بررسي عددي افزايش ظرفيت باربري قاب بتن مسلح با استفاده از كامپوزيت‌هاي سيماني مسلح اليافي توانمند

USING HPFRCC FOR INCREASING THE CAPACITY OF A R.C. FRAME

كامپوزيت‌هاي سيماني مسلح با الياف توانمند (HPFRCC) به مصالحي شامل ملات سيماني با سنگدانه‌هاي ريزدانه و الياف اطلاق مي‌شود. ويژگي شاخص اين مصالح آن است كه تحت بارگذاري كششي، رفتار سخت‌شوندگي كرنش از خود بروز مي‌دهند. اين مصالح توانمند مي‌توانند در بسياري موارد نظير بهسازي لرزه‌‌يي اعضاي ساختماني به‌كار رود. در اين نوشتار، پس از معرفي مختصر اين مصالح توانمند، قاب آزمايش‌شده توسط كرانستون آزمايش انتخاب و مورد بررسي قرار مي‌گيرد. همچنين، چشمه‌ي اتصال قاب مذكور با مصالح HPFRCC با مقاومت‌هاي كششي و فشاري متفاوت جايگزين شده است و با قاب‌هاي بتني كامل و HPFRCC كامل مقايسه مي‌شود. نتايج نشان مي‌دهند كه با استفاده از اين مصالح توانمند، ظرفيت باربري و شكل‌پذيري اين قاب‌ها افزايش مي‌يابد. در اين بررسي، با افزايش مقاومت كششي و فشاري HPFRCC ظرفيت باربري قاب به طور متوسط حدود 40% نسبت به قاب بتن مسلح معمولي افزايش يافته است. همچنين با افزايش مقاومت فشاري و كششي اين كامپوزيت، شكل‌پذيري قاب به‌طور ميانگين به ترتيب حدود 50% و 70% افزايش پيدا كرده و نحوه‌ي ترك‌خوردگي اين سازه‌ها نيز با يكديگر مقايسه شده است.

High Performance Fiber Reinforced Cementitious Composites which are called HPFRCC, are cement matrices with strain hardening response under tension loading. In these composites, the cement mortar with fine aggregates, is reinforced by random distributed fibers and could be used for various applications such as rehabilitating of structural members. In this paper, mechanical properties of HPFRCC materials are reviewed briefly. Moreover, a one-bay and one-story reinforced concrete frame which had been tested by Cranston, is selected and investigated using high performance materials with different tensile and compressive strengths instead of concrete in connection zone and the whole frame. Analytical results show that, using HPFRCC material instead of regular concrete increases the ductility and loading capacity of frame. In this investigation, with increasing in the amount of compressive and tensile strengths of HPFRCC, the loading capacity of composite frames increased about 40% compare to RC frame. Ductility factors of these composite frames increased about 50% and 70% compare to RC frame respectively too. Cracking patterns of these frames are compared to each other. Cranston’s experimental investigation was undertaken to corroborate the analytical work and lend further insight into the nature of finite element items in frame structure. Structural properties of normal concrete are constant during this investigation but structural properties of HPFRCC are variable including different compressive and tensile strengths. Compressive strengths of HPFRCC are 28, 35 and 39 MPa and tensile strengths are 3, 4 and 5 Mpa. Lateral load-deflection curves and cracking patterns of the RC (full reinforced concrete frame), RH (full reinforced HPFRCC frame) and RCH (reinforced concrete frame with HPFRCC material in its connections) frames with are investigated. Analytical results show that, maximum ductility is occurred in the RCH frame with compressive and tensile strengths of 28 and 5 MPa respectively. Maximum amount of ultimate lateral load is occurred in RH frame with compressive and tensile strengths of 28 and 5 MPa respectively. Maximum amount of ultimate lateral displacement is occurred in RH frame with compressive and tensile strengths of 35 and 3 MPa respectively. This improvement in the lateral behavior of frames with HPFRCC materials is concluded from the large ultimate tensile strain of HPFRCC material which is altered instead of normal concrete with low and consequently increases both load and deflection capacity of the structure. In addition to, another factor which has an important role in the ascending trend of the mentioned curves is the steel reinforcements. When concrete is altered by HPFRCC, because of the higher of HPFRCC, steel rebars can reach to closer amount of plastic strain value of steel and hence, lateral deflection and force increase simultaneously. In the other word, the main role of HPFRCC is to assist the work of steel by keeping unity of the HPFRCC and steel as a composite material by forming multiple cracking and avoiding from concentrated failure plane. It can be used in critical parts of a reinforced concrete frames to increase the capacity of the structure.