بررسي تاثير نوع چرخ و وزن محور بر ترك خوردگي روسازي هاي آسفالتي به روش المان محدود

Investigating the Effects of Tire Type and Axle Load on Cracking of Asphaltic Pavements Using Finite Element Method

تركهاي بالا به پايين از مهمترين خرابي هاي روسازي هاي آسفالتي محسوب ميشوند كه سبب كاهش قابل ملاحظه كيفيت روسازي ها خواهند شد. برهم كنش بين چرخ و روسازي نقشي تعيين كننده در پيدايش اين نوع از خرابيها ايفا ميكند. در اين مقاله سعي شده تا با تحليل ويسكوالاستيك و به روش المان محدود تاثير تغييرات وزن محور و همچنين تغييرات نوع چرخ بر تركهاي بالا به پايين بررسي شود. براي اين منظور تاثير سه وزن محور 5، 8.2 و 15 تن و دو تركيب چرخ مختلف (چرخ هاي زوج و عريض) بر ترك هاي بالا به پايين در روسازي مسلح شده با ژئوگريد و غيرمسلح مقايسه شد. نتايج حاصل از اين تحقيق نشان مي دهد كه در وزن محور 5 و 8.2 تن ترك هاي بالا به پايين ابتدا در لبه هاي داخلي مسير حركت چرخ ظاهر مي شوند در حالي كه، در وزن محور 15 تن اين ترك ها در فضاي بين دو چرخ زودتر از ساير نواحي ايجاد خواهند شد. همچنين مشخص شد كه استفاده از چرخ عريض به جاي چرخ هاي زوج بر ترك هاي پايين به بالا بيش از ترك هاي بالا به پايين تاثيرگذار خواهد بود. براساس نتايج به دست آمده مي توان گفت، استفاده از ژئوگريد در زير لايه آسفالتي عمدتا در كاهش ترك هاي پايين به بالا موثر خواهد بود و تاثير چنداني در كاهش ترك هاي بالا به پايين نخواهد داشت. علاوه بر اين، مقايسه نسبت خرابي به دست آمده نشان مي دهد كه هم در روسازي مسلح و هم در روسازي غيرمسلح خسارت ترك هاي بالا به پايين ناشي از چرخ عريض بيش از چرخ هاي زوج خواهد بود.

Cracking is one of the major modes of failure in asphaltic pavements. Structural cracks occur in two forms of top-down and bottom-up cracking. Bottom-up cracking occurs due to the fatigue of asphaltic materials under repetition of tensile strain at the bottom of asphaltic layer. Top-down cracking (TDC) is among the major forms of asphaltic pavement distresses that significantly affects the serviceability and development of structural failure. Interaction of tire and pavement plays a key role in the initiation of TDC. This study utilizes viscoelastic analysis -using finite element modeling- to evaluate the influence of axle loads and tire configuration on the top-down and bottom-up cracking (BUC) in typical unreinforced and reinforced asphaltic pavement structures. Reinforcing by glass-grid geogrid is selected for the reinforced structure. The highest vertical tensile strain at the surface and the highest horizontal tensile stress at the bottom of asphaltic layer are related to the TDC and BUC, respectively. Viscoelastic behavior is assumed for the asphaltic layer and linear elastic behavior is assumed for the base, sub-base and sub-grade. Prony series is used for characterizing the viscoelastic behavior of asphaltic layer. Using the tire pressure of 600kPa, effects of three axle load levels of 5, 8.2 and 15 ton and two tire configurations (conventional dual tire assembly and super single tire) on TDC and BUC have been investigated. Results show that the highest tensile strain at the surface occurs at the edge of super single tire and dual tires, with a higher values for the single tire. However, the location of the highest tensile strain shifts to the central region of dual tire with increasing axle load level. The results also show that under axle load of 5 and 8.2 ton, top-down cracking initially occurs at the inner edges of the tires, while under axle load of 15 ton its occurrence between the tires is more probable than in the other zones. For the pavement without reinforcement, the highest tensile strain at the surface is higher than that at the bottom under dual tires; however, under super single tire, the critical tensile strain at the surface is lower than that at the bottom of asphaltic layer. The results show that geogrid reinforcement is more effective in reducing the critical tensile strain at the bottom of asphaltic layer than that at the surface. This indicates that the reinforcement of pavement using geogrid at the bottom of asphalt layer is more effective on the bottom up cracking than on the top down cracking. In addition, geogrid reinforcement is more effective in reducing the critical strains under single tire than under dual tires. The rate of increase in the critical tensile strain at the surface with increasing axle load is more than that of the critical tensile strain at the bottom of asphaltic layer. Among bottom-up cracking (BUC) and TDC, BUC is more sensitive to the variations of tire type. By comparison, the super single tire created more TDC damage ratio than the dual tires assembly, in both reinforced and unreinforced pavements. However, the damage ratio due to the super single tire in unreinforced pavement is more than in reinforced pavement.