On investigation of stripping propensity of bituminous mixtures
Abstract: In this study, an experimental programme was designed toestablish a relationship between bituminous mixture constituentmaterial properties and their propensity to moisture induceddamage in form of stripping. Six bitumen types (3 from Ugandaand 3 from Sweden) with presumably varying characteristics wereevaluated basing on rheology and chemistry. Eleven aggregateswere used in this study. Seven were sourced from activequarries in Uganda and four were from Sweden. Bitumen rheology was established basing on penetration,softening point, viscosity, ductility and visco-elasticparameters obtained from dynamic mechanical analysis. Bitumenchemistry was studied using Fourier Transform InfraredSpectroscopy, Gel Permeation Chromatography (GPC) and ThinLayer Chromatography (TLC). Bituminous mixtures were reconstituted from the bitumen andaggregate combinations basing on the Swedish mix designprocedure ROAD 94 using dense graded mixtures with 16mm maximumaggregate size (AG16). Mixture sensitivity to moisture wasevaluated basing on Swedish FAS 446-98 specifications closelyrelated to the modified Lottman procedure. The investigation was done in two phases namely, (a) theeffect of aggregate properties on mixture moisture sensitivityand (b) the of cross effects of bitumen and aggregates onmixture moisture sensitivity. The results reveal chemical compositional differences inbitumens that would be considered similar basing on classicalrheological properties like penetration and viscosity. Resultsof dynamic mechanical analysis show that binders have similarvisco-elastic response around 0oC. This could be a potentialphenomenon to serve as a grading scheme for bitumen as is thecase with penetration and viscosity grading systems. The results from phase I of the study show that presence ofCa-feldspars and ferromagnesian minerals in aggregates largelyrelates to improved resistance of mixtures to moisture damage.In addition, mixtures from aggregates with high concentrationsof acid insolubles (SiO2 and Al2O3) are sensitive to moisturedamage. The results of phase II of this study show that the choiceof aggregate type is the dominant factor affecting moisturesensitivity of the resulting mixtures. Bitumen type seems notto be an important factor in determining moisture sensitivityof bituminous mixtures. Aggregates with Ca-feldspars andferromagnesian minerals seemed to be the most resistant tomoisture damage irrespective of the bitumen type. Assessment of moisture sensitivity basing on absolutestiffness values of water treated mixtures seems not to bereliable. Results from this study show that mixtures withsimilar wet resilient moduli had varying tensile strengthratios, hence varying moisture sensitivity tendencies. Modulusof resilience ratio (MRR) and tensile strength ratio (TSR)parameters show similar trends in comparing moisturesensitivity of different mixtures. However, MRR values aregenerally lower than TSR values for the same mixtures.
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