Trends in back-calculated stiffness of in-situ recycled and stabilised road pavement materials

[摘要] ENGLISH ABSTRACT: Two common methods of road pavement, granular material stabilisation used in road constructionthroughout South Africa today include Cold in Place Recycling (CIPR) and stabilisation with cementor bitumen and an active filler to create Bitumen Stabilised Materials (BSM).As part of the updating of the South African Pavement Design Method (SAPDM) an experimentalsection, investigating the structural capacity of cement and lime stabilised and BSM pavement layers,was constructed and will be monitored over a two year period. As part of this study Falling WeightDeflectometer (FWD) measurements were taken on the various experimental stabilised pavementlayers constructed. The FWD deflection data, measured at various time intervals over a 360 dayperiod, forms the basis of the study presented here.The objective of this thesis was to identify typical back-calculated layer stiffnesses and theirvariability over time for the various in-situ recycled and stabilised base layers constructed within theexperimental section. Stabiliser type, content and layer thicknesses were varied across experimentalsub-sections.Trends in back-calculated stiffness of cement stabilised base layers consistently showed significantreductions in layer stiffness subsequent to construction traffic loading. Subsequent to the initialreduction in stiffness little change in stiffness was noted under normal traffic loads.Observations on the trends in back-calculated stabilised layer stiffness per material type over timeindicated that seasonal moisture and temperature fluctuations have an effect on the stiffness of thepavement structure as a whole. BSM materials showed significant variability over time in-line withseasonal variability in the supporting subgrade stiffness in the southbound lane. BSM materials with1% cement added in the northbound lane show initial stiffness reductions due to direct rainfallapplication however a significant increase in layer stiffness occurs up to 360 days after construction.BSMs with 2% cement in the northbound lane show significant increases in layer stiffness over the360 day observation period. No significant difference in stiffness trend was observed between BSMemulsion a BSM foam materials. The BSM emulsion with 0.9% residual bitumen and 1% cement wasobserved to show rapid reduction in stiffness upon opening to traffic and reverting to stiffness valuessimilar to an unbound material of approximately 350 MPa.Cement and lime stabilised materials showed typical post 28 –day average stiffnesses per sub-sectionranging between 600 MPa and 1800 MPa. BSM foam with 1% cement added were observed to haveaverage stiffnesses per sub-section in the range of 400MPa to 2200 MPa and BSM emulsion with 1%cement with stiffnesses between 400 MPa to 1700 MPa over the 360 day period. BSMs with 2% cement added showed stiffness ranges between 900 MPa to 4300 MPa for BSM foam and 900 MPa to3900 MPa for BSM emulsions over the 360 day period.The spatial variability of back-calculated stiffness per sub-section of a particular stabilisation designwas significant and was observed, through the Co-efficient of Variation (COV), to increase over time.The effect of the observed variability when incorporated into a pavement design scenario, requiring adesign reliability of 90%, showed 50% of the pavement structure would be overdesigned by a factorof 4.With respect to the current philosophies on the development of stiffness over time of cement and limestabilised and BSM pavement layers some useful observations were made. Cement stabilisedmaterials correlate well with stiffness development theories predicted by previous studies. Theoriesrelating to the stiffness development of BSMs however did not predict the levels of variability in baselayer stiffness observed on the experimental section.The continued observation of the experimental section for another year will give greater insight to thestiffness trends of the stabilised materials discussed above.

[发布机构] Stellenbosch University