بررسي آزمايشگاهي چقرمگي شكست بتن هاي آسفالتي تحت مودهاي تركيبي كششي- برشي

Evaluation of mixed mode fracture resistance of different asphalt concretes-An Experimental study

- يكي از مهم‌ترين خرابي‌هاي ايجاد شده براي جاده ها، پديده ترك‌خوردگي رويه‌هاي آسفالتي در شرايط دمايي پايين است. تغييرات مشخصات بتن آسفالتي (مانند نوع قير، جنس مصالح، نوع دانه‌بندي و درصد فضاي خالي) تاثير زيادي در رفتار ترك‌خوردگي آن‌ها دارد. از آنجا كه اصلي‌ترين خرابي روكش‌هاي آسفالتي (ايجاد ترك‌هاي انعكاسي) تحت بارگذاري مود تركيبي كششي– برشي ناشي از عبور وسايط نقليه است، در اين پژوهش به بررسي آزمايشگاهي تاثير مشخصات بتن آسفالتي بر مقاومت ترك‌خوردگي )چقرمگي شكست( مود تركيبي آن‌ها در شرايط دمايي پايين پرداخته مي‌شود. بدين منظور، از آزمايش قطعه نيمه ديسك با ترك لبه اي(SCB) و تحت بارگذاري خمش سه نقطه‌اي استفاده شد. دو جنس مصالح سنگي، دو نوع دانه‌بندي و دو نوع قير كه معمولا در روسازي راه‌هاي ايران استفاده مي‌شود، در ساخت نمونه هاي آزمايشگاهي به كار گرفته شد. از نمونه‌هاي استوانه‌اي به دست آمده از دستگاه تراكم ژيراتوري و تحت دوران‌هاي 35 ، 70 و 90 دور چرخش، براي تهيه نمونه‌هاي SCB استفاده شد و براي در نظر گرفتن تاثير فضاي خالي از تلورانس %5/0± براي بارگذاري با مود كششي غالب‌تر و %8/0± براي بارگذاري با مود برشي غالبتر استفاده گرديد. بررسي نمودار بار-جابه‌جايي نمونه‌هاي آزمايش شده در دماي 15- درجه سلسيوس، نمايانگر اين مطلب بود كه رفتار شكست آسفالت تحت اين شرايط خطي است. نتايج به دست آمده نشان مي‌دهد كه خصوصيات بتن آسفالتي تاثير قابل توجهي بر مقدار چقرمگي شكست اين بتن‌ها دارد اما با افزايش درجه مود برشي، تاثير درصد فضاي خالي و اندازه دانه‌ها در رفتار ترك خوردگي بتن آسفالتي كاهش مي‌‌‌يابد.

Asphalt cracking is one of the major distress and deterioration modes of pavements of roads especially in the cold regions. Different types of cracks such as top-down, reflecting and alligator cracks are usually observed in the asphalt pavements. These cracks may be initiated due to improper construction and implementation of paving process, daily or seasonal cyclic thermal loads and mechanical traffic loading induced by moving vehicles. Consequently, the investigation of crack growth behavior in the asphalt overlays is an important issue for design, construction and maintenance of roads and highways. Due to the applied thermal and mechanical stresses, the cracks initiated in the asphalt layers may often experience different deformations including mode I(tensile or opening), mode II (shear or sliding) and any combinations of opening-shearing deformations (i.e. mixed mode I/II). Fracture toughness is a key fundamental parameter for investigating the crack growth behavior of cracked bodies. From the other hand, since asphalt mixtures are composite materials composed of aggregate, binder and air void, their fracture behavior might be affected by the asphalt characteristics specifications. Hence in this research the influences of asphalt characteristic specifications (including the type and size of aggregates, bitumen type, air void content and the mixture composition) is investigated experimentally on the mixed mode fracture toughness of different asphalt mixtures. For conducting the fracture toughness experiments, semi circular bend (SCB) specimens containing a vertical edge crack and subjected to asymmetric three-point bend loading was used. In order to study the influence of mixture characteristic specifications on the value of fracture toughness, two aggregate types (i.e. limestone and siliceous), two aggregate sizes (with nominal maximum aggregate sizes 19 mm and 4.75 mm), two binders (60/70 and 85/100) and two air void percentages (i.e. 4% and 6%) were considered for manufacturing asphalt mixtures. The SCB specimens were then tested under two different loading mode mixities (i.e. Me = 0.8 (mixed mode condition with dominantly tensile deformation) and Me = 0.38 (mixed mode condition with dominantly shear deformation) at -15oC. The load-displacement curves for the whole samples were linear which revealed the linear and elastic behavior of asphalt mixtures at the tested low temperature. Thus, the corresponding values of fracture toughness (KI, KII and Keff) were determined using the obtained fracture loads and the available formula. The experimental result showed that the mixed mode cracking resistance of asphalt mixtures is significantly affected by their properties. Analysis of results indicated that the specimens containing 4% air void (i.e. with the more compacted mixtures) show greater resistant against cracking than the asphalt specimens containing 6% air void. Meanwhile by increasing the maximum aggregate size the fracture toughness is also increased. Mixtures made of limestone aggregates and softer binders had more fracture toughness. Analysis of results also showed that when the contribution of mode II deformation is increased, the influence of air void and aggregate size on the fracture toughness of tested asphalt mixtures becomes negligible.