بررسي رفتار تيرهاي وصله‎دار مسلح‌شده با ميلگردهاي GFRP به كمك آزمايش‎هاي‎ استاتيكي و مودال

STUDY ON BEHAVIOR OF LAP-SPLICED CONCRETE BEAMS REINFORCED WITH GFRP BARS BY STATIC AND MODAL TESTS

مقاومت پيوستگي تيرهاي وصله‎دار مسلح‎شده با ميلگرد GFRP به دليل تفاوت در شكل سطح جانبي و مدول كشساني ماده‌ي مسلح‎كننده نسبت به تيرهاي مسلح‌شده با فولاد متفاوت است. در اين نوشتار، اثر مدول‎كشساني ميلگردهاي FRP در روابط مقاومت پيوستگي وارد و با نتايج آزمايشگاهي و آيين‌نامه‌ي ACI440.1R-03 مقايسه شده‎ است. براي اين منظور 7 نمونه‌ي‌ آزمايشگاهي تيرهاي مسلح‎شده با ميلگردهاي GFRP ساخته شده است. در نمونه‌هاي مذكور، طول وصله، مقاومت بتن و ميلگرد جانبي به عنوان متغير در نظر گرفته شده‎اند. در آزمايش استاتيكي، منحني‎هاي بار- تغييرمكان برداشت شده‎اند تا به كمك آن مقاومت پيوستگي و شكل‎پذيري نمونه‌ها بحث و بررسي شوند. در بين هر گام بارگذاري استاتيكي، آزمايش مودال انجام مي‎گيرد. با استفاده از اين آزمايش تغييرات بسامد‎هاي طبيعي در گام‎هاي خسارت ارزيابي و رابطه‌ي بين اين تغييرات با پارامترهاي وصله و شكل‎پذيري بررسي مي‎شود. نتايج مقاومت پيوستگي نشان مي‎دهند كه رابطه‌ي اصلاح‌شده در اين نوشتار مطابقت خوبي با نتايج آزمايشگاهي دارد. همچنين، شكل‎پذيري نمونه‎ها با افزايش ميلگرد جانبي، طول وصله و مقاومت بتن زياد شده است و در نمونه‎هاي شكل‎پذيرتر، افت بسامد‎ها در گام‎هاي بارگذاري بيشتر بوده است.

Fiber-Reinforced Polymer (FRP) bars can be used as a reinforcing material by nature corrosion resistant. These bars have high tensile strength and appropriate durability but are linear elastic up to failure and not ductile. Also, the bond strengths of lap-spliced concrete beams reinforced with steel and GFRP bars are different because of the elastic modulus and the surface conditions of these bars. These parameters affect the non-uniformity of bond stress along the spliced length of lap-spliced concrete beams reinforced with GFRP bars. Different studies show that by using an appropriate amount of transverse reinforcement, satisfactory bond strength and ductility response can be obtained. Some studies concluded that the bond strength of FRP bars is less than that of steel bars and the modulus of elasticity is the most significant parameter in the reduction of bond strength. In this paper, a previous proposed equation which accounts for the modulus of elasticity of reinforcing bars in bond strength is modified for FRP bars. The modified equation is compared with experimental results and ACI440.1R-03 provisions. The results show that the bond strengths calculated with the modified equation correlate well with the experimental values. In the experimental part of the study, seven beam specimens were manufactured and tested. Laboratory specimens were designed with different parameters of splice length, concrete compressive strength, amount of transverse reinforcement along the splice length and the diameter of longitudinal bars. Static test is carried out for causing damage in different levels of loading. The cracks of the specimens were mapped and test observation was recorded during loading steps and at the time of failure. Also, the relationships of force versus mid-span displacement were obtained using the static tests. Then the bond strength and the ductility of specimens were analyzed. At each step of loading, a modal test was carried out to obtain the dynamic parameters of the specimens. Changes in the dynamic parameters are evaluated by modal test results between different steps of loading. The results show that the ductility is increased by increasing of transverse reinforcement and splice length and concrete compressive strength. The bond failure mode alters from splitting to pullout due to an appropriate amount of transverse reinforcement. This alteration controls the slip of bars and increases the ductility. Frequency reduction of specimens with splitting failure is lower than that of specimens with pullout failure. Also, frequency reduction increases with increase in transverse reinforcement along splice length of lap-spliced beams. The spliced specimens with an appropriate amount of transverse reinforcement are more ductile than non-spliced specimens. This is mainly due to the slip of spliced bars in the lap-spliced specimens. In addition, frequency reduction of ductile specimens is more than that of brittle specimens in different loading steps.