بررسي آزمايشگاهي رفتار بيرون كشيدگي الياف مايل از ماتريس پايه سيماني

Experimental investigation of pull-out behavior of inclined fiber from cementitious matrix

ماتريس‌هاي پايه سيماني داراي مقاومت كششي و شكل‌پذيري ناچيز مي‌باشند. افزودن الياف به اين ماتريس‌ها سبب بهبود مشخصات مكانيكي آنها مي‌شود. مقاومت كششي پس از ترك‌خوردگي مواد مركب سيماني مسلح شده با الياف فولادي به صورت مستقيم با تعداد الياف عبور كننده از عرض ترك و رفتار بيرون كشيده شدن هركدام از الياف مرتبط مي‌باشد. بنابراين شناخت دقيق رفتار بيرون كشيدگي الياف منفرد، به‌ منظور فهم رفتار كششي تك محوره و رفتار خمشي بتن‌هاي اليافي مسلح با الياف فولادي ضروري بنظر مي‌رسد. از آنجايي‌كه الياف فولادي قلابدار يكي از كاراترين نوع الياف مورد استفاده براي كاربردهاي سازه‌اي هستند، لذا بررسي و شناخت دقيق رفتار بيرون‌كشيدگي اين نوع از الياف از اهميت بسيار بالايي برخوردار مي‌باشد. در اين مقاله يك مطالعه تجربي بر روي رفتار بيرون‌كشيدگي الياف فولادي قلابدار در حالت‌هايي كه الياف عمود بر عرض ترك و يا به صورت مايل قرار مي‌گيرند انجام گرديده و با بررسي رفتار بيرون‌كشيدگي الياف فولادي قلابدار در زواياي تمايل 0، 15، 30، 45 و 60 و در طول مدفون هاي 10، 15، 20 و 25 ميلي‌متر تاثير زاويه تمايل الياف نسبت به امتداد بارگذاري و طول مدفون الياف، بر روي پاسخ بيرون‌كشيدگي شامل؛ بيشينه نيروي بيرون‌كشيدگي، لغزش مربوط به نقطه بيشينه نيرو، انرژي بيرون‌كشيدگي، ميزان موثر بودن الياف و همچنين كنده‌شدگي ماتريس در هر حالت مورد مطالعه قرار گرفته است.

This paper presents the pullout characteristics of inclined hooked steel fiber from cementitious matrix. The effect of fiber embedded length and angle of inclination are evaluated together with the interaction of these parameters. The experimental program involved single fiber pullout test of five inclination angle and four embedded length. The studied inclination angles were 0, 15, 30, 45 and 60 degrees. The embedded lengths were 10, 15, 20 and 25 mm. Compressive strength of matrix was 40 Mpa. The length and diameter of hooked steel fibers were 50 mm and 1mm, respectively and their tensile strength was 800 Mpa. At least five specimens were prepared and tested for each combination of inclination angle and embedded length. A special mold supplemented by a cross shaped device was designed to hold the fiber in desired angle and embedded length. Xray radiography was used to verify the inclination angle and embedded length of fiber. All the specimens were tested at 28day age. Pullout test performed under displacement control condition in order to record descending branch of pullout curves. A load cell and a displacement transducer were used to acquire pullout load and slip during pullout test. Pullout load versus slip were recorded and parameters such as maximum pullout force and its associated slip, pullout energy, fiber efficiency and matrix spalling were drawn for comparison purpose. Based on the experimental results, the pullout response of hooked steel fibers is predominately influenced by fiber embedded length and inclination angle. The results indicate that the peak pullout load is maximized at approximately 30 degrees, although at greater inclination angle, the peak pullout load decreases. The fracture load also decreases as fiber inclination angle increases. The additional shear stress imposed on inclined fibers provide mechanisms favoring slip between the crystals in the steel. This causes a reduction in both yield and ultimate strength of the finer, resulting in a reduced fracture load. The results indicate that providing the hook is fully mobilized, the peak pullout load is almost independent of embedded length of fiber. The results indicate that fracture of fiber is more presumable at greater inclination angle. Slip associated with peak pullout load increases as the inclination angle increases. This can be attributed to matrix spalling. Matrix spalling also causes the drop of pullout load in pullout curves. The load drop is directly related to the size of crushed matrix. Matrix starts to spall at 30 degrees inclination angle. The results indicate that increase in embedded length and inclination angle result in increase of pullout energy. An inclined fiber with respect to the loading direction absorbs a greater amount of energy at a given slip than an aligned one, with maximum pullout energy occurring around 30 degrees. Fiber efficiency increases as the embedded length of fiber increases. Maximum fiber efficiency occurs at 30 to 45 degrees and decreases at greater inclination angle. The effect of elastic deformation of fiber during pullout test was taken into account by calculation of elastic deformation and subtracting from slip, although, its effect was negligible.