Yunus E. Harmanci

Prestressed CFRP Strips for Concrete Bridge Girder Retrofitting: Application and Static Loading Test

AbstractThis paper presents an investigation on the practicability and structural efficiency of prestressed carbon-fiber–reinforced polymer (CFRP) strips with a gradient anchorage in the framework of a bridge-strengthening application in Poland. The nonmechanical anchorage system avoids the installation of metallic bolts and plates, with the exception of a temporary support frame. Two 18.4-m-long large-scale prestressed concrete girders were produced following the drawings of the existing bridge construction. One girder served as a reference, and the second one was strengthened with two prestressed CFRP strips. In this case, the initial negative cambering was leveled out by a layer of dry shotcrete. CFRP strips with a prestrain of 0.58% were applied for flexural upgrading. Both girders with a total length of 18.4 m were finally statically loaded up to failure to assess the strengthening efficiency in flexure of the retrofitting technique used. Tensile failure of the CFRP strips was reached, indicating an ...

Structural Strengthening of Concrete with Fiber Reinforced Cementitious Matrix (FRCM) at Ambient and Elevated Temperature - Recent Investigations in Switzerland

This paper presents recent experimental investigations on structural strengthening by means of (Carbon) Fiber Reinforced Cementitious Matrix (FRCM) in Switzerland. A first test series deals with full-scale reinforced concrete slabs strengthened with one or two composite reinforcement meshes embedded in a shotcrete layer. Static load tests up to failure show the efficiency of the strengthening in terms of increased yield and ultimate load compared to the reference specimen. Due to the initially necessary straightening of the textile, the contribution at lower deflection levels is limited. Only with advanced cracking and crack opening, the mesh develops its full contribution. Ultimate load is reached after a prompt relative slip of the mesh in the shotcrete. In the post-peak domain, failure by concrete crushing was observed. To study the residual tensile strength of the carbon reinforcement after exposure to high temperatures, various tensile tests on small rovings previously cut out of a composite mesh were performed. The specimens were heated to temperatures of 300 °C, 500 °C, 700 °C, and 1000 °C, kept at that level for 30 minutes, and finally cooled down to room temperature. The subsequent tensile tests performed at room temperature revealed a significant drop in the residual tensile strength for exposure temperature higher than 300 °C. A final test was performed on a reinforced concrete slab strip strengthened with a shotcrete layer including a composite mesh as tensile reinforcement. Under a constant service load, the slab was exposed to fire with a temperature rise according to a European standard curve (ETK) for two hours. The slab could withstand the applied loads for the full two hours, during which the composite mesh reached a temperature of about 440 °C. This observation is consistent with the results from tensile tests on filaments, clearly indicating a residual tensile strength after exposure at a similar temperature. The temperature in the internal steel reinforcement did not trespass a critical value of 500 °C as proposed by current design recommendations.

Calculation Technique for Externally Unbonded CFRP Strips in Structural Concrete Retrofitting

AbstractThis paper presents a calculation procedure for externally unbonded carbon fiber–reinforced polymer (CFRP) strips anchored at their ends in flexural strengthening of existing reinforced concrete structures. Due to strain incompatibility between the composite strip and the neighboring concrete surface, the well-known conventional cross-section analysis (CSA) employed for bonded reinforcements cannot be implemented. An iterative force equilibrium and strip strain adaptation with previously defined constitutive materials laws together with an optimization procedure in a numerical computing environment are used to overcome the computational complexities. For validation purposes, two static loading tests on reinforced concrete beams with an externally prestressed CFRP strip without bond except in the anchorage zones are compared to the numerical predictions. Prior to the final evaluation, a model for the anchorage resistance is implemented in the algorithm in order to capture the ultimate load due to a...

Behaviour of Prestressed CFRP Anchorages during and after Freeze-Thaw Cycle Exposure

The long-term performance of externally-bonded reinforcements (EBR) on reinforced concrete (RC) structures highly depends on the behavior of constituent materials and their interfaces to various environmental loads, such as temperature and humidity exposure. Although significant efforts have been devoted to understanding the effect of such conditions on the anchorage resistance of unstressed EBR, with or without sustained loading, the effect of a released prestressing has not been thoroughly investigated. For this purpose, a series of experiments has been carried out herein, with concrete blocks strengthened with carbon fiber-reinforced polymer (CFRP) strips, both unstressed, as well as prestressed using the gradient anchorage. The gradient anchorage is a non-mechanical technique to anchor prestressed CFRP by exploiting the accelerated curing property of epoxy under higher temperatures and segment-wise prestress-force releasing. Subsequently, strengthened blocks are transferred into a chamber for exposure in dry freeze-thaw cycles (FTC). Following FTC exposure, the blocks are tested in a conventional lap-shear test setup to determine their residual anchorage resistance and then compared with reference specimens. Blocks were monitored during FTC by conventional and Fabry–Pérot-based fiber optic strain (FOS) sensors and a 3D-digital image correlation (3D-DIC) system during gradient application and lap-shear testing. Results indicate a reduction of residual anchorage resistance, stiffness and deformation capacity of the system after FTC and a change in the failure mode from concrete substrate to epoxy-concrete interface failure. It was further observed that all of these properties experienced a more significant reduction for prestressed specimens. These findings are presented with a complementary finite element model to shed more light onto the durability of such systems.