Latest revision as of 12:27, 24 August 2020

Abstract. The detachment of ceramic tiles in buildings is a problem that still persists in several modern constructions, despite various advances achieved in the sector of façade technology and an increasingly demanding consumer public. Studies have shown that most ceramic failures in façades occur between the interface mortar and ceramic tile.

This work intends to contribute to the study and understanding of the influence of the contact area on the interface bonding between the adhesive mortar and ceramic tile by means of the adhesion resistance evaluation from the crack propagation test in mixed mode of stress (MMF) which simulates tensile and shear forces simultaneously. A punctual progressive load was applied to a considerable number of specimens in order to promote internal tensile and shear forces. The variables incorporated into the experiment were the type of mortar employed and the mortar thickness in the specimen.

The experimental results show that the bonding strength values found by the direct drive test were 60.8% (AC II) and 58.1% (AC III) higher than the results obtained by the mechanical test in mixed mode, indicating an overestimated system result of the façade covering when the direct tensile bonding strength was verified exclusively.

Keywords: Ceramic tiles, Façade, Mixed Mode Flexure, Bond.

1 Introduction

The technology involving tiled-coated façades in Brazil; dates back to the 17^th century, according to Silva (2013), when ceramic tiles were brought from Portugal, France and Germany (Freitas et al., 2014). 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, among others. For Costa e Silva (2001), 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 the aggressiveness of the environment (Souza et al., 2018).

A literature review (Crocombe et al., 2006; Liljedahl et al., 2006 and Galbusera et al., 2014) reveals that only a residual number works analyses the adhesion resistance of tiled-coated façades using the Mixed Mode Flexure (MMF) test (Liljedahl et al., 2006). This test method is closer to the reality observed in building façades, since it considers, simultaneously, a combination of tensile (Double-Cantilever Beam DCB) and shear (ENF - End-Notched Flexure) modes, as verified in the literature (Galbusera et al., 2014). 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), the mortar thickness and the contact extension, through experimental laboratory studies employing simultaneous evaluation of the tensile and shear strength (MMF test).

2 Experimental setup

This experimental research consists on simulating situations encountered in real field during the application of ceramic coverings, especially in façade elements (see Figure 1). In this sense, the experiment was carried out with prismatic specimens (4x16 cm²) composed by two ceramic plates (of the same characteristics) filled with adhesive mortar (AC II or AC III) employing two different thicknesses (2, 4 and 7 mm) which were used to assess the performance when subjected to simultaneous tensile and shear stress using the MMF method.

Figure 1. Structure of the coating system.

The characteristic 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 (2007) and a profilometry analysis was carried out according to NBR ISO 4287 (2008). The results of the ceramic covering absorption tests used showed an average absorption of 3.5%, i.e., the ceramic tile employed in the present study is classified as BIIa. Table 1 presents the results of the average roughness (Ra) of the ceramic covering surface in the 3 conditions established.

Comparing samples impregnated with mortar, a lower surface roughness value (Ra=0.892 μm) was observed for ceramic tile with AC III, than with AC II (Ra=1.558 μm), which may indicate a better interlocking efficiency of AC III type mortars. The characterization of the bonding mortar was performed under laboratory conditions, as described by NBR 14081 (2012). The results of open time bonding strength in AC II showed that the mortar presented a satisfactory performance in relation to open time. The results for AC III mortar show that the mortar tested also met the minimum performance criteria described by NBR 14081 (2012). The ruptures occurred predominantly in the mortar-tile region, but also occurred at the substrate-mortar interface. Table 2 shows the results obtained for AC II mortar tensile strength under normal curing conditions at 28 days. All ruptures occurred at the mortar-tile interface. The AC III adhesive mortar reached an overall average of more than 1 MPa, with 100% of the ruptures at the mortar-tile interface. 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 AC II mortar.

3 Results and discussion

3.1 Influence of failure extension

Figures 2 and 3 show the most representative curves for each group as a function of the displacement obtained in the MMF test, accompanied by a figure indicating the rupture loads of the four types of failure extensions. In general, the highest values of maximum load are verified in the samples without failures, followed by those with 5, 10 and 20mm, 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 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. It is possible to observe that the bonding strength decreased to 44% when the contact area was reduced by 14%, i.e., 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%.

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

Figure 3 - Fissure propagation curves for AM III, with a thickness of (a) 4mm and (b) 7mm.

Figure 4 shows a linear trend line, 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 (2017) update.

Figure 4 - Linear trend line of resistance loss due to loss of contact area in mortar AM II and AM III.

Table 3 shows the tenacity values found and the percentage differences between the values of AC II and AC III mortars. 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 (2017), which recommends the use of AC III bonding mortar for ceramic façade covering, except in some special situations.

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 performed in the laboratory (see Figure 5). 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 mechanical capacity of the bonding mortar when subjected to MMF test.

Figure 5 - MMF Test vs. Direct Traction Test.

3.2 Influence of bonding mortar thickness

For this evaluation, the tensile versus deformation graphs obtained in the mixed mode mechanical test (MMF) are also presented, 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 the decrease of adhesion of the joint, a fact already evidenced by other researchers, such as Nascimento (2013) for polymeric adhesives and Rêgo (2008) for cement adhesives. The same situation, which was observed in AC II mortars with a 20 mm failure, occurs with AC III mortar.

Figure 6 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.

In the groups with AC III mortar adhesive in which the mortar was 4 mm thick, represented by Figure 7, 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 (Rêgo, 2008 and Nascimento, 2013). The same fact observed in AC II mortars with a 20 mm failure occurs in AC III mortar.

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. (2001) and Nascimento (2013) propose an explanation based on interfacial tensions. Azevedo et al. (2018) 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 (2008) 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.

Figure 6 - Maximum force reached for AC II with a failure of (a) 0 mm, (b) 5 mm, (c) 10 mm and (d) 20 mm.

Figure 7 - Maximum force reached for AC III with a failure of (a) 0 mm, (b) 5 mm, (c) 10 mm and (d) 20 mm.

4. Conclusions

This research intends to contribute to the study of the influence of the contact area on the adhesion of the bonding mortar-ceramic 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.

In resume, the following conclusions were obtained:

• 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;

• 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;

• 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;

• As the thickness of the adhesive increased there was a significant resistance decrease in both mortars, confirming the results of Rêgo (2008) for cementitious adhesives.

ORCID

Anne C. Melo: https://orcid.org/0000-0002-1275-9538

Ângelo J. Costa e Silva: http://orcid.org/0000-0002-0759-6439

Sandro M. Torres: http://orcid.org/0000-0003-3738-0935

João P.M.Q. Delgado: http://orcid.org/0000-0002-1026-4523

António C. Azevedo: http://orcid.org/0000-0003-4530-9880

References

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