Evaluation of dry asphalt rubber concrete in railway sub-ballast using the four point bending test

The study of bituminous mixes can be related to tyre rubber material recycling. Encouraging scrap tyre disposal and cutting down on pit employment are the environmental aims. In the field of railways, technological processes involved in the use of crumb rubber are the wet-dry processes. In the mechanical characterisation of the asphalt concrete in terms of both the fatigue resista recycled waste tires nce and the stiffness modulus it is necessary to use a design method for the sub-ballast railway material, even more so if the asphalt concrete investigated is an innovative material such as a dry asphalt rubber concrete (DARC) (i.e. a bituminous mixture with crumb rubber formed by a dry process). Such material is less known and investigated than the wet process, even if its application implies peculiar economic and environmental advantages such as no specialized equipment or significant plant modifications and a large quantity of recycled waste tires compared to the wet process. Different kinds of test are usually used in the experimental work such as bending tests or uni-axial tests, but they do not give the same answer. In the work, described in this paper, mechanical characterization was carried out by means of fatigue tests for asphalt material: a two point bending (2PB) test and four point bending (4PB) test, focused on the mechanical behaviour of DARCs with different rubber content by weight of the aggregates. Different strain controlled tests were undertaken for the same material under the same loading conditions, frequency and temperature (15Hz, 20 degrees centigrade), in particular an experimental survey was carried out in order to determine the stiffness modulus by means of the four point bending test on prismatic specimens (UNI EN 12697-26, 2004). This paper reports the experimental results for rubberized asphalt mechanical properties for a sub-ballast layer performed in a laboratory using the dry process.