The work presented in this thesis is aimed at improving the accuracy of one of the components of a flexible pavement design procedure commonly used in South Africa, namely the South African Mechanistic- Empirical Design Method. This is achieved through the development of a new design approach and permanent deformation model for the pavement subgrade. The new distress model for the pavement subgrade was developed from a comprehensive Accelerated Pavement Testing (APT) database on subgrade behaviour and permanent deformation that was generated by a fleet of Heavy Vehicle Simulators (HVSs) over 20 years of testing in South Africa. A literature review of the origin of the current subgrade design model that is used by the South African Mechanistic-Empirical Design Method revealed that that model is based on very little actual subgrade performance data. The model was also developed from the AASHO road test data and adjusted for South African conditions, based on general observations of subgrade behaviour without any calibration. Previous researchers have illustrated the potential of using Heavy Vehicle Simulator data to develop structural pavement design models and it was decided to apply a similar process to the permanent deformation of the pavement subgrade. The present investigation consisted of two components, namely, the evaluation of the resilient and the permanent deformation response of the pavement subgrade, the emphasis in this thesis being more on the permanent deformation response. A general, multi-dimensional empirical model was formulated for the permanent subgrade deformation and the characteristics of the model investigated based on previously published permanent deformation data and mathematical assessment. A set of 35 HVS tests for which suitable data were available was identified and additional field and laboratory tests were done on selected sites to improve the classification of the subgrade materials at these sites. Standard procedures were developed to present the pavement, instrumentation and load sequence data of each HVS test. A process for doing the initial analysis of the deflection and permanent MDD displacement data and presenting the data was also developed. In terms of the resilient response of the subgrade, it was shown that the vertical depth deflection and vertical strain could be modelled accurately if an appropriate set of resilient modulus values was selected for the pavement layers. A detailed investigation of the resilient response of selected HVS sections did, however, clearly illustrate the stress-dependent behaviour of subgrade material, resulting in resilient modulus values being determined, which were well outside the range that would normally be expected for natural gravel subgrade material. This research needs to be continued to enable the development of a comprehensive set of stress-dependent resilient modulus models for South African subgrade materials. The selection of an appropriate critical parameter that can be used as a predictor of permanent subgrade deformation was done by an investigation of the relationship between potential critical parameters and several permanent deformation parameters. It was found that the subgrade elastic deflection showed the best correlation with the subgrade bearing capacity (the number of load repetitions that can be sustained before a terminal rut condition is reached). The vertical subgrade strain that is currently used in the South African Mechanistic-Empirical Design Method in fact correlates poorly with subgrade bearing capacity and has to be replaced with subgrade elastic deflection. A set of subgrade bearing capacity or design models was developed for different levels of permanent subgrade deformation. These models are referred to as S-N models and form contour lines on the general permanent deformation model that was formulated. The subgrade design model accommodates loading conditions ranging from a 40 kN dual wheel load to a 100 kN dual wheel load as well as subgrade materials ranging from a material quality one class better than that which would normally be used for a subgrade, to the lowest possible material class. The model is therefore very flexible in terms of its application. / Prof. P. Pretorius

The research presented in this thesis studies the effects of foamed bitumen on the deformational behaviour and performance of pavement materials. The research was conducted in the laboratory and the field, using specific New Zealand materials. The aggregate used is a blend of a coarse aggregate imported from the Auckland region with a crushed dust from the Canterbury region. The bitumen selected for the study is an 80/100 bitumen grade, and the active filler was a Portland Cement, both commonly used for foamed bitumen stabilization in New Zealand. In the laboratory, samples of mixes with different foamed bitumen content were tested under various loading and stress conditions to investigate the effects of foamed bitumen on the deformational behaviour of the mix. The tests performed were: Indirect Tensile Strength (ITS), Indirect Tensile Resilient Modulus (ITM), Repeat Load Triaxial compression (RLT) and Monotonic Load Triaxial compression (MLT). Preliminary ITS and RLT tests conducted on mixes with 1% and 0% cement, at different foamed bitumen contents, indicated that mixes without cement performed poorly compared to the mixes with 1% cement. Therefore, the rest of the laboratory study was on mixes with 1% cement. ITS tests were conducted on 150 mm specimens prepared with 0% 1%, 2%, 3% and 4% bitumen content, with a common 1% cement. Results indicated that foamed bitumen increases the ITS values of the mix, up to an estimated optimum of 2.8% bitumen content. Similar trends were obtained with ITM tests, in which a diametrical load pulse was applied on 150 mm specimens, showing an estimated resilient modulus peak near to 2.8% bitumen content. RLT specimens were prepared at 0%, 2% and 4% bitumen content, at two compaction efforts, creating specimens at low and high bulk density. Permanent deformation RLT tests involved the application of seven stages of 50,000 load cycles each (4 Hz), with increasing deviator stress (from 75 kPa in the first stage, up to 525 kPa in the seventh stage) and at constant confining pressure of 50 kPa. Results of RLT permanent deformation tests indicated that the increase in the foamed bitumen content resulted in an increase in the permanent deformation of the material. MLT tests were conducted on specimens at 0%, 2% and 4% bitumen contents, at two compaction efforts, creating specimens of low and high bulk density, at confining pressures ranging from 50 kPa to 300 kPa, with a deformation rate of 2.1% per minute. Results indicated that the effect of foamed bitumen was a reduction of the peak vertical stress, or a reduction in the peak strength. The peak stresses obtained in MLT tests were plotted in stress diagrams, and the failure was approximated as linear function of the confining stress. The fundamental shear parameters (angle of internal friction and apparent cohesion) were estimated, and results indicated that foamed bitumen has no apparent effect in cohesion but does reduce the angle of internal friction. The reduction of the angle of internal friction explains the general trends observed in the laboratory, that on one hand the compressive strength decreases with increasing bitumen content, but on the other hand, the tensile strength increases up to an optimum. A full-scale experiment was carried out using an accelerated testing of foamed bitumen pavements at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF). In the full-scale experiments, the same materials that were tested in the laboratory (aggregates, bitumen, cement) were used to construct six different pavement sections, each with different contents of bitumen and cement. Three were constructed using foamed bitumen contents of 1.2%, 1.4% and 2.8% respectively, plus a common active filler content of 1.0% cement. Two more pavements were constructed adding cement only (1.0%), and foamed bitumen only (2.2%). In addition, one control section with the untreated unbound material was tested. Strains were collected using a 3D Emu soil strain system installed in each pavement section. The curing time between construction and pavement loading was approximately three months. The pavement response, such as surface deformation (rutting), surface deflections and strains were periodically recorded during the execution of the test. The strains were collected at different depths by using an array of Emu strain gauges. Deflections were recorded using both a Falling Weight Deflectometer (FWD) and CAPTIF Beam deflectometer, which is a modified Benkelmann beam. A total number of approximately 5.6 million equivalent standard axles were applied on the pavement sections. The rutting measured in the sections stabilised with foamed bitumen and cement was the lowest, showing that the addition of foamed bitumen significantly improved the performance of materials with 1% cement. The sections stabilised with cement only, foamed bitumen only, and the control untreated section showed large amounts of rutting and heaving by the end of the test. Deflection measurements showed that the effect of foamed bitumen content is a reduction of pavement deflections, with the lowest deflection measured in the section stabilised with 2.8% bitumen and 1% cement. The elastic pavement strains showed that foamed bitumen reduced the tensile strains in the basecourse but did not have a significant effect on vertical compressive strains. During the construction of pavements, material samples were taken for ITS and RLT testing. Results indicated that the highest ITS was measured in the section with 2.8% foamed bitumen content and 1% cement, and the ITS in the section without cement and foamed bitumen only was about 4-5 times lower than the ITS measured in specimens with cement. RLT specimens without cement performed poorly in comparison with the specimens with 1% cement. The specimens with 1% cement showed higher permanent deformation with increase in the foamed bitumen content, supporting the results from the previous laboratory study. To interpret and relate the results observed in the laboratory and the field, stress path analysis was used, in which the stress ratio of the foamed bitumen layers was calculated at different depths. The analysis showed that foamed bitumen content decreases the maximum stress ratio, hence reducing the proximity to failure and relative damage of the layer. Three-dimensional and two-dimensional finite element modelling of the CAPTIF pavements, were used to further investigate the stress and strain fields induced by the loading and to explain the pavement performance observed in the full-scale experiment.

Doctor of Philosophy / Department of Civil Engineering / Mustaque A. Hossain / Tack coat is a thin layer of asphaltic material used to bind a newly-placed lift of hot-mix asphalt (HMA) pavement to a previously-placed lift or a new HMA overlay/inlay and existing pavement. The purpose of a tack coat is to ensure that a proper bond occurs so that traffic loads are carried by the entire HMA structure. Proper bonding exists when HMA layers act as a monolithic structure, transferring loads from one layer to the next. This depends on appropriate selection of tack coat material type and application rate, and is essential to prevent slippage failure and premature cracking in the wearing surface. This study focuses on development of a three-dimensional finite element (FE) model of HMA pavement structure in order to assess HMA interface bonding. The FE model was constructed using commercially available ABAQUS software to simulate an Accelerated Pavement Testing (APT) experiment conducted at the Civil Infrastructure Systems Laboratory (CISL) at Kansas State University. Mechanistic responses measured in the CISL experiment, such as localized longitudinal strain at the interface, were used to calibrate the FE model. Once calibrated, the model was used to predict mechanistic responses of the pavement structure by varying the tack coat property to reflect material characteristics of each application. The FE models successfully predicted longitudinal strains that corresponded to APT results.

The most common use of reclaimed asphalt pavement (RAP) is in the lower layers of a pavement structure, where it has been proven as a valid substitute for virgin materials. Instead, the use of RAP in surface mixes is more limited, with a major concern being that the high RAP mixes may not perform as well as traditional mixes. To reduce risks of compromised performance, the use of RAP has commonly been controlled by specifications that limit the allowed amount of recycled material in the mixes. However, significant cost and environmental savings can be achieved if more RAP is included in the surface layer. This dissertation develops an approach that can be followed to incorporate more RAP in the surface mix while maintaining good performance. The approach is based on the results from three studies that looked at how to optimize the design of the mix, in terms of rutting and fatigue resistance, when more RAP is used. In the first study, a high RAP control mix and an optimized mix designed using different design compaction energy (65 and 50 gyrations respectively) were compared. The optimization process consisted in the definition of an alternative mix composition that supported the higher binder content allowed by the lower design compaction energy. Using Accelerated Pavement Testing and laboratory characterization it was possible to assess the potential of mix optimization with the objective of improving rutting resistance. The testing showed no indication that the optimized mixes would have rutting problems, supporting the implementation of the reduction of the design compaction energy level. The optimized mix exhibited a similar or superior rutting resistance in the full-scale setting, in the laboratory, and in the forensic investigation. The second part focused on the production of highly recycled surface mixes capable of performing well. To produce the mixes, a balanced mix design (BMD) methodology was used and a comparison with traditional mixes, prepared in accordance with the requirements of the Virginia Department of Transportation (VDOT) volumetric mix design, was performed. Through the BMD procedure, which featured the indirect tensile cracking test for evaluating the cracking resistance and the Asphalt Pavement Analyzer for evaluating rutting resistance, it was possible to optimize the selection of the optimum asphalt content. Also, it was possible to obtain a highly recycled mix (45% RAP) capable of achieving better overall performances than traditional mixes while carrying a large reduction in production cost. The final part evaluated the laboratory performance of four different highly recycled surface mixes to support their possible implementation in the state of Virginia. The mixes featured either 30% or 45% RAP, different asphalt contents, the use of a WMA additive, and a rejuvenator. To analyze the mixes' performance in great depth, a three-level (base, intermediate, and advanced) testing framework was defined. Each level was characterized by an increasing degree of complexity and included tests to characterize both the cracking resistance and the rutting resistance. The study aimed at investigating the features of the various laboratory tests. Through the review of the theoretical background, the evaluation of the test procedures, and statistical analysis of the results, it was possible to identify the strengths and weaknesses of each test and to provide guidelines to develop appropriate quality assessment criteria and mix design methodology. In summary, throughout this research, it was possible to observe that the respect of Superpave mix design requirements alone, with particular reference to gradation limits and volumetric properties, was not guarantee of satisfactory performance in terms of both cracking and rutting resistance. To increase the confidence in the RAP properties, increase the current recycling levels, and introduce more appropriate mix design specifications, BMD could be used (even with simple laboratory tests) to check performance-based criteria. / Doctor of Philosophy / Nowadays, transportation agencies are expected to perform a large number of pavement rehabilitation projects, while facing major limitations in budgetary funds. In order to have safe, efficient, and cost-effective roadways, the economic advantage of recycling is boosting an effort to increase the amount of RAP in asphalt mixtures. In addition, over the past decades, the environmental awareness of the transportation agencies and public increased significantly, pushing even more towards the use of new green technologies. The use of RAP became noticeable in the 1970s and its popularity increased significantly since that time. However, there are still many open questions which prevent larger uses of recycled materials, mainly related to the design methodology and the field performances of recycled mixtures. Therefore, today there is a large untapped potential that would grow even more the magnitude of pavement recycling and of the associated benefits. New design procedures, based on the support of laboratory tests to characterize the mixtures, and full-scale experiments are the tools that pavement engineers can use in order to enrich the knowledge of highly recycled road materials and grow the confidence of public agencies and contractors towards these new more sustainable solutions. Throughout this dissertation it was possible to evaluate new innovative ways of incorporating more RAP in the asphalt mixtures through the analysis of current state of the art and the proposition of new procedures.

Pavement macrotexture has been shown to influence a range of safety and comfort issues including wet weather friction, splash and spray, ambient and in-vehicle noise, tire wear, and rolling resistance. While devices and general guidance exist to measure macrotexture, the wide-scale collection and use of macrotexture is neither mandated nor is it typically employed in the United States. This work seeks to improve upon the methods used to calibrate, collect, pre-process, and distill macrotexture data into useful information that can be utilized by pavement managers. This is accomplished by 1. developing a methodology to evaluate and compare candidate data collection devices; 2. plans and procedures to evaluate the accuracy of high-speed network data collection devices with reference surfaces and measurements; 3. the development of a method to remove erroneous data from emerging 3-D macrotexture sensors; 4. development of a model to describe the change in macrotexture as a function of traffic; 5.finally, distillation of the final collected pavement surface profiles into parameters for the prediction of important pavement surface properties aforementioned. Various high-speed macrotexture measurement devices were shown to have good repeatability (between 0.06 to 0.09mm MPD) and interchangeability of single-spot laser dfevices was demonstrated via a limits of agreement analysis. The operational factors of speed and acceleration were shown to affect the resulting MPD of several devices and guidelines are given for vehicle speed and sensor exposure settings. Devices with single spot and line lasers were shown to reproduce reference waveforms on manufactured surfaces within predefined tolerances. A model was developed that predicts future macrotexture levels (as measured by RMS) for pavements prone to bleeding due to rich asphalt content. Finally, several previously published macrotexture parameters along with a suite of novel parameters were evaluated for their effectiveness in the prediction of wet weather friction and certain types of road noise. Many of the parameters evaluated outperformed the current metrics of MPD and RMS. / Doctor of Philosophy

" The determination of strain is an important step when using a mechanistic-empirical structural design, such as the AASHTO 2002 Design Guide. This thesis investigated the use of accelerated pavement testing system on Hot Mix Asphalt pavements to determine actual transverse and longitudinal strains under loads of varying frequency at different temperatures. A Model Mobile Load Simulator (MMLS3) was used in this study. Laboratory compacted pavement slabs were instrumented with thermocouples for monitoring the pavement's temperature, and with strain gauges in transverse and longitudinal directions at the bottom surface to measure strain. The slabs were subjected to loading by the MMLS3, running at different speeds. The pavement slab and accelerated loading equipment were enclosed in an environmental chamber to control temperatures during testing. Strains were also determined from layered elastic analysis after determining modulus values by two different methods - Resilient modulus testing and Witczak’s dynamic modulus equation. Comparisons of pavement strains calculated through the use of layered elastic design software and actual strains obtained during loading were made. The test results have shown a significant difference between strain values obtained using an instrumented pavement slab and those obtained with the use of standard resilient modulus values or dynamic modulus values determined by using a typical layered elastic design model. To avoid the discrepancies, two approaches are proposed - The first is modeling strain with accelerated pavement testing and the second one is using a correction factor. "

The study of innovative pavements is of significant importance in geotechnical engineering in Brazil, due to the continued need to increase the network of roadways. This requires optimized projects, not only for economic, but also for technical reasons. Technical solutions that use geosynthetics in asphalt overlays have been identified to minimize fatigue and reflective cracks. However, the majority of the application of this technology has ignored the possible additional structural benefits brought by the inclusion of geosynthetics as reinforcement in asphalt layers. The objective of this research is to assess the reinforcement benefits of geogrids placed within asphalt overlays on the structural performance of flexible pavements. In addition, this study investigates the tensile-strain response of geogrids under traffic conditions, induced by cyclic wheel loads generated by a new accelerated pavement testing facility (APT) that was specifically developed for this research. The APT facility consists of a large steel testing box, in which field-scale pavement layers could be constructed. Pavement materials included subgrade soil, aggregate base, hot mix asphalt concrete, asphalt emulsion and a PVA geogrid. Pavement performance was assessed by applying a cyclic wheel load pressure of 700 kPa to the pavement surface. The pavement sections investigated in this study included a geogrid-reinforced and an unreinforced asphalt overlay sections, a single new geogrid-reinforced asphalt layer, and a geogrid-reinforced asphalt overlay with reduced base course thickness. A variety of sensors were used to measure asphalt concrete strains, surface plastic and elastic displacements, and induced traffic loads. Displacements along the geogrid specimens were measured using a tell-tail system. As result, several reinforcement mechanisms of this technique could be quantified in the present study. Polymeric geogrid reinforcements were found to have considerably reduced strains developed at the bottom of asphalt layers, as well as to have reduced vertical stresses in pavement lower layers. Resistance to rutting and lateral movement induced by the geogrids were also clearly evidenced in the presented study. The measurement of displacements along the geogrid provided understanding of the distribution of strains during traffic loading. A mobilized length was identified in geogrid-reinforced sections, showing that the bonding between geogrids and asphalt layers and the stiffness of the geogrid ensured satisfactory performance of the pavement sections. The results also illustrated that the lateral restraining mechanisms effect is a governing mechanism to improve the performance of the asphalt layers by the development of shearing resistance with the geogrids. Overall, it was concluded that geogrids within asphalt overlays act as reinforcement and not merely to delay cracks, providing enhanced performance to flexible pavement structures. / O estudo de pavimentos é de grande importância na Engenharia Geotécnica brasileira devido à crescente necessidade de melhora da situação da rede rodoviária nacional. Para tanto, o desenvolvimento e a aplicação de novas técnicas são necessários, principalmente no âmbito econômico. A técnica do uso de reforços geossintéticos em capa asfáltica é identificada como uma alternativa ao aumento da vida útil do pavimento através da mitigação de trincas por fadiga e de reflexão. No entanto, a maioria das aplicações desta técnica não correlaciona os benefícios estruturais da inclusão do geossintético na capa asfáltica para a melhora do desempenho global do pavimento. O objetivo desta pesquisa é investigar os benefícios estruturais no desempenho de pavimentos flexíveis trazidos pelo reforço de geogrelhas em camadas asfálticas. Ainda neste estudo, será investigada a reposta tensão-deformação destas geogrelhas sobre as condições de tráfego através do uso de ensaios acelerados de pavimento. Um equipamento foi desenvolvido para esta pesquisa e consiste numa caixa metálica de grande porte, em que seções de pavimento em escala real podem ser construídas. O desempenho das seções de pavimento foi avaliado com a aplicação de cargas cíclicas de roda com pressão de contato de 700 kPa. Os materiais que compõem as seções de pavimento incluem solo de subleito, brita graduada simples, concreto betuminoso usinado à quente, emulsão asfáltica e geogrelha de PVA. Foram estudadas uma seção com geogrelha como reforço no recapeamento da camada asfáltica, uma seção idêntica não reforçada, uma seção com uma única capa asfáltica reforçada com geogrelha e uma seção com geogrelha no recapeamento da camada asfáltica, porém com espessura de base reduzida em relação aos demais ensaios. Sensores nas camadas do pavimento mediram tensões e deformações, e deslocamentos plásticos e elásticos na superfície. Deslocamentos ao longo da geogrelha foram monitorados utilizando o sistema tell-tales. Como resultado, mecanismos de reforço foram identificados neste estudo. O uso de uma geogrelha polimérica reduziu consideravelmente as deformações na fibra inferior da capa asfáltica, assim como as tensões verticais nas camadas subjacentes do pavimento. Resistência à formação de trilhas de roda e solevamentos laterais foram também evidenciadas. As medidas de deslocamentos ao longo da geogrelha forneceram entendimento da distribuição de deformações durante o carregamento. Foi identificado o comprimento de geogrelha mobilizado durante os ensaios, mostrando que a aderência entre a geogrelha e as camadas asfálticas e a rigidez da geogrelha asseguraram o desempenho satisfatório das seções de pavimento. Os resultados também mostraram que o efeito do mecanismo de restrição lateral é um mecanismo que governa a melhora no desempenho da capa asfáltica com o uso da geogrelha através do desenvolvimento de resitência ao cisalhamento. Estas observações permitem concluir que a geogrelha na camada asfáltica atua como reforço e não apenas reduzindo a o potencial de trincamento, levando à um aumento no desempenho de estruturas de pavimentos flexíveis.

Traditionally, flexible pavement design relies on past experience and semi-empirical methods developed through a combination of element testing and modelling. Element testing in this area especially, has centred on the performance of clean sands. This is in conflict with actual practice where a wide range of fines and soil gradations could be present in a real-world project. This research investigates the characteristics of natural sands and examines the influence of these marginal materials in pavement design using element testing and controlled modelling of an actual flexible pavement system. The element tests concentrated on separate, natural soils sourced from Kazakhstan which had similar mineralogy, but varying amounts of fines. One of the key parameters examined was equivalent void ratio and its efficiency to account for the behaviour change in granular materials which comes from increased fines content. Starting with monotonic triaxial results combined with strength-dilatancy methods it was shown that prediction of shear strength in a silty-sand could be improved by 13%. Incorporating this finding into repeat load triaxial tests, the transitions between elastic, plastic, and ratcheting failure behaviours (i.e. shakedown boundaries), commonly used to help predict the lifespan of a flexible pavement, were examined. It was seen that cycling a silty-sand, the stress path and yield surface could change depending on the fines content. The Cambridge Airfield Pavement Tester (APT) was designed and constructed to measure permanent subgrade deformation resulting from various surface loads. The number of input variables required to design flexible pavements is one of the most frequently stated problems in the field; variation of aircraft types, environmental conditions, and materials makes mechanistic design of the soil foundation problematic. Accordingly physical pavement modelling continues to be the only experimental method that allows input parameters and material characteristics to be examined simultaneously. Digital image correlation (DIC) was incorporated into the system; the first time this technology has been used in flexible pavement research. A Null Pressure System was also installed to measure soil stress distributions. It was observed that the critical failure mechanisms for thin and thick surficial layers are different, resulting in changes in the rates of surface rutting. Finally, by combining element and APT results, knowledge of the causal relationships between subsurface deformation and failure mechanisms in flexible pavement were advanced. In-situ soils, which are frequently incorporated into subgrade designs, were found to have a substantial role in the serviceability of the pavement. Correlations between element tests and APT results highlighted the complicated loading and boundary conditions present in a pavement.

Mechanistic-Empirical  (M-E)  pavement  design  procedures  are  composed  of  a  reliable  response model to estimate the state of stress in the pavement and distress models in order to predict the different types of pavement distresses due to the prevailing traffic and environmental conditions. One of the main objectives of this study was to develop a response model based on multilayer elastic  theory   (MLET)  with  improved  computational  performance  by   optimizing  the   time consuming parts of the MLET processes. A comprehensive comparison of the developed program with  two  widely  used  programs  demonstrated  excellent  agreement  and  improved  computational performance.  Moreover,  the  program  was  extended  to  incorporate  the  viscoelastic  behaviour  of bituminous materials through elastic-viscoelastic correspondence principle. A procedure based on collocation of linear viscoelastic (LVE) solutions at selected key time durations was also proposed that improved the computational performance for LVE analysis of stationary and moving loads. A comparison  of  the  LVE  responses  with  measurements  from  accelerated  pavement  testing  (APT) revealed a good agreement. Furthermore the developed response model was employed to evaluate permanent deformation models  for  bound  and  unbound  granular  materials  (UGMs)  using  full  scale  APTs.  The  M-E Pavement  Design  Guide  (MEPDG)  model  for  UGMs  and  two  relatively  new  models  were evaluated  to  model  the  permanent  deformation  in  UGMs.  Moreover,  for  bound  materials,  the simplified  form  of  the  MEPDG  model  for  bituminous  bound  layers  was  also  evaluated.  The measured  and  predicted  permanent  deformations  were  in  general  in  good  agreement,  with  only small discrepancies between the models. Finally, as heavy traffic loading is one of the main factors affecting the performance of flexible pavement, three types of characterizations for heavy traffic axle load spectrum for M-E analysis and design of pavement structures were evaluated. The study recommended an improved approach that enhanced the accuracy and computational performance. / <p>QC 20140512</p>

The study of innovative pavements is of significant importance in geotechnical engineering in Brazil, due to the continued need to increase the network of roadways. This requires optimized projects, not only for economic, but also for technical reasons. Technical solutions that use geosynthetics in asphalt overlays have been identified to minimize fatigue and reflective cracks. However, the majority of the application of this technology has ignored the possible additional structural benefits brought by the inclusion of geosynthetics as reinforcement in asphalt layers. The objective of this research is to assess the reinforcement benefits of geogrids placed within asphalt overlays on the structural performance of flexible pavements. In addition, this study investigates the tensile-strain response of geogrids under traffic conditions, induced by cyclic wheel loads generated by a new accelerated pavement testing facility (APT) that was specifically developed for this research. The APT facility consists of a large steel testing box, in which field-scale pavement layers could be constructed. Pavement materials included subgrade soil, aggregate base, hot mix asphalt concrete, asphalt emulsion and a PVA geogrid. Pavement performance was assessed by applying a cyclic wheel load pressure of 700 kPa to the pavement surface. The pavement sections investigated in this study included a geogrid-reinforced and an unreinforced asphalt overlay sections, a single new geogrid-reinforced asphalt layer, and a geogrid-reinforced asphalt overlay with reduced base course thickness. A variety of sensors were used to measure asphalt concrete strains, surface plastic and elastic displacements, and induced traffic loads. Displacements along the geogrid specimens were measured using a tell-tail system. As result, several reinforcement mechanisms of this technique could be quantified in the present study. Polymeric geogrid reinforcements were found to have considerably reduced strains developed at the bottom of asphalt layers, as well as to have reduced vertical stresses in pavement lower layers. Resistance to rutting and lateral movement induced by the geogrids were also clearly evidenced in the presented study. The measurement of displacements along the geogrid provided understanding of the distribution of strains during traffic loading. A mobilized length was identified in geogrid-reinforced sections, showing that the bonding between geogrids and asphalt layers and the stiffness of the geogrid ensured satisfactory performance of the pavement sections. The results also illustrated that the lateral restraining mechanisms effect is a governing mechanism to improve the performance of the asphalt layers by the development of shearing resistance with the geogrids. Overall, it was concluded that geogrids within asphalt overlays act as reinforcement and not merely to delay cracks, providing enhanced performance to flexible pavement structures. / O estudo de pavimentos é de grande importância na Engenharia Geotécnica brasileira devido à crescente necessidade de melhora da situação da rede rodoviária nacional. Para tanto, o desenvolvimento e a aplicação de novas técnicas são necessários, principalmente no âmbito econômico. A técnica do uso de reforços geossintéticos em capa asfáltica é identificada como uma alternativa ao aumento da vida útil do pavimento através da mitigação de trincas por fadiga e de reflexão. No entanto, a maioria das aplicações desta técnica não correlaciona os benefícios estruturais da inclusão do geossintético na capa asfáltica para a melhora do desempenho global do pavimento. O objetivo desta pesquisa é investigar os benefícios estruturais no desempenho de pavimentos flexíveis trazidos pelo reforço de geogrelhas em camadas asfálticas. Ainda neste estudo, será investigada a reposta tensão-deformação destas geogrelhas sobre as condições de tráfego através do uso de ensaios acelerados de pavimento. Um equipamento foi desenvolvido para esta pesquisa e consiste numa caixa metálica de grande porte, em que seções de pavimento em escala real podem ser construídas. O desempenho das seções de pavimento foi avaliado com a aplicação de cargas cíclicas de roda com pressão de contato de 700 kPa. Os materiais que compõem as seções de pavimento incluem solo de subleito, brita graduada simples, concreto betuminoso usinado à quente, emulsão asfáltica e geogrelha de PVA. Foram estudadas uma seção com geogrelha como reforço no recapeamento da camada asfáltica, uma seção idêntica não reforçada, uma seção com uma única capa asfáltica reforçada com geogrelha e uma seção com geogrelha no recapeamento da camada asfáltica, porém com espessura de base reduzida em relação aos demais ensaios. Sensores nas camadas do pavimento mediram tensões e deformações, e deslocamentos plásticos e elásticos na superfície. Deslocamentos ao longo da geogrelha foram monitorados utilizando o sistema tell-tales. Como resultado, mecanismos de reforço foram identificados neste estudo. O uso de uma geogrelha polimérica reduziu consideravelmente as deformações na fibra inferior da capa asfáltica, assim como as tensões verticais nas camadas subjacentes do pavimento. Resistência à formação de trilhas de roda e solevamentos laterais foram também evidenciadas. As medidas de deslocamentos ao longo da geogrelha forneceram entendimento da distribuição de deformações durante o carregamento. Foi identificado o comprimento de geogrelha mobilizado durante os ensaios, mostrando que a aderência entre a geogrelha e as camadas asfálticas e a rigidez da geogrelha asseguraram o desempenho satisfatório das seções de pavimento. Os resultados também mostraram que o efeito do mecanismo de restrição lateral é um mecanismo que governa a melhora no desempenho da capa asfáltica com o uso da geogrelha através do desenvolvimento de resitência ao cisalhamento. Estas observações permitem concluir que a geogrelha na camada asfáltica atua como reforço e não apenas reduzindo a o potencial de trincamento, levando à um aumento no desempenho de estruturas de pavimentos flexíveis.