Characterisation of material properties and behaviour of cold bituminous mixtures for road pavements

[摘要] The cold bituminous mixtures, which are the subject of this study, are obtained bymixing mineral aggregate with either bitumen emulsion or foamed bitumen atambient temperatures. These techniques are frequently used in Cold In-PlaceRecycling whereby typically the top 150 – 250 mm of the existing pavement isreworked, as a rehabilitation measure when structural maintenance is required. Todifferentiate from the cold mixes for surfacing layers the term Bitumen StabilisedMaterials (BSM's) is adopted here.The increased use of BSM's, shortcomings in the existing design guidelines andmanuals and ongoing developments in the concepts and understanding of thesematerials require further research into the fundamental properties and behaviour ofBSM's. Achieving a better understanding of the fundamental performance propertiesof BSM's is the main objective of this study, with a view to using the extendedknowledge for improvements to current mix design and structural design practices.The state-of-the-art of bitumen emulsion and foamed bitumen techniques is reviewedin a literature study. Current best practices in the design of BSM's and pavementsincorporating such materials is also included in this literature study. Shortcomingsand areas for further improvement of the design practice have been identified. Withnew environmental legislation that recently came into effect in South Africa, theimportance of BSM technology as an environmentally-friendlier and moresustainable construction technique is set to increase in the coming years.A laboratory testing programme was set-up to study the properties and behaviour ofBSM's and to establish links with the compositional factors, i.e. the type of binderused, the percentage of Reclaimed Asphalt Pavement (RAP) in the mix and theaddition of a small dosage of cement as active filler. The mineral aggregates usedwere sourced in the USA and consisted of crushed limestone rock and RAP millings.These were blended in two different proportions of crushed rock : RAP, i.e. 3:1 (with3.6 % residual binder) and 1:3 (with 2.4 % residual binder). Tri-axial testing(150 mm diamter) was carried out to determine shear parameters, resilient modulusand permanent deformation behaviour, while four-point beam testing was carried outto determine strain-at-break, flexural stiffness and fatigue behaviour.It was found that the process of bitumen stabilisation improves the shear strength ofthe material, particularly in case 1 % of cement is added as active filler. This increasein shear strength is entirely the result of increased cohesion. There is a goodcorrelation between the shear strength and the resilient modulus of BSM's. Theresilient modulus of BSM is stress-dependent and the Mr-θ model is adequate tomodel the resilient modulus of the blends with a low percentage of RAP. For theblends with a higher percentage of RAP this model cannot be applied and theresilient modulus reduces in stiffness at higher deviator stress ratios.A considerable part of the efforts of this study were dedicated to characterise andmodel the permanent deformation behaviour. The General Permanent Deformation Law as originally developed by Francken applies also to BSM's. An improved nonlinearmethod to converge at a solution for the model parameters that describe thetertiary flow part of this deformation law was developed as part of this study.Parameters that can be derived from the first stage of the permanent deformation test,i.e. initial strain and initial strain rate as defined in this study, were found thatcorrelate well with the model parameters that describe the first linear part of thedeformation law. Critical deviator stress ratios for the several mixes tested weredetermined. When BSM's are subjected to loading below these ratios, tertiary flow isunlikely to occur.A high variability was generally found in the four-point beam test results, especiallyfor the strain-at-break. Specimen preparation protocols and the quality of the beamspecimens are of utmost importance when performing four-point beam tests onBSM's. This limits the practical applications of the strain-at-break test. Trendsobserved in the strain-at-break were also inconsistent and sometimes not in line withthe other type of tests.BSM's exhibit a visco-elastic behaviour, which was determined by flexural stiffnesstesting, however, to a lesser extent than HMA. Phase angles and Black Diagramswere developed for the BSM's tested, which also made it possible to determine theparameters of the Burgers Model, which is a mechanical model describing viscoelasticbehaviour. Fatigue relationships were also developed for the BSM's tested.The fatigue performance of these mixes is lower than for selected HMA mixes. Thefoamed BSM generally showed better fatigue life than emulsion BSM, however, thelower initial stiffness of the foamed BSM's may contribute to a perceived longerfatigue life. For the mixes tested, the flexural stiffness of foamed BSM's is generallyalso lower than that of emulsion BSM'sIt is recommended that the mix design of BSM's be split into two phases. During thefirst phase the usually large number of variables could be reduced to a selected fewby means of UCS and ITS indicator testing. Subsequently, more fundamentalparameters should be determined during the second phase, such as shear strength andresilient modulus, as well as permanent deformation behaviour. The fact thatcommercial laboratories in South Africa do not have tri-axial testing facilities iscurrently a practical limiting factor. Initiatives currently underway to develop'simple shear tests are welcomed in this regard. It is proposed that classification ofBSM is based on shear strength.There are indications that shear failure in BSM is more critical than failure as a resultof fatigue. The effect of curing resulting in an increase in BSM stiffness in the periodafter construction, i.e. typically 6 to 18 months, is currently ignored in structuraldesign models. The rapid stiffness reduction of BSM's during the first period afterconstruction in the current structural design models and also found duringAccelerated Pavement Testing is not being observed in Long-Term PavementPerformance (LTPP). On the contrary, an increase in stiffness is observed in LTPP.This would indicate that stiffness reduction as a result of fatigue does not occur or isovershadowed by the effect of curing and that fatigue as a failure mechanism ofBSM's is currently over-emphasized.

[发布机构] Stellenbosch University