Influence of the contact area in the adherence of mortar – Ceramic tiles interface

Construction and Building Materials 243 (2020) 118274

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Influence of the contact area in the adherence of mortar – Ceramic tiles interface A.C. Melo a, A.J. Costa e Silva b, S.M. Torres a, J.P.M.Q. Delgado c,⇑, A.C. Azevedo d a

Universidade Federal da Paraíba (UFPB), Rod. Gov. Antonio Mariz – Conj. Pres. Castelo Branco III, João Pessoa, 58033-455 Paraíba, Brazil Universidade Católica de Pernambuco (UNICAP), Rua do Príncipe, 526, Boa Vista, Recife, Pernambuco 50050-900 Brazil c CONSTRUCT-LFC, Faculty of Engineering (FEUP), University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal d Instituto Federal de Ciências de Educação e Tecnologia de Pernambuco (IFPE), Estrada Alto do Moura, KM 3,8, S/N, Distrito Industrial III, Caruaru, Pernambuco 55040-120 Brazil b

h i g h l i g h t s
Adhesion resistance of tiled-coated façades using the Mixed Mode Flexure (MMF) test.
MMF tests versus Pull-off tests.
Adhesive thickness versus adhesive failure at the adhesive mortar-ceramic tile interface.
Influence of adhesive mortar, thickness and area on adhesion resistance of tiled-coated façades.

a r t i c l e

i n f o

Article history: Received 15 March 2019 Received in revised form 5 August 2019 Accepted 23 January 2020

Keywords: Ceramic tiles Façade Mixed mode flexure Bond

a b s t r a c t The detachment of ceramic tiles in buildings is a problem that still persists in several modern constructions, despite various advances achieved. In this work, the adhesion resistance was evaluated from the crack propagation test in a mixed mode of stress (MMF test), which simulates tensile and shear forces simultaneously. A punctual progressive load was applied to 120 specimens in order to evaluate the influence of several variables: type of adhesive mortar, mortar thickness and failure extension. The results show that the bonding strength at the interface decreases as the adhesive failure increases. The bonding strength values found by the direct-drive test were higher than the results obtained by the MMF test, indicating an overestimation. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction The technology involving tiled-coated façades in Brazil; dates back to the 17th century, according to Silva [1], when ceramic tiles were brought from Portugal, France and Germany [2]. Nowadays, this technology widely spread and improves is present in all countries and in different parts of the world. The ceramic tiles are one of the main alternatives for the protection of façades in the national scenario, mainly in the coastal regions of the country, due to advantages like waterproofing, durability, real estate value, thermal and acoustic comfort, among others. For Costa e Silva [3], the external surface coatings, especially in façades, are an exterior indicator of the intrinsic value of the property. The various factors that affect the durability of buildings and components can be subdivided into two categories: the first one related with the durability of the system; and the second one with ⇑ Corresponding author. E-mail address: [email protected] (J.P.M.Q. Delgado). https://doi.org/10.1016/j.conbuildmat.2020.118274 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

the aggressiveness of the environment. Durability is the ability of a building or its components to present the best performance when subjected to adverse environmental exposure conditions, without needing repair or replacement of its elements. On the other hand, the degradation is a consequence of the natural ageing process, associated with exposure to adverse environmental conditions [4]. The large-scale industrial production of ceramic tiles and the development of Portland cement-based adhesives are the two main aspects that have contributed to the technological development of these coverings for façades. However, even though there are a number of specific techniques for preventing the detachment of materials on façades, the detachment of ceramic tiles in buildings is still a problem that persists in several modern constructions, especially when not subject to specified periodic maintenance [2,5]. According to Gaspar and Brito [6] the breakdown occurs due to the loss of the material’s ability to respond to the demands over time, against deterioration agents, the material characteristics (depends on the production unit, materials employed and variations of the production method) and, in some

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cases, due the mortars used in reattachment of the old tiles (mortars must follow compatibility, durability and reversibility criteria) [7]. The ageing of materials and the external aggressive agents conduct to a function loss of some constituent materials over time leading to lack of mortar adhesion, lack of tile mortar/interface adhesion and gaps in glaze part of the tile. Silvestre and Brito [8] showed that ceramic tiling is one of the noblest and most versatile systems of cladding in buildings in Mediterranean and Latin countries, due to the singular importance that was attached to it throughout the centuries. Given the importance of using this type of material, the authors developed a system for inspection and diagnosis of defects in an adhesive ceramic wall or floor tiling, specifically developed for recent buildings. The adherent ceramic tiling systems, among other aspects, is the cladding solution most sensitive to the workmanship quality during the application (namely, unclean surfaces, incorrect bonding of the tiles and incorrect mixing of the bonding material), quality of materials used and precision of the building frame, when compared with cementitious and synthetic finish stuccos and stone [9]. A literature review [10-16] of the most previously published research works in this subject area reveals that only a residual number works analyses the adhesion resistance of tiled-coated façades using the Mixed Mode Flexure (MMF) test [15,16]. This test method is closer to the reality observed in building façades, since it considers, simultaneously, a combination of tensile (DoubleCantilever Beam DCB) and shear (ENF - End-Notched Flexure) modes, as verified in the literature [14–16]. A comparison with the results obtained with the pull-off test, widely used in several countries around the world, is presented. Another objective of this work is the evaluation of the factors that influence the adhesion between ceramic coverings and adhesive mortars, namely the type of adhesive mortar (AC II and AC III) in joint bonding, the thickness influence of the adhesive mortar layer and the contact extension (or area), through experimental laboratory studies employing simultaneous evaluation of the tensile and shear strength (MMF test). 2. Methodology This experimental research consists on simulating situations encountered in real field during the application of ceramic coverings, especially in façade elements. In this sense, the experiment was carried out with prismatic specimens (4  16 cm2) composed by two ceramic plates (of the same characteristics) filled with adhesive mortar (AC II or AC III) employing three different thicknesses (2, 4 and 7 mm) which were used to assess the performance when subjected to simultaneous tensile and shear stress utilizing the MMF method. All materials employed were also characterized for comparison purposes with similar products found on the market. It is also important to highlight some limitations that were not considered:
Recognizing the few precedents of the test method used for the proposed objective, a greater quantity of variables was adopted in detriment of a larger number of samples for each case in order to better understand the sensitivity of the method selected for the desired investigation. This made it impossible to carry out a more detailed probabilistic study;
The field analysis was performed only to identify the main variables to be adopted in the experiment (no work trials were performed);
Only one type of ceramic tile (group BIIa) was used for the preparation of the specimens, so the influence of water absorption on the joint was not verified. The bonding mortars for both types (AC II and AC III) were from the same supplier;


The influence of exterior conditions (relative humidity, temperature, dehydration, etc.) in the results was not evaluated, because the samples were cured in laboratory conditions;
The direct and mixed-mode adhesion strength tests for all test specimens used in this study were performed 28 days after preparation. Hence, curing time was not considered in this study. 2.1. Theoretical framework and concepts 2.1.1. Coverings This section will cover the following topics: composition of a ceramic cladding system, material properties, and stress acting upon the cladding system. Cladding systems are generally layered and can be made of diverse materials, with varying textures and shapes. In the metropolitan region of Recife, the semi-stoneware ceramic cladding, known commercially only as ceramic coating, is the most used material for the façade covering due to its efficiency in the protection of the peripheral masonry, its refractory potential, and the lower cost factor when compared to other materials such as porcelain, granite and composite aluminium panels. The coating layers are initially formed by a base or substrate, which may be concrete, masonry, etc. Then, there is a layer of spatter-dash, scratch coat, adhesive mortar or cement paste and, finally, the ceramic covering, the joints of which are filled with grout. It is extremely important that this joint behaves monolithically, i.e. as a single unit, thus minimizing the risks of pathologies that may compromise the system function. Junginger [17] describes the main function of the coating system components (see Fig. 1): - The base: the surfaces on which the layers of the coating are applied, according to NBR 13755 [18]; - The preparation layer or spatter-dash which is responsible for providing the required adhesion between the smoothing layer and the base; - The scratch coat is intended to regularize the application surface to leave it smooth and without undesirable depressions and protrusions; - The bonding layer: the main element used in this layer is the industrialized adhesive mortar which keep the ceramic tiles indirectly adhered to the substrate; - The ceramic tiles, which normally have joints filled with grout in order to guarantee beauty and façade waterproofing.

Fig. 1. Structure of the coating system.

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3. Experimental Set-Up 3.1. General aspects This experimental study was developed, starting with the confection of prismatic specimens (4  16 cm2), where two ceramic tiles were bonded in parallel with industrialized adhesive mortar, which were used to evaluate the performance under simultaneous tensile and shear stress in the Mixed Mode Flexure (MMF). In order to simulate the real field conditions, the following variables were adopted:
Type of adhesive mortar: AC II and AC III;
Thickness of the mortar: 2 mm, 4 mm and 7 mm;
Bonding mortar filler failure size: 0 mm (reference), 5 mm, 10 mm and 20 mm.

Table 1 Variables considered in the experiment.

Size of failure (mm)

Type of adhesive mortar (AM)

Thickness of the adhesive mortar (mm)

0 (reference)

AM II

2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7 2 4 7

5

AM II

AM III

10

AM II

AM III

20

AM II

AM III

Table 1 presents a description of the analysed variables which totalled 24 study groups. For each group, 5 specimens were prepared, resulting in 120 specimens tested. For a better understanding of the results, materials characterization tests were also performed as follows:
Ceramic tiles - Water absorption and profilometry;
Bonding mortar - Time open and bond strength. Regarding the failure size, it is important to point out that this problem reflects the errors commonly observed in the application of ceramic tiles, especially in façades. Therefore, the extent of adherence of the samples in this research is presented in four different ways. Fig. 2 (A) shows the condition without induced failure, (B) with induced failure of 5 mm, (C) induced failure of 10 mm and (D) induced failure of 20 mm. 3.2. Characterization tests

Variables

AM III

3

Identification of groups

AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM AM

II 2–0 II 4–0 II 7–0 III 2–0 III 4–0 III 7–0 II 2–5 II 4–5 II 7–5 III 2–5 III 4–5 III 7–5 II 2–10 II 4–10 II 7–10 III 2–10 III 4–10 III 7–10 II 2–20 II 4–20 II 7–20 III 2–20 III 4–20 III 7–20

3.2.1. Ceramic tiles Investigation of the characteristics of the tile back surface is an important item for bonding assessment. For the present study, water absorption tests were performed according to NBR 13818 [19] and a profilometry analysis was carried out according to NBR ISO 4287 [20]. In this work only one ceramic coating was used and the characteristics of this ceramic coating are presented in Table 2. 3.2.1.1. Absorption test. Table 3 shows the results of the ceramic covering absorption tests used in this study and performed in the Building Materials Laboratory of the Federal University of Paraíba. It is possible to observe an average absorption of 3.5% and according to NBR 13818 [19] classification the ceramic tile used is classified as BIIa. 3.2.1.2. Profilometry. Table 4 presents the results of the average roughness (Ra) of the ceramic covering surface in the three conditions established:
Condition (1): ceramic tile without mortar;
Condition (2): ceramic tile with AC II;
Condition 3: ceramic tile with AC III.

Fig. 2. Sample contact area: (A) without induced failure, (B) with induced failure of 5 mm, (C) with induced failure of 10 mm and (D) with induced failure of 20 mm.

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The profile readings were made in three different directions: horizontal, vertical and diagonal. To execute the test, ceramic pieces were selected from those whose rupture during the mechanical test in mixed mode occurred at the back of the adhesive mortar interface in order to verify to what degree the pores were filled, which denotes physical (or mechanical) anchorage. The results obtained showed higher average surface roughness in the case of tiles without mortar, as expected due to their manufacturing method. It should be noted that the average value found for the parameter Ra (4.87 lm) was compatible with the data obtained in the study by Parra et al. [21] where the Ra value of 6.47 lm was found for pressed ceramics. It is important to say that the average roughness value found by [21] was superior, that is, the ceramic was even rougher, since it had not yet undergone the heat process, unlike the ceramic used in the present research. Comparing samples impregnated with mortar, a lower surface roughness value (Ra = 0.892 lm) was observed for condition 3, with AC III, than in condition (2), with AC II (Ra = 1.558 lm), which may indicate a better interlocking efficiency of AC III type mortars, which also reflects the higher adhesion strength encountered with this material according to Fig. 3.

Table 2 Characteristics of ceramic coating. Features Format Thickness Class of friction Water absorption class Abrasion resistance

60x60 cm2 8.5 mm I (0.4) BIIb (semi stoneware) 3 (high average)

Table 3 Absorption test results. Ceramic format (20  20 cm)

Dry weight (g)

Saturated weight (g)

1 2 3 4 5 6 7 8 9

685 710 675 705 665 685 685 705 680 710 670 705 630 645 625 645 630 650 Average absorption (%):

Absorption (%) 3.60 4.40 3.00 2.90 4.40 5.20 2.30 3.20 3.10 3.50

Table 4 Results of the ceramic plate profile test. Condition (1)

Horizontal reading (mm)

Average (mm) Vertical reading (mm)

Average (mm) Diagonal reading (mm) Average (mm) Overall average (mm)

Condition (2)

Condition 3

Reference

Sample 1

Sample 2

Sample 3

Sample 1

Sample 2

Sample 3

3.900 6.430 3.990 4.773 6.790 5.160 5.120 5.690 3.730 4.580 4.155 4.873

1.990 2.260 1.890 1.558 1.960 1.880 1.630 1.618 2.220 1.870 1.498 1.558

1.410 0.745 1.020

1.550 1.940 1.220

1.320 0.735 0.842

1.160 0.962 0.996

1.650 1.580 1.080

2.020 1.130 1.630

0.992 1.090 0.770

1.100 0.651 0.944

1.450 1.050

1.220 1.180

0.655 0.891 0.844 0.934 0.686 0.781 0.590 0.845 0.794 0.814 0.898 0.892

1.010 1.010

0.787 0.970

Fig. 3. Profile ceramic coating (A) Condition (1): Ceramic tile without mortar; (B) Condition (2): Ceramic tile with AC II; (C) Condition 3: Ceramic tile with AC II.

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3.2.2. Bonding mortar As previously mentioned, the characterization of the bonding mortar was performed under laboratory conditions, as described by NBR 14081 [22].

3.2.2.1. Open time. Table 5 shows the results of open time bonding strength in type II bonding mortar in laboratory test performed according to [22]. Of the ten samples tested, two were discarded as they presented a deviation of more than 20% from the overall average. The results showed that the mortar presented a satisfactory performance in relation to open time, surpassing the values established in the aforementioned standard. The rupture modes occurred between the substrate-mortar and mortar-ceramic tile interfaces. The open time results for type III bonding mortar are shown in Table 5 which show that the mortar tested also met the minimum performance criteria described by [22]. The ruptures occurred predominantly in the mortar-tile region, but also occurred at the substrate-mortar interface. Only one result was discarded from the final average calculation. Interfacial adhesion is an important property for the coating/substrate structures, since poor interfacial adhesion often results in detachment of the coating from the substrate and therefore system failure. Consequently, a quantitative evaluation of the coating/substrate adhesion strength is critical to the system design [23]. The results of the mortars AC II and AC III used in this research showed that both mortars presented satisfactory results, with an open time greater than 20 min, with a tensile strength greater than 0.5 MPa. As expected, the result of the test performed with AC III mortar obtained a higher tensile strength value than the AC II mortar test.

3.2.2.2. Tensile strength. Table 6 shows the results obtained for AC II mortar tensile strength under normal curing conditions at 28 days. All ruptures of this test occurred at the mortar-tile interface. The AC III adhesive mortar reached an overall average of more than 1 MPa (minimum established by NBR 14081 [22], with 100% of the ruptures at the mortar-tile interface, as shown in Table 6. As in the open-time test, the tensile strength values were higher in the case of AC III mortar, confirming the hypothesis that the mechanical and chemical adhesion of this mortar tends to be higher than of AC II mortar. 4. Results and discussion 4.1. Mechanical test of mixed mode flexure (MMF) For a better understanding of the results that will be shown below, a model graph (see Fig. 4) will be used to exemplify each stage of the curve:
(+) Initial stage in which the load cell of the machine progressively begins to exert force on the specimen;
(j) Region showing the increase of the applied force and the displacement of the specimen in the direction of the force;
(d) Stage in which the bond between the mortar and the ceramic coating is broken, producing a macro fissure that causes the applied force to fall drop sharply. This value indicates the highest system mechanical capacity when subject to simultaneous tensile and shear stresses;
( ) The moment in which the load transfer to the ceramic tile begins;

Table 5 Results of adhesive mortar II and III – Open time. Ceramic plate

Adhesive mortar II Load (N)

1 2721.60 2 2589.74 3 2423.55 4 2258.04 5 1994.33 6 1786.24 7 2250.49 8 2115.20 9 2190.05 10 2139.23 Overall average (MPa): Open time: 20 min

MPa

1.09 1.04 0.97 0.90 0.80 0.71 0.90 0.85 0.88 0.86 0.90

Adhesive mortar III Interface (%)

Load (N)

Substrate/Mortar

Mortar/Ceramic plate

10 0 10 60 80 80 80 70 10 10

90 100 90 40 20 20 20 30 90 90

2203.79 2958.53 2936.55 2819.12 2142.67 2218.90 2950.97 2922.13 2797.83 2067.12

MPa

0.88 1.18 1.17 1.13 0.86 0.89 1.18 1.17 1.12 0.83 1.04

Interface (%) Substrate/Mortar

Mortar/Ceramic plate

0 0 0 10 70 70 0 0 0 0

100 100 100 90 30 30 100 100 100 100

Table 6 Results of adhesive mortar II and III – Adhesion strength to direct traction. Ceramic plate 1 2 3 4 5 6 7 8 9 10 Overall average (MPa): Open time: 20 min

Adhesive mortar II Load (N) 2578.07 2187.99 2421.49 1998.45 2407.75 2150.22 2370.67 1843.93 2668.03 2882.30

MPa 1.03 0.88 0.97 0.80 0.96 0.86 0.95 0.74 1.07 1.15 0.94

Adhesive mortar III Load (N) 2939.99 2637.13 3350.66 3044.37 3251.08 3258.64 2775.85 2874.74 3500.38 3196.14

MPa 1.18 1.05 1.34 1.22 1.30 1.30 1.11 1.15 1.40 1.28 1.23

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Fig. 4. Stages of the crack propagation curve.


(▲) Collapse of ceramic tile (end of test). It is important to mention that even in the stage represented by the triangle; the ceramic tile still remains attached to the adhesive mortar, except for some specimens where it is possible to verify the displacement of one of the fractured parts as shown in Fig. 5. The line identified in the image was drawn in order to highlight the place where the ceramic plate broke. For a better comprehension of the experimental data, this topic will be presented by a figure of the most representative curves for each group as a function of the displacement obtained in the mechanical test in the mixed mode (MMF), accompanied by a figure indicating the rupture loads of the four types of failure extensions. 4.2. Influence of failure extension In general, the highest values of maximum load are verified in the samples without failures, followed by those with 5, 10 and 20 mm, respectively. Such behaviour evidences the influence of the contact mortar extension on the adhesion, as expected. It is interesting to note, however, that the differences become more

Fig. 5. Group AM III 7-20 after MMF test.

expressive beginning with failures of 10 mm, which correspond to a contact loss of approximately 7%. Another interesting aspect verified is the higher load values found with AC III compared to AC II, which will be discussed in section 4.2.2. The results are presented in Figs. 6 and 7. Tables 7 and 8 show the percentages of increase or decrease of the bonding strength for each extension of adherence analysed. In the first table, as can be observed, the bonding strength decreased to 44% when the contact area of the mortar with the ceramic was reduced by 14%, in other words, when a 20 mm failure was induced. In the test specimens made with AC III mortar, the adhesion reduction was even more abrupt, reaching values up to 51%. In all cases, the greatest reduction of adhesion between the bonding mortar and the ceramic occurred when the induced failure was greater than 20 mm, as in this case. Fig. 8 shows a linear trend line based on the data of Table 7, where the loss of tensile strength due to loss of bonding area of the ceramic tiles is verified. The results indicate that the loss of tensile strength increases progressively with the reduction of the contact area between the adhesive mortar and the ceramic tile. This data reinforces the need to control, in an incisive way, the percentage of failures that occur during the application of ceramic tiles, as well as attest the assertiveness of the NBR 13755 [18] update. This data also stresses the need to recommend these checks and controls during the system execution.

4.2.1. Influence of the bonding mortar The purpose of this sub-section is to discuss the influence of the type of bonding mortar to be submitted to simultaneous tensile and shear stresses on the mechanical behaviour of the covering system. For a better visualization, the data is grouped together with the force versus displacement curve of both types of adhesive mortar for each thickness together with the graph of the maximum loads found, with information on the percentage of improvement obtained (see Fig. 9). Table 8 shows the tenacity values found and the percentage differences between the values of AC II and AC III mortars. Then, in order to facilitate a comparative complementary analytical evaluation, the force required for the fracture of the bond at the interface between the bonding mortar and the back of the ceramic tiles was calculated, beginning with the area under the strain-versus-deformation curve which was obtained in the mixed mode mechanical test (MMF), shown in Fig. 10. The observed values reinforce the established concept that the type of bonding mortar used generates an impressive improvement in the system bonding, with values that can reach up to 44% of maximum load and 48% of tenacity. These results, therefore, reinforce the importance of the type of bonding mortar and the assertiveness of the update of NBR 13755 [18], which recommends the use of AC III bonding mortar for ceramic façade covering, except in some special situations. It is important to say that the numerical tenacity value was obtained using the area under the curve of the samples tested. For this, the software AutoCAD Civil 3DÒ was initially used to define the curve and AutoCAD 2DÒ software to measure the area. Finally, another interesting analysis concerning the type of bonding mortar is the comparison between the maximum loads values found, without failures, in the three different thicknesses studied, with the result of bonding strength to direct traction (RATD) performed in the laboratory (see Fig. 10). In all cases, a loss of mechanical capacity of 66% and 61% (AC II and AC III, respectively) of the bonding mortar was observed when submitted to the mixed load test as compared to direct tension testing. This behaviour reinforces the need to know in more detail the mechan-

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Fig. 6. Fissure propagation curves for AM II, with a thickness of (a) 2 mm, (b) 4 mm and (c) 7 mm.

ical capacity of the bonding mortar when subjected to mixed tensile and shear stresses. 4.2.2. Influence of the bonding mortar thickness For this evaluation, the tensile versus deformation graphs obtained in the mixed mode mechanical test (MMF) are also pre-

sented, together with a bar graph containing the maximum loads for the two types of bonding mortar and the different contact failures studied In the groups with AC II mortar as the adhesive, a tendency of a better system performance when the mortar thickness is 4 mm can be observed. The increase in the thickness of the mortar resulted in

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Fig. 7. Fissure propagation curves for AM III, with a thickness of (a) 2 mm, (b) 4 mm and (c) 7 mm.

the decrease of adhesion of the joint, a fact already evidenced by other researchers, such as Nascimento [24] for polymeric adhesives and Rêgo [25] for cement adhesives. The same situation, which was observed in AC II mortars with a 20 mm failure, occurs with AC III mortar. The results of the maximum forces reached are numerically very close, inferring that for this type of CP, as the failure increased, the mortar thickness no longer had a significant influence factor.

Fig. 11 shows a comparison of the behaviour between the type of failure and the thickness of the adhesive when using AC II mortar. A predominant characteristic among the groups presented is the fact that the samples with the highest mortar thickness (7 mm) reached the lowest values of adhesion strength; except for the group with a 20 mm failure where the thickness variation did not indicate a significant change in the behaviour of the curves or their values.

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A.C. Melo et al. / Construction and Building Materials 243 (2020) 118274 Table 7 Increase or decrease of resistance due to the extension of adhesion, of adhesive mortar II and III. Failure (mm)

Loss area (%)

Adhesive mortar II 0 0 5 4 10 7 20 14 Adhesive mortar III 0 0 5 4 10 7 20 14

Thickness 2 mm

Thickness 4 mm

Thickness 7 mm

Resistence (N)

Increase/decrease resistance

Resistence (N)

Increase/decrease resistance

Resistence (N)

Increase/decrease resistance

956 1055 931 609

Reference 10% 3% 36%

1015 989 853 566

Reference 3% 16% 44%

791 816 666 639

Reference 3% 16% 19%

1186 1095 1016 712

Reference 8% 14% 40%

1462 1169 1001 711

Reference 20% 32% 51%

1225 1029 813 634

Reference 16% 34% 48%

Table 8 Toughness for different thicknesses: 2 mm, 4 mm and 7 mm. Failure

0 5 10 20

Thickness 2 mm

Thickness 4 mm

Thickness 7 mm

Adhesive mortar II (N/mm)

Adhesive mortar III (N/mm)

Difference (%)

Adhesive mortar II (N/mm)

Adhesive mortar III (N/mm)

Difference (%)

Adhesive mortar II (N/mm)

Adhesive mortar III (N/mm

Difference (%)

85.3 98.5 82.5 67.6

107.4 101.8 98.2 74.9

25.9 3.4 19.0 10.7

90.4 87.9 72.0 57.3

106.5 109.4 98.9 80.0

18.0 24.1 37.1 40.0

60.0 60.9 65.6 53.3

88.4 90.2 72.1 68.7

47.0 48.1 10.2 29.1

Fig. 8. Linear trend line of resistance loss due to loss of contact area in mortar AM II and AM III.

In the groups with AC III mortar adhesive in which the mortar was 4 mm thick, represented by Fig. 12, a tendency of a better system performance is observed. The increase in the thickness of the mortar resulted in the decrease of the bonding of the joint, a fact already evidenced by other researchers [24,25]. The same fact observed in AC II mortars with a 20 mm failure occurs in AC III mortar. The results of the maximum forces reached are numerically very close, inferring that for this type of CP, as the failure increased, the mortar thickness no longer had a significant influence factor. In experimental testing performed previously - those results are not presented in this research - a 40 mm failure was used. However, the specimens with the dimensions established in this project, 16x4 cm2, did not present satisfactory behaviour in the mixed mode test (MMF) with this failure, since the more accentuated absence of mortar in this situation left a very pronounced void

in the specimen. Therefore, as the punctual load was applied, only the ceramic absorbed the force and broke prematurely before the curve was obtained (force times displacement). By virtue of this experiment, the maximum failure length used in this study was 20 mm. In general, the results showed a trend towards lower mechanical response in the samples with a thickness of 7 mm (the largest thickness applied in the present study). Gleich et al. [26] and Nascimento [24] propose an explanation based on interfacial tensions. The authors demonstrated that the normal and shear stresses at the interfaces and near the ends of the overlap zone increase with increasing thickness, unlike with the same stress components in the median bonding plane. The specimens with smaller adhesive thickness presented a more uniform distribution for both tension components, whereas the ones of greater thickness showed a pronounced increase at the interfaces. Thus, the aforementioned

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Fig. 9. Fissure propagation curves of AM III  AM II, with a thickness of (a) 2 mm, (b) 4 mm and (c) 7 mm.

authors concluded that the greater probability of internal defects in samples with greater thickness of the adhesive cannot be considered as an exclusive explanation for the phenomenon, although it can contribute to it. Azevedo et al. [27] presented that the firing temperature is a variable that directly influences the properties of the red ceramic where the bricks burned at 950 °C provided greater gain of resistance to the adhesion of traction due to the high initial absorption

index compared with the temperatures of 850 °C and 750 °C. Rêgo [25] considered the influence of temperature on the adhesion of ceramic materials of different colours bonded with industrialized bonding mortar under mixed mode of tensions and found that at elevated temperatures, the increase of thickness promoted reductions of adhesion resistance higher than 35% and 40% for systems with porcelain and semi-porous ceramic (BIIb) adhered with cementitious mortars, respectively.

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Fig. 10. MMF Test vs. Direct Traction Test.

4.2.3. Statistical analysis Due to the high number of variables, the quantitative and qualitative nature as well as the corresponding interactions, the analysis of isolated effects becomes quite complex. It is important to study the multivariable statistical significance, incorporating all the parameters of the experimental planning, and evaluate the statistical significance of these parameters and their interactions on the adhesion strength of the bonded systems [25]. The evaluation of the statistical significance of the different variables in the measured parameters was based on the analysis of variance. For this, the software OriginÒ was used, where the values of all the variables are correlated (in pairs) in order to find the most significant combination, finally represented as polynomial. In all the tests performed, the results with an assessment of parameter levels were considered significant. The maximum crack propagation force (F) can be described by the following equation:

F ¼ 1211:38  33:16  a  20:76  b þ 80:96  c ð  31:41Þ ð  5:84Þ ð  1:62Þ ð  12:00Þ

ð1Þ

where F is the bonding strength in MMF (N) mixed mode, a is the bonding thickness (mm), b is the crack size or failure (mm) and c is the adhesive class AC II (-1) e AC III (+1). In this model, the bonding thickness and the failure size act negatively (negative signals), while the type of adhesive contributes positively to the value of the bonding force. The thickness variable impacted more negatively than the crack thickness. A Fisher’s coefficient equal to 80.72 indicates that the quality of the overall model is significant. With respect to the tenacity, E (N/mm2), the results showed:

E ¼ 111  4:09  a  1:24  b þ 8:97  c ð  3:86Þ ð  0:72Þ ð  0:20Þ ð  1:47Þ

ð2Þ

with a Fisher’s coefficient equal to 36.11 and a correlation coefficient R2 = 0.875 (see Fig. 13). In both equations (Force and Tenacity), the same tendency is observed, i.e., the thickness and the size of the crack influence negatively the bonding strength of the joint subjected to the mixed mode of stresses, in the same way that the type of the adhesive (AC II or AC III) had a positive performance in the analysis.

It is also understood that the correlation of the coefficients can be improved by replacing the qualitative variable (variable c – adhesive class) by a quantitative variable, for example, the coefficient 8.97 which is associated with the mortar type parameter, whose qualitative values are 1 and +1, may be more significant if this parameter is replaced by some quantitative data that is related to bonding, such as perhaps the content of some chemical component present in the adhesive. This is a perspective not addressed in this research. It is important to be in mind that energy (E) refers to the elastic pre-damage capacity while the breaking force (F) refers to the energy required for the propagation of a crack of given size. Fig. 13 shows a good correlation between strength and tenacity values from the 24 groups of this study. 5. Conclusions This research intends to contribute to the study of the influence of the contact area on the adhesion of the bonding mortarceramic tile interface. In addition to the use of the MMF test, this paper also presents a critical analysis of the pull-off test, since the results obtained in this research showed that the force values necessary to provoke the system collapse are smaller in the MMF test when compared to the values obtained in the pull-off test. The results indicating that the evaluation by this method may be masking reality, pointing to values that the system could not support when submitted to conditions other than direct drive. Another interesting point to comment on is the relationship between the adhesive thickness and the adhesive failure at the adhesive mortar-ceramic tile interface. The results showed that when adhesive failure is very large (14%), the adhesive thickness doesn’t influence the system performance, i.e., the adhesive failure is the main factor of failure. However, for 4% and 7% adhesive failures, it is possible to observe, for all samples, a similar decrease in the required breaking strength for the highest adhesive thickness (7 mm) studied. This result shows that an excessive thickness of cementitious adhesive contributes to better performance.

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Fig. 11. Maximum force reached for AC II with a failure of (a) 0 mm, (b) 5 mm, (c) 10 mm and (d) 20 mm.

A.C. Melo et al. / Construction and Building Materials 243 (2020) 118274

Fig. 12. Maximum force reached for AC III with a failure of (a) 0 mm, (b) 5 mm, (c) 10 mm and (d) 20 mm.

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R2=0.875

Force (N)

1200 1000 800 600 400 40

50

60

70

80

90

100

110

120

Tenacity (N/mm²) Fig. 13. Strength versus Tenacity of the 24 groups studied.

In resume, the following conclusions were obtained: A. General aspects:
Test of propagation of the first fissure in mixed mode of tensions was seen to be applicable to determine the adhesion strength at the interface of ceramic coverings bonded with cementitious adhesives;
The bonding results obtained through the direct traction test are about 60% greater than the results obtained by the mixed mode test on AC II and AC III mortars; B. Influence of Failure Extension:
Adhesive strength at the bonding mortar-ceramic interface of the tested samples decreases as the failure of the bonding mortar increases, presenting a loss of adhesion of up to 44.2% and 51.4%, for AC II and AC III, respectively;
Test specimens made with 20 mm induced failure showed the greatest reduction of resistance for both AC II and AC III mortar, confirming the influence of the contact area on the adhesion strength of the bonded coverings;
C. Influence of the type of adhesive mortar:
Adhesion strength verified by both the direct tensile test and the first crack propagation test (mixed mode) presented higher results for the AC III bonding mortar in relation to the AC II mortar results; D. Influence of mortar thickness:
As the thickness of the adhesive increased, there was a significant decrease in the resistance in both mortars, confirming the results obtained by [25] for cementitious adhesives.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.