Davide Lo Presti

Tyre rubber-modified bitumens development: the effect of varying processing conditions

Tyre rubber-modified binders (TR-MBs), produced through McDonalds wet process and used worldwide (e.g. asphalt rubber), have been demonstrated to provide various benefits to pavements and, moreover, they represent a good opportunity for recycling tyre rubber (TR). However, this technology is still struggling to be fully adopted in Europe, mainly because of their poor stability during high temperatures storage, which leads to high initial costs in modifying existing asphalt plants. Storage stable TR-MBs are proving to be a great option and their development could be the key to spread the recycling of TR within paving applications in Europe. This paper aims to enrich this field of research by presenting the results of a study focused on the optimisation of laboratory procedures to better understand the effect of varying processing conditions on the rheology of TR-MBs. The experimental programme has been carried out by a preliminary selection of materials, followed by the production of the TR-MBs, via practical laboratory protocols. A continuous comparison between two commercially used styrene–butadiene–styrene (SBS)-MBs, with high and medium levels of modification, and the produced TR-MBs helped to understand the effect of varying the selected processing conditions on binder properties. Results have shown that TR-MBs are a very effective alternative to commercially used SBS-MB. Nevertheless, the rheology of TR-MBs is very sensitive to the variation of the processing conditions, so appropriate selection of materials and a superior binder design are mandatory to achieve the desired level of modification.

Laboratory Mix Design of Asphalt Mixture Containing Reclaimed Material

This paper presents a study on the production of asphalt test specimens in the laboratory containing reclaimed asphalt. The mixtures considered were stone mastic asphalt concrete mixtures containing up to 30% of reclaimed asphalt. Specimens were compacted to the reference density obtained from the Marshall mix design. Gyration compaction method was used for preparing specimens for the experimental programme, while coring and cutting methods and X-ray computed tomography (CT) were used to investigate the change in properties within the specimens and to validate the selected methodology. The study concluded that gyratory compaction is suitable to produce homogeneous test specimens also for mixtures containing high amount of reclaimed asphalt. Nevertheless, preliminary trials for each material are mandatory, as well as final coring and trimming of the specimens due to side effects.

An investigation on using pre-treated tyre rubber as a replacement of synthetic polymers for bitumen modification

Rubberised bitumen obtained through a swelling process, has widely proved to be a successful technology for asphalt pavement applications and a solution to reduce the dismantling of tyre rubber on landfills. However, this technology presents two main operative issues which need the adoption of costly special equipment. First, significant high value of high-temperature viscosity (HTV), which imposes mixing and compaction difficulties, leads to increased energy consumption and emissions. Furthermore, during the hot storage period, phase separation between rubber particles and the base bitumen could occur. Developing recycled tyre rubber-modified bitumen (RTR-MB) with improved storage stability and reduced values of HTV could allow using this technology in standard asphalt plants, resulting in an environmental-friendly and cost-effective option of standard polymer-modified bitumen. In this study, two different pre-treated and one straight ambient recycled rubbers were used to produce RTR-MB. The first RTR was pre-treated by special oil and warm-mix additives and the second was partly de-vulcanised. Also, two base binders were selected with large differences in mechanical properties in order to identify the effect of base binder. The HTV was successfully reduced by using pre-treated RTR. The use of RTR together with Fischer–Tropsch wax (Sasobit®) in bitumen technology offered superior high in-service temperature properties and reduced value of HTV, and thus can be the preferred option over styrene–butadiene–styrene modification.

Linear and nonlinear fractional hereditary constitutive laws of asphalt mixtures

The aim of this paper is to propose a fractional viscoelastic and viscoplastic model of asphalt mixtures using experimental data of several tests such as creep and creep recovery performed at different temperatures and at different stress levels. From a best fitting procedure it is shown that both the creep one and recovery curve follow a power law model. It is shown that the suitable model for asphalt mixtures is a dashpot and a fractional element arranged in series. The proposed model is also available outside of the linear domain but in this case the parameters of the model depend on the stress level.

Rubberised bitumen manufacturing assisted by rheological measurements

This paper investigates the effect of processing temperature and time on the rheological properties of recycled tyre rubber-modified bitumens (RTR-MBs) produced using two different base binders and an ambient ground crumb rubber modifier (CRM). The production of the RTR-MBs was accomplished by means of a standard Brookfield rotational viscometer together with a modified impeller, dual helical impeller, to allow mixing as well as real-time viscosity measurements of the produced RTR-MBs. The rheological properties of the final RTR-MBs were determined by means of standard dynamic mechanical analysis oscillatory and multiple stress creep recovery testing using a dynamic shear rheometer. The results indicate that the low processing conditions (160°C and 60 min) are not appropriate for developing RTR-MBs with enhanced physical and rheological properties. However, allowing the crumb rubber to interact with the base binder for longer mixing times (140 min) led to the development of an enhanced rubber (polymer) network structure within the blend (i.e. swelling of the CRM particles) and superior rheological properties. At the other extreme, using high-processing conditions (200°C and 140 min) led to RTR-MBs in which the rubber network had been subjected to devulcanisation and depolymerisation with a subsequent reduction in modification.

Linear viscoelastic properties of high reclaimed asphalt content mixes with biobinders

The use of high Reclaimed Asphalt (RA) content mixtures together with binders produced from renewable resources (biobinders) is one of the current challenges in pavement engineering research. On the one hand, RA has been used for decades, but there are still some concerns about its performance, especially when high contents are used (>30%). On the other hand, biobinders are relatively new materials, which have to be deeply characterised and studied in order to develop good practices for their use. In this paper, linear viscoelastic properties of biobinders and bio-mixtures manufactured with high RA content and biobinders are analysed and discussed. High-modulus mixtures with 50% RA were selected for the mix design. Binders and mixtures were tested over a wide range of asphalt service temperatures and frequencies by means of Dynamic Shear Rheometer and two-point bending tests, respectively. Results show that biobinders have an important effect on mixtures behaviour. However, no direct links between their linear viscoelastic properties were found. Bio-asphalt mixtures still need further development for commercial exploitation; however, the take-away fact of this investigation is that it is possible to manufacture asphalt-like mixtures with acceptable viscoelastic properties while being composed only of RA and non-petroleum-based binders.

Rolling resistance contribution to a road pavement life cycle carbon footprint analysis

A bstractPurposeAlthough the impact of road pavement surface condition on rolling resistance has been included in the life cycle assessment (LCA) framework of several studies in the last years, there is still a high level of uncertainty concerning the methodological assumptions and the parameters that can affect the results. In order to adopt pavement carbon footprint/LCA as a decision-making tool, it is necessary to explore the impact of the chosen methods and assumptions on the LCA results.MethodsThis paper provides a review of the main models describing the impact of the pavement surface properties on vehicle fuel consumption and analyses the influence of the methodological assumptions related to the rolling resistance on the LCA results. It compares the CO2 emissions, calculated with two different rolling resistance models existing in literature, and performs a sensitivity test on some specific input variables (pavement deterioration rate, traffic growth, and emission factors/fuel efficiency improvement).Results and discussionThe model used to calculate the impact of the pavement surface condition on fuel consumption significantly affects the LCA results. The pavement deterioration rate influences the calculation in both models, while traffic growth and fuel efficiency improvement have a limited impact on the vehicle CO2 emissions resulting from the pavement condition contribution to rolling resistance.Conclusions and recommendationsExisting models linking pavement condition to rolling resistance and hence vehicle emissions are not broadly applicable to the use phase of road pavement LCA and further research is necessary before a widely-used methodology can be defined. The methods of modelling and the methodological assumptions need to be transparent in the analysis of the impact of the pavement surface condition on fuel consumption, in order to be interpreted by decision makers and implemented in an LCA framework. This will be necessary before product category rules (PCR) for pavement LCA can be extended to include the use phase.

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.

Rolling resistance and traffic delay impact on a road pavement life cycle carbon footprint analysis

The application of Life Cycle Assessment (LCA) to road pavements has been continuously evolving and improving over the last years, however there are several limitations and uncertainties in the introduction of some components in the framework, such as road pavement rolling resistance – in terms of pavement surface properties – and traffic delay during maintenance activities. This paper analyses the influence of methodological assumptions and the model used to estimate the increased emissions for traffic delay and road pavement rolling resistance on the results of an LCA. The Greenhouse Gases (GHG) emissions related to these two phases of a pavement LCA will be calculated for a UK case study, using different models, and a sensitivity test is performed on some specific input variables. The results show that the models used and the input variables significantly affect the LCA results, both for the rolling resistance and the traffic delay. (Galatioto et al. 2015; Huang et al. 2014). Usually, these software tools include one or more emission models able to define the impact from the work zone. A simplified approach, based on the demand-capacity (D-C) model, defined in the Highway Capacity Manual (HCM) (Transportation Research Board 2010) that describes the work zone average queue and speed. An emission model is used, based on the output provided by the traffic models. This model was used by the Federal Highway Administration (FHWA) to develop a computational approach to analyse the user cost of work zone traffic delay, in life cycle cost analysis (LCCA) (Walls III and Smith 1998). The advantage of using a simplified approach is the ease of implementation, requiring limited data input, such as hourly traffic volume, capacity and Traffic Management (TM) layout. However, the accuracy of the results could be compromised especially when the TM scheme is particularly complex (Wang et al. 2014b) or the area of impact is extensive and requires the modelling of a wider network. By contrast, an approach based on microscopic modelling is more flexible and accurate, producing disaggregated traffic data and it can readily include the wider network. However, these models are usually incorporated in commercial software that increase the cost of the analysis and require detailed traffic data, which can limit the size of the network model. The model used to calculate the emissions related to this component can affect the results, especially for high traffic volume roads. The road pavement rolling resistance is the energy loss due the pavement-vehicle interaction (PVI) and it is affected by the tire properties and by the pavement surface condition. Roughness and macrotexture, usually represented by International Roughness Index (IRI) and mean profile depth (MPD), are the pavement surface properties affecting the rolling resistance. These parameters change over the life of a pavement and their variation may be different for each lane, depending on the traffic volume and type, the surfacing type and the regional climate. In order to estimate the impact of the pavement surface properties on vehicle fuel consumption, several models have been developed (Chatti and Zaabar 2012; Hammarström et al. 2012; Wang et al. 2014a). However, there is still uncertainty concerning the lack of validated models used to analyse the vehicle emissions and the influence of specific variables and assumptions on the results. Indeed, the literature related to the influence of road surface properties on vehicle rolling resistance and emissions shows different results, possibly because road surface components are a relatively small part of the rolling resistance and of the total driving resistance and it is difficult to isolate the road surface effects from other effects and quantify their contribution and different methods of measuring rolling resistance can lead to different results. In the UK, this component is not generally included in the pavement LCA framework and there are no general pavement deterioration models to predict the deterioration rate of IRI and MPD, based on UK data. Some empirical deterioration models have been developed for specific maintenance treatments and geographical areas (Lu et al. 2009; Tseng 2012). However, in these models both the IRI and MPD values tend to increase over time, so they are not applicable to a UK case study where the MPD will generally decrease with time. The model used to investigate this impact and the input deterioration model can influence the conclusions of a study, making the results unreliable. Recent studies have included these two components (Santero and Horvath 2009; Santos et al. 2015; Trupia et al. 2016 (in press); Xu et al. 2015) and during the last Pavement LCA Symposium in 2014 (Harvey and Jullien 2014), papers related to the emissions due to the work zone traffic delay (Huang et al. 2014; Wang et al. 2014b) and the impact of the PVI rolling resistance during the use phase (Akbarian et al. 2014; Ciavola and Mukherjee 2014) were presented. Although efforts have been made to fill the research gaps related to these components, there is still a level of uncertainty concerning the methodological assumptions, the chosen methods to analyse the vehicle emissions and the parameters that can affect the results. The aim of this paper is to explore the influence of the model used and the methodological assumptions to estimate the increased emissions due to work zone traffic delay and the PVI rolling resistance phases on pavement LCA results. The GHG emissions related to these two phases will be calculated for a UK case study, using different models from the literature, and a sensitivity test is performed on specific input variables (extent of the area of impact of the work zone traffic delay and surface condition deterioration rate for the PVI rolling resistance).

Reclaimed asphalt binders and mortars fatigue behaviour

Fatigue cracking is one of the most important failure mechanisms occurring in asphalt pavements, especially when mixtures incorporate considerable amount of rReclaimed asphalt pavement (RAP). In fact, aged binders contained in RAP generally make asphalt more brittle and specifically reduce fatigue resistance of the resulting asphalt mixtures. Binders and mortars play a key role in this phenomenon, considering fatigue cracking usually starts within these asphalt components. However, performance-related tests and specifications commonly regard binders and there are no sound methodologies allowing the use of mortars to predicting fatigue performance of asphalts containing RAP. For this reason, in this paper, fatigue resistance of extracted binders from high-RAP content mixtures and of RAP mortars (passing sieve with an opening size of 0.15 mm) were assessed and compared. Binders were extracted from asphalt mixtures manufactured with 30%, 60% RAP and rejuvenators. Mixtures recipes were then reproduced to manufacture mortars accordingly. Time sweep tests in stress-controlled mode were carried out on both materials (binders and mortars) and the resulting fatigue laws were compared. As a result, a strict correlation was obtained, leading to affirm fatigue-related properties of RAP mixture could be assessed by directly testing RAP mortars. This makes the recovery of RAP binders unnecessary. Moreover, a relationship between the two fatigue laws versus the percentage of fine particles in the mortar was found. This latter relationship allows determining the fatigue law of mortars corresponding to any percentage of fine particles and therefore corresponding to any percentage of RAP.