Federal Highway Research Institute


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Asphalt pavements

Effects of the use of temperature lowered rolled asphalt on vapour/aerosol formation and durability (07.0306)
The Hazardous Substances Committee decided at its last meeting in 2019 that the occupational exposure limit value for vapours and aerosols during hot processing of bitumen would be set at 1.5 mg/m³ (bitumen condensate standard; BKS) in accordance with the recommendations for limit values for safe and healthy working environments. However, the setting has been suspended for a period of 5 years because, as things stand at present, the construction industry is not in a position to comply with this limit value on an ad hoc basis when laying asphalt. Measurements to date show exposures of the various workers during the laying of rolled asphalt between slightly more than 3 mg/m³ and almost 19 mg/m³ (BKS) and during the laying of mastic asphalt between just under 6 mg/m³ and 18 mg/m³ (BKS). Against the background of the limit value, the production of asphalt pavements is no longer possible or only possible with personal protective equipment, which, however, has further disadvantages. The present research project is intended to investigate the currently existing asphalt technology options with a potential for reducing the concentration of vapours and aerosols during the paving of rolled asphalt and to compare them with conventional paving. This is the addition of waxes and zeolites and other additives to reduce the temperature of the asphalts. The aim is to create a basis for evaluating the effects of these additives on the formation of vapours and aerosols during the paving of rolled asphalt and the durability in the life cycle of the asphalt layer according to the current state of the art and to pursue this further in the coming years.

Development of asphalt layers for road and bridge pavements under extreme climatic conditions (07.0305)
Asphalt wearing courses are directly exposed to the loads from traffic and weather and must therefore have equally high resistance to deformation and good low-temperature behaviour. Long-lasting hot spells are predicted to occur more frequently in the future, resulting in higher surface temperatures and higher temperatures throughout the asphalt package, but also less material cooling at night. For the asphalt, these boundary conditions mean increased stresses that can lead to premature damage in the form of rutting, but also accelerated fatigue cracking in the asphalt base courses. The aim of the research project is to estimate the time horizon from which a significant impairment of the service life of asphalts used today for surface courses and possibly binder courses must be expected. In addition, a new generation of very highly deformation-resistant asphalt wearing courses and possibly asphalt binder courses must be developed for the climatic conditions prevailing beyond this time horizon, which at the same time have sufficient resistance to cold and fatigue to meet the challenges of climate change.

Potential study on the use of asphalt liners (07.0290)
The use of nonwovens and grids in asphalt road construction has its origins in the use of similar product groups in earthworks. In asphalt road construction, asphalt interlayers (AE) can be used in various ways. One application is the overlaying of existing hydraulically bound base courses and (relaxed) concrete structures to prevent reflection cracking. In this application, AEs are intended to absorb stresses caused by locally occurring displacements and to transfer them over a larger area into the overlying asphalt layers. The penetration of reflection cracks should be delayed and the service life of the road structure extended. While this application is already widely used and experience has been mostly positive, the use of AE in asphalt pavement is controversial. This is partly due to the fact that the actual effect of AE in the asphalt pavement - in the field of structural road maintenance, fundamental renewal or new construction - has not yet been conclusively scientifically proven. The aim of the research project is to prove and quantify the effectiveness of AE in the asphalt pavement, taking into account the actual stress conditions.

Investigation into the influence of double coating when using asphalt granulate and soft binders on the performance properties of asphalt (3519002)
The reuse of asphalt granulate in fresh asphalt mix represents the state of the art. The amount of asphalt granulate to be added depends, among numerous other factors, on the hardness of the resulting binder. Since the binder in the asphalt granulate is usually very hard, this can limit the amount to be added. Rejuvenators are intended to remedy this situation by positively influencing the binder properties of the asphalt granulate and lowering the degree of hardness. The long-term effect of these products has not yet been conclusively proven. A possible alternative is the use of soft additive binders in the mixing process. According to the valid regulations, the degree of hardness of the additive binder may only be one grade softer than the required resulting binder of the mix. It is necessary to check whether the addition of even softer additive binders has a positive or negative influence on the performance of the asphalt. The background to this is the expected effect of double coating: the old, hard binder present in the asphalt granulate does not mix completely with the added fresh binder during the mixing process - a double coating of the aggregate occurs, formed by the old binder film and the new fresh binder film as a second layer.

Quality assurance of performance tests in asphalt road construction (3519001)
Performance-based test methods are required, among other things, for the determination of characteristic material parameters as input variables for the computational dimensioning (RDO Asphalt) and the substance evaluation (RSO Asphalt). Currently, work is also being done on the creation of a set of rules (ZTV RDO Asphalt) for the handling of construction contracts on the basis of the RDO Asphalt. These applications of performance-based test methods require the greatest possible precision in determining the material parameters and a sound knowledge of the accuracy of the method in order to avoid misinterpretations in determining the calculated service life. For this purpose, a quality assurance system must be developed for dynamic testing machines used for performance-based testing procedures. As part of this quality assurance system, the technical test specification for test equipment monitoring, Part 705 "Dynamic testing equipment for determining the stiffness and fatigue resistance of asphalt" (working title) is currently being developed. It regulates the basic procedure of a simplified routine test as well as a calibration and system check. However, a detailed technical procedure for calibration and the requirements to be formulated for a calibration service are not described in it. Calibration and system testing should be carried out by a calibration service recognised by the BASt.

Survey on the condition of roads with temperature-reduced and viscosity-modified asphalt after long periods of use (89.0347)
In the course of the current discussion on the introduction of a workplace limit value for vapours and aerosols, (further) temperature reduction – also using viscosity modifying additives – is considered a possibility to reduce vapours and aerosols from bitumen. As the observation period of at least 5 years, which is decisive for the collection of experience, is far behind the usual service life of asphalt layers and as there is no comprehensive documentation on the condition of roads paved with viscosity modified asphalts, there is uncertainty in practice about the actual durability of roads paved with viscosity modified asphalt, especially for roads paved at such low temperatures. The aim of this research project is to assess the durability of this construction method by assessing the condition of roads paved with temperature-reduced and viscosity-modified asphalt.

Determination of the properties of the bituminous binder recovered after extraction using Trichloroethylene and Perchloroethylene (07.0301)
The use of Trichloroethylene (Tri) in the extraction of asphalt is only possible until 2023. After that, other solvents must be used. In neighboring countries, Perchloroethylene (Per) is often used as a solvent. Experience has shown that this solvent shows comparable results to Trichloroethylene with regard to the binder content and the particle size distribution of the extraction. In control tests, the properties of bituminous binders after recovery are determined by the Softening Point Ring and Ball and, in the case of polymer-modified bitumen, additionally by the Elastic Recovery. It has been undisputed in expert circles for years that the Softening Point can lead to incorrect assessments, especially in the case of modified bitumen. The characteristic values determined with the aid of the Dynamic Shear Rheometer (DSR) show that the Softening Point greatly overestimates the viscosity of modified bitumens in some cases. To date, however, there is no reliable information available on the characteristic values of bituminous binders recovered from asphalt. The research project is intended to test whether the properties of the recovered binder are comparable when using the two solvents during extraction. In addition, it is to be examined whether the Softening Point can be replaced by characteristic values determined using the DSR test system.

Alternative methods for evaluating open-pore asphalt wearing courses with regard to voids content and state of compaction (07.0302)
In order to achieve good noise properties of porous asphalt wearing courses (PA), a high void content is required. At the same time, high degrees of compaction are required in order to achieve the longest possible service life. To determine the degree of compaction, the bulk densities of the core sample and the Marshall test specimen are required as reference values. To determine the void content, the bulk densities of the corresponding asphalt test specimens and the bulk density of the asphalt mixture are required. The bulk densities of PA are determined by dimensions, a method which is viewed very critically in testing practice. The aim of the project is to develop and test alternative methods for assessing PA with regard to voids content and degree of compaction. The construction industry and building authorities must be provided with a proper set of instruments for the testing of PA.

Usage behaviour of hot asphalt with asphalt granulate of different types and quantities and derivation of a simple performance criterion (07.0299)
Previous research and practice on asphalt reuse has shown that careful asphalt granulate management allows reuse at a high level of added value. Various research institutes have even found advantages in low-temperature flexibility, compaction properties and, in some cases, fatigue properties. The aim of the research project is to assess with certainty the influence of the addition of asphalt granulate on fatigue properties and also on low-temperature flexibility in terms of type, quantity and mixing time. In this context, the intensity of the asphalt granulate digestion must also be investigated. Doubts about reuse on the grounds of doubtful asphalt performance must be consistently eliminated.

Robot – Road construction 4.0 – Autonomously working road construction machines 4.0 (88.0158 to 88.0162)
The objective of the “Robot road construction 4.0” research project is to improve working and traffic safety, increase process reliability in asphalt road construction and reduce workplace stress for roadworks staff. This is to be realised by the automation of all working functions of the road construction machinery. This means that in future the road construction machinery should always be able to work in a networked and autonomous manner under the control of the machine operator. Due to the high degree of automation in the asphalt road construction process chain, the project will make an important contribution to boosting the durability of construction work, extending maintenance intervals and reducing the number of construction measures. This results in a significant improvement in the efficiency, reliability and availability of the road infrastructure and in a reduction of the life cycle and usage costs. The “Robot road construction 4.0” research project therefore represents the further development of the “PAST” and “QUAST” projects by connecting individual work functions to each other, automating them, digitally transmitting the reference variables of the work functions and allowing the overall installation process to run autonomously.

Asphalt pavement and extreme temperatures (07.0276)
Sensitivity analyses were used in a research project entitled “Projected climate change and dimensioning of pavements” to demonstrate that the useful life of the asphalt layer in the currently standardised asphalt pavement structures will drop under the impact of the projected climate change and the resistance of the asphalt surface layers to deformations at high temperatures reduced considerably in part. However, it was also possible to show by means of dimensioning calculations that the vulnerability of asphalt caused by high temperatures can be reduced by material adaptation although it is not yet possible to generalise these findings on the basis of the projects conducted so far. In order to obtain reliable statements, the aim of this research project is to reformulate existing types of asphalts in terms of their thermophysical properties in order to achieve enhanced resistance to higher temperatures. The objective is to examine the extent to which the vulnerability of asphalt to the possible effects of climate change can be reduced by material adaptation.

Epoxy III-Elvaloy (3514004)
Due to their underlying support concept and their low mortar content, porous asphalts have excellent noise-reduction characteristics. In addition, the high degree of porosity allows rainwater to run off the road quickly and safely. Due to its low mortar content and porosity, however, the road tends to suffer damage to its grain structure or to lose granular material as a result of intensive use. Furthermore, the lifetime of the asphalt surface courses in comparison to other types of asphalt surface course is considerably shorter due to the severe oxidative ageing of the binding agent. Epoxy resin could be used as a binder additive to extend the lifetime of the mixture and increase its resistance to wear and tear. Previous studies have remained unfinished for reasons of health and safety. Consequently, this project seeks to examine whether it is possible to solve or significantly improve on the challenges being faced by using the epoxy resin “Elvaloy” which has been especially conceived for use with bitumen and which has already been used successfully across the world. This project will therefore carry out asphalt-technological tests on wear and tear and road grip, as well as conventional and rheological tests on binding agents.

Asphalt wearing course on continuously reinforced concrete surface - scientific monitoring of the test sections in composite construction during the operating phase (08.0248)
Asphalt and concrete have specific material-related advantages and disadvantages in road construction. Concrete roads are characterized by high durability and longevity. Repair measures to restore the required grip and noise emissions are more complex than with asphalt pavements. Asphalt roads are characterised by their flexibility, comparatively faster production and advantages in the design of surface textures. If the positive properties of both construction methods are combined, the result is a composite construction method. The fundamental advantages were already seen in the 1990s and investigated in four test sections. The aim of this research project is to evaluate these sections. For this purpose, the damage processes are to be analysed. On this basis, recommendations for the planning and execution of the composite construction method are to be worked out.

Practical requirements for the polishing resistance of fine aggregates (06.0116)
Crushed fine aggregates may only be used in asphalt wearing course mixes if the polishing resistance of the representative coarse aggregate meets the respective requirements. If aggregates of different polishing resistance are used, the calculated polishing resistance of the respective aggregate must comply with the required Polish Stone Value (PSV). Only aggregates with a PSV greater than 42 are considered in the calculation. It is therefore not possible to include natural sand or crushed fine aggregates whose coarse aggregates do not meet the polishing resistance requirements. It has been shown that a transfer of the PSV determined on the coarse aggregates to the sand fraction is not always appropriate. The aim of this research project is to formulate practical requirements for the properties of fine aggregates.

Polyurethane asphalt on the duraBASt - INNOBOND III (07.0296)
Innovative road constructions should achieve maximum service life, minimum maintenance, minimum labour input and minimum traffic restrictions over the entire life cycle. It is possible that the current standard construction methods in the area of federal motorways are reaching their limits. Alternatively, polyurethane could fully or partially replace bitumen as a binder in road construction. Polyurethane asphalt has significantly higher deformation and fatigue resistance compared to standard asphalts and the material behaviour is only slightly temperature-dependent. Resistance to frost, freeze-thaw, heat and chemical attack has already been demonstrated. Premature material damage due to traffic loads is not to be expected. Initial laboratory tests and small-format applications have shown that there is potential. The aim of the present research project is the development of a practicable overall construction with a polyurethane asphalt surface layer in the variants "completely permeable construction" and "open-pore surface layer on an impermeable construction layer", which is realised on the duraBASt. The findings are to be evaluated with regard to an application in the network of federal roads.

Development and procurement of a mobile Wehner/Schulze measuring device (3209006)
The Wehner/Schulze polishing process is used to determine the polishing value of coarse- and fine-grained rock on the one hand, while also making the prognostic testing of roadway surface grip possible. Tests are carried out on asphalt or concrete surfaces in the laboratory. Laboratory-produced test samples or drill cores taken from the roadway may be used as laboratory samples. Testing for grip can provide an indication of the final grip values to be achieved. The removal of drill cores does, however, have an adverse effect on the roadway and is also associated with a fair amount of staff costs and organisational effort. The structural division of the Wehner/Schulze system into a polishing and a measuring station does make it possible to carry out in situ tests, using only the measuring sensor. The aim of the project is to develop and procure a mobile Wehner/Schulze measuring unit for comparison with the tyre grip prognosis obtained in a laboratory. This would make non-destructive testing of roadway surfaces possible without the removal of drill cores.