Tamon Ueda

EXPERIMENTAL STUDY ON BOND STRENGTH OF CONTINUOUS CARBON FIBER SHEET

An experimental study on bond strength of Continuous Fiber Sheet (CFS) was conducted. Based on the experimental results the bond strength and various factors are clarified. Bond strength does not increase with bond length for bond length longer than 100mm. As CFS stiffness increases, the maximum local and average bond stresses at delamination increase and CFS strain gradient decreases. CFS with a narrower width has a bond strength greater than that with a wider width. Non-uniform loading decreases the bond strength, however anchor steel plate with tensioned bolt increases it due to the bond between steel plate and CFS and confinement from the bolt. From the observed bond stress in CFS, the equation to predict the maximum local bond stress was proposed.

Experiment and modeling on axial behavior of carbon fiber reinforced polymer confined concrete cylinders with different sizes

In most experiments, small-size fiber reinforced polymer–confined concrete cylinders are used to study the axial compressive behavior. Based on the experimental results, many strength models and stress–strain relationships of confined concrete have been established. Whether the experiment-based strength models and stress–strain relationships can be directly applied to large-size circular concrete columns, however, is still an open issue. To clarify this issue, this article studies the influence of specimen sizes on the axial behavior of carbon fiber reinforced polymer confined concrete cylinders. Three types of concrete cylinders with diameters of 100, 200, and 300 mm, wrapped with one ply, two plies, and three plies of carbon fiber reinforced polymer, respectively, are cast and tested. The experimental results show that, under identical lateral confining stress, the compressive strength and axial stress–strain relationship are independent of the specimen size. Based on theoretical analysis and experimental data, an analysis-oriented stress–strain relationship and an analytical strength model are developed to predict the axial compressive behavior of fiber-reinforced polymer–confined concrete cylinders. The validity of the stress–strain relationship and strength model is verified with the experimental results and 16 existing models obtained from the literature.

Durability of FRP Concrete Bonds and Its Constituent Properties under the Influence of Moisture Conditions

AbstractStrengthening by fiber-reinforced polymers (FRPs) is one of the most common solutions to the ageing infrastructures. However, long-term durability of such systems under different environmental conditions needs to be understood properly before widely adopting these methods in the field. This paper presents research outcomes from an experimental program carried out to determine the influence of moisture on the durability of the bond between FRP and concrete along with its constituent materials. Performance was evaluated through single lap shear bond tests and various kinds of tests on the resin samples, including water absorption, mechanical characterization, and glass transition temperature (Tg) analysis. The specimens were exposed to continuous water immersion and wet-dry cycles for a maximum period of up to 24 and 18 months, respectively. The results show some deterioration on the material and the bond properties in both exposure conditions. The bond strength decreased up to 32 and 12% for high-s...

Static and Fatigue Bond Characteristics of Interfaces between CFRP Sheets and Frost Damage Experienced Concrete

Synopsis: Both short and long-term performances of repaired or strengthened concrete structures using external FRP bonding are greatly affected by states of bonding substrates, which are covercrete and may have experienced various damages. One of them is frost damage in cold regions. This paper intends to investigate how the initial frost damages in concrete influence the static and fatigue bond performances of CFRP/concrete interfaces. Concrete specimens were exposed to freeze and thaw cycles before being bonded with CFRP sheets. The initial frost damage of concrete was controlled approximately at three different levels in terms of its relative dynamic modulus of elasticity, which was 100% (non frost damage), 85% and 70%, respectively. Test results showed that failure modes of CFRP/concrete bonded joints with initial frost damage in concrete were the delamination of covercrete. By contrast the joints without initial frost damage failed in a thin concrete layer as usual. Moreover, CFRP/concrete joints with and without initial frost damage showed different manners in their interface bonding strength and stiffness. If the initial frost damage existed in concrete substrate the effective bond length of CFRP/concrete joints was increased due to the decrease of the bonding stiffness and interfacial fracture energy. Fatigue testing results indicated that the linear slopes of S-N curves of CFRP/concrete bonded joints were not influenced by the initial frost damage. The initial frost damage did not shorten the fatigue life of CFRP/concrete joints if a same relative tensile stress level was kept in the FRP sheets, where the relative tensile stress level was defined as a ratio of the applied tensile force in FRP sheets for the fatigue tests to the maximum static pullout one achieved in each test series.

Intermediate Crack Debonding of Polymer Cement Mortar Overlay-Strengthened RC Beam

The bond behavior at the polymer cement mortar (PCM)-concrete interface particularly affects the structural behavior of overlay-strengthened slabs or beams, and the debonding of the PCM overlay was one of the major failure modes. The design procedure of PCM overlay to strengthen an RC element should avoid these premature debonding failures. Because no reliable design method for retrofitting with the PCM overlay is currently available for the practitioner, a need exists to understand the mechanism of the PCM overlay debonding process to prevent failure. This paper presents an analytical approach for a PCM overlay-strengthened beam failed by intermediate crack (IC) debonding. The limit of transferred shear force along the PCM-concrete interface to avoid any occurrence of debonding is analytically provided. The reliability and accuracy of the proposed analytical procedure have been successfully verified by knowing the ratio of analytical and experimental strength of the PCM-strengthened beams compiled in a bending test database, which has a mean value of 1.10 and a standard deviation of 0.14.

New approach for usage of continuous fiber as non-metallic reinforcement of concrete

Continuous fiber such as carbon fiber, aramid fiber and glass fiber, have been accepted as a substitute for conventional steel reinforcement because of its good characteristics: high strength, lightness, anti-corrosiveness, anti-magnetism and flexibility. At the same time continuous fibers also show some major drawbacks when compared with steel. This article introduces new directions for continuous fiber to overcome its drawbacks and to utilize its unused advantage. The new directions necessitate the development of new materials.

Mesoscale Modeling of Chloride Penetration in Unsaturated Concrete Damaged by Freeze-Thaw Cycling

AbstractFor concrete structures exposed to frost attack, cracks, or microcracks induced by freeze-thaw cycling can format interconnecting flow paths and allow more water or chloride ions to penetrate into the bulk concrete. It will subsequently facilitate further deterioration of concrete structures and accelerate the corrosion of embedded reinforced steel bars. Moreover, in reality most concrete structures are rarely fully saturated, so that chloride transportation in unsaturated concrete must be studied with respect to the water moving process in order to cover the real existing service conditions. In the current work, a numerical simulation method based on the mesoscale composite structure of concrete, named the lattice network model, is established to analyze the penetration property of concrete; especially the effects of microcracking induced by freeze-thaw damage on the unsaturated flow behavior are investigated. In the mesoscale model, concrete is treated as a three-phase composite material consist...

Average Crack Spacing of Overlay-Strengthened RC Beams

This research evaluates the average crack spacing of overlay-strengthened RC beams. Current equations in different structural codes for predicting the average crack spacing of RC beams with multilayered reinforcing bars have proved inapplicable to overlay-strengthened RC beams, although both types of beams are reinforced with multiple layers. We have developed a simple, practical analytical model that can predict the average crack spacing of this type of beam. We analyzed an overlay-strengthened beam element using equilibrium and compatibility equations to formulate the average stresses of concrete, overlay, and tension reinforcement. Then we made a series of comparisons among various types of beam elements, using the experimental investigations to verify the model’s validity and reliability. The proposed model performs satisfactorily against measured responses from the experimental database.

Prediction of Punching Shear Strength of Two-Way Slabs Strengthened Externally with FRP Sheets

Strengthening two-way slabs by using fiber-reinforced polymer (FRP) is experimentally and analytically evaluated. Results show that the punching capacity of two-way slabs can increase to up to 40% greater than that of a reference specimen. A three-dimensional FEM program called 3D CAMUI, which was developed at Hokkaido University, was used to simulate the experimental slabs. Very good agreement is obtained in load-carrying capacity and modes of failure. An analytical model based on the numerical simulation, which discloses the mechanism of punching shear strength enhancement by FRP strengthening, is proposed to predict the punching shear strength of two-way slabs externally strengthened with FRP sheets. DOI: 10.1061/(ASCE)CC.1943-5614.0000177.

Fracture Criterion for Carbon Fiber Reinforced Polymer Sheet to Concrete Interface Subjected to Coupled Pull-Out and Push-Off Actions

Flexural strengthening of reinforced concrete (RC) beams using externally bonded fiber reinforced polymer (FRP) sheet is a popular application nowadays. In this application, the FRP to concrete bond interface is typically exposed to a mixed-mode loading condition at the vicinity of a critical flexural/shear crack, which includes a pull-out action parallel to the interface and introduced by the opening displacement of the flexural/shear crack together with a push-off action perpendicular to the interface and introduced by the shear sliding displacement of the flexural/shear crack. This paper presents an experimental program to evaluate the fracture properties of the FRP to concrete interface under the coupled pull-out (mode II) and push-off (mode I) loading condition. An analytical model was developed to evaluate the mode I and mode II strain energy release rates and the corresponding fracture mechanisms of the FRP to concrete interface under a pure bending action, a pure dowel action, and the coupled action of both, respectively. The reliability of the proposed analytical model was verified in comparison with the experimental results.