Sang-Wook Bae

Effective Bond Length of FRP Sheets Externally Bonded to Concrete

Strengthening and repair of concrete structures using externally bonded fiber reinforced polymer (FRP) composite sheets has been popular around the world during the last two decades. However, premature failure due to debonding of the FRP is one of the important issues still to be resolved. Numerous research studies have dealt with the debonding problem in terms of Effective Bond Length (EBL), however, determination of this length has not yet been completely assessed. This paper summarizes previous works on the EBL and proposes a new relationship of the EBL with the FRP stiffness based on the existing experimental data collected in this study.

Behavior of RC T-Beams Strengthened in Shear with CFRP under Cyclic Loading

This study investigated the shear performance of an RC beam strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) strips, subjected to a cyclic loading for 2 million cycles at 1 Hz. The stress level in the stirrups caused by the cyclic loading used in this study was higher than those typically used in fatigue studies, which could have caused the yielding of some stirrups from the beginning of cyclic loading. This was done to reflect the fact that many RC beams in need of shear strengthening do not meet the minimum stirrup requirement for the new and increased shear demand. The experimental results obtained in this study and a comprehensive review of the existing literature showed that RC beams strengthened in shear with externally bonded CFRP could survive 2 million cycles of cyclic loading without failure. Furthermore, the residual shear strength of the FRP-strengthened beam appeared to be greater, albeit slightly, than the static shear strength of the unstrengthened control beam. This study’s results also suggested that limiting the interfacial stress in CFRP strips to less than 1.5 MPa or 25% of its ultimate interfacial strength would increase fatigue life by avoiding debonding of CFRP strips.

Design of FRP Systems for Strengthening Concrete Girders in Shear

Fiber-Reinforced Polymer (FRP) systems have been used on a project-specific basis for the last two decades. They are now becoming a widely accepted method of strengthening concrete structures. The acceptance and utilization of these new strengthening techniques depend on the availability of clear design guidelines, installation procedures and construction specifications. Standard specifications exist for all commonly used traditional materials in civil engineering structures. At this time, design specifications for FRP use are still under development. The results of several experimental investigations have shown that FRP systems can be effective for increasing ductility and strength to structural members such as columns and girders. As most of the research focused on strengthening of axial members of flexural members, there are less experimental and analytical data on the use of FRP systems for shear strengthening of girders. Shear strengthening with FRP is still under investigation and the results obtained thus far are scarce and sometimes controversial. Even in traditional reinforced concrete members without FRP, the shear design is a complex challenge and uses more empirical methods as compared to axial and flexural design methods. Adding FRP to the equation, with its specific design issues, would bring another level of complication in the design. These FRP-related shear design issues and lack of comprehensive analytical and experimental models are the main motivation for this research project. Thus, a thorough understanding of the shear design problem along with the development of an American Association of State Highway and Transportation Officials (AASHTO) design method for FRP shear strengthening of concrete girders is needed. As such, the objective of this project is to develop design methods, specifications, and examples for design of FRP systems for strengthening concrete girders in shear. The proposed specifications will be in Load and Resistance Factor Design (LRFD) format and will be suitable for recommendation to the AASHTO Highway Subcommittee on Bridges and Structures for adoption.

Behavior of Various Anchorage Systems Used for Shear Strengthening of Concrete Structures with Externally Bonded FRP Sheets

AbstractEfforts to prevent the debonding failure mode of externally bonded fiber-reinforced polymer (FRP) strengthening systems have led to the investigation of many different kinds of anchorage systems. An effective anchorage system allows externally bonded FRP reinforcements to continue carrying load, even after debonding occurs. In this study, an experimental program was performed on six full-scale RC T-beams strengthened in shear using externally bonded carbon FRP sheets with three different anchorage systems, the so-called (1) discontinuous mechanical anchorage (DMA) system, (2) sandwich discontinuous mechanical anchorage (SDMA) system, and (3) additional horizontal strips (HS) system. The experimental results showed that the SDMA system performed best, followed by the DMA and HS systems. The RC T-beams strengthened with the SDMA system showed a 59–91% increase in shear strength, altering the failure mode from FRP debonding to FRP rupture. In addition, the shear contribution of internal transverse st...

Application of Microwave 3D SAR Imaging Technique for Evaluation of Corrosion in Steel Rebars Embedded in Cement-based Structures

This paper presents and discusses the attributes and results of using wideband microwave 3D SAR-based imaging technique for evaluation of reinforced cement-based structures. The technique was used to detect corrosion and thinning of reinforcing steel bars and its potential was demonstrated through experiments for different bar sizes, depth of rebar locations, and spacing between rebars. The results of a limited and preliminary investigation in which thinning of rebars with and without rust in two mortar samples were obtained at three frequency bands covering the frequency range from 8.2 GHz-26.5 GHz.

Effects of Various Environmental Conditions on RC Columns Wrapped with FRP Sheets

A majority of the existing research studies focus on the effects of individual environmental conditions, such as freeze—thaw cycles and wet—dry cycles, while investigations on the combined effects of various environmental conditions are very rare. Therefore, the objective of this study was to evaluate the effects of various environmental conditions on reinforced concrete (RC) columns wrapped with FRP sheets with focus on the combined effects. Two different scales of RC columns wrapped with carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) sheets were conditioned under various environmental conditions. Then, uni-axial compression tests were conducted in order to evaluate their strength, stiffness, and ductility. The environmental conditions considered in this study included freeze—thaw cycles, high-temperature cycles, high-humidity cycles, ultraviolet (UV) radiation, and saline solution. Based on the test results, design guidelines were suggested, which included equations to determine axial capacity of RC columns wrapped with FRP sheets, and strength reduction factors to account for the effects of environmental conditions considered in this study.

Finite Element Modeling and Simulation Analysis of a Portable Roll-Up Sign

Although a large number of crash tests have been performed between passenger cars and rigid fixed traffic signs, the number of real tests focusing on crashworthiness of portable roll-up signs is still limited. Because a standard, portable roll-up sign contains at least three kinds of dissimilar materials, such as steel for the base, fiberglass for the batten, vinyl for the sign, and because the sign’s configuration is more complicated than a rigid fixed sign, it is important to simulate the behavior of portable roll-up signs in collision. In this paper, a fine-mesh finite element model precisely representing the portable roll-up sign was created and used together with a car model to simulate the process of impact with 0 and 90 degree orientation. The simulation was performed using LS-DYNA software. Techniques for creating the finite element model were discussed. Afterwards this finite element model, being validated and verified through real tests, can be used for parametric and/or robust design.Copyright