, which does not
260
appear to significantly change the value of the first collapse force (stress applied resulting in the
261
appearance of the first crack, as shown in the non-linear stretch of the load-deformation diagram).
262
Instead, the reinforcement net (SF samples) greatly increases the ultimate collapse deformation, as
263
evidenced by the force-deformation diagram, where the last deformation is approximately 4 to 5 times
264
that of the first rupture (Figure 6).
265 266
Figure 6: Load-strain diagrams for bending tests. The SF specimen is on the right.
267
The flexural strength obtained by these tests was supplied indirectly by applying the usual formulas from
268
solids mechanics. Referring to the measured thickness values of the slabs subjected to flexural failure, the
269
following average values of the resistance module referring to a strip of 1 mm width were obtained:
270
•
WN = 1.581 mm3
271
•
WSF = 1.882 mm3 (load on ceramic side)
272
•
WSR = 1.804 mm3 (load on reinforcement side)
273
The ratio between Lam'Slab (SF and SR) and Lam's (N) slab resistance modulus changes from 1,141 to
274
1,19. Instead, if we refer to the expected minimum values of previously estimated rupture tension, the
275
ratio between slabs Lam'Slab and Lam's changes from 1.136 to 1.17. This shows the variability range of
13
276
resistance modules. Consequently, we can affirm that the contribution of the fibreglass net to the
277
resistance by flexion of the material is practically non-existent.
278
To define a characteristic flexural resistance to be used for design purposes, it is therefore necessary to
279
correlate the resistance to thickness:
280
•
281 282 283
Flexural resistance characterised by flexion compared to the average nominal thickness of 3.28 mm measured through laboratory tests: σk = 39.70 MPa.
•
Flexural resistance characterized by flexion compared to the nominal thickness stated by the manufacturer equal to 3.50 mm: σk = 34.87 MPa.
284
These values are valid for samples not submitted to ageing.
285
Furthermore, the Young’s modulus tested on N slabs provided a value in accordance with those found in
286
the literature [18] [22]; after ageing C1-8 showed a reduction of approximately 7%. In the case of slabs
287
with net reinforcement SR, ageing produced an increase of 5% in the elasticity modulus. Variations
288
caused by ageing in this case are not particularly significant (Table 3). It is important to highlight that the
289
S slab module is 17% less than that of N, but for this issue, we should note the following interpretation is
290
based on the selection of type 3 tests. Sample Type
Ageing
Mean
Std Deviation
Mean variation
1894 3971 3661 2188
-6.8% 5.2%
[Mpa]
N SR
None C1-8 None C1-8
60330 56218 49977 52576
291
Table 3: Experimental results for Young’s modulus and its variation.
292
Laboratory tests carried out to determine Young's modulus revealed a difference of 17% in the medium
293
values measured between slabs N and S. Vice versa, when referring to the result between the measured
294
Young’s modulus and the moment of inertia of the section (also a characteristic of each slab due to its
295
thickness), the tests showed a convergence of values between the two types of slabs, N and S, the
296
difference being only approximately 2.5%.
297
3.3 Type 2 tests
298
Type 2 tests were performed on assembled samples (minikit). The tests were mainly aimed at verifying
299
whether the behaviour of the sealant corresponded to that present in the literature [37], taking the values
300
indicated by the manufacturer of the structural glazing sealant as reference [38]. Tests were carried out
14
301
both by shear (further called TAG) and pull-out (further called POUT) on sample families (5 for each
302
series) that were either non-aged (C0) or subjected to the complete set of ageing (C1-8). For the statistical
303
processing of the data obtained with the tests, the procedures contained in ETAG 002 [37] have been
304
followed. The measured force values were then referred to as tensions, (normal or tangential) and divided
305
by the bond area, which was 2000 mm2 (Figure 1). Type of test
Ageing
Mean
Mean Std Ru,5 Ru,5 Ru,5 variation Deviation (Force) (Stress) variation
[N]
TAG POUT
None C1-8 None C1-8
2108 764 2415 979
95 225 158 360
-64% -59%
[N]
[MPa]
1874.1 210.1 2026.0 94.6
0.937 0.105 1.013 0.047
-88.8% -95.3%
306
Table 4: Experimental results for shear (TAG) and pull-out (POUT) test on minikits.
307
The results of tests on non-aged samples (Table 4) led to values compatible with those reported by the
308
manufacturer (Dow Corning [38] and references the following characteristic values of Ru, 5: pull-out equal
309
to 0.84 MPa and shear = 0.066 MPa, values on which the manufacturer himself recommends using a
310
safety factor of 6). For the C1-8 conditioning sample test results, they are inferior to the value provided
311
by the manufacturer for pull-out. In addition, in the case of pull-out tests, ETAG 002 [37] also admits a
312
performance decrease of up to 25% after ageing (Table 4). However, the same [37] results apply to
313
individual and non-cumulative ageing. In any case, the performance result in terms of ageing emerging
314
from the tests should be carefully evaluated. If we want to apply the results obtained on a hypothetical
315
panel of 3000x1000 mm dimensions, in which the slab (of weight = 70 N/m2) is anchored on steel
316
profiles with 2 sealant strips of length 3000 mm and width 20 mm and subjected to a wind pressure of
317
3000 N/m2 (significant action also on 40 m tall buildings facades), in this case a stress is applied on the
318
sealant strips, as follows: Pull-out = 0.073 MPa; Shear = 0.0017 MPa. The data on the pull-out in
319
operation exceeds the data measured on a characteristic rupture after ageing. This suggests, therefore, that
320
the researcher should deeply evaluate the ageing behaviour of the silicone sealant by expanding the
321
number of samples and testing them with individual ageing, in full compliance with ETAG 002 [37].
322
Indeed, the conditions in age group C1-8 are much more severe than the one proposed by regulation.
323
3.4 Type 3 tests
324
Type 3.1 tests
15
325
Tests highlighted the behaviour of elastic-fragile material in the case of load applied on the back of the
326
slab (net side) and elastic-plastic type with a load applied on the front (ceramic side). In both cases, the
327
deformation trend, until the first break occurred, was perfectly elastic (Figure 7). The deformation of the
328
slabs, in all cases, was in accordance with what theoretically obtainable by a static slab scheme supported
329
on both sides. The ratio between the deformations measured at the quarters and in the middle was
330
approximately 0.717, while the theoretical value was 0.7125. First, loading / unloading tests were carried
331
out without causing a rupture to the slab (with a pression of approximately 420 Pa): a perfectly elastic-
332
linear behaviour was then detected, with a residual lowering after the loading / unloading cycles of 1%.
333 334
Figure 7: Load test n.3.1: load lowering for fleximeter n.1. On the right, the breaking line near the specimen axis is obvious.
335
For simple mono-axial flexion, the reinforcement net contribution is essentially geometric rather than of a
336
mechanical nature (the fibreglass elastic modulus can be estimated at approximately 70-80 GPa). In
337
essence, the net brings the slab to behave as if it had a thickness of approximately 3.5 mm but without
338
appreciably improving its mechanical performance. The results of the tests carried on both slabs N and S
339
show a plate breaking at an average value of approximately 700 Pa. After the first loading and unloading
340
cycles, N and S (SF) slabs were submitted to rupture tests. In the case of the N slabs, the rupture was
341
immediate and resulted in the collapse of the sample. In the case of S slabs, even after the ceramic part
342
broke, collapse did not occur because the reinforcement supported cracked parts. At the breaking point,
343
the tests have all been interrupted. The breaks occurred in the middle of the samples. The N slabs have
344
broken at a load level of approximately 660 Pa. Instead, the slabs S (loaded on the SF front) broke at an
345
average load of 700 Pa. Considering the different thicknesses of the two types (average N slab thickness =
16
346
3.12 mm, average SF slab thickness = 3.34 mm), very similar medium rupture tension are obtained for
347
both cases in accordance with those of type 1 tests:
348
Failure tension for Type N = 50.8 MPa; Failure tension for Type SF = 47.1 MPa.
349
The slab deformation at the breaking point was approximately 48 mm in both cases (N and SF),
350
equivalent to 1/20 of the span.
351
Type 3.2 tests
352
Type 3.2 tests, whose aim was to understand the actual characteristics of the behaviour of the cantilever
353
constrained ceramic slab under load, clearly demonstrated the occurrence of second-order events such as
354
membranes thanks to the ability of the ceramic slab to develop deformations of the same order of
355
magnitude as its thickness. The presence of a constraint along the perimeter capable of developing
356
reactions in its own plane, allows the slabs, passed over once by the first phase of elastic deformation, to
357
increase their own rigidity against the orthogonal load on the plane, developing traction stress (together
358
with the flexion previously applied) that contrasts the increase in deformation. This effect does not
359
depend on the size of the slabs or on the lack of reinforcing net, as we can clearly see from the load-strain
360
diagrams in which the type of course is the same and, considering the same slab size, the level of the
361
breaking load and the relative deformation are practically the same for type N and S slabs (in case of load
362
on the SR face) (Figure 8).
363
17
364 365 366 367
Figure 8: Load test n.3.2. Load application on slab back side. Load-lowering curves in the centre point of specimen (test for ceramic without fibre reinforcement N -up- and with fibre reinforcement SR –down-. The purified curve is generated by purging the lowering of the constraint. The breaking load is not dependent on fibre reinforcement.
368
The deformation of the slab, at the breaking moment, is approximately 1/150 of the span. In this case, the
369
increase of rigidity caused by the second order effects is evident, due to the presence of perimeter
370
constraints. The value of the load rupture measured for the slabs with dimensions 1000x1000 mm was
371
approximately 2700 Pa.
372
Type 3.3 tests
373
The framed slabs show a different behaviour than the simple ones, thanks to the presence of the metal
374
profiles, although, in any case, the rupture presented in corresponds to the ceramic tile.
375
If from one side, all the steel profiles have a substantially elastic-linear deformation, on the other side of
376
the ceramic tile, thanks to its excellent elastic properties and resistance, in addition to its very low
377
thickness, has a deformation course with an increase of rigidity according to the increase of load
378
(hardening type behaviour, Figure 9). This is due to the activation of second-order events linked to the
379
extent of deformation (whose order of magnitude is comparable to that of the slab thickness) and to the
380
development of contour reactions thanks to the constraint given by the metal profiles. Obviously, such a
381
contribution has no way of developing in the type 3.1 test, in which the simple supporting constraint does
382
not offer reactions in the slab plan, while the constraint made in type 3.2 tests allows the establishment of
383
such type of behaviour.
18
384 385 386
Figure 9: Load test n. 3.3: load-lowering curves for fleximeter n. 1 on central slab centre and n.3 on first slab center (referred to Figure 5).
387
Generally, type 3.2 and 3.3 tests have shown a biaxial tension course, linked to a membrane behaviour
388
with constraints on four sides (whether the constraints are the ones directly imposed by tests 3.2 or the
389
ones generated for each square field by metallic profiles in tests 3.3), as it is quite clear from the course of
390
the breaking lines on the slabs when removed from the test instrumentation (Figure 10). In this type of
391
test, the maximum deformation at the moment of collapse, detected in the centre of the slab, is
392
approximately 1/90 of the span. Evidently, the deformation value is intermediate compared to the values
393
found in the two types of previous tests, as the reinforcing profiles generate a degree of constraint,
394
however, the cantilever rate achieved in type 3.2 tests is lower than "perfect".
19
395 396
Figure 10: Load test n. 3.3. Detail of a typical breaking line in a specimen.
397
4. Discussion
398
The results of the tests have highlighted the need to define the characteristics of the whole system
399
(ceramic slab / reinforcement net / frame) for design purposes through the synthesis of a set of parameters
400
that take into consideration some peculiarities of the system. First, the material used is not homogeneous
401
(ceramic and net). Moreover, it is necessary to consider the second-order effects to evaluate the resistance
402
of slabs due to thickness of the slab and deformation capacity of material, and finally to point out that the
403
metal frames applied to the back have a classic linear deformation behaviour. With respect the role of
404
reinforcement net tests on real size samples (both with or without frames), there is evidence that the
405
reinforcement net does not significantly increase the resistance value but rather induces a resistance
406
increase exclusively of geometrical type in the system, as if the thickness of the grid was totally porcelain.
407
The geometric role of the reinforcement net is deduced from the load of failure in both cases (tests on
408
slabs type N and S), which essentially has a proportion in accordance with the increase in the resistance
409
modulus on the outer edge of the ceramic thickness. Since the homogenization coefficient between the
410
two materials is close to the unit, the contribution of the net cannot be considered an overall increase in
411
the performance of the composite material but rather is a geometric increase in resistance due to the
412
increase of the size of the resistant section. For simplicity, using a slab with a reinforcement net having a
413
nominal thickness of 3.5 mm, and taking into account the position of the sides of the ceramic layer
414
relative to the centroid of the total section (dc = 1.75 mm towards the exposed ceramic face and df = 1.25
20
415
mm towards the face in contact with the mesh), we have the following theoretical bending moment
416
(Figure 11):
417 418
Figure 11: Stress diagram on the section of ceramic reinforced slab.
419
•
Wc = J/dc = 20.4 mm3
420
•
Wf = J/df = 28.58 mm3
421
From which the failure tensions are obtained:
422
•
σlast, c = M/Wc = [(46 + 7)/8]/0.0204 ≅ 32.5 MPa
423
•
σlast, f = M/Wf = [(70 + 7)/8]/0.02858 ≅ 33.7 MPa
424
Essentially, failure occurs in both traction cases by collapsing the most stressed ceramic fibre. The tensile
425
values obtained at the collapse appear fairly the same in the two rows of application loads, confirming the
426
proposed model. Discussing Young’s modulus, it is necessary to point out that laboratory tests do not lead
427
to a direct measurement, but they obtain the value indirectly from the applied stress and the thickness of
428
the slab (inertial characteristics). Since the thickness of the plate is quite variable (not an absolute value,
429
but a percentage regarding the stated thickness, especially for S type slabs), there is a substantial
430
variability in the indirect measurement of Young's modulus. For typical design applications, the
431
knowledge of Young's modulus is important to define the deforming state of the system, as it is also the
432
moment of inertia. Therefore, it seems reasonable to characterize, for the slab’s application as a wall
433
cladding, the EJ product, defining it as invariant, and then obtaining the normal elasticity modulus
434
according to the thickness for both cases. It has been adopted and it is applicable because the variability in
435
the measured thickness is mainly due to the variability in the thickness of the reinforcement net layer and
436
relative adhesive; conversely, the thickness of the ceramic slab is fairly uniform, having a dimension of
437
approximately 3 mm. Therefore, it is possible to apply mass geometry formulas to characterize Young’s
438
modulus considering the thickness and the contribution of net reinforcement. Referring to a material with
21
439
nominal thickness equal to the fibre reinforced material measured in the laboratory (snom = 3.28 mm) the
440
following normal elasticity modules and EJ product are obtained:
441
•
Enom = 46169 N/mm2
442
•
(EJ)k =135767 N mm2/mm (referred to a 1 mm width of the slab)
443
Conversely, referring to a material with a nominal thickness, equal to that declared by the manufacturer
444
(snom = 3.50 mm), the following normal elasticity modulus and EJ product are obtained:
445
•
Enom = 37998 N/mm2
446
•
(EJ)k =135767 Nmm2/mm (referred to a 1 mm width of the slab)
447
For the design purposes of an advanced screen ventilated facade with the material studied, it is improper
448
to speak exclusively of "ceramic cladding", since the panel is necessarily completed by a metal frame that
449
cannot be considered part of the "substructure system". The clarification is essential to understand how to
450
approach the cladding design, since two materials with radically different tensile and deformational
451
behaviours are assembled. The metal profiles have a linear type strain-deformation behaviour. Instead, the
452
ceramic slab has a strain-deformation behaviour that can only be clearly described by the means of a non-
453
linear analysis that takes into consideration the second order effects that develop on the slabs.
454
5 Conclusions
455
In conclusion:
456
•
the modulus of rupture on the material produced results that can be considered in line with
457
literature. In particular, the N slabs (C0) show characteristic values for the modulus of rupture
458
between 40 and 45 MPa (Table 2).
459
•
on cladding ageing, most impactful ageing effects were due to the decline of mechanical
460
performance (MoR expected value), specifically fatigue ageing (C1) and freeze-thaw (C2).
461
However, if these are considered individually, they do not influence more than 10% of the
462
reduction.
463
•
464 465
the ageing complex (C1-8) produces a decline that decreases the MoR (in its lowest expected value) in a range from -21.5% (type N slabs) to -70.3% (type SR slabs).
•
the elasticity modulus values found in the experimental campaign were approximately 60 GPa,
466
which is in line with the above-mentioned literature values. In the case of Young’s modulus, the
467
ageing influences the design purpose in a negligible way.
22
468
•
the tests performed on the “minikit” have results in accordance with the calculation values
469
recommended by the manufacturer of the used sealant. The obtained results are based on a
470
characteristic resistance approximately 1 MPa for both shear stress and pull-out tests.
471
•
regarding the ageing events (combined C1-C8) tests showed significant performance decrease
472
(15%) of structural glazing sealant. Considering that C1-8 comprises 8 conditioning series, which
473
partially produce a sum of effects, and that for each one the maximum admissions rate is 25%, the
474
results obtained during the test campaign are potentially compatible with the requirements [37].
475
•
the tests carried out on real-sized panel samples indicated that large slabs and thin thicknesses
476
were subject to second-order effects. They may be seen in the load-deformation test diagram as
477
characteristic hardening behaviour. The understanding and quantification of this behaviour is
478
important to better appreciate the deformations and stresses of the slabs in operation, when they
479
are subject to significant loads (wind or snow when mounted in non-vertical positions).
480
•
the role of the net applied behind the slabs is not significant for the increase of the composite
481
material ultimate strength but is certainly useful in conferring a partially ductile behaviour
482
(ductility at least 4 to the slab subjective to positive pressure) to the composite material S (the N
483
material is essentially fragile) and, at last, provides material safety after rupture in order to avoid
484
detachable parts or shards that may fall (even from high heights).
485 486 487
Acknowledgements The results presented here come from the research funded by company System SpA (headquarters in Fiorano Modenese, Italy). Authors want to thank System SpA company for collaborating. Special thanks to Andrea Gozzi and Fabrizio Marani
488 489
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Determinazione del modulo di rottura e della forza di rottura. 2014.
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S2352-7102(19)31352-X
DOI:
https://doi.org/10.1016/j.jobe.2019.101105
Reference:
JOBE 101105
To appear in:
Journal of Building Engineering
Received Date: 26 July 2019 Revised Date:
4 November 2019
Accepted Date: 2 December 2019
Please cite this article as: F. Bazzocchi, S. Bertagni, C. Ciacci, E. Colonna, V. Di Naso, Mechanical characterisation of a low-thickness ceramic tile cladding subject to ageing phenomena, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2019.101105. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Mechanical characterisation of a low-thickness ceramic tile cladding
2
subject to ageing phenomena
3
Corresponding Author:
4
Frida Bazzocchia
5
Co-authors:
6
Stefano Bertagnib, Cecilia Ciaccia, Emiliano Colonnab, Vincenzo Di Nasoa
7
a
8
[email protected]
9
b
DICEA, University of Florence, Italy, email: [email protected], [email protected],
School
of
Engineering,
University
of
Florence,
Italy,
email:
[email protected],
10
[email protected]
11
Abstract
12
In recent years, an innovative system for a rainscreen façade system with low thickness and large tiles of
13
porcelain stoneware has been patented. This paper presents the results of a large experimental campaign
14
investigating mechanical performance of a ceramic tile in 3different configurations: tile without net, with
15
fiberglass net glued on the back, and the entire cladding system (slab/net/frame). The main objective is to
16
determine material mechanical characteristics (e.g. modulus of rupture and Young’s modulus)
17
considering a protocol of artificial ageing (fatigue test, freeze-thaw, heat-rain, H2O Saturation, heat
18
treatment, accelerated corrosion SO2 and NaCl, irradiation UVB and condensation) in order to understand
19
the possible decay during the service life and consequently to design the technological solution of a
20
rainscreen façade system. The results showed that on the modulus of rupture the most influences ageing
21
effects are fatigue test and freeze-thaw while the influence on young’s modulus is negligible.
22
Furthermore, it is necessary to consider second-order effects for the evaluation of the resistance and,
23
finally, that the fiberglass net does not lead to a significant increase in resistance.
24
Keywords: porcelain stoneware, low-thickness tiles, external cladding, mechanical behaviour
25
Highlights
26
•
Test protocol for mechanical features of low-thickness ceramic tile are defined
27
•
Studies and experimental tests with artificial ageing were performed
1
28
•
Fatigue and freeze-thaw reduce the modulus of rupture of about 18% and 15%
29
•
The benefit of the reinforcement net on the back is related to ductility and safety
30
•
Accelerated ageing combinations decrease the strength of the sealant of about 15%
31
1. Introduction and literature background
32
Porcelain stoneware rainscreen façade systems are widely used in current style of architecture for many
33
reasons especially because they offer extensive compositional variety to the architectural designer [1] [2]
34
[3]. Ceramic tile could be easily used in opaque facing with exterior insulation (advanced facades,
35
ventilated facades, Vêture and ETICS -External Thermal Insulation Composite Systems- with ceramic
36
covering) to significantly improve the thermophysical performance of the envelope [4][5]. Moreover, this
37
has undoubted benefits from a life cycle cost point of view, as evidenced in the technical literature [6] [7]
38
[8]. Recently, large porcelain stoneware tiles (sizes up to 3240x1620 mm) and low thickness (minimum 3
39
mm) have been released on the market, thanks to new production technologies [9] [10] [11] [12] [13], by
40
the patented production lines of System S.p.A. in Fiorano Modenese (Italy) [14]. On the market for
41
cladding systems, there are both "simple" slabs and ceramic tiles reinforced with fibreglass net on the
42
back connected to the slab with epoxy adhesives [9] [14]. The material shows excellent eco-compatibility
43
if applied as a cladding compared to other cladding materials such as stone, glass, and aluminium. Some
44
studies confirm that the 9-10 mm traditional cladding thickness presents an excellent performance in the
45
life cycle assessment [15] [16]. Thickness and weight reduction results in increased efficiency in raw
46
material consumption, firing energy [17], packaging and energy for transporting material from gate to site
47
[16]. Some research point out that the microstructural characteristics of porcelain stoneware with low
48
thicknesses by varying the production conditions (e.g., firing temperatures) and physical characteristics
49
(density, porosity) and deriving its mechanical characteristics, mostly in terms of the modulus of rupture
50
[17] [18] [19] [20] [21] [22]. Nowadays, the current laws concern with traditional porcelain stoneware
51
could be still used, in particular the UNI EN ISO 10545 [23] and UNI 11018:2003 [24].These laws and
52
references do not exhaustively address the theme of mechanical decay over time. However, there is a lack
53
of experimental data, useful for the designer, to model the structural behaviour of the slabs when applied
54
to the substrate structures (generally bonded with sealant products). There are also no experiments that
55
can clarify the structural role of fibreglass net that may be present on the back side of the slab and any
2
56
performance modifications that the system can sustain over time. Since traditional tiles are often anchored
57
only with mechanical systems [3], in this case the ageing effects on the advanced-screen facade system
58
are not particularly felt.. All those facts have raised questions about the behaviour of the systems in terms
59
of ageing and performance decay when the envelope is naturally subjected to severe mechanical and
60
chemical stress (e.g., thermal, radiative, chemical, wind actions).
61
This paper shows the results of a large experimental campaign (carried out a few years ago) that, first,
62
performed mechanical characterization of large format slabs with low thickness, both in the version with
63
net glued to the back and in the non-net version, and then the entire cladding system consisting of the
64
slab/net/frame assembly. To verify the possible time performance decay both for slabs and for the entire
65
system, a severe protocol of artificial ageing has been conceived and implemented to reproduce the real in
66
situ conditions as realistically as possible. The purpose is providing the mechanical performance of the
67
material that the designer of the cladding system can use during the design phase in relation to the service
68
conditions.
69
2. Material and Methods
70
The tested slabs in porcelain stoneware come from a production batch of "fumo" colour lamina from the
71
series "collection", a kind of greyish tile without decorations on it. The slab with a reinforcement net
72
takes the commercial name of "Lam's – Collection Fumo" (further called slab “S”), while the one without
73
a net is called “Lam'Slab – Collection Fumo” (further called slab “N”). The manufacturer used to supply
74
the fibreglass net from the company Teximpianti (TRS300, 300 g/m2). The adhesives for the fibreglass
75
net (two-component adhesive based on polyurethane) are from the company Henkel S.p.A. (Macroplast
76
UK 8119 B3 - Mass Glue; Macroplast Hardener UK5401 - catalyst). The sealant adhesive for fixing the
77
slab to the metal structure of anchorage and support (a cold-formed galvanized sheet according UNI EN
78
10346:2015) is from Dow Corning (with trade name 993 Structural Glazing Sealant). To characterize the
79
slabs and the fixing system it was considered necessary to identify a test protocol that would establish the
80
behaviour at both the time of installation and during the period corresponding to the useful life cycle of a
81
façade system (20 years). Since the material was different from the one required by the regulations, due to
82
its small thickness, it was necessary to identify within the regulatory system those norms that best adhere
83
to the characteristics of the material.
84
The experimental campaign was organized as below:
3
85
•
Type 0 tests related to ageing processes. A varied number of ceramic tiles and assembled slabs fixed
86
to the metal support samples (named “minikit”) (Figure 1) were previously subjected to various
87
accelerated ageing tests, and then their mechanical behaviour was evaluated (named Type 1 and 2
88
tests).
89
In detail the “minikit” is assembled as follow:
90
-
ceramic tile LAM’S – Colour “Collection Fumo”;
91
-
fiberglass net: TRS 300 manufactured by the company TEXIMPIANTI;
92
-
adhesive for fiberglass net: MACROPLAST UK 5400 (catalyst); MACROPLAST UK 8119
93
(mass glue); MACROPLAST Hartvertmittler (primer) all manufactured by the company Henkel
94
S.p.A.;
95
-
cold galvanized steel plate low carbon content (UNI EN 10346-15);
96
-
sealant adhesive: 993 Structural glazing sealant manufactured by the company Dow Corning
97
Corporation (at present Dow). Type 1 tests. Tests on slab samples aiming at defining basic
98
mechanical properties (modulus of rupture, elastic modulus).
99
•
100 101 102
Type 2 tests. Tests on assembled samples (minikit) to evaluate bonding adherence (tangential and normal tensile stresses on the slabs);
•
Type 3 tests. Tests on real size prototypes of slabs assembled by bonding them on metal profiles subjected to perpendicular loads to the slab plane (Figure 5).
103
Slabs of 3 mm thickness of porcelain stoneware with glass fibre net, glued with polyurethane adhesive,
104
and slabs without this reinforcement were submitted to the tests. This procedure allowed us to verify the
105
benefits of the net reinforcement and evaluate any eventual performance decline because of accelerated
106
ageing processes.
4
107 108
Figure 1: “Minikit” scheme, and picture, of the sample used for the tests.
109
2.1 Type 0 tests (ageing)
110
To better investigate the behaviour of the ceramic slab with reinforce net (in addition to the silicon
111
adhesive), nine types of accelerated ageing treatments have been simulated:
112
•
C1- fatigue test: The intention was to evaluate stress effects on large cladding slabs, likely to be
113
applied to tall buildings, where wind action can produce "buffeting" effects due to vortex
114
separation at the downwind slabs. A study using the standard CNR-DT 207/2008 [25] (it is not
115
shown here) was preliminarily carried out over the number of cycles that simulated the conditions
116
in use (20 years of nominal life equal to approximately 105 cycles). In the laboratory, pneumatic
117
equipment specifically created for this purpose was used. In case of slabs exposed to fatigue by
118
flexion, a load with tension equal to 40% of the load required for rupture has been produced on the
119
middle. For the minikits, two different tests with 105 cycles were performed: pull-out tests and
120
traction tests with force variation from -200 to +200 N (equivalent to approximately 10% of the
121
rupture strength).
122
•
C2 – freeze-thaw: The conditioning procedure has been applied in accordance with regulation UNI
123
EN 12467:2016 [26] (category A, suitable for slabs laid on roof and subjected to severe actions)
124
with 100 cycles [27] over a temperature range from -20-23 °C.
125 126
•
C3-heat-rain: The conditioning procedure contained in UNI EN 12467:2016 [26] (category A) has been followed, as it allows the evaluation of the irradiation. The samples were subjected to
5
127
irradiation of approximately 600 W/m2 for 170 minutes alternating with water mist (1 l/min x m2)
128
for an equal period, interrupted by 10-minute pauses. The 360 minute cycle was repeated 50 times.
129
•
130 131
C4-H2O Saturation: The conditioning method does not refer to any regulation and was achieved by immersion of the samples in water at 50°C for a period of 7 days.
•
C5-Heat treatment: For this conditioning, the instructions contained in EN 1903:2015 [28] and
132
UNI EN 1296:2002 [29] have been followed. In this case, the tests were performed with a more
133
severe temperature, 150 °C, and a shorter time, 7 days, in a climatic chamber.
134
•
C6-accelerated corrosion SO2: For this conditioning, the instructions given in UNI EN ISO
135
6988:1998 [30] (which generally is applied to metal claddings) have been followed to test the
136
ageing behaviour of metal parts. The conditioning consisted of 21 cycles of 24 hours of exposure
137
in a static wet room at 45 °C with 200 ml of SO2 (exchanged at each cycle).
138
•
C7-accelerated corrosion NaCl: For this conditioning, the instructions in UNI EN ISO 9227:2012
139
[31]have been complied with in order to verify, in particular, the ageing of metal parts and
140
interfaces between the metal and sealant. The samples were inserted for 500 hours (approximately
141
21 days) in rooms at 35 °C with solution of 5% NaCl in sprayed H2O.
142
•
C8- irradiation UVB and condensation: Conditioning was inspired by the methodology of UNI EN
143
ISO 16474-3:2014 [32], in particular, the C-4 (UVB-313) method. It references an exposure of 500
144
hours at UVB (0.35 W/m2 at 340 nm), U.R. of 65% and black panel temperature of 60 °C, with
145
alternate condensation spraying. The conditioning intends to cause accelerated ageing of the
146
organic parts (glues) of the samples.
147
In addition to the single ageing conditions, a total conditioning test, defined as C1-8 (in the C1-C2-C4-
148
C5-C6-C7-C8-C3 sequence), was performed to evaluate the overall effect on accelerated ageing samples.
149
2.2 Type 1 tests (samples)
150
To determine the modulus of rupture, (MoR), tests were carried out in accordance with UNI EN ISO
151
10545-4:2014 [23], with the following numbers: 20 samples (250x100 mm) for S slabs [24], 20 samples
152
for the N slabs (250x100 mm), 7 samples [23] for each of the 9 anticipated ageing treatments (C1-C2-C3-
153
C4-C5-C6-C7-C8). The instrument (Figure 2 – Figure3) used for the tests was defined in line with UNI
154
EN ISO 1055-4:2014: each sample was placed on 2 cylindrical support rollers after placing a rubber
6
155
layer, then an increasing concentrated load was applied on the upper side in a central position until the
156
failure of the samples (Figure 4).
157
The main conditions for the performed test are the following ones:
158
•
distance between the 2 cylindrical support rollers (L):
230 mm
159
•
roller diameter (d):
20 mm
160
•
rubber thickness (t):
4.5 mm
161
•
distance between the support roller and the end of the sample (i):
10 mm
162
•
concentrated load (F):
1± 0,2 N/mm2·s
163 164
Figure 2 Section of the instrument used for the definition of MoR
165 166
Figure 3: Total view of the instrument used for the definition of MoR
7
167 168
Figure 4: Modulus of rupture determination test.
169
The MoR was tested on the N slabs by applying the load only on one side (considering symmetrical plate
170
behaviour), while on the S slabs it was evaluated by applying the load on both faces (on the outer face
171
named “F” and on the rear face named “R” with reinforcement net) in order to check if the S slabs have
172
different behaviour depending on the load direction applied (simulating, for example, the pressure and
173
depression actions on the slab due to the wind). In total, 249 samples were tested.
174
To identify characteristic values useful for the design, referring to p. 4.6.2.2.d of UNI 11018:2003 [24],
175
the minimum expected value (MEV) was obtained. The regulation [24] suggests that for MEV a
176
confidence level of 75% for a breaking value is acceptable, corresponding to the 5th lowest percentile.
177
Unfortunately, the regulation does not provide information on the probability distribution to be
178
considered, so we deferred to norm UNI EN 13161:2008 [33], which suggests a normal logarithmic
179
distribution. This statistical treatment has also been extended for other test results, unless otherwise
180
indicated.
181
For the MEV calculation, the values of ks (quantile factor) were used (Table 1), which were subjected to
182
the number of samples n tested: n ks
3 3.15
4 2.68
5 2.46
6 2.34
7 2.25
8 2.19
9 2.14
10 2.10
15 1.99
20 1.93
30 1.87
40 1.83
50 1.81
183
Table 1: Quantile factor (ks) according the number of samples (n).
184
To determine the value of the Young's modulus, the tests were conducted in accordance with UNI EN
185
843-2:2007 [34], (which is certainly the most relevant for the case), with the following numbers:
186 •
7 Samples (250x100 mm) of S slabs;
187 •
7 samples (250x100 mm) of N slabs;
8
188 •
14 samples (250x100 mm) only for the single cumulative ageing treatment (C1-8) that includes all
189
various treatments on the two types of slabs: S (7 samples) and N (7 samples).
190
The test was performed by applying a linear variable load starting at 5 N, reaching 35 N and returning to
191
5 N. During the test, the sample deformation value was measured with the help of a strain gauge at both
192
the loading and unloading stages. The Young's modulus was tested on the N slabs by applying the load
193
only on one side, while on the slabs S, it was evaluated only with the application of the load on the R face
194
(without net) due to measurement problems with the strain gauge in presence of the net reinforcement. In
195
total 28 samples were tested.
196
2.3 Type 2 tests (minikits)
197
Type 2 tests were done on composite samples that had to simulate the assembly of the slabs (both S type
198
and N type) with a galvanized cold formed steel profile through bonding with a structural glazing sealant
199
(Figure 1). For these kinds of samples, the silicon bonding joint had predetermined dimensions. The
200
bonding dimensions were defined either by the contact surface or by the thickness. Additional
201
reinforcement slabs were intended to facilitate the test with the MTS machine to avoid the occurrence of
202
parasitic flexions.
203
With such sample sizes, we intended to measure the strength of the structural glazing sealant joint with
204
respect to two prevailing actions applied: pull-out (with application of load of constant speed) and shear.
205
There are no regulations for these kinds of tests, so they have been executed in agreement with the
206
laboratory. Tests were done only with the S type slabs with bonding on the R side (thus with sealant
207
bonding on the reinforcement net), as we supposed that the best solution for the applications on facades
208
was the one with the reinforced slab, at least for security problems in case of partial detachment. Tests
209
were carried out on 10 non-conditioning samples, as well as 10 samples previously submitted to C1-8
210
treatment, for a total of 10 tested samples for pull-out tests and 10 for shear.
211
2.4 Type 3 tests (real-size prototypes)
212
To understand the mechanical behaviour of the entire system (slab/silicone/metal frame), using N type
213
and S type slabs, tests were performed on real-size prototypes with the purpose of understanding the
214
behaviour in system use. All type 3 tests were performed on non-conditioned materials. The
215
perpendicular load on the slab was simulated by applying fine sandbags or bituminous sheath sheets with
216
steps of uniform load. The slabs were collapsed after loading and unloading cycles (to also evaluate the
9
217
plasticity limit of the system). The deformations were measured with centesimal digital comparators
218
(fleximeters), with continuous monitoring of ambient temperature and humidity.
219
The tests types performed were the following:
220
•
Type 3.1: Tests on N and S slabs of 1200x1000 mm dimensions with a simple roller support on two
221
out of four sides. The constraint was made with 2 rollers placed 500 mm away from the sample axis.
222
In total, 2 samples with load on the front and 2 samples with load on the back were tested (Figure 5).
223
•
Type 3.2: Tests on both N and S slabs of 800x800 mm and 1000x1000 mm sizes, with a fixed
224
support on all 4 sides made by mechanical fixing (Figure 5). In total, 4 samples were tested for each
225
of the two sizes, applying the load both on the front and on the back side of the plate.
226
•
Type 3.3: Tests on S type cladding panels with dimensions of 3000x1000 mm fixed to frames
227
composed by profiles in cold formed galvanized steel (Figure 5), where longitudinal profiles have a
228
C-section, and transverse profiles have an omega section. The frame was then fixed on an
229
nondeformable support structure. The loads were applied both on the front (4 samples) and on the
230
back of the plate (4 samples).
10
231 232
Figure 5: From left to right: 5.1, 5.2, 5.3 schemes type test and fleximeter position. Measurements are expressed in mm.
233
3 Results
234
3.1 Type 0 tests
235
The ageing conditioning results have shown a visible and consistent non-alteration of the samples (except
236
for some limited corrosion effects of metal elements after conditionings C6 and C7, as well as C1-8). We
237
found a special issue regarding conditioning C1. The tension level limits during fatigue cycles had been
238
been calibrated before laboratory tests. With a tension level of 70% of the rupture average, samples broke
239
after 5000 cycles. Instead, with tension levels of 60% and 50%, samples were broken after 10000 and
240
20000 cycles. Only with a maximum value of 40% of the rupture average, did samples exceed the
241
predetermined reference value of 100000 (105) cycles.
242
3.2 Type 1 tests
243
The outcome of modulus of rupture tests results on non-conditioning samples, in particular, on N type
244
slabs, are in accordance with other studies [18]. The norm ISO 13006:2012 [35] provides an indicative
245
value for more traditional slabs applications, equal to 35 MPa, largely exceeded both in the mean and in
11
246
the minimum expected value (MEV) calculated per item as 4.6.1.2.d. from UNI 11018:2003 [24]. The
247
behaviour of slabs equipped with reinforcement net (S type) changed according to the direction of
248
application of the load. On S type samples loaded on the front side (indicated in the table 2 with the “SF”
249
symbol), MEV values were reduced by 12% compared to N type slabs. In the case of S type samples
250
loaded at the back side (in the table 2 indicated by "SR"), the MEV reduction was 15% lower than N type
251
slabs. Sample Type
SR
SF
N
Ageing
Mean
Std Deviation
None C1 C2 C3 C4 C5 C6 C7 C8 C1-8 None C1 C2 C3 C4 C5 C6 C7 C8 C1-8 None C1 C2 C3 C4 C5 C6 C7 C8 C1-8
[MPa] 42.94 42.26 42.95 45.86 43.46 41.96 51.51 47.76 46.12 33.43 49.91 48.25 50.32 53.43 47.62 50.06 55.80 53.85 56.46 44.57 51.67 50.02 47.20 55.00 51.01 52.09 56.72 57.00 57.69 44.71
1.73 1.58 4.02 3.13 1.28 1.71 5.46 2.33 3.03 11.24 4.99 3.98 3.16 4.50 1.84 2.74 6.04 2.63 6.29 8.16 2.71 5.87 3.74 4.20 2.79 3.83 4.41 4.39 4.04 3.99
Minimum Expected Value [MPa] 39.70 38.85 34.51 39.09 40.69 38.26 40.78 42.73 39.76 11.78 40.98 40.00 43.29 43.68 43.67 44.26 43.12 48.24 43.64 28.45 46.59 38.17 39.51 46.12 45.16 43.98 47.54 47.54 49.07 36.58
MEV variation -2.1% -13.1% -1.5% 2.5% -3.6% 2.7% 7.6% 0.2% -70.3% -2.4% 5.6% 6.6% 6.6% 8.0% 5.2% 17.7% 6.5% -30.6% -18.1% -15.2% -1.0% -3.1% -5.6% 2.0% 2.0% 5.3% -21.5%
252
Table 2: Experimental results for modulus of rupture. Minimum expected value and its variation for different ageing conditionings.
253
The conditionings may produce alternate sign variations with respect to non-conditioned samples. The
254
most significant reductions were produced by conditioning C1 (which seems to primarily afflict N type
255
samples) and C2 (which primarily afflicts N type and SF samples). Cumulative C1-8 produces sensitive
256
reductions of MEV in any case: from -21% in N type to -70% in SR samples. The verification of the
12
257
effects of performance reduction as a consequence of the cumulative classifications (C1-8 in our case)
258
was not provided either in the literature or in legislation [36] [37].
259
From the tests on non-conditioned samples, the role of the reinforcement net is clear, which does not
260
appear to significantly change the value of the first collapse force (stress applied resulting in the
261
appearance of the first crack, as shown in the non-linear stretch of the load-deformation diagram).
262
Instead, the reinforcement net (SF samples) greatly increases the ultimate collapse deformation, as
263
evidenced by the force-deformation diagram, where the last deformation is approximately 4 to 5 times
264
that of the first rupture (Figure 6).
265 266
Figure 6: Load-strain diagrams for bending tests. The SF specimen is on the right.
267
The flexural strength obtained by these tests was supplied indirectly by applying the usual formulas from
268
solids mechanics. Referring to the measured thickness values of the slabs subjected to flexural failure, the
269
following average values of the resistance module referring to a strip of 1 mm width were obtained:
270
•
WN = 1.581 mm3
271
•
WSF = 1.882 mm3 (load on ceramic side)
272
•
WSR = 1.804 mm3 (load on reinforcement side)
273
The ratio between Lam'Slab (SF and SR) and Lam's (N) slab resistance modulus changes from 1,141 to
274
1,19. Instead, if we refer to the expected minimum values of previously estimated rupture tension, the
275
ratio between slabs Lam'Slab and Lam's changes from 1.136 to 1.17. This shows the variability range of
13
276
resistance modules. Consequently, we can affirm that the contribution of the fibreglass net to the
277
resistance by flexion of the material is practically non-existent.
278
To define a characteristic flexural resistance to be used for design purposes, it is therefore necessary to
279
correlate the resistance to thickness:
280
•
281 282 283
Flexural resistance characterised by flexion compared to the average nominal thickness of 3.28 mm measured through laboratory tests: σk = 39.70 MPa.
•
Flexural resistance characterized by flexion compared to the nominal thickness stated by the manufacturer equal to 3.50 mm: σk = 34.87 MPa.
284
These values are valid for samples not submitted to ageing.
285
Furthermore, the Young’s modulus tested on N slabs provided a value in accordance with those found in
286
the literature [18] [22]; after ageing C1-8 showed a reduction of approximately 7%. In the case of slabs
287
with net reinforcement SR, ageing produced an increase of 5% in the elasticity modulus. Variations
288
caused by ageing in this case are not particularly significant (Table 3). It is important to highlight that the
289
S slab module is 17% less than that of N, but for this issue, we should note the following interpretation is
290
based on the selection of type 3 tests. Sample Type
Ageing
Mean
Std Deviation
Mean variation
1894 3971 3661 2188
-6.8% 5.2%
[Mpa]
N SR
None C1-8 None C1-8
60330 56218 49977 52576
291
Table 3: Experimental results for Young’s modulus and its variation.
292
Laboratory tests carried out to determine Young's modulus revealed a difference of 17% in the medium
293
values measured between slabs N and S. Vice versa, when referring to the result between the measured
294
Young’s modulus and the moment of inertia of the section (also a characteristic of each slab due to its
295
thickness), the tests showed a convergence of values between the two types of slabs, N and S, the
296
difference being only approximately 2.5%.
297
3.3 Type 2 tests
298
Type 2 tests were performed on assembled samples (minikit). The tests were mainly aimed at verifying
299
whether the behaviour of the sealant corresponded to that present in the literature [37], taking the values
300
indicated by the manufacturer of the structural glazing sealant as reference [38]. Tests were carried out
14
301
both by shear (further called TAG) and pull-out (further called POUT) on sample families (5 for each
302
series) that were either non-aged (C0) or subjected to the complete set of ageing (C1-8). For the statistical
303
processing of the data obtained with the tests, the procedures contained in ETAG 002 [37] have been
304
followed. The measured force values were then referred to as tensions, (normal or tangential) and divided
305
by the bond area, which was 2000 mm2 (Figure 1). Type of test
Ageing
Mean
Mean Std Ru,5 Ru,5 Ru,5 variation Deviation (Force) (Stress) variation
[N]
TAG POUT
None C1-8 None C1-8
2108 764 2415 979
95 225 158 360
-64% -59%
[N]
[MPa]
1874.1 210.1 2026.0 94.6
0.937 0.105 1.013 0.047
-88.8% -95.3%
306
Table 4: Experimental results for shear (TAG) and pull-out (POUT) test on minikits.
307
The results of tests on non-aged samples (Table 4) led to values compatible with those reported by the
308
manufacturer (Dow Corning [38] and references the following characteristic values of Ru, 5: pull-out equal
309
to 0.84 MPa and shear = 0.066 MPa, values on which the manufacturer himself recommends using a
310
safety factor of 6). For the C1-8 conditioning sample test results, they are inferior to the value provided
311
by the manufacturer for pull-out. In addition, in the case of pull-out tests, ETAG 002 [37] also admits a
312
performance decrease of up to 25% after ageing (Table 4). However, the same [37] results apply to
313
individual and non-cumulative ageing. In any case, the performance result in terms of ageing emerging
314
from the tests should be carefully evaluated. If we want to apply the results obtained on a hypothetical
315
panel of 3000x1000 mm dimensions, in which the slab (of weight = 70 N/m2) is anchored on steel
316
profiles with 2 sealant strips of length 3000 mm and width 20 mm and subjected to a wind pressure of
317
3000 N/m2 (significant action also on 40 m tall buildings facades), in this case a stress is applied on the
318
sealant strips, as follows: Pull-out = 0.073 MPa; Shear = 0.0017 MPa. The data on the pull-out in
319
operation exceeds the data measured on a characteristic rupture after ageing. This suggests, therefore, that
320
the researcher should deeply evaluate the ageing behaviour of the silicone sealant by expanding the
321
number of samples and testing them with individual ageing, in full compliance with ETAG 002 [37].
322
Indeed, the conditions in age group C1-8 are much more severe than the one proposed by regulation.
323
3.4 Type 3 tests
324
Type 3.1 tests
15
325
Tests highlighted the behaviour of elastic-fragile material in the case of load applied on the back of the
326
slab (net side) and elastic-plastic type with a load applied on the front (ceramic side). In both cases, the
327
deformation trend, until the first break occurred, was perfectly elastic (Figure 7). The deformation of the
328
slabs, in all cases, was in accordance with what theoretically obtainable by a static slab scheme supported
329
on both sides. The ratio between the deformations measured at the quarters and in the middle was
330
approximately 0.717, while the theoretical value was 0.7125. First, loading / unloading tests were carried
331
out without causing a rupture to the slab (with a pression of approximately 420 Pa): a perfectly elastic-
332
linear behaviour was then detected, with a residual lowering after the loading / unloading cycles of 1%.
333 334
Figure 7: Load test n.3.1: load lowering for fleximeter n.1. On the right, the breaking line near the specimen axis is obvious.
335
For simple mono-axial flexion, the reinforcement net contribution is essentially geometric rather than of a
336
mechanical nature (the fibreglass elastic modulus can be estimated at approximately 70-80 GPa). In
337
essence, the net brings the slab to behave as if it had a thickness of approximately 3.5 mm but without
338
appreciably improving its mechanical performance. The results of the tests carried on both slabs N and S
339
show a plate breaking at an average value of approximately 700 Pa. After the first loading and unloading
340
cycles, N and S (SF) slabs were submitted to rupture tests. In the case of the N slabs, the rupture was
341
immediate and resulted in the collapse of the sample. In the case of S slabs, even after the ceramic part
342
broke, collapse did not occur because the reinforcement supported cracked parts. At the breaking point,
343
the tests have all been interrupted. The breaks occurred in the middle of the samples. The N slabs have
344
broken at a load level of approximately 660 Pa. Instead, the slabs S (loaded on the SF front) broke at an
345
average load of 700 Pa. Considering the different thicknesses of the two types (average N slab thickness =
16
346
3.12 mm, average SF slab thickness = 3.34 mm), very similar medium rupture tension are obtained for
347
both cases in accordance with those of type 1 tests:
348
Failure tension for Type N = 50.8 MPa; Failure tension for Type SF = 47.1 MPa.
349
The slab deformation at the breaking point was approximately 48 mm in both cases (N and SF),
350
equivalent to 1/20 of the span.
351
Type 3.2 tests
352
Type 3.2 tests, whose aim was to understand the actual characteristics of the behaviour of the cantilever
353
constrained ceramic slab under load, clearly demonstrated the occurrence of second-order events such as
354
membranes thanks to the ability of the ceramic slab to develop deformations of the same order of
355
magnitude as its thickness. The presence of a constraint along the perimeter capable of developing
356
reactions in its own plane, allows the slabs, passed over once by the first phase of elastic deformation, to
357
increase their own rigidity against the orthogonal load on the plane, developing traction stress (together
358
with the flexion previously applied) that contrasts the increase in deformation. This effect does not
359
depend on the size of the slabs or on the lack of reinforcing net, as we can clearly see from the load-strain
360
diagrams in which the type of course is the same and, considering the same slab size, the level of the
361
breaking load and the relative deformation are practically the same for type N and S slabs (in case of load
362
on the SR face) (Figure 8).
363
17
364 365 366 367
Figure 8: Load test n.3.2. Load application on slab back side. Load-lowering curves in the centre point of specimen (test for ceramic without fibre reinforcement N -up- and with fibre reinforcement SR –down-. The purified curve is generated by purging the lowering of the constraint. The breaking load is not dependent on fibre reinforcement.
368
The deformation of the slab, at the breaking moment, is approximately 1/150 of the span. In this case, the
369
increase of rigidity caused by the second order effects is evident, due to the presence of perimeter
370
constraints. The value of the load rupture measured for the slabs with dimensions 1000x1000 mm was
371
approximately 2700 Pa.
372
Type 3.3 tests
373
The framed slabs show a different behaviour than the simple ones, thanks to the presence of the metal
374
profiles, although, in any case, the rupture presented in corresponds to the ceramic tile.
375
If from one side, all the steel profiles have a substantially elastic-linear deformation, on the other side of
376
the ceramic tile, thanks to its excellent elastic properties and resistance, in addition to its very low
377
thickness, has a deformation course with an increase of rigidity according to the increase of load
378
(hardening type behaviour, Figure 9). This is due to the activation of second-order events linked to the
379
extent of deformation (whose order of magnitude is comparable to that of the slab thickness) and to the
380
development of contour reactions thanks to the constraint given by the metal profiles. Obviously, such a
381
contribution has no way of developing in the type 3.1 test, in which the simple supporting constraint does
382
not offer reactions in the slab plan, while the constraint made in type 3.2 tests allows the establishment of
383
such type of behaviour.
18
384 385 386
Figure 9: Load test n. 3.3: load-lowering curves for fleximeter n. 1 on central slab centre and n.3 on first slab center (referred to Figure 5).
387
Generally, type 3.2 and 3.3 tests have shown a biaxial tension course, linked to a membrane behaviour
388
with constraints on four sides (whether the constraints are the ones directly imposed by tests 3.2 or the
389
ones generated for each square field by metallic profiles in tests 3.3), as it is quite clear from the course of
390
the breaking lines on the slabs when removed from the test instrumentation (Figure 10). In this type of
391
test, the maximum deformation at the moment of collapse, detected in the centre of the slab, is
392
approximately 1/90 of the span. Evidently, the deformation value is intermediate compared to the values
393
found in the two types of previous tests, as the reinforcing profiles generate a degree of constraint,
394
however, the cantilever rate achieved in type 3.2 tests is lower than "perfect".
19
395 396
Figure 10: Load test n. 3.3. Detail of a typical breaking line in a specimen.
397
4. Discussion
398
The results of the tests have highlighted the need to define the characteristics of the whole system
399
(ceramic slab / reinforcement net / frame) for design purposes through the synthesis of a set of parameters
400
that take into consideration some peculiarities of the system. First, the material used is not homogeneous
401
(ceramic and net). Moreover, it is necessary to consider the second-order effects to evaluate the resistance
402
of slabs due to thickness of the slab and deformation capacity of material, and finally to point out that the
403
metal frames applied to the back have a classic linear deformation behaviour. With respect the role of
404
reinforcement net tests on real size samples (both with or without frames), there is evidence that the
405
reinforcement net does not significantly increase the resistance value but rather induces a resistance
406
increase exclusively of geometrical type in the system, as if the thickness of the grid was totally porcelain.
407
The geometric role of the reinforcement net is deduced from the load of failure in both cases (tests on
408
slabs type N and S), which essentially has a proportion in accordance with the increase in the resistance
409
modulus on the outer edge of the ceramic thickness. Since the homogenization coefficient between the
410
two materials is close to the unit, the contribution of the net cannot be considered an overall increase in
411
the performance of the composite material but rather is a geometric increase in resistance due to the
412
increase of the size of the resistant section. For simplicity, using a slab with a reinforcement net having a
413
nominal thickness of 3.5 mm, and taking into account the position of the sides of the ceramic layer
414
relative to the centroid of the total section (dc = 1.75 mm towards the exposed ceramic face and df = 1.25
20
415
mm towards the face in contact with the mesh), we have the following theoretical bending moment
416
(Figure 11):
417 418
Figure 11: Stress diagram on the section of ceramic reinforced slab.
419
•
Wc = J/dc = 20.4 mm3
420
•
Wf = J/df = 28.58 mm3
421
From which the failure tensions are obtained:
422
•
σlast, c = M/Wc = [(46 + 7)/8]/0.0204 ≅ 32.5 MPa
423
•
σlast, f = M/Wf = [(70 + 7)/8]/0.02858 ≅ 33.7 MPa
424
Essentially, failure occurs in both traction cases by collapsing the most stressed ceramic fibre. The tensile
425
values obtained at the collapse appear fairly the same in the two rows of application loads, confirming the
426
proposed model. Discussing Young’s modulus, it is necessary to point out that laboratory tests do not lead
427
to a direct measurement, but they obtain the value indirectly from the applied stress and the thickness of
428
the slab (inertial characteristics). Since the thickness of the plate is quite variable (not an absolute value,
429
but a percentage regarding the stated thickness, especially for S type slabs), there is a substantial
430
variability in the indirect measurement of Young's modulus. For typical design applications, the
431
knowledge of Young's modulus is important to define the deforming state of the system, as it is also the
432
moment of inertia. Therefore, it seems reasonable to characterize, for the slab’s application as a wall
433
cladding, the EJ product, defining it as invariant, and then obtaining the normal elasticity modulus
434
according to the thickness for both cases. It has been adopted and it is applicable because the variability in
435
the measured thickness is mainly due to the variability in the thickness of the reinforcement net layer and
436
relative adhesive; conversely, the thickness of the ceramic slab is fairly uniform, having a dimension of
437
approximately 3 mm. Therefore, it is possible to apply mass geometry formulas to characterize Young’s
438
modulus considering the thickness and the contribution of net reinforcement. Referring to a material with
21
439
nominal thickness equal to the fibre reinforced material measured in the laboratory (snom = 3.28 mm) the
440
following normal elasticity modules and EJ product are obtained:
441
•
Enom = 46169 N/mm2
442
•
(EJ)k =135767 N mm2/mm (referred to a 1 mm width of the slab)
443
Conversely, referring to a material with a nominal thickness, equal to that declared by the manufacturer
444
(snom = 3.50 mm), the following normal elasticity modulus and EJ product are obtained:
445
•
Enom = 37998 N/mm2
446
•
(EJ)k =135767 Nmm2/mm (referred to a 1 mm width of the slab)
447
For the design purposes of an advanced screen ventilated facade with the material studied, it is improper
448
to speak exclusively of "ceramic cladding", since the panel is necessarily completed by a metal frame that
449
cannot be considered part of the "substructure system". The clarification is essential to understand how to
450
approach the cladding design, since two materials with radically different tensile and deformational
451
behaviours are assembled. The metal profiles have a linear type strain-deformation behaviour. Instead, the
452
ceramic slab has a strain-deformation behaviour that can only be clearly described by the means of a non-
453
linear analysis that takes into consideration the second order effects that develop on the slabs.
454
5 Conclusions
455
In conclusion:
456
•
the modulus of rupture on the material produced results that can be considered in line with
457
literature. In particular, the N slabs (C0) show characteristic values for the modulus of rupture
458
between 40 and 45 MPa (Table 2).
459
•
on cladding ageing, most impactful ageing effects were due to the decline of mechanical
460
performance (MoR expected value), specifically fatigue ageing (C1) and freeze-thaw (C2).
461
However, if these are considered individually, they do not influence more than 10% of the
462
reduction.
463
•
464 465
the ageing complex (C1-8) produces a decline that decreases the MoR (in its lowest expected value) in a range from -21.5% (type N slabs) to -70.3% (type SR slabs).
•
the elasticity modulus values found in the experimental campaign were approximately 60 GPa,
466
which is in line with the above-mentioned literature values. In the case of Young’s modulus, the
467
ageing influences the design purpose in a negligible way.
22
468
•
the tests performed on the “minikit” have results in accordance with the calculation values
469
recommended by the manufacturer of the used sealant. The obtained results are based on a
470
characteristic resistance approximately 1 MPa for both shear stress and pull-out tests.
471
•
regarding the ageing events (combined C1-C8) tests showed significant performance decrease
472
(15%) of structural glazing sealant. Considering that C1-8 comprises 8 conditioning series, which
473
partially produce a sum of effects, and that for each one the maximum admissions rate is 25%, the
474
results obtained during the test campaign are potentially compatible with the requirements [37].
475
•
the tests carried out on real-sized panel samples indicated that large slabs and thin thicknesses
476
were subject to second-order effects. They may be seen in the load-deformation test diagram as
477
characteristic hardening behaviour. The understanding and quantification of this behaviour is
478
important to better appreciate the deformations and stresses of the slabs in operation, when they
479
are subject to significant loads (wind or snow when mounted in non-vertical positions).
480
•
the role of the net applied behind the slabs is not significant for the increase of the composite
481
material ultimate strength but is certainly useful in conferring a partially ductile behaviour
482
(ductility at least 4 to the slab subjective to positive pressure) to the composite material S (the N
483
material is essentially fragile) and, at last, provides material safety after rupture in order to avoid
484
detachable parts or shards that may fall (even from high heights).
485 486 487
Acknowledgements The results presented here come from the research funded by company System SpA (headquarters in Fiorano Modenese, Italy). Authors want to thank System SpA company for collaborating. Special thanks to Andrea Gozzi and Fabrizio Marani
488 489
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26