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Superior performance benefits of multigrade bitumen asphalt with recycled asphalt pavement additive
Construction and Building Materials 230 (2020) 116963
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Superior performance benefits of multigrade bitumen asphalt with recycled asphalt pavement additive Brody R. Clark ⇑, Chaminda Gallage Faculty of Science and Engineering, Queensland Univ. of Technology, 2 George St., Brisbane, QLD 4001, Australia
h i g h l i g h t s Four-point bending testing was conducted on multigrade asphalt beams with and without RAP to examine the fatigue and modulus performance under
various temperatures. Better understanding of the materials will lead to a reduction in the use of virgin aggregate. Asphalt is very temperature dependant and multigrade offers a reliable solution that can lead to a reduction of overall pavement thickness. The current method of adopting the Shell relationship is not reliable. CIRCLY parameters determined for these mixes and can be adopted by industry.
a r t i c l e
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Article history: Received 13 May 2019 Received in revised form 2 September 2019 Accepted 13 September 2019
Keywords: Pavement engineering Recycled asphalt pavement Multigrade bitumen Asphalt fatigue testing Flexural stiffness Master curves Complex modulus
a b s t r a c t The implementation of recycled asphalt pavement (RAP) in new pavements is crucial for future sustainability, but the impact of the recycled material on the new roads properties must first be investigated to ensure future pavement performance. The introduction of RAP to high performing asphalt mixes, such as multigrade asphalt (asphalt utilising multigrade bitumen), needs to be investigated to safe guard against possible detrimental effects of the RAP. Extensive laboratory testing was conducted on multigrade asphalt beams under a range of temperatures and loading conditions. Testing has found that multigrade bitumen does perform better than expected during laboratory testing compared to conventional asphalt and the current industry standard of utilising the Shell 1978 fatigue curve for asphalt. The addition of RAP had beneficial effects on the asphalt pavements at higher temperatures (30 °C). Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction As a solution to depleting natural resources and managing demolition waste, the use of recycled materials in pavement construction has gained a significant attention in the past decade [14,8,9,15]. Virgin bitumen is a non-renewable product that we will inevitably be depleted. To extend the current supplies of virgin bitumen the use of recycled asphalt pavement (RAP) is incorporated into pavement designs. RAP is reclaimed in-service pavement that is far more aged and oxidised in comparison to the virgin binders. RAP has been shown previously to benefit the performance of conventional bitumen asphalt [10] but other testing has also
⇑ Corresponding author. E-mail addresses: [email protected] (B.R. Clark), chaminda. [email protected] (C. Gallage). https://doi.org/10.1016/j.conbuildmat.2019.116963 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
highlighted the potential negative impact that the aged binder can have on the virgin pavements [23]. These previous tests and others show the impact that RAP can have on the materials stiffness and rutting resistance but not what the RAP’s effects are on the materials fatigue resistance. Shannon, et al. [20] conducted fatigue testing on high RAP content asphalt (up to 50% RAP) and found that the RAP had no discernible difference on the pavements fatigue performance. However, these tests were only conducted at a single temperature (21 °C) and therefore do not accurately reflect the impact that changing stiffness values have on the fatigue performance of asphalt at different temperatures. Yin, et al. [24] conducted vigorous testing on high RAP inclusive asphalt and reported on the use of recycling agents to negate the potential negative impacts of using RAP in pavements. As the percentage of RAP included within the scope of this testing is considered low, the use of such additives is not considered
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appropriate. The use of low levels of RAP is considered appropriate for industries with small quantities of useable RAP stored, such as the Australian market, where 100% of the quality RAP stockpiles are currently utilised. Once greater stockpiles of RAP are gathered, the use of the recycling additives will become more prevalent. The RAP source could potentially play a large role in determining the effect of RAP of the asphalts performance and properties. As RAP is pavement that has been recycled, the binder and aggregate utilised in the old mix designs could vary significantly. The current best practice in Brisbane, Australia (the location for this testing and source of the RAP for this paper) is to stockpile and screen all RAP together. This could lead to various qualities of asphalt being implemented as RAP, and therefore adds a degree of uncertainty in the results of RAP incorporation testing. Simply put, the RAP you use today could be vastly different to the RAP you could be given next week. More analysis should be put into determining how much of an effect various RAP sources have on pavement performance and/or better stockpiling and processing of RAP should be undertaken to control RAP quality. Multigrade bitumen is a quality proprietary binder that has been ‘shelved’ due to a lack of interest in testing its capabilities. Multigrade bitumen is a chemically modified binder that is created by combining multiple binders with additional chemicals. The combination of these binders enables the pavements to have the properties of hard paving grade bitumen at high service temperatures and the properties of soft paving grade bitumen at low temperatures [19]. The actual ingredients and chemical makeup of multigrade bitumen is propriety knowledge and is not public knowledge. The multigrade bitumen utilised in this testing is M1000. M1000 has the characteristics of a conventional binder (binder with viscosity of 320 at 60 °C) at low service temperatures, with and actual typical viscosity at 60 °C of approximately 1000 Pas [22]. These observed material properties are the basis of why it is believed multigrade bitumen is less sensitive to temperature variation [19]. Previous testing [21] found through laboratory testing that multigrade bitumen can reduce required pavement thickness compared to conventional binders. However, previous testing did not investigate the effects RAP and temperature have on the laboratory fatigue of the asphalt. Bitumen is a viscoelastic material, which means that asphalt’s properties and performance are dependent on temperature and rate of loading [6]. The resistance to temperature fluctuations are what make multigrade bitumen superior to conventional asphalt binders, particularly in climate regions such as Queensland, Australia. Queensland pavement temperatures change dramatically, depending on the time of day and season [18]. The Australian Bureau of Meteorology records Queensland’s extreme temperatures as ranging between 10.6 °C and 49.5 °C [1]. Previous testing has shown that the in-service temperature of asphalt changes dramatically with depth [3]. Laws et al. [16] found that the pavements temperature generally stabilises after it extends beyond 300 mm thick. Interestingly, they also found that the highest temperature recorded for the stabilised temperature, for typical targeted air voids of 2% and 6%, is the adopted Weighted Mean Average Pavement Temperature (WMAPT). WMAPT is the nominated temperature for design of asphalt pavements in Australia. However, for pavements less than 300 mm and different air void contents, Laws et al. [16] found that the temperature extends beyond the WMAPT. Perhaps future pavements designs should adopt different fatigue curves for changing pavement thicknesses, based on the temperature at the base of the pavement layer. One of the primary failure/distress modes of asphalt pavements is fatigue cracking. These fatigue cracks initiate at the bottom of the asphalt layer and propagate to the surface. Once these cracks
are fully developed, water can easily percolate to base/sub-base, or subgrade which causes significant permanent deformation (rutting) in the pavement surface. The fatigue life of asphalt depends on a number of factors such as; complex modulus of asphalt, air voids, binder type and content/film thickness and its rheological properties, aggregate type and grading, and mode of traffic loading and conditions (the magnitude and frequency, the rest period between successive loads) [7]. The complex modulus of asphalt is an indication of the stress–strain relationship and was determined in previous testing [9] using four point bending. The most common method of determining fatigue resistance of asphalt in Australia is also by using beam fatigue testing [12]. A previous study into published fatigue results for asphalt pavements utilising multigrade bitumen and incorporating recycled asphalt pavement (RAP) found that there has been limited research conducted on these innovative materials [9]. The study concluded from the limited available data, that the impact of RAP varies and a defined correlation between RAP content and complex modulus and fatigue life has not been produced. Additionally, the study found that multigrade bitumen does appear to have a positive impact on fatigue resistance compared to conventional binders. However, due to the large number of factors affecting the performance of an asphalt pavement, previous studies have generally neglected to utilise a range of temperatures during testing. This paper endeavours to expand upon the preliminary testing results found by [9] with a significantly greater testing regime. To determine the fatigue life of the asphalt beams, under multiple testing temperatures, the four-point bending method was utilised. The four point bending test method attempts to simulate real world loading of traffic induced stresses and strains by dynamically loading rectangular beam specimens [13]. Four point bending was adopted for testing as it is found to produce the same complex modulus values as the equivalent two point bending methods, if the samples geometry and mass are taken into account [11] and due to the availability of resources. Previous testing [9] analysed the complex modulus variation of the mixes utilised in this testing by investigating the effects that loading frequency and temperature have on complex flexural modulus. These results were then used to develop a unique relationship between complex modulus and loading frequency for a reference temperature. Such relationship is called a ‘‘master curve” and can be used to obtain the modulus at any given temperature and frequency. This testing found that adding RAP to the mix increases the materials complex modulus, possibly due to the recycled material being more aged/oxidised than the virgin. The effects of RAP on the fatigue performance of multigrade bitumen asphalt is to be investigated and will be collaborated with the complex modulus master curves found in previous testing [9]. This paper investigates the effects of temperature and RAP incorporation on multigrade bitumen by analysing the fatigue performance and stiffness properties of multigrade bitumen asphalt. Fatigue testing for the specified mixes have been conducted at temperatures ranging from 10 °C to 30 °C with the intention to numerically analyse the data to create an equation that can estimate to fatigue performance at any given temperature, provided that a complex modulus master curve is available for the specific mix. To demonstrate the potential performance of the mixes CIRCLY 6.0 [17] computer program is utilised to determine the equivalent pavement thickness of the mixes under a range of traffic loading and temperatures.
2. Test materials The weighted percentage of each material included in the mix designs of the two mixes utilised in this testing can be seen in
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963 Table 1 Mix designs for asphalt samples.
Table 2 Viscosity values of binder extracted from mixtures. Asphalt mixture labels
Material
MG-0%
MG-15%
18 mm Aggregates (%) 14 mm Aggregates (%) 9 mm Aggregates (%) 7 mm Aggregates (%) Crusher Dust (%) Fine Sand (%) Rock Flour (Baghouse Fines) (%) Rap Content (%) Multigrade Bitumen Type Virgin Bitumen Content (%)
9.2 14.3 20.7 14 31.8 9 1 0 M1000 4.5
10 16.4 14.3 10.8 27 5.5 1 15 M1000 3.7
Table 1. The mixes incorporate; multigrade bitumen, aggregate, fine sand, crusher dust, rock flour and RAP. As RAP already contains a percentage of bitumen (an average of 4.37% in the case of the RAP used in this testing) the mix incorporating RAP has a reduced virgin bitumen content percentage, as shown in Table 1. The calculated percentage of virgin bitumen added to MG-15% assumes 100% of binder from the RAP is available. The grading results of the two mixes obtained from sieve analysis can be seen in Fig. 1, as well as the most recent data from the RAP source. The results of the sieve testing show that there is little variation in mix designs and it can be assumed that any inconsistencies during testing are unlikely to be caused by these differences in the mixes. The two mixes utilised virgin multigrade bitumen and targeted the optimum bitumen content of 4.5%. 4.5% was adopted as it is the typical binder content utilised for asphalt mix designs in the local industry [18]. M1000 multigrade bitumen was utilised due to it being the most commonly available multigrade bitumen in Brisbane and are believed to be supplied to meet the requirements of Australian Standard specification AS2008. In order to determine the effect of RAP on the properties of the binders, the bitumen was extracted from each mixture and tested for viscosity according to Australian Standard 2341.2. The results are given in Table 2.
MIXTURE LABEL
BITUMEN TYPE
RAP CONTENT (%)
VISCOSITY AT 60 °C (PA.S)
MG-0% MG-15%
M1000 M1000
0 15
17,250 125,000
As can be seen in Table 2, MG-0% has a lower viscosity than the mix containing 15% RAP. Adding RAP into virgin multigrade bitumen asphalt has increased the viscosity of the binder, likely due to the RAP introducing oxidised binder into the mixture. Due to current RAP stockpiling practice in Australia, the quality and consistency of the RAP is unknown and likely largely variable. The viscosity values in Table 2 show the true impact of the RAP utilised in the mixes. However, until we develop a better way of cataloguing and sorting RAP stockpiles, the recorded effect of RAP on asphalt mixes in Australia should probably continuously be monitored until an agreed result for all variation of RAP qualities are obtained.
3. Test apparatus A pneumatic four-point fatigue beam testing apparatus was utilised in testing and can be seen in Fig. 2. The apparatus loads the centre of the rectangular beam sample, with the loading applied by an actuator in the form of sinusoidal oscillation. Vertical deflection at the centre point of the sample is monitored by an onspecimen linear variable differential transformer (LVDT) with an accuracy of ±1 lm. The vertical deflection is utilised by the computer program to calculate the tensile strain at the mid-point at the base of the beam. This tensile strain is the control loading for this experiment. The testing apparatus is housed in an environmental control chamber that maintains the required temperature condition with a maximum allowable fluctuation of ±0.5 °C. The environmental chamber housing the four-point bending machine can be seen in Fig. 3. The testing software monitors the temperature of the sample by utilising a dummy sample housed inside the environmental chamber. The skin and core temperature of the dummy sample is
Fig. 1. Aggregate grading chart.
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beam can be seen housed in the four-point bending apparatus, ready for testing, in Fig. 2. 4.2. Fatigue testing procedure
Fig. 2. Pneumatic Four-Point Bending Apparatus.
The methodology adopted for this testing is taken from The Austroads Guide to Pavement Technology Test Method AGPT/ T274 [4] in conjunction with Part 2 [2] of the Austroads Guide. The testing utilised three different strain levels during testing, ranging from 100 to 300 microstrain. The samples were also tested at constant temperatures of 10 °C, 20 °C and 30 °C and maintained within a 0.5 degree variance. Sinusoidal loading was adopted for testing at a constant frequency of 10 Hz, as per Austroads test method AGPT/T274, with the failure condition being when the complex modulus of the beam is reduced to 50% of the initial stiffness. Once the samples complex modulus has reduced below 50% of the initial value, the number of cycles is recorded and an average for each load strain and temperature is calculated. Demonstrated graphically in Fig. 4 is the method of obtaining the load repetition to failure of a single beam. For a given strain amplitude and temperature, three identical beams were tested and the average failure loading cycles calculated as the corresponding loading cycles for fatigue failure. For each mix type three strain amplitudes were tested which provide three total data points for each mix and utilised to develop the fatigue curve for each temperature. For this experiment, the loading condition, temperature and incorporation of RAP were the variables, while mix design, loading frequency, deflection and virgin binder were constant. Sinusoidal loading was utilised, as previous testing [5] found that haversine loading becomes sinusoidal after around 50 cycles, but at half the strain. The results of the laboratory testing will be transformed to fit into a single function that can derive the loading cycle at any strain for a given modulus for a specific mix design. The adopted model fitting function can be seen in Eq. (1), with the definitions of variables provided beneath. Eq. (1) was derived by extensive testing carried out by the Australian road authority Austroads in the development of AGPT/T274.
Nlab ¼ EXP c1 ln3 ðE Þ þ c2 ln2 ðE Þ þ c3 lnðE Þ þ c4 þ c5 lnðleÞ
ð1Þ where; N lab ¼ Number of cycles to failure in the laboratory flexural fatigue test E ¼ complex modulus ðMPaÞ at the test frequency and test temperature Fig. 3. Environmental Chamber Housing Testing Apparatus.
le ¼ strain in laboratory flexural fatigue test ðlm=mÞ
monitored to ensure that the asphalt is sufficiently conditioned to the required testing temperature for at least four hours prior to testing. 4. Methodology The methodology to investigate the effects temperature and RAP has on multigrade bitumen asphalt fatigue performance consists of sample preparation and a testing program for fatigue performances. 4.1. Sample preparation Each mixture in Table 2 was compacted into a slab using the slab compactor to achieve a target air void content of 5%. Four beams were cut from each slab each measuring a length of 390 mm, height of 50 mm and width of 63 mm. An example of a
Fig. 4. Demonstration of How to Find Load Repetition to Failure.
B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
le ¼ load induced tensile strain at base of the asphalt ðmicrostrainÞ
c1 ; c2 ; c3 ; c4 & c5 ¼ fitting parameters To demonstrate how multigrade bitumen asphalt is currently under-utilised in industry the results of testing were compared to equivalent values calculated using the Shell equation. The Shell equation was developed in 1978 with testing conducted on a range of asphalt mixes, incorporating various bitumen types, in order to obtain a general relationship for the fatigue performance of an asphalt pavement [2]. The Shell relationship currently utilised by Australian road authorities can be seen in Eq. (2). This paper will analyse how accurate the Shell relationship is for determining the performance of the multigrade bitumen asphalt mixes used in this testing. Note that the reliability factor (RF) is ignored in this testing, as it is a safety factor applied to the equation to allow for construction variability, environment, traffic variability, etc.
N ¼ RF where;
" #5 6918ð0:856V b þ 1:08Þ E0:36 le
5
ð2Þ
RF ¼ reliability factor V b ¼ percentage by v olume of bitumen in the asphalt ð%Þ E ¼ asphalt modulus ðMPaÞ N ¼ allowable number of repetitions of the load 4.3. Equivalent pavement thickness modelling
In order to represent the findings of this paper and to highlight the possible implications that they will have on the design of modern roads, the pavement performance calculation software CIRCLY 6.0 [17] is utilised. The software calculates the fatigue and rutting performance of an inputted pavement structure and determines its applicability for the assigned traffic conditions. For the use of this modelling, the programs auto-calculate pavement thickness feature is used to determine the equivalent pavement thickness for each mix under the same traffic loading. Each mix design was analysed as a full depth asphalt pavement on top of a subgrade with a California Bearing Ratio (CBR) of 5%, which equates to a modulus of
Fig. 5. Fatigue results of mixes at 10 °C.
Fig. 6. Fatigue results of mixes at 20 °C.
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
Fig. 7. Fatigue results of mixes at 30 °C.
50 MPa for the subgrade. The pavements were designed to endure traffic of 10^6 Design Equivalent Standard Axles (DESA). In order to model the effects of changing temperature and velocities CIRCLY 6.0 requires the modulus and fatigue performance exponents of each mix for each scenario. For each mix and temperature, the fatigue results produce the CIRCLY 6.0 input parameters ‘b’ and ‘k’ and the modulus input of the material is gained from producing a modulus master curve. Previous testing [8] has produced the required modulus master curves for the mixes utilised in this testing. The master curves are used to derive the modulus of the material at any given temperature and v. The input performance exponent values (b & k) are simply derived from fitting a power curve to each of the fatigue curves found from testing. 5. Results and discussion 5.1. Effects of RAP on fatigue performance of multigrade bitumen asphalt Figs. 5–7 demonstrate the fatigue results obtained during testing for the two mixes at ten, twenty and thirty degrees Celsius respectively. It is clear in Fig. 6 that at 20 °C the mixes appear to perform similarly, with the only significant difference at the lower strain level. The difference at the lower strain level could be due to possible outliers in the data or that the virgin mix is more capable of self-healing than the possibly over-oxidised RAP mix. More testing should be conducted to determine if the ‘perpetual pavement’ theory is the cause of the difference at this strain. It is clear that at the lower temperature (10 °C) the mix without RAP performs better than the mix with RAP. However, at 30 °C the mix with RAP performs significantly better than the mix without RAP. This could be due to the difference in complex modulus. As asphalt is a viscoelastic material, the binder relies on having a balance between ductility and stiffness to provide structural support to traffic loading, while having the ability to be flexible. As the ductility of the mix without RAP increases and the stiffness decreases at thirty degrees, the material loses its ability to resist the loading as well as the mix with RAP. As the RAP increases the stiffness of the material, it enables the material to resist the loading at a higher temperature – compared to the mix without RAP. It is clear that for warmer climates, such as most parts of Queensland, Australia, RAP can provide benefits to asphalt roads subject to high service temperatures.
It is also interesting to see that the two mixes perform significantly better than the Austroads fatigue equation. This could mean that roads utilising multigrade bitumen are possibly over-designed when it comes to fatigue life. These results demonstrate that more road agencies should investigate the benefits of multigrade bitumen and RAP and implement these innovative materials into more designs.
5.2. Effects of temperature on fatigue performance of multigrade bitumen asphalt Fatigue testing was conducted at 10 °C, 20 °C and 30 °C under three strain loads for the two mixes and the results of which can be seen in Table 3. The results of the fatigue testing are compared against the Austroads (Shell) fatigue relationship (Eq. (2)), which is current best practice for asphalt pavement design. To determine the equivalent number of loading cycles utilising the Austroads (Shell) equation, the modulus adopted was obtained from the complex modulus master curve derived in previous testing [8] for each mix. Fig. 8 demonstrates the results of the fatigue testing for MG-0%. The graph clearly shows that the material does not perform significantly differently at 10 °C and 20 °C, but loses fatigue capacity at 30 °C. These results are surprising, as previous testing [8] showed that for this material and loading frequency, each 10 °C increase in temperature roughly halved the materials modulus. This can possibly be because at these two temperatures the materials modulus is almost symmetrically placed on the bell curve for optimum material modulus for fatigue life. More testing should be conducted to develop the bell curve for optimum material modulus with regards to fatigue life to determine if this finding is correct. Table 3 Loading cycles to failure fatigue testing results. Mix
MG-0%
MG-15%
Micro-strain
150* 200 300 150* 200 300
Temperature (°C) 10
20
30
5.25E + 05 6.72E + 04 1.44E + 04 1.52E + 05 1.71E + 04 5.29E + 03
9.00E + 06 7.25E + 04 1.06E + 04 2.57E + 05 5.32E + 04 1.35E + 04
3.03E + 05 3.49E + 04 1.49E + 04 1.61E + 06 1.04E + 05 2.58E + 04
*Note: For 20 degrees, testing was conducted at 100 micro-strain.
B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
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Fig. 8. MG-0% Fatigue testing results.
Fig. 9. MG-15% Fatigue testing results.
Fig. 9 demonstrates the results of the fatigue testing conducted on MG-15%. The graph shows a clear performance difference between temperatures, with the highest tested temperature, 30 °C, outperforming the other two temperatures. It appears from this testing that as temperature increases, the mix with RAP seems to perform better during fatigue testing. It appears to be the converse for the mix without RAP. Much like MG-0%, MG-15%’s complex modulus significantly decreases with every increase in temperature, but as previous testing found, MG-15% is on average 70% stiffer than the virgin binder mix under these testing conditions. What is clear in both Figs. 8 and 9 is the unreliability of the Austroads (Shell) fatigue equation (Eq. (2)). Both graphs demonstrate a clear uncertainty in the use of the fatigue equation in predicting the laboratory fatigue life of these mixes. That is likely due to the
Austroads (Shell) equation being derived from out-dated mix designs utilising conventional binders and not applicable to modern innovative pavement materials. 5.3. Laboratory fatigue model fitting The results obtained from testing were transformed to fit a fatigue model that is able to be used to predict the laboratory fatigue life of these mixes at a specific loading strain and stiffness. The fitting function is given by Eq. (1) in the Methodology section of this paper and the applicable fitting parameters determined during testing are provided in Table 4. To demonstrate the accuracy of the fitting function, Fig. 10 compares the fatigue results of MG0% at 10 °C and 20 °C with the derived results from Eq. (1). The fitting function achieves a coefficient of determination (also known
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Table 4 Fatigue function fitting parameters.
MG-0% MG-15%
c1
c1
c1
c1
c1
0.328 0.412
8.833 10.137
79.229 77.771
192.516 140.964
6.140 4.990
Fig. 10. Comparison of derived and determined fatigue results.
Fig. 11. The effects of temperature and RAP on pavement thickness.
as R2) of 95%. This provides some confidence that this equation can be utilised to derive an estimate of the laboratory fatigue life cycle count for this mix. 5.4. Impact of results on pavement thickness The results obtained from previous complex modulus master curve testing [8] were combined with the results of the fatigue function derived for each mix from this testing to determine the
impact that these laboratory testing results have on required pavement thickness. The data was analysed and computed using the pavement design program CIRCLY 6.0 to determine a comparative pavement thickness for each mix, under a range of temperatures and traffic speeds. The pavement program calculates the required pavement thickness for a given mix by analysing both fatigue and rutting performance. For the purpose of comparing the mixes, a nominal subgrade with a California bearing ratio (CBR) of 5% was adopted and the mix is represented as a full depth pavement. The
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963 Table 5 CIRCLY 6.0 input parameters derived from testing. Mix
Temp (°C)
Performance Exponent (b)
Performance Exponent (k)
Velocity (km/h)
E* (MPa)
MG-0%
10
6.13
1.29E03
20
6.13
1.30E03
30
6.13
1.27E03
10
5
1.24E03
20
5
1.58E03
30
5
2.56E03
12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66
9772.5 11,635 13142.25 14552.75 4321.75 5718.5 6927.5 8134.25 1789 2673 3332 3954.5 14753.75 16220.75 17384.5 18484.5 9171.5 10554.25 11707.5 12,838 4644.5 5737.5 6725.75 7550.75
MG-15%
result of this process can be seen in Fig. 11. The CIRCLY input parameters utilised in the program, which were obtained from testing, can be seen in Table 5. Fig. 11 clearly reflects the results obtained during the fatigue and complex modulus testing. MG-0% at low temperatures performed significantly better than MG-15%, but as temperature increases, MG-15% clearly has a greater resistivity to fatigue. This is because MG-15% is clearly a stiffer mix across all temperatures and is able to better resist fatigue and rutting failure at higher temperatures. MG-15% doesn’t perform as well at lower temperatures potentially because the mix is too stiff, which causes the pavement to be more brittle. 6. Conclusion This study utilised fatigue testing to investigate the environmental benefits and sustainability opportunities of adopting multigrade bitumen asphalt with and without fifteen percent recycled asphalt pavement (RAP) under a range of temperatures. A fatigue fitting function was developed for each mix, which is able to approximate the laboratory fatigue cycle number of these mixes at any given temperature and loading and can be adapted to create equivalent pavement designs. The main conclusions of this study are the following: The use of a performance based specification for multigrade asphalt, rather than conventional design guides, can greatly reduce the required pavement thickness. For Australian climates, where pavement temperature is generally above 20 degrees Celsius, the multigrade asphalt benefitted from the inclusion of 15% RAP, as it was more capable of resisting the fatigue degradation of the testing at higher temperatures - likely due to the increase in material modulus. The use of RAP will further reduce demand for virgin aggregate. At low temperatures (10 degrees Celsius and below) the asphalt mix with RAP had a reduced fatigue life compared to the mix without RAP, likely due to the RAP mix being too stiff. The multigrade bitumen asphalt mixes outperformed current design expectations for laboratory fatigue life at the temperatures tested.
The current best practice use of the Shell fatigue equation for designing asphalt pavement mixes should be reconsidered or potentially redeveloped to suit individual binder types. Alternatively, the process should only be used as a means of providing confidence in laboratory test results and not the primary design tool. Asphalt designers should consider a range of temperatures when designing roads, as the chosen pavement may perform well at the nominated weighted mean average pavement temperature, but fail prematurely at other temperatures. More testing should be conducted to analyse the impact of this theory. For design comparison purposes, the input parameters required for the CIRCLY pavement design tool have been provided. Note that these values are specific to the mix designs detailed in this paper and the local materials incorporated. Additional testing should be conducted prior to adoption of these values to confirm that there is no variation from the qualities of the local materials. There are no conflicts of interest with this publication. Research was conducted independently through a collaboration between government body and the Queensland University of Technology. Neither party benefits from the published results, other than increasing the knowledge available to the engineering world. A financial assistance scholarship was awarded by the Australian government to assist the lead researcher with living expenses. The Australian government had no involvement with the outcome of the research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was partially supported by an Australian Government Research Training Program Scholarship. We thank our colleagues from Queensland University of Technology who provided
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insight and expertise that greatly assisted the research. We would also like to thank the employees of Brisbane City Council who played an invaluable role in providing guidance and the testing materials. References [1] Australian Bureau of Meteorology (BOM), Rainfall and Temperature Records, 2014. Retrieved from http://www.bom.gov.au/climate/extreme/records.shtml. [2] Austroads, Guide to Pavement Technology – Part 2: Pavement Structural Design, third ed., Austroads, Sydney, NSW, 2012. AGPT02-12. [3] Austroads, Improved Design Procedures for Asphalt Pavements: Pavement Temperature and Load Frequency Estimation AP-T248-13, Austroads, Sydney, NSW, 2013. [4] Austroads, Austroads Test Method AGPT/T274 – Characterisation of Flexural Stiffness and Fatigue Performance of Bituminous Mixes, Austroads, Sydney, NSW, 2016. [5] Austroads, Austroads Research Report AP-R511-16 – Improved Design Procedures for Asphalt Pavements, Austroads, Sydney, NSW, 2016. [6] E. Behzadfar, S. Hatzikiriakos, ‘‘Viscoelastic Properties and Constitutive Modelling of Bitumen, Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, British Columbia, V6T 1Z3, Canada, 2012. [7] P. Baburamani No. ARR 334, in: Asphalt Fatigue Life Prediction Models – A Literature Review, ARRB Group Limited, Victoria, Australia, 1999, p. 334. [8] B.R. Clark, C.P. Gallage, J. Yeahman, Temperature susceptibility of multigrade bitumen asphalt and an approach to account for temperature variation through deep pavements, Int. J. Urban Civ. Eng. (World Academy of Science, Engineering and Technology) 12 (6) (2018) p712–p717. [9] B. Clark, C. Gallage, L. Piacere, Effects of recycled asphalt pavement on the stiffness and fatigue performance of multigrade bitumen asphalt, J. Mater. Civ. Eng. (2018), https://doi.org/10.1061/(ASCE)MT.1943-5533.0002150. [10] S.B. Cooper Jr, L.N. Mohammad, M.A. Elseifi, Laboratory performance of asphalt mixtures containing recycled asphalt shingles, reclaimed asphalt pavement, and recycling agents, J. Mater. Civ. Eng. (2017), https://doi.org/10.1061/(ASCE) MT.1943-5533.0001658. [11] H. Di Benedetto, M.N. Partl, L. Franken, De La Roche, C. Saint Andre, Recommendations of RILEM TC 182-PEB performance testing and evaluation of bituminous materials: stiffness testing for bituminous mixtures, Mater. Struct. 34 (236) (2001) 66–70.
[12] H. Hakimelahi, S. Saadeh, J. Harvey, Investigation of fracture properties of California asphalt mixtures using semicircular bending and beam fatigue tests, Road Mater. Pavement Des. 14 (2013) 252–265, https://doi.org/10.1080/ 14680629.2013.812835. [13] A.M. Hartman, M.D. Gilchrist, Evaluating four-point bend fatigue of asphalt mix using image analysis, J. Mater. Civ. Eng. (2004), https://doi.org/10.1061/ (ASCE)0899-1561(2004) 16:1(60). [14] S. Jayakody Arachchige, C.P. Gallage, J.M. Ramanujam, Effects of reclaimed asphalt materials on geotechnical characteristics of recycled concrete aggregates as a pavement material, Road Mater. Pavement Des. 20 (4) (2019) p754–p772. [15] S. Jayakody Arachchige, C.P. Gallage, A. Kumar, Assessment of recycled concrete aggregates as a pavement material, Geomech. Eng. 6 (3) (2014) p235–p248. [16] N. Laws, K. Westaway, J. Yeaman, G. White, Characterisation of brisbane asphalt mixtures for dynamic modulus, 17th AAPA International Flexible Pavements Conference. Melbourne, Australia, 2017. [17] Pavement Science, CIRCLY 6.0. https://pavement-cience.com. au/softover/circly/, 2017, Jan. 25, 2018. [18] L. Petho, G. Stephenson, Stiffness variability of multigrade asphalt in Brisbane, 26th ARRB Conference – Research driving efficiency, Sydney, New South Wales, 2014. [19] J. Read, D. Whiteoak, The Shell Bitumen Handbook, Shell Bitumen, 2003. [20] C. Shannon, A. Mokhtari, H. Lee, S. Tang, C. Williams, S. Schram, Effects of high reclaimed asphalt-pavement content on the binder grade, fatigue performance, fractionation process, and mix design, J. Mater. Civ. Eng. (2017), https://doi.org/10.1061/(ASCE)MT.1943-5533.0001694. [21] D. Sybilski, W. Ban´kowski, Asphalt pavement design using results of laboratory fatigue tests of asphalt mixtures, Road Mater. Pavement Des. 3 (2) (2002) 183– 194, https://doi.org/10.1080/14680629.2002.9689921. [22] P.J. Vardanega, T.J. Waters, R.E. Spies, J.M. Ramanujam, A. Nataatmadja, Ranking binder creep performance using the ARRB Elastometer, J. Mater. Civ. Eng. (2010), https://doi.org/10.1061/(ASCE)MT.1943-5533.0000043. [23] M.J. Watson, J. McGraw, E. Johnson, D. Linell, S. Dai, The effect of recycled asphalt materials on hot mixed asphalt pavement performance, Green Streets and Highways 2010. An Interactive Conference on the State of the Art and How to Achieve Sustainable Outcomes, 2010. [24] F. Yin, F. Kaseer, E. Arámbula-Mercado, A. Epps Martin, Characterising the long-term rejuvenating effectiveness of recycling agents on asphalt blends and mixtures with high RAP and RAS contents, Road Mater. Pavement Des. 18 (Supp. 4) (2017) 273–292.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Superior performance benefits of multigrade bitumen asphalt with recycled asphalt pavement additive Brody R. Clark ⇑, Chaminda Gallage Faculty of Science and Engineering, Queensland Univ. of Technology, 2 George St., Brisbane, QLD 4001, Australia
h i g h l i g h t s Four-point bending testing was conducted on multigrade asphalt beams with and without RAP to examine the fatigue and modulus performance under
various temperatures. Better understanding of the materials will lead to a reduction in the use of virgin aggregate. Asphalt is very temperature dependant and multigrade offers a reliable solution that can lead to a reduction of overall pavement thickness. The current method of adopting the Shell relationship is not reliable. CIRCLY parameters determined for these mixes and can be adopted by industry.
a r t i c l e
i n f o
Article history: Received 13 May 2019 Received in revised form 2 September 2019 Accepted 13 September 2019
Keywords: Pavement engineering Recycled asphalt pavement Multigrade bitumen Asphalt fatigue testing Flexural stiffness Master curves Complex modulus
a b s t r a c t The implementation of recycled asphalt pavement (RAP) in new pavements is crucial for future sustainability, but the impact of the recycled material on the new roads properties must first be investigated to ensure future pavement performance. The introduction of RAP to high performing asphalt mixes, such as multigrade asphalt (asphalt utilising multigrade bitumen), needs to be investigated to safe guard against possible detrimental effects of the RAP. Extensive laboratory testing was conducted on multigrade asphalt beams under a range of temperatures and loading conditions. Testing has found that multigrade bitumen does perform better than expected during laboratory testing compared to conventional asphalt and the current industry standard of utilising the Shell 1978 fatigue curve for asphalt. The addition of RAP had beneficial effects on the asphalt pavements at higher temperatures (30 °C). Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction As a solution to depleting natural resources and managing demolition waste, the use of recycled materials in pavement construction has gained a significant attention in the past decade [14,8,9,15]. Virgin bitumen is a non-renewable product that we will inevitably be depleted. To extend the current supplies of virgin bitumen the use of recycled asphalt pavement (RAP) is incorporated into pavement designs. RAP is reclaimed in-service pavement that is far more aged and oxidised in comparison to the virgin binders. RAP has been shown previously to benefit the performance of conventional bitumen asphalt [10] but other testing has also
⇑ Corresponding author. E-mail addresses: [email protected] (B.R. Clark), chaminda. [email protected] (C. Gallage). https://doi.org/10.1016/j.conbuildmat.2019.116963 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
highlighted the potential negative impact that the aged binder can have on the virgin pavements [23]. These previous tests and others show the impact that RAP can have on the materials stiffness and rutting resistance but not what the RAP’s effects are on the materials fatigue resistance. Shannon, et al. [20] conducted fatigue testing on high RAP content asphalt (up to 50% RAP) and found that the RAP had no discernible difference on the pavements fatigue performance. However, these tests were only conducted at a single temperature (21 °C) and therefore do not accurately reflect the impact that changing stiffness values have on the fatigue performance of asphalt at different temperatures. Yin, et al. [24] conducted vigorous testing on high RAP inclusive asphalt and reported on the use of recycling agents to negate the potential negative impacts of using RAP in pavements. As the percentage of RAP included within the scope of this testing is considered low, the use of such additives is not considered
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appropriate. The use of low levels of RAP is considered appropriate for industries with small quantities of useable RAP stored, such as the Australian market, where 100% of the quality RAP stockpiles are currently utilised. Once greater stockpiles of RAP are gathered, the use of the recycling additives will become more prevalent. The RAP source could potentially play a large role in determining the effect of RAP of the asphalts performance and properties. As RAP is pavement that has been recycled, the binder and aggregate utilised in the old mix designs could vary significantly. The current best practice in Brisbane, Australia (the location for this testing and source of the RAP for this paper) is to stockpile and screen all RAP together. This could lead to various qualities of asphalt being implemented as RAP, and therefore adds a degree of uncertainty in the results of RAP incorporation testing. Simply put, the RAP you use today could be vastly different to the RAP you could be given next week. More analysis should be put into determining how much of an effect various RAP sources have on pavement performance and/or better stockpiling and processing of RAP should be undertaken to control RAP quality. Multigrade bitumen is a quality proprietary binder that has been ‘shelved’ due to a lack of interest in testing its capabilities. Multigrade bitumen is a chemically modified binder that is created by combining multiple binders with additional chemicals. The combination of these binders enables the pavements to have the properties of hard paving grade bitumen at high service temperatures and the properties of soft paving grade bitumen at low temperatures [19]. The actual ingredients and chemical makeup of multigrade bitumen is propriety knowledge and is not public knowledge. The multigrade bitumen utilised in this testing is M1000. M1000 has the characteristics of a conventional binder (binder with viscosity of 320 at 60 °C) at low service temperatures, with and actual typical viscosity at 60 °C of approximately 1000 Pas [22]. These observed material properties are the basis of why it is believed multigrade bitumen is less sensitive to temperature variation [19]. Previous testing [21] found through laboratory testing that multigrade bitumen can reduce required pavement thickness compared to conventional binders. However, previous testing did not investigate the effects RAP and temperature have on the laboratory fatigue of the asphalt. Bitumen is a viscoelastic material, which means that asphalt’s properties and performance are dependent on temperature and rate of loading [6]. The resistance to temperature fluctuations are what make multigrade bitumen superior to conventional asphalt binders, particularly in climate regions such as Queensland, Australia. Queensland pavement temperatures change dramatically, depending on the time of day and season [18]. The Australian Bureau of Meteorology records Queensland’s extreme temperatures as ranging between 10.6 °C and 49.5 °C [1]. Previous testing has shown that the in-service temperature of asphalt changes dramatically with depth [3]. Laws et al. [16] found that the pavements temperature generally stabilises after it extends beyond 300 mm thick. Interestingly, they also found that the highest temperature recorded for the stabilised temperature, for typical targeted air voids of 2% and 6%, is the adopted Weighted Mean Average Pavement Temperature (WMAPT). WMAPT is the nominated temperature for design of asphalt pavements in Australia. However, for pavements less than 300 mm and different air void contents, Laws et al. [16] found that the temperature extends beyond the WMAPT. Perhaps future pavements designs should adopt different fatigue curves for changing pavement thicknesses, based on the temperature at the base of the pavement layer. One of the primary failure/distress modes of asphalt pavements is fatigue cracking. These fatigue cracks initiate at the bottom of the asphalt layer and propagate to the surface. Once these cracks
are fully developed, water can easily percolate to base/sub-base, or subgrade which causes significant permanent deformation (rutting) in the pavement surface. The fatigue life of asphalt depends on a number of factors such as; complex modulus of asphalt, air voids, binder type and content/film thickness and its rheological properties, aggregate type and grading, and mode of traffic loading and conditions (the magnitude and frequency, the rest period between successive loads) [7]. The complex modulus of asphalt is an indication of the stress–strain relationship and was determined in previous testing [9] using four point bending. The most common method of determining fatigue resistance of asphalt in Australia is also by using beam fatigue testing [12]. A previous study into published fatigue results for asphalt pavements utilising multigrade bitumen and incorporating recycled asphalt pavement (RAP) found that there has been limited research conducted on these innovative materials [9]. The study concluded from the limited available data, that the impact of RAP varies and a defined correlation between RAP content and complex modulus and fatigue life has not been produced. Additionally, the study found that multigrade bitumen does appear to have a positive impact on fatigue resistance compared to conventional binders. However, due to the large number of factors affecting the performance of an asphalt pavement, previous studies have generally neglected to utilise a range of temperatures during testing. This paper endeavours to expand upon the preliminary testing results found by [9] with a significantly greater testing regime. To determine the fatigue life of the asphalt beams, under multiple testing temperatures, the four-point bending method was utilised. The four point bending test method attempts to simulate real world loading of traffic induced stresses and strains by dynamically loading rectangular beam specimens [13]. Four point bending was adopted for testing as it is found to produce the same complex modulus values as the equivalent two point bending methods, if the samples geometry and mass are taken into account [11] and due to the availability of resources. Previous testing [9] analysed the complex modulus variation of the mixes utilised in this testing by investigating the effects that loading frequency and temperature have on complex flexural modulus. These results were then used to develop a unique relationship between complex modulus and loading frequency for a reference temperature. Such relationship is called a ‘‘master curve” and can be used to obtain the modulus at any given temperature and frequency. This testing found that adding RAP to the mix increases the materials complex modulus, possibly due to the recycled material being more aged/oxidised than the virgin. The effects of RAP on the fatigue performance of multigrade bitumen asphalt is to be investigated and will be collaborated with the complex modulus master curves found in previous testing [9]. This paper investigates the effects of temperature and RAP incorporation on multigrade bitumen by analysing the fatigue performance and stiffness properties of multigrade bitumen asphalt. Fatigue testing for the specified mixes have been conducted at temperatures ranging from 10 °C to 30 °C with the intention to numerically analyse the data to create an equation that can estimate to fatigue performance at any given temperature, provided that a complex modulus master curve is available for the specific mix. To demonstrate the potential performance of the mixes CIRCLY 6.0 [17] computer program is utilised to determine the equivalent pavement thickness of the mixes under a range of traffic loading and temperatures.
2. Test materials The weighted percentage of each material included in the mix designs of the two mixes utilised in this testing can be seen in
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963 Table 1 Mix designs for asphalt samples.
Table 2 Viscosity values of binder extracted from mixtures. Asphalt mixture labels
Material
MG-0%
MG-15%
18 mm Aggregates (%) 14 mm Aggregates (%) 9 mm Aggregates (%) 7 mm Aggregates (%) Crusher Dust (%) Fine Sand (%) Rock Flour (Baghouse Fines) (%) Rap Content (%) Multigrade Bitumen Type Virgin Bitumen Content (%)
9.2 14.3 20.7 14 31.8 9 1 0 M1000 4.5
10 16.4 14.3 10.8 27 5.5 1 15 M1000 3.7
Table 1. The mixes incorporate; multigrade bitumen, aggregate, fine sand, crusher dust, rock flour and RAP. As RAP already contains a percentage of bitumen (an average of 4.37% in the case of the RAP used in this testing) the mix incorporating RAP has a reduced virgin bitumen content percentage, as shown in Table 1. The calculated percentage of virgin bitumen added to MG-15% assumes 100% of binder from the RAP is available. The grading results of the two mixes obtained from sieve analysis can be seen in Fig. 1, as well as the most recent data from the RAP source. The results of the sieve testing show that there is little variation in mix designs and it can be assumed that any inconsistencies during testing are unlikely to be caused by these differences in the mixes. The two mixes utilised virgin multigrade bitumen and targeted the optimum bitumen content of 4.5%. 4.5% was adopted as it is the typical binder content utilised for asphalt mix designs in the local industry [18]. M1000 multigrade bitumen was utilised due to it being the most commonly available multigrade bitumen in Brisbane and are believed to be supplied to meet the requirements of Australian Standard specification AS2008. In order to determine the effect of RAP on the properties of the binders, the bitumen was extracted from each mixture and tested for viscosity according to Australian Standard 2341.2. The results are given in Table 2.
MIXTURE LABEL
BITUMEN TYPE
RAP CONTENT (%)
VISCOSITY AT 60 °C (PA.S)
MG-0% MG-15%
M1000 M1000
0 15
17,250 125,000
As can be seen in Table 2, MG-0% has a lower viscosity than the mix containing 15% RAP. Adding RAP into virgin multigrade bitumen asphalt has increased the viscosity of the binder, likely due to the RAP introducing oxidised binder into the mixture. Due to current RAP stockpiling practice in Australia, the quality and consistency of the RAP is unknown and likely largely variable. The viscosity values in Table 2 show the true impact of the RAP utilised in the mixes. However, until we develop a better way of cataloguing and sorting RAP stockpiles, the recorded effect of RAP on asphalt mixes in Australia should probably continuously be monitored until an agreed result for all variation of RAP qualities are obtained.
3. Test apparatus A pneumatic four-point fatigue beam testing apparatus was utilised in testing and can be seen in Fig. 2. The apparatus loads the centre of the rectangular beam sample, with the loading applied by an actuator in the form of sinusoidal oscillation. Vertical deflection at the centre point of the sample is monitored by an onspecimen linear variable differential transformer (LVDT) with an accuracy of ±1 lm. The vertical deflection is utilised by the computer program to calculate the tensile strain at the mid-point at the base of the beam. This tensile strain is the control loading for this experiment. The testing apparatus is housed in an environmental control chamber that maintains the required temperature condition with a maximum allowable fluctuation of ±0.5 °C. The environmental chamber housing the four-point bending machine can be seen in Fig. 3. The testing software monitors the temperature of the sample by utilising a dummy sample housed inside the environmental chamber. The skin and core temperature of the dummy sample is
Fig. 1. Aggregate grading chart.
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beam can be seen housed in the four-point bending apparatus, ready for testing, in Fig. 2. 4.2. Fatigue testing procedure
Fig. 2. Pneumatic Four-Point Bending Apparatus.
The methodology adopted for this testing is taken from The Austroads Guide to Pavement Technology Test Method AGPT/ T274 [4] in conjunction with Part 2 [2] of the Austroads Guide. The testing utilised three different strain levels during testing, ranging from 100 to 300 microstrain. The samples were also tested at constant temperatures of 10 °C, 20 °C and 30 °C and maintained within a 0.5 degree variance. Sinusoidal loading was adopted for testing at a constant frequency of 10 Hz, as per Austroads test method AGPT/T274, with the failure condition being when the complex modulus of the beam is reduced to 50% of the initial stiffness. Once the samples complex modulus has reduced below 50% of the initial value, the number of cycles is recorded and an average for each load strain and temperature is calculated. Demonstrated graphically in Fig. 4 is the method of obtaining the load repetition to failure of a single beam. For a given strain amplitude and temperature, three identical beams were tested and the average failure loading cycles calculated as the corresponding loading cycles for fatigue failure. For each mix type three strain amplitudes were tested which provide three total data points for each mix and utilised to develop the fatigue curve for each temperature. For this experiment, the loading condition, temperature and incorporation of RAP were the variables, while mix design, loading frequency, deflection and virgin binder were constant. Sinusoidal loading was utilised, as previous testing [5] found that haversine loading becomes sinusoidal after around 50 cycles, but at half the strain. The results of the laboratory testing will be transformed to fit into a single function that can derive the loading cycle at any strain for a given modulus for a specific mix design. The adopted model fitting function can be seen in Eq. (1), with the definitions of variables provided beneath. Eq. (1) was derived by extensive testing carried out by the Australian road authority Austroads in the development of AGPT/T274.
Nlab ¼ EXP c1 ln3 ðE Þ þ c2 ln2 ðE Þ þ c3 lnðE Þ þ c4 þ c5 lnðleÞ
ð1Þ where; N lab ¼ Number of cycles to failure in the laboratory flexural fatigue test E ¼ complex modulus ðMPaÞ at the test frequency and test temperature Fig. 3. Environmental Chamber Housing Testing Apparatus.
le ¼ strain in laboratory flexural fatigue test ðlm=mÞ
monitored to ensure that the asphalt is sufficiently conditioned to the required testing temperature for at least four hours prior to testing. 4. Methodology The methodology to investigate the effects temperature and RAP has on multigrade bitumen asphalt fatigue performance consists of sample preparation and a testing program for fatigue performances. 4.1. Sample preparation Each mixture in Table 2 was compacted into a slab using the slab compactor to achieve a target air void content of 5%. Four beams were cut from each slab each measuring a length of 390 mm, height of 50 mm and width of 63 mm. An example of a
Fig. 4. Demonstration of How to Find Load Repetition to Failure.
B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
le ¼ load induced tensile strain at base of the asphalt ðmicrostrainÞ
c1 ; c2 ; c3 ; c4 & c5 ¼ fitting parameters To demonstrate how multigrade bitumen asphalt is currently under-utilised in industry the results of testing were compared to equivalent values calculated using the Shell equation. The Shell equation was developed in 1978 with testing conducted on a range of asphalt mixes, incorporating various bitumen types, in order to obtain a general relationship for the fatigue performance of an asphalt pavement [2]. The Shell relationship currently utilised by Australian road authorities can be seen in Eq. (2). This paper will analyse how accurate the Shell relationship is for determining the performance of the multigrade bitumen asphalt mixes used in this testing. Note that the reliability factor (RF) is ignored in this testing, as it is a safety factor applied to the equation to allow for construction variability, environment, traffic variability, etc.
N ¼ RF where;
" #5 6918ð0:856V b þ 1:08Þ E0:36 le
5
ð2Þ
RF ¼ reliability factor V b ¼ percentage by v olume of bitumen in the asphalt ð%Þ E ¼ asphalt modulus ðMPaÞ N ¼ allowable number of repetitions of the load 4.3. Equivalent pavement thickness modelling
In order to represent the findings of this paper and to highlight the possible implications that they will have on the design of modern roads, the pavement performance calculation software CIRCLY 6.0 [17] is utilised. The software calculates the fatigue and rutting performance of an inputted pavement structure and determines its applicability for the assigned traffic conditions. For the use of this modelling, the programs auto-calculate pavement thickness feature is used to determine the equivalent pavement thickness for each mix under the same traffic loading. Each mix design was analysed as a full depth asphalt pavement on top of a subgrade with a California Bearing Ratio (CBR) of 5%, which equates to a modulus of
Fig. 5. Fatigue results of mixes at 10 °C.
Fig. 6. Fatigue results of mixes at 20 °C.
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
Fig. 7. Fatigue results of mixes at 30 °C.
50 MPa for the subgrade. The pavements were designed to endure traffic of 10^6 Design Equivalent Standard Axles (DESA). In order to model the effects of changing temperature and velocities CIRCLY 6.0 requires the modulus and fatigue performance exponents of each mix for each scenario. For each mix and temperature, the fatigue results produce the CIRCLY 6.0 input parameters ‘b’ and ‘k’ and the modulus input of the material is gained from producing a modulus master curve. Previous testing [8] has produced the required modulus master curves for the mixes utilised in this testing. The master curves are used to derive the modulus of the material at any given temperature and v. The input performance exponent values (b & k) are simply derived from fitting a power curve to each of the fatigue curves found from testing. 5. Results and discussion 5.1. Effects of RAP on fatigue performance of multigrade bitumen asphalt Figs. 5–7 demonstrate the fatigue results obtained during testing for the two mixes at ten, twenty and thirty degrees Celsius respectively. It is clear in Fig. 6 that at 20 °C the mixes appear to perform similarly, with the only significant difference at the lower strain level. The difference at the lower strain level could be due to possible outliers in the data or that the virgin mix is more capable of self-healing than the possibly over-oxidised RAP mix. More testing should be conducted to determine if the ‘perpetual pavement’ theory is the cause of the difference at this strain. It is clear that at the lower temperature (10 °C) the mix without RAP performs better than the mix with RAP. However, at 30 °C the mix with RAP performs significantly better than the mix without RAP. This could be due to the difference in complex modulus. As asphalt is a viscoelastic material, the binder relies on having a balance between ductility and stiffness to provide structural support to traffic loading, while having the ability to be flexible. As the ductility of the mix without RAP increases and the stiffness decreases at thirty degrees, the material loses its ability to resist the loading as well as the mix with RAP. As the RAP increases the stiffness of the material, it enables the material to resist the loading at a higher temperature – compared to the mix without RAP. It is clear that for warmer climates, such as most parts of Queensland, Australia, RAP can provide benefits to asphalt roads subject to high service temperatures.
It is also interesting to see that the two mixes perform significantly better than the Austroads fatigue equation. This could mean that roads utilising multigrade bitumen are possibly over-designed when it comes to fatigue life. These results demonstrate that more road agencies should investigate the benefits of multigrade bitumen and RAP and implement these innovative materials into more designs.
5.2. Effects of temperature on fatigue performance of multigrade bitumen asphalt Fatigue testing was conducted at 10 °C, 20 °C and 30 °C under three strain loads for the two mixes and the results of which can be seen in Table 3. The results of the fatigue testing are compared against the Austroads (Shell) fatigue relationship (Eq. (2)), which is current best practice for asphalt pavement design. To determine the equivalent number of loading cycles utilising the Austroads (Shell) equation, the modulus adopted was obtained from the complex modulus master curve derived in previous testing [8] for each mix. Fig. 8 demonstrates the results of the fatigue testing for MG-0%. The graph clearly shows that the material does not perform significantly differently at 10 °C and 20 °C, but loses fatigue capacity at 30 °C. These results are surprising, as previous testing [8] showed that for this material and loading frequency, each 10 °C increase in temperature roughly halved the materials modulus. This can possibly be because at these two temperatures the materials modulus is almost symmetrically placed on the bell curve for optimum material modulus for fatigue life. More testing should be conducted to develop the bell curve for optimum material modulus with regards to fatigue life to determine if this finding is correct. Table 3 Loading cycles to failure fatigue testing results. Mix
MG-0%
MG-15%
Micro-strain
150* 200 300 150* 200 300
Temperature (°C) 10
20
30
5.25E + 05 6.72E + 04 1.44E + 04 1.52E + 05 1.71E + 04 5.29E + 03
9.00E + 06 7.25E + 04 1.06E + 04 2.57E + 05 5.32E + 04 1.35E + 04
3.03E + 05 3.49E + 04 1.49E + 04 1.61E + 06 1.04E + 05 2.58E + 04
*Note: For 20 degrees, testing was conducted at 100 micro-strain.
B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
7
Fig. 8. MG-0% Fatigue testing results.
Fig. 9. MG-15% Fatigue testing results.
Fig. 9 demonstrates the results of the fatigue testing conducted on MG-15%. The graph shows a clear performance difference between temperatures, with the highest tested temperature, 30 °C, outperforming the other two temperatures. It appears from this testing that as temperature increases, the mix with RAP seems to perform better during fatigue testing. It appears to be the converse for the mix without RAP. Much like MG-0%, MG-15%’s complex modulus significantly decreases with every increase in temperature, but as previous testing found, MG-15% is on average 70% stiffer than the virgin binder mix under these testing conditions. What is clear in both Figs. 8 and 9 is the unreliability of the Austroads (Shell) fatigue equation (Eq. (2)). Both graphs demonstrate a clear uncertainty in the use of the fatigue equation in predicting the laboratory fatigue life of these mixes. That is likely due to the
Austroads (Shell) equation being derived from out-dated mix designs utilising conventional binders and not applicable to modern innovative pavement materials. 5.3. Laboratory fatigue model fitting The results obtained from testing were transformed to fit a fatigue model that is able to be used to predict the laboratory fatigue life of these mixes at a specific loading strain and stiffness. The fitting function is given by Eq. (1) in the Methodology section of this paper and the applicable fitting parameters determined during testing are provided in Table 4. To demonstrate the accuracy of the fitting function, Fig. 10 compares the fatigue results of MG0% at 10 °C and 20 °C with the derived results from Eq. (1). The fitting function achieves a coefficient of determination (also known
8
B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963
Table 4 Fatigue function fitting parameters.
MG-0% MG-15%
c1
c1
c1
c1
c1
0.328 0.412
8.833 10.137
79.229 77.771
192.516 140.964
6.140 4.990
Fig. 10. Comparison of derived and determined fatigue results.
Fig. 11. The effects of temperature and RAP on pavement thickness.
as R2) of 95%. This provides some confidence that this equation can be utilised to derive an estimate of the laboratory fatigue life cycle count for this mix. 5.4. Impact of results on pavement thickness The results obtained from previous complex modulus master curve testing [8] were combined with the results of the fatigue function derived for each mix from this testing to determine the
impact that these laboratory testing results have on required pavement thickness. The data was analysed and computed using the pavement design program CIRCLY 6.0 to determine a comparative pavement thickness for each mix, under a range of temperatures and traffic speeds. The pavement program calculates the required pavement thickness for a given mix by analysing both fatigue and rutting performance. For the purpose of comparing the mixes, a nominal subgrade with a California bearing ratio (CBR) of 5% was adopted and the mix is represented as a full depth pavement. The
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B.R. Clark, C. Gallage / Construction and Building Materials 230 (2020) 116963 Table 5 CIRCLY 6.0 input parameters derived from testing. Mix
Temp (°C)
Performance Exponent (b)
Performance Exponent (k)
Velocity (km/h)
E* (MPa)
MG-0%
10
6.13
1.29E03
20
6.13
1.30E03
30
6.13
1.27E03
10
5
1.24E03
20
5
1.58E03
30
5
2.56E03
12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66 12.57 31.42 62.83 125.66
9772.5 11,635 13142.25 14552.75 4321.75 5718.5 6927.5 8134.25 1789 2673 3332 3954.5 14753.75 16220.75 17384.5 18484.5 9171.5 10554.25 11707.5 12,838 4644.5 5737.5 6725.75 7550.75
MG-15%
result of this process can be seen in Fig. 11. The CIRCLY input parameters utilised in the program, which were obtained from testing, can be seen in Table 5. Fig. 11 clearly reflects the results obtained during the fatigue and complex modulus testing. MG-0% at low temperatures performed significantly better than MG-15%, but as temperature increases, MG-15% clearly has a greater resistivity to fatigue. This is because MG-15% is clearly a stiffer mix across all temperatures and is able to better resist fatigue and rutting failure at higher temperatures. MG-15% doesn’t perform as well at lower temperatures potentially because the mix is too stiff, which causes the pavement to be more brittle. 6. Conclusion This study utilised fatigue testing to investigate the environmental benefits and sustainability opportunities of adopting multigrade bitumen asphalt with and without fifteen percent recycled asphalt pavement (RAP) under a range of temperatures. A fatigue fitting function was developed for each mix, which is able to approximate the laboratory fatigue cycle number of these mixes at any given temperature and loading and can be adapted to create equivalent pavement designs. The main conclusions of this study are the following: The use of a performance based specification for multigrade asphalt, rather than conventional design guides, can greatly reduce the required pavement thickness. For Australian climates, where pavement temperature is generally above 20 degrees Celsius, the multigrade asphalt benefitted from the inclusion of 15% RAP, as it was more capable of resisting the fatigue degradation of the testing at higher temperatures - likely due to the increase in material modulus. The use of RAP will further reduce demand for virgin aggregate. At low temperatures (10 degrees Celsius and below) the asphalt mix with RAP had a reduced fatigue life compared to the mix without RAP, likely due to the RAP mix being too stiff. The multigrade bitumen asphalt mixes outperformed current design expectations for laboratory fatigue life at the temperatures tested.
The current best practice use of the Shell fatigue equation for designing asphalt pavement mixes should be reconsidered or potentially redeveloped to suit individual binder types. Alternatively, the process should only be used as a means of providing confidence in laboratory test results and not the primary design tool. Asphalt designers should consider a range of temperatures when designing roads, as the chosen pavement may perform well at the nominated weighted mean average pavement temperature, but fail prematurely at other temperatures. More testing should be conducted to analyse the impact of this theory. For design comparison purposes, the input parameters required for the CIRCLY pavement design tool have been provided. Note that these values are specific to the mix designs detailed in this paper and the local materials incorporated. Additional testing should be conducted prior to adoption of these values to confirm that there is no variation from the qualities of the local materials. There are no conflicts of interest with this publication. Research was conducted independently through a collaboration between government body and the Queensland University of Technology. Neither party benefits from the published results, other than increasing the knowledge available to the engineering world. A financial assistance scholarship was awarded by the Australian government to assist the lead researcher with living expenses. The Australian government had no involvement with the outcome of the research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was partially supported by an Australian Government Research Training Program Scholarship. We thank our colleagues from Queensland University of Technology who provided
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