Konrad Mollenhauer

Evaluation of hot-mix asphalt susceptibility to temperature-induced top-down fatigue cracking by means of Uniaxial Cyclic Tensile Stress Test

A dominant failure mode in hot-mix asphalt (HMA) layers is material fatigue, occurring when the asphalt layer is repeatedly loaded by tensile stresses. The maximum stress arises at the bottom of the asphalt layer and, in consequence, crack development is initiated at the bottom. Various techniques are known that simulate bottom-up fatigue cracking in the laboratory. Fatigue failure is also observed on top of the pavement. Especially in cold climates, temperature-induced top-down cracking initiated at the pavement surface is a well-known failure mode. Usually, horizontal tensile stresses in the surface layer are smaller than stresses in the bottom layer. At cold temperatures and when temperature falls within a short period of time, the traffic-induced stress is superposed by temperature-induced stress, and the total stress may come up to the tensile strength of the material. In order to test the materials susceptibility to temperature-induced top-down fatigue cracking in the laboratory, the Uniaxial Cyclic Tensile Stress Test (UCTST) was established and recently introduced in European Standards (prEN 12697-46). In this test, a prismatic shaped HMA beam is subjected to a constant tensile load (representing the temperature-induced stress) which is superposed by a sinusoidal tensile load (representing the traffic load). This loading situation results in a visco-elastic strain response on the one hand, and in an accumulation of visco-plastic strain on the other. Different failure modes are observed in UCTST. In this paper, the failure modes occurring in UCTST, i.e. cracking of the specimen, stiffness decrease, and creep, are discussed based on the results from laboratory testing of nine different HMA and on variation of loading conditions.

Influence of asphalt compaction procedure on 3D deformation properties

In this study, the effect of various laboratory procedures for compacting hot-mix asphalt to form test specimens is investigated. Based on the triaxial cyclic compression test (TCCT) according to the European specification EN 12697-25, the resistance to permanent deformation of the specimen obtained from different types of laboratory compaction is studied. The deformation behaviour of the laboratory-compacted specimens is always compared with the deformation properties stated in TCCT for the asphalt mix specimen taken from on-site roller-compacted road pavement. As a result, the effect of the choice of laboratory compaction procedure on the resulting deformation behaviour in TCCT is assessed.

Precision of Iatroscan Method for Assessment of SARA Compounds in Bitumen

Bitumen consists of thousands of different chemical molecules which are difficult to distinguish. Iatroscan method using thin layer chromatography with flame ionization detection (TLC/FID) is a comparably simple test method to characterize bituminous binder with regard to the colloidal bitumen structure in saturates, resins and asphaltenes (SARA). In a round robin test, the general precision of the test procedure was confirmed. However, especially the reproducibility of the test procedure is comparably especially in terms of the proportion of aromatic compounds. Within a sensitivity study, important test parameters were identified which will be further controlled in new RRT studies in order to improve the test precision.

Effect of wax crystallization on complex modulus of modified bitumen after varied temperature conditioning rates

Most of European roads are paved with asphalt materials. Mechanical properties as well as durability depend on bituminous binder properties. To influence viscous binder properties wax additives are applied in asphalt mixture for reducing temperature during production process. The crystallization of wax additives results in a rapid viscosity changes within a small temperature span. This allows the reduction of asphalt mix temperatures as well as affects the complex modulus within the performance temperature range. In order to evaluate the effect of wax crystallization in bituminous binders, three binders of different viscosity are modified with 0%, 1.5% and 3% Fisher-Tropsch wax. For the rheological characterization complex shear modulus and phase angle are measured by variation of the cooling rate after sample trimming. Furthermore physical properties were determined by softening point ring and ball again with varied cooling of the bitumen sample after specimen preparation.

Benefits of F-T wax based warm asphalt mixes for short-term binder aging and pavement durability

Wax based warm mix additives are increasingly applied to enable asphalt mixing and paving at lower temperatures or to improve the workability and compactability of hot-mix asphalt at conventional temperatures. The effects of these wax additives on the rheological behaviour and mechanical properties of bitumen and asphalt were already extensively investigated. Effects on the chemical properties of bitumen, such as aging, are largely unknown. For evaluating the effects of F-T wax, 50/70 bitumen was modified with F-T wax and analysed after short term aging (RTFOT). RTFOT at 163°C simulated the asphalt mixing and paving at conventional temperature. Warm mix conditions were simulated by RTFOT at 143°C in order to consider the lower mixing temperatures in the plant. The aging of the binders was characterised by measuring rheological properties (DSR, penetration, softening point), chemical analysis (IR spectroscopy; SARA chromatography) and analysis of the colloidal components (asphaltenes fractionation). The results show that the chemical composition is only determined by the bitumen characteristics. F-T wax modification significantly influences the mechanical properties after ageing. Compared to the unmodified binder, wax modification results in decreased effects of ageing on the shear properties in the complete service temperature range. The warm mix aging simulation indicated the significantly reduced aging and the resulting potential for increased pavement durability by applying wax additives.

Categories for stiffness and fatigue based on cyclic indirect tensile tests and their applicability in construction contracts

The cyclic indirect tensile test (CIT-CY) was recently introduced as additional test procedure to European standardized asphalt test procedures. In Germany the pavement design can be based on material characteristics obtained on stiffness modulus and fatigue functions which are evaluated with CIT-CY. In order to adopt the rules provided by European construction products directive for these performance-based material properties, categories were introduced to the product standards which will allow the definition of requirements on these important characteristics. Based on material properties obtained during various research and practical pavement construction projects, categories could be defined which allow the classification of these asphalt mixtures regarding properties which were applied in pavement design calculations. In order to check the applicability of these categories in construction contracts, former research projects where evaluated regarding the effect of systematically varied asphalt properties on the resulting stiffness and fatigue properties. In addition to this, asphalt mixtures were sampled as loose asphalt mix after industrial production, cored from the completed pavements and also mixed in laboratory with using the constituent materials. Based on the found variability in the performance properties a concept for the contractual implementation of requirements for stiffness and fatigue is proposed.

Tolerances for inhomogeneity of pavement structure for in-situ cold recycling

Cold recycling technique is a road construction method for producing a new base layer from existing road material. For in-situ cold recycling, a recycler mills the existing road structure in a depth up to 30 cm by mixing these materials with bituminous emulsion or foamed bitumen and/or hydraulic binder (e.g. cement). The composition of the mix granulates results from the structure of the recovered pavement and may contain different proportions of reclaimed asphalt, reclaimed cement concrete and reclaimed unbound material. The mix design of the new cold-recycling material is optimised for the site-specific mix granulate composition. Though, pavement structures may show inhomogeneities due to partly conducted road maintenance, road widening or former excavation works. In this study it is evaluated, in what extend inhomogeneities in pavement structure will influence the mechanical properties of cold recycling materials. Therefore, cold recycling mixtures are produced with constant binder content by varying the mix granulate composition (reclaimed asphalt, reclaimed cement concrete and reclaimed unbound material) to evaluate the sensitivity of the material performance on differing pavement structures. As a bias, it is evaluated if the binder of the reclaimed asphalt materials affects the properties of the new cold recycled material. In total eight different cold recycling mixes were produced in laboratory by varying the composition of the mix granulate material. All mixtures were produced with the same grading, a constant residual virgin bitumen content of 4% and cement content of 2%. After static compaction, indirect tensile strength after 7 and 28 days of conditioning, water susceptibility and CBR properties were tested. Limits of pavement inhomogeneity could be evaluated which may be tolerated during cold recycling mix application. Further the test results indicate a significant effect of old RA bitumen on the performance of the cold recycled material.

Mix designs for cold recycled pavement materials considering local weather and traffic conditions

Cold recycling is a road rehabilitation procedure/technique, where the reclaimed road material from rehabilitated pavements is recycled completely and used in the new structure with only small contents of new road materials. This is done preferably in-situ to save time, costs and environment. However, internationally various mix design procedures were developed since decades resulting in diverse contents of bituminous binders (emulsion or foamed bitumen) and/or mineral binders (cement or hydraulic road binder). The different material compositions result in diverse mechanical material properties and demand for different pavement designs. Based on an international comparison of cold recycling experience, commons and differences were elaborated during European CoRePaSol project funded by the CEDR. The existing definitions of various cold recycled materials were assessed and supplemented in order to introduce clear material definitions in future European specification documents. Based on intensive test campaigns suitable assessment procedures are proposed to address these materials. At the same time based on local traffic and weather conditions as well as availability of source materials, a decision model is proposed for choosing the optimum cold recycling material for the given rehabilitation project.

End-of-life strategies for cold recycled mixtures and the multiple recycling approach

Cold recycling is a road rehabilitation procedure, where the reclaimed road material from demolished pavements is recycled completely in the new structure with only small contents of new road materials. Cold recycling is applied worldwide since several centuries but little is known about end-of-life strategies for this kind of pavement materials. However, European construction regulation demands for taking the recycling strategies into consideration already during the mixture and pavement design phase. Therefore the multiple recyclability of cold recycled materials was analyzed during European CoRePaSol project. Therefore, artificially aged cold recycled material was prepared in laboratory in order to simulate end-of-life characteristics of these road materials. On these material options for multiple cold recycling as well as the application of hot and warm recycling in new asphalt mixtures was analyzed designing suitable mixes with reduced binder content and RAP content of up to 30 %. The results indicate that cold recycled pavement layers can be rehabilitated again by applying cold recycling at least once. However, each applied cold recycling cycle will raise the bituminous binder content which may lead to lower stiffness and reduced resistance against permanent deformation. It is necessary to focus more intensively on combining these approaches with rejuvenation and better reactivation of the used bitumen. Nevertheless the cold recycled material also can be recycled in new warm and hot-mix asphalt where the bitumen originating from the initial asphalt pavement as well as from the applied cold recycling cycles can be reactivated in future. Here common recycling strategies can be applied without the need for modifications. As a result, the recyclability of cold recycled materials is similar to the recyclability of road layers composed of hot-mix asphalt and don’t indicate a constraint against the application of this methodology in pavement rehabilitation.