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Quantifying the impact of reclaimed asphalt pavement on airport asphalt surfaces
Construction and Building Materials 197 (2019) 757–765
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Quantifying the impact of reclaimed asphalt pavement on airport asphalt surfaces Greg White School of Science and Engineering, University of the Sunshine Coast, 90 Sippy Downs Drive, Sippy Downs, Queensland 4556, Australia
h i g h l i g h t s RAP is no more variable than new quarried crushed rock sources. RAP is more finely graded than the asphalt it is recovered from. Asphalt containing RAP is moderately stiffer than asphalt without RAP. Asphalt containing RAP has significantly lower surface friction than asphalt without RAP. Inclusion of RAP from temporary ramps is not detrimental to airport asphalt.
a r t i c l e
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Article history: Received 6 March 2018 Received in revised form 19 September 2018 Accepted 15 November 2018
Keywords: Airport Asphalt RAP Quantified
a b s t r a c t Airports desire sustainable infrastructure solutions and airport pavements provide an opportunity for increased reuse and recycling of materials. The recycling of reclaimed asphalt pavement (RAP) in airport pavement surfaces is attractive and viable, but has been resisted by many airports. Two asphalt mixtures, both produced with and without RAP, were compared based on Marshall mixture design properties, laboratory performance-indicative test results and full-scale asphalt production properties. No testing indicated any detrimental effect associated with the inclusion of low-risk RAP sources at 5–10% RAP content. However, the surface friction was significantly reduced by the inclusion of RAP. It is recommended that further research consider the cause of the reduced surface friction associated with low-risk RAP inclusion, as well as the influence of other RAP sources on airport asphalt surface performance. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Flexible airport pavement surfaces are predominantly comprised of dense graded, Marshall designed asphalt. The aggregates are usually fully crushed, newly quarried hard rock and the bituminous binder is usually a premium or modified product. Importantly, the binder content is high compared to road and highway asphalt, typically 5.4–5.8% by mass of the asphalt mixture. In addition, some jurisdictions and airports have developed alternate airport surface types. For example, some European countries use stone mastic asphalt [5], open graded friction course [8] or gap graded mixtures [23]. However, the USA, Australia, New Zealand, the Middle East and South Africa continue to favour dense graded mixtures designed using the Marshall approach. Regardless of the asphalt mixture type, increased environmental sustainability is desired by many airports [14,6]. Environmental
E-mail address: [email protected] https://doi.org/10.1016/j.conbuildmat.2018.11.131 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
sustainability can be achieved by recycling, reduced emission generation or increased surface life [20]. The incorporation of recovered asphalt, known as reclaimed asphalt pavement (RAP) is arguably the most common and well-established sustainability initiative in asphalt production and construction. However, some airports have resisted the use of RAP, citing concerns regarding performance. The research aims to quantify the impact of incorporating lowrisk RAP into asphalt for runway surfacing. The low-risk RAP was sourced from the texturing of the underlying asphalt surface and the removal of temporary ramps by cold planing machines. The RAP was therefore produced to the same mixture design and as part of the same surfacing project. Asphalt was sampled and tested from two different Australian airport resurfacing projects and the findings are intended to be extended to other RAP sources in the future.
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2. Background 2.1. Airport asphalt As stated above, airport asphalt in Australia and many other countries is designed based on the Marshall method [22] with samples compacted by 75 blows of a Marshall hammer. Runway asphalt is generally 14 mm maximum nominal size and is typically constructed in 50–80 mm thick layers. Airport asphalt is densely graded with a high bitumen content and hydrated lime is often added as an anti-stripping agent (Table 1). Although common in road pavements and the airports of some countries, Australian airports have generally resisted the use of RAP, particularly in the surface layer of flexible runway pavements. This resistance reflects concerns regarding the potential impact on surface performance, generally indicated by resistance to cracking, resistance to deformation and resistance to moisture damage [20,18–19]. Furthermore, surface durability and the risk of coarse aggregate ravelling out of the surface and becoming a hazard to fragile jet engines is always a concern [1]. 2.2. Sustainability and recycling Like other modern-day infrastructure owners, private airport owners are becoming ever more focussed on environmental sustainability, as well as achieving a sustainable return on their assets, and reducing the whole of life infrastructure cost. It follows that interest in environmentally responsible and sustainable pavement solutions has increased, including airport asphalt surfaces. The primary opportunities for improved asphalt surface environmental sustainability in airport pavements relate to increasing the period between resurfacing (surface preservation), reducing the volume of new aggregate and binder (recycling or extending) and reducing the energy input (warm mix technology) [20]. Materials recycled to extend asphalt include crushed reclaimed asphalt [3], crushed concrete [13], crushed (gusset) glass [12] and blast furnace slag [2]. Of these, RAP is arguably the most common and wellestablished asphalt extender for increased asphalt surface sustainability. 2.3. Reclaimed asphalt pavement Recycling of pulverised and processed existing asphalt surfaces as RAP is well-established in the asphalt industry and includes reincorporation of bitumen-coated aggregate into new asphalt pro-
duction [3]. Every tonne of RAP incorporated into an asphalt surface saves almost a tonne of new quarried aggregate production and reduces the new bitumen content, resulting in substantial sustainability benefits. Moreover, avoiding the disposal of cold planing material as waste further reduces the environmental impact of asphalt resurfacing. By definition, RAP is less controlled than newly produced material and may have variable grading or binder content. This creates a challenge for asphalt mixture design and production [16]. For high quality and tightly specified asphalt mixtures, engineered RAP (free of contamination, screened and pulverised) is less variable and is more appropriate for the tight specifications associated with airport asphalt surface production. In the USA, RAP has been incorporated into airport asphalt surfaces for over fifteen years [11]. A field inspection of three runways containing RAP found good to excellent performance of seven to ten year old surfaces. Guercio & McCarthy [10] reported that airport surface asphalt containing both RAP and warm mixed asphalt presented a saving of up to 27% and recommended incorporation into future specifications. In Australia, RAP sourced from temporary ramps was used in the surface layer as part of the rehabilitation of the Whitsunday Coast Airport runway pavement [21]. It is expected that use of RAP in airport asphalt will increase as confidence in the resulting surface performance improves. It is important to understand that there are different potential sources of RAP. For a particular application, potential RAP sources include the removal of temporary works, texturing of the underlying asphalt layer and RAP from other projects. The lowest risk source of RAP is from the removal of temporary works that were constructed from the same nominal asphalt mixture. For airport resurfacing works using mobiles production facilities, this includes the removal of the temporary construction ramps and the texturing of any underlying surface of the same mixture. These are the most homogenous and consistent RAP available and present the lowest risk source of RAP for airport asphalt surfacing. It is also important to recognise the practical limits of RAP in airport asphalt surfacing. Significant recent research has investigated RAP contents of 50% and greater [16,7,15,17]. However, where RAP is sourced from the texturing of the underlying surface and removal of temporary ramps, the amount of RAP generated is only 5–15% of the typical asphalt produced, making higher RAP contents impracticable.
3. Materials, methods and results 3.1. Materials
Table 1 Typical Australian airport asphalt characteristics [9]. Asphalt Property
Typical; value
Bitumen content (% by mix mass) Target Marshall air voids (%) Target voids filled with bitumen (%) Filler Content (% by aggregate) Minimum Marshall Stability (kN) Maximum Marshall Flow (mm) Percentage passing AS sieve (mm) 13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075
5.8 4 75 1.5 of hydrated lime 12 3 Target by volume (%) 100 82 70 60 44 33 25 16 10 6
Two different asphalt mixtures were produced, both with and without RAP. All asphalt mixtures were designed to be dense graded, Marshall-designed 14 mm nominal maximum aggregate sized and meeting the requirements summarised in Table 1. The RAP for the first asphalt, Mixture P, was recovered from texturing of the underlying layer, as well as removal of temporary ramps. The underlying asphalt layer was produced as part of the same project, to the same mixture design and with the same ingredients, but excluded RAP, and was constructed approximately eight weeks prior. The RAP was recovered in an expedient manner and then screened and crushed prior to re-use. The RAP content was nominally 10%, which comprised 4% from ramps and 6% from texturing of the underlying layer. Mixture P was used to surface a runway catering for up the B737-800 sized aircraft. The second asphalt, Mixture D, was recovered from the removal of temporary ramps. The cold planing machines were operated slowly to maximise the degree of pulverisation of the RAP during the ramp removal process and the RAP was not subsequently
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crushed. However, large pieces of RAP were removed prior to reuse. Nominally 5% RAP was added to the surface, reflecting the smaller volume of RAP generated. Mixture D was used to surface a runway accommodating turbo-prop (Dash 8–400) and regional jet (Embaer RJ 135) aircraft. Both mixtures targeted the same overall volumetric composition and the various fractionated aggregate sizes were adjusted to account for the grading and binder content of the different RAP sources (Table 2). Both mixtures were produced using a proprietary plastomeric polymer modified binder that typically achieves PG 70-22 for extreme traffic rating under the multiple stress creep recovery based performance grading system [19].
3.2. Methods Asphalt mixtures were produced with and without RAP. The two asphalt mixtures used different RAP sources and the RAP was recovered and processed by different methods. This allowed and initial comparison of the two recovery and processing approaches and assessment of the impact on the resulting RAP properties. Secondly, the four mixtures were compared to determine the impact of RAP on the volumetrics, Marshall properties and laboratory performance testing. Finally, all RAP sources and asphalt mixtures were sampled during full-scale production in a real asphalt plant, allowing comparison of RAP property consistence, asphalt production property consistence and the finished surface properties. Simple statistics, including average, standard deviation, coefficient of variation and the Student’s T-test for differences of means, were used to quantify the impact of including low-risk RAP in airport asphalt. Physical testing used Australian standard methods where available, and when not available, international test methods were adopted (Table 3). Because the results
were obtained from real asphalt production, the number of results, with and without RAP, were sometimes unbalanced. 3.3. Results Mixtures with and without RAP were tested for the Marshall Stability and Flow, as well as the performance-indicative properties for stiffness, crack resistance, deformation resistance and moisture damage resistance (Table 4). During the Mixture P design process, three samples of RAP were recovered from temporary ramp removal and three samples of RAP were recovered from texturing of the underlying surface. The results for binder content and grading were averaged (Table 5) for input to the mixture composition. For Mixture D, RAP was only recovered from temporary ramp removal and three samples were tested for grading and bitumen content (Table 6) and the results averaged for the design of the mixture containing RAP. During real asphalt production, recovered and processed RAP was sampled and tested periodically for grading, binder content and moisture content (Table 7). This allowed the consistence of recovered RAP to be evaluated, providing an indication of the potential impact of RAP variability on asphalt production properties. Asphalt production test results are summarised for Mixture P, with and without RAP (Table 8) as well as for Mixture D (Table 9). Comparison allowed assessment of the impact of RAP on asphalt production consistence. Finally, following construction of the surface, friction testing was performed along the full length of the runway surface constructed with Mixture P. The 10 m averages of the continuously recorded results are summarised in Table 10. Surface texture was also measured in areas of the surface with and without RAP (Table 10).
Table 2 RAP and non-RAP mixture components. Component
Nominal 14 mm minus aggregate fraction Nominal 10 mm minus aggregate fraction Nominal 7 mm minus aggregate fraction Natural sand Crusher dust Hydrated lime New binder content RAP Total binder content
Units
% % % % % % % % %
of aggregate volume of aggregate volume of aggregate volume of aggregate volume of aggregate volume of aggregate volume as mass of combined aggregate as mass of asphalt mixture as mass of combined aggregate
Mixture P
Mixture D
No RAP
With RAP
No RAP
With RAP
22.0 13.0 15.0 13.0 35.2 1.8 6.0 0 6.0
19.8 9.6 9.0 12.6 38.1 0.9 5.3 10 6.0
27.4 7.1 7.1 17.0 35.1 0.7 5.6 0 5.6
27.4 6.6 6.6 15.0 33.1 0.7 5.5 5 5.6
Table 3 Test methods. Tested property
Test Method
Description and notes
Grading (% aggregate volume) Binder content (% mass) Moisture content (% mass) Air Voids (% volume) Maximum density (kg) Marshall Stability (kN) Marshall Flow (mm) Resilient Modulus (MPa) Fatigue life (cycles) Wheel track rutting (mm) Tensile Strength Ratio (%) Grip Number at 65/95 km/hr
AS 1141.11 AS/NZS 2891.3.1 AS/NZS 2891.10 AS/NZS 2891.8 AS/NZS 2891.7.1 AS/NZS 2891.5 AS/NZS 2891.5 AS/NZS 2891.13.1 AG:PT/T274 AG:PT/T231 AG:PT/T232 CAA [4]
Particle size distribution
Surface Texture Depth (mm)
AG:PT/T250
Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Indirect tensile modulus at 25 °C Four-point repeated beam bending at 20 °C Copper wheel tracker at 60 °C Modified Lottman test Griptester Trailer with 1 mm water film using New Zealand protocol, which is based on international recommendations. Pestle method sand circle
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Table 4 Laboratory Marshall and performance properties. Asphalt Property
Mixture P
Marshall Stability (kN) Marshall Flow (mm) Resilient Modulus (MPa)
Fatigue life (cycles)
Wheel track rutting (mm) Tensile Strength Ratio (%)
Mixture D
No RAP
With RAP
No RAP
With RAP
19.1 3.2 4062 4899 4350 >1,000,000 >1,000,000 >,1,000,000 1.9 97
16.5 2.5 5018 5134 5155 >1,000,000 >1,000,000 >,1,000,000 1.4 96
13.6 2.8 5002 5089 5382 >1,000,000 >1,000,000 >,1,000,000 1.5 97
15.1 2.7 6675 6723 6656 >1,000,000 >1,000,000 >,1,000,000 0.8 94
Table 5 Mixture P RAP properties.
Table 7 Mixture P and mixture D RAP production characteristics.
RAP Property
From texturing
From ramps
Percentage passing AS sieve (mm)
% of aggregate by volume
13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075 Binder content (% mass)
100 89 76 66 47 35 27 19 10 6.6 5.4
100 91 83 72 51 36 28 20 11 7.2 5.4
Combined
RAP Characteristic
100 90 80 69 49 35 28 19 11 6.9 5.4
Binder content (% mass) 5.8 0.3 5.1 Moisture content (% mass) 1.5 0.7 0.2 Percentage passing AS sieve (mm) (% of aggregate by volume) 13.2 99.8 0.5 100.0 9.5 91.3 2.4 90.7 6.7 81.4 3.7 78.8 4.75 69.6 4.4 67.8 2.36 49.9 4.1 51.2 1.18 36.4 2.8 37.2 0.600 28.2 1.7 26.1 0.300 20.4 1.2 17.7 0.150 11.3 1.0 12.2 0.075 7.3 0.7 9.4
Table 6 Mixture D RAP properties. RAP Property
Sample 1
Sample 2
Percentage passing AS sieve (mm)
% of aggregate by volume
13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075 Binder content (% mass)
100 96 86 77 58 41 28 18 13.1 10.1 4.8
100 98 91 83 63 45 30 20 13.7 10.6 4.9
Sample 3
Average
100 98 89 80 60 43 30 20 14 10 4.8
4. Discussion To quantify the impact of RAP on airport asphalt mixtures, the results with RAP and without RAP were compared for both Mixture P and Mixture D. Quantitative comparisons included the different methods of RAP recovery and processing, asphalt mixture design properties, RAP production consistence, asphalt production property consistence and friction and texture of the finished asphalt surface. 4.1. Impact of RAP recovery and processing on RAP properties Mixture P RAP, recovered from the removal of temporary ramps and from texturing of the surface, were similar with regards to binder content (Table 2) and grading (Fig. 1). The three samples of
Mixture D (19 samples)
Average
Average
Std. Dev.
Std. Dev. 0.2 0.1 0.0 3.7 5.4 6.2 4.8 3.3 2.0 1.5 0.9 0.8
Table 8 Mixture P asphalt production characteristics. Asphalt Characteristic
100 99 90 80 59 44 31 21 14.1 9.8 4.8
Mixture P (18 samples)
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results
No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
6.0 4.6 2443 17.2 2.6 100
0.2 0.5 31 1.9 0.1
6.0 4.7 2452 16.3 2.6 47
0.2 0.4 7 1.1 0.1
Mixture D RAP from temporary ramp removal were also consistent with regards to binder content, with a coefficient of variation of just 1.2%, and comparable grading (Fig. 2). Furthermore, the two design gradings were similar, reflecting the same mixture specification, but the two RAP sources deviated from the mixture design by different degrees (Fig. 3). RAP from Mixture P was recovered in a typical and expedient manner. Consequently, the RAP was coarse and required pulverisation, which resulted in a grading marginally finer than that of the mixture design. However, the Mixture D RAP was recovered by the slow operation of the cold planing machine, which resulted in a consistently finer RAP than for the mixture design. For example, both mixture designs targeted 59% of the aggregate passing the 4.75 mm sieve. The Mixture P RAP averaged 69% passing 4.75 mm and the Mixture D RAP averaged 80%. Regarding binder content, the Mixture P RAP recovery and processing reduced the binder content by a consistent 0.5% and the Mixture D RAP recovery reduced the binder content by a consistent 0.8%. These differences must be considered during the asphalt mixture design, but are not detrimental as long as they are accounted for.
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4.3. Consistence of RAP production
Table 9 Mixture D asphalt production characteristics. Asphalt Characteristic
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results
No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
5.6 3.3 2532 15.4 3.5 41
0.1 0.2 5 0.7 0.2
5.5 3.4 2535 15.2 3.7 29
0.1 0.2 5 0.8 0.2
Table 10 Mixture P surface friction and texture characteristics. Friction Characteristic
Grip Number at 65 km/hr Grip Number at 95 km/hr Surface Texture Depth (mm) Number of Friction Results Number of Texture Results
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No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
0.70 0.55 0.44 440 8
0.07 0.11 0.05
0.62 0.47 0.43 1040 8
0.08 0.10 0.05
One of the risks cited by airports that resist the inclusion of RAP in runway surfaces is the variability of RAP and its impact on asphalt production. The summary of Mixture P RAP production results indicate that the coefficients of variation associated with the percentages passing the various sized sieves ranged from 2 to 9% over the 18 RAP samples, calculated from the statistics in Table 7. The critically important binder content had a lower coefficient of variation at less than 5%. However, the moisture content, although acceptably low, had a higher variation of almost 50%. The moisture in the RAP comes from the water used to prevent overheating of the profiler teeth during the cold planing process, and the variability likely reflects the amount of water used, as well as the drying time between cold planing and subsequent RAP sampling for testing. Similarly, Mixture D RAP had grading coefficients of variation of 4–9% (Table 7) which is comparable to Mixture P. However, for Mixture D, the binder content was less variable, 3.0% compared to 4.6%, and the moisture content was lower, averaging 0.2% compared to 1.5%. Overall, the variability of the produced RAP was no greater than is typically observed with new aggregate sources.
4.2. Impact of RAP on mixture properties
4.4. Impact of RAP on asphalt production
The target volumetric properties of the two mixtures, with and without RAP, were nominally identical. However, when the mixtures were tested for Marshall properties and the performance properties, some differences were noted (Table 4). For Mixture P, the addition of RAP was associated with a decrease in the Marshall Stability, indicating a less stiff mixture. In contrast, the Marshall Flow and wheel track rutting both reduced and the modulus increased, all suggesting a stiffer mixture. In the case of Mixture D, all mixture design properties indicated a stiffer mixture associated with inclusion of RAP. However, the increase in stiffness was not detrimental, with the resilient modulus within the normal range for all mixtures and the fatigue life of all mixtures exceeding the terminal 1,000,000 cycles.
The binder content, air void content and Marshall Flow (Table 8) were not significantly different for Mixture P, with RAP and without RAP (Table 11), and as shown in Fig. 4. However, the Marshall Stability was significantly lower and the asphalt maximum density was significantly higher with RAP (Table 11). The increase in density likely reflects small changes in the overall asphalt composition (Table 2) and differences in the specific gravity of the various aggregate sources. The likely cause of the Marshall Stability decrease is not known, although it is clear that this is a more variable test than others (with a coefficient of variation approximately 9–10%). Similar trends were observed in the Mixture D asphalt production results, except only the maximum density was significantly different when RAP was added (Table 11).
Fig. 1. Mixture P ramp removal and texturing RAP aggregate grading.
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Fig. 2. Mixture D ramp removal RAP aggregate gradings.
Fig. 3. Mixture design and recovered RAP aggregate gradings.
Table 11 Mixture P and mixture D asphalt production property changes. Asphalt Production Property
Change in average results and associated Student’s T-test for difference of means with and without RAP Mixture P
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results compared
Mixture D
Change in mean
P-value
Change in mean
P-value
No detectible change Increase 0.1 with RAP Increased 9 with RAP Decreased 0.9 with RAP No detectible change 100 without RAP and 47 with RAP
0.41 0.12 <0.01 <0.01 0.90
Increased 0.1 with RAP Increased 0.1 with RAP Increased 3 with RAP Decreased 0.2 with RAP Increased 0.2 with RAP 41 without and 29 with RAP
0.21 0.82 0.01 0.34 0.22
Note: p-values exceeding 0.05 indicate that there is not a statistically significant difference.
G. White / Construction and Building Materials 197 (2019) 757–765
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Fig. 4. Mixture P asphalt production properties with and without RAP.
Overall, the inclusion of RAP had minimal impact on the asphalt production properties. This likely reflects the RAP having been produced only weeks prior to its recovery and from asphalt that was produced to the same design, with the same aggregate sources and the same binder type. The decrease in Marshall Stability (5.0%) and increase in maximum density (0.3%) observed when RAP was introduced to Mixture P indicates the potential for changes in these properties, particularly when aggregates of significantly different specific gravity are used to produce the asphalt. This was consistent for Mixture D, where the inclusion of RAP increased the maximum density by 3 kg (or 0.1%). However, these properties are not indicators of asphalt performance, although they must be controlled for consistent asphalt production. Furthermore, there was no indication that the introduction of RAP reduced the consistence of the manufactured asphalt. This is important and likely reflects the consistent source of RAP added to the two mixtures and the relatively modest RAP contents of low-risk RAP available. 4.5. Impact of RAP on surface friction and texture
the measured surface friction at both 65 km/hr and 95 km/hr test speeds (Table 12). For example, the 95 km/hr average friction result decreasing from 0.55 to 0.47 where RAP was included (Table 10). The reduction in friction was observed despite the measured surface texture being unaffected, with averages of 0.44 mm and 0.43 mm without RAP and with RAP, respectively (Table 10). The friction reduction was statistically significant at both test speeds, while the difference in the surface texture was not (Table 12). Furthermore, Figs. 5 and 6 show the clear reduction in friction measured at 65 km/hr and 95 km/hr, respectively, at the commencement of the construction of Mixture P with RAP. The reduction in Mixture P surface friction associated with the inclusion of RAP did not reflect a significant reduction in surface texture. It is therefore likely that a double coating (in binder) of the fine RAP aggregate occurred during the second asphalt production. Whether this impact was specific to Mixture P was not determined. It is important to note that the levels of friction measured exceeded the minimum requirement, before the runway surface was grooved. Consequently, even though the inclusion of RAP reduced both the 65 km/hr and the 95 km/hr results, there was no impact to the functional suitability of the runway surface.
Interestingly, the Mixture P friction testing results indicate that the addition of RAP was associated with a significant reduction in 5. Further research
Table 12 Mixture P surface friction and texture changes p-values. Asphalt Production Property
Change in average results and associated Student’s T-test for difference of means with and without RAP Change in mean
Grip Number at 65 km/hr Grip Number at 95 km/hr Surface Texture Depth (mm) Number of friction results compared Number of texture results compared
P-value
Reduced 0.08 with RAP <0.01 Reduced 0.08 with RAP <0.01 Reduced 0.02 with RAP 0.78 440 without RAP and 1040 with RAP 8 without RAP and 8 with RAP
Note: p-values exceeding 0.05 indicate that there is not a statistically significant difference.
This research objectively quantified the effect of incorporating low (5–10%) contents of low-risk RAP in two typical dense graded airport asphalt mixtures. In both cases, the RAP was recovered from asphalt produced to the same mixture design as the asphalt to which it was added. Furthermore, it was recovered from the removal of ramps and the texturing of the underlying surface constructed not more than eight weeks prior. Consequently, conclusions can only reflect the use of similarly low-risk RAP sources. Therefore, further research is required to objectively assess the impact of incorporating higher portions of other RAP sources. Additional research is also required to better understand the reduction in measured surface friction, without a commensurate reduction in surface texture, when RAP was introduced.
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Fig. 5. Mixture P 65 km/hr surface friction with and without RAP.
Fig. 6. Mixture P 95 km/hr surface friction with and without RAP.
6. Conclusions Comparison of the two typical airport asphalt mixtures indicates there is no detrimental effect associated with the inclusion of low-risk RAP. That is, airports should not resist the recycling of the 5–10% of low-risk RAP available from the removal of temporary ramps. Where the underlying surface is produced to the same asphalt mixture design, cold planings from existing surface texturing should also be incorporated as RAP. Consequently, it is recommended that low-risk RAP sources be incorporated into airport asphalt surfaces, at up to 10% RAP content, without requiring a mixture design inclusive of representative RAP. It is also recom-
mended that further research consider other sources of RAP, specifically RAP from existing and aged airport asphalt surfaces, as well as the impact of RAP on runway surface friction. Conflict of interest None. References [1] AAA 2017, Airfield Pavement Essentials, Airport Practice Note 12, Australian Airports Association, Canberra, Australia, April, viewed 10 January 2018,.
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[14] R.L. Keast, D.C. Baker, K. Brown, Sustainable Airport Infrastructure: Balancing Infrastructures for the airport metropolis, Chapter 7, Sustainable Urban and Regional Infrastructure Development: Technologies, Applications and Management, Information Science Reference, Hershey, New York, New York, USA, 2010. [15] A. Norouzi, M. Sabouri, Y.R. Kim, Evaluation of the fatigue performance of high RAP asphalt mixtures, in: 12th ISAP Conference on Asphalt Pavements, June 15, 2014, Raleigh, North Carolina, USA. [16] G. Valdés, F. Pérez-Jiménez, R. Miró, A. Martínez, R. Botella, Experimental study of recycled asphalt mixtures with high percentages of reclaimed asphalt pavement (RAP), Constr. Build. Mater. 25 (2011) 1289–1297. [17] R.K. Veeraragavan, R.B. Mallick, M. Tao, M. Zaumanis, R. Frank, R.L. Bradbury, Laboratory comparison of rejuvenated 50 percent reclaimed asphalt pavement and hot-mix asphalt with conventional 20 percent RAP mix, Transp. Res. Rec.: J. Transp. Res. Board 2633 (2017) 69–779. [18] G. White, Towards a performance-based airport asphalt specification, in: International Conference on Highway Pavements and Airfield Technology, Philadelphia, Pennsylvania, USA, 27-30 August, 2017. [19] G. White, K. Embleton, Validation of a new generation bitumen for airport asphalt, in: 17th AAPA International Flexible Pavements Conference, Melbourne, Victoria, Australia, 13–16 August, 2017. [20] G. White, A. Jamshidi, Incorporating environmentally sustainable asphalt surfaces into airport pavements, in: 4th International Symposium on Asphalt Pavements & Environment, Toyosu, Tokyo, Japan, 20-21 November, 2017. [21] G. White, H. Fairweather, A. Jamshidi, Sustainable runway pavement rehabilitation: a case study of an Australian airport, J. Cleaner Prod. 204 (2018) 380–389. [22] T.D. White, Marshall procedures for design and quality control of asphalt mixtures, Proc. Asphalt Pavement Technol. 54 (1985) 265–285. [23] I. Widyatmoko, J. Richardson, C. Ferguson, Long-term monitoring of high performance airfield pavement surfacing, Int. J. Pavement Res. Technol. 6 (5) (2013) 582–587.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Quantifying the impact of reclaimed asphalt pavement on airport asphalt surfaces Greg White School of Science and Engineering, University of the Sunshine Coast, 90 Sippy Downs Drive, Sippy Downs, Queensland 4556, Australia
h i g h l i g h t s RAP is no more variable than new quarried crushed rock sources. RAP is more finely graded than the asphalt it is recovered from. Asphalt containing RAP is moderately stiffer than asphalt without RAP. Asphalt containing RAP has significantly lower surface friction than asphalt without RAP. Inclusion of RAP from temporary ramps is not detrimental to airport asphalt.
a r t i c l e
i n f o
Article history: Received 6 March 2018 Received in revised form 19 September 2018 Accepted 15 November 2018
Keywords: Airport Asphalt RAP Quantified
a b s t r a c t Airports desire sustainable infrastructure solutions and airport pavements provide an opportunity for increased reuse and recycling of materials. The recycling of reclaimed asphalt pavement (RAP) in airport pavement surfaces is attractive and viable, but has been resisted by many airports. Two asphalt mixtures, both produced with and without RAP, were compared based on Marshall mixture design properties, laboratory performance-indicative test results and full-scale asphalt production properties. No testing indicated any detrimental effect associated with the inclusion of low-risk RAP sources at 5–10% RAP content. However, the surface friction was significantly reduced by the inclusion of RAP. It is recommended that further research consider the cause of the reduced surface friction associated with low-risk RAP inclusion, as well as the influence of other RAP sources on airport asphalt surface performance. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Flexible airport pavement surfaces are predominantly comprised of dense graded, Marshall designed asphalt. The aggregates are usually fully crushed, newly quarried hard rock and the bituminous binder is usually a premium or modified product. Importantly, the binder content is high compared to road and highway asphalt, typically 5.4–5.8% by mass of the asphalt mixture. In addition, some jurisdictions and airports have developed alternate airport surface types. For example, some European countries use stone mastic asphalt [5], open graded friction course [8] or gap graded mixtures [23]. However, the USA, Australia, New Zealand, the Middle East and South Africa continue to favour dense graded mixtures designed using the Marshall approach. Regardless of the asphalt mixture type, increased environmental sustainability is desired by many airports [14,6]. Environmental
E-mail address: [email protected] https://doi.org/10.1016/j.conbuildmat.2018.11.131 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
sustainability can be achieved by recycling, reduced emission generation or increased surface life [20]. The incorporation of recovered asphalt, known as reclaimed asphalt pavement (RAP) is arguably the most common and well-established sustainability initiative in asphalt production and construction. However, some airports have resisted the use of RAP, citing concerns regarding performance. The research aims to quantify the impact of incorporating lowrisk RAP into asphalt for runway surfacing. The low-risk RAP was sourced from the texturing of the underlying asphalt surface and the removal of temporary ramps by cold planing machines. The RAP was therefore produced to the same mixture design and as part of the same surfacing project. Asphalt was sampled and tested from two different Australian airport resurfacing projects and the findings are intended to be extended to other RAP sources in the future.
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2. Background 2.1. Airport asphalt As stated above, airport asphalt in Australia and many other countries is designed based on the Marshall method [22] with samples compacted by 75 blows of a Marshall hammer. Runway asphalt is generally 14 mm maximum nominal size and is typically constructed in 50–80 mm thick layers. Airport asphalt is densely graded with a high bitumen content and hydrated lime is often added as an anti-stripping agent (Table 1). Although common in road pavements and the airports of some countries, Australian airports have generally resisted the use of RAP, particularly in the surface layer of flexible runway pavements. This resistance reflects concerns regarding the potential impact on surface performance, generally indicated by resistance to cracking, resistance to deformation and resistance to moisture damage [20,18–19]. Furthermore, surface durability and the risk of coarse aggregate ravelling out of the surface and becoming a hazard to fragile jet engines is always a concern [1]. 2.2. Sustainability and recycling Like other modern-day infrastructure owners, private airport owners are becoming ever more focussed on environmental sustainability, as well as achieving a sustainable return on their assets, and reducing the whole of life infrastructure cost. It follows that interest in environmentally responsible and sustainable pavement solutions has increased, including airport asphalt surfaces. The primary opportunities for improved asphalt surface environmental sustainability in airport pavements relate to increasing the period between resurfacing (surface preservation), reducing the volume of new aggregate and binder (recycling or extending) and reducing the energy input (warm mix technology) [20]. Materials recycled to extend asphalt include crushed reclaimed asphalt [3], crushed concrete [13], crushed (gusset) glass [12] and blast furnace slag [2]. Of these, RAP is arguably the most common and wellestablished asphalt extender for increased asphalt surface sustainability. 2.3. Reclaimed asphalt pavement Recycling of pulverised and processed existing asphalt surfaces as RAP is well-established in the asphalt industry and includes reincorporation of bitumen-coated aggregate into new asphalt pro-
duction [3]. Every tonne of RAP incorporated into an asphalt surface saves almost a tonne of new quarried aggregate production and reduces the new bitumen content, resulting in substantial sustainability benefits. Moreover, avoiding the disposal of cold planing material as waste further reduces the environmental impact of asphalt resurfacing. By definition, RAP is less controlled than newly produced material and may have variable grading or binder content. This creates a challenge for asphalt mixture design and production [16]. For high quality and tightly specified asphalt mixtures, engineered RAP (free of contamination, screened and pulverised) is less variable and is more appropriate for the tight specifications associated with airport asphalt surface production. In the USA, RAP has been incorporated into airport asphalt surfaces for over fifteen years [11]. A field inspection of three runways containing RAP found good to excellent performance of seven to ten year old surfaces. Guercio & McCarthy [10] reported that airport surface asphalt containing both RAP and warm mixed asphalt presented a saving of up to 27% and recommended incorporation into future specifications. In Australia, RAP sourced from temporary ramps was used in the surface layer as part of the rehabilitation of the Whitsunday Coast Airport runway pavement [21]. It is expected that use of RAP in airport asphalt will increase as confidence in the resulting surface performance improves. It is important to understand that there are different potential sources of RAP. For a particular application, potential RAP sources include the removal of temporary works, texturing of the underlying asphalt layer and RAP from other projects. The lowest risk source of RAP is from the removal of temporary works that were constructed from the same nominal asphalt mixture. For airport resurfacing works using mobiles production facilities, this includes the removal of the temporary construction ramps and the texturing of any underlying surface of the same mixture. These are the most homogenous and consistent RAP available and present the lowest risk source of RAP for airport asphalt surfacing. It is also important to recognise the practical limits of RAP in airport asphalt surfacing. Significant recent research has investigated RAP contents of 50% and greater [16,7,15,17]. However, where RAP is sourced from the texturing of the underlying surface and removal of temporary ramps, the amount of RAP generated is only 5–15% of the typical asphalt produced, making higher RAP contents impracticable.
3. Materials, methods and results 3.1. Materials
Table 1 Typical Australian airport asphalt characteristics [9]. Asphalt Property
Typical; value
Bitumen content (% by mix mass) Target Marshall air voids (%) Target voids filled with bitumen (%) Filler Content (% by aggregate) Minimum Marshall Stability (kN) Maximum Marshall Flow (mm) Percentage passing AS sieve (mm) 13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075
5.8 4 75 1.5 of hydrated lime 12 3 Target by volume (%) 100 82 70 60 44 33 25 16 10 6
Two different asphalt mixtures were produced, both with and without RAP. All asphalt mixtures were designed to be dense graded, Marshall-designed 14 mm nominal maximum aggregate sized and meeting the requirements summarised in Table 1. The RAP for the first asphalt, Mixture P, was recovered from texturing of the underlying layer, as well as removal of temporary ramps. The underlying asphalt layer was produced as part of the same project, to the same mixture design and with the same ingredients, but excluded RAP, and was constructed approximately eight weeks prior. The RAP was recovered in an expedient manner and then screened and crushed prior to re-use. The RAP content was nominally 10%, which comprised 4% from ramps and 6% from texturing of the underlying layer. Mixture P was used to surface a runway catering for up the B737-800 sized aircraft. The second asphalt, Mixture D, was recovered from the removal of temporary ramps. The cold planing machines were operated slowly to maximise the degree of pulverisation of the RAP during the ramp removal process and the RAP was not subsequently
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crushed. However, large pieces of RAP were removed prior to reuse. Nominally 5% RAP was added to the surface, reflecting the smaller volume of RAP generated. Mixture D was used to surface a runway accommodating turbo-prop (Dash 8–400) and regional jet (Embaer RJ 135) aircraft. Both mixtures targeted the same overall volumetric composition and the various fractionated aggregate sizes were adjusted to account for the grading and binder content of the different RAP sources (Table 2). Both mixtures were produced using a proprietary plastomeric polymer modified binder that typically achieves PG 70-22 for extreme traffic rating under the multiple stress creep recovery based performance grading system [19].
3.2. Methods Asphalt mixtures were produced with and without RAP. The two asphalt mixtures used different RAP sources and the RAP was recovered and processed by different methods. This allowed and initial comparison of the two recovery and processing approaches and assessment of the impact on the resulting RAP properties. Secondly, the four mixtures were compared to determine the impact of RAP on the volumetrics, Marshall properties and laboratory performance testing. Finally, all RAP sources and asphalt mixtures were sampled during full-scale production in a real asphalt plant, allowing comparison of RAP property consistence, asphalt production property consistence and the finished surface properties. Simple statistics, including average, standard deviation, coefficient of variation and the Student’s T-test for differences of means, were used to quantify the impact of including low-risk RAP in airport asphalt. Physical testing used Australian standard methods where available, and when not available, international test methods were adopted (Table 3). Because the results
were obtained from real asphalt production, the number of results, with and without RAP, were sometimes unbalanced. 3.3. Results Mixtures with and without RAP were tested for the Marshall Stability and Flow, as well as the performance-indicative properties for stiffness, crack resistance, deformation resistance and moisture damage resistance (Table 4). During the Mixture P design process, three samples of RAP were recovered from temporary ramp removal and three samples of RAP were recovered from texturing of the underlying surface. The results for binder content and grading were averaged (Table 5) for input to the mixture composition. For Mixture D, RAP was only recovered from temporary ramp removal and three samples were tested for grading and bitumen content (Table 6) and the results averaged for the design of the mixture containing RAP. During real asphalt production, recovered and processed RAP was sampled and tested periodically for grading, binder content and moisture content (Table 7). This allowed the consistence of recovered RAP to be evaluated, providing an indication of the potential impact of RAP variability on asphalt production properties. Asphalt production test results are summarised for Mixture P, with and without RAP (Table 8) as well as for Mixture D (Table 9). Comparison allowed assessment of the impact of RAP on asphalt production consistence. Finally, following construction of the surface, friction testing was performed along the full length of the runway surface constructed with Mixture P. The 10 m averages of the continuously recorded results are summarised in Table 10. Surface texture was also measured in areas of the surface with and without RAP (Table 10).
Table 2 RAP and non-RAP mixture components. Component
Nominal 14 mm minus aggregate fraction Nominal 10 mm minus aggregate fraction Nominal 7 mm minus aggregate fraction Natural sand Crusher dust Hydrated lime New binder content RAP Total binder content
Units
% % % % % % % % %
of aggregate volume of aggregate volume of aggregate volume of aggregate volume of aggregate volume of aggregate volume as mass of combined aggregate as mass of asphalt mixture as mass of combined aggregate
Mixture P
Mixture D
No RAP
With RAP
No RAP
With RAP
22.0 13.0 15.0 13.0 35.2 1.8 6.0 0 6.0
19.8 9.6 9.0 12.6 38.1 0.9 5.3 10 6.0
27.4 7.1 7.1 17.0 35.1 0.7 5.6 0 5.6
27.4 6.6 6.6 15.0 33.1 0.7 5.5 5 5.6
Table 3 Test methods. Tested property
Test Method
Description and notes
Grading (% aggregate volume) Binder content (% mass) Moisture content (% mass) Air Voids (% volume) Maximum density (kg) Marshall Stability (kN) Marshall Flow (mm) Resilient Modulus (MPa) Fatigue life (cycles) Wheel track rutting (mm) Tensile Strength Ratio (%) Grip Number at 65/95 km/hr
AS 1141.11 AS/NZS 2891.3.1 AS/NZS 2891.10 AS/NZS 2891.8 AS/NZS 2891.7.1 AS/NZS 2891.5 AS/NZS 2891.5 AS/NZS 2891.13.1 AG:PT/T274 AG:PT/T231 AG:PT/T232 CAA [4]
Particle size distribution
Surface Texture Depth (mm)
AG:PT/T250
Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Compacted by 75 blows of a Marshall hammer Indirect tensile modulus at 25 °C Four-point repeated beam bending at 20 °C Copper wheel tracker at 60 °C Modified Lottman test Griptester Trailer with 1 mm water film using New Zealand protocol, which is based on international recommendations. Pestle method sand circle
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Table 4 Laboratory Marshall and performance properties. Asphalt Property
Mixture P
Marshall Stability (kN) Marshall Flow (mm) Resilient Modulus (MPa)
Fatigue life (cycles)
Wheel track rutting (mm) Tensile Strength Ratio (%)
Mixture D
No RAP
With RAP
No RAP
With RAP
19.1 3.2 4062 4899 4350 >1,000,000 >1,000,000 >,1,000,000 1.9 97
16.5 2.5 5018 5134 5155 >1,000,000 >1,000,000 >,1,000,000 1.4 96
13.6 2.8 5002 5089 5382 >1,000,000 >1,000,000 >,1,000,000 1.5 97
15.1 2.7 6675 6723 6656 >1,000,000 >1,000,000 >,1,000,000 0.8 94
Table 5 Mixture P RAP properties.
Table 7 Mixture P and mixture D RAP production characteristics.
RAP Property
From texturing
From ramps
Percentage passing AS sieve (mm)
% of aggregate by volume
13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075 Binder content (% mass)
100 89 76 66 47 35 27 19 10 6.6 5.4
100 91 83 72 51 36 28 20 11 7.2 5.4
Combined
RAP Characteristic
100 90 80 69 49 35 28 19 11 6.9 5.4
Binder content (% mass) 5.8 0.3 5.1 Moisture content (% mass) 1.5 0.7 0.2 Percentage passing AS sieve (mm) (% of aggregate by volume) 13.2 99.8 0.5 100.0 9.5 91.3 2.4 90.7 6.7 81.4 3.7 78.8 4.75 69.6 4.4 67.8 2.36 49.9 4.1 51.2 1.18 36.4 2.8 37.2 0.600 28.2 1.7 26.1 0.300 20.4 1.2 17.7 0.150 11.3 1.0 12.2 0.075 7.3 0.7 9.4
Table 6 Mixture D RAP properties. RAP Property
Sample 1
Sample 2
Percentage passing AS sieve (mm)
% of aggregate by volume
13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075 Binder content (% mass)
100 96 86 77 58 41 28 18 13.1 10.1 4.8
100 98 91 83 63 45 30 20 13.7 10.6 4.9
Sample 3
Average
100 98 89 80 60 43 30 20 14 10 4.8
4. Discussion To quantify the impact of RAP on airport asphalt mixtures, the results with RAP and without RAP were compared for both Mixture P and Mixture D. Quantitative comparisons included the different methods of RAP recovery and processing, asphalt mixture design properties, RAP production consistence, asphalt production property consistence and friction and texture of the finished asphalt surface. 4.1. Impact of RAP recovery and processing on RAP properties Mixture P RAP, recovered from the removal of temporary ramps and from texturing of the surface, were similar with regards to binder content (Table 2) and grading (Fig. 1). The three samples of
Mixture D (19 samples)
Average
Average
Std. Dev.
Std. Dev. 0.2 0.1 0.0 3.7 5.4 6.2 4.8 3.3 2.0 1.5 0.9 0.8
Table 8 Mixture P asphalt production characteristics. Asphalt Characteristic
100 99 90 80 59 44 31 21 14.1 9.8 4.8
Mixture P (18 samples)
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results
No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
6.0 4.6 2443 17.2 2.6 100
0.2 0.5 31 1.9 0.1
6.0 4.7 2452 16.3 2.6 47
0.2 0.4 7 1.1 0.1
Mixture D RAP from temporary ramp removal were also consistent with regards to binder content, with a coefficient of variation of just 1.2%, and comparable grading (Fig. 2). Furthermore, the two design gradings were similar, reflecting the same mixture specification, but the two RAP sources deviated from the mixture design by different degrees (Fig. 3). RAP from Mixture P was recovered in a typical and expedient manner. Consequently, the RAP was coarse and required pulverisation, which resulted in a grading marginally finer than that of the mixture design. However, the Mixture D RAP was recovered by the slow operation of the cold planing machine, which resulted in a consistently finer RAP than for the mixture design. For example, both mixture designs targeted 59% of the aggregate passing the 4.75 mm sieve. The Mixture P RAP averaged 69% passing 4.75 mm and the Mixture D RAP averaged 80%. Regarding binder content, the Mixture P RAP recovery and processing reduced the binder content by a consistent 0.5% and the Mixture D RAP recovery reduced the binder content by a consistent 0.8%. These differences must be considered during the asphalt mixture design, but are not detrimental as long as they are accounted for.
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4.3. Consistence of RAP production
Table 9 Mixture D asphalt production characteristics. Asphalt Characteristic
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results
No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
5.6 3.3 2532 15.4 3.5 41
0.1 0.2 5 0.7 0.2
5.5 3.4 2535 15.2 3.7 29
0.1 0.2 5 0.8 0.2
Table 10 Mixture P surface friction and texture characteristics. Friction Characteristic
Grip Number at 65 km/hr Grip Number at 95 km/hr Surface Texture Depth (mm) Number of Friction Results Number of Texture Results
761
No RAP
With RAP
Average
Std. Dev.
Average
Std. Dev.
0.70 0.55 0.44 440 8
0.07 0.11 0.05
0.62 0.47 0.43 1040 8
0.08 0.10 0.05
One of the risks cited by airports that resist the inclusion of RAP in runway surfaces is the variability of RAP and its impact on asphalt production. The summary of Mixture P RAP production results indicate that the coefficients of variation associated with the percentages passing the various sized sieves ranged from 2 to 9% over the 18 RAP samples, calculated from the statistics in Table 7. The critically important binder content had a lower coefficient of variation at less than 5%. However, the moisture content, although acceptably low, had a higher variation of almost 50%. The moisture in the RAP comes from the water used to prevent overheating of the profiler teeth during the cold planing process, and the variability likely reflects the amount of water used, as well as the drying time between cold planing and subsequent RAP sampling for testing. Similarly, Mixture D RAP had grading coefficients of variation of 4–9% (Table 7) which is comparable to Mixture P. However, for Mixture D, the binder content was less variable, 3.0% compared to 4.6%, and the moisture content was lower, averaging 0.2% compared to 1.5%. Overall, the variability of the produced RAP was no greater than is typically observed with new aggregate sources.
4.2. Impact of RAP on mixture properties
4.4. Impact of RAP on asphalt production
The target volumetric properties of the two mixtures, with and without RAP, were nominally identical. However, when the mixtures were tested for Marshall properties and the performance properties, some differences were noted (Table 4). For Mixture P, the addition of RAP was associated with a decrease in the Marshall Stability, indicating a less stiff mixture. In contrast, the Marshall Flow and wheel track rutting both reduced and the modulus increased, all suggesting a stiffer mixture. In the case of Mixture D, all mixture design properties indicated a stiffer mixture associated with inclusion of RAP. However, the increase in stiffness was not detrimental, with the resilient modulus within the normal range for all mixtures and the fatigue life of all mixtures exceeding the terminal 1,000,000 cycles.
The binder content, air void content and Marshall Flow (Table 8) were not significantly different for Mixture P, with RAP and without RAP (Table 11), and as shown in Fig. 4. However, the Marshall Stability was significantly lower and the asphalt maximum density was significantly higher with RAP (Table 11). The increase in density likely reflects small changes in the overall asphalt composition (Table 2) and differences in the specific gravity of the various aggregate sources. The likely cause of the Marshall Stability decrease is not known, although it is clear that this is a more variable test than others (with a coefficient of variation approximately 9–10%). Similar trends were observed in the Mixture D asphalt production results, except only the maximum density was significantly different when RAP was added (Table 11).
Fig. 1. Mixture P ramp removal and texturing RAP aggregate grading.
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Fig. 2. Mixture D ramp removal RAP aggregate gradings.
Fig. 3. Mixture design and recovered RAP aggregate gradings.
Table 11 Mixture P and mixture D asphalt production property changes. Asphalt Production Property
Change in average results and associated Student’s T-test for difference of means with and without RAP Mixture P
Binder Content (% mass) Air Voids (% volume) Maximum density (kg/m3) Marshall Stability (kN) Marshall Flow (mm) Number of results compared
Mixture D
Change in mean
P-value
Change in mean
P-value
No detectible change Increase 0.1 with RAP Increased 9 with RAP Decreased 0.9 with RAP No detectible change 100 without RAP and 47 with RAP
0.41 0.12 <0.01 <0.01 0.90
Increased 0.1 with RAP Increased 0.1 with RAP Increased 3 with RAP Decreased 0.2 with RAP Increased 0.2 with RAP 41 without and 29 with RAP
0.21 0.82 0.01 0.34 0.22
Note: p-values exceeding 0.05 indicate that there is not a statistically significant difference.
G. White / Construction and Building Materials 197 (2019) 757–765
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Fig. 4. Mixture P asphalt production properties with and without RAP.
Overall, the inclusion of RAP had minimal impact on the asphalt production properties. This likely reflects the RAP having been produced only weeks prior to its recovery and from asphalt that was produced to the same design, with the same aggregate sources and the same binder type. The decrease in Marshall Stability (5.0%) and increase in maximum density (0.3%) observed when RAP was introduced to Mixture P indicates the potential for changes in these properties, particularly when aggregates of significantly different specific gravity are used to produce the asphalt. This was consistent for Mixture D, where the inclusion of RAP increased the maximum density by 3 kg (or 0.1%). However, these properties are not indicators of asphalt performance, although they must be controlled for consistent asphalt production. Furthermore, there was no indication that the introduction of RAP reduced the consistence of the manufactured asphalt. This is important and likely reflects the consistent source of RAP added to the two mixtures and the relatively modest RAP contents of low-risk RAP available. 4.5. Impact of RAP on surface friction and texture
the measured surface friction at both 65 km/hr and 95 km/hr test speeds (Table 12). For example, the 95 km/hr average friction result decreasing from 0.55 to 0.47 where RAP was included (Table 10). The reduction in friction was observed despite the measured surface texture being unaffected, with averages of 0.44 mm and 0.43 mm without RAP and with RAP, respectively (Table 10). The friction reduction was statistically significant at both test speeds, while the difference in the surface texture was not (Table 12). Furthermore, Figs. 5 and 6 show the clear reduction in friction measured at 65 km/hr and 95 km/hr, respectively, at the commencement of the construction of Mixture P with RAP. The reduction in Mixture P surface friction associated with the inclusion of RAP did not reflect a significant reduction in surface texture. It is therefore likely that a double coating (in binder) of the fine RAP aggregate occurred during the second asphalt production. Whether this impact was specific to Mixture P was not determined. It is important to note that the levels of friction measured exceeded the minimum requirement, before the runway surface was grooved. Consequently, even though the inclusion of RAP reduced both the 65 km/hr and the 95 km/hr results, there was no impact to the functional suitability of the runway surface.
Interestingly, the Mixture P friction testing results indicate that the addition of RAP was associated with a significant reduction in 5. Further research
Table 12 Mixture P surface friction and texture changes p-values. Asphalt Production Property
Change in average results and associated Student’s T-test for difference of means with and without RAP Change in mean
Grip Number at 65 km/hr Grip Number at 95 km/hr Surface Texture Depth (mm) Number of friction results compared Number of texture results compared
P-value
Reduced 0.08 with RAP <0.01 Reduced 0.08 with RAP <0.01 Reduced 0.02 with RAP 0.78 440 without RAP and 1040 with RAP 8 without RAP and 8 with RAP
Note: p-values exceeding 0.05 indicate that there is not a statistically significant difference.
This research objectively quantified the effect of incorporating low (5–10%) contents of low-risk RAP in two typical dense graded airport asphalt mixtures. In both cases, the RAP was recovered from asphalt produced to the same mixture design as the asphalt to which it was added. Furthermore, it was recovered from the removal of ramps and the texturing of the underlying surface constructed not more than eight weeks prior. Consequently, conclusions can only reflect the use of similarly low-risk RAP sources. Therefore, further research is required to objectively assess the impact of incorporating higher portions of other RAP sources. Additional research is also required to better understand the reduction in measured surface friction, without a commensurate reduction in surface texture, when RAP was introduced.
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Fig. 5. Mixture P 65 km/hr surface friction with and without RAP.
Fig. 6. Mixture P 95 km/hr surface friction with and without RAP.
6. Conclusions Comparison of the two typical airport asphalt mixtures indicates there is no detrimental effect associated with the inclusion of low-risk RAP. That is, airports should not resist the recycling of the 5–10% of low-risk RAP available from the removal of temporary ramps. Where the underlying surface is produced to the same asphalt mixture design, cold planings from existing surface texturing should also be incorporated as RAP. Consequently, it is recommended that low-risk RAP sources be incorporated into airport asphalt surfaces, at up to 10% RAP content, without requiring a mixture design inclusive of representative RAP. It is also recom-
mended that further research consider other sources of RAP, specifically RAP from existing and aged airport asphalt surfaces, as well as the impact of RAP on runway surface friction. Conflict of interest None. References [1] AAA 2017, Airfield Pavement Essentials, Airport Practice Note 12, Australian Airports Association, Canberra, Australia, April, viewed 10 January 2018,
G. White / Construction and Building Materials 197 (2019) 757–765 [2] G.D. Airey, A.C. Collop, N.H. Thom, Mechanical performance of asphalt mixtures incorporating slag and glass secondary aggregates, in: Proceedings 8th Conference on Asphalt Pavement for Southern Africa, 2004, Sun City, South Africa, 12–16 September. [3] Austroads 2015, Maximising the use of Reclaimed Asphalt Pavement, Technical Report AP-T286-15, 2 February, viewed 10 January 2018,
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[14] R.L. Keast, D.C. Baker, K. Brown, Sustainable Airport Infrastructure: Balancing Infrastructures for the airport metropolis, Chapter 7, Sustainable Urban and Regional Infrastructure Development: Technologies, Applications and Management, Information Science Reference, Hershey, New York, New York, USA, 2010. [15] A. Norouzi, M. Sabouri, Y.R. Kim, Evaluation of the fatigue performance of high RAP asphalt mixtures, in: 12th ISAP Conference on Asphalt Pavements, June 15, 2014, Raleigh, North Carolina, USA. [16] G. Valdés, F. Pérez-Jiménez, R. Miró, A. Martínez, R. Botella, Experimental study of recycled asphalt mixtures with high percentages of reclaimed asphalt pavement (RAP), Constr. Build. Mater. 25 (2011) 1289–1297. [17] R.K. Veeraragavan, R.B. Mallick, M. Tao, M. Zaumanis, R. Frank, R.L. Bradbury, Laboratory comparison of rejuvenated 50 percent reclaimed asphalt pavement and hot-mix asphalt with conventional 20 percent RAP mix, Transp. Res. Rec.: J. Transp. Res. Board 2633 (2017) 69–779. [18] G. White, Towards a performance-based airport asphalt specification, in: International Conference on Highway Pavements and Airfield Technology, Philadelphia, Pennsylvania, USA, 27-30 August, 2017. [19] G. White, K. Embleton, Validation of a new generation bitumen for airport asphalt, in: 17th AAPA International Flexible Pavements Conference, Melbourne, Victoria, Australia, 13–16 August, 2017. [20] G. White, A. Jamshidi, Incorporating environmentally sustainable asphalt surfaces into airport pavements, in: 4th International Symposium on Asphalt Pavements & Environment, Toyosu, Tokyo, Japan, 20-21 November, 2017. [21] G. White, H. Fairweather, A. Jamshidi, Sustainable runway pavement rehabilitation: a case study of an Australian airport, J. Cleaner Prod. 204 (2018) 380–389. [22] T.D. White, Marshall procedures for design and quality control of asphalt mixtures, Proc. Asphalt Pavement Technol. 54 (1985) 265–285. [23] I. Widyatmoko, J. Richardson, C. Ferguson, Long-term monitoring of high performance airfield pavement surfacing, Int. J. Pavement Res. Technol. 6 (5) (2013) 582–587.