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  25  th  ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012 © ARRB Group Ltd and Authors 2012 1 EFFECT OF GEOGRID REINFORCEMENT ON CRITICAL RESPONSES OF BITUMINOUS PAVEMENTS Satish Pandey, CSIR-Central Road Research Institute, India K. Ramachandra Rao, Indian Institute of Technology Delhi, India Devesh   Tiwari, CSIR-Central Road Research Institute, India ABSTRACT  A series of finite element (FE) simulations are carried out to evaluate the benefits of integrating a high modulus geogrid as reinforcement into the pavement layers. This paper presents a two dimensional axisymmetric finite element model that analyzes the behavior of unreinforced and geogrid reinforced bituminous pavement subjected to static and dynamic loading conditions. The critical pavement responses such as fatigue (horizontal) strain, rutting (vertical) strain and vertical surface deflection are calculated for unreinforced and geogrid reinforced flexible pavement using a pavement response model developed through a commercially available finite element program PLAXIS. Parametric studies are performed by varying the location of geogrid reinforcement i.e. base – bituminous concrete interface and the base sub-grade interface. The structural benefits of geogrid reinforcement over fatigue and rutting strain criteria have been quantified. The results obtained are qualitatively compared with the results of published literature and fairly good agreement is found in fatigue and rutting strains in the reinforced pavement. It has been found that placing geogrid reinforcement at the base-bituminous concrete interface leads to the highest reduction in fatigue (horizontal) strain. The highest decrease of vertical strain occurs when the reinforcement is placed at the interface of base and sub grade layers. Keywords: Finite Element Analysis; Flexible pavement; Geogrid; Reinforced pavement; Bituminous concrete; Dynamic loading INTRODUCTION  A large variety of detrimental factors affect the service life of flexible pavement. These factors include environmental factors affecting properties of pavement material, sub grade conditions, traffic loading, aging etc. All these factors cause an equally wide variety of distress which is manifested in the form of fatigue, rutting, differential settlement and reflective cracks in pavement. The fundamental objective of pavement design is to prolong the service life of a pavement structure and thus reducing the life-cycle cost. In the last few decades, geosynthetic reinforcement, particularly high modulus polymeric geogrid, have been increasingly utilized within pavement layers to improve the structural performance of both, newly constructed and rehabilitated flexible pavement. A biaxial extruded polymer geogrid consisting connected parallel sets of tensile ribs (with apertures of sufficient size to allow strike-through of the surrounding soil, stone, or other geotechnical material), offer confinement to the aggregate material used in pavement layers. It restricts lateral flow of pavement layers materials during loading. Most of the aggregates used in pavement structure are stress-dependent materials, improved lateral confinement results in an increase in the modulus of the base course material. The effect of increased modulus of the base course is improved vertical stress distribution on subgrade and a corresponding reduction in the vertical strain on top of the subgrade. Besides reducing the rutting strain, geogrids also resist fatigue (horizontal) strain through the tensioned membrane effect, induced in the bituminous concrete layer.  25  th  ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012 © ARRB Group Ltd and Authors 2012 2 Finite element analysis can handle complex geometry, different boundary conditions and material properties with ease. Extensive research has been carried out to model flexible pavements using finite element analysis techniques, with defined boundary conditions, under static and dynamic loading. In the present study a finite element program, PLAXIS, which has proved its efficacy in geotechnical application (Kazemien et al. 2010), is used to model the unreinforced and geogrid reinforced flexible pavement. The finite element method is validated by comparing the results obtained through a linear elastic layer based program KENLAYER. The aim of this paper is to investigate the benefits provided by geogrid reinforcement to the fatigue and rutting resistance of a flexible pavement system in conjunction with pavement design life. LITERATURE REVIEW Numerous researchers studied the impact of geosynthetic reinforcement over structural performance of paved roads through laboratory, field, and numerical modelling methods. Laboratory experiments carried out by Haas (1984), under cyclic loading condition showed a considerable reduction of about 30% in the maximum horizontal tensile strain transmitted underneath of the asphalt-concrete (AC) layer and 20 % to 40% reduction in the maximum vertical compressive stress at the top of subgrade, as a result of placing polymeric grid at the bottom of the AC layer of varying thickness. The influence of the geogrid location over fatigue and rutting strain were not studied. Barksdale, Brown and Chan (1989) compared the structural performance of unreinforced and geogrid reinforced pavement subjected on cyclic loading condition through laboratory experimentation. Performance characterization of the unreinforced and geogrid reinforced pavement was carried out on the basis vertical permanent deformation. The results of the study indicated that for a stronger pavement, the stiff geogrid at the bottom of the granular base did not produce any significant improvement. Their results further indicated that placing the geogrid in the middle and bottom of the base layers, despite its lower stiffness, resulted in better performance against permanent deformation than the use of a geotextile. Numerical simulation carried out by them using finite element analysis techniques showed that the benefits of geosynthetic reinforcements are more pronounced for weaker subgrades. Virgile et al. (2009) studied the flexural behavior of bi-layer bituminous system reinforced with geogrid (polyester and glass fiber) through laboratory experiments. All bi-layer bituminous systems were tested by means of a four-point bending test under repeated loading cycles. The failure criterion was defined as the number of loading cycles corresponding to the flex point of the permanent deformation evolution, i.e. where the permanent deformation evolution inverts it trend (from decreasing to increasing rate). Geogrid was placed at the interface of bituminous layer. The laboratory study showed that the reinforced system improved the resistance to repeated load cycles by 66% to 100 % and delayed the inversion from decreasing to increasing rate of the permanent deformation evolution curve. However, this study did not include the effect of geogrids on fatigue resistance of asphalt layers. Dondi (1994) used the finite element program ABAQUS to model a geosynthetic reinforced flexible pavement. Three dimensional static analysis was carried out using linear and non linear constitutive material models. Bituminous concrete layer and geosynthetic reinforcement were modeled using a linear elastic material model based on Hook’s law while Drucker-Prager and Cam Clay material models were used to model base course and subgrade layers. The results of their study indicated 15-20% reduction in vertical displacement under the load in reinforced section and 2-2.5 times increase in fatigue life of reinforced sections compared to unreinforced sections. Another example of a finite element study on a geosynthetic reinforced flexible pavement is that conducted by Wathugala, Huang and Pal (1996). They used the ABAQUS finite element program to explore the decrease in the permanent deformation as a result of placing geosynthetic membrane at the base-subgrade interface of a flexible pavement system. An axisymmetric analysis was adopted in their simulations which introduced the hierarchical single surface (HiSS) model for subgrade. The asphalt concrete and the crushed stone base layers were modeled by Drucker-Prager material. No special interface models were used between the geogrid and the surrounding soil. The inclusion of  25  th  ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012 © ARRB Group Ltd and Authors 2012 3 the geogrid reduced the permanent deformation by approximately 20% for a single cycle of load. Moayedi et al. (2009) studied the effect of geogrid reinforcement location in paved road improvement using axisymmetric pavement response model developed through the finite element program PLAXIS. Bituminous concrete layer and geogrid were modeled as a linear elastic isotropic material while the Moho-Coulomb material model was used to simulate granular layers. Pavement responses were determined under static loading condition. They showed that the geosynthetic reinforcement placed at the bottom of bituminous concrete layer leads to the highest reduction in vertical pavement deflection. Some researchers believe that geogrid should be placed at the top of base course while others have found that geogrid should be placed at the base – subgrade interface. Considerable variation in fatigue and rutting resistance of geogrid reinforced pavement has been found under static and dynamic loading conditions in the literature. Studies quantifying the effect of geogrid reinforcement over pavement design life under varying axle load are still an area for further research. FINITE ELEMENT ANALYSIS Flexible pavements in India are designed on the basis of California bearing ratio of soil subgrade and traffic loading. Indian Roads Congress code entitled Guideline for the Design of Flexible Pavement   (IRC:37:2001) is used in the present study to find out the layer composition and overall pavement thickness for the soil subgrade of 7% C.B.R value and cumulative traffic loading of 10 million standard axles (msa), during the design life. The pavement is modeled as a multilayer structure of linear elastic material subjected to circular loading of static and dynamic conditions. Bituminous Concrete (BC) and Dense Bituminous Macadam (DBM) have been clubbed together and considered as a single layer for finite element modeling. Two dimensional axisymmetric pavement response models are developed for unreinforced and geogrid reinforced pavement using commercially available finite element program PLAXIS. Various researchers Moayedi et al. (2009), Kazemien et al. (2010),   Howard and Warren (2009), have also used axisymmetric modeling and it was selected because it could simulate circular loading and did not require excessive computational time under dynamic loading conditions. A typical axisymmetric pavement response model has been shown in Figure 1 with their material layers and boundary conditions in Figure 2. Physical and mechanistic properties of the pavement response model are given in Table 1. Figure 1: Axisymmetric finite element model  25  th  ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012 © ARRB Group Ltd and Authors 2012 4 Figure 2: Material layers and boundary conditions for axisymmetric finite element model Table 1: Parameters used in axisymmetric finite element modelling   Section Thickness (mm) Unit wt. (kN/m 3 )  Young’s modulus E (MPa) Poisson’s ratio ( ν ) Rayleigh damping coefficients  R  ,    R  Material properties Bituminous layer (40 BC+60 DBM) 100 22.8 2579 0.3 0.9659, 0.00021 Isotropic and linear elastic Granular base 480 21.2 196 0.35 0.9402, 0.00032 Isotropic and linear elastic Sub grade 6420 19.6 61 0.4 0.7356, 0.00061 Isotropic and linear elastic  25  th  ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012 © ARRB Group Ltd and Authors 2012 5 Boundary conditions and meshing criteria Conventional kinematic boundary conditions have been adopted, i.e. roller support on all vertical boundaries of the mesh and fixed support at the bottom of the mesh. Such boundary conditions have been successfully used by Saad, Mitri and Poorooshasb (2006) and   Kazemien et al. (2010) .  The modelled domain must be large enough to avoid any edge error. Domain size analysis is carried out using PLAXIS program to find out the optimum size of pavement response model, which yield desirable pavement responses with reasonable degree of accuracy. It has been found that domain size exceeding 25 times the radius of contact area in horizontal direction and 35 times the radius of contact area in vertical direction yield the critical pavement responses comparable with the pavement responses obtained through elastic layered program KENLAYER. Accordingly pavement response model of 5 m length in horizontal direction and 7 m in height in vertical direction has been selected for axisymmetric analysis. When performing a dynamic analysis, it is also important to choose a model size having dimensions covering a significant distance away from the vibration source. This helps to avoid unwanted and unrealistic reflection of ground shock waves. Hence, to avoid these spurious reflections, absorbent boundaries are applied at the bottom and right-hand side boundary during dynamic analysis.  All natural systems subjected to moving loads show some degree of damping. In soils, damping is mainly due to loss of energy resulting from internal friction in the material and viscous properties.  A dynamic analysis is more realistic than a static analysis. PLAXIS offers a dynamic analysis facility for a problem in which inertia effects are important. Implicit direct integration is provided in the PLAXIS to solve the equation of motion as shown in Equation (1) (1)Where M, C, K are the mass, damping and stiffness matrices. The displacement, u, the velocity ů  and acceleration vector, ü  can vary with time. F(t) is the external load vector. Mathematically (1) represents a system of linear differential equations of second order. In direct integration, the equation is integrated using a numerical step-by-step procedure. For linear dynamic analysis through PLAXIS Rayleigh damping parameters are used to introduce natural material damping. Rayleigh damping term is defined as a damping matrix formed as a linear combination of the mass and stiffness matrices as shown in Equation (2). C =   R   x  M +   R. x   K (2)Here  R,   and    R    are the Rayleigh coefficients. Rayleigh coefficients used in the present study are given in Table 1. As the loading on the pavement surface is localized, the finest mesh is required near the loaded area to capture the steep stress and strain gradient in these areas. The subdivision is carried out so that the element aspect ratio remains close to one where the strain and stress gradients are high to achieve faster convergence in these areas. Pavement layers are modelled using 15-noded triangular elements. The geosynthetic reinforcement is modelled using 5-noded triangular tension elements. Tension elements have the ability to carry loads in tension but have no bending stiffness or ability to carry load in compression. Material behaviour and constitutive laws The contribution of the bituminous layer to surface rutting is dependent on the material properties. In this research study, bituminous layer properties have been considered at 30 °C temperature. At this temperature, for a given load amplitude, the vertical permanent deformation of the bituminous layer is considered to have insignificant contribution to the total surface deflection. Furthermore, a
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