Curing and ageing of biofluxed bitumen: a physicochemical approach

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Curing and ageing of biofluxedbitumen: a physicochemical approachMarkus Simonen a , Timo Blomberg b , Terhi Pellinen c , MichalinaMakowska c & Jarkko Valtonen ca A-Insinöörit Suunnittelu Oy, Bertel Jungin aukio 9, 02600, Espoo,Finland;b Nynas Oy, Äyritie 12 B, FI-01510, Vantaa, Finland;c Department of Civil and Environmental Engineering, AaltoUniversity, P.O. Box 12100, FI-00076, Aalto, FinlandVersion of record first published: 15 Jan 2013.

To cite this article: Markus Simonen , Timo Blomberg , Terhi Pellinen , Michalina Makowska &Jarkko Valtonen (2013): Curing and ageing of biofluxed bitumen: a physicochemical approach, RoadMaterials and Pavement Design, 14:1, 159-177

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Road Materials and Pavement Design, 2013Vol. 14, No. 1, 159–177, http://dx.doi.org/10.1080/14680629.2012.755933

Curing and ageing of biofluxed bitumen: a physicochemical approach

Markus Simonena, Timo Blombergb, Terhi Pellinenc*, Michalina Makowskac

and Jarkko Valtonenc

aA-Insinöörit Suunnittelu Oy, Bertel Jungin aukio 9, 02600 Espoo, Finland; bNynas Oy, Äyritie 12B, FI-01510 Vantaa, Finland; cDepartment of Civil and Environmental Engineering, Aalto University,P.O. Box 12100, FI-00076 Aalto, Finland

Bitumen solutions, comprising of four bioflux-modified bitumens and one traditional slow-curing cut-back bitumen as a reference, were stabilised with two-phased process consistingof recovery and stabilisation parts. The oxidative long-term ageing (LTA) of the stabilisedbitumen solutions was performed by a pressure ageing vessel (PAV). The generic fractionsof the base bitumens were determined after LTA by thin-layer chromatographic method withflame-ionisation detector (IATROSCAN MK-6s). Complex shear moduli and phase anglesof both base bitumens and bitumen solutions were examined with Physica 301 rheometer inoscillatory mode. The curing and ageing behaviour of the biofluxed bitumens differed notablyfrom the traditional cut-back bitumen. Based on PAV and rheometer testing, the effects of ageingon the rheological properties of biofluxed binders may be solely bound to the evaporationof bioflux. Sigmoidal functions were introduced as an option for evaporation models withlimited evaporation. However, interpretation of chemical composition of cut-back bitumens asanalysed by thin layer chromatography-flame ionisation detector leaves a reasonable doubt forthis method to give ultimate composition result.

Keywords: bioflux; cut-back bitumen; stabilisation; pressure ageing vessel; rheology;sigmoidal function

1. Introduction1.1. BackgroundBiofluxed bitumens were introduced in Finland as an alternative for the cut-back bitumens in 2008by Nynas Oy (Valtonen, Pellinen, & Blomberg, 2010). The cut-back bitumens are widely used instockpiled cold-patching mixtures, and they contain significant amount of volatile organic com-pounds (VOCs). VOCs are considered to be harmful to the environment (IPCC, 2001). Bioflux,a mixture of linear and branched C10–C20 alkanes, is less volatile than the lighter solvent of thecut-back bitumens. It is produced through hydrogenation of renewable material (vegetable oilsand animal fats) by a chemical process called NExBTL, which is patented by Neste Oil. Figure 1shows a suggestive boiling point distribution of bioflux compared to the solvents of cut-back bitu-mens, kerosene and gas oil. The suitability of bituminous binders for certain application dependsboth on the properties of fresh product and on ageing-induced changes in the physical and chemi-cal properties of binder. The relationship between the composition and rheological characteristicsof fresh biofluxed bitumens has already been established in the earlier research conducted bySimonen, Blomberg, Pellinen, and Valtonen (submitted); thus this study focuses on curing andageing of bitumen solutions.

*Corresponding author. Email: [email protected]

© 2013 Taylor & Francis

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160 M. Simonen et al.

Figure 1. Boiling point distribution for kerosene, bioflux and gas oil (suggestive).

The physicochemical properties of bituminous binders evolve continuously due tocompositional and structural changes in binder. The mechanisms of these changes may be dividedinto two distinct but simultaneous processes, irreversible and reversible ageing. Irreversible ageingcomprises chemical changes through oxidation, polymerisation, volatilisation of light componentsand exudation of oily components into aggregate (Bell, 1989; Branthaver et al., 1993; Traxler,1963). Oxidation is generally established as the primary reason for in-service ageing, and oxidativetreatment by pressure ageing vessel (PAV) is the long-term ageing (LTA) procedure implementedin specifications (e.g. Euro norm (EN) and Superpave). Reversible ageing originates from time-dependent development of bitumen microstructure (De Moraes, Pereira, Simão, & Leite, 2010;Masson, Price, & Collins, 2001). Recent studies show that reversible ageing substantially affectsthe structure and properties of bitumen at and below room temperature (Masson, Collins, &Polomark, 2005; Simon, 2007).

Volatilisation is considered to form only a minor part in ageing of paving-grade bitumens(Lesueur, 2009; Mastrofini & Scarsella, 2000); however, evaporative short-term ageing procedures(simulating hot-plant mixing), such as thin-film oven test (TFOT) and rolling thin-film oven test(RTFOT), are included in aforementioned specification as well. Although short-term ageing byTFOT or RTFOT has widely been accepted as evaporative treatment, they may have pronouncedoxidative impact on bituminous binders as well (Lu & Isacsson, 2002). Besides, Migliori and Corté(1998) reported that the effects of RTFOT on the physicochemical properties of five penetration-grade bitumens equalled fixed time under PAV treatment. Nevertheless, volatilisation represents animportant contributor to changes in physical properties of bitumen solutions (cut-back and fluxedbitumens). The volatilisation of bitumen solutions may be referred as curing (or stabilisation),since it is partly desired to ensure adequate increase in pavement stability.

The effects of ageing on chemistry and rheological properties of bituminous binders have beenextensively studied both after laboratory conditioning and in situ ageing. Oxidation induces chem-ical reactions leading to increases in carbonyl (C=O) and sulfoxide (S=O) functional groups,which in turn increases the overall polarity, molecular weights and molecular association in bitu-men (Lamontagne, Dumas, Mouillet, & Kister, 2001; Le Guern, Chailleux, Farcas, Dreessen, &Mabille, 2010; Lu & Isacsson, 2002; Siddiqui, 1999). The aforementioned consequences maybe tracked in the changes of the generic fractions (saturates, aromatics, resins and asphaltenes);detailed characterisations provided elsewhere by Lesueur (2009), which may be quantified bya chromatographic fractionation method based on polarity (Jewell, Albaugh, Davis, & Ruberto,1974). Observations have proven that saturate content remains unchanged and aromatic con-tent decreases while resin and asphaltene contents increase (transformation towards more polarfractions) during ageing (Le Guern et al., 2010; Lu & Isacsson, 2002; Siddiqui, 1999). Ageingof bitumens have been observed to affect rheological properties unambiguously. Generally, theageing processes increase the stiffness and elasticity of bituminous binders (Lu & Isacsson, 2002;

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Road Materials and Pavement Design 161

Mastrofini & Scarsella, 2000; Maccarone & Tiu, 1998; Vargas, 2008). Although the direction ofchanges in rheological properties appears obvious, the ageing susceptibility among bitumens mayvary significantly.

The framework for specifying cut-back and fluxed bituminous binders in Europe includes two-phase stabilisation (curing) procedure and an accelerated LTA conditioning with PAV (SFS-EN,2009). The actual scope of stabilisation is to prepare cut-back and fluxed bitumens to furthertesting (SFS-EN, 2006), while PAV conditioning is an actual ageing treatment (SFS, 2005).Characteristics of PAV conditioned bitumen samples correspond to those exhibited by binderafter 4–8 years ageing in field (Anderson et al., 1994). In this study, stabilisation is utilised asa laboratory curing method for simulating the short-term ageing. Curing of bitumen solutionsensures the adequate increase in stability of asphalt mixtures, while increase in stability shouldnot result in undesired decrease in workability for stockpiled mixtures during storage.

1.2. ObjectiveThe objective of this study was to evaluate the effect of stabilisation and LTA on the compositionsand rheological properties of biofluxed bitumens used for stockpiled mixtures. The effect of thebase bitumen on the evaporation rate of volatile components is not known; therefore this studyincludes a section concentrating on evaporation rates of volatile components from various basebitumens.

2. Experimental section2.1. MaterialsStudied bitumen solutions consisted of four biofluxed bitumens and one traditional slow-curingcut-back bitumen as a reference. Biofluxed binders were prepared by blending four base bitu-mens (Table 1) with bioflux (viscosity of circa 2 mm2/s at 60◦C), so that the blends had viscosityof 600 mm2/s at 60◦C. Each of biofluxed bitumens was blended at 90–95◦C in two batches,which were afterwards mixed together to a single homogenous blend. The compositions andbasic properties of biofluxed bitumens and the cut-back bitumen (from production) are presentedin Table 2.

Table 1. Kinematic viscosities of base bitumens in mm2/s.

Capillary viscosity (mm2/s)

Temperature V15A V30A V60A B20A

60◦C 1420 2920 5660 36 700135◦C 37.3 52.5 71.6 171

Table 2. Compositions of bitumen solutions studied.

Bitumen Base Composition (basesolution bitumen Solvent bitumen/solvent) (%/%)

V15BF V15A Bioflux 94.5./5.5V30BF V30A Bioflux 90.8/9.2V60BF V60A Bioflux 88.5/11.5B20BF B20A Bioflux 82.0/18.0BL2K –a Kerosene + gas oil 86.0/7.0 + 7.0

Note: aClose to V60A (exact information was not available).

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Table 3. Composition of biofluxed bitumens in evaporation ratemeasurements.

Composition, baseBitumen solution Base bitumen bit./bioflux (%/%)

V15BF-ET V15A 88.8/11.2V30BF-ET V30A 88.8/11.2V60BF-ET V60A 88.8/11.2B20BF-ET B20A 88.8/11.2

Additionally, four other biofluxed bitumens were prepared for the evaporation rate mea-surements. Table 3 presents the composition (100.0 g of bitumen and 12.6 g of bioflux) of theevaporation samples that were mixed at 100◦C, and thereafter, poured into evaporation containers.

2.2. Methods2.2.1. Evaporation rateThe evaporation rates were observed from samples with film thicknesses of 1.0 and 3.0 mm. Thediameters of the evaporating trays were 70.4 mm for 1.0 mm samples and 76.0 mm for 3.0 mmsamples. Two replicates were prepared for each bitumen solution and film thickness. After theinitial weighting the trays were subjected to heat of 50◦C for 15 min to level the surfaces. The actualevaporation was performed on a levelled table at room temperature (2326◦C). The weightingswere continued for 111 days and the samples were weighted total of 19 times. The scale (PresicaXR 405A) was calibrated according to the manufacturer’s instructors and its resolution was 1 mg.

2.2.2. Stabilisation and LTABitumen solutions were stabilised according to EN-standard 14895. The stabilisation processwas two-phased consisting of recovery and stabilisation parts and its objective was to evaporatesolvents. The oxidative LTA of the stabilised bitumen solutions was performed by a PAV afterEN-standard 14769. Table 4 presents the conditioning circumstances.

2.2.3. Composition2.2.3.1. Saturate, aromatic, resin, and asphaltene (SARA) fractions. The generic fractionsof the base bitumens were determined after LTA by thin-layer chromatographic method withflame-ionisation detector (IATROSCAN MK-6s). Bitumen samples were dissolved in chloro-form and applied on chromarods. Saturates were eluted with n-heptane, aromatics with solutionof toluene (80%) and n-heptane (20%) and resins with solution of dichloromethane (95%) andmethanol (5%).

Table 4. Circumstances under conditionings.

Conditioning Circumstances (film thickness)

Recovery RT – 24 h and 50◦C – 24 h (1.0 mm)Stabilisationa 85◦C – 24 h (1.0 mm)LTAb 85◦C–65 h – 2.1 MPa (3.2 mm)

Notes: aIncludes recovery.bIncludes recovery and stabilisation.

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2.2.3.2. Distillation characteristics. Determination of distillation characteristics according toEN-13358 (current method for cut-back bitumens in Finland) had previously proven problematicwith biofluxed bitumens due to incidents of sample ignition during distillation. Instead, bitumensolution samples were analysed by simulated distillation (AC HT SIMDIS Analyzer) based ongas-chromatographic method (EN-15199-2).

2.2.4. ViscosityViscosities of bitumen solutions were measured in capillary method (SFS-EN 12595 or 12596).

2.2.5. Complex shear modulus and phase angleComplex shear moduli and phase angles of both base bitumens and bitumen solutions were exam-ined with Physica 301 rheometer in oscillatory mode. Measuring geometry was parallel plates25 mm in diameter. Frequencies for oscillation were 0.01–10 Hz (4 pts. per decade) and strains0.1 (0.01 Hz)–0.001 (10 Hz). Test temperatures ranged from −20 to 90◦C, and measurementswere initiated from the lowest temperature with 10◦C increments. Conditioning time before eachtemperature was 10 min.

3. Results3.1. EvaporationFigure 2 shows the measurement data from evaporation study of bitumen blends. Differencesbetween the evaporation of bioflux from the four base bitumens proved negligible, thus thewhole data-set is handled as uniform herein. Fingas (1997) studied evaporation rates of varioushydrocarbons, including crude oils and diesel, by tray evaporation at around room temperature.Fingas found that power functions (Equation (1)) fit well on gathered evaporation data and thatthe magnitude of exponent depends on the number of evaporating components.

y = kxa. (1)

In Equation (1), y is the amount of evaporated material and x is time. The two fitting coefficients,k and a, depend on studied material.

Figure 2 includes the least-square fits of power function to obtained evaporation data. Accordingto Fingas (1997) power-law exponents around 0.65 would suggest 3–4 dominant volatile compo-nents for bioflux. Although the power function fits are satisfactory for the first 1000–1500 h, thefits start to deviate from data after longer evaporation times. Since biofluxed bitumens containonly a certain fraction of volatile components, there must be a limiting value for evaporation. Alimiting value may be incorporated using a sigmoidal model (Equations (2) and (3)).

y = δ + α/(1 + exp(β + γ log t)). (2)

In Equation (2), y is the amount of evaporated material, coefficient δ is the lower asymptote,coefficient α is the span of evaporation (δ + α equals to the upper asymptote or total evaporation)and t is time. The shape coefficients, β and γ , depend on materials. Equation (2) simplifies toEquation (3), since the initial evaporation and thus the lower asymptote, δ, is zero. In Equation (3),coefficient α denotes both the span and the upper asymptote. Later regression coefficient α isreferred as the limiting value for evaporation.

y = α/(1 + exp(β + γ log t)). (3)

In addition to already presented power function fits, least-square sum fits of data to sigmoidalmodels are presented in Figures 3 and 4. The first fits, labelled as Sigm. (11.2%), have a fixed

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Figure 2. Evaporation data and power function fits (Pow.) for bitumen blends listed in Table 3.

Figure 3. Power (Pow.) and sigmoidal (Sigm.) function fits on evaporation data (1 mm film thickness).

limiting value (α = 11.2%) for the total evaporation, which equals the proportion of bioflux inthe samples. In the second sigmoidal fits, labelled as Sigm. (7.95%) and Sigm. (4.73%), thelimiting values (alphas 7.95% and 4.73%) were obtained among the other coefficients with theleast-square sum fit method. The shadowed areas in Figures 3 and 4 illustrate the importance ofthe limiting value. The advantages of sigmoidal models are obvious, and Figure 5 shows highcorrelations between measured and modelled values. Additional research would be needed toassess the goodness of the sigmoidal evaporation models and to determine model coefficients onphysicochemical bases under varying circumstances. Asymmetric sigmoidal model, known asgeneralised or Richards’ sigmoidal model (1959) could be utilised as well.

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Figure 4. Power (Pow.) and sigmoidal (Sigm.) function fits on evaporation data (3 mm film thickness).

Figure 5. Correlations between measured and modelled (sigmoidal models with limiting values by theleast-square sum model estimations) evaporation data.

3.2. Effects of ageing on the composition3.2.1. Solvent contentThe recovery and stabilisation procedures greatly influenced the compositions of the bitumensolutions. Table 5 and Figure 6 present the development of solvent content during recoveryand stabilisation procedures. Measured solvent contents of bitumen solutions are included in both

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Table 5. Evaporation of solvents from bitumen solutions during recovery and stabilisationwith the original solvent contents of bitumen solutions by simulated distillation.

Bitumen During During Remained Initial solventsolution recovery (%)a stabilisation (%)a (%)a content (%)b

V15BF 8.3 52.1 39.6 4.8V30BF 19.5 58.5 22.0 8.2V60BF 20.2 58.7 21.2 10.4B20BF 18.2 51.2 30.6 17.0BL2K 49.6 17.7 32.6 14.1

Notes: aWeight percent of solvent.bWeight percent of bitumen solution.

Figure 6. Evaporation of solvents from bitumen solutions during recovery and stabilisation.

Table 5 (initial solvent content) and Figure 6 (the heights of the columns). During recovery biofluxcontents decreased 8.3% (V15BF) to 20.2% (V60BF) compared to the original bioflux contents.The highly volatile solvent in BL2K evaporated completely during recovery (evaporation equalsto original 50% kerosene content). Respectively, the most of bioflux and only a minor part ofthe heavier solvent of BL2K evaporated during stabilisation. Differences in evaporation duringstabilisation may be explained by the distinct distillation characteristics of bioflux, keroseneand gas oil (see Figure 1). Figure 6 demonstrates that the evaporation of bioflux have probablysignificantly decreased or ceased when the bioflux content had lowered to about 2% (V15BF,V30BF and V60BF).

3.2.2. Generic fractionsTable 6 shows the generic fractions of original base bitumens and long-term aged biofluxedbitumens. The LTA decreased aromatic contents and increased resin contents significantly. Thus,

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Table 6. The generic fractions of original base bitumens and long-term aged biofluxed bitumens.

% Saturates % Aromatics % Resins % Asphaltenes

Bitumen Base bit. PAVa Base bit. PAVa Base bit. PAVa Base bit. PAVa

V15A 13.7 14.4 60.3 46.2 19.5 33.9 6.3 5.5V30A 10.8 12.6 56.8 43.8 23.7 36.4 8.5 7.1V60A 9.8 10.4 54.0 42.5 25.4 37.4 10.7 9.7B20A 5.4 5.4 52.5 39.8 26.9 42.0 15.2 12.7

Note: aPAV treated bitumen solution.

the general polarity of studied binders increased during the treatment. Observations concerningaromatics and resins were similar to the experiences found in literature. However, during LTA theasphaltene contents of biofluxed bitumens decreased, which contradicts the anticipated behaviour.The saturate contents remained somewhat unchanged as expected.

3.3. Effects of ageing on the rheological propertiesTable 7 presents the capillary viscosities of original, recovered, stabilised and long-term aged(PAV treated) bitumen solutions. Lower bioflux content and softer base bitumen led to a slighterincrease in viscosity during recovery. The viscosity of cut-back bitumen increased substantiallyduring recovery, which may be addressed to the total volatilisation of the kerosene component. Theviscosities of stabilised bitumen solutions were close to the viscosities of the base bitumens apartfrom B20BF. LTA did not increase the viscosity of V15BF and V30BF above the correspondingbase bitumens, whereas V60BF and BL2K exhibited a noticeable increase in viscosities. Theviscosity of long-term aged B20BF was more than 3 times higher than its base bitumen and thusdiffered significantly from other bitumen solutions.

The rheology of studied bitumen solutions and base bitumens was found to be complex in theearlier research by Simonen et al. (submitted). Softer base bitumen and lower bioflux content ledto more elastic and stiffer binder. In addition, viscosity-graded bitumens (V15A, V30A and V60A)showed thermo-rheologically complex characteristics meaning that time–temperature superpo-sition principle could not be applied either to viscosity-graded bitumens or to their biofluxedproducts.

Figures 7 and 8 demonstrate the fundamental differences between bitumen solutions. In Figure 7,recovered BL2K had the highest stiffness in the whole temperature range, which complies withobserved solvent evaporations and capillary viscosities. Although the viscosities of recoveredbiofluxed bitumens followed the order of viscosities of base bitumens at 60◦C (see Table 7),the consistencies were opposite at temperatures below 30◦C. Thus, the exceptional stiffnesses

Table 7. Effect of ageing on the viscosities of bitumen solutions (capillary method at 60◦C).

Bitumen Original Recovered Stabilised PAV Base bitumensolutions (mm2/s) (mm2/s) (mm2/s) (mm2/s) (mm2/s)

V15BF 674 764 1290 1330a 1420V30BF 587 847 2650 2850a 2920V60BF 622 1010 5170 7330a 5660B20BF 646 1160a 17,800a 124,000a 36,700a

BL2K 551 2920 5910 11,700a –

Note: aCalculated from dynamic viscosity.

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Figure 7. Isochronal plots of complex shear modulus for recovered bitumen solutions at 1 Hz.

Figure 8. Isochronal plots of phase angle for recovered base bitumens at 1 Hz.

of viscosity-graded bitumens observed by Simonen et al. (submitted) were also present withinrecovered biofluxed bitumens.

The phase angles of recovered bitumen solutions presented in Figure 8 resemble those oforiginal bitumen solutions with wide divergences. Even after recovery V15BF was the mostelastic bitumen solution. The inflection temperature for phase angle was around 20◦C for recoveredV15BF, while the inflection temperature for original V15BF was 10◦C and for its base bitumen

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Figure 9. Isochronal plots of phase angle for original V15BF, recovered (V15BFR), stabilised (V15BFS)and long-term aged (V15BFP) bitumen solution at 1 Hz.

Figure 10. Isochronal plots of complex shear modulus for original (B20BF), recovered (B20BFR), sta-bilised (B20BFS), long-term aged (B20BFP) B20BF bitumen solution and its base bitumen (B20A) at1 Hz.

30◦C, respectively. Figure 9 demonstrates the transition of inflection temperature as the stiffnessof the V15BF increased.

As already stated, ageing generally increases stiffness and elasticity of bituminous binders.Figure 10 shows the effects of fluxing (B20A → B20BF), stabilisation (B20BF → B20BFR →

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Figure 11. Isochronal plots of phase angle for original B20BF, recovered (B20BFR), stabilised (B20BFS),long-term aged (B20BFP) bitumen solution and its base bitumen (B20A) at 1 Hz.

Figure 12. Isochronal plots of complex shear modulus for base bitumens V15A and V60A and long-termaged V15BF and V60BF bitumen solutions at 1 Hz.

B20BFS) and LTA (B20BFS → B20BFP) on the stiffness of biofluxed B20A bitumen. Respec-tively, the effect on the elasticity is illustrated in Figure 11. The effects of ageing on the rheologicalproperties of the other bitumen solutions did not differ significantly from B20BF apart from long-term aged V15BF, V30BF and V60BF, on which PAV treatment had only minor or no influence(see Figure 12).

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4. DiscussionAs the biofluxed cut-back binder blends are intended to be used in cold and warm mixtures,oxidative ageing, typical for the hot-mix production, was not considered in the experimentalsetup for the evaporation testing. It could be argued that at room temperature, 1-mm thick filmcould be affected by oxidation but for the 3-mm tick film evaporation from the bulk most likelydominates. A research by Petersen and Glaser (2011) suggest that at 45◦C the thin film oxidationwould correspond to approx. 0.16% mass increase due to oxygen addition, whereas competitiveprocess of volatilisation of base bitumen could result in mass decrease up to 0.5% (Siddiqui &Ali, 1999). Mass changes due to evaporation procedure in base bitumen were assumed negligible.However, we acknowledge possible effect of oxidation and volatilisation, due to viscosity increasein laminar surface layer and possible decrease in solvents’ mobility through that region. Theevaporation rates of bioflux from bitumen solutions were found equal at room temperatures,regardless of the viscosity of the base bitumen. The evaporation decelerated through the wholeobservation period. Mackay and Matsugu (1973) stated that in multicomponent hydrocarbonliquids the more volatile components evaporate first leading to a fall in the rate of evaporation,which was later supported by Fingas (1997). Moreover, the surface layers of the samples mayhave become less permeable during evaporation, due to oxidation of surface layers (Petersen &Glaser, 2011). Evaporation during recovery and stabilisation suggested that bioflux concentrationgoverned the evaporation rate and that evaporation significantly decreased or ceased when thebioflux content had decreased to about two percent in the given circumstances. Flashpoint testssupport the importance of bioflux concentration, since higher bioflux contents resulted in lowerflashpoints (see Table 8). Altogether decreasing bioflux concentration, increasing diffusion pathlengths within film, evaporation of light components and less permeable surface due to someoxidation, may contribute to the observed decreasing evaporation.

The two mathematical models fitted to the evaporation data, the power-law and sigmoidalfunctions will obviously give different results when measured data are extrapolated. Based onphysical observations, sigmoidal models fit better the evaporation data of the bitumen solutionswith limited capability to evaporate. The obtained results do not tell whether all of bioflux oronly part of it evaporates after very long period of time, which information would be neededto set limiting values for the sigmoidal fits. More research on physicochemical bases should beconducted before sigmoidal models could be utilised in actual evaporation models (reviewed anddiscussed, e.g. by Fingas, 1995).

The evaporation of bioflux differed greatly from the solvents of BL2K. However, the resultscomplied with the distillation characteristics of bioflux, kerosene and gas oil (see Figure 1). Therecovery (according to SFS-EN 14895) of binders had more pronounced effect on the compositionof BL2K than on the compositions of the biofluxed bitumens, which was observed from rheological

Table 8. Basic properties of bitumen solutions studied.

Bitumen Viscosity, Viscosity, Density, Flashpointbsolution 60◦Ca (mm2/s) 135◦Ca (mm2/s) 50◦C (Kg/m3) (◦C)

V15BF 674 25.2 945.8 141V30BF 587 24.1 945.2 127V60BF 622 25.2 948.0 122B20BF 646 24.7 945.6 110BL2K 551 25.0 942.2c 80

Notes: aCapillary method.bPensky–Martens closed-cup method.c60◦C.

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properties as well. The properties of stabilised (according to SFS-EN 14895) bitumen solutions,apart from B20BF, resembled those of the base bitumens. The high bioflux content of B20BFeven after stabilisation probably caused the relatively low viscosity and stiffness.

Initial field studies suggest that 0–20% from the bioflux evaporates during the first 2 years(exact results are to be published), thus recovery would simulate 2 years of curing in field atmaximum. As there is a lot of variation associated with the field testing, zero evaporation isyet to be confirmed. Apilo (1996) and Toropainen (1989) observed that 60–70% of the solventsused in BL2K evaporate within the first 2 or 3 years. Additionally, Apilo suggested that solventcontent will not change dramatically after three years. Thus, stabilisation according to 14,895would simulate ageing during the whole life time of bitumen solution (before reuse).

The evaporation characteristics of solvents greatly affect the designing of cut-back and fluxedbitumens. With two solvents used in BL2K the short-term and long-term curing could be decidedindependently. The design approach for extremely slow-curing bioflux bitumens is different, sincethe rates of short-term and long-term curing are the same. Thus, biofluxed bitumens and mixturesmade of them are compromises between the short-term (adequate for traffic) and long-term (lowenough for workability) stabilities.

LTA increased the polarity of bitumen solutions. The aromatic content decreased substantiallywhile the resin content increased respectively, while changes in saturate and asphaltene contentsremained moderate. Contradictory to observations made for the paving-grade bitumens, the PAVtreatment decreased the asphaltene contents of biofluxed bitumen solutions. Further research iswarranted to study the influence of bioflux on readings of SARA fractions. A supplementary dataanalysis, presented in Appendix 1 formulates three theories to explain the ambiguous findingsshown in Table 6. These theories will be pursued in our future work. Although the relative changesin the generic fractions were of the same magnitude for the binder solutions studied, only B20BFexhibited pronounced increases in viscosity, complex shear modulus and phase angle. Reasonsfor such differences remained unsolved within this study.

The rheological properties of the cured and long-term aged bitumen solutions were similar tothe properties of the base bitumens and fresh bitumen solutions. The highest stiffness and elasticityof V15BF at temperatures below 30◦C even after recovery was surprising, since its kinematicviscosity at 60◦C was notably lower than the viscosities of the other bitumen solutions. Presumablythe rigid crystalline networks observed in the softest bitumens (Simonen et al., submitted) werepresent and emphasised in the recovered solutions. Additionally, the melting temperature ofcrystalline material depended on the viscosity of the bitumen solution, since curing increasedthe starting temperatures for melting. These changes in crystalline structure are evidenced by theinflection temperatures shown in Figures 8 and 9.

5. ConclusionsBoth curing and ageing had significant effects on the physicochemical properties of bitumensolutions. In addition, the curing and ageing behaviour of the biofluxed bitumens differed notablyfrom the traditional cut-back bitumen used in Finland.

The evaporation of different solvents from bitumen solutions at ambient temperature compliedwith the distillation characteristics of solvents. The lighter solvent (kerosene) of BL2K evaporatedcompletely during recovery, while during stabilisation bioflux evaporated more readily than theheavier solvent (gas oil) of BL2K. There were no observed differences between evaporation ratesof bioflux from different base bitumens. However, bioflux content was found to relate to theevaporation rates. The bioflux contents approached 2% during stabilisation implying that somebioflux will remain in solutions even after a long period of time. Finally, sigmoidal functions wereintroduced as an option for evaporation models with limited evaporation.

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The rheological properties of the cured and long-term aged bitumen solutions resembled thoseof the fresh solutions. In general, curing and LTA increased the stiffness and elasticity of the softbitumen solutions, as would be expected for the paving-grade bitumens. Interestingly, after curingand LTA the final product of the cut-back blends gave rheological response similar to that of theinitial base bitumens.

LTA by PAV induced changes in the chemical composition of the bitumen solutions, which wasobserved as increases in the polarity of the bitumen solutions. Contradictory to observations madefor the paving-grade bitumens, the PAV treatment decreased the asphaltene contents of biofluxedbitumen solutions. However, interpretation of chemical composition of cut-back bitumens asanalysed by thin layer chromatography-flame ionisation detector (TLC-FID) leaves a reasonabledoubt for this method to give ultimate composition result. The viscosities, complex shear moduliand phase angles of V15BF, V30BF and V60BF remained practically unchanged despite thenoticeably increased polarity. Thus, the effects of ageing on the rheological properties of V15BF,V30BF and V60BF may be solely bound to the evaporation of bioflux solvent.

Cut-back blends created with bioflux as a solvent have good workability in comparison withblends used up to date. Evaporation of the solvent stays within standards and is predictable. Flashpoints for this novel blends are much higher than for BL2K, making them safer to use.

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Appendix 1. SARA fraction analysis with supplementary dataThe contradictory finding of lower concentration of asphaltene fraction in biofluxed samples after LTAprocedure of PAV could be explained by three theories: (A) miscibility of non-interacting liquids of differentinitial concentrations; (B) carboids precipitation or (C) measurement artefact due to presence of foreignsolvent (bioflux). These theories are pursued by additional analysis for the SARA fractioning shown inTables A1–A3.

Table A1 presents algorithms to back-calculate generic fraction concentrations for the base bitumens andfor the bioflux solvent in addition of the calculations to predict the concentrations for the cut-back bitumenblends from the amount of base bitumen and solvent used. Table A2 presents the back-calculated biofluxconcentrations as a response in TLC-FID SARA fractioning on the basis of data collected after blending stage(prior to stabilisation, evaporation or ageing). Finally, Table A3 presents ratios of generic SARA fractionsas measured by TLC-FID technique.

Table A1. Rows 1–5: back-calculation of SARA fractions for V1500 and prediction of SARAfraction response for the blend of two bitumens (REB + V1500). Rows 6–7: back-calculations ofSARA fractions for bioflux. (Supplementary REB data obtained from Lehtimäk, 2012). Examplecalculations below are for aromatic fraction.

No. Description Asph. % Res. % Arom. % Sat. %

1 RAP extracted binder (REB), measured 17.6 40.6 37.9 3.82 V1500, measured 6.3 19.5 60.3 13.73 V1500, back-calculateda 8.6 15.27 59.9 16.134 REB 70% + V1500 30%, measured 14.9 33.0 44.5 7.55 REB 70% + V1500 30%, predictedb 14.2 34.3 44.6 6.86 REB 93% + BF 7%, measured 15.1 43.5 35.3 6.17 Bioflux (BF100) back-calculatedc −18.11 82.03 0.76 36.66

Notes: aAromatic fraction of V1500 (data from lines 1 and 4): (44.5 − 0.7 × 37.9)/0.3 = 59.9.bAromatic fraction of blend REB 70% + V1500 30% (data from lines 1 and 2): (0.7 × 37.9) + (0.3 × 60.3) =26.5 + 18.1 = 44.6.cAromatic fraction of Bioflux (BF100) (data from lines 1 and 6): (35.3 − 0.93 × 37.9)/0.07 = 0.76.

Table A2. Bioflux back-calculated as a response in TLC-FID SARA fractioning on thebasis of data collected after blending stage (prior to stabilisation, evaporation or ageing).

Description Asph. % Res. % Arom. % Sat. % Biofluxa %

Bioflux BF100 from V15 26.30 68.59 −1.52 11.88 5.5Bioflux BF100 from V30 19.37 18.27 26.37 36.89 9.2Bioflux BF100 from V60 12.10 −7.99 76.45 24.79 11.5Bioflux BF100 from B20 14.09 9.12 51.94 24.84 18.0

Note: aBioflux concentration in the cut-back blend.

Table A3. Ratios of generic SARA fractions as measured by TLC-FID technique.

No. Description Asph. % Res. % Arom. % Sat. %

1 V15BF-PAV/V15A from Table 6 0.87 1.74 0.77 1.052 V30BF-PAV/V30A from Table 6 0.84 1.54 0.77 1.173 V60BF-PAV/V60A from Table 6 0.91 1.47 0.79 1.064 B20BF-PAV/B20A from Table 6 0.84 1.56 0.76 1.005 REB + BF/REBa 0.86 1.07 0.93 1.60

Note: aEffect of bioflux on aged bitumen (REB), sample is water tempered (Lehtimäki, 2012).

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Calculations are utilising supplementary data from study by Lehtimäki (2012), which investigated thelightweight oil products as rejuvenators for the reclaimed asphalt pavement (RAP). Aim in Lehtimäki’s studywas to investigate the effect of bioflux on naturally aged reclaimed bitumen. Binder from RAP was extractedwith dichloromethane and recovered according to procedure described in specification (EN 12697-3).Reclaimed extracted bitumen (REB) samples were mixed with softening agent (bioflux or soft bitumenV1500) at temperatures of 90–95◦C and then allowed to cool down. Samples were further tempered in waterfor Penetration test (SFS-EN 5289). Afterwards TLC-FID SARA fractions were conducted. In our opinion,tempering in water allowed for the removal of any excess unbound (easily evaporable) bioflux from theblend, as bioflux density is lower than that of water and the two substances are immiscible. Fraass BreakingPoint measurements were conducted on the samples after water tempering according to SFS-EN 12593.

1.1. Miscibility of non-interacting liquids of different initial concentrationsIn attempt of SARA fraction prediction and deeper understanding of lower concentration of asphaltenephenomena, we postulated that a blend of two bitumens or bitumen and a solvent will behave as a mixtureof two solutions of different concentrations, and processed data obtained accordingly. Mixing two solutionsof higher and lower asphaltene concentration should result in intermediate value, easily calculable from theweight concentrations. Despite being correct for the mixture of two bitumens (e.g. extracted binder fromRAP and fresh low viscosity bitumen V1500, Table A1, rows 3–5), this assumption proved to be insufficientexplanation for the blends containing bioflux (Table A1, rows 6–7) and other low molecular weight solvents.

Nevertheless, ability to estimate SARA fractions of blend of two base bitumens is important in termsof predicting fracture temperature of asphalt binders. Isaacson and Zeng (1997) have proposed a modelwhich relates the cumulative Resin and Asphaltene fraction to the low-temperature fracture of bitumen. Tofurther evaluate generic fraction characteristics of bitumen blends, we incorporated the Fraass breaking pointmeasurements. We found that both calculated and measured SARA fractions (REB + V1500), according tothe Isacsson et al. model, should give Fraass breaking point of −22◦C, whereas actual measured fracturetemperature for this blend was −18◦C (which remains within standard deviation of the model). However,the cut-back blends with bioflux do not follow this model, leading us to explore other explanations of theirSARA fractioning results.

1.2. Carboids precipitationKuszewski, Gorman, and Kane (1997) using TLC-FID method, report that extreme ageing of bitumen (80 hof thin film oxidation at 163◦C) leads to creation of carboids from asphaltenes. These substances are notsoluble in initial solvent used for sample preparation prior to spotting on chromarods. In their work, lowervalues of asphaltenes were assigned to this effect. Although, our study incorporates 65 h of PAV ageing(which is considerably more than in other PAV ageing studies by Sá da Costa, Farcas, Santos, Eusébio, andDiogo (2010) and Siddiqui (1999), no observation of carboids formation during sample preparation wasreported.

1.3. Measurement artefact due to presence of foreign solvent (bioflux)Siddiqui (1999) provides a comprehensive study on the PAV and RTFOT ageing in respect of changes inSARA fractions. Unfortunately, as this study incorporates ASTM D-4124 procedure of SARA analysis, basedon Kharrat, Zacharia, Cherian, and Anyatonwu (2007), it should not be compared between laboratories, andeven more so to the TLC-FID method. However, Sá da Costa et al. (2010) present a study by TLC-FIDSARA, incorporating equal times and temperatures employed by Siddiqui (1999), in which an agreement isfound for the increased response for asphaltenes and resins, along the decrease of aromatics and change ofsaturates, all within typical standard deviation of measurements reported by Masson, Polomark, and Collins(2002).

In Table A2, we presented the back-calculated effect of bioflux on the SARA measurement (followingalgorithm c presented in Table A2), after the stage of blending (before stabilisation or evaporation or PAVageing). Seemingly, addition of bioflux affects to all the fractions and, as the concentration of bioflux in

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cut-back bitumens remains variable, response is not uniform. We believe that lack of bioflux in referencesample development may cause rise or fall of the baseline in measurement during TLC-FID SARA testing.Bioflux, as the least mobile of all the used solvents during the developing stage, is most likely still presenton chromarods during signal reading. It is unlikely that bioflux contains asphaltenes, resins and aromatics asit is a mixture of short-chain alkanes. However, its evaporation in the measurement chamber may contributeto creating artefact during measurements, dependable on the initial concentration in the mixture (i.e. speedof elution from chromarod). FID counts carbon ions after ionisation of residue from chromarod. Signal is afunction of hydrocarbon chemical composition (Holm, 1999). If bioflux (mixture of alkanes) continues toevaporate in the measurement chamber, it becomes ionised and counted by detector. Divergences in signalare expected, which will translate into altered shape of a chromatogram.

Additionally, according to Baginska and Gawel (2004), use of n-decane (C10 asphaltenes) instead ofn-heptane (C7 asphaltenes) during the classical SARA fractioning will precipitate approximately 78% wt.of C7 asphaltenes. The C10 asphaltenes will be higher molecular mass than C7 asphaltenes. If longerchain alkane was used during development, lower molecular mass asphaltenes, with relatively lower mobil-ity in developing solvents than saturates and aromatics, would most likely be transferred into the regionthat is assigned to resin fraction in classical bitumen analysis by TLC-FID. Bioflux is a mixture of lin-ear and branched C10–C20 alkanes and we believe its presence affects precipitation of asphaltenes duringmeasurement.

The PAV ageing procedure was designed to simulate LTA in the field (Bahia & Anderson, 1995), in orderto minimise volatile compound loss during procedure and maximise oxygen diffusion, i.e. LTA of bitumen.The effect of the PAV on bioflux alone is not clear. It is unlikely that bioflux reduced asphaltenes during theprocess. In our case, the PAV was conducted on stabilised samples (exhausted of excess bioflux). However,seemingly a different factor is observed (Table A3) for aged bitumen mixed with bioflux (REB + BF), thanfor fresh bitumen blended with bioflux prior to LTA (V15BFP, V30BFP, V60BFP and B20BFP). Aromaticfraction is not affected vastly by addition of bioflux, strong change is observed after the PAV treatment.

The lowered intensity of aromatic fraction in the PAV treated samples is consistent with typically reportedresearch on ageing of bitumens (Isaacson & Zeng, 1997; Siddiqui, 1999; Sá da Costa et al., 2010). However,effect is stronger than that of the reported work and we predict it being due to the extended PAV ageing timechosen in our work.

To the best of our knowledge, neither bioflux nor mixtures of bioflux and solvents used in developing stagesof TLC-FID SARA fraction analysis have been studied previously in respect of asphaltene precipitation.TLC-FID SARA analysis is not only material dependent but also solvent and developing process-dependenttechnique (Cagniant et al., 2001). Therefore, optimisation of the technique is necessary in order to characterisecut-backs as a separate material from base bitumens and we intend to investigate this effect in detail in ourfuture work.

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Documents

Curing and ageing of biofluxed bitumen: a physicochemical approach