Martin Hugener

Synthesis of standards and procedures for specimen preparation and in-field evaluation of cold-recycled asphalt mixtures

The use of recycled asphalt (RA) materials in pavement rehabilitation processes is continuously increasing as recycling techniques, such as cold recycling (CR), are being utilised in increasing magnitude and greater awareness for use of recycled materials and consideration of sustainable practices is becoming common in the construction industry. The focus of this paper is on developing a state of the art and state of the practice summary of processes used for classification of RA as well as the curing and specimen preparation practices for cold-recycled asphalt mixtures. A variety of topics were explored through an exhaustive literature search, these include RA production methods, definition of RA materials, stockpiling practices, industrial operations, specimen curing and preparation practices and in-field evaluation of cold-recycled rehabilitation. This paper was developed through efforts of CR task group (TG6) of RILEM Technical Committee on Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems (TC-237 SIB).

Emissions of tar-containing binders: a laboratory study.

In Switzerland, hot recycling of tar-containing pavements is a subject of much dispute between environmentalists, road authorities and constructors. The main reason for this controversy comes from a lack of knowledge about the amount of hazardous compounds emitted, especially polycyclic aromatic hydrocarbons (PAHs), and the resulting health risk for road workers. On this background we decided to initiate a research project to study the emission behaviour of tar-containing materials. Mixtures of tar and bitumen with variable PAH content were heated in an open reactor flask at different temperatures to identify and quantify the key parameters of PAH emissions. The expected linear correlation between PAH concentration in the fumes and in the binder was found only for binder mixtures with PAH concentrations above 5000 ppm. This was traced back to the problem that a change of the PAH content in the binder was always accompanied by a change of other parameters, like viscosity. In the experiments with temperature variation, emissions of individual PAHs correlated well with vapor pressure. However, for Naphthalene and, in a lesser degree, for 3-ring PAHs too, a partial depletion of these compounds in the vapour was observed in some experiments. The effect is a slower increase with temperature for these compounds compared to 4–6-ring PAHs. This is one reason why the commonly used set of EPA-PAHs, which includes naphthalene and 3-ring PAHs, is considered inappropriate for the assessment of the health hazard in the case of tar-containing materials in hot recycling.

Cold asphalt recycling with 100% reclaimed asphalt pavement and vegetable oil-based rejuvenators

Cold in-place recycling methods are used to different extents but are often not very suitable for small construction sites. Therefore, a new process was developed using vegetable oil-based rejuvenators to reactivate the old binder of the reclaimed asphalt pavement (RAP) material. Briefly summarised, RAP is produced on-site from the old pavement, then sprayed with water and rejuvenator before it is mixed thoroughly and immediately compacted. Process parameters have been optimised in laboratory tests with gyratory compacted specimens. It turned out that rejuvenators with used cooking oil did give comparable results to virgin rapeseed and linseed oil. Only the outcome of the water sensitivity tests was not satisfactory, as the wet specimens showed a considerable loss of tensile strength. However, the damage was only temporary and the stability recovered to the original value after drying of the water-saturated specimens. In a second step, four cold mixtures with different rejuvenator compositions were compared in a trial field section with conventional hot mix asphalt concrete. In situ testing with the mobile load simulator MMLS3 showed good performance for three of four cold mixture pavements, which was comparable to the rutting behaviour of hot mix asphalt. Several wearing courses but also surface layers for small secondary roads have been constructed during the last three years. No significant damage has been reported for these cold mixture pavements so far. Particularly, the use of used cooking oil as a rejuvenator is promising due to its low price. Since only recycling products are used and no transport of the cold mixture is necessary for in situ recycling, this new cold recycling system is highly ecological as well.

Dynamic X-ray radiography for the determination of foamed bitumen bubble area distribution

Foam bitumen is highly efficient in wetting and coating the surface of mineral aggregate at lower temperature. In order to improve understanding and characterization of the bitumen foam, X-ray radiography was used to study the formation and decay of bitumen foam in 2D representation. Image segmentation analysis was used to determine the foam bubble size distribution. In addition, the main parameters influencing foam bitumen formation, water content, and temperature were also investigated. The results demonstrate the influence of the water content on morphology and expansion of foam bitumen bubbles. Adding more water in the foaming process leads to quick collapse of bubbles and intensifies coalescence of foam bitumen. Higher temperatures produces larger bubbles at early foaming stage compared to lower temperature. Moreover the morphology of bubble formation depends on the types of bitumen used. An exponential function has been implemented to represent the bubble area distribution.

Hot-recycling of Tar-containing Asphalt Pavements. Emission Measurements in the Laboratory and in the Field

ABSTRACT Hot recycling in pavement construction leads to the emission of hazardous compounds when tar-containing recycled asphalt pavement (RAP) is used. This is due to the relatively high content of substances such as phenols and polycyclic aromatic hydrocarbons (PAH) in tar. The latter are of special interest, because some PAHs have carcinogenic and/or mutagenic properties. High tar content in combination with high pavement temperatures during construction increases the potential health risk for workmen. Switzerland does not currently consider a complete ban of RAP in hot recycling for economical and ecological reasons. Since RAP might always contain some tar, there is a need to fix a scientifically sound limit for the maximum tar content. However, scientific data on the emissions of PAH, as well as the resulting occupational health risk are rare, which motivated the present research. In the laboratory, several parameters, such as temperature, PAH content and viscosity, were varied in an experimental setup to quantify their effect on the resulting emissions. It was shown that temperature is the major factor determining the emissions in the 140 to 240°C range. A PAH content of up to 20000 ppm EPA-PAH in RAP had no significant effect on total particulate matter (TPM) emissions. However, emissions of PAH in the fumes increased proportionally with the PAH concentration in the RAP. Three field tests were carried out with RAP containing up to 5600 ppm EPA-PAH. Occupational health data for the workmen were collected with personal samplers. TPM concentrations were between 2–4 mg/m3, close to the occupational health limit value for bitumen fumes (10 mg/m3). Concentrations of Benzo(a)pyrene (BaP), which is often used as a lead compound, were in all cases below the occupational health limit value of 2 μg/m3 by at least a factor of three. This study indicates that the current Swiss recommended maximum value of 5000 ppm for EPA-PAH in the binder of RAP may be considered as adequate. However, conditions in the field trials did not include a worst-case scenario and the margin to occupational health limit values is small.

Fracture Toughness Testing Aspects for Assessing Low Temperature Behaviour of Bituminous Binders

Thermal cracking of asphalt roads usually occurs in cold regions and areas with frequent temperature changes. Hence, a complete comprehensive understanding about the behaviour of bituminous binders at very low temperatures is required for predicting the performance of the asphalt road. Since conventional methods have not shown reliable results evaluating polymer modified binders, the fracture toughness test (FTT) has been developed as an alternative method for the determination of the low temperature behaviour. This work presents a complementary study where the influence of different parameters on the results in the experimental FTT procedure is analysed. It is shown that preparation and conditioning of the beams before the test can have a significant effect on the fracture temperature. In order to achieve repetitive results in the assessment of different types of binders, the experimental procedure has been improved.

Emissions of tar-containing binders: Field studies

This study describes the measurement of emissions during field construction of asphalt pavements using tar-containing recycled asphalt pavement (RAP), which is known to release harmful substances, such as polycyclic aromatic hydrocarbons (PAH). At three different test sites, the main emission sources were identified and the total emission rates of fumes and PAHs of the paving process were determined. For this purpose, the paver was temporarily enclosed. While the screed area was the main emission source, the hopper area and freshly compacted pavement were also significant. In comparison with previous laboratory tests, the binder composition and the resulting emissions were comparable, except for Naphthalene. Benzo(a)pyrene (BaP) as a representative for carcinogenic PAHs was identified as a good leading compound, correlating well with the toxicity weighted sum of PAHs. In contrast, the unweighted, mass related sum of all EPA PAHs does not seem to be a good parameter to assess workplace concentrations because emissions by mass are dominated by the less hazardous 2-, 3- and 4-ring PAHs. Workplace concentrations for bitumen fumes and PAHs were below limit values in all three field studies. However, the margin was not large and the field tests were done under favourable meteorological conditions. Therefore, we suggest maintaining the current Swiss limit of 5000 mg EPA-PAH per kg binder in the RAP-containing hot mix.

Cold Recycling of Reclaimed Asphalt Pavements

Pavement engineers have in front of them multiple challenges linked to addressing issues related to social development and society’s expanding needs. One of the most substantial of these issues is perhaps how to effectively rehabilitate and/or maintain the existing road network while preserving and sustaining limited natural resources. The re-usage of existing pavement materials to reconstruct/rehabilitate our future pavements is the solution that is now more and more selected by the different road administrations around the world. However, upon closer inspection, one can find many areas and details, not negligible issues, that are simply extensions of HMA technology (i.e. mix design process in cold recycling) or empirical arrangements; in particular RAP still does not have an internationally recognized classification. So SIB – TG6 decided to develop a classification protocol of RAP, depending on its intended application. The objective has been followed by considering the procedures generally utilized to classify the natural aggregates: tests able to identify the main components (i.e. the geometrical and mechanical properties of aggregates and the characterization of recovered bitumen for RAP) and provide information on their behaviour under specific conditions, near to real life usage (e.g. the Los Angeles test for aggregates gives an idea of the potential behaviour of aggregates under the action of a roller compactor). The following sections illustrate and explain the actions of the TG in order to achieve the goals outlined above: the review of current standards, the protocol designed to classify RAP and the round robin tests carried out to validate the protocol.

Simulating repeated recycling of hot mix asphalt

Addition of reclaimed asphalt pavement (RAP) to asphalt mixes is a standard procedure today in many countries. With increasing use of RAP it becomes more probable that asphalt material is recycled multiple times. This paper deals with the change of binder and asphalt mix properties after multiple recycling steps. A reference dense hot mix asphalt (HMA) containing no recycled asphalt was aged in the laboratory to produce artificial RAP. This was afterwards used in a proportion of 40% together with virgin mineral aggregates and bitumen to produce a new asphalt mixture with binder properties and volumetric characteristics that matched the reference mixture as closely as possible. This mixture was aged again and used for a second recycling mixture until finally three recycling cycles have been carried out. In addition, a second series simulating 100% recycling without addition of mineral aggregates or bitumen, but solely adding a rejuvenating agent, was performed. In the case of 40% recycling, most of the binder and mixture properties remained virtually constant except for the water sensitivity, where a significant decrease in the indirect tensile strength ratio (ITSR) was detected. Analysis of the binder by means of Fourier transform infrared spectroscopy (FTIR) showed that the potential functional groups in the RAP were almost completely oxidised and could not provide information on the ageing degree of RAP. With 100% recycling the binder in the mixture hardened more during each recycling stage. Despite this, the mixture demonstrated good properties with regard to water sensitivity, fatigue behaviour and plastic deformation. Obviously the binder characteristics of penetration and softening point ring and ball are not sufficient to assess the performance of recycled pavements. There is evidence that the mixture properties should be taken into account in the design of recycled pavements, at least when additives like rejuvenators or polymers are added.

Effects of Rejuvenator on Reclaimed Asphalt Binder: An Exploratory Study of the RILEM TC 264-RAP Task Group 3

This paper presents the preliminary experimental activity conducted to develop the round robin test (RRT) plan on the use of rejuvenators for the Task Group (TG) 3 on Asphalt Binder for Recycled Asphalt Mixtures as part of the RILEM TC 264-RAP. For this purpose, a reference Reclaimed Asphalt (RA) binder was extracted from a single RA source. This material was then fully characterised with conventional experimental methods and with Fourier Transform Infrared Spectroscopy (FTIR) for evaluating the chemical structure. The RA binder was found to be very hard compared to target binder of PEN50/70 grade and the use of virgin binder cannot restore its properties. In this condition, a rejuvenator dosage of 9% per weight of RA binder is foreseen as optimum value to restore the binder properties close to target PEN50/70 grade for the final binder-rejuvenator blend over a RA content ranging from 60% and 100%.