Flexibility and performance properties of bitumen stabilised materials

[摘要] ENGLISH ABSTRACT: This research investigates the flexibility and the performance properties of bitumen stabilisedmaterials under the influence of mix variables. The laboratory testing consisted of two main phases.During the first phase (mix design), the strength and the flexibility of the mixes were assessedthrough ITS (Indirect Tensile Strength), UCS (Unconfined Compressive Strength), displacement atbreak, strain at break and fracture energy. The second phase consisted of a series of triaxial testsdone to assess the performance properties (shear strength: cohesion and angle of internal friction; andstiffness: resilient modulus) of the mixes.The mineral aggregates used in this study were milled from different locations of the R35, nearBethal. This was a blend of granular material (dolerite, from various locations of the existing baseand subbase layer of the R35) and Reclaimed Asphalt (RA) milled from the existing surfacing.During the mix design phase, two types of bituminous binders were used (bitumen emulsion andfoamed bitumen) at bitumen content ranges of 2%, 2.4% and 2.8% each. Two types of active fillerwere used separately and in combination at a proportion of 1% and 2%. Finally, specimens weretested in wet and dry conditions for each mix combination. During the triaxial testing phase, only theoptimum bitumen content of 2.4% was used, both for bitumen emulsion and foamed bitumen, withonly cement as active filler in a proportion 1% and 2%. The specimens were tested at differentranges of densities and saturation levels.The flexibility of the mix was assessed through the fracture energy, the strain and the displacement atbreak parameters. An analysis of variance (ANOVA) was conducted on the data to assess thesignificance of experimental variables on this property. This property was found to be very sensitiveto bitumen and cement content added to the mix. When assessing the combined effect and thesignificance of the variables on the flexibility of the mixes, it was found that fracture energy ismostly influenced by the cement content, followed by the bitumen content, then the type of treatmentand finally the testing condition. However, the level of significance was not in the same order for theother two parameters (displacement and strain at break). It was also found that the combined effectof some independent variables (cement content + testing condition, type of treatment + cementcontent + bitumen content) had a significant effect on the fracture energy and the strain at breakrespectively.From the ITS and UCS tests, an increase in strength was noticed with the increase of cement content.On the other hand, the increase in bitumen content led to a decrease in strength of the material. Thestatistical analysis on the ITS and UCS values show that the independent variable with the most significant effect on the ITS is the cement content, followed by the testing conditions, then thebitumen content and finally the type of treatment. The combined effect of cement content + bitumencontent was found to be significant both for ITS and UCS.In the second phase triaxial tests were performed in order to evaluate the performance properties ofthe mixes. It was found that the increase of the active filler content significantly improves the shearstrength of the material. It was also found that at a fixed cement content, specimens tested at lowdensity and/or high level of saturation show low shear strength. The Mr-θ model was used to modelthe resilient modulus of the mixes and the model coefficients used to evaluate the effect ofexperimental variables on the resilient modulus. It was found that the resilient modulus of the mixesincreases as the bulk stress increases. This confirms the stress dependent behaviour of bitumenstabilised materials. The analysis show that increasing the percentage of active fillers content resultsin a significant increase in the resilient modulus values. An increase in relative density also resultedin an increase in the resilient modulus of the mixes, while the opposite effect was observed with theincreased of the saturation level.Besides the engineering properties and the mechanical test parameters, other parameters such as theTensile Strength Ratio (TSR) was calculated in order to evaluate the moisture sensitivity of themixes. Weakening due to moisture was found to be more predominant in the mixes with less activefiller. In addition, bitumen emulsion mixes were found to have a better resistance to moistureweakening effects compared to foamed bitumen. In addition, a comparison between the rapid curingand the accelerated curing was done. Higher ITS and UCS results were obtained for specimens curedusing long term curing compared to specimens cured using the accelerated curing method.In conclusion, flexibility is an important property of bitumen road construction material (bitumenstabilised material include) however, it is not an easy property to measure. Although,displacement/strain at break and fracture energy from ITS and UCS were able to give us someindications on the main factors governing the flexibility of bitumen stabilised materials (the bitumenand active filler content), more accurate and adequate tests are required to evaluate the parameter.

[发布机构] Stellenbosch University