Influence of durability properties on performance of bitumen stabilised materials

[摘要] ENGLISH ABSTRACT: In both developing and developed countries, to ensure sustained economic growth the quest foroptimal roads performance is an extremely high priority. A global increase in the use of foamedbitumen and bitumen emulsion materials (BSMs) as a solution to roads maintenance,rehabilitation, and upgrading has become evident. This is driven by environmental policiesaimed at conserving energy and limiting the exploitation of new borrows pits. It has thereforebecome imperative that BSMs are used optimally, and, in order to achieve this, practitionersneed to understand the mechanisms that influence durability and long-term performance.The changes in the behaviour of materials and the failure mechanisms of BSM mixesare long-term phenomena. This implies that the study of the physicochemical and mechanicalproperties of the mixes is vital. Therefore, a fundamental understanding of the moisture damageand age-hardening characteristics, which are related to materials' properties, is required. Themain objective of this study is to advance BSM technology by assessing the influence of theselected materials on durability behaviour and long-term performance in all phases ofapplication (i.e. mix design, construction, and in-service condition).This study begins with a comprehensive literature review of research dealing with theinteractions of binder and mineral aggregates. The properties of bitumen (foamed bitumen orbitumen emulsion) and mineral aggregates were reviewed. This was followed by review into thecolloidal behaviour of foam and emulsion and physicochemical and mechanical interaction withmineral aggregates. Factors influencing the interaction of BSMs were then identified. Finally, thefundamental theories on thermodynamics, hydrodynamics, and electrokinetics were used todescribe the step-by-step process by which adhesive bonding and cohesion occur in BSMs.The mixture durability in terms of moisture damage was investigated. To achieve thisaim, the physical and mechanical moisture-induced damage process was analysed. The testcontrol parameters were established and a laboratory device to quantify these parametersdesigned. New moisture conditioning procedures were developed and demonstrated in thisstudy. From the moisture induction simulation test (MIST) procedure, it became evident thatpulsing water pressures into compacted and cured BSM mixes simulates the hydrodynamiceffect that occurs in the field due to dynamic traffic loading. The different mix matrices typicallyapplicable to the recycling processes – such as Hornfels-RAP and Quartzite crushed stone,stabilised with either foamed bitumen or bitumen emulsion and the addition of active filler(cement or lime) – were investigated. It was found that a new moisture-conditioning procedureusing the MIST device and monotonic triaxial testing can distinguish those BSM mixes that areresistant to moisture damage from those that are less resistant. The validation of the MIST andmonotonic test results was done using the APT device, which is the MMLS3 wet trafficking test.The results on both tests showed good correlations in evaluating and screening BSMs in termsof moisture susceptibility.Field temperature data was collected and a model to accurately simulate the curing ofBSMs was identified and proposed for further investigation and validation. It was found from thefield temperature data collected in this study that the temperature gradient on the study sitevaried according to the depth of the BSMs (that is, 10oC-17oC during winter and 17oC– 47oCduring summer). Understanding the influence of the temperature conductivity and rate ofevaporation is important for inferring moisture damage and age-hardening behaviour and properselection of BSMs.The age-hardening behaviour of BSMs is linked to the durability properties and longtermperformance of these materials. The fundamental characteristics associated with shortandlong-term age hardening were investigated in this study. The short-term dimension involvedassessing the age-hardening characteristics of the binder (foamed bitumen colloids and bitumen emulsion droplets) prior to the production of BSMs. The long-term study involved extracting andrecovering the binder from the briquettes (made from different mixes) compacted in thelaboratory and cores extracted from different field pavement sections which were in service for8-10 years. The study found that the length of time bitumen is kept in circulation in thelaboratory plant at elevated temperature (170oC–180oC) before making BSM-foam contributesto the ageing of the binder, especially after eight hours. The foaming process in itself was foundnot to alter the bitumen properties. It is recommended that a temperature range between 160oC-165oC be used for the production of foamed bitumen with softer bitumen. This will notcompromise its quality. In addition, the time of circulation of bitumen in laboratory plant shouldnot be longer than three (3) hours.The rheological properties of the bitumen recovered from laboratory briquettes and coresfrom field pavement show that age hardening on foamed bitumen and bitumen emulsion duringin-service life occurred. The ageing also seemed to be dependent on the effect of traffic, withtrafficked areas (i.e. on-wheel path and inner-wheel path) experiencing more ageing than untraffickedareas (i.e. between-wheel path). However, the extraction and recovery process wasfound to be complex, and produce uncertain results. Although the results show that binders inBSMs undergo age hardening, its distinct behaviour in BSM performance was not obvious fromthe extensive tests carried out in this study.The last part of the study contains its conclusions and recommendations. The studyprovides an insight into fundamental material durability properties, and this will assist inimproving the current procedure for selection, combining and formulation of the mix matrices forBSMs. In addition, the study provides guidelines that will enable practitioners to confidentlyapply a mix that is durable and long-lasting. The specific durability-related issues addressed inthis study are substance for future research. This novel solution to the application of BSMs willbenefit all parties involved in the development of pavement recycling technology.

[发布机构] Stellenbosch University