Daniel Steiner

Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen

Ageing of bitumen leads to increased stiffness and brittleness. Thus, bituminous bound pavements become more prone to failure by low-temperature and fatigue cracking. Therefore, the ageing behaviour of bitumen has a crucial impact on durability, as well as recyclability of pavements. To assess ageing of bitumen, the rolling thin film oven test and pressure ageing vessel are standardised methods for short-term and long-term ageing in the lab. For lab-ageing of hot mix asphalt (HMA), various methods have been developed in the last decades. This paper presents a study on the potential of employing a highly oxidant gas for simulating the long-term oxidative ageing of asphalt mix specimens in the lab. Based on the results, an optimised lab-ageing procedure (Viennese Ageing Procedure – VAPro) for compacted HMA specimens to assess mix performance of long-term lab-aged specimens is developed. Thus, it is possible to optimise mix design not only for short-term performance but to take into account effects of oxidative ageing during its in-service life. VAPro is based on a triaxial cell with forced flow of a gaseous oxidant agent through the specimen. The oxidant agent is enriched in ozone and nitric oxides to increase the rate of oxidation. It is shown by stiffness tests of unaged and lab-aged specimens, as well as by Dynamic Shear Rheometer tests of recovered binder from aged specimens that asphalt mixes can be long-term aged at moderate temperatures (+60°C) and within 4 days and a flow rate of 1 l/min by applying VAPro. Thus, an ageing procedure is at hand that can simulate long-term ageing at conditions that are representative of conditions that occur in the field within an efficient amount of time.

Using Highly Oxidant Gas for Simulating Long-Term Ageing of Asphalt Mix Specimens in the Lab

Ageing of bitumen leads to increased stiffness and brittleness. Thus, bituminous bound pavements become more prone to failure by low-temperature and fatigue cracking. Therefore, the ageing behavior of bitumen has a crucial impact on durability, as well as recyclability of pavements. To assess ageing of bitumen, RTFOT and PAV are standardized methods for short-term and long-term ageing in the lab. For lab-ageing of hot mix asphalt (HMA), various methods have been developed in the last decades. This paper presents an optimized lab-aging procedure (Viennese Aging Procedure—VAPro) for compacted HMA specimens to assess mix performance of long-term lab-aged specimens. Thus, it is possible to optimize mix design not only for short-term performance but to take into account effects of oxidative aging during its in-service life. VAPro is based on a triaxial cell with forced flow of a gaseous oxidant agent through the specimen. The oxidant agent is enriched in ozone and nitric oxides to increase the rate of oxidation. It is shown by stiffness tests of unaged and lab-aged specimens, as well as by DSR tests of recovered binder from aged specimens that asphalt mixes can be long-term aged at moderate temperatures (+60 °C) and within 4 days and a flow rate of 1 l/min by applying VAPro. Thus, VAPro can simulate long-term ageing at conditions that are representative of conditions that occur in the field within an efficient amount of time.

Short Term Aging - Influence of Mixing Time at Laboratory Specimen Production

The viscoelastic behavior of hot mix asphalt (HMA) is influenced by several factors, e.g. by the binder behavior, which has crucial impact on the pavement performance. The behavior changes due to aging during its in-service life within years, as well as during HMA production and compaction processes within a few hours. Therefore, aging processes are classified into long-term aging (LTA) and short-term aging (STA). This paper presents a study, analyzing STA processes at laboratory productions of HMA slabs. Six production cycles, with up to three slabs each were analyzed. The mix of binder and aggregates were prepared at once. This requires storing the hot and prepared mix inside the laboratory mixer while the first and the second slab are compacted, respectively. Bitumen was recovered for each produced slab. |G*| stiffness tests (DSR) are carried out afterwards at 52–82°C and 1.592 Hz. The results show that mixing times lead to a stiffness increase of 1.5 compared to the virgin bitumen, what is slightly below RTFO aging level. Recovered binder from Slab 2 and Slab 3 show increases of |G*| of 1.9 and 2.5. Therefore, for the used binder, longer mixing times have a significant impact on STA. The results can be used to back-calculate STA levels for specimens that are used for LTA aging studies. Furthermore, both binder and additional HMA stiffness results can serve as basis for multi-scale stiffness modelling by coupling the results.

Analysis of Bitumen and PmB Using Fluorescence Spectroscopy and Microscopy

Physicochemical characterization of bitumen and polymer modified bitumen has been a research subject for the last decades. Various microscopic and spectroscopic techniques have been used to unravel the bitumen microstructure and to establish the link to its chemical composition. Over the years, the usage of bitumen has risen. This is particularly true for polymer modified bitumen (PmB) which has primarily been blended with styrene-butadiene-styrene (SBS) polymers. Since there is a demand for homogeneous blending and an overall quality control, suitable methods need to be developed. Fluorescence spectroscopy and fluorescence microscopy were found useful tools to achieve these goals. This paper focuses on the analysis of a 160/220 pen-graded base bitumen and SBS modified bitumen. Complementary analysis of specific surfaces enables a better understanding of chemical composition (spectroscopic information) and microstructure (microscopy).

Chemical Composition and Microstructure of Bitumen – a Matter of Terminology?

Bitumen is an organic material derived from crude oil refinery. It exhibits a highly complex chemical composition, time- and temperature dependent viscoelastic material behavior and distinct microstructural features. Ongoing efforts try to link these three material characteristics by combining physico-chemical with mechanical analysis. Two different theses that explain which chemical constituents trigger the materials’ microstructure: one group of researchers is in favor of the idea that waxes are responsible, the other group favors asphaltenes as the crucial factor. Both groups base their assumptions on experimental evidence. However, no final proof for either theses exists. A recent study on high mass resolution analysis of bitumen and its constituents shows that in fact both groups might be referring to the same thing and simply using different terminology. This paper suggests a temporary solution for overcoming the differences by substituting the controversial terms “wax” and “asphaltene” by the more general but still correct term “n-heptane insolubles” for the time being.

Viennese Aging Procedure – Behavior of Various Bitumen Provenances

Bitumen changes its properties in the course of time under natural and anthropogenic influences due to its organic origin. These processes are commonly called “aging”. The material becomes stiffer and more brittle, resulting in less favorable low temperature and fatigue behavior. For this reason, it is important to simulate the aging of the material in the laboratory in an accelerated way to study the change in material behavior and minimize damage on the road. On the bitumen level, the standardized methods RTFOT (Rolling Thin Film Oven Test) and PAV (Pressure Aging Vessel) are used. Various methods have been developed in the past to simulate aging of asphalt mixes or compacted specimens. The study presented in this paper evaluated the aging method “Viennese Aging Procedure” (VAPro) for applicability with the aid of a parameter study with bitumen of different origin. VAPro uses realistic boundary conditions (temperature: +60 °C, pressure: ~0.3 bar) and increases the rate of aging of the compacted asphalt mix specimen by perfusing (1.0 l/min) with ozone and nitric oxides enriched air for three days. The state of aging of the extracted bitumen is assessed using the Dynamic Shear Rheometer (DSR). Significant differences between the employed bitumen are determined, which is probably caused by their different initial stiffness and their origin. The stiffness after VAPro of the extracted bitumen is between 1.2 and 2.6 times the RTFOT+PAV-aged state.

Impact of Loading Rate and Temperature on Tensile Strength of Asphalt Mixtures at Low Temperatures

Low temperature cracking of an asphalt mix with specific design is primarily influenced by the loading rate and the temperature. For laboratory testing of hot mix asphalt (HMA), several test methods have been developed and standardised in the last decades to assess the resistance to low-temperature cracking. In order to characterise a mixture efficiently and economically, the test has to be kept within acceptable timeframes. Therefore, low temperature test methods apply fixed parameters, as Uniaxial Tensile Strength Test (UTST) and Relaxation Test (RT) according to EN 12697-46. To study impact of time and temperature on the low temperature behaviour, loading rate at UTST and initial stresses at RT are systematically varied at four temperatures (−20 | −10 | 0 | +10 °C). UTST are carried out at 3 strain rates (1.6 × 10−4–1.6 × 10−2 mm/s). The tensile strength βt and failure strain efailure are determined and evaluated. Furthermore, RTs are performed at 3 different initial stresses, depending on the strength determined by UTST (0.75 | 0.5 | 0.35* βt). This will reveal how the level of initial stress influences the relaxation time and ratio. The results of this study can be used for modelling and simulation of pavement structures employed on bridges for expansions joints with low deformation rates and high absolute deformations.