Andrea Graziani

Adhesive and Cohesive Properties of Asphalt-Aggregate Systems Subjected to Moisture Damage

ABSTRACT The bond strength between asphalt and aggregate plays a fundamental role in evaluating the moisture sensitivity of HMA Mixtures. In this study the effect of water on adhesive and cohesive properties of asphalt-aggregate systems was investigated using a modified version of the PATTI. The device was used to measure the pull-off strength on different asphalt-aggregate combinations and to evaluate the influence of water immersion at two different temperatures. In particular, six asphalt binders were employed in combination with two aggregate types, having different asphalt affinity. The effect of the aggregate surface temperature during specimen preparation was also tested. In the first phase of the study the within-laboratory repeatability of the test procedure was investigated. The results showed the PATTI test is able to evaluate with good precision the pull-off strength and that its repeatability depends on the failure type (adhesive or cohesive). In the second phase of the study a full factorial experiment was employed to verify the reliability of the test for routine use in determining the adhesive and cohesive properties of asphalt-aggregate combinations and the effects of moisture damage. The results showed that, in the dry condition, the test was able to measure the internal cohesion of the asphalt binders. The results also showed the effects of water damage on the pull-off strength and the decisive role of asphalt-aggregate affinity was clearly highlighted. Using wet conditioning of the PATTI samples it was proven that water affects the adhesive bond between asphalt and aggregate much more than the asphalt cohesion. Moreover, the results indicate that aggregate temperature during sample preparation has only a limited effect on the adhesive strength.

Compactability and thermal sensitivity of cement–bitumen-treated materials

Full-depth reclamation is one of the most used rehabilitation methods for subbase courses in high-traffic roads. The use of both cement and bituminous binders as binding agents for reclaimed pavement materials can lead to mixtures having high bearing capacity and resistance to permanent deformation, avoiding premature cracking due to shrinkage. This article focuses on two main topics: compactability and thermal sensitivity of cement–bitumen-treated materials (CBTMs). The dosage of liquids in CBTMs is a key parameter in order to obtain an effective compaction. The volumetric characteristics and the optimum liquid content of CBTMs were studied by means of two compaction methods: Proctor and Shear Gyratory Compactor. The temperature susceptibility of CBTMs can be a valuable factor in both design and construction quality control. The influence of temperature on the stiffness modulus of CBTMs was investigated using two testing methods: indirect tensile stiffness modulus and ultrasonic pulse velocity (UPV). While ITSM provided reference modulus values at strain levels and rates typical of traffic loads, UPV was used to estimate Youngs modulus at very low strain levels and high frequencies.

Performance evaluation of a cold-recycled mixture containing high percentage of reclaimed asphalt

Cold recycling of asphalt pavements proved to be an effective maintenance and rehabilitation technology for both environmental and economic reasons. Nevertheless, the use of cold-recycled (CR) asphalt mixtures requires a careful assessment of their mechanical properties, especially when they are designed to replace traditional hot-mix asphalt concrete (AC) mixtures. In this study, the potential use of a CR asphalt mixture as base course of an Italian motorway was evaluated. The studied mixture was produced in a central plant employing high-reclaimed asphalt (RA) content and used to construct two experimental sections along an in-service Italian motorway. In particular, a special mixing procedure, involving the use of water vapour and bituminous emulsion, was tested. A third experimental section was constructed with the same layer thickness using the AC mixture currently used in rehabilitation projects, incorporating 30% of RA. Volumetric properties, stiffness, resistance to permanent deformation and fatigue behaviour of mixtures were investigated by performing tests on samples cored from the three test sections and on laboratory-compacted samples. Results of the mechanical tests showed that CR mixtures provide lower stiffness modulus and lower resistance to repeated loading, but better resistance to permanent deformation when compared with AC. This behaviour can be explained due to the presence of cementitious bonds that reduce thermal sensitivity and viscous response.

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).

Curing and temperature sensitivity of cement–bitumen treated materials

In the present study, the curing process of cement–bitumen treated materials (CBTM) was investigated by analysing the influence of cement dosage and curing temperature on moisture loss and evolution of complex modulus. Moreover, the study aimed to characterise the thermo-rheological behaviour of cured CBTM. Results showed that moisture loss by evaporation controls the increase in stiffness of the mixtures. However, excessive evaporation can hinder the full potential of the cement hydration process. Results also showed that the quantitative effects of curing time and loading frequency on stiffness can be superposed. Similar to hot-mix asphalt, CBTM showed a viscoelastic and thermo-dependent response. In particular, results suggested that at higher frequencies, the iso-thermal viscoelastic response is mainly affected by the aged binder whereas, at lower frequencies, the response of the mixtures depended mainly on the behaviour of the fresh binder.

Influence of reclaimed asphalt content on the mechanical behaviour of cement-treated mixtures

For the rehabilitation of asphalt pavements the upper distressed layers are usually milled before overlaying to eliminate reflection cracking–related problems and to preserve the pavement geometry. This maintenance technique generates a large amount of reclaimed asphalt (RA) as a product of the milling operation. The recycling of RA in cement-treated base and subbase courses represents a valuable solution in terms of technical, economic and environmental benefits. However, the influence of RA on the mechanical properties of cement-treated materials (CTMs) is still not completely understood. As a consequence, CTMs using a high content of RA have not yet been widely applied. The present paper shows the findings of an experimental analysis on CTMs including 50% and 80% RA in comparison with the reference CTM consisting of 100% mineral aggregates. In particular, indirect tensile tests and unconfined compressive tests were conducted to evaluate the resistance characteristics of the CTMs. In addition, complex modulus tests and ultrasonic pulse velocity tests were performed to investigate the stiffness properties of CTMs. The investigation shows promising results as regards the use of high percentages of RA in CTMs and offers a substantial contribution for the understanding of the mechanical behaviour of CTMs.

Complex modulus characterisation of cold-recycled mixtures with foamed bitumen and different contents of reclaimed asphalt

Cold-in place recycling with foamed bitumen (FB) is currently employed for the rehabilitation of road pavements, from local roads to heavy-duty motorways. The mechanical properties of FB mixtures may vary from that of an improved granular material to that of an asphalt concrete, depending on their composition. The objective of the present study was to characterise the linear viscoelastic (LVE) response of FB mixtures with different contents of reclaimed asphalt (50%, 70% and 0%). This objective was tackled by measuring the complex Youngs modulus by means of uniaxial cyclic compression tests, in a range of temperatures (from −20°C to 55°C) and frequencies (from 0.03 to 10 Hz). The thermo-rheological behaviour was modelled using the Huet–Sayegh model, commonly used for bituminous mixtures. In order to focus on the influence of reclaimed asphalt content, the same grading and FB content were used for all the mixtures. In addition, an effort was made to produce mixtures with similar volumetric properties. Considering all the tested mixtures, the stiffness modulus ranged between 171 and 4075 MPa, whereas the phase angle ranged between 4° and 18°. Though these values are well below those normally measured on asphalt mixtures, the frequency and temperature variations clearly highlight a typical asphalt-like behaviour. In addition, the time–temperature superposition principle can be considered valid, regardless of the reclaimed asphalt content. Overall the experimental results showed that it is possible to characterise the LVE response of cold-recycled FB mixtures using the same experimental and analytical approach adopted for asphalt mixtures.

The evolution of the mechanical behaviour of cold recycled mixtures stabilised with cement and bitumen: field and laboratory study

Road construction and maintenance involve huge amounts of materials that can include wastes from the demolition of old asphalt pavements. Recycling allows environmental and economic benefits to be achieved, by reducing the consumption of natural resources. These advantages are maximised with cold-recycling, in particular when full depth reclamation (FDR) is adopted as road rehabilitation technique. The aim of this research was to compare different FDR techniques, which produce cement-treated material and cement–bitumen treated material (CBTM) combining the use of cement and bitumen emulsion or foamed bitumen. The FDR was applied in a trial section built along an in-service highway and monitored with annual falling weight deflectometer (FWD) surveys in order to evaluate the performance evolution. Results highlighted that, because of the effect of temperature, the FWD deflections were not suitable to accurately evaluate the evolution of the mechanical behaviour of the cold recycled mixture (CRM) layers. However, the temperature-corrected FWD moduli allowed to conclude that the performance of the CRM layers was similar.

The Effect of Curing on the Mechanical Behavior of Cement-Bitumen Treated Materials

The re-use of pavement materials is an efficient and cost-effective solution in road rehabilitation and construction activities, especially when the availability of high-quality virgin aggregates is limited. In this context, cold recycling of bituminous pavements is becoming one the of most attractive and low environmental impact techniques. The use of cold-recycled pavement mixtures requires a careful assessment of their mechanical properties, which are influenced by both compositional and environmental factors. In particular, regardless of aggregate nature, binder type and dosage, a distinctive feature of cold recycled mixtures is the requirement for a certain curing period to develop the ultimate values of strength and stiffness. In this study, the mechanical behaviour of cement-bitumen treated materials (CBTM), containing high percentage of reclaimed asphalt (RA), was evaluated considering the influence of curing time and temperature. Two CBTM containing 1.0 and 2.5 % cement and 2.0 % of fresh bituminous binder were analyzed. Cylindrical specimens were compacted using a gyratory compactor and cured at 25 and 40 °C; moisture loss and indirect tensile strength (ITS) were measured at increasing curing times. Results showed that the curing temperature and time (curing conditions) significantly affect the moisture loss by evaporation that therefore can be considered a good estimator of the curing process. Moreover, the mechanical characterization indicated that the moisture loss and the cement content control the increase in strength properties of the investigated CBTM. Results also showed that the cement content strongly affects the moisture loss in addition to assure improved mechanical behavior.

Influence of Specimen Dimension and Test Speed on the Shear Strength of Bituminous Interfaces

In view of the fact that pavements are multilayer systems, achieving high bonding between layers is a key element to increase service life. Interface debonding is mainly responsible for the slipping failure of pavements that leads to high rehabilitation and maintenance costs. The bonding between asphalt layers is usually evaluated by testing the interlayer shear strength and is affected by several parameters such as test speed, test temperature, normal stress applied and specimen diameter. This paper focuses on the effect of test speed and specimen diameter on the shear strength evaluated through the Leutner equipment, for a typical dense graded asphalt mixture. Leutner tests were carried out on double-layered specimens with a diameter of 100 and 150 mm and with interlayer deformation rates corresponding to nominal test speeds of 1, 2.5, 5, 10, 25 mm/min. The effective interlayer deformation rate was calculated by measuring the deformation through an external transducer in order to perform a reliable data analysis. Results showed a steady increase in the shear strength with the increase in the interlayer deformation rate. Moreover, a clear scale effect was observed at any test speed resulting in higher values for shear strength measured on specimens with diameter of 100 mm.