An alternative method for short- and long-term ageing for bitumen binders

Abstract

Short-term ageing of bituminous binders is a well-accepted concept that represents ageing of binders during plant mixing, production, transportation and construction. Simulation of this ageing is carried out in controlled laboratory conditions and these aged binders are used to predict binder rutting performance. Similarly, the simulation of long-term ageing of binders accounts to ageing of the binder during service life of the pavements and these binders are used to characterise the binder response to fatigue performance in the SUPERPAVE test protocol. Two well-established laboratory test methods are generally practised for evaluating the short-term ageing of bitumen binders: (i) the thin film oven (TFO) test method where a pan filled with bitumen is placed on a rotating pan on the horizontal plane and (ii) the rolling TFO (RTFO) method that uses cylindrical glass bottles filled with bitumen which are rolled in the vertical plane. In the case of long-term ageing, simulation is performed in a pressurised ageing vessel (PAV) under constant conditioning of pressure and temperature. The RTFO and PAV are the part of the test protocol of SUPERPAVE specifications. These equipments are costly and may not be available to different agencies and thereby it has become difficult to adopt any studies involving short- and long-term ageing of binders. Any alternative procedure developed that provides similar ageing effects comparable to the SUPERPAVE method would help highway agencies to adopt and study the effect of ageing on binders. Keeping this as the main concern, an attempt is made to develop a cheap and generalised method for short-term and long-term ageing effects using a normal laboratory oven. Six binder types consisting of two each from viscosity grades, penetration-grade polymer-modified binders (PMB) and crumb rubber-modified binders, were considered in this study. Binders were aged in RTFO and PAV ageing and tested for rheological properties using the dynamic shear rheometer. Similarly, these binders were also aged in a normal oven kept at a constant temperature for varying periods of ageing. Test results were compared on the basis of rutting and fatigue criteria to arrive at the time of ageing required in a normal oven that would be similar to RTFO and PAV ageing, respectively. The study revealed that it is possible to correlate ageing methods of SUPERPAVE with a normal oven method and arrive at an alternative method of ageing. In order to compare the actual short-term ageing of one of the modified binders, bituminous mixes were collected from the field during mix production, transportation and compaction, and binders from these were extracted. Rheological parameters of the extracted bitumen from field samples collected during different stages of construction, RTFO and normal oven-aged samples were evaluated and compared. It was found that the actual ageing is different from RTFO and a normal oven ageing and thereby there is a need to carryout a detailed study on ageing characteristics of Indian binders.