Finite Element Analysis Programmes for Pavements

The application of finite element modelling has been widely used for pavement structure analysis and design. The reason behind its getting widespread attention from pavement modelling is its remarkably varied implementation of mechanical properties (Kim et al., 2010). In this part of the chapter, some of two and three-dimensional finite element solutions that used for pavement analysis are summarised. As shown in Figure (2-21), three sorts of analysis models, axisymmetric, 2-D plane, and 3-D, are typically adopted by researchers to investigate the performance of multilayered pavement structures.

Figure (2-21) Three Typical FE Analysis Models for Pavements (Kim et al., 2010)

2.7.3.1 Axisymmetric and Two-dimensional Finite Element Analysis

Hicks (1970) made a model of three-layer pavement section including an asphalt concrete of a thickness of 102 mm, 305 mm of a granular base and the lower layer is a clay subgrade. A uniformly distributed load was applied on a circular area. He adopted the finite element method and used two types of material resilient modulus modelling according to the way of computing, the first one was by bulk stress and the other one by the confining pressure. The solution involved an application of four equal wheel load increments. He found that fair change in the resilient modulus of pavement layers was observed as a result of considerable variations in the pavement structure responses to the applied load.

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Raad and Figueroa (1980) developed the ILLI-PAVE finite element programme at the University of Illinois, and Harichandran et al. (1989) developed the MICH-PAVE that based on finite element method as well at the Michigan State University. Both programmes are widely used for flexible pavement analysis and adopt axisymmetric modelling method using the K-θ model for the granular materials and the bilinear approximation for fine-grained subgrade soils. The principal stresses in the pavement layers (granular and subgrade) were governed by Mohr-Coulomb theory, so they did not exceed materials strength. In MICH-PAVE programme, an adjustable boundary of a limited depth under the subgrade surface, rather than fixed boundary that located deeper in the subgrade layer and that reduced the time and storage consuming as compared to other programmes.

Thompson and Garg (1999) proposed an engineering Procedure for estimation of critical pavement responses adopting the superposition of single wheel pavement responses. They employed two analysis approaches, axisymmetric finite element and multilinear elastic layered system to calculate responses. Their approach used the average value of resilient modulus of the pavement layer that acquired from axisymmetric ILLI-PAVE finite element programme. The results of ILLI- PAVE finite element analysis were used as input data for the multilayered elastic analysis.

2.7.3.2 Three-Dimensional Finite Element Analysis

Chen et al. (1995) investigated the influence of the tire inflation pressure and heavy axle loading on the performance of flexible pavement by making a three-dimensional finite element model. The pavement structure layers were considered to be homogeneous and linear elastic materials. They compared the results of their study to an elastic layered programme, ELSYM5 (Kopperman et al., 1986), for a circular uniformly distributed pressure and they found that there was a good agreement between the two approaches.

Helwany et al. (1998) considered a three-layer flexible pavement system with application of several sorts of axle loading, with different configurations and tire pressures. They used two- dimensional (DACSAR) and three-dimensional (NIKE3D) finite element programmes. The preliminary analysis of one layer pavement model using both programmes showed an agreement the Boussinesq’s solutions. The study suggested that adopting finite element modelling of pavements could be remarkably beneficial to predict accurate pavement structure responses.

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Shoukry et al. (1999) employed three-dimensional finite element model for back- calculation of modulus of elasticity of pavement structures and they compared the outcomes with the predictions of back-calculation programmes such as EVERCALC, MODCOMP, and MODULUS. The displacements were calculated from the falling weight deflectometer tests. The pavement layers were modelled as linear elastic layers with 8-noded solid brick elements. They found that the proposed three-dimensional finite element analysis had a decent agreement with the back- calculated layer modulus of elasticity estimated by the other programmes.

Wang (2001) used three-dimensional finite element analysis to study the flexible pavement structures responses including different materials, model dimensions and various loadings. He developed an effective meshing tool for a three-dimensional model consolidating multiple layers, interface bonding, and several loading configurations. He found that spatially different tire and pavement pressures significantly influenced the stresses and strains in the flexible pavement model.

2.7.3.3 Finite Element Pavement Analysis Programmes Characteristics

The finite element analysis technique extends the best way of investigation for the multilayered pavement structures. Three and two-dimensional as well as axisymmetric finite element models have different element creation and consider variable directional mechanisms of stresses and strains. Three-dimensional finite element analysis can analyse all three directional response components and should predict more accurate pavement responses (Kim, 2007).

2.7.3.4 General Purpose Finite Element Programmes

ANSYS, ABAQUS, and ADINA are widely used general-purpose finite element programmes that can afford decent analyses of different engineering problems. Despite the fact that the modelling of pavement structure has dramatically progressed in recent years, the analysis of pavements by general-purpose programmes has not been employed for modelling of flexible pavement frequently. Formerly, a few researchers have studied the pavement responses using the general-purpose finite element programmes.

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Hu et al. (2015) used using ANSYS programme for the development of a linear elastic three-dimensional model of flexible pavement with fully bonding between layers to compute the deformation of each asphalt concrete layer regarding some variable like the asphalt concrete layer modulus, the asphaltic layer thickness, wheel loads, and tire inflation pressure. They found that the deformation in pavement significantly influenced by resilient modulus of asphaltic layers, while the variation in the pavement thickness and wheel loading had less influence on pavement displacement.

Ying and Yiling, (2015) made a three-dimensional model by employing ANSYS programme with linear elastic properties of pavement materials and briefly described the calculation of the stress distribution and displacement on the surface under the applied tire pressure.

Ševelová and Florian (2013) used the ANSYS software as an FE modelling tool to develop a two- dimensional model of low volume roads for to determination of stress and deformation in any point of the pavement structure and hence produce data required for the reliability analysis. The low volume roads usually built of compacted stones and granular coarse and fine aggregate layers with no need for asphalt concrete layer of high modulus of elasticity. Their model was of four layers; cover, base, subbase and subgrade. They compared two material characteristics for modelling of unbound non-homogenous materials used in low volume roads. The first one is linear elastic model according to Hook theory (H model), and the second model is nonlinear elastic-plastic Drucker- Prager (D-P model). They found that the Stresses predicted by the Drucker-Prager method are generally lower than the stresses obtained from H model particularly for small values of resilient modulus.

Shi and Guo (2008) performed a linear three-dimensional finite element method to present design parameters for highway tunnel flexible pavement with an application of dual wheel load on a rectangular contact area. The model included three layers, and the layers are surface asphalt concrete, base course and bedrock course. They investigated the horizontal tensile stress at the surface of the asphalt layer, the horizontal tensile stress at the bottom of the asphalt concrete layer and the vertical shear stress at the surface of the asphalt layer. They concluded that the increase in asphalt layer thickness had a slight effect on reducing the damage caused by the vertical shear stress and its optimum value was 14 cm, and base course with higher stiffness modulus should be agreed, and its thickness adjusted base on the bedrock modulus.

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Sinha et al. (2014) studied the influence of subbase material type on the flexible pavement life. Three kinds of commonly occurring materials were used; coarse sand, conventional subbase material (CSM) and stone dust with four types of industrial waste materials. They carried out a finite element analysis of the pavement structure using ANSYS software, and they found that using of industrial waste in the subbase layer decreased the life to 60 to 83 percent. They made suggestions for design possibilities to counterbalance the decline in the life of the pavement.