Bitumen ageing testsPrint
Bitumen ageing tests
It is clearly desirable that there should be laboratory tests that quantitatively determine the resistance of bitumens to hardening at the various stages during the production process. A number of tests already exist to measure the effect of heat and air on bitumen. The main aim of these tests is to identify bitumens that are too volatile or are too susceptible to oxidation to perform well in service (for more information on this testing see section 5.5.4). The thin-film oven test (Lewis and Welborn, 1940) simulates practical conditions. In this test, the bitumen is stored at 1638C for 5 h in a layer 3.2 mm thick. It is claimed that in this test the amount of hardening that takes place is about the same as that which occurs in practice. However, diffusion in the bitumen film is also limited, and it is not possible to obtain homogeneous hardening or ageing. Accordingly, the test is far from ideal. This test was adopted initially by the American Society for Testing Materials (ASTM) in 1969 as method ASTM D1754 and has been modified since that time to include improvements (ASTM, 2009). In 1963, the State of California Department of Public Works, Division of Highways, developed a test that more accurately simulates what happens to a bitumen during mixing. It is called the rolling thin-film oven test (RTFOT) (Hveem et al., 1963). In this test, eight cylindrical glass containers each containing 35 g of bitumen are fixed in a vertically rotating shelf. During the test, the bitumen flows continuously around the inner surface of each container in a relatively thin film, with preheated air blown periodically into each glass jar. The test temperature is normally 1638C for a period of 75 min. The method ensures that all the bitumen is exposed to heat and air, and continuous movement ensures that no skin develops to protect the bitumen. A homogeneously aged material, similar to that which is produced during full scale mixing, is obtained. Clearly, 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 orrelates reasonably well with that observed in a conventional batch mixer. However, the mixing conditions in fficient modern drum mix plants are less aggressive, and bitumen hardening during asphalt production in such plants is less than predicted by the RTFOT. The RTFOT was accepted in 1970 by the ASTM as method ASTM D2872- 12e1 (ASTM, 2012) and was included as part of the European specification for paving grade bitumens in BS EN 12591:2000 (BSI, 2000) and is part of the Superpave specification used in the USA (AI, 1997). Australia utilises the RTFOT apparatus artificially to age samples of bitumen to a specified apparent viscosity level (SAVL – taken to be 5.67 log Pa.s at 458C). The SAVL is considered to equate to the viscosity of a bitumen in a sprayed seal at the end of its service life, and is measured using the Shell sliding plate viscometer (Standards Australia, 1997a). A correlation between the time taken for a bitumen to reach the SAVL and the in situ service life of a seal was proposed by the Australian Road Research Board (ARRB) from data obtained from bitumen ecovered from aged sprayed seals. The assessment is called the ‘durability test’, and is applied by a number of road agencies in Australia as a control point on the suitability of bitumen for use in sprayed seals (Standards Australia, 1997a). Australia is one of the few countries where a specific test is employed to quantify the durability of bitumen (Standards Australia, 1997b). Over the years there have been a number of attempts to simulate the long term ageing of bitumen in asphalts but this has proved to be extremely difficult because of the number of variables that affect binder ageing – void content, mixture type, aggregate type, etc. The Strategic Highway Research Program (SHRP) investigated accelerated ageing techniques and identified the pressure ageing vessel (PAV) (Anderson et al., 1994) as the preferred apparatus to simulate long term ageing of bitumen. The US Superpave specification (AI, 1996) uses the RTFOT to simulate initial ageing, followed by ageing over 20 h at elevated temperature (90, 100 or 1108C) and pressure 2070 kPa in a PAV. After this ageing procedure, the residue is used for dynamic shear rheometry, bending beam rheometry and direct tension testing. The use of the PAV, using modified conditions, is currently being considered in Europe as a method for ageing bitumens in the laboratory. The artificial ageing of binders in the PAV to simulate ageing in situ has still to be fully validated but the technique is now widely accepted as a satisfactory approach.