¹ PhD. M.Sc., Chemical Engineer. Environmental Engineering Program Faculty of Engineering, Universidad Manuela Beltrán. Bogotá D.C, Colombia. Research Group Tecnología de Aprovechamiento de Residuos y Energías Renovables. xiomara.vargas@umb.edu.co.

² PhD. MSc. Civil Engineer. Civil Engineering Department, Faculty of Engineering, Pontificia. Centro de Estudios en Carreteras Transporte y Afines (CECATA), Pontificia Universidad Javeriana. fredy.reyes@javeriana.edu.co.

ABSTRACT

This work presents a state of the art revision of the results from studies of asphalt aging found by researchers aimed at deepening understanding of this complex phenomenon. The background shows the evolution of asphalt aging, initially considering it as physical hardening and progressively recognizing it as a complex phenomenon having repercussions on asphalt durability and physical-chemical properties. This document aims to be a guide for understanding future investigations for developing new types and mixtures of asphalt having improved properties.

Keywords: aging, oxidation, asphalt.

Received: may 20th 2009 Accepted: november 11th 2010

Introduction

Asphalt´s physical-chemical properties become altered due to its aging and therefore asphalt pavement durability is affected, causing economic loss due to premature asphalt layer deterioration. This complex phenomenon has consequently been widely studied for over a hundred years and can be defined as a slow process involving changes in the asphalt´s chemical composition. Youtcheff and Jones (1994) defined its oxidative aging as a reaction of weak groups of asphalt with oxygen; however, this process involves other changes at structural level.

The properties of asphalt change with time and, due to this, the specifications used for designing road networks based on initial physical properties do not ensure good performance after the asphalt has been mixed with the aggregate, applied and laid to support the mechanical effects of transport. When asphalt is mixed, asphalts become oxidised by reaction with atmospheric oxygen and high mixing temperatures, leading to aging starting immediately and then being induced by various climatic factors affecting road surfaces. Therefore, to find an asphalt layer having longer durability, one has to consider the effect of chemical composition change in asphalt cement during hot mixing and during service time. First of all, it is necessary to study asphalt oxidation to include this effect since petroleum binder oxidation characteristics are those determining pavement behaviour and durability after it has been prepared, as well as the initial chemical composition.

This article thus explores the current situation of studying the influenc of aging on asphalt´s physiochemical properties which is irreversible and promotes a tendency to form molecular associations leading to various problems during production, refining, operation and application of heavy petroleum fractions, causing this topic to be of great interest and the object of investigations during the past few years.

Asphalt aging

A. W. Dow first documented the study of asphalt aging (1903); he related the heating of asphalt extracted from asphalt mixtures to weight reduction and penetration values.

Brooks et al., studied the effect of sulphur on hydrocarbon oxidation of petroleum and asphalt hydrocarbons in 1917. They found that sulphur compounds accelerated hydrocarbon oxidation of petroleum in the presence of air at higher temperatures and noted that the time required for obtaining a medium grade of asphalt hardening was reduced by adding a certain amount of sulphur.

On the other hand, in the oil industry, Waters (1921) studied the effect of various catalysts on the oxidation of two crudes and their relationship to the amount of deposits generated in internal combustion engines. He found that metals like zinc and aluminium did not increase the amount of precipitate and that bronze and copper caused a notable acceleration of oxidation by contrast with iron, nickel, cobalt and steel.

Researchers focused on studying the phenomenon of asphalt hardening due to short- and long-term aging during the 1930s. The aging method used was based on the heating of air-blown asphalts at different times and temperatures. Changes were subsequently detected by estimating the degree of hardening obtained from the values of empirical properties such as asphalt penetration and ductility (Welborn, 1979).

The thin film oven test (TFOT) aging method was introduced in the 1940s, defining the aging index concept which relates viscosity measured before and after an asphalt sample´s aging and whose use extended for several years up to the present day. In the same decade, asphalt aging in service and in the laboratory began to be correlated. Table 1 presents a summary of asphalt aging and evaluation methods used by researchers from 1903 until 1989.

Parallel to these investigations, Thurston et al., (1941) studied asphalt constituents and its oxidation (photo-oxidation) at various operating temperatures. The methodology consisted of exposing asphalt samples contained in sealed Pyrex containers and filled with oxygen to lamp heating at temperatures of about 77°C. They identified constituents like asphaltenes, resins, paraffins and typical naphthenic compounds from petroleum residues.

They noticed that all these compounds absorbed oxygen, especially the naphthenic compounds and resins. They applied this method to asphalts used in coatings and paving.

One year later, based on the fact that the study of bituminous material aging only focused on hardening³, Ebberts (1942) complemented studies of asphalt aging considering oxidation as the most influential factor in this phenomenon. He oxidized thin films of asphalt with potassium permanganate, discarding the use of ultraviolet light applied by other researchers in the same decade. He confirmed that asphalt resins and naphthenic fractions oxidized quickly and noted that the oxygen demand decreased significantl in asphalts having high viscosity.

Anderson et al., (1942) used two methods for measuring the stability of asphalt samples at different processing temperatures: the heating test and the study of oxygen reaction at higher pressure in asphaltic benzene solutions. They calculated the resistance to hardening and the deterioration index from penetration values obtainned at two oxidation periods and temperatures in standard conditions. They found that asphalts having deterioration index values less than 15 had good performance in service, and related values equal to or greater than 20 to asphalts having poor durability.

Rescorla et al., (1956) described a methodology for optimising asphalt oxidation time using a mechanical stirrer for this purpose. They studied the effect of stirring speed on oxidation time at various operating temperatures. They found that produced asphalt quality was better than that obtained by conventional methods in terms of properties like penetration and softening point. In 1957,Vallerega et al., reported studies using ultraviolet and infrared light for aging asphalt films. Treatment with ultraviolet light resulted in more effective change of asphalt softening point, ductility and penetration after oxidation.

Griffin and Simpson evaluated the influence of the asphaltic pavement´s chemical composition on viscosity, temperature susceptibility and durability in 1959, relating asphalt´s molecular weight to its viscosity. They concluded that, due to the complexity of the chemical composition of asphalt, it is not easy to establish a simple method for its specification from it.

Hughes (1962) oxidized thin films of asphalt at high temperatures (>165°C) and studied the combined effects of oxidation time and temperature, as well as asphalt film thickness in change, percentage of weight, the fractions of asphaltenes and resins, saturates and insoluble material, detected by liquid chromatography as well as oxygen content change by elemental analysis. He managed to relate the effects of temperature and time to an Arrhenius-type expression and defined three stages of oxidation in the range of temperatures studied: peroxide decomposition, oxygen diffusion and dehydrogenation.

Traxler listed five factors influencing asphalt hardeningin 1961; in order of importance, they were oxidation, volatilization, time, polymerization induced by reactions with free radicals and condensation. In 1963 he expanded the list of factors by oxidation (in darkness), photo-oxidation (under direct and reflected light), volatilization, photochemistry (direct and reflected light), polymerization, internal structure development (thixotropy), paraffin exudetion (syneresis), changes in nuclear energy, outdoor conditions, absorption by solids, adsorption of components on the solid surface, chemical and catalyzed reactions, effects on the interface and microbiological spoilage. He thus enunciated where the hardening phenomenon occurred, on the surface or in the complete material, if it is influenced by weather, heat, oxygen or sunlight, and ways of slowing down these effects.

Davis and Petersen (1966) adapted the inverse gas-liquid chromatography technique to predict the durability of asphalts exposed to oxidation. Campbell and Wright (1965, 1966) studied the oxidetion products of air-blown asphalts using infrared spectroscopy, microviscosimetry and the change of softening point. Behl, Traxler et al., (1969) used the scattered light technique to oxidize asphalt films. They found a direct relationship between the aging index and the proportion of scattered light from the asphalt. It highlights the use of analytical instruments combined with the determination of empirical properties like the softening point and the asphalt´s viscosity before and after oxidation.

Meanwhile, Sisko and Burnstrum (1968, 1969) were the first to compare the effects of oxidative asphalt aging in the laboratory with those of aging in real conditions using viscoelastic property characterisation techniques. They used a rheometer with cone and plate geometry to measure the complex modulus of aged and nonaged asphalts in a wide range of temperatures and frequencies, concluding that oxidation changed the material´s dependency on temperature and that the changes tended to increase with increasing temperature, coinciding with the results obtained by Majidzadeh (1969). Dickinson and Witt (1969), and later Maccarrone (1987), concluded that oxidative aging results in a distortion in the form of asphalt´s master curve viscoelastic response when compared to the curves of samples without aging. Such changes depend on temperature and, more importantly, the type of aging.

Researchers like Speros, Speight and Moschopedis (1974, 1975, 1978) made important contributions to the study of the oxidation of crude, petroleum and its heavy fractions from a chemical point of view, analyzing the influence of oxygenated functional groups present in resin and asphaltene fractions, the asphaltene´s molecular weight and its relationship with resins and these factors´ influence on the operations of recuperation, conversion and transport of crudes, petroleum and its derivatives. Likewise, they studied the influence of metals on bitumen oxidation, asphalt sample sulphoxidation and asphaltene thermal decomposition.

On the other hand, Dorrence and Petersen (1974) reported the formation of significant amounts of ketones during asphalt aging and the detectable presence of aldehydes based on the interpretations made for absorption bands in asphalt infrared spectra before and after oxidation. A year later, they identified dicarboxylic anhydrides in oxidised asphalts, determined by chemical reactivity and infrared spectroscopy. They recognised that the main products of asphalt oxidation are carbonyl type functional groups which are a mixture of several kinds of chemical compounds.

The use of asphalt aging tests on thin film was extended during the 1970s and 1980s, presenting various modifications of the method until the late 1980s. The researchers noticed that the level of aging caused by the hardening of the developed samples was less than under real conditions. The evaluation methods used in these two decades included parameters like asphaltene content, percentage of oxygen and chemical analysis, as well as elastic modulus, creep tests and calculation of modules estimated before and after aging

Gorshkov et al., (1980) studied the changes taking place in the generic fractions of aged thin film asphalt evaluated by micro-chromatograph and infrared spectroscopy. They found that the aging of each asphalt component (such as resins and asphaltenes) was accompanied by significant chemical conversions which are specific to those components. They found increments in hetero-structure content and an accumulation of aromatic structures in the asphaltene fraction´s infrared spectrum. They concluded that aromatic structures were the most reactive materials in asphalt aging and that reactivity increased with the components´ degree of condensation and molecular weight.

Petersen listed three important factors in 1984 that caused asphalt hardening and its mixtures like the loss of petroleum components by volatility and absorption, changes in the composition of atmospheric oxygen reactions and the molecular structuring that produces thixotropic effects (steric hardening)

Goodrich (1985) listed some commonly-used methods for analysing asphalt composition, considering them potential binder performance indicators. These methods are fractionation by precipitation (precipitation by solvents), fractionation by distillation (vacuum distillation, thermogravimetric analysis), chromatographic separation (gas, inverse gas-liquid and liquid (adsorption, ion interchange, size exclusion, thin films) chromatography), chemical analysis with spectrophotometric techniques (ultraviolet, infrared, nuclear magnetic resonance, x-ray, emission, fluorescence), gravimetric techniques and elementals analysis, molecular weight analysis by mass spectrometry, osmometry and size-exclusion chromatography. Most of these methods are still used for characterising asphalts.

Jakubowicz used thermo-mechanical analysis in 1987 as a new evaluation technique for asphalt aging characteristics applied to coatings. He analysed the effects of heat and ultraviolet radiation on the degradation rate of the asphalts being studied.

Petersen oxidized asphalt samples in 1989, using accelerated TFOT and observed that saturate fraction content estimated by Corbett fractionation remained constant at the end of oxidation and that the level of oxidative aging (aging index) and hardening in accelerated aging was lower regarding oxidation in actual service conditions, deducing that these simulated oxidative tests reflected the aging occurring in hot mixture asphalts. He also stated that the simulated aging kinematics of asphalt were different from those carried out in real conditions.

Simultaneously, Bell (1989) studied the aging of asphalt-aggregate systems and concluded that the most recommended methods for evaluating durability over a longer period of time were oven-aging, pressure oxidation, treatment with ultraviolet light, treatment with humidity and heating and microwave treatment for a short period of time. He also divided the tests for evaluating asphalt-aggregate mixtures into destructive and non-destructive experiments, some of the latter being the dynamic modulus, elastic modulus and the indirect tension test. Meanwhile, Vankeerbergen and Trhyrion (1989) studied the oxidation of petroleum asphalt and asphaltenes at 85°C. They concluded that the asphalt oxidation rate depended on its origin and that the oxidation rate decreased by extending the reaction.

In 1990, Quddus and Khan evaluated and optimised some effects, such as reactor design, temperature and air flow, on the treatment of air-blown asphalts to obtain high softening point industrial asphalts. The reactions were evaluated with the changes observed at the softening and penetration point at various intervals.

Bell et al., reported the research results of a study in 1994 of aging tests applied to asphalt-aggregate mixtures under the strategic highway research programme (SHRP). They presented and discussed the effect of aging temperature and duration from elastic modulus measurements. They found that asphalt mixtures presented an increased elastic modulus value with short- and long-termoven aging tests by contrast with the oxidation tests carried out at low pressure.

In their studies, Anderson et al., (1994) and Petersen et al., (1994) developed functions obtained by 10 rad/s strain sweeps in dynamic shear rheometer (DSR) which relate stress and deformation data in the complex modulus G* (Pa) for a range of aged asphalts in various conditions. The pertinent literature lists numerous models describing asphalt´s viscoelastic response regarding frequency and temperature, mostly based on developing master curves as proposed by authors like Vinogradov (1977), Christensen and Anderson (1992), Stastna and Zanzotto (1994), Lesueur et al., (1997) and Polacco et al., (2003).

Lin (1995) studied the formation of asphalt and its impact on the physical-chemical properties of asphalt. He proposed a model for describing the increased viscosity of asphalt samples intended for standard aging regarding asphalt formation and oxidation of carbonyl groups. On the other hand, Liu et al., (1996) examined the effect of asphaltene content on viscosity. They concluded that the aging of conventional asphalt in service results from the increase of carbonyl groups and that this reaction produces asphaltenes, which harden the material.

Herrington and Ball (1996) examined asphalt oxidation temperature dependency by gel permeation and infrared spectroscopy. They did not find any correlation between the concentration of carbonyl groups and species containing sulfoxides. Additionally, they reported that asphalt presenting the same viscosity at different temperatures had different oxidation products detected by gel permeation chromatography. They confirmed that the oxidation mechanism depended on temperature.

Afanasieva and Álvarez began a study of the aging of Colombian asphalt in natural conditions in 1997 by evaluating changes in physical and mechanical properties like viscosity, ductility, penetration, softening point and initial density, as well as after thermooxidative processes for a period of 3 years. They used instrumental techniques like x-ray diffraction, infrared, ultraviolet and visible spectrometry, liquid chromatography and nuclear magnetic resonance for estimating chemical and structural changes occurring during natural thermo-oxidation and detected an increase in the content of asphaltene fractions and changes in its structure, reflected in crystallite parameters. They also reported variations in asphalt functional group characteristic (relative aromaticity, branching and degree of condensation, relative ketone content) and in generic fraction content, such as saturates, aromatics and resins.

Lin et al., published the results of a study of generic asphalt fractions´ oxidation kinetics as well as the relationship between oxidetion and the composition of the asphalt being studied in the SHRP programme in 1998. They aged samples at various temperatures to obtain kinetic parameters and concluded that the kinetic characteristics of asphalt oxidation were determined by the kinetic characteristics of their fractions and the molecular interactions and associations between them.

The same year, Herrington described the asphalt oxidation reaction rate at constant oxygen concentration by a simple firstorder equation in terms of increased viscosity and the content of the carbonyl groups present. The estimated parameters were used for comparing the durability of oxidised asphalt at different operating temperatures.

Bonemazzi and Giavarini (1999) evaluated changes in the sol-gel colloidal structure of acid-treated oxidised asphalt, although not being universally accepted, following variations of the Cole-Cole diagram (η'' viscous component of complex viscosity η* vs. η' elastic component of complex viscosity) and loss tangent (tan = G""/G"), which is an indicator of the relationship between the viscous and elastic modules of asphalt obtained by rheological analysis in dynamic shear rheometer. They reported that air-blown and acid-treated asphalt shows the same colloidal sol to gel structure deduced from tan vs. temperature and Cole-Cole diagram data, so they concluded that the gel structure ensured a better performance at high temperatures in terms of permanent deformation.

Mastrofini and Scarsella (2000) used rheological analysis to evaluate three vacuum-aged bottoms. They studied the influence of aging on rheological properties, vacuum residue thermal susceptibility and their malthene fractions. They concluded that aging produces fundamental changes in the colloidal structure of vacuum and malthene bottoms and that the asphalts and the malthene fraction play an important role in the viscoelastic response of the vacuum bottoms.

Chipps et al., (2001) proposed a model for oxidative aging of rubber modified asphalt and analysed the sample´s performance using rheological analysis as well as susceptibility to hardening, which relates changes in viscosity with an increase in the area of carbonyl groups in infrared space. The rubber modified asphalts showed superior aging characteristics, like low oxidation rate in the range of simulated aging.

Yutai (2002) evaluated the aging of petroleum asphalt and its compounds by determining their molecular weights and structure. He concluded that asphaltenes and resins were the most instable fractions and found that there some types of asphalt absorb more oxygen than others by comparing the asphalt samples. Yutai studied asphalt reaction kinetics and composition changes during continuous aging with heating and the presence of air during the same year. He found that the fraction of saturates remained constant, aromatic content decreased and asphaltene content increased gradually during continuous aging. He calculated apparent activation energy and velocity constants and explained the changes observed through a series of kinetic models.

Afanasieva, Alvarez and Ortiz (2002) characterised the rheological properties of three types of industrial asphalt produced in Colombia after being exposed to 18 months of natural aging in the environmental conditions of the Piedecuesta area (Santander department). They measured the complex modulus regarding applied deformation and reported a decrease in complex flow index, regar ded as an indicator of the change of the colloidal change of aged binders.

Shenoy (2002) proposed a methodology for predicting the rheological properties of asphalt aged at high temperatures using flow data from non-aged asphalt to eliminate the aging treatment in the laboratory. The proposed method involved the use of unified curves for each rheological function and volumetric flow rates from which Shenoy managed to predict the dynamic rheological properties of aged asphalt.

Ruan, Davison and Glover (2003) analysed the effect of long-term oxidation on the rheological properties of polymer-modified asphalt. They found that polymer modification increased the complex modulus of asphalt at high temperatures and decreased it at low temperatures, as well as the fact that oxidative aging decreesed susceptibility to aging in asphalts, deteriorated the polymer network and reduced its effectiveness in increased asphalt ductilety.

X. A. Vargas (2004) described the behaviour of pseudoplastic flows using the Sisko model of asphalt samples and their naturally-aged malthene fraction. The model parameters so calculated were used to evaluate the influence of asphaltene concentration and temperature. She concluded that change in asphaltene fraction complex structure can influence the flow of thermo-oxidised petroleum heavy fractions.

So far, the approach to describing the study of oxidative aging by some researchers has been described from chemical, physical and rheological points of view. They can be summarised as follows.

Asphalt material aging was initially considered as material hardening and was evaluated by the changes observed in ductility and penetration and asphalt weight reduction. Researchers identified the constituents of asphalt as the fraction of asphaltenes, resins, naphthenics and paraffins in the 1940s and likewise concluded that aging was not only due to physical hardening but also to oxidation.

The concept of the aging index was introduced in the following decade and evaluated from the capillary viscosity values estimated before and after oxidation. A way to find a relationship between the asphalt´s chemical composition and its viscosity was also sought. Ten years later, researchers studied the effect of oxidation on the change of generic fraction content in asphalt, relying on infrared spectroscopy results.

The implementation of asphalt´s rheological evaluation from elastic modulus and yield curve measurements began in the 1970s. It was continued by estimating capillary viscosities, ductility and penetration as indicators of oxidation change, although these tests were entirely empirical and inadequate for describing asphalt´s linear viscoelastic properties.

Researchers continued measuring penetration and ductility in the 1980s and used plate viscosimeter to take viscosity measurements

Several years later, they began to evaluate the viscoelastic response of bituminous materials to relate their physical and chemical properties and, fundamentally, to develop a specification of asphalt types related to their performance from dynamic mechanical properties determined by dynamic shear rheometer.

New procedures and equipment for evaluating bituminous materials´ rheological properties were developed by the SHRP. The Brookfield rotational viscosimeter replaced capillary tube viscosimeters for determining viscosity-temperature curves to estimate asphalt properties at mixing, compactation and pumping temperatures. The dynamic shear rheometer (DSR) was selected for measuring asphalt stiffness at high and intermediate temperatures reached in pavements and the torsion beam rheometer was chosen to measure asphalt pavement flow properties at low temperatures. These two devices were used for measuring the rheological properties of asphalt at a wide range of temperatures and loading times and have been used by various researchers.

Meanwhile, Choplin and Marchal (1996, 1997, 1999) developed the concept of the rheo-reactor for in situ monitoring of physical and chemical processes. The methodology was based on reproducing or simulating a process or one of its parts in mini or microreactors coupled to rheometers and the instrument equipped this way was called a process rheometer. The first step consisted of searching for relevant rheological information, followed by rheological follow-up in situ.

One of the rheo-reactor applications conducted by Choplin and Marchal (1999) was the formulation of a polymer and sulphur modified asphalt through viscosity change by using helical ribbon type mixing under stable rotation. The researchers were able to follow the effect of incorporating other compounds during formulation which, in this case, consisted of dispersion and cross-linking by monitoring absolute viscosity values. Once formulation was finished, they evaluated the material´s performance by oscillatory tests with the same mixing device; the complex modulus (G*) was determined as a function of temperature, obtaining permanent (rutting) deformation criteria data proposed by SHRP in minimum time and without the need for sampling.

Researchers such as Airey et al., (2004), Navarro et al., (2004, 2005) and Ruan et al., (2003), found that the rheological responses of pure and modified asphalt depended on oxidation conditions and agreed with other authors that asphalt oxidation in the laboratory at high temperatures may influence the material´s rheology differently than when oxidation is achieved in real conditions.

The results of the study on the effect of TFOT and rolling thin film oven test (RTFOT) oxidation on the rheology of pure and modified asphalt reported in other publications (Lu and Isacsson (2002); Polacco et al., (2004); Shenoy (2002); Navarro et al., (2004); Cortizo et al., (2004); Airey et al., (2004); Pérez-Lepe et al., (2003)) agree on an increase in complex modulus (G*), storage modulus (G´) and decrease in phase angle ( ) after oxidation aging. They found thataging has a minimal effect on the modulus of the asphalt obtained at low temperatures (<0°C), but at higher temperatures (0-70°C) they observed a sharp decrease of phase angle and concluded that the dynamic modules of asphalt are less susceptible to temperature after aging (Lu and Isacsson 2002).

Lesueur (2002) suggested that the rheological properties of asphalt needed to obtain good material performance should include good pumping facility at high temperatures (160°C), sufficient stiffness at moderate service temperatures (60°C) to avoid viscous flow and should be properly soft at low service temperatures to resist material fatigue cracks. Achieving these properties simultaneously in bituminous materials is not easy and therefore it is used for asphalt modification and will continue to be part of future investigations.

Authors such as Sirin et al., (1998, 2000) found difficulties when they tried to evaluate polymer-modified asphalts using the RTFOT technique like irregular aging, formation of films on the asphalt´s surface and no homogeneity in the asphalt-polymer systems. In 1998, Bahia et al., documented a new RTFOT protocol for modified asphalts. Another alternative to the aging method was proposed by Glover et al., (2003), who proposed a new method for simulating aging, in which asphalt is exposed to hot-mixing plants called stirred air-flow test (SAFT) which oxidise asphalt by agitation and air-blow thereby avoiding the problems encountered in the modified asphalts. With this new method, the authors attempted to eliminate problems like inconsistencies due to the formation of membranes on asphalt surface, difficulties in processing polymermodified asphalt and in asphalt recuperation and cleaning of the equipment after the experiments. The procedure takes half the time required for RTFOT.

X.A. Vargas (2007), X.A. Vargas et al., (2008) associated structural changes in asphalt samples exposed to in situ thermo-oxidation in a rheo-reactor with changes in its viscoelastic properties to obtain information on the evolution of the complex structure of asphalts induced by aging. The study proposed a new asphalt aging methodology using continuous agitation in a rheo-reactor in which it was possible to monitor in situ viscosity changes of asphalt as it was aging in controlled conditions. Such in situ aging in a rheo-reactor allowed simulating hot mixing asphalt in controlled conditions close to reality. The author found that the asphalt´s initial and aged viscoelastic response could be represented by a power law in the experimental frequency window for the first time; its decreasing exponent 'n´ values suggested the asphalt´s degree of aging. This research proved that variation in this exponent 'n´ value was an indicator of the evolution of the asphalt structure caused by aging and the decrease of temperature inducing a change in asphalt´s viscoelastic behaviour. The new rheological model represents the increasing connectivity between thermo-aged asphalt molecules. A pseudo-gel point and critical 'n´ relaxation exponent similar to that observed in the polymers by the so-called Winter-Chambon criterion were observed in samples of thermo-oxidised asphalt in the rheo-reactor for the first time. The minimum phase angle observed in the aged asphalt´s Black diagram showed relaxation towards a higher structuring of asphalt due to aging.

However, the TFOT and RTFOT tests are still used today as standard tests for evaluating the effect of some asphalt modifiers on resistance to aging. Some examples would be Chunfa et al., (2006), Feng et al., (2010), Jian-Ying et al., (2009) and Peiliang et al., (2010).

Conclusions

The background shows the evolution of the study of asphalt aging which was initially considered a physical hardening process; over time, the phenomenon´s complexity has become recognised, trying at first to estimate it indirectly from the change in physiochemical properties and empirical tests like ductility, penetration, ring and ball, and penetration indices, temperature susceptibility etc., until reaching rheological determinations like the material´s viscoelastic properties once the authors understood the complex rheological behaviour of asphalt. Tests developed for simulating asphalt oxidative aging have been similarly evolving to aging tests in continuous agitation with in situ measures of the viscoelastic properties, since some authors have proved that the traditional tests are not suitable for aging simulations of either pure or modified asphalt.

In situ aging in controlled agitation conditions may be extended to replacing conventional aging techniques which, besides permitting in situ rheology, allow kinetic studies in controlled agitation conditions for asphalt aging and modification in different conditions of interest.

Analysis of asphalt´s chemical composition is still used in the Fourier transform infrared spectroscopy (FTIR), x-ray, ultraviolet and visible diffraction, nuclear magnetic resonance, liquid chromatography and thermogravimetric analysis to estimate chemical composition changes in asphalt before and after aging and material modification.

Future work will be aimed at searching for new asphalt modifiers leading to improving its rheological properties and durability. Nanocomposite asphalt is being developed on a global level but remains a largely unexplored area of research in Colombia.

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