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J. Shanghai Jiaotong Univ. (Sci.), 2008, 13(2): 206210DOI: 10.1007/s12204-008-0206-5Mechanical Properties of Asphalt PavementStructure in Highway TunnelSHI Chun-xiang1(石春香),GUO Zhong-yin2(郭忠印)(1. Shanghai Institute of Technology, Shanghai 200233, China; 2. Tongji University, Shanghai 200092, China)Abstract: A linear full 3D finite element method (FEM) was performed in order to present the key designparameters of highway tunnel asphalt pavement under double-wheel load on rectangular loaded area consideringhorizontal contact stress induced by the acceleration/deceleration of vehicles. The key design parameters arethe maximum horizontal tensile stresses at the surface of the asphalt layer, the maximum horizontal tensilestresses at the bottom of the asphalt layer and the maximum vertical shear stresses at the surface of the as-phalt layer were calculated. The influencing factors such as double-wheel weight; asphalt layer thickness; basecourse stiffness modulus and thickness; and the contact conditions among the structure layers on these keydesign parameters were also examined separately to propose construction procedures of highway tunnel asphaltpavement.Key words: tunnel asphalt pavement structure; three-dimensional finite element method; horizontal force;horizontal tensile stress; vertical shear stressCLC number: U 452.2Document code: AIntroductionAlmost all mechanistic flexible pavement design pro-cedures determine the fatigue life of pavements by con-sidering the tensile stress or strain at the bottom ofthe asphalt layers.They implicitly assume that fa-tigue cracks originate at the bottom of the asphalt lay-ers and propagate upwards towards the surface of thepavement. Assuming a uniform normal contact pres-sure distributed over a circular contact area betweenthe tyre and the pavement surface, layered elastic the-ory predicts that the maximum horizontal tensile stress(strain) occurs at the bottom of bound layers directlyunder the load, and the maximum horizontal compres-sive stress (strain) occurs at the surface of the pavementdirectly under the load1.However, observations from cracked roads in theUK2, the US35, Japan6and in Northwest area ofChina7have shown that the cracking originated fromthe surface of the pavement rather than at the base,particularly for thick flexible constructions.More recently, researchers have hypothesized thatthe surface cracking phenomenon may be related tothe highly non-uniform three dimensional contact stressdistribution measured between the tyre and the pave-ment, inducing large horizontal tensile stresses (strains)in the top section of the pavement structure8.Mea-surements from free-rolling car and truck tyres haveReceived date: 2007-05-28Foundation item: WesternTrafficTechnologyFunds(No. 2002-318-000-23)E-mail: shown that, in addition to the normal contact pressure,there can be large transverse and longitudinal shearstresses acting in the contact area.Traditional layered elastic models of a pavementstructure typically assume that the load is applied asa uniform vertical contact pressure distributed over acircular contact area. Under these conditions the max-imum horizontal tensile stress and strains are usuallypredicted at the bottom of the asphalt layers, due tothe bending stresses induced by the loading. However,on the tunnel asphalt pavements, it should be notedthat horizontal tensile stresses at the surface away fromthe loaded area (due to the negative curvature of thesurface).The elastic stiffness modulus of highway tunnelbedrocks is high; the deterioration of well-constructedpavement is not structural. Furthermore, when vehi-cles enter the tunnels, drivers will decelerate vehiclesbecause of weak light and narrow space, and acceler-ate vehicles when leaving the tunnels. There exist highhorizontal stresses (in the direction of travel) betweenthe tyre and the pavement surface.The objective of this paper is to use a linear elas-tic full 3D finite element (FE) program to present thekey design parameters of highway tunnel asphalt pave-ment under double-wheel loads considering horizontalcontact stress induced by the acceleration/decelerationof vehicles. The following key design parameters wereto be calculated: the maximum horizontal tensilestresses maxat the surface of the asphalt layer; themaxat the bottom of the asphalt layer; the maxi-mum vertical shear stresses maxat the surface of theJ. Shanghai Jiaotong Univ. (Sci.), 2008, 13(2): 206210207asphalt layer.The influencing factors such as double-wheel weight;asphalt layer thickness; base course stiffness modulusand thickness; and the contact situation among thestructure layers on these key design parameters werealso examined separately in order to propose signifi-cant construction problems should be considered in thedesign procedure of the tunnel asphalt pavement.1Traffic Loading FormTraditional layered elastic models of a pavementstructure typically assume that the load is applied asa uniform vertical contact pressure distributed over acircular contact area. The FEM in this paper, on theother hand, the loading conditions were simplified usinga uniform vertical considering horizontal contact pres-sures distributed over a rectangular contact area whichis close to the true contact area between the tyre andpavement. The numeric values of tyre print and tyrepressure are given in the Table 1. Spacing interval be-tween tyres is 34 cm. Tyre contact stresses have beenrelated to double-wheel weight and tyre internal pres-sure using an empirical formula of form10:p = 0.0042P?+ 0.29pi+ 0.145,(1)where, p is the tyre contact stress; P?is the double-wheel weight; piis the tyre internal pressure.Table 1Tyre print and tyre pressure12P?/kNpi/MPap/MPawidth/cmlength/cm200.60.402211.3400.70.522217.6500.750.5732219.8600.80.632221.7800.90.742422.51001.00.8552424.4In general, the horizontal contact stress is defined as30% of the vertical contact stress, so defined in thispaper for calculating.2Description of the Full 3D FEM andPavement Structure VariantsTunnel asphalt pavements generally include asphaltlayers, base course and bedrock. The full 3D FEM de-veloped for the pavement structure is shown in Fig. 1.The width along x-direction and y-direction is 3.0 m.The depth along z-direction is varied at the pavementstructure thickness. 8-node element is adopted in themodel.The schematic representation with boundaryconditions of the pavement analysis problem is shownin Fig. 2.z-direction is fixed on the bottom of themodel; x-direction is fixed on the left and right planes;y-direction is fixed on the front and back planes; thesurface is the free plane.Contact conditions amongpavement layers are also considered particular layer ischaracterized by thickness (hi), elastic stiffness modu-lus (Ei) and Poissons ration (vi). Following variants oftunnel pavement structures were given in the table 2.Fig. 1The full three-dimensional finite element modelzyxLoaded areaAsphalt layerBase courseBedrockFig. 2Schematic representation of the pavementanalysis problemTable 2Pavement structure variantsLayerhi/cmEi/GPaviAsphalt layer8,10,12,14,16,201.50.35Base course0,10,15,25,302.0,0.50.2Bedrock2505.0,0.50.23Analysis of Calculated PavementResponses3.1Effect of Double-wheel Weight onPavement ResponsesPavement responses at various double-wheel weightswere to be considered. Contact conditions among lay-ers are completely sticking. Following variants of thetunnel pavement structure were given in the table 3.Table 3Pavement structure variants-ALayerhi/cmEi/GPaviAsphalt layer141.50.35Base course152.00.2Bedrock2505.00.2Figures 35 present results of the asphalt layer at var-ious double-wheel weights w considering and not con-sidering the horizontal stresses.208J. Shanghai Jiaotong Univ. (Sci.), 2008, 13(2): 20620.40.52040506080100w/kNmax/MPaNot considering horizontal stressConsidering horizontal stressFig. 3maxat the pavement surface2040506080100w/kNNot considering horizontal stressConsidering horizontal stressmax/MPaFig. 4maxat the pavement surface2040506080100Not considering horizontal stressConsidering horizontal stressw/kNmax/MPaFig. 5maxat the bottom of the asphalt layerAs can be seen, the stresses of the asphalt layer in-crease with increasing double-wheel weight level. Theeffect of horizontal forces on the maxat the bottomof the asphalt layer is less than that on the maxandvertical shear stresses at the surface of the asphaltlayer.These figures indicate that horizontal stressesbetween the tyre and the pavement surface acceleratethe deterioration of the pavement surface.3.2Effect of Asphalt Layer Thickness onPavement ResponsesPavement responses at various asphalt layer thick-nesses h1set on the different stiffness modulus basecourse were to be considered.Contact conditionsamong layers are completely sticking. Following vari-ants of the tunnel pavement structure were given in thetable 4.Table 4Pavement structure variants-BLayerhi/cmEi/GPaviw/kNAsphalt layer8,10,12,14,16,201.50.35Base course152.0,0.50.250Bedrock2505.00.2Figure 6 shows the results of the asphalt layer at var-ious asphalt layer thickness set on the semi-rigid basecourse with E = 2 GPa.50.4581012141620h1/cmmax, max/MPamax (surface)max (surface)max (bottom)Fig. 6Effect of asphalt layer thickness on pavementstresses on the semi-rigid base courseAs can be seen:(1) When asphalt layer thickness is set 810 cm, themaxand maxat the surface of the asphalt layer dueto the negative curvature at the wheel interspaces sur-face are higher in magnitude compared to those at thebottom of the asphalt layer.(2) The maxat the surface of the asphalt layer de-crease with increasing asphalt layer thickness level, butthe rate of decay decreases when asphalt layer thicknessis more than 14 cm.(3) The maxat the surface of the asphalt layerincrease with increasing asphalt layer thickness level,which indicates that increasing asphalt layer thicknesslevel is not favorable for preventing the damage inducedby the vertical shear stress.(4) The recommended optimal asphalt layer thicknessis 14cm on the semi-rigid base course.Figure 7 presents results of the asphalt layer at var-ious asphalt layer thickness on the flexible base coursewith E = 500 MPa.0.81.01.281012141620max (surface)max (bottom)max (surface)h1/cmmax, max/MPaFig. 7Effect of asphalt layer thickness on pavementstresses on the flexible base courseAs can be seen:(1) The maximum stresses of the asphalt layer de-crease with increasing asphalt layer thickness level.(2) The magnitude of stresses is more than that onthe semi-rigid base course, so semi-rigid base courseswith higher stiffness modulus should be set in the de-sign procedures of tunnels asphalt pavement structure.J. Shanghai Jiaotong Univ. (Sci.), 2008, 13(2): 2062102093.3Effect of Base Course Thickness onPavement ResponsesPavement responses at various base course thick-nesses h2with the different bedrock stiffness moduluswere to be considered. CCAL are completely sticking.Following variants of the tunnel pavement structurewere given in the table 5.Table 5Pavement structure variants-CLayerhi/cmEi/GPaviw/kNAsphalt layer141.50.35Base course0,10,15,25,302.00.250Bedrock2505.0,0.50.2Figure 8 shows results of the asphalt layer at variousbase course thicknesses on the rigid bedrock with E = 5GPa. As can be seen, the change in parameters due tothe effects of base course thickness is minute.If thetunnel bedrock stiffness modulus is high, base courseshould not be set or the base course thickness needsnot to be set too thick when assuring the constructionsmoothness.0.4010152530h2/cmmax (surface)max (bottom)max (surface)max, max/MPaFig. 8Effect of base course thickness on pavement stresseson the rigid bedrockFigure 9 presents results of the asphalt layer at vari-ous base course thicknesses on the flexible bedrock withE = 500 MPa. As can be seen, the change in the maxat the bottom of the asphalt layer due to the effectsof base course thickness is significant, so base courseshould be set and the recommended optimal base coursethickness is 15 cm on the flexible bedrock.00.81.0010152530h2/cmmax (surface)max (bottom)max (surface)max, max/MPaFig. 9Effect of base course thickness on pavement stresseson the flexible bedrock3.4Effect of Contact Conditions on PavementResponsesInfluence of different CCAL on pavement responseswere to be considered.No. 1 stands for completelysticking among layers; No. 2 for completely sticking be-tween the asphalt layer and the base course while com-pletely sliding between the base course and the bedrock;No. 3 for completely sliding between the asphalt layerand the base course while completely sticking betweenthe base course and the bedrock; No. 4 for completelysliding among layers.Following variants of the tunnel pavement structurewere given in the table 6.Table 6Pavement structure variants-DLayerhi/cmEi/GPaviw/kNAsphalt layer141.50.35Base course152.00.250Bedrock2505.00.2Figure 10 shows results of the asphalt layer at dif-ferent CCAL. As can be seen, the change in the maxat the bottom of the asphalt layer due to the effects ofCCAL is significant. The most advantageous situationis completely sticking among layers; the most disadvan-tageous situation is completely sliding among layers, sostrengthening the bound among pavement layers espe-cially between the asphalt layer and the base course caneffectively control cracking at the bottom of the asphaltlayer. 0.70.81234CCALmax (surface)max (bottom)max (surface)max, max/MPaFig. 10Effect of CCAL on the pavement stresses4ConclusionThe mechanic analysis has been performed in orderto present the key design parameters and propose con-struction procedures of highway tunnel asphalt pave-ment as follows:(1) Inclusion of the horizontal stress between the tyreand the pavement surface has been shown to producehigher local values of horizontal tensile stress and ver-tical shear stress inducing the surface damaging on thepavement surface, at the same time, the magnitude ofthe horizontal tensile stress at the bottom of the pave-ment is high. So the key design parameters are the max210J. Shanghai Jiaotong Univ. (Sci.), 2008, 13(2): 206210at the surface of the asphalt layer, the maxat the bot-tom of the asphalt layer and the maxat the surface ofthe asphalt layer.(2) Semi-rigid base courses with higher stiffness mod-ulus should be set, and its thickness should be adjustedaccording to the bedrock stiffness.(3) Increasing asphalt layer thickness level on thesemi-rigid base courses is not favorable for preventingthe damage induced by the vertical shear stress. Theoptimal asphalt layer thickness is recommended 14 cmon the semi-rigid base course.(4) Strengthening bounding among pavement layerse