See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/245326204 State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials Article in International Journal of Pavement Engineering · September 2003 DOI: 10.1080/1029843042000198568 CITATIONS READS 319 3,313 1 author: Gordon D. Airey University of Nottingham 299 PUBLICATIONS 9,592 CITATIONS SEE PROFILE All content following this page was uploaded by Gordon D. Airey on 09 December 2016. The user has requested enhancement of the downloaded file. International Journal of Pavement Engineering State Of The Art Report on Ageing Test Methods for Bituminous Pavement Materials G. D. Airey School of Civil Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD UK 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, 165176 To link to this article: http://dx.doi.org/10.10850/1029843042000198568 1 State Of The Art Report on Ageing Test Methods for Bituminous Pavement Materials G.D. Airey Senior Lecturer Nottingham Centre for Pavement Engineering University of Nottingham University Park Nottingham NG7 2RD UK Tel: +44 115 9513913 Fax: +44 115 9513909 Email: [email protected] 2 ABSTRACT 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, PAV 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 3 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, Vallerga et 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, rolling thin film oven test, 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 oxidative ageing (e.g. Iowa durability test, SHRP-PAV, HiPAT, 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, 4 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 the thin film oven test (TFOT), the rolling thin film oven test (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 hours at 163C. The TFOT was adopted by AASHTO in 1959 and by ASTM in 1969 (ASTM D1754) as a means of evaluating the hardening of bitumen during plant mixing. 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 Edler et al., (1985), where the binder film is reduced from 3.2 mm to 100 m with an additional increased exposure time of 24 hours. 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. 5 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 undergoes 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 (Hveem et 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 163C for 75 minutes. The vertical circular carriage rotates at a rate of 15 revolutions/minute and the air flow is set at a rate of 4000 ml/minute. 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. Several modifications have also been made to the RTFOT, however most of them have been relatively minor, for example Edler et al. (1985) used an extended time period of 8 hours rather than 75 minutes in their extended rolling thin film oven test (ERTFOT), while Kemp and Predoehl (1981) used 5 hours. 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 6 or emulsion binders (MCDHW 1998). The procedure uses a temperature of 85C 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 minimise 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 minutes at a temperature of 165C. 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, 5 microns, film of bitumen is aged for 2 hours on a glass plate at 107C (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 (Wellborn, 1979), except for the work done on the ZacaWigmore test roads (Zube and Skog, 1969). Simpson et 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 20 microns and the exposure time to 24 hours with a slight reduction in temperature to 99C. These alterations did demonstrate an indirect relationship between field and laboratory hardening. Additional, slight variations were made by Traxler (1961) and Halstead and Zenewitz (1961), who increase the binder film thickness from 5 to 15 microns. 7 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 20 micron film of bitumen which is then aged at 99C for 24 hours. 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 during the ageing of asphalt mixture specimens at 60C. A 1.04 mm diameter opening was selected and in addition the ageing time was increased from 24 to 48 hours. 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 per 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 1.06 higher at the front to prevent bitumen migrating from the bottles (Kemp and Predoehl, 1981). In addition, the TODT uses a lower temperature and longer time for ageing compared to the RTFOT, namely 168 hours at 113C. This level of ageing approximates to that found for pavement mixtures after 2 years in hot desert climates (Petersen, 1989). In addition, Kemp and Predoehl (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. 8 A similar modification was reported by McHattie (1983) with test conditions of 100 hours at 115C. Both methods (168 hours at 113C and 100 hours at 115C) were evaluated by Santucci et al. (1981), who found the tests at 168 hours and 113C 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 15 micron thick bitumen film, coated on Teflon particles, was oxidised in a gas chromatographic column at 130C for 24 hours 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 130C for 24 hours produced the same degree of oxidative ageing found for the RMFOT as well as for 11 to 13 year 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 113C but for a longer period of 3 days compared to the one day test at 130C. 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 9 test. In addition, some binders have a tendency to roll out of the bottles. To overcome these problems, Bahia et 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 hours and a temperature of 163C, 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. 10 Iowa Durability Test The Iowa Durability Test (IDT) is one such test that combines thin film ageing with oxidative ageing (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 65C for up to 1000 hours. 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 five year period. Based on this hyperbolic relationship and considerable field and laboratory data, Lee concluded that 46 hours of ageing with the IDT is equivalent to 60 months field ageing for Iowa conditions (Lee, 1973). Pressure Oxidation Bomb Edler et al. (1985) used a similar approach to that used by Lee, where residue from their eight hour 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 mm by 40 mm glass plates coated with 30 micron bitumen films are positioned horizontally. The test consists of ageing the bitumen residue at a pressure of 2.07 MPa at 65C for 96 hours. Accelerated Ageing Test Device / Rotating Cylinder Ageing Test 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 11 has been based on a theoretical kinetic approach to ageing (Verhasselt, 1996; Verhasselt, 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 Figure 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 per minute and flows oxygen through the aperture at a rate of 4 to 5 litres per hour (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 70C and 110C. At discreet intervals, approximately 20 g to 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 found that ageing bitumen at 85C for 144 hours reflects field ageing with regard to the formation of asphaltenes. He also noted that temperatures less than 100C 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, Francken et al. (1997) found that longer ageing times than 240 hours were required to simulate field ageing of porous mixtures. Pressure Ageing Vessel The SHRP-A-002A research team developed a method using the pressure ageing vessel (PAV) to simulate the long-term, in-service oxidative ageing of bitumen in the field (Christensen and Anderson, 1992). The method involves hardening of bitumen 12 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 (3.2 mm binder film) within the heated vessel, pressurised with air to 2.07 MPa for 20 hours at temperatures between 90 and 110C (AASHTO PP1) (see Figure 2). Migliori and Corte (1999) investigated the possibility of simulating RTFOT (shortterm ageing) and RTFOT + PAV (long-term ageing) simply by means of PAV testing for unmodified penetration grade bitumens. They found that 5 hours of PAV ageing at 100C and 2.07 MPa was equivalent to standard RTFOT ageing, and that 25 hours of PAV ageing at 100C and 2.07 MPa was equivalent to standard RTFOT + PAV ageing. Verhasselt and Vanelstraete (2000) compared the relative accelerated ageing obtained using the PAV at 100C and the RCAT at 85C 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 hours of PAV ageing approximately corresponds to 178 hours of RCAT ageing. However, they did find that the higher temperature of the PAV did result 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 85C and a longer duration of 65 hours (Hayton et al., 1999). The reason for these modifications was the concern that the temperatures used in the PAV procedure were unrealistically 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 13 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). The procedure consists of maintaining samples of emulsion or cutback bitumen at 85C for two hours in the RTFOT with nitrogen gas flow followed by a further 22 hours 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 nanometres (nm) (Bocci and Cerni, 2000). Approximately 7 percent of the solar radiation that reaches the surface of the earth is ultraviolet (UV) radiation (180 - 400 nm), 42 percent is within the visible band (400 - 800 nm) and 51 percent 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 (3 microns) but that the effect decreases for thicker films. Montepara et 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 14 to 140C and spread on glass plates (25 cm x 20 cm) to obtain a binder film thickness of 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-day 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 polymerisation 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 standardised ageing procedure. The procedure attempts to simulate UV radiation exposure corresponding to 4.6 years to 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 15 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 100 micron bitumen films, coated on 50 mm by 50 mm glass plates, at 65C during a 2 hour cycle comprising a 102 minute cycle of UV light only and 18 minutes of UV light and water spray at a pressure of 300 kPa. Test durations of 32.5 hours, 73.5 hours, 7 days and 14 days were used. Kuppens et 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 hour period, to 16¼ hours of UV light at 50C, 4 hours rain with NaCl at 40C, 1 hour water at 20C and 2¾ hours dry at -20C. 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 Bishara et 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 147C and an air pressure of 3.08 MPa for 4.5 hours 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 RTFOT + PAV ageing. 16 Steric Hardening Traxler (1963) identified molecular structuring (thixotropy), 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 (eg stiffness, viscosity, strength etc). Extended heating procedures typically expose the mixture to high temperatures for a specified period(s) of time before suitable testing (eg compressive testing, tests on recovered binder, etc). Oxidation tests typically utilise a combination of high temperature and pressure oxidation to laboratory age specimens. Ultraviolet/infrared treatment involves exposing specimens to either ultraviolet or infrared radiation. Most of the initial studies on asphalt mixture ageing involved tests on the recovered binder as detailed by Hubbard and Gollomb (1937) and Shattuck (1952). Understandably these tests relied on acceptable and sound procedures for extracting and recovering bitumen from the asphalt mixtures (Abson, 1933). 17 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 mm by 50 mm cylinders of an Ottawa sand mixture to 163C (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 mm thick by 40 mm diameter specimens at 150C for 5 hours. After this accelerated ageing, the samples were cooled to 25C for 72 hours and subjected to resilient modulus tests. Kemp and Predoehl (1981) aged Ottawa sand mixtures in an oven at 60C for up to 1200 hours. 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 100C for 4 or 7 days under either dry atmosphere or 80 percent 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 et al. (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 18 at 135C in a force draft oven for periods of 8, 16, 24 and 36 hours. 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 et al. (1988). The procedure requires loose mixtures, prior to compaction, to be aged in a forced draft oven for 4 hours at 135C (AASHTO PP2). The process was found to represent the ageing occurring during mixing and placing and also represents pavements of less than two years (Bell et al., 1994; Monismith et al., 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 135C or related to the desired compaction temperature, whichever is higher, and that the period of conditioning is limited to two hours (Brown and Scholz, 2000). Von Quintas et al. (1988) also investigated long-term ageing using a force draft oven where compacted asphalt mixture specimens were aged for 2 days at 60C followed by 3 days at 107C. 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-A003A project, namely 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 85C for 5 days (AASHTO PP2) (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 DryFreeze climate. However, field validation of the LTOA indicates that 8 days at 85C 19 is equivalent to over 9 years for Dry-Freeze and over 18 years for Wet-No-Freeze; 2 days at 85C is equivalent to 2 to 6 years for both Dry-Freeze and Wet-No-Freeze; and 4 days at 85C 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). The details of the LPO procedure are given in section 2.2.2. 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 85C for 120 hours (Brown and Scholz, 2000). Oxidative (Air Blowing) Procedures Kumar and Goetz (1977) developed a method consisting of ageing specimens at 60C 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 et al. (1988) also used a pressure oxidation treatment. The procedure consisted of conditioning compacted specimens at 60C at a pressure of 0.7 MPa for 5 to 10 days. Kim et al. (1986) used a similar pressure oxidation treatment on compacted specimens of Oregon mixtures. Samples were subjected to oxygen at 60C 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 20 the ageing procedure. This was attributed to a loss of cohesion in the samples at the temperature of 60C 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-A003A 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 60C or 85C for a period of 5 days. Khalid and Walsh (2001) 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 Figure 3). A test temperature of 60C 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). Korsgaard et al. (1996) used the PAV to age gyratory compacted dense asphalt mixture specimens rather than bitumen. Based on recovered binder properties they determined an optimum ageing procedure consisting of PAV ageing for 72 hours at 2.07 MPa and 100C, but concede that 60 hours may be more appropriate for more porous mixtures. Ultraviolet and Infrared Light Treatments Hveem et 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 60C and a maintained air stream across the specimen of 41C. The size distribution of the sand and the binder content of 2 percent ensures a uniform film thickness of 5 to 7 microns. Based on the 21 calibration of the test, 1000 hours of exposure in the weathering machine is approximately equal to 5 years field ageing. Kemp and Predoehl (1981) also used an actinic light weathering test at a temperature of 35C for 18 hours duration with 1000 MW/cm2 Angstrom actinic radiation. The authors note that the weathering test only measures the hardening within the outer 5 microns of the bitumen film irrespective 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 hours 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 considerable lower. Tia et al. (1988) used a series of ageing procedures consisting of convection oven ageing at 60C, force draft oven ageing at 60C and ultraviolet light ageing at 60C for various periods. They recommended an improved ageing procedure incorporating both ultraviolet light and forced draft oven heating. In addition they identified UVlight 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 (Hveem et 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 23C. The remaining two specimens are tested in the same way after being cured at 60C for 24 hours. 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 22 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-term 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 23 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 longterm oven ageing method, pressure oxidation, using low pressure oxidation as well as pressurised procedures, and ultraviolet and infrared light 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 to 2 mm of the surface and can generally be neglected. 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. REFERENCES Abson, G. (1933). “Apparatus for the recovery of asphalts.” Proc., American Society for Testing Materials, Part II, Vol. 33. 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(1981). “The effect of crude source and additives on the long term oven aging of paving asphalts.” Proc., Assn. of Asphalt Paving Technologists, 50, 560-571. 28 Schmidt, R.L. and Santucci, L.E. (1969). “The effects of asphalt properties on the fatigue cracking of asphalt concrete on the Zaca-Wigmore Test Project.” Proc., Assn. of Asphalt Paving Technologists, 38, 39-64. Schmidt, R.L. (1973). “Laboratory measurement of the durability of paving asphalts.” ASTM STP 532, American Society of Testing and Materials, 79-99. Scholz, T.V. (1995). “Durability of bituminous paving mixtures.” PhD Thesis, School of Civil Engineering, University of Nottingham. Shattuck, C.L. (1940). “Measurement of the resistance of oil asphalts (50-60 pen) to changes in penetration and ductility at plant mixing temperatures.” Proc., Assn. of Asphalt Paving Technologists, 11, 186-203. Shell Bitumen Review. (1973). “The rolling thin film oven test.” No. 42, 18-19. Simpson, W.C., Griffin, R.L. and Miles, T.K. 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(1969). “Final report on the Zaca-Wigmore asphalt test road.” Proc., Assn. of Asphalt Paving Technologists, 38, 1-38. 30 TABLE I Bitumen Ageing Methods Test method Temperature Duration Sample size Film thickness Extra features Thin film oven test (TFOT) (Lewis and Welborn, 1940) – ASTM D1754, EN 12607-2 Modified thin film oven test (MTFOT) (Edler et al., 1985) Rolling thin film oven test (RTFOT) (Hveem et al., 1963) – AASHTO T240, ASTM D2872, EN12607-1 Extended rolling thin film oven test (ERTFOT) (Edler et al., 1985) Nitrogen rolling thin film oven test (NRTFOT) (Parmeggiani, 2000) Rotating Flask Test (RFT) – DIN 52016, EN12607-3 163C 5 hours 50 g 3.2 mm 163C 24 hours - 100 m - 163C 75 minutes 35 g 1.25 mm 163C 8 hours 35 g 1.25 mm 163C 75 minutes 35 g 1.25 mm 165C 150 minutes 100 g - Shell microfilm test (Griffin et al, 1955) 107C 2 hours - 5 m Air flow – 4000 ml/min Air flow – 4000 ml/min N2 flow – 4000 ml/min Flask rotation – 20 rpm - Modified Shell microfilm test (Hveem et al., 1963) Modified Shell microfilm test (Traxler, 1961; Halstead and Zenewitz, 1961) Rolling microfilm oven test (RMFOT) (Schmidt and Santucci, 1969) Modified RMFOT (Schmidt, 1973) 99C 107C 24 hours 2 hours - 20 m 15 m - 99C 24 hours 0.5 g 20 m 99C 48 hours 0.5 g 20 m Tilt-oven durability test (TODT) (Kemp and Predoehl, 1981) Alternative TODT (McHattie, 1983) 113C 168 hours 35 g 1.25 mm Benzene solvent 1.04 mm opening - 115C 100 hours 35 g 1.25 mm - - 31 TABLE I Bitumen Ageing Methods (continued) Test method Temperature Duration Sample size Film thickness Extra features 130C or 113C 163C 24 hours 72 hours 75 minutes 4g 160 m 35 g 1.25 mm 3 mm opening Steel rods 65C 1000 hours 3.2 mm Pressure oxidation bomb (POB) (Edler et al., 1985) 65C 96 hours Accelerated ageing test device / Rotating cylinder ageing test (RCAT) (Verhasselt and Choquet 1991) Pressure ageing vessel (PAV) (Christensen and Anderson 1992) 70C to 110C 144 hours TFOT residue – 50 g ERTFOT residue 500 g 3.2 mm High pressure ageing test (HiPAT) (Hayton et al., 1999) 85C RTFOT or TFOT residue – 50 g RTFOT residue – 50 g 2.07 MPa – pure oxygen 2.07 MPa – pure oxygen 4 to 5 l/hr – pure oxygen 2.07 MPa – air 3.2 mm 2.07 MPa – air Thin film accelerated ageing test (TFAAT) (Petersen, 1989) Modified rolling thin film oven test (RTFOTM) (Bahia et al., 1998) Iowa durability test (IDT) (Lee 1973) 90C to 110C 20 hours 65 hours 30 m 2 mm 32 TABLE II Asphalt Mixture Ageing Methods Test method Temperature Duration Sample size Extra features Production ageing (Von Quintas et al., 1988) 135C 8, 16, 24, 36 hours Loose material - SHRP short-term oven ageing (STOA) 135C 4 hours Loose material - Bitutest protocol (Scholz 1995) 135C 2 hours Loose material - Ottawa sand mixtures (Pauls and Welborn 1952) 163C Various periods 50 mm x 50 mm - cylinders Plancher (1976) 150C 5 hours 25 mm x 40 mm - Ottawa sand mixtures (Kemp and Predoehl 1981) 60C 1200 hours - - Hugo and Kennedy (1985) 100C 4 or 7 days - 80% relative humidity Long-term ageing (Von Quintas et al., 1988) 60C 2 days Compacted specimens - 107C 3 days SHRP long-term oven ageing (LTOA) 85C 5 days Compacted specimens - Bitutest protocol (Scholz 1995) 85C 5 days Compacted specimens - Kumar and Goetz (1977) 60C 1, 2, 4, 6, 10 days Compacted specimens Air at 0.5 mm of water Long-term ageing (Von Quintas et al., 1988) 60C 5 to 10 days Compacted specimens 0.7 MPa - air Oregon mixtures (Kim et al., 1986) 60C 0, 1, 2, 3, 5 days Compacted specimens 0.7 MPa - air SHRP low pressure oxidation (LPO) 60C or 85C 5 days Compacted specimens Oxygen – 1.9 l/min Khalid and Walsh (2001) 60C Up to 25 days Compacted specimens Air – 3 l/min PAV mixtures (Korsgaard et al., 1996) 100C 72 hours Compacted specimens 2.07 MPa - air 33 FIGURE 1 Rotating Cylinder Ageing Test (after Verhasselt, 2000) 34 FIGURE 2 Pressure Ageing Vessel (after Christensen and Anderson, 1992) 35 FIGURE 3 Low Pressure Oxidation Technique for Porous Asphalt (after Khalid and Walsh, 2002) 36 View publication stats
