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Permanent deformation (rutting) is a pavement distress condition visible in the surfacing layer of a pavement. It occurs along the wheel path and results from the accumulation of load-induced permanent deformation developed from all individual pavement layers, including the subgrade. It is one of the major distress conditions in flexible pavements. Plenty of research regarding permanent deformation in flexible pavements exists, but it is mainly focused on asphalt surface layers and granular base, subbase, and subgrade layers. The South African National Roads Agency Ltd (SANRAL) completed the construction of seven flexible pavement sections on the R104, between the east of Pretoria and Bronkhorstspruit, during 2013. In-situ pavement response and environmental related data have been collected from these test sections ever since on a number of occasions. The seven flexible pavement structures include a natural gravel (G4) base, a high-quality graded crushed stone (G1) base, a Foam Treated Base (FTB), an Emulsion Treated Base (ETB), a Cement Treated Base (CTB), a Bitumen Treated Base (BTB), and a High Modulus Asphalt (HiMA)/Enrobe̹s A̳℗ Module Eleve̹ (EME) base. The permanent deformation behaviour of different flexible pavements relative to each other was investigated by processing, validating, and analysing the relevant in-situ pavement response and environmental related data collected from each of the SANRAL test sections. With the focus on total and base layer deformation, it was found that in terms of a short-term loading response and under normal operating conditions, bituminous pavements show superior performance to cement/bitumen stabilised pavements, while the latter performs better than granular pavements. CTB and ETB pavements are very similar with FTB pavements closely behind. The only granular exceptions are inverted crushed stone pavements, which should closely follow bituminous pavements at the top end of the performance range. For permanent deformation behaviour in terms of a longer-term recovering response, it was found that bituminous pavements tend to recover a larger amount of the permanent deformation attained after load application than granular pavements, probably due to the delayed elasticity (visco-elastic properties) of bituminous materials. The possibility of a transfer function for linking the permanent deformation behaviour of a pavement to its structural integrity was also investigated by determining a representative pavement number for each of the SANRAL test sections. It was found that the permanent deformation behaviour of flexible pavements relates relatively well to their structural integrity as a general decrease in permanent deformation (rut rate) was observed with an increase in pavement number. A negative power function for linking permanent deformation behaviour to structural integrity was proposed (y = 76.657x-0.752, R2 = 0.77). Additionally, it was found that post-compaction trafficking has a significant effect on the permanent deformation behaviour of flexible pavements during the initial stages of their life cycle; temperature variations can have a major influence on the in-situ performance and behaviour of bituminous layers, and the permanent deformation behaviour of flexible pavements correlates positively with the corresponding dynamic response as an increase in permanent deformation (rut rate) was observed with an increase in maximum dynamic deflection (positive linear function, y = 0.0361x - 2.5687, R2 = 0.92).
This doctoral dissertation presents the results of an extensive research performance into the mechanical behavior of granular and lightly bound base materials and sand sub-base materials and the associated performance of pavements in which such materials are applied. To this end a very large number of cyclic and static load triaxial tests have been performed on sands, mix granulates and hydraulic mix granulates. In addition the obtained stress dependent failure, resilient and permanent deformation behavior has been implemented in Finite Element calculations to demonstrate the performance of a variety of pavement structures in relation to the behavior of these materials
Rutting is recognized to be the major distress mechanism in flexible pavements as a result of increase in tire pressures and axle loads. Rutting is caused by the accumulation of permanent deformation in all or some of the layers in the pavement structure. The accumulation of permanent deformation in the asphalt surfacing layer is now recognized to be the major component of rutting in flexible pavements. This is a consequence of increased tire pressures and axle loads, which subjects the asphalt surfacing layer nearest to the tire-pavement contact area to increased stresses. Thus the study of permanent deformation properties of asphalt mixtures has become the focus of research, which aim to mitigate or reduce rutting in flexible pavements. The research work reported in this thesis aims to contribute towards understanding of the material properties and factors affecting permanent deformation in asphalt mixtures, mechanisms of the permanent deformation, and methods of its prediction. The specific objectives of this research work include; review and evaluation of available models for permanent deformation of asphalt concrete mixtures, investigation of the effect of volumetric composition, loading and temperature conditions on the permanent deformation of asphalt concrete, and the identification and definition of simple measures of resistance to permanent deformation. To meet the objectives of the study a laboratory investigation is conducted on several asphalt concrete specimens with varying volumetric composition. Two testing procedures are adopted; the repeated load triaxial and triaxial creep and recovery tests. The tests were conducted at two temperature levels of 25 and 50oC under varying stress conditions. A review of literature on factors affecting permanent deformation and available models for prediction of the permanent deformation is also conducted. The literature review indicated that most of the research work done so far concentrated on evaluation of the effect on permanent deformation response of component material properties such as aggregate gradation, aggregate angularity and binder type (or grade). Most of the studies conducted on permanent deformation properties of asphalt mixtures were also found to be based on different testing procedures and methods of evaluation, which makes it difficult to compare them and draw firm conclusions. The literature also indicated that, as yet, there is no comprehensive model for deformation of asphalt concrete. Results of tests conducted in this study are analysed to investigate the effect of volumetric composition, particularly binder content and void content, and loading conditions on the permanent deformation response of the mixture. Both the binder content and void content are found to significantly influence the permanent deformation characteristics. The effect of loading conditions, i.e., the confining stress and the deviatoric stress, is also found to be significant. Throughout this study emphasis is placed on methods and parameters that are used to evaluate mixtures for their resistance to permanent deformation. The traditionally used parameters such as the slope and intercept of the power model are evaluated for their sensitivities to changes in volumetric composition. This evaluation is based on the premises that any measure of resistance to permanent deformation should be sensitive to changes in volumetric composition to be good enough. It is found that most of these parameters are not sensitive to changes in volumetric composition and therefore are not suitable for comparison of mixtures made from the same materials but with varying proportion of the components. Permanent deformation in asphalt concrete is caused by both densification and shear deformation. The mode of deformation in asphalt concrete pavements, for greater part of their service life, is considered to be the shear deformation. Therefore it is necessary to evaluate mixtures for their susceptibility to shear deformation. The shear deformation manifests itself in the form of large lateral deformation relative to axial deformation. It is found that one dimensional analysis, which does not take the lateral deformation into account may lead to misleading results regarding the resistance to permanent deformation of mixtures. Therefore parameters which include volumetric and lateral strain are proposed for use in evaluation of mixtures. Substantial effort is put into modelling the accumulation of permanent deformation under repeated loading. For this purpose two approaches were selected: the cyclic hardening model based on bounding surface plasticity concept and an elasto-viscoplastic model based on strain decomposition approach. The bounding surface plasticity approach is found to be a convenient method to model the accumulation of permanent deformation. It is demonstrated that deformations calculated using cyclic hardening model based on bounding surface plasticity fits the measured deformation quite well. The elasto-viscoplastic model, which is based on strain decomposition approach, provides a suitable method for analysis of creep and recovery test results. Deformations calculated using this model also fit the measured deformation quite well. Finally a new composite measure of resistance to permanent deformation is developed. The resistance index is based on strain decomposition approach and is simple to calculate. The index incorporates a parameter related to shear susceptibility of mixtures and is sensitive to changes in volumetric composition. It is believed that this index can be used to compare and select mixtures at mixture design stage. If its applicability to other materials is proved by further research, it can also be linked to performance related specifications, as a simple measure of performance with regard to rutting.