بررسي آزمايشگاهي تأثير مقدار و طول الياف فولادي بر مشخصات مكانيكي و ضربهاي بتن توانمند

استفاده از الياف براي تسليح بتن از جمله تلاشهايي است كه توسط پژوهشگران بهمنظور بهبود رفتار مقاومتي و افزايش عمر سرويسدهي سازههاي بتني صورت گرفته است. بررسيها نشان ميدهد با بكارگيري انواع الياف در بتنهاي مختلف ميتوان عملكرد مكانيكي آن در مقابل نيروهاي ضربهاي و ديناميكي را بهبود داد. در همين راستا سعي شده تا با انجام تحقيق حاضر، تأثير مقادير مختلف الياف فولادي دو سر قلاب موجدار بر مشخصههاي مكانيكي بتن توانمند با مقاومت بالا بررسيشود. علاوه بر آن تأثير طول الياف بر 1 الياف فولادي با دو طول متفاوت 30 و 50 / 1 و 5 ،0/ رفتار بتن سخت شده نيز توجه شده است. بدين منظور از سه درصد حجمي 5 ميليمتر در طرح اختلاط بتن استفاده شد. پس از طي دوره عملآوري، نمونهها در سنين 7 و 28 روزه تحت آزمايشهاي مقاومت فشاري، كششي و ضربهاي قرار گرفتند. نتايج بهدستآمده بيانگر آن است كه استفاده از الياف فولادي سبب افزايش قابلملاحظه مقاومت 1 درصد الياف فولادي به طول 50 ميليمتر به بتن، اين مشخصه افزايش 40 درصدي نسبت / كششي بتن شده، به طوريكه با افزودن 5 به بتن بدون الياف را نشان ميدهد. همچنين بكارگيري الياف فولادي با طول بلندتر تعداد ضربات تا اولين ترك و ضربات تا شكست نهايي در تست مقاومت ضربه را به ميزان قابلتوجهي افزايش داده بطوريكه تعداد ضربات مورد نياز براي ايجاد اولين ترك تا شكست نهايي در درصدهاي بالاي الياف، تا 80 درصد بيشتر ميشود. همچنين مشخص شد افزودن الياف الگوي شكست بتن را به شكل مطلوبي تغيير ميدهد.

Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibers. Fiber reinforced concrete are of different types and properties with many advantages. Fibers are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also lower the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete. Generally fibers do not increase the flexural strength of concrete. Some fibers reduce the strength of concrete. Various fibers have been used by researchers to reinforce concrete and extend the service life of structures, also researchers are using different fibers to improve behavior of different concretes, especially high-strength concrete, against impact and dynamic loads. This study investigate the effect of different contents and lengths of steel fiber on the mechanical properties of high-strength concrete. In this study, wavy hooked-ended fibers of two length (30, 50 mm) with three volume fraction 0.5%, 1% and 1.5% were added to concrete mixes and 150×300 mm cylindrical specimens were made. Mixing of fiber reinforced concrete needs careful conditions to avoid balling of fibers, segregation and in general the difficulty of mixing the materials uniformly. Increase in the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify the difficulties and balling tendency. Then different tests were performed for determination of compressive strength, splitting tensile strength and impact resistance at 7 and 28 days, in accordance with standards and procedure proposed. The impact resistance (strength) test was s carried out by using drop weight method recommended by ACI Committee. The drop-weight test equipment was fabricated according to A SASTM standards. The number of blows to the first visible cracks on the top surface of the disc is defined as the firstcrack strength. The results indicated that the addition of different contents and lengths of steel fiber caused significant change in the mechanical properties of high-strength concrete. In the best case, using 1.5% of 50 mm long steel fibers, increased compressive strength and splitting tensile strength by 25% and 40%, respectively, compared to non-fibrous high-strength concrete. A remarkable improvement was observed in impact resistance of the fibrous concretes, as compared with the reference materials. By incorporating steel fibers into the mixtures, specially longer fibers, a conclusive increase in the number of blows required for first and final cracking (as compared to reference values) was observed, as well as the number of blows from the first cracks to the final failure in the high percentage of the fiber increased up to 80%. Moreover, it can be concluded that, by adding fiber, the failure crack pattern was changed from a single crack to a group of narrow crack, which demonstrate the beneficial effect of fiber reinforced concrete when subjected to impact loading. Addition of steel fibers to concrete significantly enhances modulus of rupture and toughness, and resists cracking in high-strength concrete, and restrains damage during the process of impact by complementally mechanisms.