potential and protect steel from corrosion [11]. As a result of which the total structural mass, and therefore weight, is increased. Substituting the steel for corrosionfree, tensile load carrying polymer fibres would therefore, reduce overall structural weight and construction cost [2, 5], ease construction [5], and reduce CO 2 -emissions [12]. These benefits, however, are still limited as the tensile load carrying ability of the polymer fibres in cured FRC is less that that of steel-reinforced concrete. As such, a total substitution of steel has only been achieved in structures that are non-essential for structural support [2, 5]. To realise the aforementioned benefits of FRC technology, current fibre design focuses on the matrix-bonding of the fibres and their mechanical properties as the paramount design criteria to improve the fibres tensile load carrying ability [8, 9]. To ensure that none of the fibre properties optimised during development are lost during FRC processing, tribological characterisation of the fibres is of interest. Here, information gained about wear characteristics can be Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 1 Introduction For several years polymer fibres haven been used as a reinforcement material to improve the mechanical properties of concrete [1-3]. Fibre-reinforced concrete (FRC) is a composite material [2], produced by mixing concrete with reinforcing fibres using a mixing process to ensure homogenous fibre dispersion [3, 4]. Polymer fibres, polypropylene (PP) fibres in particular, are used as reinforcing fibres in FRC to control temperature and shrinking induced cracking, thus improving its material toughness [1, 5-7]. These improved properties of FRC, however, are dependent on the fibres matrix-bonding and mechanical properties which in turn influence the fibres’ tensile load carrying ability [8, 9]. Previous studies [3, 4] have shown that these properties are negatively influenced by the tribological stresses that occur during the concrete mixing process, causing fibre wear by abrasion and material deterioration. The result of which can be seen in the reduction of the materials tensile load carrying ability [3, 4, 7]. With improvements in fibre design, focusing on fibre optimisation as well as tribological characterisation techniques [8-10], further application fields of FRC are opened up. This includes, but is not limited to, the reduction in the overall amount of concrete used. Today, a considerable amount of concrete is used in steelreinforced structures to reinforce its tensile load carrying A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding Justus Rüthing, Frank Haupert, Regine Schmitz, Michael Sigrüner, Nicole Strübbe* A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding conditions is developed. The production process of the tested polymer fibres is described and an introduction in fibre-reinforced concrete is given. Tribotesting is done on an optimised tribometer capable of measuring the friction and wear behaviour of polymer fibres with diameters of a few 100 µm under lubricated conditions. Three extruded polypropylene macro fibres with varying diameters are characterised under tribological conditions found in an industrial concrete mixing process. It is shown that detailed friction and wear data of polymer fibres can be gathered. Keywords polymer-fibres, fibre reinforced concrete, Pin-on- Disc, abrasive wear, water lubrication, hydrodynamic sliding Abstract * B. Sc. Justus Rüthing; Orcid-ID: https: / / orcid.org/ 0000-0001-7615-4979 Prof. Dr. -Ing. Frank Haupert; Orcid-ID: https: / / orcid.org/ 0000-0002-3312-6844 Dr.-Ing. Regine Schmitz; Orcid-ID: https: / / orcid.org/ 0000-0002-4510-2559 Hamm-Lippstadt University of Applied Sciences, Marker Allee 76 - 78, 59063 Hamm M. Sc. Michael Sigrüner; Orcid-ID: https: / / orcid.org/ 0000-0002-0644-023X Prof. Dr.-Ing. Nicole Strübbe; Orcid-ID: https: / / orcid.org/ 0000-0002-2084-9031 Rosenheim Technical University of Applied Sciences, Hochschulstraße 1, 83024 Rosenheim used to predict fibre wear and, as a result, positively influence fibre design. In order to aid this design process, tribological characterisation of polymer fibres under conditions found in industrial concrete mixing processes has been undertaken, with the development of a tribological characterisation process capable of modelling these conditions being the overall aim of this study. Continuing the development described by Schmitz et al. [10], a new approach for the friction and wear characterisation of polymer fibres is developed using an optimised Pin-on-Disc test rig. Using this test rig, three different types of single extruded macro PP-fibres, with varying cross-sections of a few 100 µm 2 , are characterised under conditions found in industrial concrete mixing processes. The results gathered from this characterisation are then used to compare the fibres’ tribological properties using the coefficient of friction (COF) and the steadystate wear rate. 2 Materials 2.1 Pin-on-Disc test rig used for tribological fibre characterisation For the characterisation of the polymer fibres an optimised, in-house designed and built, Pin-on-Disc test rig is used (Figure 1). Normal force is applied by a mechatronic controlled load and test unit. The load and test unit is able to apply precise force to the specimen in the range from 1 to 1000 N. During measurement a precise normal force application (± 0.3 N) is realised using a stepper motor and corrected if any deviations of the target normal force are detected. Utilising a load cell friction force is measured. To measure wear, a µm-accurate laser sensor is used to measure specimen wear depth. Counterpart rotation is realised by a servomotor. The temperature sensors and heating cartridges are located underneath the counterpart. Using the heating cartridges, heating of the counterpart is possible while the temperature sensors are used to monitor the counterpart’s overall temperature. To conduct measurements under lubricated conditions, a programmable peristaltic pump (Ismatec ® Reglo ICC) is used. Using this set up, lubricants can be applied directly onto the counterpart’s surface with rates in the range of 0.01 to 5.70 ml/ min over the water injection attachment, attached to the peristaltic pump using a tube with an inner diameter of 1.02 mm. Surrounding the counterpart, there is a liquid enclosure protecting the tribometers fragile electronics. All of the features of the described test rig are controlled by a MATLab based in-house designed operating system which is operated over a graphical user interface (GUI). During the conduced tests, applied normal force [F N ], friction force [F x ], coefficient of friction [µ], friction speed [v], specimen wear distance [s] and temperature [T] are measured and displayed as a function over time using said GUI. 2.2 Specimen-holder Preparing the fibres for tribotesting, the extruded fibres are individually fixated onto a sample holder, designed for the tribological characterisation of fibres (Figure 1 and 2). During testing, the fibre is held in place using screw-in mounts on each side of the specimen holder. Using the specimen holder described, a fibre-counterpart contact-length of 20 mm is realised. Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Figure 1: Optimised Pin-on-Disc test rig for the tribological characterisation of single polymer fibres Figure 2: Specimen-holder with PP-fibre for tribological fibre characterisation 2.3 Specimen - Extruded PP-fibres 2.3.1 Fibre Production and Draw Down Fibre production was executed on an HAAKE Polydrive single screw extruder with a length/ diameter ratio of 25 and a screw diameter of 19 mm (Figure 3). All polymer granulates were pre-dried and a round shaped strand of approximately 3 mm diameter was extruded. The strand was cooled through a combination of water bath and air cooling and continued into a Dr. Collin GmbH MDO laboratory drawing machine with a take up speed of 3 m/ min (v 1 ). The drawing machine resembles a onestep drawing process, consisting of two roll packages and a heating oven. The oven temperature was set at 157 °C for all compounds. A second roll package with higher speed (v 2 ) stretched the polypropylene to its individual maximum draw ratio to accomplish high mecha- Figure 4. Additionally, the mechanical properties of the fibres are given in Table 2. 2.4 Counterpart - Alumina-Disc To recreate the abrasive conditions found in industrial concrete mixing processes, a counterpart investigation was undertaken in conjunction with the aforementioned experiments to model the abrasive conditions of concrete mixing. As multiple experiments using counterparts with varying average surface roughness’s and topographical properties were conducted, the Aluminacounterpart was chosen to be the most suitable to model Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 nical properties. The draw ratio (DR) was defined as the ratio of roll speed differences. (1) 2.3.2 Dimensions and Mechanical Properties Using the production and draw down process described in 2.3.1, three oval-shaped PP-fibres with draw ratios of 1: 10, 1: 14 and 1: 17 were produced. The exact vertical and across diameters of the fibres are given in Table 1. A schematic cross section of the PP-fibres can be seen in ed PP-fibres and Draw Down = nical Properties Figure 3: Schematic description of the fibre draw down process PP-fibres raw Down anical Properties Figure 4: Schematic cross section of an extruded oval-shaped PP-fibre Figure 5: 3D surface topography image of the alumina counterpart used (white light interferometry, section: 1x1 mm) image taken after the grinding procedure Table 1: Dimensions for the across and vertical diameter of the fibre draw ratios 1: 10, 1: 14 and 1: 17 the abrasive conditions of fresh concrete during mixing. The counterpart selected consists of an Alumina (Al 2 O 3 ) disc with an average surface roughness (R a ) of 1.6 ± 0.1 µm (Figure 5). To ensure the same surface properties apply for each measurement, the disc surface is grinded using a diamond grinding disc (Schmitz Metallography, stated grain size 0080) before each tribotest. The average surface roughness of the ceramic disc is measured using a white light interferometer (FRT Mirco-Prof ® ) and is controlled in defined intervals. Table 2: Mechanical Properties: tensile strength, E modulus and elastic strain for PP-fibres 1: 10, 1: 14. and 1: 17 3 Methods 3.1 Run-in-period and data acquisition As the polypropylene fibre is abraded during tribological characterisation, a frequent geometry dependent reduction in the applied surface pressure of the oval PP-fibre can be measured. This frequent change is present within fibre wear depths of up to 100 µm, with its magnitude changing for each fibre-diameter characterised. To account for this, a run-in-period was defined as t start = 0 µm to t 100 = 100 µm under dry sliding conditions. After the wear depth of 100 µm is reached, the data acquisition of the friction and wear data is initiated with the change in surface pressure now being a marginal factor during fibre characterisation (Figure 6). A more detailed description of the run-in-period and its causes are outlined in Schmitz et al. [10]. 3.2 Testing Parameters In order to ensure consistency across experiments under dry, mixed and hydrodynamic sliding conditions, the pvproduct was controlled and kept at 0.16 MPa m/ s across all tests conducted. To account for the varying fibre diameters, the applied normal force is adapted and three lubrication rates are used to model the sliding conditions, see Table 3. The tests were conducted under temperatures between 24 °C and 25 °C to which no adjustments were made during the experiments. 3.3 Experimental procedure To model the varying sliding conditions, similar to those found in industrial concrete mixing processes, an experimental procedure is developed (Figure 7). As the starting phase is predetermined by the run-in-period, as defined in 3.1, characterisation of the three sliding conditions is as follows. The dry sliding phase is modelled using a lubrication rate of 0.00 ml/ min. The end of the tribological characterisation under dry sliding conditions was defined as the wear depth (s) at t 150 = 150 µm. The friction and wear data are thus taken in the wear range of 50 µm, as this was found to contain sufficient enough data to characterise the fibres by previous experiments. As t 150 is reached, water as a lubricant is added in the rate of 0.50 ml/ min to model the mixed sliding phase for 25 min. The mixed sliding test phase, in which data is acquired, is defined as t 150 + 15 min to t 150 + 25 min, to ensure sufficient lubrication conditions. As the test time of t 150 + 25 min is reached, the experiment proceeds to the hydrodynamic sliding condition by increasing the lubrication rate to 4.00 ml/ min. Equivalent to the mixed sliding phase, the relevant fibre characterisation data is taken in the last 10 min, of the 25 min test time, at t 150 + 40 min to t 150 + 50 min. As the test time of t 150 + 50 min is reached, the tribological characterisation of the polymer fibre under dry, mixed and hydrodynamic sliding conditions is complete. As absolute test time is predetermined by the run-in-period, test time varies between each experiment conducted. Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Table 3: Applied tribological testing parameters: normal force, sliding speed and lubrication rate 3 Methods 3.1 Run-in-period and data acquisition 3.2 Testing Parameters 3.3 Experimental procedure Figure 6: Schematic image of the cross section of an extruded PP-fibre with the first 100 µm marked as the run-in period (green line) and the range from 100 µm to the fibres’ maximum diameter (horizontal dotted line) marked as the data acquisition period Figure 7: Flow diagram of the experimental procedure for the tribological fibre characterisation over test time the lubrication rate is increased to 4.00 ml/ min, another reduction in the COF is visible. This transition point is marked by a dotted line at t = 30 min. During the modelling of the hydrodynamic sliding phase, an overall reduction of the COF by approx. 15 % is recorded. In the example case shown in Figure 8, the total test time ends at t = 55 min as the predefined endpoint of t 150 + 50 min is reached. Figure 9 shows the COF results of PP-fibre 1: 10 (in black), PP-fibre 1: 14 (in blue) and PP-fibre 1: 17 (in red) under dry, mixed and hydrodynamic sliding conditions gathered in this study. The average values as well as the standard deviation (in brackets) taken from five experiments each are displayed in Table 4. Examining the three curves of Figure 9, an overall trend can be seen in a reduction in the COF that occurs with each increase in lubrication rate. Thus, the modelling of the dry sliding phase results in the highest COF and the modelling of the hydrodynamic sliding phase results in the lowest COF per fibre tested. Comparing the fibres on an individual level, PP-fibre 1: 10 records the highest COF across all fibres tested, across all modelled sliding conditions, with a COF of 0.39 being the maximum recorded during the experiments. This can Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 4 Findings 4.1 Friction behaviour Figure 8 provides an example of the typical curved relationship between the COF relative to test time (t) of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding conditions. The curve is created using the experimental procedure described in 3.3 using the parameters shown in Table 3. The COF is given as the ratio between friction and normal force. Overall, five experiments were carried out for each of the thee fibres tested. The curve shows a COF of about 0.36 in the first five minutes of test time. Here the run-in-period is completed and the dry sliding phase is being modelled. Fluctuation in the recorded COF-curve can be seen during the run-in-period, although this decreases and remains at a steady level throughout modelling of the dry sliding phase. As the predefined wear-depth is reached at t 150 , water as a lubricant is added in the rate of 0.50 ml/ min resulting in an immediate reduction of the COF by approx. 30 %. During the time frame in which the COF data is taken for tribological characterisation, the COF decreased by approx. 47 % compared to the COF data taken under dry sliding. At the end of t 150 + 25 min, as Figure 8: Coefficient of friction over test time of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding further be seen by the comparison of the COF under hydrodynamic sliding of PPfibre 1: 10 with the COF taken under mixed sliding conditions of PP-fibres 1: 14 and 1: 17 as all three recorded COF record an equal value of 0.19 despite the lower relative lubrication rate applied to PP-fibre 1: 10. While PP-fibres 1: 14 and 1: 17 show a lower overall COF than fibre 1: 10, a distinction between these two fibres cannot be made due to the overlap in their standard deviation. Table 4: Coefficient of friction data of PP-fibres 1: 10, 1: 14 and 1: 17 under dry, mixed and hydrodynamic sliding conditions. COF value is given as an average of 5 with standard deviation in brackets 4.2 Wear behaviour Figure 10 shows an example wear curve of PP-fibre 1: 14 with the wear distance (s) on the y-axis and the test time (t) on the x-axis. The coefficient chosen to characterise the fibres is the steady-state wear rate (w const. ), which is calculated using the wear curve gradient given in µm/ h. Overall, five experiments were carried out for each of the three fibres tested. Starting with high wear PP-fibre 1: 14, represented by a steep rise in the wear distance curve, the run-in-period of the experimental procedure is displayed. During this run-in-period, in particular the initial 50 µm wear depth, that the fibre is subjected to its maximum wear rate. This, however, reduces by a noticeable margin in the latter 50 µm of the run-in-period. A further reduction in wear distance over time can be seen during the period under dry sliding. This reduction can be explained by the increase in fibre-counterpart contact area as a direct result of the increase in wear depth displayed in Figure 3. As the wear-depth of 150 µm is reached at t 150 and the mixed sliding condition is modelled, a decrease in w const. by approx. 97 %, moving from 1555 µm/ h under dry sliding to 57 µm/ h (under mixed sliding) is recorded. As the lubrication rate is increased to 4.00 ml/ min moving from t 150 + 25 min onwards, no further change in w const. is apparent form the recorded data. As the wear distance and the COF are recorded simultaneously during the experiments, the total test ends at t = 55 min when the predetermined terminus of t 150 + 50 min is reached. Figure 11 shows the steady-state wear rate results of PPfibre 1: 10 (in black), PP-fibre 1: 14 (in blue) and PPfibre 1: 17 (in red) under dry, mixed and hydrodynamic sliding conditions gathered in this study. The average w const. is taken out of five experiments conducted for each fibre and is presented in Table 5 alongside the standard deviation of each measurement. Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Figure 9: Comparison of the coefficient of friction of PP-fibres 1: 10 (black), 1: 14 (blue) and 1: 17 (red) over the variation of lubrication rate Figure 10: Wear distance over test time of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding gathered using the optimised Pin-on-Disc test rig under varying lubrication rates and sliding conditions. During the experiments, the PPfibre with the draw ratio of 1: 10 exhibits the lowest steady-state wear rate and thus provides better wear resistance under dry sliding conditions, while PP-fibres 1: 14 and 1: 17 exhibit increased wear resistance under mixed and hydrodynamic sliding conditions relative to fibre 1: 10. A reduction in the COF was examined across all fibres tested as water was added in modelling of mixed and hydrodynamic sliding conditions, with PP-fibres 1: 14 and 1: 17 showing the lower overall COF across all modelled sliding conditions. While a compression of the tribological characteristics of PP-fibres 1: 10, 1: 14, and 1: 17 was described in section 4, no significant distinction between fibres 1: 14 and 1: 17 could be made in this study due to the overlap in the standard deviation of these fibres. Using the optimised Pin-on-Disc test rig in the configuration described, it has been shown that detailed friction and wear information of single polymer fibres can be gathered under tribological conditions similar to those found in industrial concrete mixing processes. Applying the information gathered from this suit of experiments, it has been shown that the selection of polymer fibres for particular construction and end-use scenarios can be made on the bases of their tribological properties, allowing for further development of future design processes. 6 Acknowledgements The authors thank the German federal ministry of education and research (BMBF) for the funding of this study as part of the FHprofUnt project ConPlasite - 13FH068PB6. Further, the authors would like to show their gratitude to Mr. S. S. Fellows for comments that greatly improved the manuscript. Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 As well as with the COF results described in 4.1, a reduction of the steady-state wear rate occurs with each increase in lubrication rate. This decrease is most apparent when progressing from dry to mixed sliding conditions across all tested fibres. The dry sliding phase records the highest steady-state wear rates while the hydrodynamic sliding phase records the lowest wear rates overall per fibre tested. Comparing the experimental results of each fibre under dry sliding conditions, PP-fibre 1: 14 experiences the highest overall wear rate (1555 µm/ h) and PP-fibre 1: 10 experiences the lowest overall wear rate with PP-fibre 1: 17 in between. Moving to the mixed sliding conditions, the correlation of wear rate properties of the fibres is changed, with PP-fibre 1: 10 experiencing the highest overall wear rate; PP-fibres 1: 14 and 1: 17 on the other hand experience a significant decrease in their wear rate and the curves intersect. This overlap continues into the hydrodynamic sliding conditions, however, whereas PP-fibre 1: 10 experiences notable wear in this phase PP-fibres 1: 14 and 1: 17 show no significant change in wear rate. 5 Summary The results show that reliable friction and wear data of three 500 - 700 µm thick single polymer fibres could be Figure 11: Comparison of the steady-state wear rate of PP-fibres 1: 10 (black), 1: 14 (blue) and 1: 17 (red) over the variation of lubrication rate Table 5: Steady-state wear rate data of PP-fibres 1: 10, 1: 14 and 1: 17 under dry, mixed and hydrodynamic sliding conditions. Steady-state wear rate value is given as an average of 5 with standard deviation in brackets Literature [1] Z. Zheng, „Synthetic fibre-reinforced concrete“, Progress in Polymer Science, 20, No. 2, p. 185-210, 1995. [2] M. Di Prisco, G. Plizzari und L. Vandewalle, „Fibre reinforced concrete: new design perspectives“, Mater Struct, 42, No. 9, p. 1261-1281, 2009. [3] O. Czoboly und G. L. Balázs, „Possible mechanical deterioration of fibres influenced by mixing in concrete“, Concrete Structures, 16, p. 18-23, 2015. [4] O. Czoboly und G. L. Balázs, „Are fibers sensitive to mixing? “, Structural Concrete, 18, p. 19-28, 2017. [5] S. Yin, R. Tuladhar, F. Shi, M. Combe, T. Collister und N. Sivakugan, „Use of macro plastic fibres in concrete: A review“, Construction and Building Materials, 93, p. 180- 188, 2015. [6] N. Thibodeaux, D. E. Guerrero, J. L. Lopez, M. J. Bandelt und M. P. Adams, „Effect of Cold Plasma Treatment of Polymer Fibers on the Mechanical Behavior of Fiber- Reinforced Cementitious Composites“, Fibers, 9, No. 10, 2021. [7] J. O. Lerch, H. L. Bester, A. S. van Rooyen, R. Combrinck, W. I. de Villiers und W. P. Boshoff, „The effect of mixing on the performance of macro synthetic fibre reinforced concrete“, Cement and Concrete Research, 103, p. 130-139, 2018. [8] M. Sigrüner, D. Muscat und N. Strübbe, „Investigation on pull-out behavior and interface critical parameters of polymer fibers embedded in concrete and their correlation with particular fiber properties“, J. Appl. Polym. Sci., 138, No. 28, 2021. [9] Sigrüner M., Muscat D., Strübbe N., „Investigations of the bonding behavior of modified polypropylene to a concrete matrix by single fiber pull out tests“, SAMPE Europe Conference 2020 Amsterdam - Netherlands, 2020. [10] R. Schmitz, F. Haupert, J. Rüthing, M. Sigrüner und N. Strübbe, „Tribologische Charakterisierung von Polymerfasern unter Trockenreibung, Mischreibung und Hydrodynamik mittels einer optimierten Pin-on-Disc-Prüfmethode“, TuS, 68, 2021. [11] DIN EN 1992-1-1: 2011-01, Eurocode_2: Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken_- Teil_1-1: Allgemeine Bemessungsregeln und Regeln für den Hochbau; Deutsche Fassung EN_1992-1- 1: 2004_+ AC: 2010, Berlin. [12] C. Camille, D. K. Hewage, O. Mirza, F. Mashiri, B. Kirkland und T. Clarke, „Evaluation of Macro-Synthetic Fibre Reinforced Concrete as a Sustainable Alternative for Railway Sleepers“ in Lecture Notes in Civil Engineering, CIGOS 2019, Innovation for Sustainable Infrastructure, C. Ha-Minh et al., Hg., Singapore: Springer Singapore, 2020, p. 471-476. Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034