Fabrizio Cardone

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.

Full-depth reclamation for the rehabilitation of local roads: a case study

Full-depth reclamation (FDR) techniques for pavement construction and rehabilitation have gained general recognition because of their technical, economical and environmental advantages. The use of cement–bitumen-treated material (CBTM) has rapidly increased over the last 10 years mainly in motorways. Public administrations have recognised the advantages of FDR, and this technique has also begun to be applied for the rehabilitation of local and/or rural roads. This paper shows the results from a trial section built to verify the suitability of FDR for the rehabilitation of local roads. The design planned the in situ stabilisation with styrene-butadiene-styrene (SBS)-modified bituminous emulsion and cement of hot mix asphalt and foundation course. The mechanical characteristics of CBTM were evaluated by means of stiffness modulus tests at different temperatures and fatigue tests. The obtained results offer good prospects for the application of FDR in the maintenance project of local roads.

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.

Innovative Testing Protocol for Evaluation of Binder-Reclaimed Aggregate Bond Strength:

The durability of asphalt mixtures is strongly related to the adhesion properties developed at the interface between binder and aggregates. The loss of adhesion implies a rapid deterioration (e.g., stripping or raveling) of pavement layers under traffic loads, especially when the pavement is affected by the presence of moisture. Adhesion is a complex phenomenon related to the mineralogical and morphological nature of aggregates as well as to the chemical binder composition and the environmental conditions. The evaluation of adhesion has become even more complicated as an increasing percentage of reclaimed asphalt pavement (RAP) is used in the production of new asphalt mixes. Therefore, adhesion properties are also related to the mechanisms developed at the interface between virgin binder and aged binder that coats the RAP aggregate surface. An innovative procedure to evaluate the compatibility of the system virgin binder–RAP aggregate was proposed in this study. This procedure allowed the substrate of a RAP aggregate to be simulated in the laboratory and could integrate the binder bond strength test currently used to investigate bonding properties and water sensitivity of the system binder and virgin aggregates. Tests were performed with various aggregate sources, several modified binders, and two conditioning types (dry and wet). It was found that the procedure was able to discriminate different test configurations and variables. In particular, the artificial reclaimed aggregate substrate ensured higher adhesion performance compared with the virgin aggregate, especially in the wet condition, regardless of the modification level of the virgin bitumen.

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.

Fatigue rheological characterization of polymer-modified bitumens and mastics

Fatigue is one of the major distresses of flexible pavements and is mainly related to the rheological properties of the bituminous components of mixtures. In particular, bitumen and mineral filler create a blend called mastic that significantly influences the service life of asphalt pavements depending on its nature and composition. The purpose of this study is to investigate the effects of different polymer types and mineral fillers on the rheological behavior of a plain bitumen. Two types of polymer (an elastomer and a plastomer) were employed to produce polymer modified bitumens (PMBs) through laboratory mixing. Moreover, two fillers characterized by a different mineralogical nature (limestone and basalt) were selected in order to obtain several mastics. The dynamic shear rheometer (DSR) was used to study the fatigue behavior of all materials. Experimental data show that the effect of both polymer types is similar on mastics and bitumens as the presence of the elastomer leads to an improvement in fatigue life whereas the presence of the plastomer leads to a slight decrease in fatigue performance with respect to the plain bitumen, regardless of the mineral filler type. Moreover, the stiffening effect of mineral fillers was found to be significant regardless of filler mineralogy and bitumen type leading to a decrease in fatigue life with respect to bitumens. All mastics were less sensitive to the strain level applied as compared to the corresponding bitumens.

Resilient behaviour of unbound granular materials through repeated load triaxial test: influence of the conditioning stress

The present paper reports the results of a laboratory investigation aimed to investigate the resilient behaviour of unbound granular materials (UGMs) for pavement unbound layers under repeated triaxial loading. The first part of research was focused on the effects of stress history, in terms of loading conditioning phase and stress paths, on the resilient response of the investigated UGM. The obtained results highlighted that the samples that experienced several stress paths after an initial stress conditioning showed reduced resilient moduli as well as higher stress dependency of the resilient properties as compared with samples not subjected to any specific stress history. The second part of research aimed to evaluate the influence of the conditioning stress level on the resilient modulus. The results showed poorer resilient characteristics along the several stress paths as the selected conditioning stress increases. Whereas, a sort of “loading memory effect”, resulting in a similar mechanical behaviour, can be recognised as a similar stress state between conditioning phase and subsequent stress path is considered. The study confirms the high dependency of the resilient response of UGM on the stress condition and in particular, the main role of conditioning confining pressure on the stable mechanical behaviour of these materials is highlighted. Finally, a simplified approach that would allow the intrinsic resilient properties of UGM to be predicted for a specific combination of conditioning stresses and stress paths is proposed.

Time–temperature superposition principle for interlayer shear strength of bituminous pavements

Poor interlayer bonding leads to a reduction in service life of bituminous pavements, thus the identification and measurement of the parameters affecting interlayer shear strength (ISS) are becoming increasingly important. This study focuses on the effects of test temperature and interlayer deformation rate (IDR) on the ISS of double-layered asphalt concrete specimens, with the aim of comparing two different shear testing devices. Specifically, tests were performed by means of Ancona Shear Testing Research and Analysis and Leutner devices, at temperatures ranging from 5°C to 40°C and deformation rates ranging from 1 to 50 mm/min. Moreover, two interface conditions (with and without tack coat) were investigated. Experimental data showed that for both shear testing devices, higher IDR results in higher ISS, because of the time-dependent response of bituminous mixture and that the effect of IDR and temperature on ISS can be superposed allowing a master curve to be satisfactorily obtained using a three-parameter sigmoidal model.

Mechanical behaviour of cement-bitumen treated materials containing different amounts of reclaimed asphalt

Cold recycling techniques have become one of the main sustainable solutions in road construction and maintenance. The use of reclaimed asphalt (RA) as a constituent material for bound mixtures offers two main advantages: it eliminates problems concerning disposal and it allows natural resources to be preserved. Among the most widespread recycled mixtures, the use of cement–bitumen-treated materials (CBTM) for subbase courses has rapidly increased over the last 10 years. This paper deals with the influence of RA content on the mechanical characteristics of CBTM. Several mixtures were produced by combining different dosages of binders (cement and bituminous emulsion) and RA contents. The behaviour of recycled mixtures was evaluated and compared in terms of indirect tensile strength, water sensitivity and stiffness modulus. In addition, the effects of temperature on the stiffness properties were investigated. Experimental results showed that the RA content did not significantly affect the compactability and the strength properties of CBTM. The presence of RA determined an increase in mix deformability and thermodependence, but the effect can be inhibited by increasing cement dosage. The findings offer good prospects for increasing the use of RA and encourage administrations to employ this material as an effective construction solution.