Construction and Building Materials 25 (2011) 3966–3971

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Construction and Building Materials

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Slate flexural and anchorage strength considerations in cladding design

Vera Pires a,⇑, P.M. Amaral a, L.G. Rosa a, R.S. Camposinhos b

a Department of Materials Engineering, Instituto Superior Técnico, T.U. Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugalb Department of Civil Engineering, Instituto Superior de Engenharia do Porto, Rua Dr. Bernardino de Almeida 431, 4200-072 Porto, Portugal

a r t i c l e i n f o

Article history:Received 10 May 2010Received in revised form 14 March 2011Accepted 11 April 2011Available online 5 May 2011

Keywords:SlateFlexural strengthAnchorage strengthStone

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.04.029

⇑ Corresponding author. Tel.: +351 938436402; faxE-mail address: vera.pires@ist.utl.pt (V. Pires).

a b s t r a c t

This paper describes an experimental investigation on the flexural and anchorage strength of slate forcladding. The study has been conducted on sawed slate specimens, all showing the same surface finish-ing. Slate flexural strength was compared for two distinct situations: (i) using a 3-Point flexure loadingconfiguration in batches of materials with larger cross-sectional specimen dimension (50 � 30 mm2);and (ii) using a 4-Point flexure loading configuration in the same batch of materials but with smallercross-sectional dimensions (30 � 25 mm2). The 4-Point bending specimens were tested in three differentdirections considering slate anisotropy planes. Load was applied along the direction perpendicular to theplanes of schistosity; and also along two directions parallel to the planes of schistosity. Slate anchoragestrength has been determined on slate slabs with 400 � 200 � 30 mm3 with dowel anchorage in 8 mmdiameter cylindrical holes with 35 mm depth. Test load was applied perpendicularly to the schistosityplanes. Cladding stone in building facades and its supporting systems must be compatible with thebehaviour and performance of other interfacing systems, such as curtain walls and superstructure frames.In this sense, a properly executed dimensional stone cladding should be designed and installed within thecapabilities and limitations of the slate’s support system to resist all active forces or actions. The results ofthis work reveal the importance of complementary characterization techniques for dimension stone clad-ding, particularly for anisotropic rocks as slates. From the results it is possible to conclude that schistosityplanes have an utter influence on either anchorage or flexural strength.

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1. Introduction

Stone has been used as a building material for thousands ofyears. Its aesthetical characteristics, together with the long-lasting(‘‘perpetual’’) effect produced, made it a popular material, espe-cially among architects and builders. Throughout history, most ofthe more significant buildings have been constructed of stoneand the evolution of stone facades closely parallels the evolutionof building construction and technologies employed. Economic as-pects and alternative building systems have led to numerous vari-ations in the installation of stone on building facades over the past100 years [1]. A complete understanding of the material and fixingtechniques is critical for the proper design and installation of stonecladding systems, especially when non-isotropic materials are em-ployed (e.g., slates, quartzites and several limestones) [2,3].

In what concerns with non-isotropic natural stone materials,some cladding systems are more suitable than others. Numerousconfigurations of discontinuous ‘‘kerfs’’ or undercut systems (hid-den fixing systems on the back side of slate slabs) are usually rec-ommended for non-isotropic stones [4,5]. Examples of thesecladding systems are shown in Fig. 1a–d.

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: +351 218418132.

In this particular work, instead of these systems we have stud-ied the dowel–hole fixing system. Despite the fact that this is oneof the less recommended cladding systems for the great majorapplications with vibration, wind, and non-linear expansionrequirements. We were particularly interested in this fixing systemin order to investigate if it could be accepted under certainconditions.

This paper overviews the mechanical testing methodology thatmay be conducted in a slate used as an anisotropic cladding mate-rial, emphasizes the difficulties to define a rupture model for thisparticular type of stone and has the aim of identify suitable charac-terization guidelines for a proper design and installation of thisparticular case (dowel–hole fixing system for slate or other stonewith similar physical–mechanical properties).

2. Materials and testing methodology

The present study has been conducted on sawed slate specimens from Brazilshowing an as-sawn surface or a honed surface finishing (example is shown inFig. 2).

Slate flexural strength was compared for two distinct situations:

(i) using a 3-Point flexure loading configuration for specimens with largercross-sectional dimensions (50 � 30 mm2; specimen length = 150 mm);

Fig. 1. Examples of ‘‘kerf’’ and undercut cladding systems.

Fig. 2. Visual aspect of Brazilian Slate honed surface.

V. Pires et al. / Construction and Building Materials 25 (2011) 3966–3971 3967

(ii) using a 4-Point flexure loading configuration for specimens of the samebatch but with smaller cross-sectional dimensions (30 � 25 mm2; specimenlength = 150 mm).

All flexural tests were carried out in an Instron 4302 machine, at a constantcrosshead displacement speed of 2.0 mm/min.

In the tests with the 4-Point configuration the specimens were loaded in threedifferent orientations considering slate anisotropy planes (see Fig. 3). Load was ap-plied perpendicularly to the planes of schistosity (orientation A); and also in twodirections parallel to the planes of schistosity (orientation B and orientation C).The testing methodology was conducted according to EN 13161:2008 [6]. 3-Pointbending tests were conducted only with specimen orientation A and according toEN 12372:2006 [7].

The 4-Point flexural tests were performed considering slate anisotropy planesbecause these values were the ones used, in this particular study, for dimensioningslate slabs. 3-Point flexural test values for orientation A were used just to make arough comparison with 4-Point values.

The dowel anchorage rupture load was determined for slate slabs with dimen-sions 400 � 200 � 30 mm3, containing cylindrical holes of 8 mm diameter and35 mm depth. The anchorage tests were conducted applying a load perpendicularlyto the schistosity planes, according to the ASTM 1354-96 standard [8].

These tests were performed in a Seidner FORM + TEST machine, at a constantcrosshead displacement speed of 0.1 mm/min.

Tests were performed according to ASTM instead of the corresponding Euro-pean standard EN 13364:2002 [9]. The European standard requires that the dowelis fixed to the hole using a specific mortar prepared with cement CEM I 52.5 R inaccordance with EN 197-1 and a water/cement ratio of (0.6 ± 0.1) by mass (normal-ized test conditions).

In this particular study, for the validation of specific conditions for dowel–holecladding system for slate, it was useful to perform the tests in a similar way to thefinal cladding application, which used plastic caps for stainless steel pins withoutmortar. The specimens’ dimensions were also chosen according to the finalapplication.

3. Results and discussion

Results of flexural strength are summarized in Table 1. 4-Pointflexural tests were performed on 18 specimens for each specimenorientation. 3-Point flexural tests for specimen orientation A werecarried out just for a rough comparison with 4-Point flexuralstrength results, and therefore only 8 specimens were tested in3-Point bending. Fig. 4 shows the aspect of the 8 specimens after3-Point flexural test. Examples of the slate specimens after 4-Pointflexural test are depicted in Fig. 5a (specimen orientation A) and b(specimen orientation B).

As it was expected, results show higher flexural strength for ori-entation A (load applied perpendicularly to the planes of schistos-ity) when compared with orientations B and C (load parallel to theplanes of schistosity). The main reasons to explain this behaviourare intrinsically related with the materials’ microstructure; slateis a fine grained, homogeneous, foliated, anisotropic metamorphicrock derived from sedimentary rocks through low grade metamor-phism [10].

Fig. 3. Specimens orientations used in the 4-Point bending tests. Loading is appliedin the vertical direction. The planes of schistosity are schematically shown.

Fig. 4. Slate specimens after 3-Point flexural test load applied perpendicularly tothe planes of schistosity, i.e. specimen orientation A.

Table 1Slate flexural strength results for different specimen orientation.

Specimen # 4-Point flexural strength (MPa) 3-Point flexuralstrength (MPa)

OrientationA

OrientationB

OrientationC

Orientation A

1 43.48 27.36 0.48 17.752 37.76 36.75 0.49 26.713 44.90 46.53 0.46 27.544 44.07 28.04 0.48 33.275 48.37 31.59 0.49 26.976 47.24 21.14 0.50 19.047 42.23 35.57 0.49 24.778 39.20 35.76 0.49 33.989 47.54 25.47 0.4810 50.77 32.14 0.5011 35.76 41.55 0.5012 37.21 40.24 0.3813 35.99 26.45 0.4914 39.19 40.81 0.5015 48.26 28.34 0.5016 47.29 19.29 0.5017 38.32 23.56 0.5018 36.76 41.16 0.50Mean value 42.5 32.3 0.49 26.3Standard deviation 5.0 7.9 0.01 5.8

3968 V. Pires et al. / Construction and Building Materials 25 (2011) 3966–3971

Flexural strength for orientation A is an important value to con-sider when dimensioning an exterior facade subjected to wind andimpacts. On the other hand, orientation C is important to take intoaccount anchorages gathered through a dowel–hole fixing system,where a section with a relevant decreasing strength is always ob-served contributing for a potential decrease in the claddingstrength of the overall system. In the present case, the results fromthe 4-Point flexural tests indicate a very substantial drop from42.5 MPa (mean value for orientation A) to 0.49 MPa (mean valuefor orientation C) which is about 99% less.

Examples of plots representing flexural strength (MPa) vs.stroke (mm) for the different orientations (A, B and C) are depictedin Fig. 6. Plots evidence the strong influence of schistosity on flex-ural strength. Flexural strength values shown in Fig. 6 are: 50.77for orientation A, 25.47 MPa for orientation B and 0.38 MPa for ori-entation C.

Tables 2 and 3 present results of rupture load and rupture an-gles obtained in slate slabs submitted to anchorage strength tests.All slabs were 400 � 200 � 30 mm3 and contained cylindrical holesof 8 mm diameter and 35 mm depth. Results obtained from slateslabs with the load applied in a honed surface of the slab, perpen-dicularly to the schistosity planes, are presented in Table 2;whereas Table 3 summarizes the results obtained from slate slabswith the load applied in as-sawn surface of the slab (also perpen-dicularly to the schistosity planes). Load was applied in this direc-tion due to the fact that this was the most unfavorableconfiguration for the dowel–hole fixing system.

As it is possible to observe from specimens fracture surfaces(e.g. in Fig. 7), rupture areas are irregular and very different fromeach other, and due to that it was not possible to define a rupturemodel for this group of specimens.

Contrarily to traditional Portuguese marbles where rupture an-gles of about 39� are normally found [11], the results shown in Ta-bles 2 and 3 reveal a strong variation of rupture-angle values in theslate slabs. The majority of fracture surfaces in slate slabs (seeFig. 7) presented irregular rupture angles and irregular fracturepaths, all characteristic from foliated rocks which indicate the needto consider an extra safety factor when dimensioning slabs withdowel anchorage for slate stone.

Moreover, rupture loads were slightly higher for honed faces,which can be mainly attributed to the fact that this finishing treat-ment causes a more uniform surface with less superficial defects.This enforces the need to define harmonized characterizationguidelines for cladding applications of this slate (or for other slateswith similar properties).

Fig. 5. Slate specimens after 4-Point flexural test: (a) specimen orientation A; (b) specimen orientation B.

Fig. 6. Examples of plots obtained in 4-Point flexural tests (flexural strength (MPa) vs. stroke (mm)).

Table 2Results obtained from anchorage strength tests conducted on slate slabs, with theload applied in a honed surface of the slab, perpendicularly to the schistosity planes.

Specimen # Rupture load (kN) Rupture angle

Right Left

1 3.44 0� 0�2 5.99 0� 0�3 4.60 0� 16�4 4.67 8� 9�5 4.63 0� 8.5�6 4.59 15� 15�7 4.49 16� 17�8 4.66 39� 12�9 4.60 0� 0�10 4.40 0� 0�Average value 4.61 7.8� 7.8�Standard deviation 0.61 12.7� 7.2�Coefficient of variation 13.2% 163.2% 92.9%

Table 3Results obtained from anchorage strength tests conducted on slate slabs, with theload applied in an as-sawn surface of the slab, perpendicularly to the schistosityplanes.

Specimen # Rupture load (kN) Rupture angle

Right Left

1 1.46 14� 10�2 3.42 0� 0�3 2.46 0� 0�4 4.37 0� 0�5 2.95 0� 0�6 2.57 0� 0�7 3.79 17� 18�8 3.79 21� 14�9 3.34 0� 0�10 3.65 0� 0�Average value 3.18 5.2� 4.2�Standard deviation 0.84 8.5� 7.0�Coefficient of variation 26.4% 164.1% 167.2%

V. Pires et al. / Construction and Building Materials 25 (2011) 3966–3971 3969

Fig. 7. (a–d) Examples of slate fracture su

3970 V. Pires et al. / Construction and Building Materials 25 (2011) 3966–3971

In our opinion, general cladding characterization guidelines forthis slate should include:

(i) Mechanical characterization

� Flexural resistance under constant moment (for the dif-

ferent directions considering slate anisotropy planes)for specimens with the same finishing treatment as thefinal product

� Flexural resistance under concentrated load (with thesame conditions defined above)

� Pure shear tests� Anchorage strength test with load applied perpendicu-

larly to slab final application plane, with the same finish-ing treatment and for the selected fixing system. If valuesbelow 3 KN are obtained, another slate orientationshould be considered

� Analysis of load vs. stroke graphs obtained on the abovetests

(ii) Physical characterization

� Apparent density and open porosity� Water absorption at atmospheric pressure� Thermal expansion coefficient

(iii) Thin-section analysis and macroscopic description shouldalso be included.

(iv) Special considerations should also be made for the slatematerial:

� Discontinuous ‘‘kerf’’ fixing systems or undercut systems

are preferred over dowel–hole configurations speciallyfor high buildings where wind and seismic loads arecritical

� Dowel–hole configuration can be used in particular appli-cations, specially near-ground and northern exposures(with less sun exposure and, due to that, lower tempera-ture gradients)

� Slabs must have a minimum thickness of 30 mm

4. Conclusions

rfaces after anchorage strength tests.

The determination of flexural strength for three different spec-imen orientations (in what concerns schistosity planes) is one ofthe most important tasks when dimensioning cladding systemsfor some anisotropic rocks, such as slates. In this study a relevantdecrease from 42.5 to 0.5 MPa representing a drop in the flexuralstrength of about 99% was observed when comparing failure stres-ses on specimens where load is perpendicular to the planes ofschistosity with those where load is parallel to the planes ofschistosity.

Although flexural strength perpendicular to schistosity planes isan important property to consider when dimensioning an exteriorfacade subject to wind and exterior impacts, it is also essential toregard the anchorage strength (and the rupture angles) for dow-el–hole fixing systems or other.

Rupture detachments obtained from anchorage strength tests(performed according to ASTM C1354-96 standard) were very het-erogeneous (all very different from each other) and thus a properrupture model could not be defined for this type of slate from Bra-zil. This demonstrates the importance of performing a completecharacterization and evaluation of the material before its applica-tion in building facades.

Dowel–hole fixing system is, among all possibilities, oneof the less recommended cladding systems for the great majorapplications with vibration, wind, and non-linear expansionrequirements. However, taking into account both flexural andanchorage strength results, the system can be accepted for someparticular applications (near-ground and northern exposureswith less sun exposure and because of that, lower temperaturegradients).

The evaluation of the material should always follow the charac-terization guidelines proposed for this slate (or others with similarphysical–mechanical characteristics).

V. Pires et al. / Construction and Building Materials 25 (2011) 3966–3971 3971

Models that are usually applied to describe anchorage rupturesurfaces for marbles or limestone cannot be applied in this partic-ular type of slate [12].

Dowel anchorage strength test seems to be influenced by shear,and therefore, pure shear tests should be conducted in future workto correctly characterize the stone.

References

[1] Camposinhos R, Amaral PM. Manual técnico para a aplicação, uso emanutenção de rochas ornamentais. Lisboa: ASSIMAGRA; 2007.

[2] Moreiras STJF, Paraguassú AB, Ribeiro RP. Dimension stone for buildingfaçades: methodology for structural design. Bull Eng Geol Environ2008;67(1):53–7.

[3] ASTM C1242-03. Standard guide for selection, design, and installation ofexterior dimension stone anchors and anchoring systems; 2003.

[4] Lammert BT, Hoigard KR. Material strength considerations in dimension stoneanchorage design. ASTM STP 1499: dimension stone use in buildingconstruction 2007;55:40–57.

[5] West DG, Heinlein M. Anchorage pullout strength in granite: design andfabrication influences. ASTM STP 1394: dimension stone cladding: design,construction, evaluation and repair;2000. p. 121–134.

[6] EN 13161. Natural stone test methods. Determination of flexural strengthunder constant moment; 2008.

[7] EN 12372. Natural stone test methods. Determination of flexural strengthunder concentrated load; 2006.

[8] ASTM C1354-96. Standard test method for strength of individual stoneanchorages in dimension stone; 1996.

[9] EN 13364. Determination of the breaking load at the dowel hole. Brussels: CEN– European committee for standardization; 2002.

[10] Fernandes JC, Pires V, Amaral PM, Rosa LG. Analysis of strength scaling effect inportuguese limestone: comparison between three- and four-point bendingtests. Mater Sci Forum 2010;636–637:1336–41.

[11] Silva MF. Estudo do revestimento de fachadas de edifícios em pedra naturalfixada mecanicamente. Tese de Mestrado em Georecursos, IST-ISEP, Portugal;2008.

[12] Camposinhos RS, Camposinhos RPA. Dimension stone cladding design withdowel anchorage. in: Proc ICE – constru mater 2009;162(3):95–104.