Influence of freezing test methods, composition and microstructure on frost durability assessment of clay roofing tiles

Construction and Building Materials 25 (2011) 2888–2897

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Influence of freezing test methods, composition and microstructure on frost durability assessment of clay roofing tiles M.I. Sánchez de Rojas a, F.P. Marín b,⇑, M. Frías a, E. Valenzuela b, O. Rodríguez a a b

Eduardo Torroja Institute (CSIC), C/Serrano Galvache n. 4, 28033 Madrid, Spain Department of Chemical and Energetic Technology, Universidad Rey Juan Carlos, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain

a r t i c l e

i n f o

Article history: Received 18 May 2009 Received in revised form 18 December 2010 Accepted 23 December 2010 Available online 19 January 2011 Keywords: Clay roofing tiles Frost durability Lifetime Mechanical properties Microstructure Porosity

a b s t r a c t Clay roofing tiles are sensitive to frost action and require testing by repeated freezing and thawing cycles. The research purpose is to predict frost durability, taking into account the effect of clay mineralogy, microstructure (studied by DRX, FRX and mercury porosimetry) and manufacturing parameters, drawing an empirical comparison between methods C and E suggested by the European Standard. Method E is more severe and selective than method C, showing earlier failure and smaller dispersion, which may be explained due to the different saturation process, cooling kinetic and the presence of cloth. The damage increased with the number of cycles, it is substantially affected by the fired clay porosity and calcium content, being consistent with the thermodynamics and kinetics of the mechanisms that lead to failure. The findings provide guidance to improve frost durability based on clay characterisation and industrial process control. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Clay roofing tiles have been widely used in building coverage of any nature. But, when they are used in cold districts, frost damage, which is caused by the freezing of water which has permeated into the pores of the material, become a serious problem which reduces mechanical integrity and necessitates expensive repair or replacement [1–4]. It develops with the number of freezing–thawing cycles and can take several aspects according to the nature of the clay and the shaping method used. Water contained in some of the pores, following rainfall freezes into ice due to a decrease of temperature, then local high tensile stress will arise around the pores, and micro-cracks will be generated. These micro-cracks will grow and join with each other until the final fracture occurs. The main parameters influencing the durability of fired-clay materials subjected to freezing–thawing cycles are according to the nature of the material [3,5], porosity, modulus of elasticity, permeability, as well as the imposed boundary conditions. The frost resistance is determined by the pore-size, pore type, and pore distribution, which depend on firing temperature [2]. Conventional wisdom has suggested that the requirement for numerous freeze–thaw cycles was due to a fatigue fracture mechanism. Stresses leading to frost damage could be developed by a variety of mechanisms: hydraulic pressures due to volume ⇑ Corresponding author. Tel.: +34 630029534; fax: +34 914887338. E-mail address: [email protected] (F.P. Marín). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.12.041

increase by ice formation, stress due to supercooling, pressures at the interfaces between the phases, and even stresses between ice crystals of different size [6,7]. The damage induced by frost action is initiated by nucleation, crystal growth, and followed by the interactions of the microscopic cracks. These processes, which usually occur inside the pores, are expressed by the volumetric expansion during cooling [8,9]. The first explanations of the strain observed were based on the development of a hydraulic pressure created by ice formation. The harmful effects were attributed purely to the pressure created by the increase of 9% in volume as water freezes. Later, an osmotic pressure was added to the hydraulic pressure because, in the vicinity of the formed ice, the degree of saturation of the aqueous solution contained in the pores increases. Consequently water in neighbouring pores migrates towards the sites of ice formation to establish thermodynamic equilibrium. Litvan [8] suggests that, below 0 °C, water migrates under the effect of the vapour pressure gradient to places where it can freeze, be it in large pores or outside. This induces desorption of the small pores. The damage occurs when desorption cannot compensate for the non-equilibrium created by the decrease of temperature. This behaviour is similar in all porous materials studied, so, clay may replace other materials without significant change in their frost properties [10,11]. By reproducing conditions of moisture and temperature close to those generally found on site, it is possible to show up the damage that can appear when frost resistance is insufficient. A good frost resistance at the laboratory has to be a guarantee for the behaviour

M.I. Sánchez de Rojas et al. / Construction and Building Materials 25 (2011) 2888–2897

on the roofs [5,12]. This assessment method represents a very approximate way of estimation. However, it does put the requirements on some sort of logical basis bearing in mind that natural weathering itself is a very variable phenomenon depending on location, aspect, shading and so on. European standards establish the requirements for the products and the tests to measure them. The first European standard for clay tiles kept four freezing tests methods: A for Benelux, B for Germany, C for Southern Europe (France, Greece, Italy, Portugal and Spain) and D for UK. It is not clear that the mode of failure in all of the tests is similar to the types of frost failure that can be experienced in real roofs [13], so they will progressively be replaced by the method E, suggested in the revised European standard and described in the next section. 2. Description of method E In method E the saturation procedure, the kinetic of the freezing–thawing cycles and their number are well defined and a wet cloth is placed on the back face of the tile (Fig. 1) in order to create temperature differences between the two faces of the sample. In fact, the roof modifies the incident climatic conditions due to the flows of wind and wind driven rain [14]. For saturated products it has been stated that the water is subjected to an overpressure during the freezing test and a wet cloth placed in contact with the tiles increased its magnitude. This pressure may be broken down into two terms. The first one of them can be deduced from the pore size distribution and the variation in the volumetric fraction of ice formed upon freezing. The second term expresses the additional pressure due to the liquid phase. The pore size distribution plays a secondary role in comparison with the pressure developed by the liquid phase, which is conditioned by the capacity of the water to move within the pore volume. The distribution of the ice crystals formed and the liquid permeability of the porous material seem to be the dominant parameters. Method E leads to laboratory results which are in very good agreement with the behaviour of products on different European sites [9]. Nevertheless despite these good results, the E-method

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presents the disadvantage, from the industrials point of view, to be time consuming, considering the great number of cycles which are expected. 3. Experimental procedure 3.1. Materials and specimens description Clay roofing tiles under investigation were produced in different works. They meet all the specifications required in European standard EN 1304 [15]. The nine different models selected include different sizes of over and under tiles made by extrusion and interlocking tiles made by extrusion and pressing, without any surface finishing such as engobe or glaze, except model M8, produced with a water repellent treatment of polymethyl silicate, applied by immersion on an aqueous solution of potassium methyl siliconate, which reacts with atmospheric carbon dioxide to form polymethyl silicate. Their properties and production process are given in Table 1. Models M5 and M6 vary only in dimensions, they are made using the same raw materials and process conditions, in order to investigate the possible effect of clay flow in the extrusion process. Models M3 and M9 share raw materials but they have been produced in different factory installations, taking special care in the case of M9 to control the most common causes of frost failure (vacuum extrusion, correct flow of material in the pressing and control of firing temperatures). 3.2. Testing procedures The chemical composition of clay mixtures used as raw materials were studied with X-ray fluorescence techniques using a Panalytical PW 1404 wavelength-dispersive spectrometer. Their mineralogical compositions were studied with X-ray diffraction techniques using a Philips PW.1730 diffractometer (Eindhoven, The Netherlands). X-ray diffraction analysis were also performed on fired tiles. The mercury porosimetry technique was used to measure total porosity, apparent density, and median pore diameter in fired clay tiles. These tests were performed in a Micromeritics Pore Sizer 9500 (Norcross, GA). Laboratory sample consists of 40 tiles of each model which was split in 2 sets of 20 units selected in a way such that each tile has an equal chance of being chosen, in order to test one set according to method C and another set according to method E. This number was established in order to get information enough to apply statistical techniques; the standard requires only 10 specimens for method C and 6 for method E. Test pieces underwent visual inspection for possible imperfections which were marked with waterproof ink. Each test sample was subjected to two types of frost tests, methods C and E, such as established in the European standard EN 539-2 [16], chapters 7 and 9, respectively, using a freeze–thaw unit ‘‘Dycometal CHD 525’’. Both test methods require, before freeze–thaw cycles, to saturate tiles using two different soaking processes: – Method C: 1 h immersion in water under vacuum to absolute pressure of 6.13  104 Pa. – Method E: test specimens are progressively immersed in water during 1 week. Samples were placed in the freezing chamber on a rack standing on their short side with a minimum distance between them and the side of the unit of 60 mm. The temperature of the immersed tiles is at first stabilized at about 11 °C. The surrounding air is then cooled; the rate of cooling must be controlled so that the temperature of the tiles drops down to freezing temperature following the normalized cycle shown in Table 2. The actual temperature values in the chambers’ air and inside the tile in cycles C and E are shown and compared in Fig. 2. Thawing was carried out in water at 11 °C until the temperature stabilized. The tiles were covered by a cloth on one face, for E-method. The test cycles were carried out until failure occurs in all specimens or up to 520 cycles. Statistical techniques based on software Minitab 15.1 were used to help the analysis and interpretation of the results.

4. Results

Fig. 1. Tile with cloth placed on the back face.

The chemical composition of clay mixtures used as raw materials, studied by X-ray fluorescence is shown in Table 3 and their mineralogical composition, studied by X-ray diffraction is shown in Fig. 3A. The semiquantitative analysis revealed a similarity in the mineralogical composition of raw materials. Clays had an illitic or illitic-kaolinitic character (37–43%) with a high sand content (quartz content of 20–31%), and a chlorite content of about 6– 15%. The average content of Fe-oxyhydroxides (hematite) never exceeds 4%. The content is SO3 is rather high for specimens M3, M8 and M9.

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Table 1 Selected clay roofing tiles: production process and properties. Model

M1

M2

M3–M9

M4

M5

M6

M7

M8

Shaping Firing temperature (°C) Surface finishing Type of tile Individual length (mm) Maximum width (mm) Roof tiles number in m2 Mass (kg) Impermeability category Thermal conductivity k (W/m K) Reaction to fire class External fire performance class

Extrusion 930 None Over/under 400 154 36.0 1.25 1 1.0 A1 BROOF

Extrusion 1010 None Over/under 490 210 19.0 2.40 1 1.0 A1 BROOF

Ext + pressing 925 None Interlocking 428 258 12.5 3.00 1 1.0 A1 BROOF

Ext + pressing 945 None Interlocking 458 284 10.5 3.70 1 1.0 A1 BROOF

Extrusion 935 None Over/under 400 195 28.0 1.75 1 1.0 A1 BROOF

Extrusion 935 None Over/under 450 195 22.0 1.90 1 1.0 A1 BROOF

Ext + pressing 945 None Interlocking 460 286 10.5 3.70 1 1.0 A1 BROOF

Ext + pressing 1050 Polysilicate Interlocking 435 258 12.5 3.00 1 1.0 A1 BROOF

Table 2 Comparison between the freeze–thaw cycles of methods C and E. Method

C

Program

Temperature (°C)

Time (min)

Temperature (°C)

Time (min)

Cooling

12 ± 3 to 4 ± 2 4 ± 2 to 5 ± 2 5 ± 2 to 15 ± 5 15 ± 5 15 ± 5 to 12 ± 3

50 ± 10 55 ± 15 45 ± 15 15 ± 10 15 ± 5

11 ± 6 to 1 ± 0.5 1 ± 0.5 to 3 ± 0.5 3 ± 0.5 to 16 ± 3 16 ± 3 16 ± 3 to 11 ± 6

10–20 34–48 30–40 P30 30

Freezing Thawing

E

X-ray diffraction analyses performed on fired tiles are shown in Fig. 3 B. It is consistent with the mineralogical composition of clay mixtures used as raw materials. The increase in temperature for singly fired ceramic clays produces a series of reactions and transformations that lead to the formation of new phases and the disappearance of others. Jordan et al. [17] reported the persistence of illite up to at least 900 °C during firing. Mixtures of illitic clays containing CaCO3, such as M1, M5, M6 and M8 clays, form gehlenite, anorthitic plagioclases, akermanite, mullite and diopside. Samples poor in CaCO3, such as M4 and M7, give a very simple mineralogical composition (quartz and Fe-oxyhydroxides). The total amounts of phyllosilicates vary in a narrow range. Table 4 shows total porosity, apparent density, and median pore diameters in fired clay tiles obtained by the mercury porosimetry

technique and Fig. 4 shows incremental and cumulative volume versus pore diameter. Almost no differences could be found in XRD analysis and porosimetry between tiles model M5 and M6 and between tiles M3 and M9, so their curves are shown together in Figs. 3 and 4. Table 5 shows the water content after soaking when tiles are soaked according to method C and to method E. The absorption using method E is slightly bigger than using method C; when tiles have a coating (model M8) method E gets a much higher absorption than method C. The frost resistance of each tile model is different owing to both design and different raw materials or production process. The tiles tested cover a wide range of frost durability. In general, every model of tile shows similar type of damages when tested according method C or E but, depending on model properties they appear in the same range of cycles or not. In other words, the type of test also gives different frost resistance for the same model. Both test methods seem to be reliable. The degree of cracking increased with the number of cycles, and damage was greater using test method E, with the presence of the cloth, resulting in earlier failure. The damage generally occurred in the form of delamination (exfoliation), cracks and chips, see Fig. 5, other damages, such as peeling, loss of ribs and loss of nibs could also be found. Fig. 6 shows the percentage of tiles damaged for every model of tile in different range of cycles, drawing a comparison between methods C and E.

15 10

Temperature (ºC)

5 0 -5 -10 -15 -20 0

30

60

90

120

150

180 210 Time (min)

240

270

300

Method E - Air Temperature

Method E - Inside tile Temperature

Method C - Air Temperature

Method C - Inside tile Temperature

Fig. 2. Actual cycles E and C comparison.

330

360

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M.I. Sánchez de Rojas et al. / Construction and Building Materials 25 (2011) 2888–2897 Table 3 Chemical and mineralogical composition of raw materials. Majority constituent (%)

M1

M2

M3–M9

M4

M5–M6

M7

M8

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 MnO SO3 P2O5 LOI

45.2 17.4 5.39 11.0 2.31 0.78 3.55 0.65 0.06 0.03 0.13 13.5

56.0 17.9 6.67 4.30 2.34 0.71 3.43 0.87 0.09 0.03 0.14 7.52

54.2 15.4 6.37 3.50 5.15 0.57 4.79 0.74 0.08 0.22 0.13 8.85

58.0 18.0 6.01 0.67 4.71 1.26 4.03 0.90 0.08 0.09 0.22 6.03

47.4 16.4 5.24 10.8 2.32 0.86 3.38 0.63 0.06 0 0.13 12.8

58.6 16.9 5.26 0.85 4.39 1.06 3.71 0.85 0.11 0.06 0.20 8.00

45.9 12.6 4.67 14.4 2.40 0.47 3.32 0.60 0.04 0.56 0.10 15.0

29 31 12 6 8 – 6 – 4 4

37 31 – 15 2 6 – – 4 6

29 20 12 14 – – 11 9 3 2

22 26 17 5 15 5 – 5 3 2

30 21 12 12 – – 9 9 3 4

28 28 – 7 25 – 4 – 2 6

Semiquantitative Mineral analysis (%) Illite (+muscovite) 30 Quartz 24 Kaolinite 10 Chlorite 10 Calcite 18 Dolomite 3 Plagioclase (Albite) – Potassic feldspar – Fe oxyhydroxides 3 Other 2

5. Discussion 5.1. C and E methods comparison When the minimum (standard requirement) and mean frost resistances are compared, the test method E seems to be more severe than the test method C, at least for models M1, M2, M3, M4, M5, M6 and M9. The test method E seems to be more selective, showing smaller dispersion, measured by the standard deviation, the range and the coefficient of variation (see Table 6), for models M1, M2, M4, M5, M6, M7, M8 and M9; in model M3 the dispersion is similar. The frequency distributions are closer to the normality. In model M7 it looks like test method E gives better frost resistance than method C, but when the damage is analyzed, we can see that it is due to the loss of ribs. This damage is clearly seen in the front ribs, but ribs and nibs in the back are hidden behind the cloth. When only failure in the front side is taken into account, both test methods give similar assessment (Table 6 row M7 Front). After 520 cycles, 15% tiles in method C and 25% tiles in method E kept their ribs and nibs because they were correctly shaped, but nascent cracks were found in tiles tested under method E. The frost failure of ribs and nibs in model M7 (and partially in model M3) looks to be due to clay flow problems in the pressing process, because cracks at the base of the ribs were found before testing (Fig. 7A). When the rib is detached, it is possible to see how the crack starts at the base of the rib and it advances in parallel to tile surface following the flow of the clay (Fig. 7B). In most cases it produces the loss of the rib or nib and sometimes the structural crack of the whole tile. In this case both test methods have given more similar results, because the clay do not delaminate. It is possible to conclude that the frost durability may increase considerably if the flow were better controlled in the shaping process. Model M8 allows drawing a comparison between both test methods when the pores of the surface are blocked due to the water repellent treatment with polymethyl silicate applied by tile immersion. The frost behaviour is similar in both tests; this fact may be explained because when pores are blocked in the surface, the effect of the cloth is negligible because it does not contribute with any additional strain. It is possible to do a regression analysis between methods E and C when tiles have no surface coating and the type of failure (delamination, crack, chip, peeling) is due to the freeze–thaw cycles,

excluding thus models M7 and M8, respectively. Fig. 8 shows the regression analysis for models M1, M2, M3, M4, M5, M6 and M9, where mean and minimum frost resistance may be fit using a linear or quadratic regression:

Linear : E ¼ 15:68 þ 0:5502C

R2 ¼ 86:8%

Quadratic : E ¼ 2:58 þ 0:9393C  0:001165C 2

ð1Þ R2 ¼ 89:9% ð2Þ

where C is the number of freeze–thaw cycles that tiles resist without failure under method C, and E is the number of freeze–thaw cycles that tiles resist without failure under method E. In order to have a rough approximation, method E reduces the frost resistance about 40% compared to method C. The difference between C and E test methods may be explained in part due to the different kinetic of the cooling process (faster cooling rate and prolonged freezing time of method E). As it has been largely described in the literature [6,7,18], the conditions of equilibrium between vapour, water, and ice are influenced by the curvature of their interfaces, the presence or absence of species in water, and the pressures applied to the phases. Relations between pore diameter and freezing or melting point can be deduced from the thermodynamic models based on equilibrium between the phases. Since the process of crystal formation is exothermic, the heat flow increases proportionally to the quantity of ice formed. The ice crystals within the pores do not form at the temperatures at which they are stable, but at markedly lower temperatures, this fluctuation of the temperature at which ice begins to form means that the water is supercooled. Consequently, and in contrast to the case of fusion which always takes place at the temperature of equilibrium of the system for a given pressure, crystallization proves to be a phenomenon governed by kinetics of nucleation for which one can only define the ‘‘most probable’’ temperature of crystallization. Brun et al. [19,20] proved that the variation DT between the temperature of crystallization T and the triple point temperature T0 related to the radius of the ice–water interface (the water being assumed to be pure) can be expressed as

T  T 0 ¼ DT P

2m1 c1s DSf Rs1

ð3Þ

where m1 is the molar volume of water, DSf the molar entropy of fusion, c1s the free extension energy of the ice–water interface, and Rs1 the radius of this interface. For materials containing essentially

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s (A) Raw material

A: albite Ca: calcite Cl: chlorite Do: dolomite F: potassic feldspar He: hematite I: illite/muscovite K: kaolinite Q: quartz

Q

Ca

Q Q I

K

Cl

Cl

Cl

FA

He I

Do

I

Ca Q

Q

Ca

Ca

Q

Q

Q

Ca M8 M7 M5-M6 M4 M3-M9 M2 M1

5

10

15

20

25

30

35

40

45

50

55

2 - Theta

(B) Fired clay tiles

Ak: akermanite An: anorthite Cr: cristobalite Di: diopside F: potassic feldspar Ge: gehlenite He: hematite I: illite/muscovite Mu: mullite Q: quartz

Q

Q I

Mu

Cr,An

Mu

F,An Ge, Ak

He Di

Q

Q

M8 M7 M5-M6 M4 M3-M9 M2 M1

5

15

25

35 2- Theta

45

55

Fig. 3. X-ray diffraction patterns for raw materials and fired clay tiles.

Table 4 Porosity of the fired clay tiles. Sample

Total porosity (%)

Median pore diameter (lm)

Apparent density (g/ml)

% Pores 0.25– 1.4 lm

M1 M2 M3 M4 M5 M6 M7 M8 M9

35.32 27.72 27.40 24.71 35.12 35.13 24.63 34.33 27.37

0.6313 0.6313 0.2358 0.5439 0.4207 0.4211 0.4972 0.5704 0.2401

2.75 2.69 2.62 2.63 2.73 2.73 2.64 2.76 2.62

89.8 91.9 67.7 52.1 88.1 88.2 52.6 88.5 67.8

micropores (pore radius < 50 nm), the terms of the above relationship are equal and no supercooling is observed. However, ice starts to form very close to 0 °C and the formation of ice is influenced by the dimensions of the specimen and also the kinetic of the cooling process.

Large differences have been reported between the behaviour with and without the cloth, more significant strains were observed in the presence of cloth. In order to explain the effect of the cloth, additional cause of earlier failure during freezing cycles in method E, strain analysis reported [9] reveals the existence of three periods, corresponding to three temperature intervals. In liquid water (T P 0.5 °C) the contraction of the saturated material is purely thermal and when all the pores are filled with ice (T 6 9 °C) the practically linear strains observed are the result of thermal contraction of the fired clay. In period (0.5 °C P T P 9 °C) the water contained in the porous material freezes progressively. As temperature falls, nucleation occurs and the nuclei formed then grow by transformation of the water in contact with them. A series of mechanisms are then involved, which lead to the cryo-deformation of materials: (i) difference of density between liquid water and ice crystal, (ii) interfacial effects between the different constituent parts, (iii) drainage of liquid water expelled from the freezing sites, (iv) cryo-suction driving water towards frozen pores, (v) thermomechanical coupling between the different phases [21–23]. When a cloth, containing a large quantity of water in bulk state, is placed

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0,80

0,70

Log incremental volume (mL/g)

0,60

0,50

0,40

0,30

0,20

0,10

0,00 1000

100

10

1 Pore diameter (µm)

0,1

0,01

0,001

0,2 M1

M2

M3-M9

M4

M5-M6

M7

M8

0,18

Cumulative volume (mL/g)

0,16 0,14 0,12 0,1 0,08 0,06 0,04 0,02 0 1000

100

10

1 Pore diameter (µm)

0,1

0,01

0,001

Fig. 4. Pore Distribution.

Table 5 Absorption: Water content after soaking. Sample

Method C (vacuum) %

Method E (progressively 1 week) %

M1 M2 M3 M4 M5 M6 M7 M8 M9

14.9 11.4 11.5 9.5 16.7 16.8 10.1 0.82 11.1

15.1 11.8 12.9 9.5 17.5 17.7 10.5 4.6 12.6

on one face of the sample under test, water transforms to ice at this face first. The ice formed on the surface of the tiles may block a part of the water movement towards the outside which would other-

wise compensate for the volumetric variation between water and ice [8]. The pressure increase of the liquid phase will be more significant in the presence of a cloth than without it. 5.2. Effect of tile properties and manufacturing parameters on frost resistance The analysis of frost resistance (minimum and mean values in both test methods) versus the water content after soaking (Fig. 9A) shows how the frost resistance decreases when the water content increases, except when pores are blocked due to a water repellent treatment (M8). The frost resistance of a brick or a tile and its may be estimated by the saturation coefficient, which is expressed as a percentage of cold water or boiling water absorption. It is assumed that if the value of this coefficient is lower than 0.78, then the product should have the required frost resistance. It im-

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Fig. 5. Damages: delamination, crack, chip.

Fig. 6. Percentage of tiles damaged of every tile model in different range of cycles.

plies that the presence of small pores, where water can intrude only during boiling, may improve frost resistance, although this assumption is controversial and it has been put in doubt by recent studies [24]. According to Sveda [3], the frost resistance is influenced not only by pore volume, but also by the median pore radius. The highest frost resistance is reached when the pore volume decreases and at the same time that the pore radius median increases. Fig. 9B shows minimum and mean values versus the total porosity, it is consistent with the observation of frost durability and total porosity is a better predictor of frost behaviour than

water content after soaking. It has been reported by Ravaglioli [18] that the roofing tiles with pore-size of 0.25–1.4 lm suffer from more frost damage, in order to analyze this fact, Fig. 9C shows minimum and mean values versus the percentage of pores in the range 0.25–1.4 lm, which confirms Ravaglioli’s assumption. The roofing tile is a complex, heterogeneous material consisting of quartz, feldspar, Ca-silicates, amorphous/vitreous phases and numerous pores. It is considered that the change of the constituent minerals and microstructure of the bodies with a densification process influences the frost behaviour [17,25,26]. Mixtures of illitic

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M.I. Sánchez de Rojas et al. / Construction and Building Materials 25 (2011) 2888–2897 Table 6 Statistical summary of the frost resistance (number of cycles). Model

Minimum

M1 M2 M3 M4 M5 M6 M7 M7 Fr. M8 M9

Mean

Std. deviation

Range

Coeff. variation

E

C

E

C

E

C

E

C

E

C

50 85 70 125 53 53 404 404 150 181

70 85 85 250 60 68 270 416 120 197

62.0 102.8 114.3 172.3 61.8 62.5 468.1 468.1 160.4 201.1

92.3 160.8 131.5 353.0 82.3 105.8 356.7 464.5 134.6 308.1

10.21 15.24 32.90 43.89 9.82 10.15 38.37 38.37 17.25 27.49

16.05 57.20 34.71 61.27 13.69 17.20 56.20 52.85 21.91 67.16

20 40 110 125 27 27 116 116 53 95

55 165 95 149 53 52 250 185 50 211

0.1647 0.1483 0.2880 0.2548 0.1591 0.1625 0.0820 0.0820 0.1075 0.1367

0.1740 0.3559 0.2640 0.1736 0.1665 0.1627 0.1576 0.1138 0.1628 0.2180

Cycles

(A) 500

Min E

450 400 350 300 250 200 150 100 50 0

Min C Mean E Mean C

M4 M7

M8 4

5

6

7

8

M2 M9 M3

9 10 11 12 13 Water content (%)

M5M6

M1 14

15

16

17

18

Cycles

(B) 500

450 400 350 300 250 200 150 100 50 M7 M4 0 24 25

M1 M9 M3M2 26

27

28

29 30 31 32 Total porosity (%)

M8 M5M6 33

34

35

36

Cycles

(C) 500 450

Fig. 7. Ribs. Before and after frost tests.

M9

200 M9

Cycles method E

M4

400 350 300 250 200 150 100 50 M4 M7 0 50 55

M8

M3 M9 60

65 70 75 80 Pores 0,25-1,4 m (%)

M5M6 M1 M2 85

90

95

Fig. 9. Frost durability versus water content, total porosity and pores 0.25–1.4 lm.

M4 M3

M2

100 M2 M5 M3 M6 M1 M5 M1

Minimum Mean Quadratic Linear

M6

0 0

100

200 Cycles method C

300

400

Fig. 8. Comparison methods C and E (Tiles without surface finishing. Damages: delamination, crack, chip).

clays containing CaCO3, such as M1, M5, M6 and M8 clays, form calcium and magnesium silicoaluminates gehlenite, anorthitic plagioclases, akermanite, in smaller proportion mullite, diopside and wollastonite from temperatures of 950 °C upwards while calcium phases disappears and illite decreases, providing a poor frost durability. Clays poor in CaCO3, such as M4 and M7, which give a very simple mineralogical composition (quartz, feldspars, mullite, diopside and Fe-oxyhydroxides) improve the frost durability. Fig. 10 shows the number of cycles versus the CaO content, it could be seen that the freeze resistance increases when the CaO content de-

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in the pressing and control of firing temperatures), which may double the frost performance with independence of the test used to measure it.

500 450 Mean C Mean E

400

Cycles

350

6. Conclusions

300 250 200 150 100 50 M4 M7

0 0

M5 M6 M1

M3 M9 M2

5

CaO content

10

M8

15

Fig. 10. Frost durability versus CaO content.

The test method E seems to be more severe and selective than the method C, showing smaller dispersion and standard deviation and frequencies distribution closer to the normality. The degree of cracking increased with the number of cycles, and damage was greater using test method E, resulting in earlier failure. The damage generally occurred in the form of delaminations, cracks and chips. The difference between C and E test methods may be explained due to: – Method E provides more water absorption after soaking than method C, especially when tiles have a water repellent treatment. – The different kinetic of the cooling process (faster cooling rate and prolonged freezing time of method E). – The presence of cloth on one side of the tiles, which prevents liquid movements in one way or another, thus inducing an increase in the internal liquid pressure, this effect is negligible and may be ignored when tiles have the pores blocked due to a water repellent treatment because it does not contribute with any additional strain. The effect of raw materials, clay mineralogy, microstructure and manufacturing parameters, for the clay tiles tested, shows:

Fig. 11. Exfoliation after extrusion – sheets separated by hand.

creases. Model M8 has a better behaviour than it could be foreseen, perhaps due to the higher firing temperature. The total amount of phyllosilicates vary in a narrow range and the comparison of X-ray diffraction patterns of raw materials and fired clay does not show a clear difference which can explain the different behaviour on frost resistance of the tiles. This fact leads to the conclusion that the main factors which affect the frost resistance are: – fired clay porosity, such as it has been previously discussed; – the layered fabric of the phyllosilicates (illite, kaolinite, chlorite) of M1, M5 and M6 models which cause marked exfoliation in the tiles after extrusion, allowing to separate the layers by hand (Fig. 11) may contribute to the poor frost resistance of these models. The comparison between models M3 an M9 shows the effect of the control focused on the process parameters which are common causes of frost failure (extrusion vacuum, correct flow of material

– Frost durability is substantially affected by the fired clay porosity. It decreases when the total porosity and the water content after soaking increases. Tiles with pore sizes in the range 0.25–1.4 lm suffer from more frost damage. – Frost resistance is also affected by the mineral composition of the clay. Clays poor in CaCO3 improve the frost durability. The worst frost resistance founded may be explained due to the layered fabric of the phyllosilicates in their composition, which cause marked exfoliation in the tiles after extrusion. – The control focused on the extrusion vacuum, correct flow of material in the pressing and firing temperatures may double the frost resistance. – The pressing process seems to increases the frost resistance, although cracks in the ribs due to incorrect flow of clay in the moulds may cause an earlier failure. References [1] Sadunas A, Bure D. Water migration processes in heavy clay ceramics under cycling freezing-thawing. Ind Ceram (Italy) 2000;20:153–9. [2] Ikeda K, Kim H, Kaizu K, Higashi A. Influence of firing temperature on frost resistance of roofing tiles. J Eur Ceram Soc 2004;24:3671–7. [3] Sveda M. Frost resistance of brick knowledge about the relationship between pore structure and frost resistance is the first step to the production of high frost-resistance brick products. Am Ceram Soc Bull 2001;80:46–8. [4] Sveda M. Effect of water absorption on frost resistance of clay roofing tiles. Br Ceram Trans 2003;102:43–4. [5] Hansen W, Kung JH. Pore structure and frost durability of clay bricks. Matér Constr/Mater Struct 1988;21(126):443–7. [6] Setzer MJ. Mechanical stability criterion, triple-phase condition and pressure differences on matter condensed in a porous solid. J Colloid Interface Sci 2001;235:170–82. [7] Setzer MJ. Micro-ice-lens formation in porous solid. J Colloid Interface Sci 2001;243:193–201. [8] Litvan GG. Phase transition of adsorbates: VI. Effect of deicing agents on the freezing of cement paste. J Am Ceram Soc 2001;58(1-2):26–30. [9] Wardeh G, Perrin B. Freezing-thawing phenomena in fired clay materials and consequences on their durability. Constr Build Mater 2008;22(5):820–8. [10] Sánchez de Rojas MI, Marín FP, Frías M, Rivera J. Viability of utilization of waste materials from ceramic products in precast concretes. Mater Constr 2001;51(263-264):149–61.

M.I. Sánchez de Rojas et al. / Construction and Building Materials 25 (2011) 2888–2897 [11] Sánchez de Rojas MI, Marín FP, Frías M, Rivera J. Properties and performances of concrete tiles containing waste fired clay materials. J Am Ceram Soc 2007;90(11):3559–65. [12] Maage M. Frost resistance and pore distribution in bricks. Matér Constr/Mater Struct 1984;17(1016):345–50. [13] Hamilton KA. Proposed European standards for clay and concrete roofing tiles. Br Ceram Trans J 1991;90(1):34–6. [14] Marín FP, Sánchez de Rojas MI. Standard test method to determine the performance of tiled roofs to wind driven rain. Mater Constr 2008;58(291):111–7. [15] Standard EN 1304:2005. Clay roofing tiles and fittings – product definitions and specifications. [16] Standard EN 539-2:2006. Clay roofing tiles for discontinuous laying – determination of physical characteristics – part 2: test for frost resistance. [17] Jordán MM, Boix A, Sanfeliu T, De la Fuente C. Firing transformations of cretaceous clays used in the manufacturing of ceramic tiles. Appl Clay Sci 1999;14:225–34. [18] Ravaglioli A. Evaluation of the frost resistance of pressed ceramic products based on the dimensional distribution of pores. Trans Br Ceram Soc 1976;75:92–5.

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