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A comparison of results from portable and laboratory floor slipperiness testers
International Journal of Industrial Ergonomics 39 (2009) 353–357
Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
A comparison of results from portable and laboratory floor slipperiness testers R. Ricotti*, M. Delucchi, G. Cerisola DICheP, University of Genoa, P.le Kennedy 1, 16129 Genoa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2008 Received in revised form 25 June 2008 Accepted 26 July 2008 Available online 14 September 2008
The slip resistance of many commercial floor coverings used in a wide range of applications was measured with the British Pendulum Tester, the Tortus II and the Ramp test under dry and wet conditions. The repeatability and validity of the results of these methods were considered. The results showed that British Pendulum is able to diversify between dry and wet conditions, moreover ANOVA showed that it is possible to discriminate different materials. Results obtained with Tortus revealed that this device cannot distinguish between dry and wet measurements, but it is able to discriminate dissimilar materials. Ramp test can differentiate materials, but it produces different results depending on the test person involved; its repeatability can be seriously questioned. Generally, the results indicated that the ranking of materials depends highly on the slipmeters and surface conditions.
Keywords: Slip resistance Friction Tribometers Tortus British Pendulum Ramp
Relevance to industry: It is impossible to find a univocal correlation between results obtained by different methods, but flooring manufacturers need to verify and optimise the anti-slipperiness of their products. Then a new approach to obtain a ‘‘gold’’ standard that avoid to reconcile the differences in numerical results of the various tribometers and based on a set of external calibration materials could be a more useful flooring resistance tester. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Slipping incidents result from one or more factors including the person involved; the activity performed; environmental factors such as contaminants (water, grease, frost, dust), distractions, temperature and lighting; and the characteristic of the footwear and of the walking surface (Kim et al., 2001). All of these elements can combine to determine whether traction (or slip resistance) is adequate to prevent a slip. Several authors studied the tribological phenomena occurring at the surface/shoe interface when a subject is walking normally (Cham and Redfern, 2002; Gronqvist, 1989; Strandberg and Lanshammar, 1981; Hanson et al., 1999; Marpet, 2002). Other authors studied the nature of the mating materials, floor surface and footwear (Chang and Matz, 2001; Kim and Smith, 2000; Kim, 2004a,b). Every surface has some degree of roughness, though the scale of the roughness might be microscopic (Chang, 1999). Traction between two rough surfaces (soling and floor) results in two types of interactions: adhesion and interlock. Adhesion results from molecular attraction between the two surfaces and exists even when both surfaces are quite smooth. Interlock is
* Corresponding author. Tel.: þ39 01 03 536022; fax: þ39 01 03 536028. E-mail address: [email protected] (R. Ricotti). 0169-8141/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2008.07.004
a mechanical interaction involving peaks and valleys that may be microscopically small. A lubricant can defeat both adhesion and interlock to some extent. Centuries ago, scientists noticed that frictional force resisting movement between two hard surfaces is proportional to the force pressing the surfaces together. ‘‘Coefficient of friction’’, CoF, is the term used for the constant ratio between the frictional force and the pressing force. It is now known that traction, or slip resistance, is not as simple as this, particularly when water or oil separates the surfaces, but the concept of a coefficient of friction is still used. Coefficient of friction for a given surface is not a unique number, but depends on the test method used to determine it. The minimum coefficient of friction needed for good traction thus also depends on the test method. A lot of different test devices were identified in the literature (Gronqvist et al., 1999; Leclercq, 1999). Their output quantity may be CoF, or force, torque, loss of energy or angle of inclination. Two criteria for determining the acceptability of a method were proposed: (a) its reproducibility, which was assessed in terms of the dispersion of the measurements obtained using a given method when the measurement conditions do not vary; and (b) its validity.
354
R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357
Strandberg (1983) defined a valid method as one that provides pertinent measurements that are correlated with the real slip resistance during walking. In the present study the repeatability and precision of three well known and widely used floor slipperiness testing methods were evaluated. The assessed devices were the British Pendulum Tester, the Tortus II Floor Friction Tester and the Ramp. A total of 36 commercially available floorings were considered to cover a wide range of applications and, then, a wide range of slipperiness (from footing tracks to swimming pool edges). 2. Materials and methods 2.1. British Pendulum Tester The British Pendulum Tester (also known as the ‘portable skid resistance tester’ or ‘TRRL pendulum’) is a dynamic pendulum impact-type used to measure the energy loss when a rubber slider edge is propelled over a test surface. The values measured, BPN (British Pendulum Number), represent the frictional properties of the surface, either in the field or in the laboratory, obtained with the apparatus and the procedures standardised in ASTM E 303-93. The pendulum slider, generally made by 4S rubber (standard simulated shoe sole) for internal floor, is positioned to barely come in contact with the test surface prior to conducting the test. The pendulum is raised to a locked position, and then released, thus allowing the slider to make contact with the surface. A drag pointer indicates the BPN. The greater the friction between the slider and the test surface, the more the swing is retarded, and the larger the BPN reading (Fig. 1). 2.2. Tortus II Tortus II is a microprocessor controlled precision instrument which measures directly the dynamic coefficient of friction, CoF, as it traverses a surface or flooring material used by pedestrians. It provides an instantaneous reading of CoF on a digital display as it moves across the surface and displays the average value of CoF at the completion of the test. Tortus II uses a friction foot mounted on
a leaf spring assembly which is held in contact with the surface under examination by a fixed vertical load. As the instrument moves forward at a constant velocity the horizontal force deflects the foot. The deflection is measured by strain gauges attached to the spring assembly. Micro controllers are used to convert the signal to a digital value of CoF. The standard rubber used as foot material is type 4S and the diameter of foot is 9.5 mm (to reproduce a heal). Guidelines that have been generally used within the European Union indicate that friction values less than 0.4 are unsatisfactory and values below 0.2 are dangerous. Surfaces having values in the range 0.4–0.75 are considered suitable for areas where special care is required. The dynamic coefficient value of 0.4 has also been set as the minimum requirement for pedestrian surface in the Australian/ New Zealand standard. Gronqvist et al. (1999, 2003) noticed that digitised dragsleds, like this instrument, are subject to the problem of water squeezing at the interface between the test foot and the walkway surface. This phenomena creates a temporary bond between these surfaces known as sticktion. It is generally deemed that the cause of sticktion is ‘‘residence time’’ or the delay between the time the test foot of the slipmeter contacts the floor (vertical force due to gravity) and the time of application of the horizontal force. Recently High (2007) did not find a correlation between residence time and sticktion force. 2.3. The Ramp test For the determination of anti-slip properties using Ramp test a test person (TP) moves backwards and forwards in an upright position on a variable-angle ramp. The ramp is repeatedly raised and lowered until the smallest angle is found at which the walker becomes unsteady. The angle of inclination is determined on a floor covering which is subjected to a continuous stream of a contaminant (DIN 51097, 1992; DIN 51130, 2004). There are different procedures depending on what is being tested: floorings for areas where people will be using footwear or floorings for barefoot areas. For the first test the ramp is coated with a specified weight of motor oil to make it slippery, for the latter test running soapy water (1 g/L) covers the flooring and the walker is barefoot. The angle of
Fig. 1. (a) British Pendulum Tester. (b) Tortus II. (c) The Ramp.
R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357
3. Results and discussion
1.25 dry wet
1.00
dynamic CoF
inclination is used to assess the anti-slip properties. To account for human variables, the laboratory first checks the performance of each walker every test day. If he or she rates the reference floorings outside of certain tolerances, that walker cannot test that day. Actually the ramp method is considered the ‘‘gold standard’’ for pedestrian traction tests (Sotter, 2000). In our work, the angle of inclination is determined using four test persons four times each, each time starting with the floor covering in the horizontal position.
355
0.75
0.50
0.25
3.1. British Pendulum Tester
3.2. Tortus II Fig. 3 reports the dynamic CoF obtained for the different materials in dry and wet conditions. It is worth noticing that almost all the results are characterised by good repeatability (mean standard deviation equal to 0.02 in both dry and wet conditions). Only the surface under the friction foot was wetted during the test to maintain adequate wheel traction. Despite this attention,
1 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
0.00
Fig. 2 reports the BPN obtained for the different materials in dry and wet conditions. It is worth noticing that almost all the results are characterised by good repeatability (mean standard deviation equal to 2.07 and 1.97 in dry and wet conditions, respectively). According to the fact that lubricated floorings are more potential slip hazards than dry ones, the dry BPN always resulted higher than the corresponding wet BPN. The test is able to discriminate the skid resistance properties of pedestrian floors, in fact Student’s t-test confirms that the differences in the BPN when measured in dry and wet conditions were statistically significant and the changes in the results were not due to the chance (p < <0.001). Moreover an analysis of variance (ANOVA) showed that Pendulum is able to realise the difference between different materials in both dry and wet conditions (p < <0.001). Considering that the operating instructions of the Wessex skid tester indicate that the slipperiness is low when the BPN is higher than 35 and extremely low when it is higher than 65 (test limits represented by the grey area in Fig. 2): (i) in dry conditions all the tested materials showed low slipperiness and only a few of them showed a extremely high slip resistance; (ii) in wet conditions almost all the floorings showed moderate or high slipperiness (BPN ranging between 25 and 35 or BNP lower than 25, respectively).
sample Fig. 3. Dynamic CoF for different materials in dry and wet conditions.
Tortus II often produced results indicating greater slip resistance of a specific material on wet surfaces than on dry ones. These results, clearly unrealistic, suggest that the application of this device with this surface condition can be seriously questioned. As a confirmation of this hypothesis, the Student’s t-test showed that the results found in dry and wet conditions could be members of the same population (p ¼ 0.26 [ 0.01). On the other hand, ANOVA showed that Tortus II is able to realise the difference between different materials in both dry and wet condition (p 0.001). Considering that the operating instructions of the Tortus II indicate that the surfaces are good when the CoF is higher than 0.4 (test limit represented by the grey area in Fig. 3): (i) in dry conditions all the tested materials showed low slipperiness in accordance to the results obtained with Pendulum; (ii) in wet conditions all the floorings showed good performance but this result can be considered unreliable.
3.3. The Ramp Fig. 4 reports the angle of inclination obtained by different test persons (TP) for the different materials in wet conditions. Considering a specific material, the value reported for each operator is the arithmetic mean of four measurements. The repeatability of these mean values is confirmed by the low standard deviation found for the different TPs (mean standard deviation in the range 0.63–0.99).
35.00 30.00
90.00 80.00
angle of inclination (degree)
100.00 dry wet
70.00
50.00 40.00 30.00
20.00 15.00 10.00 5.00
20.00 10.00
0.00 1 2 3 4 5 6 12 13 14 15 16 24 25 26 27 28 29 30 31 32 33 34 35 36
0.00 1 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 3
BPN
60.00
25.00
TP1 TP2 TP3 TP4 Mean
9
sample
sample Fig. 2. BPN for different materials in dry and wet conditions.
Fig. 4. Angle of inclination in Ramp test for different materials and different test persons in wet conditions.
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R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357 1.20
Tortus CoF
1.00 0.80 0.60 0.40
y = 0.013x 0.20
R2 = 0.328
0.00
0
20
40
60
80
BPN Fig. 5. CoF taken with Tortus versus BPN under dry conditions.
Differently considering the results obtained by the different test persons on a specific flooring, the reproducibility is not good (mean standard deviation equal to 2.39). ANOVA showed that also Ramp test is able to realise the difference between different materials in wet condition (p 0.001). DIN 51097 allocates the floor coverings in different quality groups (A, B, C) on the basis of the mean angle of inclination; considering as acceptable all the three quality groups, the limit of 12 can be fixed to draw the grey area in Fig. 3. About 50% of the mean values are placed in this area. 3.4. Tortus II vs Pendulum Due to the lack of statistical significance, the values of CoF obtained with Tortus under wet conditions were eliminated in the present comparison. Fig. 5 reports the comparison between the results obtained with Tortus II and British Pendulum under dry conditions. It is known that there are some differences in the coupling effect related to the slider edge of the Pendulum and the friction foot of Tortus II. As a consequence, the generally suggested correlation between the two set of results CoF ¼ BPN/100 cannot be considered meaningful; in fact the linear regression of the experimental data CoF ¼ BPN 0.013 has a coefficient of determination equal to 0.33 (the statistical significance of this value was proved by test F, p ¼ 0.005 < 0.01). On the other hand, it is evident that all the results are placed in the region illustrated in Fig. 5, indicating floorings with acceptable anti-slip properties. 3.5. Pendulum vs Ramp Since the CoF is not a constant for any particular material, but is typical of two materials sliding against each other under a given set
Fig. 6. BPN versus angle of inclination taken with Ramp test.
of environmental conditions, the comparison between results obtained with ramp and pendulum can be done only in terms of acceptability. However, even if a comparison is tried, the coefficient of determination is very low (R2 ¼ 0.13) and it lacks of statistical significance (test F, p ¼ 0.09 > 0.01). The acceptability of a material, according to the ramp test, corresponds to its belonging to a quality group (A, B or C), then at angles of inclination higher than 12 . On the other hand BPN defines different groups based on the risk of slipperiness. Even if the ‘‘moderate’’ classification of flooring (25 < BPN < 35) is considered as acceptable, the two methods give the same results only in 60% of cases (Fig. 6). On this basis, if the ramp has to be considered the gold standard, the British Pendulum cannot be considered designed for testing pedestrian walkways. 4. Conclusions The slip resistance of 36 commonly used floor coverings was measured with the British Pendulum Tester, the Tortus II and Ramp under dry and wet conditions. The results of a one way ANOVA indicated that the precision and ability of the British Pendulum Tester to discriminate material slipperiness was acceptable under both dry and wet conditions. The precision and ability to discriminate material slipperiness of the Tortus II was acceptable only under dry conditions. In wet conditions it performed poorly, probably because it was subjected to sticktion. The repeatability of Ramp results can be seriously questioned. Its classification of flooring slipperiness is based on the subjective evaluation of the operator and the low number of test persons required in the standard produce a not statistically significant result. It should be pointed out that it is impossible to find a univocal correlation between results obtained by different methods. Then a new approach to obtain a ‘‘gold’’ standard could avoid to reconcile the differences in numerical results of the various tribometers and identify a set of external calibration materials. If it represents the range (low to high) of pedestrian slip resistance situations, a valid floor slip resistance tester should rank these materials in the proper order, thereby developing a customised calibration curve. Acknowledgments The authors are grateful to Api S.p.A. for its useful cooperation in the research activities. References ASTM E 303-93, Standard test method for measuring surface frictional properties using the British Pendulum Tester. Cham, R., Redfern, M., 2002. Heel contact dynamics during slip events on level and inclined surfaces. Safety Science 40, 559–576. Chang, W., 1999. The effect of surface roughness on the measured of slip resistance. International Journal of Industrial Ergonomics 24, 299–313. Chang, W., Matz, S., 2001. The slip resistance of common footwear materials measured with two slipmeters. Applied Ergonomics 32, 549–558. DIN 51097, 1992. Testing of foor coatings – determination of anti-slip properties, wet-loaded barefoot areas. Walking method – Ramp test. DIN 51130, 2004. Testing of floor coverings – determination of anti-slip properties – workrooms and field of activities with slip danger. Walking method – Ramp test. Gronqvist, R., 1989. An apparatus and a method for determining the slip resistance of shoes and floors by simulation of human foot motions. Ergonomics 32, 979–995. Gronqvist, R., Hirvonen, M., Tohv, A., 1999. Evaluation of three portable floor slipperiness testers. International Journal of Industrial Ergonomics 25, 85–95. Gronqvist, R., Hirvonen, M., Rajama¨ki, E., Matz, S., 2003. The validity and reliability of a portable slip meter for determining floor slipperiness during simulated heel strike. Accident Analysis and Prevention 35, 211–225.
R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357 Hanson, J., Redfern, M., Mazumdar, M., 1999. Predicting slips and falls considering required and available friction. Ergonomics 42, 1619–1633. High, S., 2007. Slip Resistance Testing Research Activities. Available from:
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Kim, I., 2004b. Development of a new analyzing model for quantifying pedestrian slip resistance characteristics: part II – experiments and validations. International Journal of Industrial Ergonomics 33, 403–414. Leclercq, S., 1999. The prevention of slipping accidents: a review and discussion of work related to the methodology of measuring slip resistance. Safety Science 31, 95–125. Marpet, M., 2002. Improved characterization of tribometric test results. Safety Science 40, 705–714. Sotter, G. (Ed.), 2000. Stop Slip and Fall Accidents!. Sotter Engineering Corporation. Strandberg, L., Lanshammar, H., 1981. The dynamics of slipping accidents. Journal of Occupational Accidents 3, 153–162. Strandberg, L., 1983. Ergonomics applied to slipping accidents. In: Kvalseth, T.O. (Ed.), Ergonomics of Workstation Design. Butterworths, London, pp. 201–228.
Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
A comparison of results from portable and laboratory floor slipperiness testers R. Ricotti*, M. Delucchi, G. Cerisola DICheP, University of Genoa, P.le Kennedy 1, 16129 Genoa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2008 Received in revised form 25 June 2008 Accepted 26 July 2008 Available online 14 September 2008
The slip resistance of many commercial floor coverings used in a wide range of applications was measured with the British Pendulum Tester, the Tortus II and the Ramp test under dry and wet conditions. The repeatability and validity of the results of these methods were considered. The results showed that British Pendulum is able to diversify between dry and wet conditions, moreover ANOVA showed that it is possible to discriminate different materials. Results obtained with Tortus revealed that this device cannot distinguish between dry and wet measurements, but it is able to discriminate dissimilar materials. Ramp test can differentiate materials, but it produces different results depending on the test person involved; its repeatability can be seriously questioned. Generally, the results indicated that the ranking of materials depends highly on the slipmeters and surface conditions.
Keywords: Slip resistance Friction Tribometers Tortus British Pendulum Ramp
Relevance to industry: It is impossible to find a univocal correlation between results obtained by different methods, but flooring manufacturers need to verify and optimise the anti-slipperiness of their products. Then a new approach to obtain a ‘‘gold’’ standard that avoid to reconcile the differences in numerical results of the various tribometers and based on a set of external calibration materials could be a more useful flooring resistance tester. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Slipping incidents result from one or more factors including the person involved; the activity performed; environmental factors such as contaminants (water, grease, frost, dust), distractions, temperature and lighting; and the characteristic of the footwear and of the walking surface (Kim et al., 2001). All of these elements can combine to determine whether traction (or slip resistance) is adequate to prevent a slip. Several authors studied the tribological phenomena occurring at the surface/shoe interface when a subject is walking normally (Cham and Redfern, 2002; Gronqvist, 1989; Strandberg and Lanshammar, 1981; Hanson et al., 1999; Marpet, 2002). Other authors studied the nature of the mating materials, floor surface and footwear (Chang and Matz, 2001; Kim and Smith, 2000; Kim, 2004a,b). Every surface has some degree of roughness, though the scale of the roughness might be microscopic (Chang, 1999). Traction between two rough surfaces (soling and floor) results in two types of interactions: adhesion and interlock. Adhesion results from molecular attraction between the two surfaces and exists even when both surfaces are quite smooth. Interlock is
* Corresponding author. Tel.: þ39 01 03 536022; fax: þ39 01 03 536028. E-mail address: [email protected] (R. Ricotti). 0169-8141/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2008.07.004
a mechanical interaction involving peaks and valleys that may be microscopically small. A lubricant can defeat both adhesion and interlock to some extent. Centuries ago, scientists noticed that frictional force resisting movement between two hard surfaces is proportional to the force pressing the surfaces together. ‘‘Coefficient of friction’’, CoF, is the term used for the constant ratio between the frictional force and the pressing force. It is now known that traction, or slip resistance, is not as simple as this, particularly when water or oil separates the surfaces, but the concept of a coefficient of friction is still used. Coefficient of friction for a given surface is not a unique number, but depends on the test method used to determine it. The minimum coefficient of friction needed for good traction thus also depends on the test method. A lot of different test devices were identified in the literature (Gronqvist et al., 1999; Leclercq, 1999). Their output quantity may be CoF, or force, torque, loss of energy or angle of inclination. Two criteria for determining the acceptability of a method were proposed: (a) its reproducibility, which was assessed in terms of the dispersion of the measurements obtained using a given method when the measurement conditions do not vary; and (b) its validity.
354
R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357
Strandberg (1983) defined a valid method as one that provides pertinent measurements that are correlated with the real slip resistance during walking. In the present study the repeatability and precision of three well known and widely used floor slipperiness testing methods were evaluated. The assessed devices were the British Pendulum Tester, the Tortus II Floor Friction Tester and the Ramp. A total of 36 commercially available floorings were considered to cover a wide range of applications and, then, a wide range of slipperiness (from footing tracks to swimming pool edges). 2. Materials and methods 2.1. British Pendulum Tester The British Pendulum Tester (also known as the ‘portable skid resistance tester’ or ‘TRRL pendulum’) is a dynamic pendulum impact-type used to measure the energy loss when a rubber slider edge is propelled over a test surface. The values measured, BPN (British Pendulum Number), represent the frictional properties of the surface, either in the field or in the laboratory, obtained with the apparatus and the procedures standardised in ASTM E 303-93. The pendulum slider, generally made by 4S rubber (standard simulated shoe sole) for internal floor, is positioned to barely come in contact with the test surface prior to conducting the test. The pendulum is raised to a locked position, and then released, thus allowing the slider to make contact with the surface. A drag pointer indicates the BPN. The greater the friction between the slider and the test surface, the more the swing is retarded, and the larger the BPN reading (Fig. 1). 2.2. Tortus II Tortus II is a microprocessor controlled precision instrument which measures directly the dynamic coefficient of friction, CoF, as it traverses a surface or flooring material used by pedestrians. It provides an instantaneous reading of CoF on a digital display as it moves across the surface and displays the average value of CoF at the completion of the test. Tortus II uses a friction foot mounted on
a leaf spring assembly which is held in contact with the surface under examination by a fixed vertical load. As the instrument moves forward at a constant velocity the horizontal force deflects the foot. The deflection is measured by strain gauges attached to the spring assembly. Micro controllers are used to convert the signal to a digital value of CoF. The standard rubber used as foot material is type 4S and the diameter of foot is 9.5 mm (to reproduce a heal). Guidelines that have been generally used within the European Union indicate that friction values less than 0.4 are unsatisfactory and values below 0.2 are dangerous. Surfaces having values in the range 0.4–0.75 are considered suitable for areas where special care is required. The dynamic coefficient value of 0.4 has also been set as the minimum requirement for pedestrian surface in the Australian/ New Zealand standard. Gronqvist et al. (1999, 2003) noticed that digitised dragsleds, like this instrument, are subject to the problem of water squeezing at the interface between the test foot and the walkway surface. This phenomena creates a temporary bond between these surfaces known as sticktion. It is generally deemed that the cause of sticktion is ‘‘residence time’’ or the delay between the time the test foot of the slipmeter contacts the floor (vertical force due to gravity) and the time of application of the horizontal force. Recently High (2007) did not find a correlation between residence time and sticktion force. 2.3. The Ramp test For the determination of anti-slip properties using Ramp test a test person (TP) moves backwards and forwards in an upright position on a variable-angle ramp. The ramp is repeatedly raised and lowered until the smallest angle is found at which the walker becomes unsteady. The angle of inclination is determined on a floor covering which is subjected to a continuous stream of a contaminant (DIN 51097, 1992; DIN 51130, 2004). There are different procedures depending on what is being tested: floorings for areas where people will be using footwear or floorings for barefoot areas. For the first test the ramp is coated with a specified weight of motor oil to make it slippery, for the latter test running soapy water (1 g/L) covers the flooring and the walker is barefoot. The angle of
Fig. 1. (a) British Pendulum Tester. (b) Tortus II. (c) The Ramp.
R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357
3. Results and discussion
1.25 dry wet
1.00
dynamic CoF
inclination is used to assess the anti-slip properties. To account for human variables, the laboratory first checks the performance of each walker every test day. If he or she rates the reference floorings outside of certain tolerances, that walker cannot test that day. Actually the ramp method is considered the ‘‘gold standard’’ for pedestrian traction tests (Sotter, 2000). In our work, the angle of inclination is determined using four test persons four times each, each time starting with the floor covering in the horizontal position.
355
0.75
0.50
0.25
3.1. British Pendulum Tester
3.2. Tortus II Fig. 3 reports the dynamic CoF obtained for the different materials in dry and wet conditions. It is worth noticing that almost all the results are characterised by good repeatability (mean standard deviation equal to 0.02 in both dry and wet conditions). Only the surface under the friction foot was wetted during the test to maintain adequate wheel traction. Despite this attention,
1 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
0.00
Fig. 2 reports the BPN obtained for the different materials in dry and wet conditions. It is worth noticing that almost all the results are characterised by good repeatability (mean standard deviation equal to 2.07 and 1.97 in dry and wet conditions, respectively). According to the fact that lubricated floorings are more potential slip hazards than dry ones, the dry BPN always resulted higher than the corresponding wet BPN. The test is able to discriminate the skid resistance properties of pedestrian floors, in fact Student’s t-test confirms that the differences in the BPN when measured in dry and wet conditions were statistically significant and the changes in the results were not due to the chance (p < <0.001). Moreover an analysis of variance (ANOVA) showed that Pendulum is able to realise the difference between different materials in both dry and wet conditions (p < <0.001). Considering that the operating instructions of the Wessex skid tester indicate that the slipperiness is low when the BPN is higher than 35 and extremely low when it is higher than 65 (test limits represented by the grey area in Fig. 2): (i) in dry conditions all the tested materials showed low slipperiness and only a few of them showed a extremely high slip resistance; (ii) in wet conditions almost all the floorings showed moderate or high slipperiness (BPN ranging between 25 and 35 or BNP lower than 25, respectively).
sample Fig. 3. Dynamic CoF for different materials in dry and wet conditions.
Tortus II often produced results indicating greater slip resistance of a specific material on wet surfaces than on dry ones. These results, clearly unrealistic, suggest that the application of this device with this surface condition can be seriously questioned. As a confirmation of this hypothesis, the Student’s t-test showed that the results found in dry and wet conditions could be members of the same population (p ¼ 0.26 [ 0.01). On the other hand, ANOVA showed that Tortus II is able to realise the difference between different materials in both dry and wet condition (p 0.001). Considering that the operating instructions of the Tortus II indicate that the surfaces are good when the CoF is higher than 0.4 (test limit represented by the grey area in Fig. 3): (i) in dry conditions all the tested materials showed low slipperiness in accordance to the results obtained with Pendulum; (ii) in wet conditions all the floorings showed good performance but this result can be considered unreliable.
3.3. The Ramp Fig. 4 reports the angle of inclination obtained by different test persons (TP) for the different materials in wet conditions. Considering a specific material, the value reported for each operator is the arithmetic mean of four measurements. The repeatability of these mean values is confirmed by the low standard deviation found for the different TPs (mean standard deviation in the range 0.63–0.99).
35.00 30.00
90.00 80.00
angle of inclination (degree)
100.00 dry wet
70.00
50.00 40.00 30.00
20.00 15.00 10.00 5.00
20.00 10.00
0.00 1 2 3 4 5 6 12 13 14 15 16 24 25 26 27 28 29 30 31 32 33 34 35 36
0.00 1 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 3
BPN
60.00
25.00
TP1 TP2 TP3 TP4 Mean
9
sample
sample Fig. 2. BPN for different materials in dry and wet conditions.
Fig. 4. Angle of inclination in Ramp test for different materials and different test persons in wet conditions.
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R. Ricotti et al. / International Journal of Industrial Ergonomics 39 (2009) 353–357 1.20
Tortus CoF
1.00 0.80 0.60 0.40
y = 0.013x 0.20
R2 = 0.328
0.00
0
20
40
60
80
BPN Fig. 5. CoF taken with Tortus versus BPN under dry conditions.
Differently considering the results obtained by the different test persons on a specific flooring, the reproducibility is not good (mean standard deviation equal to 2.39). ANOVA showed that also Ramp test is able to realise the difference between different materials in wet condition (p 0.001). DIN 51097 allocates the floor coverings in different quality groups (A, B, C) on the basis of the mean angle of inclination; considering as acceptable all the three quality groups, the limit of 12 can be fixed to draw the grey area in Fig. 3. About 50% of the mean values are placed in this area. 3.4. Tortus II vs Pendulum Due to the lack of statistical significance, the values of CoF obtained with Tortus under wet conditions were eliminated in the present comparison. Fig. 5 reports the comparison between the results obtained with Tortus II and British Pendulum under dry conditions. It is known that there are some differences in the coupling effect related to the slider edge of the Pendulum and the friction foot of Tortus II. As a consequence, the generally suggested correlation between the two set of results CoF ¼ BPN/100 cannot be considered meaningful; in fact the linear regression of the experimental data CoF ¼ BPN 0.013 has a coefficient of determination equal to 0.33 (the statistical significance of this value was proved by test F, p ¼ 0.005 < 0.01). On the other hand, it is evident that all the results are placed in the region illustrated in Fig. 5, indicating floorings with acceptable anti-slip properties. 3.5. Pendulum vs Ramp Since the CoF is not a constant for any particular material, but is typical of two materials sliding against each other under a given set
Fig. 6. BPN versus angle of inclination taken with Ramp test.
of environmental conditions, the comparison between results obtained with ramp and pendulum can be done only in terms of acceptability. However, even if a comparison is tried, the coefficient of determination is very low (R2 ¼ 0.13) and it lacks of statistical significance (test F, p ¼ 0.09 > 0.01). The acceptability of a material, according to the ramp test, corresponds to its belonging to a quality group (A, B or C), then at angles of inclination higher than 12 . On the other hand BPN defines different groups based on the risk of slipperiness. Even if the ‘‘moderate’’ classification of flooring (25 < BPN < 35) is considered as acceptable, the two methods give the same results only in 60% of cases (Fig. 6). On this basis, if the ramp has to be considered the gold standard, the British Pendulum cannot be considered designed for testing pedestrian walkways. 4. Conclusions The slip resistance of 36 commonly used floor coverings was measured with the British Pendulum Tester, the Tortus II and Ramp under dry and wet conditions. The results of a one way ANOVA indicated that the precision and ability of the British Pendulum Tester to discriminate material slipperiness was acceptable under both dry and wet conditions. The precision and ability to discriminate material slipperiness of the Tortus II was acceptable only under dry conditions. In wet conditions it performed poorly, probably because it was subjected to sticktion. The repeatability of Ramp results can be seriously questioned. Its classification of flooring slipperiness is based on the subjective evaluation of the operator and the low number of test persons required in the standard produce a not statistically significant result. It should be pointed out that it is impossible to find a univocal correlation between results obtained by different methods. Then a new approach to obtain a ‘‘gold’’ standard could avoid to reconcile the differences in numerical results of the various tribometers and identify a set of external calibration materials. If it represents the range (low to high) of pedestrian slip resistance situations, a valid floor slip resistance tester should rank these materials in the proper order, thereby developing a customised calibration curve. Acknowledgments The authors are grateful to Api S.p.A. for its useful cooperation in the research activities. References ASTM E 303-93, Standard test method for measuring surface frictional properties using the British Pendulum Tester. Cham, R., Redfern, M., 2002. Heel contact dynamics during slip events on level and inclined surfaces. Safety Science 40, 559–576. Chang, W., 1999. The effect of surface roughness on the measured of slip resistance. International Journal of Industrial Ergonomics 24, 299–313. Chang, W., Matz, S., 2001. The slip resistance of common footwear materials measured with two slipmeters. Applied Ergonomics 32, 549–558. DIN 51097, 1992. Testing of foor coatings – determination of anti-slip properties, wet-loaded barefoot areas. Walking method – Ramp test. DIN 51130, 2004. Testing of floor coverings – determination of anti-slip properties – workrooms and field of activities with slip danger. Walking method – Ramp test. Gronqvist, R., 1989. An apparatus and a method for determining the slip resistance of shoes and floors by simulation of human foot motions. Ergonomics 32, 979–995. Gronqvist, R., Hirvonen, M., Tohv, A., 1999. Evaluation of three portable floor slipperiness testers. International Journal of Industrial Ergonomics 25, 85–95. Gronqvist, R., Hirvonen, M., Rajama¨ki, E., Matz, S., 2003. The validity and reliability of a portable slip meter for determining floor slipperiness during simulated heel strike. Accident Analysis and Prevention 35, 211–225.
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