Towards a performance related seal design method for Bitumen and modified road seal binders

[摘要] ENGLISH ABSTRACT:Bitumen based road surfacing seals and asphalt wearing courses have been used by society's Engineers to counter the damage to the existing unsurfaced roadways by the newly developed automobile with its rubber driving wheels since the early 1900's. Early experiments were conducted with both tar and bitumen to find a suitable material to alleviate the situation, and ongoing research has been carried out through the past century and into the new millennium, throughout the world, examining improvements, from materials used, to design and construction methods. However, there is still much to be understood, improved and refined, when considering road surfacing seal design.Pavement designers have the choice of utilizing either an asphalt (graded aggregate remanufacturedwith a bitumen binder and applied as a complete product) or a surfacing seal (including variations of bitumen binder sprayed onto the road surface, with the addition of single sizestones, either in one or two layers of binder and aggregate, i.e. single or double seals) as apavement wearing course.Current road surfacing seal design practice depends on empirical analysis and experience, being primarily a volumetric based assessment of bitumen application. This research project assessesSouth African seal design philosophy, investigates design areas where review or updating isrequired to accommodate changing bitumen sources and types, and traffic loading. Sealperformance criteria are examined, with the development of a matrix of influences on sealperformance. Using this, the need for a seal design method based on mechanistic materialproperties is proposed, and the prototype example of such a numerical model using finite elementmethod is presented.To contribute further towards a performance related seal design method, the feasibility of modellingof road surfacing seals using mechanistic principles was examined. The potential of developingfailure and fatigue criteria or relationships to enable assessment of the expected seal performance, with inclusion of different component material characteristics and variations, varying traffic andenvironmental conditions, was also examined.From assessment of literature, and understanding of the components of the seal, pavement, and influencing factors, a choice of numerical model of seal performance was made. The Finite ElementMethod (FEM) Analysis was selected for the purpose of modelling seal performance. The model was developed to enable examination of the interaction of individual seal components (i.e. stone and bitumen), at micro-mechanic scale.The prototype 3-dimensional numerical seal model was undertaken in 2002 and 2003 at TechnicalUniversity Delft, using the CAPA research program. On the basis of the linear calculations thedeveloped numerical prototype model is able to provide insight into seal behaviour and distinctionbetween mechanical (seal geometry) and chemical (components) seal aspects, and insight intostress and strain development in the different seal types. Simulations of different seal,environmental and traffic scenarios are provided to demonstrate the potential of the model(excluding seal aggregate interlock and embedment effects at prototype stage).In order to provide data for the verification of the prototype numerical model, and to furthercontribute to the development of a performance related seal design method, performance tests weredeveloped, with a new tool for assessment of comparative seal performance using the Model MobileLoad Simulator Accelerated Pavement Testing apparatus. The performance of each different sealbinder type - Penetration grade Mumen, SBS, SBR, EVA and Bitumen Rubber - was undertaken. Amethodology for the assessment of in-service seal performance was developed, and theperformance of the respective seals reported. The results of this examination showed that eachbinder type has its unique contribution to seal performance.These new performance tests will be able to assist designers in the added determination of thefundamental binder properties on seal performance, and the seals' ability to contribute to the overallperformance of the pavement.An additional comparative performance test method was developed to enable assessment of theeffect of ageing and moisture, to complement the MMLS results.In summary, the performance testing has assisted in identifying the critical parameters a sealdesigner should consider during the design process.From this research, it is evident that the current seal design method requires further development toable designers to predict the effect of:Varying axle loads, tyre pressures and design speed;Varying characteristics of the different binders, (i.e. temperature - viscosity relationships,adhesion and visco-elastic behaviour);on the performance of seals.The major areas for suggested improvement in current seal design methods towards a performancebased design method are:inclusion of variable traffic load and environmental characteristics, including temperature andmoisture influences, andinclusion of mechanistic material characteristics into the design methodology.

[发布机构] Stellenbosch University