بررسي تاثير نسبت آب به مواد سيماني و مقدار دوده سيليسي بر رفتار شكست و مشخصات تردي بتن مقاومت بالا با روش اثر اندازه

اين تحقيق به مطالعه و تحليل رفتار شكست بتن مقاومت بالا با مقادير مختلف دوده سيليسي بهمراه تغيير در نسبت آب به مواد سيماني با روش اثر اندازه (SEM) مي پردازد. در اين تحقيق آزمايشگاهي، در مجموع 10 طرح اختلاط مورد آزمايش قرار گرفته است. براي بررسي اثرات نسبتهاي مختلف آب به مواد سيماني در محدوده مربوط به بتن مقاومت بالا و تاثير مقادير دوده سيليسي، نسبتهاي كم آب به مواد سيماني (w/c) 24/.، 3/.، 35/. و 4/. مورد برسي قرا گرفته است و علاوه بر آن در دو نسبت آب به مواد سيماني 24/. و35/.، يك طرح مربوط به بتن مقاومت بالابدون افزودني دوده سيليسي و مابقي براي درصد نسبت وزني دوده سيليسي نسبت به مواد سيماني با مقادير 5%، 10%و 15% بوده است. براي تعيين خصوصيات شكست بتنها در اين تحقيق، در مجموع تعداد 120 عدد تير مورد آزمايش قرار گرفته است. نتايج نشان داده است كه با كاهش نسبت آب به مواد سيماني از مقدار 35/.، مقدار انرژي شكست اوليه (Gf) كاهش مي يابد و از سوي ديگر پارامترهاي ديگر شكست شامل طول موثر منطقه صدمه ديده پيش رونده ترك (Cf) و جابجايي بحراني نوك ترك (CTODc) كاهش يافته است، و در طرف مقابل باعث افزايش عدد تردي (β) گرديده است. همچنين با افزايش مقدار دوده سيليسي در هريك از نسبتهاي آب به مواد سيماني 35/. و 24/.، انرژي شكست، طول منطقه صدمه ديده نوك ترك، چقرمگي شكست (KIC) و جابجايي بحراني نوك ترك كاهش مي يابد و در طرف مقابل عدد تردي افزايش يافته است .نتايج مورد بررسي نشان داده اند با استفاده از پارامترهاي شكست بدست آمده از روش اثر اندازه مي توان حداكثر بار وارده بر بتنهاي مقاومت بالا را بصورت صحيح پيش بيني نمود.

The use of high strength concrete (HSC) is increasing due to the expansion of the construction technology of these concretes. In structural engineering, concrete is known as a material with brittle behavior that the tensile strength of which is negligible compared to its compressive strength and show low resistance to crack propagation. Moreover, the concrete can be considered as a quasi-brittle material, which is due to the type of behavior related to the crack propagation and is also existed around the crack tip of fracture process zone (FPZ) that involves a set of microcracks. From the perspective of structural behavior, the size effect of the structure is one of the most important concepts provided by the fracture mechanics; therefore, it is important to provide an equation between concrete fracture properties such as fracture toughness (KIC) and fracture energy (Gf) and its correlation with the size effect mechanism. The fracture energy is one of the most important characteristics for analysis of fracture behavior in concrete, evidenced to be a concrete property, showing its strength to cracking and fracture toughness. Given that the fracture energy (Gf) is sufficient to calculate the fracture behavior evaluation for the brittle materials in range of linear fracture mechanics, for the quasi-brittle materials such as concrete, this is not a sufficient parameter due to the presence of microcracks in the fracture process zone, and the length of fracture process zone (Cf) is one of the important properties of fracture in the unlimited-size structures. For determining the fracture parameters of concrete, various methods have been proposed. One of the most important methods that is presented by Bazant is the size effect method (SEM). This research studies and analyzes the fracture behavior of high strength concrete (HSC) with various amounts of silica fume, along with a change in water to cementitious materials ratio (w/c) with SEM. In this experimental study, a total of 10 mixing designs have been tested. To investigate the effects of different w/c ratios in the range of HSC and the effect of silica fume, four w/c ratios of 0.24, 0.3, 0.35 and 0.4 Examined and in two w/c ratios of 0.24 and 0.35 a mix design for plain concrete without silica fume and three mix designs prepared with silica fume content of 5%, 10% and 15% by weight of cement. To determine the fracture characteristics of concrete, a total of 120 beams were tested. The results showed that by decreasing the w/c ratio from 0.35 in the range of HSC, the value of initial fracture energy (Gf), the effective length of fracture processing zone (Cf) and the critical crack tip opening displacement (CTODc) decreased and on the other hand the brittleness number (β) increased. Also, by increasing the amount of silica fume in the w/c ratios of 0.35 and 0.24, the fracture energy, the length of the fracture processing zone, the fracture toughness (KIC), the critical crack tip opening displacement reduced, and the brittleness number increased. Also, the results indicated that by using the fracture parameters obtained from the SEM, the maximum load on the high strength concrete specimen can be predicted correctly.