EXPERIMENTAL ASPHALT SECTIONS IN THE RUNWAY TOUCH DOWN ZONE ON JOHANNESBURG INTERNATIONAL AIRPORT

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1 EXPERIMENTAL ASPHALT SECTIONS IN THE RUNWAY TOUCH DOWN ZONE ON JOHANNESBURG INTERNATIONAL AIRPORT P.B. Joubert 1, L. Gounder 2 and S. van Wyk 1 1 Stewart Scott International 2 Norwich Close, Sandton, South Africa. 2 Airports Company South Africa Johannesburg International Airport, South Africa. ABSTRACT The performance of four asphalt mix types were studied under the severe operational conditions in the Touch Down Zone of a high volume runway on a major international airport, Johannesburg International Airport. The asphalt mixes were a medium and a coarse continuously graded mix, an open graded mix and a stone mastic asphalt (SMA) mix. The performance criteria included the durability of the mixes to ensure that the integrity of the surfacing was maintained, as well as the surface texture and skid resistance as affected by the rubber build-up on the surfacing. The study showed that the SMA mix gave the best performance in terms of the overall criteria. The continuously graded mixes had good durability but did not perform as well in terms of retention of skid resistance. The open graded mix developed integrity problems due to aggregate loss. Keywords: Durability, porous asphalt, skid resistance, SMA, surface texture, rubber deposit 1. INTRODUCTION An experimental section using different asphalt surfacing mixes was constructed in the Touch Down Zone (TDZ) of Runway 03R/21L at Johannesburg International Airport in December This was designed and managed by Stewart Scott International for the Airports Company South Africa (ACSA), in order to assess the performance of the different types of asphalt wearing courses under the severe operational conditions of the high volume TDZ. The objectives of the study were to: Evaluate the skid resistance and texture depth of each surface type over time. Evaluate integrity and durability of each surface type. Compare sections by visual assessment (including rubber deposit build-up). The outcome of the assessment of the experimental section was used to make a recommendation on the most appropriate type of wearing course for the use on the TDZ of high volume runways. The recommended asphalt type should retain its integrity while minimising the rate of rubber deposit build-up, and therefore reduce the frequency of rubber removal. This paper discusses the results and observations made during the observation period from 1999 to 2000 and also summarises recent observations in Proceedings of the 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) September 2004 ISBN Number: Sun City, South Africa Proceedings produced by: Document Transformation Technologies cc

2 2. BACKGROUND 2.1 Resistance to Skidding This section provides a brief overview of aspects that influence the resistance to skidding on a runway surface. Frictional resistance is essential for an aircraft to be able to decelerate and turn. If the frictional resistance proves to be insufficient the wheel begins to slide and the aircraft is said to be skidding. When this occurs the pilot s control over the aircraft is drastically reduced. Friction between aircraft tyre and runway surface has been found to consist of two components, namely adhesion and deformation (hysteresis) (Visser, 1974). The frictional force due to adhesion is the result of the shear resistance resulting from the molecular interaction of the rubber and stones and its magnitude is determined by the nature of the two materials in contact. It is generally accepted that dry pavement surfaces seldom cause skid resistance problems. However once wet, the skid characteristics of surfaces may change drastically and will inevitably be lower. Under wet runway conditions the hysteresis component provides the major portion of skid-resistance. (Kolbe, 1998) Hydroplaning or aquaplaning Experiments by the National Aeronautics and Space Agency (USA) and others have shown that this hydroplaning can certainly occur whenever the runway surface is covered with a film of water of adequate thickness and the aircraft travelling thereon travel with sufficient speed. The Runway must have adequate longitudinal and/or transverse drainage to prevent the accumulation of deep water over a lengthy distance. Experiments also indicated that in the case of runways, grooving of the runways may effectively reduce the probability of the occurrence, possibly by improving the drainage characteristics of the runway Surface texture The surface texture refers to the existence or not of open hollows between the surface aggregates. These hollows will allow for the dissipation of water between the wheels and surface, thereby preventing aquaplaning. Surface texture can also be defined as a combination of micro-texture (detailed surface characteristics of aggregate) and macro-texture (largerscale surface profile, visible to eye) of the surface of the wearing course of a pavement (See Figure 1a). Microtexture 0-0.2mm Macrotexture mm Figure 1(a). Illustration of texture types (Rogers and Gargett, 1991). For good low-speed skidding resistance, a harsh micro-texture is essential to provide friction between an aircraft tyre and a wet runway surface. It assists in penetrating the last thin film of water and provides effective contact between runway and tyre. However, a very harsh micro-texture can cause increased tyre wear without a proportional increase in wet friction. During skidding the tyre is in contact with the runway surface at all times. Figure 1b indicates the effect of surface texture on tyre-surface coefficient of friction. A pronounced macro-texture is important for high-speed skidding resistance and to facilitate the rapid removal of water before

3 the action of the micro-texture can be brought into effect. With modern low-resilience tyres, aggregate is able to deform the tyre tread and further improve skidding resistance. The highest skidding resistance is obtained with a rough macro-texture, harsh micro-texture and a low-resilience tyre. The build-up of rubber deposits due to initial tyre contact in the TDZ reduces the wet-weather skid resistance of the runway as it affects both micro- and macro-texture. Figure 1(b). Effect of surface texture on tyre-surface friction (ICAO, 1995). 3. THE EXPERIMENTAL SECTION CONSTRUCTION 3.1 Alternative Asphalt Types Four alternate types of asphalt wearing courses were selected for design and construction. These four types are: The existing runway surface consisting of a SBR modified continuous (dense) medium (13mm) graded asphalt mix. (Existing thickness 40mm). The existing rubber build-up was partially removed during construction of the experimental section. The rubber deposits were only partially removed due to inadequacies of the removal system. In this mix, illustrated in Figure 2 as dense graded, the larger aggregate floats in a continuous mix of finer aggregate, filler and binder. Continuously (dense) coarse (19mm aggregate) graded asphalt wearing course mix. (50mm thick). This mix is the same as the medium mix but with a larger maximum aggregate size. Stone Mastic Asphalt (SMA; 9mm aggregate) including 0,3% cellulose fibre. (30mm thick). SMA consists of discrete single sized aggregates forming a stone skeleton, which is glued together to support themselves by rich mastic comprising of bitumen, fines and a stabilising agent (micro fibres). Open graded mix (13mm aggregate) as specified in the South African standard specification (COLTO; Type 1 in Table 4202/9), using 0,3% cellulose fibre. The target voids in the mix was 18%. (30mm thick). This mix is the same as the SMA except that the voids in the stone skeleton are not completely filled. A thick bitumen coating of a modified binder is used to glue the skeleton in place.

4 3.2 Location and Layout Figure 2. Comparison of the asphalt mix types (Cambell, 1999). The experimental section was constructed inside the southern TDZ of RWY 03R/21L. This area is well suited as it has a high frequency of landings. The SMA and Open graded mixes were placed on the same side (western) of the centreline. The coarse graded mix was placed east of the centreline. The additional thickness of this mix, resulted in a lower spread rate per ton and it was therefore placed over a shorter length. A section of the existing medium graded mix was left on the east as part of the experimental section. 3.3 Construction The construction work commenced at approximately 14:00 and the asphalt paving work was completed by ±24:00 the same night. The actual areas of asphalt paved in 6m wide strips were: Coarse graded: 900m²; SMA: 1440m²; Open graded: 960m². The rubber build-up on the existing SBR modified medium continuous graded asphalt surfacing was cleaned using the available high pressure water jet method. As the method was not completely effective, a full removal of all rubber deposits was not achieved. This existing asphalt was therefore not representative of a newly constructed finish. The surface was also previously grooved to improve surface drainage and reduce the possibility of aquaplaning. 3.4 Asphalt Characteristics All three new asphalt mixes were sampled and tested during and after the construction to determine the asphalt quality and characteristics of the mixes. A summary of the results is given in Table 1. The results indicate that the target asphalt qualities were achieved. The binder content of the SMA mix was higher than targeted. However, it did not affect the mix negatively. The void content of the Open Graded mix was lower than expected at ±15,7%. This can be considered as a good result as it reduced the chances of stripping while it had very little effect on the surface texture.

5 Table 1. Summary of asphalt test results. Asphalt Test Stone Mastic Asphalt (SMA) Continuous Coarse (CC) Open Graded Asphalt (OA) Target Actual Target Actual Target Actual Binder Type (Pen) Content (%) 40/50 6,5 40/50 7,1 60/70 4,5 60/70 4,4 60/70 6,0 60/70 6,3 BRD (kg/m³) TRMD (kg/m³) Voids in Mix (%) 6,6 5,7 3,9 3,5 17,7 15,7 Stability (kn) 7,8 6,7 14,2 13,9 5,8 7,1 Flow (mm) 3,5 4,0 4, ,9 3,0 ITS (kpa) ,36mm Sieve Cellulose Fibre (%) 0,3 0, ,3 0,3 Densities (%) Quality of Paving The general quality of the paving operation was good. Although the riding quality was not measured, it was assessed to be of adequate quality through visual observations and a high-speed drive-over. 4. THE EXPERIMENTAL SECTION MONITORING As planned in the method statement, various aspects of the experimental section were monitored on a regular basis. These were as follows: Visual rating of rubber deposit build-up on a scale of 0 to 5, where 0 indicates no rubber deposits and 5 indicates total coverage of the surface by rubber deposits. Eight numbered predetermined positions were selected on each surface type. Four positions were at 3m from the centreline and four were at 5m from the centreline. Each evaluation position covers an area of one square meter. Measurement of skid resistance using the Grip Tester (GN measurement). Measurement of skid resistance using the Pendulum Test equipment. Measurement of the surface texture depth using the Sand Patch Test. Assessment of operations under wet conditions (incidents monitored with the manager, flight safety). Taking photos of the texture and rubber deposit build-up at each evaluation position. Assessment of surface integrity. The open graded mix could be susceptible to aggregate loss and requires close scrutiny. Other possible distress types, e.g. bleeding are also monitored. The monitoring actions are summarised in Table 2. The monitoring was done for a five-month period.

6 Table 2. Summary of monitoring actions. Activity Description Location Time of day Frequency 1. Rubber rating Visually rate degree of build-up of rubber deposits 2. Photos Take close-up photos of evaluation areas. 3. Skid resistance Grip tester At 8 pre-selected spots per asphalt mix Daylight Monthly As for (1) Daylight Monthly On 3m and 5m from centreline 4. Pendulum Static Tester At 4 pre-selected spots per asphalt mix 5. Surface texture 6. Operations (incidents) 7. Surface integrity Anytime Anytime Monthly Monthly Sand patch test As for (4) Anytime Monthly Liase with Manager: Flight Operations Assess surface for possible distress Full TDZ - Monthly Full experimental area Daylight Weekly (SS) Daily (JIA) 5. RESULTS OF THE EXPERIMENTAL SECTION This section presents the results of the monitoring actions after the 5-month period. Monitoring Activity Rubber Build-up Rating Initial / 5 months: Skid Resistance (GN) Table 3. Summary of monitoring results. Existing SBR Medium Grade Coarse Continuous Grade Stone Mastic Asphalt Open Grade Asphalt 2 / 4,5 0 / 3,9 0 / 2,4 0 / 2 0,34 / 0,28 0,79 / 0,38 0,64 / 0,71 0,73 / 0,73 Initial / 5 months: Pendulum Test Initial / 5 months: 81 / 63 61/ / / 66 Surface Texture Initial / 5 months: 0,86 / 0,62 0,40 / 0,30 1,33 / 0,90 2,66 / 0,89 Operations (Incidents) Two Nil Nil Nil Surface Integrity 5.1 Visual Rating of Rubber Build-Up Very Good Very Good Good Scattered Aggregate Loss A noticeable degree of rubber build-up occurred on all surface types during the five-month monitoring period, as shown in Figure 3(a). With the existing traffic conditions and surface types (continuous medium), the rubber build-up would normally require removal at least once during a five-month period

7 5.1.1 SBR medium graded Rubber build-up was removed just after construction of the experimental section. As mentioned before, the rubber could not be removed completely. Rubber build-up rapidly redeveloped to a full coverage (rating: 4 to 5) during the monitoring period Coarse grade This mix has also shown a significant build-up (rating up to 4) although noticeably lower than that for the SBR Medium Grade SMA The SMA has significant rubber deposits between the coarse aggregates. However because it has not grown above the top of the large aggregates it is rated at a minor rubber build-up (rating up to 2) Open grade Similar but slightly lower rubber build-up than the SMA mix. The results therefore show a distinct lower build up of rubber deposits on the two surfaces with a more open (coarser texture). 5.0 AVERAGE RUBBER BUILD-UP PER SURFACE TYPE TDZ Trial Section (JIA) Open Graded SMA SBR Med Cont.Coarse Rubber Rating Dec 21-Dec 5-Jan 20-Jan 4-Feb 19-Feb 5-Mar 20-Mar 4-Apr 19-Apr 4-May Date 5.2 Skid Resistance: Grip Tester Figure 3a. Visual indication of rubber build-up with time. The Grip Tester simulates that skid resistance at various travelling speeds with the addition of water during the test. It provides a Grip Number (GN) for which standards have been set. The ICAO standards for a testing speed of 95 km/h are set at: Good : 0,70 Maintenanc intervention level : 0,37 Minimum allowable level : 0,24

8 The results of skid resistance tests measured at 95km/h with 1mm water film are presented in Figures 3(b) and 3(c). AVERAGE GN VALUE PER SURFACE TYPE TDZ Trial Section (JIA) 0.90 Open graded SMA SBR med. Cont. Coarse GN value Dec 16-Dec 31-Dec 15-Jan 30-Jan 14-Feb 29-Feb 15-Mar 30-Mar 14-Apr 29-Apr Date Figure 3b. Change in GN values over time GN VALUE AT 3-METER OFFSET: TDZ Trial Section (JIA) 06/12/ /02/ /05/ /05/ GN Value Open SMA SBR Medium Coarse 0.00 Asphalt types Figure 3c. Skid resistance per surface type SBR medium grade Low GN values were recorded from the start due to the rubber remaining on the surface and smooth texture. The values were close to the minimum allowable level Coarse grade Good initial results but deteriorated noticeably over the 5-month period (0,79 to 0,38).

9 5.2.3 SMA and open grade Both mixes retained a good skid resistance and did not deteriorate significantly in the monitoring period. The results show a slight improvement in the SMA mix. This is due to a bitumen film on the surface aggregates directly after construction. This bitumen wears off with traffic action and results in a slight increase of skid resistance. Figure 3(c) also provides the results of measurements taken in 2004 after a recent surface cleaning operation. It showed that all the surface types could be restored to an acceptable skid resistance level with a thorough cleaning operation. The Open and SMA mixes achieved slightly better GN values than the dense graded mixes. 5.3 Skid Resistance: Pendulum The Pendulum test simulates skid resistance for slow moving vehicles. The majority of the results remained in a band between 60 and 70. They did not have any clear pattern or give any advantage to one of the mixes. At a slow speed the skid resistance may in fact increase due to the interaction of the rubber tyre with the rubber finish. 5.4 Surface Texture The surface texture is measured through the sand patch test. The results are presented in Figure 3d. TEXTURE DEPTH PER SURFACE TYPE TDZ Trial Section (JIA) /12/99 06/05/ Texture depth (mm) Open graded SMA SBR med.(grooved) Coarse Asphalt type Figure 3d. Texture depth per surface type SBR medium grade The texture depth is very low due to the rubber left on the surface. The grooves in the surface affected (increased) the texture depth and also made it difficult to measure the texture accurately. The actual values should actually be lower than the coarse mix, but the results were affected by the grooves Coarse grade This mix has a low texture depth (0,40mm), which decreased with time as the rubber deposits filled up the texture.

10 5.4.3 SMA The SMA hasa significant initial texture depth of 1,33mm that dropped to 0,90mm over the monitoring period Open grade As expected, this mix gave the best initial texture depth (2,66mm initial) but also dropped with time. To contain initial aggregate loss, slurry was applied over some sections of the open grade. This influenced the texture depth readings. The huge drop to 0,89 is therefore not related to the initial texture. 5.5 Operations The normal use of the TDZ by aircraft was not effected at all by the construction and monitoring of the experimental section. Two incidents outside the TDZ were however reported over the monitoring period. Pilots reported brake loss in wet weather while exiting the runway at Tango taxiway. 5.6 Surface Integrity The surface integrity refers to defects that may appear on the surface and lead to problems such as the breaking up of the surfacing layer SBR medium grade No significant problems were observed. Only minor spalling occurred on the edges of the grooves Coarse grade No problems were observed SMA No significant problems were observed. Minor aggregate loss was observed initially but the mix performed well during the monitoring period Open grade This mix experienced significant chip loss within a week after construction. To reduce the risk of loose chips lying on the surface, isolated slurry patching of some areas were undertaken. Further slurry patching was necessary over larger areas during April The observations showed that the open graded mix is vulnerable to aggregate loss under the impact of the undercarriage of large aircraft. Recent (2004) visual observations showed no further significant deterioration in the integrity of any of the sections. Very minor chip loss was however evident on the surface of the SMA and Open Graded sections. 6. RUBBER REMOVAL Cleaning of the medium and coarse continuously graded mix surfaces was performed in June The method and end product was acceptable and removed almost 100% of rubber that was deposited during the five month period. The rubber was removed using water jetting at 1500 bar pressure. 6.1 SBR Medium Grade and Coarse Grade No significant problems were observed. There were however fine sand particles that were blasted out of the asphalt layer with the rubber deposits. This loss of fines was only observed on the surface and will not affect the impermeability and integrity of the asphalt mixes

11 6.2 SMA No cleaning was required because the skid resistance is still well above the maintenancerequired level. However, this surface was cleaned only over a short section to monitor the effect of the high pressure water jetting. No problems were experienced and this wearing course can be cleaned using high pressure water jetting. After the cleaning process the SMA was still intact and had an even better skid resistance based on visual appearance. 6.3 Open Grade No cleaning was performed on this mix. It was not necessary because the skid resistance is still well above the maintenance-required level. It was also not done because the slurry that was applied in April 2000 could be removed resulting in further surface integrity problems. 7. OTHER SMA EXPERIENCE ON AIRSIDE PAVEMENTS A study undertaken by a researcher in Australia (Cambell, 1999) indicated that Stone Mastic Asphalt (SMA) has been used overseas in more than 25 countries. Usage on aircraft pavements for a number of years is limited to less than 15 countries mainly in Europe. Overseas experience suggests that the use of SMA has been successful. Its use in aircraft pavements spread from Norway (biggest user) and Sweden to Germany, Austria, Belgium, the United Kingdom (England and Ireland) and China. Although the initial costs are higher than conventional dense graded asphalt, these costs should be offset in the longer term through longer life. The longer life is a result of increased rut resistance, increased fatigue resistance, increased durability and increased resistance to crack propagation. SMA also has good texture and wear resistance. According to NCAA grooving is not necessary. Perceived disadvantages of SMA include: Increased costs Reduced production rate Delayed opening to traffic to ensure binder temperature lower than 40 C to prevent flushing of the binder. Initial lower skid resistance until binder film is worn off the top of the surface by traffic. Potential segregation problems at low mastic contents. Potential low shear strength under turning movements at sharp curves. The research report does not provide any reference to the rate of rubber build-up on SMA surfaces. SMA experiments were undertaken in 1998/9 at Cairns and Sydney International Airports. It is reported that these trials were in their infancy and conclusive results on performance are probably two to three years away. Problems were experience with quality due to wrong construction techniques. SMA has been used successfully on a number of road projects in South Africa (e.g. R21 from JIA to Benoni) and in 2002 for new taxiways at JIA. The change in surface friction with speed is also of interest. Richardson (Richardson, 1999) showed the change in grip test number for a SMA mixture with an aggregate nominal size of 10mm that was laid on an airfield runway and that typically has an initial texture depth of between 1.0 and 1.5mm. The minimum friction level for the new runway in this case was 0.52 at 65km/h with 0.8 set as a target. The required friction level was achieved, but perhaps the more interesting feature is that the friction on the SMA surface did not appear to be influenced by speed to any great degree over the range examined. This might suggest that texture depth may not be quite so critical for this type of material, but further work would be needed to substantiate such a claim.

12 8. CONCLUSIONS The surface integrity and change in skid resistance and texture depth was measured during a five-month period. A difference in skid resistance performance (rubber build-up) between the different asphalt mixes was noted. The Open Graded and SMA asphalt mixes performed significantly better than the rest. The coarse graded asphalt surfacing performed well initially. However the loss in skid resistance increased in the last 2 months of the monitoring period. The result after 5 months indicated that maintenance cleaning was required at that stage. The skid resistance on the existing SBR medium graded grooved asphalt surfacing was very low and stayed in the maintenance zone, close to the rejection limit. In considering the integrity of the different mixes, the open graded asphalt showed early signs of stone loss as discussed above. The other asphalt mixes were all stable. Water jet cleaning was performed on the coarse grade and existing SBR medium asphalt mixes during the first week of June GN measurements was not done after cleaning but visibly judged. Based on the visual judgments the surfaces had adequate skid resistance for the next 3 to 4 months. Based on the findings, the following conclusions were made: The SMA mix is considered as the best surfacing due to the low rubber build-up rate, high skid resistance and surface integrity. Grooving will not be necessary Rubber build-up can be reduced but not completely eliminated. Provision must therefore still be made for future rubber removal at an estimated rate of at least once a year on the SMA asphalt mix. The dens graded mixes require more frequent rubber removals (4-5months). As expected, open grade mixes are sensitive to damage such as aggregate loss and measures may be required to prevent chip loss. Dense graded mixes performs very well as far as durability and integrity is concerned but requires more frequent interventions to maintain an adequate level of skid resistance. 9. ACKNOWLEDGEMENTS The authors wish to thank the following personnel of ACSA (JIA) for permission and encouragement to perform the trial and publish this paper as well as for assistance with the execution thereof: Mr C Henry: Manager: Maintenance and Engineering Mr A Rossouw: Department Head, Surface Maintenance Any views expressed are not necessarily those of the Airports Company of South Africa. 10. REFERENCES Kolbe R., Measurement of skid resistance and its variability across the pavement surface. (Course Code CIVN 420), University of the Witwatersrand, Johannesburg. Rogers M P and Garget T., A skidding resistance standard for the national road network, TRRL. International Civil Aviation Organisation ICAO International standards and recommended practices AERODROMES Annexe 14 Volume 1 Aerodrome Designs and Operations.

13 Cambell C., The use of stone mastic asphalt on aircraft pavements. Deakin University. European Asphalt Pavement Association. Heavy duty surfaces, The Netherlands. Visser A T., An investigation of skidresistance under South African conditions, University of Pretoria. Richardson J.T.G., Stone Mastic Asphalt In The UK, Society of Chemical Industry Lecture Papers Series, Wolverhampton, West Midlands, UK.