Woo-Tai Jung

Carbon-fiber reinforced polymer (CFRP) tendons have attracted increasing attention as corrosion-resistant prestressing elements for prestressed concrete and cable-supported structures; however, their practical implementation requires reliable verification of long-term mechanical performance and anchorage reliability. In this study, a 9.5 mm pultruded CFRP tendon and compression-type anchorage system were developed and experimentally evaluated through relaxation, creep rupture, and fatigue tests. The tendon exhibited a tensile strength of 2501 MPa and an elastic modulus of 132.5 GPa. Relaxation tests were conducted at an initial load corresponding to 70% of the ultimate tensile capacity, and the measured relaxation loss after 1000 h was 1.02%. Based on logarithmic regression of the measured data, the relaxation loss at 1,000,000 h was estimated to be 2.11%; however, this value should be interpreted as an extrapolated long-term estimate rather than a directly verified result. Creep rupture tests performed at load ratios of 82.4–100.0% yielded an estimated 1,000,000 h creep rupture load ratio of approximately 80%, although the prediction is subject to uncertainty because of the limited number of specimens and scatter in rupture times. Fatigue tests indicated that the CFRP tendon–anchorage assembly maintained stable performance up to 2,000,000 cycles without measurable degradation in elastic stiffness under the adopted loading conditions. These results suggest that the developed CFRP tendon–anchorage system has promising potential for prestressing applications, while further long-term tests with a larger number of specimens are required to improve the statistical reliability of the extrapolated relaxation and creep rupture predictions.

Carbon-fiber-reinforced polymer (CFRP) cables have emerged as promising alternatives to conventional prestressing tendons because of their high tensile strength, excellent corrosion resistance, and low self-weight. Their use is particularly advantageous in infrastructure exposed to aggressive environments, such as chloride-induced corrosion, where improved durability and reduced maintenance are critically required. In this study, a 10 mm diameter round-bar-type CFRP cable was developed using a pultrusion process, and its applicability to structural systems was comprehensively evaluated through material testing and field implementation. Mechanical performance was assessed through tensile, relaxation, and fatigue tests. The developed CFRP cable exhibited an average tensile strength of 3019 MPa and an elastic modulus of 176.9 GPa, demonstrating mechanical properties comparable to or better than those of conventional prestressing tendons. The final relaxation ratio was measured as 2.25%, satisfying the low-relaxation criterion specified in KS D 7002. In the fatigue test, the cable sustained 2,000,000 loading cycles under a stress range corresponding to 60–66% of the ultimate tensile strength without fracture or significant stiffness degradation, confirming its excellent fatigue durability. In addition, the developed CFRP cable was implemented in a cable-net structure to verify its constructability and structural applicability in practice. The field application confirmed that the lightweight CFRP cable enabled convenient transportation and installation, while stable prestress introduction was achieved using the same tensioning procedure as that for conventional steel cable systems. The results demonstrate the integrated feasibility of the developed CFRP cable in terms of both material performance and practical structural application. This study provides experimental evidence supporting the structural use of CFRP tendons and offers a technical basis for the future development of design provisions and broader infrastructure applications.

Structural steel and concrete are essential materials for the construction of social infrastructures. However, these materials undergo degradation over time, thereby causing steel corrosion. To address this problem, a fiber-reinforced polymer (FRP) is used for reinforcement. In this study, tensile tests were performed to evaluate the material properties for the application of the FRP to cable bridge structures. These tests aimed to investigate various parameters to improve bond performance. Based on experiments with different parameters, sufficient bond performance could be achieved if the following conditions are met: mortar water ≤16%, regardless of the manufacturer; a depth of splitting and steel pipe length ratio ≥75%; upward/downward directions for the mortar injection; and the use of fiber-sheet reinforcement. In addition, the steel pipe used in the test (length of 410 mm and outer diameter of 42.7 mm) performed the best in terms of workability and cost effectiveness. By conducting more accurate tests to study the basic properties of materials, more accurate conditions to accomplish sufficient bond performance can likely be achieved. This will contribute to improved cost effectiveness and safety in the use of carbon FRP cables in cable bridge constructions.

Prestressed concrete composed of steel materials is increasingly used in various social infrastructures, such as bridges (cables), nuclear containment structures, liquefied natural gas (LNG) tanks, and structural reinforcements. This study aimed to substitute the steel in bridge cables with fiber-reinforced polymers (FRPs) to prevent the damage caused by the performance degradation of corroded prestressed steel. An optimized single-anchorage system was derived by applying multiple variables, such as the surface treatment, number of insert layers, and sleeve processing companies, to improve the maximum load and bonding with the anchorage system sleeve using the carbon FRP (CFRP) cable. The B-L-4 specimen (sleeve specifications of company B, longitudinal surface treatment, and four insert layers) was determined to be the optimized single-anchorage system. When the tensile test was conducted after applying the optimized single-anchorage system to the three- and seven-multi-anchorage systems, the tensile performances of B-L-4 were 100 and 95% of the one-multi-anchorage system, respectively. Considering that the problems associated with the construction of three- and seven-multi-anchorage systems have been addressed, these systems can be applied to actual bridges in the future, and can significantly benefit their maintenance.

Concrete bridge structures require reinforcement, as their performance deteriorates over time. In this regard, this study evaluated the effect of additional prestressing using fiber-reinforced polymers (FRPs) and strands applied to a demolished, deteriorated bridge. In particular, specimens were prepared for a bridge subjected to non-, near-surface mounted (NSM), and external prestressing (EP) strengthening to evaluate the stiffness and safety of the structure. In the 200–400 kN load range, the EP method exhibited the highest stiffness (15 kN/mm), followed by non-strengthening (8.5 kN/mm) and the NSM method (5.45 kN/mm). The EP method increased the stiffness by approximately two times; however, the NSM method decreased the stiffness by 0.6 times. In the 400–800 kN load range, the EP and NSM methods yielded stiffness values of 2.58 and 0.7 kN/mm, respectively. These results confirm that the EP method reinforces the structure. The results of this study are expected to be used as basic data to reinforce deteriorated bridges in actual operation.

Various methods for strengthening existing structures have been developed owing to the increase in human and property damages caused by the deterioration of structures. Among the various reinforcing methods, the external prestressing method increases the usability and safety of a structure by directly applying tension to the weak tensile area that suffers the greatest deflection during the structure usage. The external prestressing method is advantageous in reducing cracks caused by the introduced tension and restoration of the deflection. Since the strengthening method is applied to deterioration bridges, the strengthening effect is affected by the condition of the existing structure. However, studies on the strengthening effect according to the degree of deterioration are insufficient. Therefore, the behavior according to the strengthening status was analyzed, and the strengthening effect was identified in this study by simulating the deteriorated bridge, reducing the compressive strength and reinforcement quantity, and conducting a four-point loading test. As a result of the experiment, a reinforcement effect of 215% crack load, 161% yield load, and the difference in behavior according to the reinforcement parameters were confirmed.

The external prestressing method was developed to improve the load carrying capacity of concrete bridges damaged by ageing or other external environmental factors. Therefore, it is necessary to examine the performance of the structure before and after strengthening, and verification and confirmation of the actual strengthening effect should be preceded. However, for reasons such as securing deteriorated bridges and the absence of design data, the strengthening effect of an actual bridge is rarely analyzed. Hence, in this study, the strengthening effect was verified by conducting a set of material property tests and four-point loading tests before and after strengthening of an actual bridge, which had been utilized for over 45 years. As a result, it was determined that confirming the strengthening effect before cracking is challenging due to the nature of the external prestressing method, which has an insignificant effect on the stiffness; however, the effect of increasing the crack load of the external prestressing method was experimentally verified. The result of the study is expected to contribute to the determination of the load carrying capacity and required strengthening amount when strengthening existing prestressed concrete girder bridges.

Reinforced concrete (RC) structures age with time, which results in performance degradation and cracks. These performance degradations do not recover easily, but a performance higher than the existing structures can be expected through reinforcement. There are various reinforcement methods for RC structures. This study selected four reinforcement methods: near-surface mounting (NSM), external prestressing (EP), external bonding (EB), and section enlargement (SE). In the past, steel bars were often used as reinforcements. However, this study uses fiber-reinforced polymer (FRP), which is an alternative to steel bars owing to its high tensile strength, and its non-corrosive and lightweight properties. It is a basic strengthening material, along with a carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) in bar and sheet forms. Various strengthening materials such as a CFRP, GFRP, and prestressing (PS) strand are applied to the NSM, EP, EB, and SE methods, followed by flexural experiments. In addition, changes in the ductility of the RC structures were examined. The concrete EP and near-surface mounting prestressing (NSM(P)) methods have a stiffness that is almost double the non-strengthened specimen. However, because the EP and EB methods are brittle, the NSM(P) method with ductile behavior is considered the most effective.