بررسي مقاومت شكست مخلوط‌هاي آسفالتي اصلاح‌شده با استفاده از آزمايش نيم‌ديسك تحت بار خمشي

Evaluation of Fracture Resistance of Modified Asphalt Mixtures Using Semi-Circular Bending Test

ترك‌هاي ناشي از دماي پايين، اصلي‌ترين خرابي روسازي‌هاي آسفالتي در فصل زمستان است. يكي از روش‌هاي مناسب و متداول براي بهبود خصوصيات عملكردي روسازي‌هاي آسفالتي، به ويژه در شرايط سرويس‌دهي دماي بالا، به‌كار بردن اصلاح‌كننده‌ها و افزودني‌هاست. با اين حال در مناطق با آب و هواي سرد، كه معمولاً روسازي‌ جاده‌ها دماهاي زير صفر را تجربه مي‌كنند، مكانيسم شكست لايه‌هاي آسفالتي عمدتاً به دليل شكست ترد كشسان و رشد ترك‌هاي آغازشده در درون روسازي اتفاق مي‌افتد. از آنجايي كه هدف اصلي اين مطالعه بررسي اثر افزودني‌هاي مختلف شامل: اسيد پلي‌فسفريك (PPA)، استايرن- بوتادين- استايرن (SBS)، آنتي ‌استريپينگ، پودر لاستيك و ساسوبيت در دماي پايين در مقاومت شكست مخلوط‌هاي آسفالتي تحت مود I بوده است، يك سري از نمونه‌هاي آسفالتي با درصدهاي مختلفي از افزودني‌هاي ذكرشده ساخته شدند و سپس نمونه‌هاي آزمايشگاهي در دماي معين زير صفر درجه‌ي سانتي‌گراد (C?15-)، تحت بارگذاري مونوتنيك متقارن با استفاده از فيكسچر خمش سه‌نقطه‌‌يي قرار گرفتند .

Low temperature cracking is the main distress in asphalt pavements in winter. Use of modifiers or additives in the asphalt mixture is a suitable and common method for improving its mechanical properties, especially under high temperature service conditions. However, for cold climates, where road pavement usually experience subzero temperatures, the overall failure mechanism of asphalt layers may occur mainly due to elastic brittle fracture and the growth of initiated cracks inside the pavements. Hot mix asphalt (HMA) concrete is perhaps the most complicated material in flexible pavement systems since its properties depend upon temperature and loading conditions. Fracture toughness is the most important parameter for characterizing crack growth and the failure of cracked materials and structures such as asphalt pavements. Cracked semi-circular specimens subjected to three-point bending have been recognized as appropriate test specimens for conducting fracture tests in brittle materials. The manufacturing and pre-cracking of the specimen are simple. A non complicated loading fixture is also required for a fracture test. Hence, the main aim of this research is to study the effect of different additives, including Poly phosphoric acid (PPA), Styrene butadiene styrene (SBS), Anti striping agent (ANTI), Crumb rubber (CR) and FT-paraffin wax (sasobit), on the low temperature mode I fracture resistance of asphalt mixtures. A series of asphalt samples with different percentages of the mentioned additives were manufactured in the shape of semi circular specimens containing vertical edge cracks. The test samples were then loaded monotonically using a symmetric three-point bend fixture at a constant subzero temperature of -15oC. The value of mode I fracture toughness (KIC) was determined by recording the critical fracture loads of tested specimens. It is shown that all the investigated additives can increase the low temperature fracture toughness of the asphalt mixture, and maximum increase in the value of KIC occurs when the sasobit and CR additives are used.