Friction after polishing – initial experiences of new test method to predict skid resistance of Irish road surface materials

Authors: Dr Shaun Friel, business development manager, Hillstreet Quarries Ltd, Arigna, Co Roscommon, Ireland; Dr David Woodward, reader in infrastructure engineering and director for the Centre of Subjects Allied to Built Environment Research (SABER) at Ulster University, School of the Built Environment, Jordanstown, Shore Road, Newtownabbey, Co Antrim, BT37 0QB The friction after polishing (FAP) test has recently been included as a requirement in the new NRA Specification for Road Works Series 900 [1]. This paper reports one of the first studies into the use of the FAP test using aggregate and asphalt mixtures commonly used in Ireland. The study was carried out at the French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR) located in Nantes as the FAP test equipment is currently not available in Ireland. This study is important as the specification requirements for aggregate and asphalt road construction surfacing materials are being made CPR compliant. Prior to this study, Ireland had almost no experience with the FAP test. This paper compares the three types of asphalt mix commonly used in Ireland and the UK i.e. Stone Mastic Asphalt of 14mm and 10mm nominal size and Hot Rolled Asphalt with 20mm rolled-in chippings. The data produced during the FAP testing of HRA and SMA mixes produced new insight into skid resistance compared to what the PSV test offers. This new information allows a method to optimise the use of aggregate in different types of asphalt mix and to show their frictional speed dependency. However, visual assessment of the FAP roller / asphalt test specimen interface raises questions relating to what the FAP test is actually measuring.

Introduction

Throughout Europe, the specification and testing of aggregate and road construction products are currently being revised due to changes associated with CE marking and the need to declare the performance of the products used. The reason for this is because aggregate and road construction products are two of 35 products specifically named in the Construction Products Regulation (CPR). This European Regulation gives seven generic requirements for all named products, one of which is safety. With regard to meeting this requirement for safety and be able to declare a level of performance, there must be tests for both aggregate and the actual road surface material used i.e. the mixture of aggregate and bitumen. The polished stone value (PSV) test was developed over 70 years ago to test aggregate and has become a harmonised European Standard [2]. However, until recently there has been no harmonised European Standard test method for the testing of asphalt surface mixes. With regard to declaring the performance of an asphalt mix, this is important as the PSV test only assesses the 10/6.3mm size-fraction aggregate, whereas there are many different nominal sized mixes being used, typically ranging from 14mm to 6mm. The PSV test has been the subject of many studies [3, 4, 5, 6]. Although these studies show the PSV test as being able to rank aggregate, the standard PSV test does not fully predict what may happen when the aggregate is subjected to different simulated in-service stressing conditions. These studies have concluded that if the standard laboratory test regime is altered e.g. increase the test load or test speed to reflect different in-service conditions, the aggregate particle / test tyre interface is changed causing new equilibrium conditions reflected by a different pendulum tester value. There are general trends based on rock-type e.g. some types are more susceptible to polishing than would be predicted in the standard test whilst others are able to maintain similar equilibrium conditions irrespective of what test condition is altered.

Background

Over 30 years ago, two German Professors - Wehner and Schulze - developed a test method that assesses both single size aggregate and asphalt mixes. Their test became known as the Wehner Schulze test (WS). In 2014 their method became adopted as a harmonised European Standard test method and is now known as the Friction after Polishing test [7]. Similar to the PSV test, the FAP test is in two parts (i) subject test specimens to simulated trafficking and (ii) measure the effect of this simulated trafficking by measuring wet skid resistance. [caption id="attachment_27198" align="alignright" width="300"]

Figure 1. FAP test equipment[/caption] Studies to assess the FAP test have been undertaken in a number of European and other countries. These have been important as experience with the test equipment had predominately only been confined to Germany. Other countries may use different asphalt mixes across their respective highway networks to those used in Germany. One of the main objectives of these studies was to determine whether the FAP test could be used to assess their range of asphalt mixes and give meaningful predictions of friction. In France, Do et al. [8] studied the evolution of skidding resistance using core specimens extracted from three pavement surfaces shortly after laying. The materials extracted were 10mm asphalt concrete (10mm AC), 10mm very thin asphalt concrete (10mm VTAC), and 6mm VTAC made with spilite PSV 53, gabbro PSV 51 and rhyolite PSV 55 respectively. These materials are widely used on French main and secondary roads. The rock types would typically not be used in the UK and Ireland due to their low PSV values. Analysis of the data found an initial increase in friction to a maximum value, followed by a decrease towards an equilibrium condition. Cores were extracted from the sites after two years trafficking and the data compared with data from the initially extracted specimens. Good agreement was found between the two sets of site cores. Allen et al. [9] compared the PSV and WS test methods for a range of UK asphalt mix and aggregate types using the WS equipment located in Technical University of Berlin. Most of the 11 mixes were proprietary, made with aggregates with a range of declared PSVs. It was found that mixes made with declared PSV 55 granite aggregate outperformed similar mixes made with declared PSV 60 andesite aggregate. Hot rolled asphalt (HRA) mixes had lower friction coefficient values as the application of 20mm pre-coated chippings was increased. Maximum friction coefficients were found to occur at approximately 20,000 cycles for most mixes, with equilibrium conditions not achieved after the standard 180,000 cycles. For the 11 mixes assessed there was no meaningful correlation between WS of the asphalt mixes and declared PSV of the aggregate used in their manufacture. Declared PSV 55 granite used in 6mm and 10mm nominal size asphalt mixes had higher WS friction coefficient values than declared PSV 65 gritstone used as pre-coated 20mm chippings applied to HRA. This last investigation is particularly relevant to Ireland as similar mixes and rock types are used in both countries. Further important studies into the friction after polishing test include work undertaken at the Transport Research Laboratory (TRL) [10, 11] and by Professor Huschek of the Berlin Technical University [12].

Materials assessed in this study

The three asphalt materials assessed in this study were CE marked harmonised asphalt mixes made in accordance with EN 13108. They were sampled during production at an asphalt mixing plant in Northern Ireland. The aggregate used in all three mixes was a Silurian greywacke with declared PSV 63. The asphalt mixes were Stone Mastic Asphalt of 14mm and 10mm nominal size (SMA14 and SMA10) [13] and Hot Rolled Asphalt with 20mm chippings (35/14HRA) [14]. Table 1 summarises the found composition of each asphalt mix. Sample boxes of each asphalt mix were reheated in the laboratory before asphalt test slabs 305 x 305 x 50 mm in size were prepared using a Cooper roller compactor. Three slabs were prepared for each asphalt mix. Cores of 225mm diameter were extracted from each slab by IFSTTAR technicians in Nantes, France for FAP testing.

Sieve size (mm)	Percentage passing (%)
Sieve size (mm)	SMA 14	SMA 10	35/14 HRA
20	100	-	100
14	97	100	97
10	51	99	77
8	39	74	68
6.3	34	44	67
4	31	35	66
2	22	26	65
1	17	18	63
0.500	15	15	59
0.250	12	12	44
0.125	10	10	15
0.063	7.6	7.7	7.4
Bitumen (%)	6.0	6.2	7.3

Table 1. Composition of the asphalt mixes used

Test methodology

[caption id="attachment_27200" align="alignright" width="243"]

Figure 2. Polishing head[/caption] The FAP test equipment is shown in Figure 1. It consists of two rotary heads. One head polishes the specimens with the second head used for friction measurement. The specimens for testing are 225mm in diameter and can either be mosaics of aggregate only (similar to the PSV test method) or asphalt cores. The cores can either be obtained from a road surface or extracted from laboratory compacted slabs. The polishing head is shown in Figure 2 and consists of three solid rubber conical rollers with slots to represent the treads present in a typical tyre. The rollers rotate at 500 rpm giving a linear speed of 17 km/h. The contact pressure is 0.4 N/mm². One rotation of the rotary head equals three roller passes. There is a slip between the cone and specimen surface of between 0.5 per cent and 1 per cent. A water-abrasive mix is projected on to the test specimen surface during the polishing process. [caption id="attachment_27203" align="alignright" width="243"]

Figure 3. Measuring head[/caption] The friction measurement head is shown in Figure 3 and consists of three rubber slider pads 30mm x 14.5mm in size. The measuring head is rotated to a speed of 2,700 rpm (90 km/h) at which point water is projected on to the surface of the specimen to give a theoretical water film thickness of 0.5mm. When the rotating head reaches 3,000 rpm (100 km/h) the motor is turned off and the head is dropped making contact with the specimen surface. The contact pressure is 0.2 N/mm². The rubber pads interact with the specimen surface and the rotating head decelerates to a stop. The frictional forces recorded are used to calculate a friction coefficient. In the standard FAP test the friction coefficient corresponding to 60 km/h is reported.

Test results from this study

[caption id="attachment_27209" align="alignright" width="300"]

Figure 4. Effect of asphalt mix type and deceleration speed after 180,000 roller passes[/caption] With regard to improving the prediction of performance, the FAP test can show how friction coefficient changes as the measuring head decelerates from 100 to 0 km/h. This provides valuable insight into how the rubber sliders interface with the test specimen surface with respect to speed under simulated braking conditions. In comparison, the pendulum tester used in the PSV test gives a single interface speed measurement i.e. a relatively slow 10 km/h. Figure 4 shows example friction coefficient deceleration curves from 100 km/h to 0 km/h as the head decelerates to a stop. The curves shown in Figure 4 are after 180,000 roller passes. The curves clearly show that the FAP friction coefficient is related to both speed and asphalt type. Table 2 compares friction coefficient values recorded at 60 km/h and 10 km/h to illustrate this speed and asphalt mix dependency.

Material	Friction coefficient
Material	60 km/h	10 km/h
HRA	0.30	0.63
SMA 14	0.40	0.71
SMA 10	0.46	0.74

Table 2. Comparison of mix type and FAP friction coefficient at 60 km/h and 10 km/h [caption id="attachment_27211" align="alignright" width="300"]

Figure 5. Friction coefficient for all asphalt specimens up to 180,000 polishing cycles[/caption] Figure 5 plots the development of friction coefficient for the 3 asphalt mix types with 9 test specimens in total. This data relates to a deceleration measurement speed of 60 km/h. Even though the same Silurian greywacke aggregate from the same source was used to make these specimens, the data shows friction coefficients to range from 0.30 to 0.45 after 180,000 polishing cycles. Figure 6 plots the data on a log scale to better illustrate the early development of friction coefficient during testing. Figure 7 plots the development of friction coefficient for the HRA specimens on a log scale. This shows the three HRA test specimens to follow a similar trend i.e. initial decrease in friction coefficient followed by an increase to peak friction. The lower friction coefficient values for HRA throughout the polishing process are most likely due to S3 having slightly more 20mm chippings than the other two HRA specimens. This suggests that the increased interaction between the rubber rollers and slider pads results in a reduction in friction coefficient. [caption id="attachment_27213" align="alignright" width="300"]

Figure 6. Friction coefficient on a log scale to better illustrate early life changes[/caption] Figure 8 plots the development of friction coefficient for the three SMA 14 test specimens. This shows the three SMA 14 specimens to follow a similar trend i.e. initial increase in friction coefficient to peak friction followed by a decrease towards equilibrium. After 180,000 passes there was a considerable range in the three friction coefficient values. Figure 9 plots the development of friction coefficient for the SMA 10 specimens. This shows the SMA 10 test specimens to follow similar trends to the SMA 14 i.e. initial increase in friction coefficient to peak friction followed by a decrease to equilibrium conditions. After peak friction, test specimen S9 gave much lower values compared to the other two test specimens.

Discussion

[caption id="attachment_27216" align="alignright" width="300"]

Figure 7. Friction coefficient for HRA test specimens[/caption] This study has compared three asphalt mixes i.e. HRA, SMA 14 and SMA 10 all made with the same PSV 63 aggregate, using the FAP test. HRA, SMA and BBA HAPAS proprietary high stone content products derived from SMA are important mixes in Ireland and the UK. Although HRA is a standardised European Standard asphalt mix, it is not used in any other European country. For example, researchers at IFSTTAR had very little knowledge of HRA and this investigation was the first time HRA had been FAP tested at IFSTTAR. This makes the investigation summarised in this paper significant as it offers new experience and ways to compare and optimise the use of construction products. [caption id="attachment_27221" align="alignright" width="300"]

Figure 8. Friction coefficient for SMA 14 test specimens on a log scale[/caption] The FAP testing carried out in this study found different deceleration curves and friction coefficients for the same aggregate when used in three different asphalt mixes. This questions the current philosophy in Ireland and the UK of a specification for skidding resistance based on aggregate PSV. It raises issues ranging from over specifying / underspecifying to not optimising the potential of an aggregate source. Testing the asphalt mix rather than simply the 10/6mm size aggregate suggests that skidding resistance for an aggregate can be better optimised by using it in the most appropriate asphalt mix. [caption id="attachment_27218" align="alignright" width="300"]

Figure 9. Friction coefficient for SMA 10 test specimens on a log scale[/caption] The first measurement of friction coefficient was carried out after 1,600 polishing cycles. Friction coefficient within the very early life time period (0 to 1,600 polishing cycles) may have been different to that shown in the plots for HRA and SMA. Despite this the FAP data shows similar trends in the development of friction coefficient dependant on mix type. Both the SMA 14 and SMA 10 test specimens followed similar trends in the development of friction coefficient (measured at the 60 km/h point of the deceleration curve). The friction coefficient of both mixes increased to a maximum value at approximately 15,000 polishing cycles. The HRA data shows an initial decrease in friction coefficient followed by an increase to a maximum value. [caption id="attachment_27219" align="alignright" width="300"]

Figure 10. Bitumen removal showing the FAP roller / HRA 20mm chipping interface[/caption] The increase in SMA friction coefficient may be explained by steady removal or stripping of the binder/mastic component surrounding the coarse aggregate in contact with the polishing rollers. The initial decrease in friction coefficient for the HRA is probably related to a smaller contact area for the 20mm chippings interface. The low penetration grade bitumen (50 pen) surrounding the pre-coated chippings may have undergone a short period of initial polishing giving the early life decrease. The bitumen then starts to wear off the 20mm chippings exposing the microtexture of the aggregate. Figure 10 shows the interface between the FAP rollers and the 20mm chippings of a HRA test specimen. This shows the FAP polishing rollers not to have fully draped around the chippings i.e. there is still bitumen adhered to the surface of the aggregate particles. Rather, the polished interface is the top surface of the 20mm chippings. It is assumed that the same happens as the FAP friction measuring rubber sliders interface the chippings. Comparison with visual observation of trafficked HRA shows this issue of limited interface is not comparable to real life where greater amounts of pneumatic tyre rubber drape occurs at this interface. [caption id="attachment_27227" align="alignright" width="300"]

Figure 11. Bitumen removal showing the FAP roller / SMA14 interface[/caption] Figures 11 and 12 show the FAP roller / SMA14 and SMA10 interfaces respectively. Both mixes have similar looking interfaces. The figures show the SMA interfaces to be very different to that of the HRA. There is relatively little coarse aggregate exposure for both nominal sized SMA mixes. The FAP polishing rollers have modified the original surface texture and created a new type of macrotexture consisting of discrete patches of exposed coarse aggregate and mastic. These interface images show the test specimen surface textures and contact areas to be different to what would be observed in real life with real trafficking. The lower friction coefficient found for the HRA test specimens is most likely caused by greater loading of the smaller contact area by the polishing rollers. [caption id="attachment_27240" align="alignright" width="300"]

Figure 12. Bitumen removal showing the FAP roller / SMA10 interface[/caption] A similar issue relating to FAP roller contact was found by Dunford [15] during an investigation of the polishing process for aggregate mosaic test specimens. Dunford used a paint erosion technique to show FAP roller contact with 10mm aggregate. Figure 13 shows a painted aggregate mosaic test specimen after 90,000 polishing cycles. This shows a considerable amount of the supposedly polished test specimen aggregate to still be covered in paint. Figure 14 is the same aggregate particle before and after polishing and shows where the paint has been removed at this interface. This paint removal technique was repeated at Ulster University using PSV test samples and similar results found [16].

Conclusions

[caption id="attachment_27242" align="alignright" width="300"]

Figure 13. Polished mosaic showing paint erosion during testing [15][/caption]This paper has reported one of the first studies in Ireland into the use of the new harmonised European Standard Friction after Polishing test to assess asphalt surfacing mixes made with the same declared PSV 63 aggregate. As the FAP test equipment was not available in Ireland the research was carried out at the IFSTTAR research institute in Nantes, France. This study was the first time HRA had been FAP tested at the national IFSTTAR research institute. This illustrates how much is known about the FAP test and the testing of asphalt mixes that are not used in Germany or France. The PSV test assesses the 10/6.3mm size fraction aggregate and the standard test has been used for many years to specify aggregate for use in road surfaces. However, the PSV test is not the ultimate state of polish for an aggregate but rather produces a result that is dependent on the test conditions. If the tests conditions are kept standard, then different laboratories should produce similar values of PSV. If the test conditions are changed then different interface conditions are created and a different value of skid resistance may occur. This is a significant problem if the PSV test on a single size of aggregate is expected to predict performance as stated in the Declaration of Performance required for CE Marking. [caption id="attachment_27244" align="alignright" width="159"]

Figure 14. Aggregate particle before and after paint erosion [15][/caption]The data produced during the FAP testing of HRA and SMA mixes produced new insight into skid resistance compared to what the PSV test offers. FAP testing allows the assessment of both aggregates and asphalt mixes. The measurement process produces a deceleration curve that shows changes in friction coefficient as the slider pads decelerate to a stop. This new information allows a method to optimise the use of aggregate in different types of asphalt mix and to show their frictional speed dependency. These are characteristics that may be argued to offer better prediction of skid resistance for Declaration of Performance and CE Marking purposes. However, visual assessment of the FAP roller / asphalt test specimen interface raises questions relating to what the FAP test is actually measuring. The same questions have recently been raised for painted PSV test specimens. So, whilst the overall finding of this study is positive regarding improved optimisation of aggregate / asphalt mix use and prediction of skid resistance, the FAP test requires substantial further work to better understand what is happening at the test specimen interface during polishing and measurement of friction.

References

[1] NRA, 2015. NRA Specification for Road Works. Volume 1, Series 900, Road Pavements – Bituminous Materials. National Roads Authority, Dublin. [2] I.S. EN 1097-8:2009. Tests for mechanical and physical properties of aggregates - Part 8: Determination of the polished stone value. National Standards Authority of Ireland, Dublin. [3] Woodward, D., 1995. Laboratory prediction of surfacing aggregate performance. DPhil Thesis, School of the Built Environment, Ulster University. [4] Perry, M. J., 1996. A study of the factors influencing the polishing characteristics of gritstone aggregate. DPhil Thesis, School of the Built Environment, Ulster University. [5] Jellie, J., 2003. A study of factors affecting skid resistance characteristics. DPhil Thesis, School of the Built Environment, Ulster University. [6] Friel, S., 2013. Variation of the friction characteristics of road surfacing materials with time. DPhil Thesis, School of the Built Environment, Ulster University. [7] EN 12697-49: 2014. Bituminous mixtures - Test methods for hot mix asphalt - Part 49: Determination of friction after polishing. British Standards Institution, London. [8] Do, M.-T., Tang, Z., Kane, M. and de Larrard, F., 2009. Pavement polishing – Development of a dedicated laboratory test and its correlation with road results. Wear, Volume 263, pp. 36-42. [9] Allen, B., Phillips, P., Woodward, D. and Woodside, A., 2008. Prediction of UK surfacing skid resistance using Wehner Schulze and PSV. In: International Conference Managing Road and Runway Surfaces to improve Safety, Cheltenham, England. [10] Woodbridge, M. and Dunford, A. R. P., 2006. Wehner-Schulze machine: First UK experiences with a new test for polishing resistance in aggregates. Published Project Report PPR144, TRL Ltd, Crowethorne. [11] Dunford, A., 2013. Friction and the texture of aggregate particles used in the road surface course. DPhil Thesis, University of Nottingham. [12] Huschek, S., 2006. Traffic simulation by Wehner/Schulze-Method on nine asphaltic mixes. Report No. 719. Report for Aggregate Industries, Derbyshire, U.K. [13] I.S. EN 13108-5:2006. Bituminous Mixtures - Material Specifications - Part 5: Stone Mastic Asphalt. National Standards Authority of Ireland, Dublin. [14] I.S. EN 13108-4:2006. Bituminous Mixtures - Material Specifications - Part 4: Hot Rolled Asphalt. National Standards Authority of Ireland, Dublin. [15] Dunford, A.M., Parry, A.R., Shipway, P.H. and Viner H.E., 2012. Three-dimensional characterisation of surface texture for road stones undergoing simulated traffic wear. Wear 292-293, pp. 188-196. [16] Woodward, D., Millar, P., McQuaid, G., McCall, R. and Boyle, O. 2015. PSV Tyre / Test Specimen Contact. 8th RILEM International Symposium on Testing and Characterisation of Sustainable and Innovative Bituminous Materials, Ancona Italy, 7th to 9th October DOI 10.1007/978-94-017-7342-3.