State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials

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International Journal of Pavement Engineering

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State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials

G.D. Airey

a Nottingham Centre for Pavement Engineering, University of Nottingham, University Park, NGRD, 7 2, Nottingham, UK

Version of record first published: 31 Jan 2007.

To cite this article: G.D. Airey (2003): State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials, International Journal of Pavement Engineering, 4:3, 165-176

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State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials

G.D. AIREY*

Nottingham Centre for Pavement Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK (Received 29 July 2002; Revised 18 December 2003)

The findings of an extensive literature review on bitumen and asphalt mixture ageing test methods are presented in the paper. The primary factors affecting the durability of bituminous paving mixtures, assuming they are constructed correctly, are age hardening and moisture damage. Ageing of the bituminous binder is manifested as an increase in its stiffness (or viscosity). Water damage is generally manifested as a loss of cohesion in the mixture and/or loss of adhesion between the bitumen and aggregate interface (stripping). Short-term ageing is primarily due to volatilisation of the bitumen within the asphalt mixture during mixing and construction, while long-term ageing is due to oxidation and some steric hardening in the field. Of the tests used to simulate short-term ageing, the extended heating procedures of the thin film oven test (TFOT) and the rolling thin film oven test (RTFOT) are the most frequently used binder methods. In regard to long-term binder ageing, the oxidative pressure ageing vessel (PAV) test and the rotating cylinder ageing test (RCAT) have shown the greatest potential.

Asphalt mixture ageing is primarily limited to extended heating methods for loose bituminous material prior to compaction and combinations of extended oven ageing, high and low pressure oxidation and ultraviolet and infrared light treatments.

Keywords: Bitumen; Ageing; Asphalt mixtures; Oxidation; TFOT; RTFOT

INTRODUCTION

The primary factors affecting the durability of bituminous paving mixtures, assuming they are constructed correctly, are age hardening and moisture damage. Ageing of the bituminous binder is manifested as an increase in its stiffness (or viscosity). Water damage is generally manifested as a loss of cohesion in the mixture and/or loss of adhesion between the bitumen and aggregate interface (stripping).

Ageing (hardening) is primarily associated with the loss of volatile components and oxidation of the bitumen during asphalt mixture construction (short-term ageing) and progressive oxidation of the in-place material in the field (long-term ageing). Both factors cause an increase in viscosity of the bitumen and consequential stiffening of the mixture. Other factors may also contribute to ageing, such as molecular structuring over time (steric hardening) and actinic light (primarily ultraviolet radiation, particularly in desert conditions). Oxidation, volatile loss and thixotropic effects (steric hardening) tend to be universally accepted as the three dominant factors affecting age

hardening. However, the precise list of factors differs with Petersen (1984) listing the three composition-related factors mentioned above, Vallergaet al.(1957) suggesting six factors while Traxler (1963) suggests an additional nine factors. Age hardening can have two effects, either increasing the load-bearing capacity and permanent deformation resistance of the pavement by producing a stiffer material or reducing pavement flexibility resulting in the formation of cracks with the possibility of total failure (Vallerga, 1981).

Tests related to ageing of bituminous materials can be broadly divided into two categories, namely; tests performed on bitumens and tests performed on bituminous (asphalt) mixtures. Much of the research into the ageing of bitumen utilises thin film oven ageing to age the bitumen in an accelerated manner (e.g. thin film oven test (TFOT), rolling thin film oven test (RTFOT), rolling microfilm oven test, tilt-oven durability test). Typically, these tests are used to simulate the relative hardening that occurs during the mixing and laying process (i.e. short-term ageing). To include long-term hardening in the field, thin film oven ageing is typically combined with pressure

ISSN 1029-8436 print/ISSN 1477-268X onlineq2003 Taylor & Francis Ltd DOI: 10.10850/1029843042000198568

*E-mail: [email protected]

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oxidative ageing (e.g. Iowa durability test, SHRP-PAV, HiPAT and Rotating Cylinder Ageing Test (RCAT)).

This paper contains a critical review of existing test methods, protocols and techniques for assessing the age hardening of bituminous paving materials. Both binder and mixture tests have been reviewed with particular attention being given to highlighting the advantages and disadvantages of the different methods and the suitability of the tests for both modified and unmodified binders.

AGEING TESTS FOR BITUMINOUS BINDERS Numerous attempts have been made by researchers over the last seventy years to correlate accelerated laboratory ageing of bitumen with field performance. Most of this research has used thin film ovens to age the bitumen in an accelerated manner, with most of the thin film oven ageing methods relying on extended heating (oven volatilisation) procedures. The ageing tests are presented in Table I.

Extended Heating Procedures

Extended heating procedures tend to be used to simulate short-term ageing (hardening) of bitumen associated with asphalt mixture preparation activities. The most commonly used standardised tests, to control the short-term ageing of conventional, unmodified bitumen, are TFOT, RTFOT and the rotating flask test (RFT).

Thin Film Oven Test

The TFOT was first introduced by Lewis and Welborn (1940) to differentiate between bitumens with different volatility and hardening characteristics. In the TFOT, a 50 ml sample of bitumen is placed in a flat 140 mm diameter container resulting in a film thickness of 3.2 mm.

Two or more of these containers are then positioned on a rotating shelf (5 to 6 rpm) in the oven for 5 h at 1638C.

The TFOT was adopted by AASHTO in 1959 and by ASTM in 1969 (ASTM D1754, 1995a) as a means of evaluating the hardening of bitumen during plant mixing.

TABLE I Bitumen ageing methods

Test method

Temperature

(8C) Duration

Sample size

(g) Film thickness Extra features Thin film oven test (TFOT)

(Lewis and Welborn, 1940)—ASTM D1754, EN 12607-2

163 5 h 50 3.2 mm –

Modified thin film oven test (MTFOT) (Edleret al., 1985)

163 24 h – 100mm –

Rolling thin film oven test (RTFOT) (Hveemet al., 1963)—AASHTO T240, ASTM D2872, EN12607-1

163 75 m 35 1.25 mm Air flow—4000 ml/min

Extended rolling thin film oven test (ERTFOT) (Edleret al., 1985)

163 8 h 35 1.25 mm Air flow—4000 ml/min

Nitrogen rolling thin film oven test (NRTFOT) (Parmeggiani, 2000)

163 75 m 35 1.25 mm N2flow—4000 ml/min

Rotating Flask Test (RFT)—DIN 52016, EN12607-3

165 150 m 100 – Flask rotation—20 rpm

Shell microfilm test (Griffinet al., 1955) 107 2 h – 5mm –

Modified Shell microfilm test (Hveemet al., 1963)

99 24 h – 20mm –

Modified Shell microfilm test

(Traxler, 1961; Halstead and Zenewitz, 1961)

107 2 h – 15mm –

Rolling microfilm oven test (RMFOT) (Schmidt and Santucci, 1969)

99 24 h 0.5 20mm Benzene solvent

Modified RMFOT (Schmidt, 1973) 99 48 h 0.5 20mm 1.04 mmfopening

Tilt-oven durability test (TODT) (Kemp and Prodoehl, 1981)

113 168 h 35 1.25 mm –

Alternative TODT (McHattie, 1983) 115 100 h 35 1.25 mm –

Thin film accelerated ageing test (TFAAT) (Petersen, 1989)

130 or 113 24 or 72 h 4 160mm 3 mmfopening

Modified rolling thin film oven test (RTFOTM) (Bahiaet al., 1998)

163 75 m 35 1.25 mm Steel rods

Iowa durability test (IDT) (Lee, 1973)

65 1000 h TFOT residue—50 3.2 mm 2.07 MPa—pure oxygen

Pressure oxidation bomb (POB) (Edleret al., 1985)

65 96 h ERTFOT residue 30mm 2.07 MPa—pure oxygen

Accelerated ageing test device/Rotating cylinder ageing test (RCAT) (Verhasselt and Choquet, 1991)

70 – 110 144 h 500 2 mm 4 – 5 l/h—pure oxygen

Pressure ageing vessel (PAV) (Christensen and Anderson, 1992)

90 – 110 20 h RTFOT or TFOT residue—50

3.2 mm 2.07 MPa—air High pressure ageing test (HiPAT)

(Haytonet al., 1999)

85 65 h RTFOT residue—50 3.2 mm 2.07 MPa—air

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However, a major criticism of the TFOT is that the thick binder film which results in a large volume to exposed surface area for the aged binder. As the bitumen is not agitated or rotated during the test, there is a concern that ageing (primarily volatile loss) may be limited to the

“skin” of the bitumen sample.

This concern over the testing of bitumen in relatively thick films meant that there was a move, from the 1950s, to develop or modify ageing tests to age and test bitumen in microfilm thicknesses. One such example is the modified thin film oven test (MTFOT), used by Edleret al.(1985), where the binder film is reduced from 3.2 mm to 100mm with an additional increased exposure time of 24 h.

This minor modification of the TFOT was done in order to increase the severity of the ageing process to include oxidative hardening of the binder as well as volatile loss.

Rolling Thin Film Oven Test

The RTFOT is probably the most significant modification of the TFOT involving the placing of bitumen in a glass jar (bottle) and rotating it in thinner films of bitumen than the 3.2 mm film used in the TFOT. The RTFOT, therefore, simulates far better the hardening which bitumen under- goes during asphalt mixing (Hveem et al., 1963; Shell Bitumen Review, 1973).

The RTFOT was developed by the California Division of Highways and involves rotating eight glass bottles each containing 35 g of bitumen in a vertically rotating shelf, while blowing hot air into each sample bottle at its lowest travel position (Hveemet al., 1963). During the test, the bitumen flows continuously around the inner surface of each container in relatively thin films of 1.25 mm at a temperature of 1638C for 75 min. The vertical circular carriage rotates at a rate of 15 revolutions/min and the air flow is set at a rate of 4000 ml/min. The method ensures that all the bitumen is exposed to heat and air and the continuous movement ensures that no skin develops to protect the bitumen. The conditions in the test are not identical to those found in practice but experience has shown that the amount of hardening in the RTFOT correlates reasonably well with that observed in a conventional batch mixer (Whiteoak, 1990). The RTFOT was adopted by ASTM in 1970 as ASTM D2872 (1995b).

Several modifications have also been made to the RTFOT. However, most of them have been relatively minor, for example, Edleret al.(1985) used an extended time period of 8 h rather than 75 min in their extended rolling thin film oven test (ERTFOT), while Kemp and Prodoehl (1981) used 5 h. A more recent modification is the development of a nitrogen rolling thin film oven test (NRTFOT) to determine more accurately the actual loss of volatiles during the test (Parmeggiani, 2000).

The procedure is identical to the standard test except that nitrogen, rather than air, is blown over the exposed surface of the bitumen samples.

A similar application of the RTFOT with nitrogen gas is the rapid recovery test (RRT) used to obtain a quantity of

“recovered binder” from modified or unmodified cutback or emulsion binders (MCDHW, 1998). The procedure uses a temperature of 858C with the RTFOT to evaporate water and/or the light solvent or highly volatile fraction of emulsions or cutback binders. Nitrogen gas is used instead of air to minimize ageing effects.

Rotating Flask Test (DIN 52016)

The RFT method consists of ageing a 100 g sample of bitumen in the flask of the rotary evaporator for a period of 150 min at a temperature of 1658C. As the flask is rotated at 20 rpm, the material forming the surface of the specimen is constantly replaced thus preventing the formation of a skin on the surface of the bitumen.

Shell Microfilm Test

The shell microfilm test is another variation of the principal used with the TFOT. In this test a very thin, 5mm, film of bitumen is aged for 2 h on a glass plate at 1078C (Griffin et al., 1955). The thinner film thickness was chosen to simulate the film thicknesses that exist in asphalt mixtures. The bitumen is evaluated on the basis of viscosity before and after testing to provide an “ageing index”. However, there is limited reported correlation between field performance and laboratory ageing using the shell microfilm test (Welborn, 1979), except for the work done on the Zaca-Wigmore test roads (Zube and Skog, 1969). Simpsonet al.(1959) compared the viscosity data for bitumen recovered from the two test roads with the shell microfilm test and found a definite correlation between field and laboratory data.

The Shell microfilm test was modified slightly by Hveem et al. (1963) and Skog (1967) by increasing the film thickness to 20mm and the exposure time to 24 h with a slight reduction in temperature to 998C. These alterations did demonstrate an indirect relationship between field and laboratory hardening. Additional, slight variations were made by Traxler (1963) and Halstead and Zenewitz (1961), who increased the binder film thickness from 5 to 15mm.

Rolling Microfilm Oven Test

The rolling microfilm oven test (RMFOT) is a modification of the RTFOT in order to obtain much thinner films of bitumen for ageing (Schmidt and Santucci, 1969). The test consists of dissolving bitumen in benzene (solvent), coating the inside of the RTFOT bottles with this solution and then allowing the benzene to evaporate. The result of this process is the creation of a 20mm film of bitumen which is then aged at 998C for 24 h.

The RMFOT was modified by Schmidt (1973) in order to reduce the amount of volatile loss during ageing. This was accomplished by placing a capillary in the opening of the RTFOT bottle and calibrating the capillary size to match the volatile loss from the bottle to that achieved

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during the ageing of asphalt mixture specimens at 608C.

A 1.04 mm diameter opening was selected and in addition the ageing time was increased from 24 to 48 h.

The modified RMFOT was found to have good correlation with field cracking of the Zaca-Wigmore pavements as well as with other field and laboratory aged asphalt mixtures. The primary disadvantage of the test is the small amount of aged bitumen (0.5 g/bottle) that can be used for subsequent binder testing.

Tilt-oven Durability Test

An additional modification to the RTFOT is found in the California tilt-oven durability test (TODT) where the oven is tilted 1.068 higher at the front to prevent bitumen migrating from the bottles (Kemp and Prodoehl, 1981).

In addition, the TODT uses a lower temperature and longer time for ageing compared to the RTFOT, namely 168 h at 1138C. This level of ageing approximates to that found for pavement mixtures after 2 years in hot desert climates (Petersen, 1989). In addition, Kemp and Prodoehl (1981) aged laboratory produced specimens in four distinct climates in the field and concluded that the TODT could be used to predict hot climate hardening of bitumen.

A similar modification was reported by McHattie (1983) with test conditions of 100 h at 1158C. Both methods (168 h at 1138C and 100 h at 1158C) were evaluated by Santucci et al.(1981), who found the tests at 168 h and 1138C to be more severe.

Thin Film Accelerated Ageing Test

A modification of the RMFOT is the thin film accelerated ageing test (TFAAT), developed by Petersen (1989), which has the advantage of providing an increased amount of aged binder as it uses a sample size of 4 g of binder compared to the 0.5 g of the RMFOT. Whereas extended heating tests, such as the TFOT and RTFOT, reflect only the ageing (mainly volatile loss) that occurs during hot- plant mixing, the TFAAT was developed to produce a representative level of volatilisation and oxidation to simulate the level of oxidative age hardening typically found for extended pavement ageing.

The TFAAT was developed to complement a column oxidation procedure developed by Davis and Petersen (1967) where a 15mm thick bitumen film, coated on Teflon particles, was oxidised in a gas chromatographic column at 1308C for 24 h by passing air through the column. As the TFAAT uses eight times more binder than the RMFOT, with subsequent increased binder films, the TFAAT either has to have longer ageing times or higher test temperatures to achieve the same degree of oxidative ageing as that found for the RMFOT. Petersen (1989) found that performing the test at 1308C for 24 h produced the same degree of oxidative ageing found for the RMFOT as well as for 11 – 13 years old pavements. As with the RMFOT, the 31 mm diameter opening for the standard

RTFOT bottle was reduced to 3 mm to restrict excessive volatile loss. The TFAAT can also be performed at the lower temperature of 1138C but for a longer period of 3 days compared to the one day test at 1308C.

Modified Rolling Thin Film Oven Test

One of the main problems with using the RTFOT for modified bitumens is that these binders, because of their high viscosity, will not roll inside the glass bottles during the test. In addition, some binders have a tendency to roll out of the bottles. To overcome these problems, Bahiaet al.

(1998) developed the modified rolling thin film oven test (RTFOTM).

The test is identical to the standard RTFOT except that a set of 127 mm long by 6.4 mm diameter steel rods are positioned inside the glass bottles during oven ageing.

The principle is that the steel rods create shearing forces to spread the binder into thin films, thereby overcoming the problem of ageing high viscosity binders. Initial trials of the RTFOTM indicate that the rods do not have any significant effect on the ageing of conventional penetration grade bitumens (Bahia et al., 1998). However, recent work at the Turner-Fairbanks research centre has indicated that using the metal rods in the RTFOTM does not solve the problem of roll-out of modified binder and further validation work is required before the technique can be accepted.

The rapid recovery test (RRT) uses a similar mechanism to prevent the roll-out of emulsions or cutback binders but instead of steel rods the procedure uses 120 mm long by 12.2 mm diameter stainless steel or PTFE screws (MCDHW, 1998). The direction of the screw is such that the sample is drawn to the rear of the bottle during rotation in the RTFOT carousel. Oliver and Tredrea (1997) also used a roller with a screw thread to age polymer modified bitumens (PMBs) in the RTFOT where, as the bottle rotated, the roller “screwed” the binder towards the back wall of the bottle. Using their modified RTFOT with a exposure time of 9 h and a temperature of 1638C, Oliver and Tredrea were able to produce similar changes in the rheological properties of polymer modified and unmodified bituminous binders to those found after 2.5 years exposure in a sprayed seal in a hot climate.

Oxidative (Air Blowing) Procedures

Although thin film oven tests can adequately measure the relative hardening characteristics of bitumens during the mixing process they generally fall short of accurately predicting long-term field ageing. Attempts have been made to overcome this by combining thin film oven ageing with oxidative ageing.

Iowa Durability Test

The Iowa Durability Test (IDT) is one such test that combines thin film ageing with oxidative ageing

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(Lee, 1973). The test consists of ageing binder residue from a standard TFOT in a pressure vessel at 2.07 MPa using pure oxygen at a temperature of 658C for up to 1000 h. As the residue binder from the TFOT is not transferred from its container, the film thickness during the pressure-oxidation treatment is still 3.2 mm.

Lee found that ageing bitumen using the IDT produced a hyperbolic relationship similar to that found for binders aged in the field over a 5 year period. Based on this hyperbolic relationship and considerable field and laboratory data, Lee concluded that 46 h of ageing with the IDT is equivalent to 60 months field ageing for Iowa conditions (Lee, 1973).

Pressure Oxidation Bomb

Edleret al.(1985) used a similar approach to that used by Lee, where residue from their 8 h ERTFOT was followed by oxidation under pressure using the pressure oxidation bomb (POB). The POB consists of a cylindrical pressure vessel fitted with a screw-on cover containing a safety blow-off cap, pressure gauge and stopcock. The vessel houses a metal support where twelve 40£40 mm²glass plates coated with 30mm bitumen films are positioned horizontally. The test consists of ageing the bitumen residue at a pressure of 2.07 MPa at 658C for 96 h.

Accelerated Ageing Test Device/RCAT

Similar in concept to the RTFOT is the accelerated ageing test device developed at the Belgium Road Research Centre (BRRC) (Verhasselt and Choquet, 1991). Although standard tests such as the RTFOT and RFT can adequately simulate construction ageing, their high temperatures make them unsuitable for simulating field ageing. This has lead to the development of the accelerated ageing device which has been based on a theoretical kinetic approach to ageing (Verhasselt, 1996; 2000).

The device consists of a fairly large cylinder (tube), with an internal diameter of 124 mm and a length of 300 mm, which is capped at both ends but with a central aperture of diameter 43 mm at one end, where bitumen can be introduced and extracted (see Fig. 1) (Verhasselt, 2000).

After charging the cylinder with up to 500 g of bitumen, a stainless steel roller, 296 mm in length and 34 mm in diameter, is placed into the cylinder. The cylinder is then placed in a frame which rotates the cylinder at 1 revolution/min and flows oxygen through the aperture at a rate of 4 – 5 l/h (75 ml/min). Rotation of the roller within the cylinder distributes the bitumen into an even 2 mm thick film on the inner wall of the cylinder. Tests are conducted at temperatures between 70 and 1108C. At discrete intervals, approximately 20 – 25 g of bitumen is removed from the cylinder for subsequent testing. Due to the large initial quantity of bitumen, the procedure allows numerous evaluations to be made and progressive changes in the bitumen chemistry and physical properties to be investigated.

Using the accelerated ageing test device, now known as the rotating cylinder ageing test (RCAT) (Verhasselt, 2000), Choquet (1993) found that ageing bitumen at 858C for 144 h reflects field ageing with regard to the formation of asphaltenes. He also noted that temperatures less than 1008C were essential in accelerated ageing tests in order to produce chemical and rheological changes similar to those found in the field. Verhasselt (1997) also found mutual agreement between in-service ageing in the field and laboratory ageing using the RCAT for dense mixtures.

However, Franckenet al.(1997) found that longer ageing times than 240 h were required to simulate field ageing of porous mixtures.

Pressure Ageing Vessel (PAV)

The SHRP-A-002A research team developed a method using the PAV to simulate the long-term, in-service oxidative ageing of bitumen in the field (Christensen and Anderson, 1992; Petersen et al., 1994). The method involves hardening of bitumen in the RTFOT or TFOT followed by oxidation of the residue in a pressurised ageing vessel. The PAV procedure entails ageing 50 g of bitumen in a 140 mm diameter pan (ø3.2 mm binder film) within the heated vessel, pressurised with air to 2.07 MPa for 20 h at temperatures between 90 and 1108C (AASHTO PP1, 1993) (see Fig. 2).

Migliori and Corte (1999) investigated the possibility of simulating RTFOT (short-term ageing) and RTFOTþPAV (long-term ageing) simply by means of PAV testing for unmodified penetration grade bitumens.

They found that 5 h of PAV ageing at 1008C and 2.07 MPa was equivalent to standard RTFOT ageing, and that 25 h of PAV ageing at 1008C and 2.07 MPa was equivalent to standard RTFOTþPAV ageing.

Verhasselt and Vanelstraete (2000) compared the relative accelerated ageing obtained using the PAV at 1008C and the RCAT at 858C for a range of unmodified and polymer modified binders. They found that the changes observed (rheological properties, IR spectra) and reaction mechanisms involved are quite similar for both techniques. They established an equivalency between the two methods such that 20 h of PAV ageing

FIGURE 1 Rotating cylinder ageing test (after Verhasselt, 2000).

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approximately corresponds to 178 h of RCAT ageing.

However, they found that the higher temperature of the PAV resulted in some segregation of the polymer in some of the PMBs.

High Pressure Ageing Test

The high pressure ageing test (HiPAT) is a modification of the PAV procedure using a lower temperature of 858C and a longer duration of 65 h (Haytonet al., 1999). The reason for these modifications was the concern that the temperatures used in the PAV procedure were unreali- stically high compared to expected pavement temperatures. In addition it was felt, particularly for modified binders, that the procedure was liable to significantly alter the binders to an unrepresentative extent to that found in the field.

Initial studies to predict long-term ageing in the field have suggested that the HiPAT process may be more severe than the natural ageing process for a dense asphalt mixture with a 10 year service life (Hayton et al., 1999). However, the procedure shows potential as a tool to identify binders that age excessively in service.

An alternative to the HiPAT procedure is the extended recovery test which is an extension of the RRT used to age emulsions or cutbacks containing highly volatile oil (MCDHW, 1998). This procedure consists of maintaining samples of emulsion or cutback bitumen at 858C for 2 h in the RTFOT with nitrogen gas flow followed by a further 22 h with an air supply.

Ultraviolet and Infrared Light Treatments

The sun beams energy in the form of electromagnetic radiation in a wavelength band between 200 and 3000 nm (Bocci and Cerni, 2000). Approximately 7% of the solar radiation that reaches the surface of the earth is ultraviolet (UV) radiation (180 – 400 nm), 42% is within the visible band (400 – 800 nm) and 51% is infrared (IR) radiation (800 – 3000 nm). In the UV range, three different sub- ranges of increasing wavelength can be identified: UVC band (240 – 280 nm), UVB band (280 – 315 nm) and UVA band (315 – 400 nm). The relative importance of the three bands is governed by their intensity and wavelength with the shorter ones being more destructive.

The use of UV and IR light to age bitumen has been reported by Vallerga et al. (1957), where bitumen films were aged in TFOT containers. The UV treatment was found to be more effective in terms of changing the physical properties of the bitumen compared to the use of infrared light.

Traxler (1963) used actinic light to simulate the photochemical ageing of bitumen. His data shows that the photochemical reaction has a significant effect on thin films of bitumen (3mm) but that the effect decreases for thicker films.

Monteparaet al.(1996) developed an ultraviolet ageing chamber for the long-term ageing of conventional paving grade bitumen. The chamber uses a mercury gas lamp with a frequency band between 180 and 315 nm (UVC and UVB). Bitumen is heated to 1408C and spread on glass plates (25 £ 20 cm²) to obtain a binder film thickness of

FIGURE 2 Pressure ageing vessel (after Christensen and Anderson, 1992).

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approximately 1.5 mm. The plates are then positioned on an ageing bench at a set distance below the lamp and aged for 450 days (equivalent to approximately 2000 solar days). At 20-days intervals, bitumen samples from the glass plates are subjected to standard physical tests (penetration, softening point and viscosity) as well as nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) testing. The results show clear evidence of volatilisation, oxidation and polymeri- sation of the bitumen due to ageing under UV radiation.

Montepara and Giuliani (2000) compared the relative ageing produced by RTFOT, UV radiation and PAV ageing. They subjected two conventional penetration grade bitumens to UV radiation using a 2000 W, high UV ray density emission lamp for equivalent solar exposure periods of 1, 2, 6 and 10 years after RTFOT ageing.

The results show that UV ageing produces a reduced ageing effect compared to PAV ageing.

Bocci and Cerni (2000) developed an alternative UV standardized ageing procedure. The procedure attempts to simulate UV radiation exposure corresponding to 4.6 – 14.5 years as measured at 40 reference stations throughout western Europe. This accumulated radiation corresponds to a fixed energy quantity of 360,000 Wh/m². In the UV ageing method, 30 g of bitumen is placed in a container and heated to produce a uniform layer of 1 mm.

Identical containers are then placed in a specially prepared radiation room equipped with an iron vapour light at high UVA, UVB and UVC radiation emissions. The bitumen samples are then aged for between 12 and 35 days, depending on their position in the room relative to the lamp, in order to accumulate energy equal to 360,000 Wh/m².

Initial trials with the UV ageing procedure, using a range of binders, show that standard extended heating and oxidative procedures (RTFOT followed by PAV) produce different ageing effects to that obtained from the photochemical process. This indicates that the ageing results obtained by photochemical treatments cannot generally be reproduced by thermal-oxidative treatments, particularly for binders that are susceptible to UV ageing.

There may therefore be a need to combine photochemical techniques with extended heating and oxidative procedures to simulate long-term field ageing of bituminous materials.

Edler et al. (1985) developed a weatherometer to simulate climatic conditions on the road, with part of the test comprising UV light treatment. The weatherometer consists of a cabinet housing, a revolving sample holder, a temperature controlled environment, an ultraviolet light source and a sprinkling device. The test consists of ageing 100mm bitumen films, coated on 50 £ 50 mm glass plates, at 658C during a 2 h cycle comprising a 102 min cycle of UV light only and 18 m of UV light and water spray at a pressure of 300 kPa. Test durations of 32.5 h, 73.5 h, 7 days and 14 days were used.

Kuppenset al.(1997) used a special climate chamber (oven) to simulate the ageing of porous asphalt under

Dutch climatic conditions. The procedure consists of subjecting bitumen, over a 24 h period, to 16 1/4 h of UV light at 508C, 4 h rain with NaCl at 408C, 1 h water at 208C and 2 3/4 h dry at2208C. The procedure therefore, attempts to simulate both field ageing and water damage and can be repeated as often as required. However, evaluation of the procedure showed a very poor correlation with field performance.

Microwave Ageing

Bisharaet al.(2000) have developed a microwave method of ageing neat, unmodified bitumen to give a product equivalent to that produced by the combined ageing achieved with RTFOT followed by PAV ageing. The one- step approach consists of subjecting bitumen to microwave radiation at a temperature of 1478C and an air pressure of 3.08 MPa for 4.5 h at an output power of approximately 1000 W. Based on physical as well as chemical analysis, the results from the microwave method were found to be comparable to those obtained for RTFOTþPAV ageing.

Steric Hardening

Traxler (1963) identified molecular structuring (thixo- tropy), which results in steric hardening, as one of his 15 effects that reduce the binding properties of bitumen.

Steric hardening is mostly reversed by heating or mechanically working the bitumen, but a portion may be permanent depending on the composition of the bitumen.

However, there are currently no test methods that address steric hardening.

AGEING TESTS FOR ASPHALT MIXTURES In addition to artificially ageing binders, a number of methods also exist for artificially ageing the bituminous (asphalt) mixture. These can broadly be divided into four categories:

. Extended heating procedures;

. Oxidation tests;

. Ultraviolet/Infrared treatment; and . Steric hardening.

The basic procedure is to artificially age the mixture and then assess the effect of ageing on key material parameters (e.g. stiffness, viscosity, strength etc). Extended heating procedures typically expose the mixture to high temperatures for a specified period(s) of time before suitable testing (e.g. compressive testing, tests on recovered binder, etc). Oxidation tests typically utilize a combination of high temperature and pressure oxidation to laboratory age specimens. Ultraviolet/infrared treatment involves exposing specimens to either ultraviolet or infrared radiation.

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Most of the initial studies on asphalt mixture ageing involved tests on the recovered binder as detailed by Hubbard and Gollomb (1937) and Shattuck (1940).

Understandably these tests relied on acceptable and sound procedures for extracting and recovering bitumen from the asphalt mixtures (Abson, 1933).

A large percentage of the initial laboratory ageing procedures used Ottawa sand as a standard “aggregate”

with the tests being done with ultraviolet light as well as extended exposure to heat and air (Lang and Thomas, 1939). A list of asphalt mixture ageing tests is presented in Table II.

Extended Heating Procedures

Pauls and Welborn (1952) exposed 50 £ 50 mm² cylinders of an Ottawa sand mixture to 1638C (TFOT and RTFOT ageing temperature) for various time periods.

The compressive strength of the cylinders, as well as the consistency of the recovered binder, were compared to that of the original (unaged) material. Results from this study indicated that bitumen recovered from laboratory aged specimens or aged in the TFOT could be used to assess the hardening properties of bitumens. However, the results did not suggest that the TFOT could predict long- term field ageing.

Plancher et al. (1976) used a similar oven ageing procedure to age 25 £ 40 mm diameter specimens at 1508C for 5 h. After this accelerated ageing, the samples were cooled to 258C for 72 h and subjected to resilient modulus tests. Kemp and Prodoehl (1981) aged Ottawa sand mixtures in an oven at 608C for up to 1200 h.

The bitumen was then recovered and tested. However, they preferred to use the TODT to age bitumen as it produced much larger quantities of bitumen compared to the Ottawa sand mixtures.

Hugo and Kennedy (1985) oven aged asphalt specimens that had been cored from laboratory compacted slabs at 1008C for 4 or 7 days under either dry atmosphere or 80%

relative humidity conditions. Bitumen, recovered from

the aged cores, was then subjected to viscosity testing.

In addition, the samples were weighed before and after ageing and the weight loss used to indicate volatile loss.

Most of the methods used for laboratory ageing of asphalt mixtures involve the ageing of compacted asphalt mixture specimens. However, Von Quintas (1988) investigated the use of force draft oven ageing to simulate short-term “production” hardening on loose mixture samples. In this method, loose asphalt material was heated at 1358C in a force draft oven for periods of 8, 16, 24 and 36 h. Although this method showed similar levels of ageing to those found in the field, there was considerable scatter in the laboratory data.

Short- and long-term ageing procedures were also developed under the SHRP-A-003A project. The SHRP short-term oven ageing (STOA) procedure is based on the work done by Von Quintas (1988). The procedure requires loose mixtures, prior to compaction, to be aged in a forced draft oven for 4 h at 1358C (AASHTO PP2). The process was found to represent the ageing occurring during mixing and placing and also represents pavements of less than two years (Bellet al., 1994; Monismithet al., 1994; Bell and Sosnovske, 1994).

Scholz (1995) developed a similar short-term ageing procedure to simulate the amount of hardening which occurs during the construction process for continuously graded mixtures. The procedure is similar to the SHRP STOA procedure except that the temperature is either 1358C or related to the desired compaction temperature, whichever is higher, and that the period of conditioning is limited to 2 h (Brown and Scholz, 2000).

Von Quintas (1988) also investigated long-term ageing using a force draft oven where compacted asphalt mixture specimens were aged for 2 days at 608C followed by 3 days at 1078C. However, Bell (1989) comments that the elevated temperature level used in the test may cause specimen disruption, particularly for high void content and/or high penetration grade asphalt mixtures.

Two alternative long-term ageing procedures were developed under the SHRP-A-003A project, namely

TABLE II Asphalt mixture ageing methods

Test method Temperature (8C) Duration Sample size Extra features

Production ageing (Von Quintas, 1988) 135 8, 16, 24, 36 h Loose material –

SHRP short-term oven ageing (STOA) 135 4 h Loose material –

Bitutest protocol (Scholz, 1995) 135 2 h Loose material –

Ottawa sand mixtures (Pauls and Welborn, 1952) 163 Various periods 50£50 mm²cylinders –

Plancheret al.(1976) 150 5 h 25£40 mm²f –

Ottawa sand mixtures (Kemp and Prodoehl, 1981) 60 1200 h – –

Hugo and Kennedy (1985) 100 4 or 7 days – 80% relative humidity

Long-term ageing (Von Quintas, 1988) 60 2 days Compacted specimens –

107 3 days

SHRP long-term oven ageing (LTOA) 85 5 days Compacted specimens –

Bitutest protocol (Scholz, 1995) 85 5 days Compacted specimens –

Kumar and Goetz (1977) 60 1, 2, 4, 6, 10 days Compacted specimens Air at 0.5 mm of water

Long-term ageing (Von Quintas, 1988) 60 5 to 10 days Compacted specimens 0.7 MPa—air

Oregon mixtures (Kimet al., 1986) 60 0, 1, 2, 3, 5 days Compacted specimens 0.7 MPa—air

SHRP low pressure oxidation (LPO) 60 or 85 5 days Compacted specimens Oxygen—1.9 l/min

Khalid and Walsh (2000) 60 Up to 25 days Compacted specimens Air—3 l/min

PAV mixtures (Korsgaard, 1996) 100 72 h Compacted specimens 2.07 MPa—air

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long-term oven ageing (LTOA) of compacted specimens in a force draft oven and low pressure oxidation (LPO) of compacted specimens in a modified triaxial cell.

The LTOA procedure requires that after STOA, the loose material should be compacted and placed in a force draft oven at 858C for 5 days (AASHTO PP2, 1994) (Harrigan et al., 1994). The parameters used for LTOA are meant to represent 15 years of field ageing in a Wet-No-Freeze climate and 7 years in a Dry-Freeze climate. However, field validation of the LTOA indicates that 8 days at 858C is equivalent to over 9 years for Dry-Freeze and over 18 years for Wet-No-Freeze; 2 days at 858C is equivalent to 2 – 6 years for both Dry-Freeze and Wet-No-Freeze; and 4 days at 858C is equivalent to 15 years of field ageing in a Wet-No-Freeze climate and 7 years in a Dry-Freeze climate (Bell et al., 1994; Monismith et al., 1994; Bell and Sosnovske, 1994). The details of the LPO procedure are given in section “Pressure Oxidation Bomb”.

In association with his short-term procedure, Scholz (1995) developed a long-term oven ageing procedure for compacted asphalt mixture specimens. The procedure is identical to the SHRP LTOA procedure consisting of force draft oven ageing of compacted specimens at 858C for 120 h (Brown and Scholz, 2000).

Oxidative (Air Blowing) Procedures

Kumar and Goetz (1977) developed a method consisting of ageing specimens at 608C for periods of 1, 2, 4, 6 and 10 days while “pulling” air through the compacted specimens at a constant head of 0.5 mm of water. The low head was used to avoid turbulence in the air flow through the specimen.

A valuable feature of the research undertaken by Kumar and Goetz is the quantifying of film thickness and permeability. For open graded mixtures, the ratio of film thickness to permeability is the best predictor of resistance to ageing. However, for dense mixtures, permeability is the best predictor. It should be noted that Goode and Lufsey (1965) also concluded that permeability was

a better indicator of ageing susceptibility than void content. In addition to oven ageing of loose material and compacted specimens, Von Quintas (1988) also used a pressure oxidation treatment. The procedure consisted of conditioning compacted specimens at 608C at a pressure of 0.7 MPa for 5 – 10 days.

Kim et al. (1986) used a similar pressure oxidation treatment on compacted specimens of Oregon mixtures.

Samples were subjected to oxygen at 608C and 0.7 MPa for 0, 1, 2, 3 and 5 days. The effects of ageing were evaluated by indirect tensile stiffness and indirect tensile fatigue. Although the stiffness results generally increased with ageing, some mixtures showed an initial decrease in stiffness in the early part of the ageing procedure. This was attributed to a loss of cohesion in the samples at the temperature of 608C used in the ageing test. Similar results were found by Von Quintas et al.(1988) and therefore, some confinement of the samples may be desirable at the elevated temperatures used in these tests. This will probably not be an issue for high modulus materials.

One of the long-term ageing procedures that were developed under the SHRP-A-003A programme was a LPO procedure, carried out on compacted specimens after they had been short-term aged. The procedure consists of passing oxygen through a confined triaxial specimen at 1.9 l/min at either 60 or 858C for a period of 5 days.

Khalid and Walsh (2002) developed a LPO test for accelerated ageing of porous asphalt. The system consists of feeding compressed air, at a flow rate of 3 l/min, through a series of heat exchange coils and then through a number of porous asphalt samples (see Fig. 3). A test temperature of 608C was used and a rubber membrane was fitted over the samples to ensure that air flowed through the samples instead of around its periphery. The technique has been shown to recreate the ageing effect produced by the SHRP LTOA procedure, although due to its lower temperature, longer ageing times are required (Khalid and Walsh, 2000).

Korsgaardet al. (1996) used the PAV to age gyratory compacted dense asphalt mixture specimens rather than

FIGURE 3 Low pressure oxidation technique for porous asphalt (after Khalid and Walsh, 2002).

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bitumen. Based on recovered binder properties they determined an optimum ageing procedure consisting of PAV ageing for 72 h at 2.07 MPa and 1008C, but concede that 60 h may be more appropriate for more porous mixtures.

Ultraviolet and Infrared Light Treatments

Hveemet al.(1963) describe an infrared weathering test for Ottawa sand mixtures. The test consists of subjecting the sand-bitumen mixture, in a semi-compacted state, to infrared radiation at a constant mass temperature of 608C and a maintained air stream across the specimen of 418C. The size distribution of the sand and the binder content of 2% ensures a uniform film thickness of 5 – 7mm. Based on the calibration of the test, 1000 h of exposure in the weathering machine is approximately equal to 5 years field ageing.

Kemp and Prodoehl (1981) also used an actinic light weathering test at a temperature of 358C for 18 h duration with 1000 MW/cm² Amgtro¨m actinic radiation. The authors note that the weathering test only measures the hardening within the outer 5mm of the bitumen film irrespectively of different bitumen film thicknesses.

Hugo and Kennedy (1985) used two approaches to evaluate the effect of UV radiation on asphalt mixtures.

The first method was similar to that used by Traxler (1963) to age bitumen and used 54 h of UV exposure. The second method used an Atlas weatherometer for a period of 14 days. Compared to the weatherometer used by Edler et al.(1985) to age pure binder, the levels of ageing were found to be considerably lower.

Tia et al. (1988) used a series of ageing procedures consisting of convection oven ageing at 608C, force draft oven ageing at 608C and ultraviolet light ageing at 608C for various periods. They recommended an improved ageing procedure incorporating both ultraviolet light and forced draft oven heating. In addition they identified UV-light as a major cause of mixture ageing although the resultant effect is a surface one, or at least not at any significant depth into the mixture.

Steric Hardening

The only test method that attempts to measure the steric hardening of paving grade bitumens is the cohesiograph test (Hveemet al., 1963). The test involves making four 305 mm long semi-cylindrical specimens using Ottawa sand. Two of the specimens are tested immediately in the cohesiograph whereby the long, slender specimens are extruded out of a support such that they act as cantilevers and break into short sections at the test temperature of 238C. The remaining two specimens are tested in the same way after being cured at 608C for 24 h. Any differences between the two sets can be attributed to oxidative ageing, volatile loss or “structuring” of the bitumen. However, if the cured (second set) specimens are remoulded and re-tested and the readings reduce to that of the unaged

(first set) specimens then any differences can be attributed to steric hardening rather than oxidative ageing or volatile loss.

SUMMARY AND CONCLUSIONS

The ageing of asphalt mixtures occurs essentially in two phases, namely short- and long-term. Short-term ageing is primarily due to volatilisation of the bitumen within the asphalt mixture during mixing and construction, while long-term ageing is due to oxidation and some steric hardening in the field. Tests related to the ageing of bituminous materials can be divided into tests performed on the bitumen and those performed on the asphalt mixture.

The most commonly used short-term binder ageing tests are the high temperature TFOT and RTFOT used to simulate the hardening that occurs during asphalt mixture production. Considerable evidence exists to indicate that the RTFOT and similar extended, high temperature, heating test methods are able to simulate short-term ageing for conventional bituminous binders. However, operational difficulties associated with the ageing of PMBs has necessitated the modification of the RTFOT testing procedure and apparatus with the positioning of steel rods within the glass bottles to reduce binder films and prevent roll-out. In addition, bitumen aged in the TFOT and RTFOT experience higher volatile loss during testing compared to that experienced during low temperature field ageing of pavement mixtures, while the levels of oxidative ageing in the tests is considerably lower than that found during field ageing. Therefore, these extended heating tests have a limited ability to estimate the long-term ageing of bitumen in asphalt pavements.

Based on the inability of these high temperature oven ageing tests to predict field ageing, tests such as the shell microfilm test, RMFOT, TFAAT and others were introduced with reduced temperatures and increased ageing times. However, most of these tests tend to produce relatively small quantities of aged bitumen for further testing or require excessively long ageing times to age larger quantities of binder. Currently, the most commonly used binder tests to simulate long-term ageing are the PAV and RCAT. In terms of long-term ageing, no one test seems to be satisfactory for all cases and the RCAT method, based on a kinetic approach to ageing, is probably the most acceptable. Like the RCAT method, the HiPAT procedure makes use of a lower temperature and extended time to simulate long-term ageing, compared to the PAV.

The most promising methods for short-term ageing of asphalt mixtures are extended heating of the loose material and extended mixing. The most promising methods for long-term ageing of mixtures include extended oven ageing, such as the SHRP long-term oven ageing method, pressure oxidation, using low pressure oxidation as well as pressurised procedures, and ultraviolet and infrared light

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treatments. In terms of sun radiation, the high absorption coefficient of bitumen in the ultraviolet range means that the influence of UV is limited to the top 1 – 2 mm of the surface and can generally be neglected (Verhasselt, 1991). However, the influence of infrared radiation should not be neglected as its absorption results in considerable increase in mean temperature which simulates oxidative reactions in the bitumen.

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