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Process engineering

Introduction

The sustainability of functional proper-ties of ceramic surfaces is increasingly an important issue. The European Construc-tion Products Regulation implemented in 2013 requires durability proof of essential product properties and the compilation of a performance declaration for CE-marked products (replacing the conformity dec lar-ation) for their economic life cycle [1]. The durability has to be declared or proven by a corresponding method, if available. Internationally implemented tests on slip resistance durability [2] however lack an objective evaluation of actual wear. Based on the growing awareness of the need for specification of long-term per-formance in the market, FGK/DE has per-formed customer-driven projects, focussed

on the investigation of practice-relevant wear simulation methods, aided by topog-raphy measurements as an objective char-acterization tool, to simultaneously analyse changes in slip resistance as well as clean-ing behaviour as a function of the actual wear on site.

Slip resistance – a “slippery” subject

The ever high number of slipping and trip-ping accidents, beside minor injuries even leading to invalidity or fatalities, also has a substantial economic impact due to re-habilitation and compensation measures. Numerous renowned health and safety organizations as well as testing institutes concern themselves with measurement and evaluation of slip risks in the laboratory and

on site, with a large variety of slip measure-ment methods (Fig. 1). Due to non-matching reference or calibra-tion materials and different slip mechanisms

The replacement of the European Construction Products Directive by the European Building Products Regulation in 2013 [1] has put more pressure on the manufacturers and users of flooring materials to concern themselves with the aspects of durability characteristics as slip resistance and cleaning. Experts as well as testing institutes are increasingly commissioned to evaluate the durability of slip resistance and cleaning behaviour as well as the actual soiling properties of tile surfaces during use. This paper addresses the background, the current status and the potential of new practice-based measurement and evaluation methods for slip resistance as well as the cleaning behaviour of tile surfaces and their durability based on surface topography measurements.

Marcel Engels

FGK Forschungsinstitut für Anorganische

Werkstoffe – Glas/Keramik – GmbH

56203 Höhr-Grenzhausen, Germany

E-mail: marcel.engels@fgk-keramik.de

www.fgk-keramik.de

Keywords: floor slip resistance, cleaning,

durability, surface topography

*Paper presented at Qualicer 2016

New Practice-Oriented Testing Possibilities Regarding the Durability of Slip Resistance and Cleanability*M. Engels

Fig. 1 Slip resistance test methods: the walking method on the ramp (l.), the dynamic coefficient of friction measurement with the GMG 200 (middle) and the pendulum test (r.)

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lished as a method to evaluate and control slip resistance on site under occupational conditions [4].These differences underline the need for a separate objective evaluation of slip resist-ance over the range of available surfaces, and especially for changing slip resistance due to surface wear.

Cleaning behaviour and susceptibility to soiling

Another aspect always incorporated in the discussion regarding increased slip re-sist ance is the cleaning behaviour of the surface. The general rule “the higher the slip resistance, the more cleaning effort is needed” puts extra pressure on meeting performance requirements. Staining char-acteristics are measured according to the standard (EN ISO 10545-14), with staining agents, not relevant in most problematic applications, especially for areas with fre-quent pedestrian traffic, thus in practice providing no differentiation of actual dif-ferences in soiling and cleaning properties. Therefore some manufacturers have already been applying own indicative manual tests with reference dirt. These results however are not accessible to the customers. FGK was commissioned by a large internationally operating discounter to initiate a new approach to this problem, initiated by complaints on large scale grey shading and locally visible mosaic effects in the tile surfaces in stores, although all tiles had passed the highest staining class (Fig. 2). The developed test addresses two aspects of the problem: besides the cleaning effort also the long-term soiling behaviour had to be investigated: cleaning with relative low effort, but necessary on a frequent basis is a complaint often formulated in assessments of unsatisfactory flooring performance. To address this aspect, a reference test based on recommendations of the German Indus-trial Association for Personal Hygiene and Detergents for testing all-purpose cleaning agents [5] has been adapted to the ceramic surface [6]. This reference test is based on a mixture of peanut oil, kaolin and soot in order to simu-late adhesive greasy soil with dirt particles as trodden in from the street, wear residues from shoe soles as well as shopping trol-leys and contaminations from groceries and

selection and the availability of these slabs is a constant discussion issue. Another drawback is that the method can be used only in the laboratory. The pendulum test, well established in Great Britain and in Spain, was developed originally to simulate the braking effects of car tires on roads. It is based on the de-celeration of a rubber slider, mounted on a spring-loaded slider unit on a pendulum arm which swings over a surface with a de-fined contact path. The arm overswing after contact is meas-ured. These values are compared to speci-fied thresholds, each indicating a different expected slip risk. The pendulum can also be used on site, but puts high demands on the operator experience and training. Smooth surfaces can be easily overestimated due to stick effects, whereas on highly profiled surfaces the exact positioning of the impact point influences the measurement. The dynamic coefficient of friction depends on the resistance to move of a slider when pulled over the surface in dry or wet con-ditions (with water or water with a sur-factant). The slider material generally is rub-ber or leather (for dry applications). The measurement of the friction dynamic coefficient can also be performed on site using self-propelling devices. This is less dependent on human influences, but also leads to an over-estimation on smooth sur-faces due to sticking, as well as an under-estimation at a certain profile depth due to the contact area loss. Therefore, results have to be treated with caution. In Germany safety threshold values have been estab-

which however, depend on the surface type, they do not always offer a comparable estim ation of the actual slip risk. Although slip resistance has been specified as essen-tial characteristic for the CE marking of floor tiles, no corresponding harmonized single test could yet be defined. In 2011 the European Standardization Com-mittee CEN/TC 339 on Measurement of Slip Resistance in Pedestrian Areas put forward a technical specification, including three es-tablished but different testing methods: the walking method on an inclined ramp, the pendulum test method and a method for the measurement of the dynamic coefficient of friction (CEN/TS 16165 [3]). These methods differ strongly dependent on the surface tested: the ramp method, gener-ally used in Germany, is based on the de-termination of a critical inclination angle at which a person slips on an oil covered sam-ple using reference shoes for occupational areas (providing the known R-slip resistance classification, for higher requirements com-bined with additional measurement of the displacement volume) or a sample flooded with water with a surfactant for barefoot applications in swimming pools, shower areas and in spa environments (generating the A, B and C classification for application with different amounts of loading water). The ramp method incorporates static and dynamic friction during human walking, leading to a high uncertainty at lower angles, but providing the only possibility to measure highly profiled surfaces. For this measurement, operators have to qualify themselves using reference slabs. Here the

Fig. 2 Heavy shading of the surface, leading to strong colour-transitions on the floor (l.) and highly visible mosaic effects (r.)

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profile above the mean line and being rep-resentative for protruding peaks in the pro-file (the grip component) were established as significant parameters. The reduced val-ley depth Pvk below the core roughness and the calculated oil retention volume in this depth range were found to define the actual cleaning and soiling behaviour.A clear correlation of the surface topog-raphy with the slip risk to be expected for different surface types could be established. This interpretation made possible to gener-ate a surface classification independent of the different slip resistance interpretations as a result of using different test methods. In the SlipSTD project, this classification was combined and correlated with the assess-ment of the type and amount of contamin-ation expected, the conditions to be met re-garding control measures and the cleaning procedures implemented for a specific ap-plication. This generates a holistic basis for a surface based slip risk classification [10]. A significant result of the project was the specification and verification of the suitabil-ity of the different slip test methods regard-ing different surface types which now can

average of the roughness profile) and Rz (mean roughness depth) represent wave-filtered profile parameters which can hide actual relevant differences in surface pro-files. Another important aspect is that es-pecially for relatively rough-surface tiles, contact measurements are pushed to their limits and can no longer be used to image the characteristic height of tile profiles. Optical measurement with chromatic aber-ration, the spectral shift of backscattering spectrum of focussed white light enables handling of a necessary larger measure-ment area (up to 56 mm × 56 mm) with an adjustable height range (up to 1 cm) by research within the project specifically de-fined to characterize tile surfaces with ac-ceptable effort [9]. To interpret the surface amplitude param-eters shape factors as well as parameters specifying the material ratio curve in the depth of the profile were incorporated (Fig. 3). After an extensive statistical cor-relation analysis, the core roughness Pk defining the main load carrying part of the profile (the friction component) as well as the parameter Pp defining the height of the

spills. After application and drying of the soiling agent on the surface, the assessment of the cleaning properties is performed in an automatic multi-track scrub tester by wiping with brushes, sponges or cloths pre-pared with the maintenance cleaning agent used in practice. The number of strokes needed to clean the surface (if possible) and the transition sharpness of the cleaned/clean area provide a measure for the removal of coarse dirt as well as the possibility to perform an assess-ment of how the tile surface will react to soiling on a long term by dirt penetration into the surface, leading to a grey discolor-ation which was shown to correlate very well to the situation in practice [7].It is important to notice that these results can be very different if the cleaning system is adjusted: using other brushes or pads for specific surfaces may lead to better results, indicating how the regular floor cleaning and maintenance can be optimised. Based on these results the actual optimised test procedure was transferred into a working description, available for customers, insti-tutes and development departments in the tile sector.

Topography as an objective validation tool

To assess the slip resistance effects in combination with the cleaning and soiling behaviour, especially when these effects have to be regarded over the period of use, another objective parameter is necessary in order to be able to describe the nature and changes of the actual surface, independent of its functional behaviour. In the scope of the European Collective Research Project SlipSTD – Development of Slip-Resistant Standard Surfaces, as part of an international consortium of leading institutes active in the field of ceramics, tri-bology, work safety and statutory accident insurance as well as architect organizations and tile manufacturers [6], FGK developed a method for the characterization of ceramic surfaces by means of optical profilometry based on the international Geometrical Product Specifications [8, 9].The topographic data on which this char-acterization is based are unfiltered profile values, which can deliver an useful descrip-tion of the actual profile. It has to be noted that the well-known values Ra (arithmetic

Fig. 3 Relevant topography parameters defining the slip resistance and cleaning and soiling behaviour

Fig. 4 Suitability of different slip test methods dependent on the surface characteristics

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like slipping medium, measurement speed and slider type are more significant than the actual topography and that, as rough-ness increases in significance, correlations between the methods can be established. At higher roughness values this correl ation is lost again due to the contact surface loss and the more important role of profile shape parameters. This has an important effect on the assessment of slip resistance durability.

Slip resistance durability and cleaning behaviour

The implementation of the surface topog-raphy characterization creates a basis to objectively assess and evaluate the actual slip resistance changes as well as cleaning behaviour in time. To be able to use this method for installed tiles, a duplication technique was investi-gated and implemented, accurate enough to establish relevant changes in surface topography without the need for tiles ex-traction from the actual floor. A silicone du-plication material able to duplicate with an accuracy of 2 µm by pouring a 2-component mixture on the surface was used ready to be extracted after ca. 20 min (Fig. 5). To simulate the so specified actual changes due to use and surface abrasion on site, a suitable abrasion method had to be devel-oped. As the surface abrasion according to the EN ISO 10545-7 test method (PEI, small scale circular abrasion using an abrasive mixture of iron balls and alumina grains) was proven to generate a non-uniform abrasive pattern (with a stronger abrasive effect at the tested surface rim, leading to an exaggerated interpretation of actual abrasion for dark coloured surfaces) an-other approach had to be taken. As international approaches are based on linear abrasion and small treated sur-faces, radial abrasion for larger samples had to be considered – as in Germany the ramp method requiring sample sizes of 50 cm × 100 cm and established as refer-ence method. This sample size provides a surface which can also be measured by the other methods as incorporated in the European technical specification [3]. The use of specified abra-sive pads in a single-pad cleaning device was optimised and validated using topog-raphy measurements in order to compare

be differentiated quantitatively using the mentioned surface parameters as shown in Fig. 4. This result confirmed the aforementioned differences between the methods in specific

surface ranges and explains why it is not possible to establish correlations between these methods over the complete span of the existing surface topography. It confirms that on smooth surfaces other parameters

Fig. 5 Duplication technique using a 2-component-based silicone material (l.) and the visualization of the surface duplicate (r., inverted)

Fig. 6 Validation of the abrasion simulation method by topographical comparison with PEI results and an actual surface on site

Fig. 7 Topographical presentation of the wear effects on a microrough surface (group 2): above: 2D-representation, profile and characteristic values, and the actual cleaning behaviour as measured in the laboratory (below: cleaned surface)

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sions in the surface, which are not relevant for slip resistant (Fig. 9). While the slip re-sistance tends to decrease, cleaning behav-iour of the surface can also show a negative development in the first abrasion stages

can be explained: it could be established that increased dirt susceptibility was not only depending on the roughness as such, but also, if not prominently, on the percent-age and the depth of the pores and depres-

the laboratory results with identical surfaces in use on site. It could be established that specified abra-sion patterns (indicated as cycles) over the surface can be correlated to actual wear effects: 20 abrasive cycles in laboratory cor-respond to the wear effects after 1,5 years in a highly trafficked area with abrasive dirt (discount store, mall, train station hall). Here, similar changes of the profile core roughness and the reduced peak heights, so smoothing the surface, could be detected in the same magnitude as found in the surface on site, as shown in Fig. 6. In the abrasion test, an initial increase of the Pvk value can be seen which by topographic analysis could be referred to the pores and inclusions opening as can be seen in Fig. 7 before smoothening the surface. Here also the mentioned roughening effect of the PEI test can be seen in the continuous increase of the Pvk value (scratching and damaging the surface).In an extensive study of different floor sur-face types, the method showed that most surfaces designed for slip resistance re-quirements show a significant decrease in the slip resistance in their first 1,5 year of use: reductions of 30–50 % or even higher for the results of the ramp method are no exception (Fig. 8). Even for highly profiled tile surfaces small changes in the sharpness of the protruding parts have a significant influence on the slip resistance and can lead to its significant de-crease. In some cases this leads to the trans-gression of slip thresholds implying a differ-ent slip class of the actual surface as it was the case when the tile surface was installed. This indicates the critical character of this ef-fect in regard to the required long-term per-formance as incorporated in the CE marking requirements and emphasizes the need for further developing this method in regard to the European Building Products Regula-tion. This approach has been systematically implemented to support the design and de-velopment of durable slip resistant surfaces.The measurement of slip resistance durabil-ity can now simultaneously be assessed with the durability of cleaning effects. The abraded samples were tested and rated us-ing the aforementioned cleaning behaviour test method and investigated with the help of microscopy and the topographic charac-terization method. So non-obvious effects

Fig. 8 Slip resistance decrease due to abrasion as measured with the ramp method according to DIN 51130 [3]

Fig. 9 Surface roughness relevance of regarding the cleaning effort and susceptibility to soiling

Fig. 10 Microscope image of the soiled microrough tile surface (l.) and SEM image of the surface after cleaning (r.); the inhomogeneity of the surface coating can be clearly seen on the right

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[2] Strautins, C.J.: Sustainable slip resistance:

an opportunity of innovation. Proc. Qualicer

2008, Castellón, Spain

[3] CEN/TS 16165 : 2012: Determination of slip

resistance of pedestrian surfaces – methods of

evaluation

[4] Information zur Bewertung der Rutschgefahr

unter Betriebsbedingungen BGI/GUV-I 8687

der Deutschen Gesetzlichen Unfallversi-

cherung DGUV. Fachausschuss Bauliche Ein-

richtung, 2011

[5] Fitzner, A.; Aßmus, U.: Empfehlung zur Qua-

li täts bewertung der Produktleistung von All-

zweckreinigern. SÖFW-J. 130 (2004) [10] 83

–93

[6] Engels, M.: Development of a process for

the application-oriented investigation of the

cleanability of tile surfaces. cfi/Ber. DGK 90

(2013) [3] E1–E4

[7] Engels, M.; Tenaglia, A.; Tari, G.: The classifica-

tion of hard floor coverings according to slip

risk: a new approach for ceramic floor cover-

ings. Proc. of Qualicer XI, Global Forum on Cer-

amic Tiles (CDROM)

[8] EN ISO 4287:2009: Geometrical product

speci fi cations (GPS) – surface texture: Profile

method – terms, definitions and surface tex-

ture parameters (ISO 4287:1997 + Cor 1:1998

+ Cor 2:2005 + Amd 1:2009)

[9] EN ISO 4288:1997: Geometrical product

speci fi cations (GPS) – surface texture: Profile

method – rules and procedures for the assess-

ment of surface texture (ISO 4288 : 1996)

[10] Tari, G.; et al.: SlipSTD publicly available speci-

fication (SlipSTD PAS) – classification of hard

floor coverings according to their contribu-

tion to reduce the risk of pedestrian slipping.

Authorised by the SlipSTD Steering Committee,

2009, www.slipstd.com

related to the wear on site by topography measurements. Furthermore, the method offers the pos-sibility to correlate different slip durability measurement methods on the same sur-faces as well as to simultaneously evaluate the cleaning properties using the newly de-veloped test for cleaning and soling behav-iour. In this way, a basis is created for the development of durable slip resistant and easy to clean surfaces with low dirt sus-ceptibility. Transfer of the abrasion method into a laboratory set-up further reducing the manual impact by automated abrasion cycles will help to improve reproducibility and repeatability of the method. Further studies on the wear effects in dif-ferent application areas can lead to a better understanding of the wear effects in differ-ent practice situations. They offer an optimal basis to address the requirements as put forward by the European Building Products Regulation. At present, the process is already used by tile manufacturers working in collaboration with FGK as a practice-relevant develop-ment aid and support of production control where the standard test for staining resist-ance testing is failing to address actual cleaning problems.

References

[1] EU Regulation No. 305/2011 of the European

Parliament and the Council on 9.3.2011: A lay-

ing down harmonised conditions for the mar-

keting of construction products and repeal-

ing Council Directive 89/106/EEC. Text with

EEA relevance. Official J. of the Europ. Union,

4.4.2011, L 88/5

by removal of the top layer of the surface: pores and inclusions are uncovered leading to an initial deterioration of cleaning be-haviour. Later on these pores, depressions and holes can be smoothened out again in which case the cleaning is improved and the soiling is reduced (Fig. 7) while the slip resistance stays low. This change can be quantified by the change of the reduced valley depth Pvk which ini-tially increases and then is reduced indicat-ing the decreasing depth of pores and holes. It was demonstrated that these phenomena play a major part in cleaning problems even in the case of polished surfaces. Non-homogeneous factory-applied finishes for unglazed tiles, meant to improve the stain resistance, could be detected as the possible cause for the dirt susceptibility (Fig. 10). These effects can now be analysed quantitatively, enabling the development of design and production criteria to obtain easy to clean surfaces. For example, for microrough tiles, a reduced value depth with a maximum of 6 µm or an oil retention volume sV0, the displace-ment area in this depth range of maximum 0,3 µm3/µm2 could be measured as critical values. Exceeding this value leads to a strong in-crease in dirt susceptibility. Here mechan-ic al cleaning is ineffective, it requires con-siderable cleaning effort with the use of deep- action cleaning agents containing surfactants.

Discussion

The abrasion method described is proven to simulate wear effects which can be cor-

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