مقاومت پيوستگي بين آرماتورهاي پليمري مسلح اليافي (FRP) و بتن در تيرهاي وصله‌دار

Bond Strength between Concrete and FRP Bars for Lap-Spliced Concrete Beams

براي استفاده از آرماتورهاي پليمري مسلح اليافي (FRP) در سازه‌هاي بتن‌آرمه بايد روابط طراحي جديدي براي آن‌ها ارايه شود. ايجاد پيوستگي كافي بين بتن و آرماتورهاي FRP در محل وصله، يكي از بحث‌هاي مهمي است كه در طراحي سازه‌هاي بتن‌آرمه بايد توجه شود. هدف از اين مقاله ارايه رابطه‌اي براي تعيين مقاومت پيوستگي آرماتورهاي FRP وصله شده در تيرهاي بتن‌آرمه است. به‌اين منظور نخست به‌كمك نتايج آزمايش‌هاي بيرون‌كشيدگي محققان ديگر، روابطي براي تعيين مقاومت پيوستگي موضعي و مدول تغيير مكان آرماتورهاي FRP ارايه مي‌شود. سپس به‌كمك نتايج آزمايش‌هاي وصله كششي در تيرها، رابطه‌اي براي تعيين مقاومت پيوستگي در طول وصله آرماتور FRP به‌‌دست مي‌آيد. اين رابطه اثر پوشش بتن و آرماتور جانبي وصله را در مقاومت پيوستگي وارد مي‌كند. مقايسه رابطه پيشنهادي و نتايج آزمايشگاهي نشان مي‌دهد كه رابطه پيشنهادي با دقت مناسبي مقاومت پيوستگي آرماتورهاي FRP در نمونه‌هاي تيري وصله‌دار را پيش‌بيني مي‌كند. اين رابطه با روابط ارايه شده در ديگر آيين‌نامه‌ها و مراجع نيز مقايسه شده است.

Steel is considered to be one of the desirable materials used for reinforcing concrete structural members. However, the corrosion of steel bars has been always a threat for the service life of reinforced concrete members in corrosive environments. Fiber Reinforced Polymer (FRP) bars can be used as reinforcing materials due to their corrosion resistance. FRP reinforcing bars are available in different grades of tensile strength and modulus of elasticity. These bars have high tensile strength and durability and display linear elastic behavior up to their failure. The behavior of concrete beams reinforced with FRP bars is different from that of steel reinforced concrete beams. Concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars exhibit large deflections and crack widths as compared with steel reinforced concrete beams due to the low modulus of elasticity of GFRP. In addition, the bond between concrete and FRP bars is different from steel bars because of the difference in their surface geometries and mechanical characteristics. This paper proposes an equation for the bond strength of lap-spliced concrete beams reinforced with FRP bars. First, equations for displacement modulus and local bond strength of FRP bars are formulated by pullout test results, tested by other researchers. Then, using the local bond strength equation and based on the experimental results of lap-spliced FRP reinforced concrete beams, an equation for bond strength of splices is derived. In the formulation of this equation, the non-uniform distribution of the bond stress along the splice length is considered. The effects of concrete cover and transverse reinforcement are also taken into account in the proposed equation. Transverse reinforcement has an important role in the bond strength of beams with spliced bars. Transverse reinforcement confines developed and spliced bars by limiting the progression of splitting cracks and increases the uniformity of bond stress distribution along the splice length and thus, increasing the bond strength. The bond strengths calculated by the proposed equation are compared with the experimental values. The comparison shows that the proposed equation predicts the splice strength accurately. Also, calculated bond strengths are compared with the values predicted by different code provisions and other models. The average and standard deviation of the experimental over calculated bond strength ratios obtained by the proposed equation are 1.00 and 0.14, respectively. These ratios are 0.65 and 0.19 for the ACI440.1R-06 code, 0.55 and 0.15 for the CAN/CSA-S6-00, 0.67 and 0.16 for the CAN/CSA S806-02 code and 0.99 and 0.36 for the Aly equation. Compared to Aly equation and design guidelines, the proposed equation for calculating the bond strength shows better agreement with experimental values. In addition, code equations overestimate the bond strength of GFRP bars in splices of beams.