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Research into improvements of hot mix asphalt (HMA) materials, mix designs, and methods of pavement evaluation and design, including laboratory and field-testing can provide extended pavement life and significant cost saving in pavement maintenance and rehabilitation.
Silicone rubber has been one of the most promising materials because of its unique properties including temperature flexibility, excellent elastic recovery after load application, biocompatibility, oxidation resistance, thermal stability, water repellency; climate resistance, low surface tension and unique high permeability because of the unique structure of polysiloxane. Silicones are semi-inorganic polymers consisting of silicone, oxygen, and organic molecules. Silicones can be fluids, gels or rubber like solids. ). Al-an T.M.A et al (2009) investigated the effect of rubber silicone additive on physical properties of asphalt cement. Results showed that rubber silicone has more effects on performance of asphalt mixture by increasing the Marshal stability, air voids, and reducing the flow and bulk density compared with the original mix. Results also showed that rubber silicone increases the flexibility properties of the asphalt mix and this appears to reduce the permanent deformation at test temperature (60C).
This potential of silicone rubber modified asphalt mixtures to enhance permanent deformation will be investigated further in this study based on finite element method Abaqus 6.14 software and laboratory experiment.
Rutting is a main distress encountered in asphalt pavements, especially when the temperature is high. Rutting is caused by the accumulation of permanent deformation in all pavement layers under the action of repeated traffic loading. Among the contributions of rut depth by various pavement layers, the cumulative permanent deformation in the surface course of asphalt pavement is known to be responsible for a major portion of final rut depth measured on flexible pavements as indicated by the permanent deformation or rut depth along the wheel paths. The width and depth of rut are widely affected by structural characters of pavement layers (thickness and material quality), traffic loads and environmental conditions (Al-khateeb A.L et al (2011). Construction of pavement structures with modified chacteristics can offer better performance as well as longer service life.
Conventional asphalt binder has been used for many years but exhibits poor performance in harsh environments for example areas of vehicle acceleration or deceleration, steep curves etc. Therefore, in recent years, researches have focused on the use of additives or modifiers to improve the properties of the binder. These additives and modifiers include fillers, extenders, polymers (rubber and plastic and/or its combination); fibers, oxidants, hydrocarbon, antistripping agents, waste materials and miscellaneous materials such as silicones (Brown, R. E et al, 2009)
Polymer modified asphalt binders (PMBs) are becoming wider spread in road building to meet today’s high traffic loading. Many efforts are directed towards modifying the asphalt or paving mixture properties to get superior performance and serviceability under local conditions and to economize the construction of pavement. Modification of bitumen with polymers decreases its temperature susceptibility chiefly by increasing its ring and ball softening point, increases its cohesion and modifying its rheological characteristics
Modification of asphalt mixtures with elastomers (rubbers) improve asphalt binder such that when subjected to an applied load (stress) exhibits initial deformation (strain), then as the load is eventually removed the binder demonstrates some elastic recovery. However, overall, the binder suffers a degree of permanent deformation, which would ultimately result in rutting under continued traffic loading. Polymers are mainly used to improve the binder’s elastic component, thereby delaying the onset of permanent deformation by enhancing the binder’s ability to recover after each load cycle is removed. (Robinson L.H, 2004). Modification of binder with polymers also decreases its temperature susceptibility chiefly by increasing its ring and ball softening point, increases its cohesion and modifying its rheological characteristics.
Traditionally, asphalt pavements are analysed by multilayer linear method, in which, asphalt layers are modelled as an elastic material. However, under many service conditions, asphaltic materials show viscoelastic behavior. Studies show that assuming viscoelastic behavior for asphaltic mixtures is more realistic and consistent with experimental and field results.
Liao and Sargand 2010 used 3D finite element modelling in Abaqus with viscoelastic behavior for asphaltic layer to compute stresses, strains and deflections of pavement under traffic load at different temperatures and speed levels. Good correlation was obtained by comparing the results of modelling with field measurements. In addition, they compared the responses obtained by viscoelastic and elastic finite-element modelling, which, results showed that, at different speeds and temperatures, viscoelastic modelling could predict the pavement responses more accurately than the elastic modelling. Elastic modelling is not capable of providing accurate solutions over a wide spectrum of speeds and temperatures
Elseifi et al. (2006) undertook studies to determine the viscoelastic properties of asphalt pavements at moderate and high temperatures. Using the parameters obtained from experimental data, a 3D finite-element model of pavement was made in ABAQUS, and the results from modelling were compared with field data, which were found to be consistent. Viscoelastic modelling of asphaltic pavements is able to capture the time dependent recoverable deformation of the pavement and the related responses. In addition, viscoelastic finite-element modelling is capable to predict the surface deflection at a point, before and after passing the load. Noticeable differences were observed between the responses obtained by elastic finite-element modelling and field data. At moderate and high temperatures, the responses obtained from the elastic finite-element analysis were higher than those measured in field. In addition, the elastic analysis is not able to predict the permanent deformation of the pavement and the time-dependent responses of the pavement.
Under many in service conditions, asphaltic materials show viscoelastic behaviour and using an appropriate method is essential for computing the accurate responses of the pavement. A variety of mechanical models are available to characterise the viscoelastic behaviour of asphaltic mixtures. These models are commonly composed of a spring, representing the elastic behaviour, and, a damper, which is used to simulate the time-dependent viscous behaviour of the materials. Maxwell and Kelvin models are very simple models, which can simulate the viscoelastic behaviour of asphaltic materials. The Maxwell mechanical model is a combination of a spring and a dashpot in series, and, a Kelvin mechanical model is that in which a spring and a damper are connected in parallel. E is the relaxation modulus of the spring and η is the viscosity of the damper. It should be mentioned that these models suffer from some shortcomings.
For instance, the Maxwell mechanical model cannot well simulate the creep behaviour of asphaltic mixture, while the Kelvin mechanical model does not show the effect of time on the behaviour of the asphaltic mixture (Liao, 2007).
Due to the limitations of the Maxwell and Kelvin models, more complex models have been developed for better and more accurate characterisation of viscoelastic materials (Pei et al., 2016).
Generalised Maxwell model is one of the complex models, which is able to simulate the behaviour of any viscoelastic material, including asphaltic mixtures. This model is a combination of a spring with the relaxation modulus of Ee and m number of Maxwell elements connected in parallel. The relaxation of asphaltic mixtures under moving loads can be well described by this model (Liao, 2007).