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Pavement performance prediction in terms of fatigue cracking and surface rutting are essential for any mechanistically-based pavement design method. Traditionally, the estimation of the expected fatigue field performance has been based on the laboratory bending beam test. Full-scale Accelerated Pavement Testing (APT) is an alternative to laboratory testing leading to advances in practice and economic savings for the evaluation of new pavement configurations, stress level related factors, new materials and design improvements. This type of testing closely simulates field conditions; however, it does not capture actual performance because of the limited ability to address long-term phenomena. The same pavement structure may exhibit different response and performance under APT than when in-service. Actual field performance is better captured by experiments such as Federal Highway Administration's Long-Term Pavement Performance (LTPP) studies. Therefore, to fully utilize the benefits of APT, there is a need for a methodology to predict the long-term performance of in-service pavement structures from the results of APT tests that will account for such differences. Three models are generally suggested to account for the difference: shift factors, statistical and mechanistic approaches. A reliability based methodology for fatigue cracking prediction is proposed in this research, through which the three models suggested previously are combined into one general approach that builds on their individual strengths to overcome some of the shortcomings when the models are applied individually. The Bias Correction Factor (BCF) should account for all quantifiable differences between the fatigue life of the pavement site under APT and in-service conditions. In addition to the Bias Correction Factor, a marginal shift factor, M, should be included to account for the unquantifiable differences when predicting the in-service pavement fatigue life from APT. The Bias Correction Factor represents an improvement of the currently used "shift factors" since they are more general and based on laboratory testing or computer simulation. By applying the proposed methodology, APT performance results from a structure similar to an in-service structure can be used to perform four-point bending beam tests and structural analysis to obtain an accurate estimate of the necessary Bias Correction Factor to estimate in-service performance.

The principal objective of this study is to determine the accuracy of calculating stresses in concrete pavements and unbonded overlays under different loading conditions. The computed stresses for the single layer pavements are obtained based on Westergaard theory, the finite-element model (ILLI-SLAB), and the elastic layered model (ELSYM5). For the two-layer unbonded systems, stresses are estimated based on the finite-element theory and the Portland Cement Association design method. The experimental results for the comparison were available from a series of tests done on model scale concrete pavements and unbonded overlays. The results of present investigation show that the computed stresses for the single layer pavement slab are in good agreement with the observed stresses selected for this study, when the slab is loaded at the interior. When the pavement slab is loaded at the edge, the analytical methods give lower stresses and further investigation is recommended. On the other hand, the unbonded overlay of concrete pavement is thoroughly explored. The stresses as given by the analytical methods are, in general, of smaller magnitude than the observed ones in the unbonded overlay. The difference in the magnitude of stresses is considered attributable to the manner in which the stresses are distributed in the slab through the contact area with the load. A revised equation for the determination of the equivalent contact area is suggested to bring the analytical stresses in line with the experimental values. This revised formula is further verified in its application to some of the experimental stresses obtained from other tests and is found to give satisfactory results.

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Thesis (MEng)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: Accelerated pavement testing (APT) is the application of a wheel loading, to a prototype or actual layered, structural pavement system to determine pavement response and performance under controlled, accelerated accumulation of damage in a compressed time period. It is a tool used for the evaluation of performance of new pavement materials, distress mechanisms, pavement distress, and selection of rehabilitation strategies. In this study, the research work that was done in Jacksboro, Texas (USA) with the third-scale Model Mobile Load Simulator (MMLS3) as part of the APT programme of the Texas Department of Transportation (TxDOT) is presented. The primary objective was to evaluate the performance ofthe different asphalt concrete (AC) materials (Dustrol and Remixer) used on highway US 281, in terms of rutting and distress due to wet trafficking. The other objective was to investigate the difference in rutting between the MMLS3 and the full-scale Texas Mobile Load Simulator (TxMLS). The scope of the study included fieldwork, laboratory work and theoretical analysis. On average, the performance of the layer from the Dustrol process was found to be poorer than that of the Remixer process. The Dustrol process was more susceptible to moisture damage and less resistant to permanent deformation compared to the Remixer process. The MMLS3 and TxMLS permanent deformations in the upper 90 mm surface layers correlated well in terms of the respective vertical stresses imposed by the two APT devices, after allowing for the difference in environmental conditions during trafficking. Overall, the study demonstrated that the MMLS3, used in conjunction with nondestructive field and laboratory testing, is a significant cost-effective APT device that can be used for evaluating the response and performance of the (surface) layers of full-scale, in-service pavements. / AFRIKAANSE OPSOMMING: Versnelde Plaveisel Toetsing bestaan uit die gebruik van 'n wiellas op 'n prototipe of 'n bestaande gelaagde plaveiselstruktuur om die plaveiselgedrag onder beheerde en versnelde toename in skade in 'n verkorte periode te bepaal. Dit word gebruik om die gedrag van nuwe plaveiselmateriale, swigtingsmeganismes, plaveiselswigting en die keuse van rehabilitasie strategieë te evalueer. Die navorsingswerk met die derde skaal Model Mobile Load Simulator (MMLS3) wat onderneem is in Jacksboro, Texas (VSA), en deel uitmaak van die Versnelde Plaveisel Toetsingsprojek van die Texas Departement van Vervoer (TxDOT), word uiteengesit. Die hoofdoel van die studie was om die gedrag van twee verskillende asfaltmateriale, naamlik Dustrol en Remixer, wat gebruik is op die US 281-snelweg, in terme van sporing en swigting as gevolg van nat belastingstoestande te evalueer. 'n Verdere doelstelling was om die verskil in die gemete sporing tussen die MMLS3 en die volskaal Texas Mobile Load Simulator (TxMLS) te ondersoek. Die studie het veld- en laboratoriumtoetse en teoretiese analise behels. Die gedrag van die Dustrol laag is oor die algemeen swakker as die Remixer lae. Die Dustrol laag, in vergelyking met die Remixer lae, is meer vatbaar vir vogskade en spoor makliker. Nadat die invloed van verskillende omgewingstoestande gedurende asbelasting inaggeneem is, korreleer die sporing vir die MMLS3 en die TxMLS in die lae van die boonste 90mm van die plaveisel goed in terme van die vertikale spannings soos opgewek deur die twee toetstoestelle. In die geheel word bewys dat die MMLS3 tesame met nie-destruktiewe veld- en laboratoriumtoetse 'n belangrike koste-effektiewe versnelde plaveisel toetstoestel is, wat aangewend kan word in die evaluasieproses van die gedrag van (oppervlak) lae van volskaalse plaveisels in gebruik.

Pavement foundations for major roads in the UK were historically designed on an empirical basis, related to a single design chart, restricting the incorporation of superior performing materials or materials for which the empirical data sets were not available. The adoption of performance based specifications was promoted as they are perceived to 'open up' the use of alternative materials (including 'local' sources of primary materials) or techniques, and allow for the incorporation of superior performance into the overall pavement design. Parallel developments to the performance based design of pavement foundations (allowing for superior performance) and in situ testing required the support of laboratory based performance tests. These laboratory based tests were required to determine material performance parameters (elastic modulus and resistance to permanent deformation) for both unbound and hydraulically bound pavement foundation materials. A review of the available laboratory apparatus indicated that they were either; unrealistic (and hence unable to provide the required material performance parameters), or overly complex and more suited to fundamental research. Therefore, the requirement for developmental research work was identified. The research reported herein details the development, manufacture and initial evaluation of simplified laboratory apparatus (the Springbox for unbound materials and static stiffness test for hydraulically bound mixtures) designed to produce the performance parameters of elastic modulus and relative resistance to permanent deformation for pavement foundation materials. The equipment and test procedure evaluation was undertaken across a range of materials, giving initial guidance on likely in situ performance. The innovative laboratory apparatus and materials guidance (including the potential to use recycled and secondary aggregates) was incorporated into key Highways Agency specification and guidance documents, which in turn influence construction practice outside of motorways and major trunk roads. This research concludes by outlining a number of recommendations for continued development and evaluation, including feeding back data sets from long term in situ performance testing for subsequent refinement of assumptions.