A design function for bitumen stabilised material performance based on laboratory and field evaluation

[摘要] ENGLISH ABSTRACT: The use of stabilised granular materials for road pavements are ever increasing throughout SouthAfrica. There are currently two commonly used methods for the stabilisation of granular materialsin road construction. The first method is stabilisation with cement or lime to create a cementedlayer with increased tensile strength. The second is stabilisation with bitumen and an active fillerto create Bitumen Stabilised Material (BSM).BSMs are constructed by stabilising granular material using either foamed bitumen or bitumenemulsion. Unlike asphalt where all the aggregate is coated with bitumen, the bitumen in a BSM isdispersed among the fine particles of the aggregate. When a BSM is compacted, the bitumen-richfines are forced against each other resulting in localised bonds (spot welds). This produces amaterial with improved engineering properties. Although the material is treated with a binder, itfails in permanent deformation similar to a granular material.The need for improved design methods for BSMs is driven by the increased use of this material bythe industry. This study aims to produce a mechanistic-empirical design function for BSMs whichrelates the mechanical properties to in-field performance. This function can then be used to designBSMs based on material properties and stress conditions.The new transfer function for BSMs has similar architecture to the function developed forwaterbound macadam by Theyse et al. (2000). This function calculates the number of loadrepetitions the layer can sustain before a specified limit of permanent strain is reached. Thefunction was altered to incorporate material properties that are specified for the design of BSMs.The factors influencing the new BSM transfer function are density, moisture susceptibility, deviatorstress ratio and the permanent strain limit.Data was gathered from fourteen pavement structures that formed part of a long-term pavementperformance study. These pavements incorporate BSM base or subbase layers and were analysedat different stages of their design lives. The data obtained from these studies was used to calibratethe new transfer function for BSMs.The calibrated transfer function was adjusted to predict the data at a range of reliabilities i.e. 95%,90%, 80% and 50%. These reliabilities were based on the required reliability levels of the designcategories.The new transfer function was validated by investigating its ability to predict the life of a BSMthat did not form part of the calibration process. The N7 highway near Cape Town was selectedfor the comparison as a substantial body of information was available to investigate the effect ofthe different factors on the transfer function.FWD data was obtained from the N7 at different points in time. Back calculation of layer stiffnessesallowed insight into the resilient modulus development of the BSM. Cores were drilled from thispavement for triaxial testing. The triaxial tests provide insight into the shear properties of thebase layer. The shear properties and stiffnesses obtained from the triaxial tests show that this baselayer does not behave like a typical BSM. It was, however, still used for validation as it providesvaluable information into the transfer function's sensitivity to these factors.The new transfer function was compared to other design methods to validate its predictions. Thefunction was found to produce less conservative results at lower stress ratios. The function showedsignificant sensitivity to the deviator stress ratio induced by loading.In conclusion, the new transfer function for BSMs was calibrated to best describe the data fromthe fourteen long-term studies. The function was altered for design by adjusting it to differentlevels of reliability. Limits were recommended on the minimum and maximum values for the inputvariables to ensure a realistic result. The validation of the function yielded representative andrealistic results.

[发布机构] Stellenbosch University