Finite Element Analysis of Pavement Design Using ANSYS Finite Element Code

Melaku, S.1, Hongsheng Qiu2 1 School of transportation, Wuhan University of Technology, 430063, Wuhan Hubei China 2 School of transportation, Wuhan University of Technology, 430063, Wuhan Hubei China

12690035285@qq.com, 2qiuhhss@163.com

Abstract: Dimensional finite-element programs have been employed in the past two decades for analyzing road pavement response. In recent years three dimensional finite-element (3DFE) analysis emerged as a powerful tool which is capable of capturing pavement response. Then the study of the effect vehicle load response on road pavement using ANSYS software was presented. ANSYS is a finite element method based on software. Analysis for deformation stresses-strain state have been done using both linear and non-linear material properties between top and bottom of pavement structures. Rectangular prism has been used as the geometry of the road pavement in different layer structures including surfaces, subsurface, capping and subgrade. The element type is solid 45 with six degrees of freedom. 7M pa concentrated load was applied as vehicle load in road pavement. The model was used to perform parametric studies involving effect of vehicle load on road pavement on different layer of structures. High compressive stress was observed on road pavement due to the vehicle load which is equivalent to 0.4 Mpa. The vertical deformation of the road pavement value due to the vehicle load obtained by ANSYS is 0.6 mm which is very high and deteriorates the road pavement Keywords: ANSYS; Stress, Strain; Finite element; Deformation

1. Introduction

A highway pavement is composed of a system of overlaid strata of chose processed materials that is positioned on the in-situ soil, termed the subgrade Its basic requirement is the provision of a uniform skid-resistant running surface with adequate life and requiring minimum maintenance. The chief structure purpose of the pavement is the support of vehicle wheel loads applied to carriageway and the distribution of them to the subgrade immediately underneath. If the road is in cut, the subgrade will consist of the in-situ soil. If it constructed on fill, the top layers of the embankment structure are collective termed the subgrade. The pavement designer must develop the most economical combination of layers that will guarantee adequate dispersion of the incident wheel stresses so that each layer in the pavement does not become overstressed during the design life of the highway. The major variables in the design of a highway pavement are: The thickness of each layer in the pavement the material contained within each layer of the pavement the type of vehicles in the traffic stream the volume of traffic predicted to use the highway over its design, the strength of the underlying subgrade soil. The major structure layers of the road

pavement described as follows. 1) Foundation

The foundation consists of the native subgrade soil and the layer of graded stone (Sub base and possibly capping) immediately overlaying it. The function of the sub base and capping is to provide a platform on which to place the road base material as well as to insulate the subgrade below it against the effects of inclement weather. These layers may form the temporary road surface used during the construction phase of the highway. 2) Road base

The road base is the main structural layer whose main function is to with stand the applied wheel stresses and strains incident on it and distribute them in such a manner that the materials beneath it do not become overloaded. 3) Surfacing

The surfacing combines good riding quality with adequate skidding resistance, while also minimizing the probability of water infiltrating the pavement with consequent surface cracks. Texture and durability are vital requirements of a good pavement surface as are surface regularity and flexibility. For flexible pavements, the surfacing is normally applied in two layers – base-course and wearing course – with the base course an extension of the road base layer but providing a regulating course on which the final layer is applied. In the case of rigid pavements, the structural function of both the road base and surfacing layers are integrated within the concrete slab. In broad terms, the two main pavement types can be described briefly as:

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a) Flexible pavements is the surfacing and road base materials, bound with bitumen binder, overlay granular unbound or cement-bound material b) Rigid pavements Pavement quality concrete, used for the combined surfacing and road base, overlays granular cement-bound material. The concrete may be reinforced with steel.

Stresses are developed in rigid pavements as a result of several factors, including the action of traffic wheel loads, the expansion and contraction of the concrete due to temperature changes, yielding of the sub base or subgrade supporting the concrete pavement, and volumetric changes. For example, traffic wheel loads will induce flexural stresses that are dependent on the location of the vehicle wheels relative to the edge of the pavement, whereas expansion and contraction may induce tensile and compressive stresses, which are dependent on the range of temperature changes in the concrete pavement. These different factors that can induce stress in concrete pavement have made the theoretical determination of stresses rather complex, requiring the following simplifying assumptions.

A study at University of Minnesota [10] was reported. The goal is to develop an analysis method combined with a non-destructive testing procedure for the evaluation of load transfer for joints in concrete pavements. The basic analysis involved a frequency response analysis by dynamically loading the joints. A three-dimensional finite element method was used to analyze various joint conditions for load transfer ranging from full to partial load

transfer. Stoner reported on research plans to develop a3Dfiniteelement program to study concrete pavements and to

simulate truck actions. The objectives were to develop a truck simulation model, to develop a model for doweled concrete pavement, and to implement these two models in an interactive fashion. The analysis was intended to obtain performance relationships based on damages predicted by the program. Actual damages recorded on Interstate 80 were used to verify the predictions [9].

Barksdale reported a study of compressive stress pulse at different depths in a flexible pavement. Loadings for variable vehicle speed were considered. A series of pseudo-dynamic linear and nonlinear elastic analyses were conducted. It was concluded that linear elastic finite element was adequate for the analysis. Dynamic effects including damping and inertial forces were neglected in the study and hence, a correction factor was introduced in order to match the results of AASHTO road tests [12].

Paterson reported on the finite element analysis of an asphalt overlay of a cracked airport concrete pavement in combination with a thin interlayer of elastomeric asphalt. Predictions were obtained in terms of an equivalent thickness of asphalt overlay which yields the same performance of the inter layer system [14].

Measured pavement deflections in combination with a three-dimensional finite element analysis were used to evaluate overlay requirement and pavement performance in a study reported by Bala and Kennedy 1986[11]. The response predictions of the finite element analysis were calibrated against surface deflections measured by a deflector graph. The calibration process included adjustments determined through a parametric study. The properties assumed for the materials in the analysis were compared with laboratory determined dynamic test results for in situ samples.

Zaghloul and White 1993 a reported results of a three-dimensional dynamic finite element analysis of flexible pavements. The analysis simulated actual truck loads moving at different speeds. Linear and non-linear material properties were used to model different paving materials and subgrades. An extended Drucker-Prager model was used to model granular materials, and an extended Cam-Clay model was used for the clayey soils. Asphalt mixtures were modeled as viscoelastic materials. Including these material models lead to the capability to obtain accurate elastic and plastic pavement responses. With this capability, they are able to predict or to interpret pavement performances under a variety of loading conditions and for different material characteristics [13].

The 3D finite element analysis was verified by comparing the predictions with a multi-layer elastic system, assuming linear elastic properties and static loads. A linear correlation was found between the results obtained by the finite element predictions and the multi-layer elastic analysis. To verify the dynamic, nonlinear finite element analysis, the results were compared with actual measurements of pavement deflection.Agreementat95%confidence level was obtained between the deflection predictions and the measurements.

A sensitivity study was performed by using the 3D finite elements for the effect of cross section and load parameters on pavement responses. It was found that the speed of moving vehicle load has a significant effect on elastic and plastic pavement responses. The confinement of shoulders has the effect of reducing pavement deflections. A crack along the pavement/shoulder joint results in an increase in pavement deflection. Temperature affects the asphalt layer and hence the overall pavement responses. The loading time and the rate of loading were found to have significant effect also. When a subgrade is subjected to a high stress level, higher than its yield stress, rutting increases significantly.

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The primary objective is to develop a three-dimensional finite element program for the analysis of general pavement problems. The program considers conventional static and dynamic loading conditions including harmonic excitations, pulse loadings, ramp loadings, and multiple step loadings. Provisions are made for the convenience of handling non-conventional loadings such as falling weights and general time-dependent load histories generated from non-destructive testing used for pavement structural evaluation. One particular important aspect of the programs to develop a general material library. Common soil and asphalt material models in the form of linear and nonlinear elastic materials, elastic-plastic materials with hardening, and viscoelastic materials are included in the material library.

2. Materials and Methods

2.1. Finite element method

The finite element method is a very world leading structural tool, but several factors should be kept in mind when performing analysis. First, the method is an approximate analytical procedure, whose accuracy will usually depend on the level of discretization of the mesh. Second; the accuracy of the results will depend on whether or not the major influences of the problem behavior are included in the analytical idealization. Finally, considerable skill and knowledge is still required to interpret the finite element simulations properly and make significant and correct.

APDL stands for ANSYS Parametric Design Language, a scripting language that you can use to automate common tasks or even build your model in terms of parameters (variables). APDL also encompasses a wide range of other features such as repeating a command, macros, if-then-else branching, do-loops, and scalar, vector and matrix operations, particularly worth mentioning of which is macro. You can record a frequently used sequence of ANSYS commands in a macro file (these are sometimes called command files). Creating a macro enables to, in effect, create your own custom ANSYS command. In addition to executing a series of ANSYS commands, a macro is called GUI (graphical user interface) functions.

The modelling of the structure involved the creation of the geometrical model; the assignment of the materials, properties, mesh, contacts, the appropriate boundaries and the loadings is done using ANSYS. The finite element model is presented in Figure-1 and was meshed with 4560 of solid elements and 5440 nodes in ANSYS Pre-processor.

Fig. 1 Model of road pavement

2.2 Material properties

Material properties used for performing ANSYS analysis of the road pavement and each layers model were obtained from the literature. PALSTIC- KINEMATIC (material type16) was used to represent the surface and subsurface of superstructure of the road pavement i.e. Reinforced Cement Concrete material model. Finally modelling and representing the foundation soil subgrade and formation soil was modelled using*VISCOELASTIC MATERIALS (material type 24). The vehicle load is applied as concentrated load 7Mpa for each of two vehicles.

3. Finite element analysis result and discussion

3.1. Pavement deformation

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The total deformation of road pavement was calculated by ANSYS along successive structures including surface, subsurface, capping and subgrade respectively. The maximum deformation of the vehicle load is equivalent to 6 mm. From the calculated result the analysis of deformation is very high and leads to deterioration of the road. The modelling result is depicted in the following Fig.2

Fig. 2 Road pavement deformation

3.2. Road pavement stress and strain state

Contour stress and plastic strain were also presented as the result of the analysis of finite element simulation. The study gives information about the behavior of the road pavement stress- strain state. The maximum stress from model analysis is 0.4MPa and similarly the maximum strain is 2mm. These result indicate that the impact of the vehicle load under the road pavement structure. Therefore the result is a good input for maintenance plan and materials selection. Figure 3a and 3b is depicted the stress and strain state.

3.3 Road pavement shear stress

The finite element analysis of ANSYS Element code could analyze the shear stress of the road pavement under the impact of the vehicle load. The shear stress has significant influence in road pavement deterioration and failures of the subgrade to carry the stress of superstructures. The study analysis result showed the shear stress that about 1.2 MPa

Fig.3a. Stress state

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Fig. 3b. Strain state

Figure 4. Shear stress.

4. Conclusion and Recommendation

All of the effort made in this research is to develop the applicable procedures for evaluating pavement layer conditions from the geometry and material properties of road pavement. The characteristics of the road pavement under vehicle load are acquired through numerical simulation when vehicles pass through the pavement under axial load with different layers. This research can provide reference for the improvement of road pavement and materials design. The analysis result showed that the vertical deformation of the road pavement and stress- strain state & the shear stress to the vehicle load response of the structures. The result can be used as pavement vehicle road design response with finite element model using ANSYS finite element code. Therefore the method can be considered as one of the methods to calculate the vehicle load response of road pavement

References

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[2] P. Selvi. Fatigue and rutting strain analysis on lime stabilized subgrades to develop a pavement design chart. Journal of Transportation Geotechnics, 2015:PP.86-98 [3] M.A.P. Taylor, M. L. Philp. Investigating the impact of maintenance regimes on the design life of Road Pavements in a Changing Climate and The Implications. Journal of Transport policy, 2015:PP.117-135. [4] A. Adel. A.Azzawi. Finite element Analysis of Flexible Pavements Strengthened with Geogrid. ARPN Journal of Engineering and Applied Sciences, 2012 [5] H.Behbahani, S. A.Sahaf. Designing a Mathematical Model for Predicting the Mechanical Characteristics of Asphalt Pavements Using Dynamic Loading [6] M. I. Khan, M. A. Qadeer, A. B. Harwalkar. Mechanistic Analysis of Rigid Pavement for Temperature Stresses Using Ansys. IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), 2014:PP.90-107. [7]J.A.Boateng ,J.Maina.Permanent Deformation Testing for a new South African Mechanistic Pavement Design Method.Journal of Construction and Building Materials,2012:PP.541-546. [8]. P. Mackiewicz. Thermal Stress analysis of Jointed Plane in Concrete pavements. Journal of Applied Thermal Engineering, 2013:PP.169-1176. [9] Stoner, T.W., Bhatti, M.A., Kim, S.S., Kou, J.K., Milinas-Vega,I. and Amhof, B. Dynamic Simulation Methods for Evaluating Motor Vehicle and Roadway Design and Resolving Policy Issues, Iowa University Public Policy Center, Iowa City, Iowa, January 1990. [10] Koubaa, A., Krauthammer, T. Numerical Assessment of Three- Dimensional Rigid Pavement Joints Under Impact Loads. MN/RD-91/03, Minnesota Department of Transportation, Maplewood, Minnesota, August 1990. [11] Bala, K.V. and Kennedy, C.K., the Structural Evaluation of Flexible Highway Pavements Using the Deflectograph. Proceedings, 2nd International Conference on the Bearing Capacity of Roads and Airfields, Plymouth, England, September 16-18, 1986. [12] Barksdale, R.D. Compressive Stress Pluse Tunes in Flexible Pavement for Use in Dynamic Testing. Highway Research Record No.345, Highway Research Board, Washington, D.C., and January 1971:PP. 32-44. [13]  Paterson, W.D.O. Design Study of Asphalt Membrane-Overlay for Concrete Runway Pavement. Transportation Research Record No. 930, Transportation Research Board, Washington, D.C., January 1983: PP. 1-11. [14] Zaghloul, S. and White, T.D. Use of a Three Dimensional-Dynamic Finite Element Program for Analysis of Flexible Pavements. Submitted to Transportation Research Board, National Academy of Sciences, Washington, D.C., and January 1993.

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