This paper investigates experimentally how various bond configurations influence the flexural response of FRP-strengthened RC beams. A total of seven beams(200×200×2200mm) were tested. One was the reference without strengthening. Two were strengthened with CFRP sheets using hard and soft adhesive bonding, respectively, along the whole test span. The remaining four were applied a combination of hard and soft adhesives but in two different ways. One way is to use the soft adhesive at the flexure-dominating zone only and another is to use it for the shear-dominating zone only. Pair tests were conducted for these four specimens under both normal and low temperature (- 10°C). It is revealed that a sole use of soft adhesive improves the ultimate member strength and ductility but leads to a loss of stiffness. Using the soft adhesive at the flexure-dominating zone can not improve the member strength but improve the member ductility only. However, use of hard adhesive at the flexure-dominating zone and the soft one for the remaining parts can improve the member strength and ductility while avoiding loss of member stiffness. This test program provides an in-depth understanding of the debonding mechanisms in the FRP-strengthened RC beams. In addition, it is verified that that a low temperature of -10°C does not influence the global flexural response of FRP strengthened beams using the combined bonding system.

Aiming at improving bond strengths between FRP sheet and concrete, the FRP sheet bonding method using high elongation elastic resin as a buffer layer was developed. In this study, bonding tests was conducted to clarify the basic bond characteristics of FRP strand sheet-concrete interface with a polyurea resin. As a result of the bonding tests, it was found that the use of the polyurea resin as a buffer layer significantly improved the bond strength and the interfacial fracture energy between FRP sheets and concrete. In addition, bond stress-slip relationships with a polyurea resin were proposed. Finally, numerical analyses of bonding CFRP strand sheets and concrete were conducted so as to verify validity of the proposed bond stress-slip relationships. This paper is an extended version in English from the authors' previous work [Kobayashi, A., Ozaki, M., Sato, Y., Arazoe, M., Tateishi, A. and Komori, A., (2020). “Study on bonding behavior of FRP sheets and concrete bonded with high elongation elastic resin.” Journal of Structural Engineering, JSCE, 66A, 855-867. (in Japanese)].

In this study, authors investigated a fatigue life prediction method for RC slabs considering effects of support condition and stepped incremental load based on experimental data of traveling wheel-type loading test. At first, to properly predict the fatigue life of RC slabs under constant cyclic loading test, a coefficient which represents influence of support condition was introduced to previous prediction method that authors had proposed. The modified method can cover three types of support conditions; simply support along longitudinal direction and elastic support along transverse direction, simple support along longitudinal direction and free ends along transverse direction, and simple support along all directions. Besides considering fatigue damage caused by preceding loading block as reduction of shear strength due to cyclic loading, a new evaluation method for stepped incremental loading test was proposed.

Studying about fatigue in RC beams without stirrup is one of valid approaches to investigate fatigue problem in RC slabs because the transverse cracked slab can be considered as plural RC beams lined up in parallel. Therefore, in this paper, authors tried to clarify the fatigue damage in RC beams keeping the problem of RC slabs in mind. Non-linear three-dimensional finite element analysis (3D-FEA) of RC beams without stirrup is conducted under three types of loading conditions; static load, fatigue load and variable amplitude fatigue load. By comparing static and fatigue analysis results, authors examine the principal compressive stress distribution and the stiffness reduction. This paper presents that static and fatigue loading showed different trends in both the stress distribution and stiffness reduction. It can be also revealed that when variable amplitude load with different two load blocks is given in fatigue analysis, the cyclic loading in the first load block influences on the decreasing rate of stiffness.

The authors conducted on-site load test and 3D Finite Element Analyses (3D-FEA) of an existing prestressed post-tensioned concrete bridge with four post-tensioned T-girders, whose span length was 36 m, constructed in 1960 so as to clarify structural behaviour and its capacity of the bridge superstructure. It was found that the 3D-FEA combined with the linear surface spring elements which represents restraint of adjacent members can well simulate the actual structural behavior, the capacity and failure mode. Besides it is also discussed about the reason why capacity of a girder in superstructure becomes greater than a single girder with same cross-sectional properties.

In this paper, a weakly singular boundary integral equation method is developed for the stress analysis of an anisotropic, linearly elastic, cracked whole space possessing a plane of symmetry. This study should offer an alternative powerful tool essential for the modeling of both near-surface and deeply embedded defects in a rock/soil medium. A system of governing equations is established using a pair of weakly singular, weakform, displacement and traction integral equations for the cracked whole space along with the symmetric condition. The final equations contain only unknown crack-face data in a lowerhalf of the whole space. In addition to their capability to treat cracks of arbitrary shape, material anisotropy and general loading conditions, all involved kernels are weakly singular allowing all integrals to be interpreted in the sense of Riemann. A symmetric Galerkin boundary element method together with the Galerkin approximation is implemented to solve the governing integral equations for the unknown crack-face data. To further enhance the accuracy and efficiency of the proposed scheme, special basis functions are introduced to approximate the near-front field and the interpolation technique is adopted to evaluate all kernels for generally anisotropic materials. The solved crack-face displacement data is then utilized to postprocess for the essential fracture information along the crack front. Various scenarios are employed to verify the proposed technique and a selected set of results is presented to demonstrate its accuracy and computational robustness.

An experimental program was carried out to investigate the shear capacity of High-Performance Fiber-Reinforced Concrete (HPFRC) I-beams. The main parameters were assigned as the fiber content and presence of shear reinforcement. To study the effect of these main parameters on the shear capacity, testing of six I-beams and other control specimens was conducted. It can be observed from the results of the experimental study that the presence of fibers and shear reinforcement significantly improves the ultimate capacity and structural behavior of HPFRC members. Finally, the experimental results are discussed, and the shear capacity of HPFRC can be estimated by extending the code provisions stated in AFGC-Sétra 2013.

The experimental and analytical study are attempted to clarify the static and fatigue behavior of mortar with blast furnace slag (BFS) sand as fine aggregates in compression in air and water. The cylindrical BFS mortar specimens were used for static and fatigue compression tests in air and water and results are compared with those of mortar using crushed sand (CS). The fatigue load was applied in the form of sine signal with a constant amplitude and the frequency of 5 Hz. The stress level is varied from 60%, 70% and 80% of the uniaxial compressive strength. The experimental results disclose that BFS mortar exhibits more fatigue life compared to CS mortar in air, while it is similar in water. Thereafter, the stress-strain relationships for each mortar under monotonic loading are formulated. Moreover, a simplified fatigue model for the assessment of plastic strain, total strain and fracture parameter at different number of loading cycles and for prediction of fatigue life is also proposed.

This study presents the experimental investigation carried out to evaluate the performance of non air-entrained concrete with blast furnace slag (BFS) sand as full amount of fine aggregates under freeze thaw cycles in comparison with conventional air-entrained normal concrete. Cuboidal specimens were used for freeze-thaw test. The concrete specimens were subjected to freeze-thaw cycles (FTC) in 3% NaCl solution at the age of about 4 months. The duration of each freeze-thaw cycle was 8 h with temperature variation between +20 °C and -25 °C. The strain variation during each FTC was recorded using strain gauges attached to the concrete surface and change of relative dynamic modulus of elasticity was measured at certain number of FTC using ultrasonic pulse velocity method. The damage progress is discussed based on plastic tensile strain accumulated during freeze-thaw cycles. A particular number of specimens were taken out from freeze-thaw chamber after 50, 125 and 225 FTC. Thereafter, the static compressive strength tests were performed to determine the change in mechanical properties of concrete with blast furnace slag fine aggregates compared with normal concrete at different number of FTC. BFS concrete showed more compressive strength and Young's modulus for same FTC number compared to normal concrete, however, the rate of reduction of mechanical properties of air-entrained normal concrete is slightly slow.

This paper aimed to assess structural performance of superstructures of typical RC bridge in Thailand. The static load test was performed to measure both horizontal and vertical displacement in Reinforced Concrete (RC) girder using Image Measurement System (IMS). The RC girder's stiffness obtained from the IMS results is compared with that obtained from 3D Linear Finite Element Method. The stiffness from IMS is smaller than that from 3D FEM, indicating deterioration of RC girder. Besides, to predict the failure mode and capacity of RC girder, 2D Non-linear Finite Element Method. It was founded that the RC girder occurred shear and anchorage failure. In this paper, furthermore, the Load and Resistance Factor Rating (LRFR) is conducted to evaluate the performance of the bridge which has been designed by old LRFD specification. The results show that the moment rating factor and the shear rating factor for the designed load rating is equal to 1.20 and 1.01, respectively. From LRFR, the bridge still has sufficient capacity to carry both the AASHTO LRFD-design live load HL-93 and the AASHTO legal loads.

This study presents a new bond model based on the deterioration mechanisms observed during uniaxial tensile tests of reinforced concrete prismatic specimens subjected to freeze-thaw cycles. The test results demonstrated that the bond stress reduced, even though the specimens were exposed only to temperature fluctuation without the damage in concrete cover. The proposed model considers the bond deterioration mechanism caused by the temperature fluctuation in addition to the damage in concrete cover. Finally, the proposed model shows good agreement with the test results regardless of the dominant deterioration mechanism.

An examination including an evaluation of mechanical properties of concrete that had suffered combined deterioration from fatigue and frost damage was done using cylindrical specimens. The order of deterioration and degree of deterioration of the specimens were used as variables. The examination clarified that certain mechanical properties of concrete that had undergone combined deterioration were able to be evaluated by measuring the propagation speed of ultrasonic waves. The decrease in the elastic modulus and the ultrasonic velocity that accompanies the increase in the number of freeze-thaw cycles was smaller in the specimens that experienced fatigue first and then frost damage than in the specimens that experienced frost damage only. The values for mechanical properties (e.g., compressive strength, elastic modulus, and shrinkage strain) of the specimen that experienced frost damage first and then fatigue had already greatly decreased after the application of freeze- thaw cycles. Therefore, the decrease in the mechanical properties was small even with increases in the number of loading cycles. The decrease in the fatigue life of the specimen in the fatigue test was proportional to the deterioration from the preceding frost damage.

Concrete structures can be deteriorated in extreme service conditions especially for fire incidents. For the evaluation of the post-fire performance, analytical methods are very powerful and useful. However, widely applicable mechanical models of fire-damaged material have not yet been proposed. To develop such models, the scale of control volume considering strength, stiffness and deformability is one of important factors. This paper presents an experimental study in which visual observations and mechanical properties of oven-dried mortar after exposed to the fire curves of ISO 834 and ASTM E119 for 90 minutes and to be left to cool down in air were conducted. Mortars were made from two different types of fine aggregate and cement. Experimental results indicate that the raw material made mortar is a prominent factor to introduce the different fire damage characteristics. Meanwhile, the mechanical properties of mortar in fire problem are strongly related to temperature that mortars were experienced to.

Bond performance of RC strengthened with polyurea resin and strand sheet depends on the concrete strength and the amount of polyurea resin. The present experimental study was undertaken in order to confirm the effect of above factors in the bond behaviour of concrete - strand sheet with a layer of polyurea resin by means of one side pull-out bond test method. Consequently, the specimen of the low strength concrete showed lower ultimate bond strength than that of the normal strength concrete at the same amount of polyurea resin regardless of presence or absence of the polyurea resin. However, the specimens with polyurea resin showed far higher ultimate strength than that of the specimens without polyurea resin regardless of the concrete strength. Furthermore, the bond strength was almost the same in the range of polyurea resin coating amount of 0.5-3kg/m2.

The fiber-reinforced polymers (FRP) were used over the past years to strength the reinforced concrete (RC) beams and slabs by attaching it to their tension face. Despite considerable research over the last decade, reliable model to simulate the performance of strengthened beams still need more research. In this paper, a nonlinear numerical analysis code based on the rigid body spring model (RBSM) was extended to simulate the behavior of (RC) beams strengthened with externally bonded (FRP). The code supports the nonlinear constitutive laws for the different materials and nonlinear bond stress-slip relationships for steel - concrete and FRP-concrete interfaces. Previous experimental programs were used to validate the the model and confirm its ability to simulate the experimental observations. In all cases, very good agreement between the analytical and the experimental observations is obtained in terms of deflections, strains, internal forces, and failure mode, showing the capabilities of the model to evaluate the efficiency of proposed strengthening solutions.

The purpose of this study is to grasp the change in mechanical behavior of meso-scale mortar specimens damaged by Ca leaching. Particularly, thin mortar specimens were immersed in demineralized water and NaCl solution for 0, 56, 112 and 168 days. After immersion tests, three points bending tests and double direct shear tests were carried out in order to determine elastic modulus, tensile strength, fracture energy and tensile softening curve from load displacement curves using back analysis and shear strength. In addition, chemical analysis was conducted so as to clarify how amount of calcium hydroxide and C-S-H and in the porosity quantitatively. Consequently, it was found that the change in mechanical behavior was strongly related to the change in the amount of cement hydrate and porosity. Furthermore, mechanical models that can consider the change in porosity were constructed and were evaluated adequacy itself.

This paper presents the flexural strengthening effect of CFRP sheets on prestressed concrete (PC) beams having strands ruptured at the midspan. The increase in the number of sheets resulted in the reduction of tensile stress of the remaining prestressing strands. However, the increase in the number of sheets with insufficient sheet lengths did not provide a higher flexural strength because of the debonding from the sheet ends. The increase in the sheet lengths not only prevented the latter failure but also significantly improved the beam stiffness. More importantly, the debonding of the sheets was avoided by wrapping the transverse sheets for the whole beam span.

Previously, beam bending tests had been conducted on strengthened RC beams with polyurea resin layer, which has low elastic modulus and high elongation, between the CFRP strand sheet (strand sheet for short) and concrete surface. It was found out that polyurea resin layer improves the bonding ability between the strand sheet and concrete surface. In this study, in order to examine the bonding ability of strand sheets with polyurea resin layer, a series of bonding test was carried out. Especially to verify the applicability of this method at elevated temperature for structure such as a chimney, bonding test at different temperature were selected. As a result, it was found out that bonding strength with polyurea resin was about 2.8 times more than that without polyurea resin at room temperature. Furthermore, it was found out that sufficient bonding performance can be achieved even at high temperature.

How volume change of the cement hydrates affects mechanical characteristics of mortar after calcium leaching are discussed and prediction equations for elastic modulus, tensile strength, fracture energy and shear strength are proposed. Furthermore an analysis of mass transfer with chemical reaction based on a finite volume method is developed and is coupled with a mechanical analysis based on the RBSM. The validity of the coupled analytical method is discussed.

A new bond model for reinforced concrete members considering material deterioration induced by freezing-thawing action has been developed. The developed model with the extensive applicability from non-damaged to severely damaged members can reasonably describe deterioration mechanism between steel bar and concrete.

Reinforced concrete T-beam is widely used in civil engineering structures and buildings. However, the evaluation method based on the shear resistance mechanism considering the effects of the top fange area has not been established in current structural design codes. The purpose of this study is to clarify the shear failure mechanism of RC T-beams with stirrups. The shear failure behaviours of the RC T-beams were investigated numerically with a three-dimensional nonlinear finite element method in which the width of the top flange, shear reinforcement ratio were varied. As a result, increasing in flange width of T-beam gives higher shear capacity with nonlinear relationship. The principal strain distribution and stirrups strain development of T-beams were different from the rectangular beam. In T-beams, the average shear stresses of web area in top flange were almost the same at the ultimate state, regardless of the flange width. In addition, the effect of shear reinforcement with respect to the increase of the average shear stress in fange area was not observed. The RC T-beam has a unique shear failure mechanism which is caused by T-shape cross-section and the horizontal crack.

On RC beam members damaged by freeze-thaw cycles, the authors performed nonlinear finite-element analysis using distributed reinforcement and smeared cracking models, toward evaluating the structural properties of such RC beams. We then compared the analysis results to test results. The comparison found that the accuracy of analysis depends on the severity of concrete deterioration. It was clarified that the test values do not agree with the analysis values for RC beams that experience shear rigidity reduction from freezing damage to the compressive area or for RC beams that experience decreased rebar-concrete bonding strength from freezing damage to the tensile area. It was also clarified that the analysis may enable the evaluation of the rigidity to yield the failure mode and the maximum load for RC beams that have experienced no major decreases in shear rigidity and bonding strength. © 2014 4th International Conference on the Durability of Concrete Structures.

A strengthened method using CFRP strand sheets, which consist of bunch of hardened continuous fiber strands, and polyurea resin (shortly polyurea) with low elastic modulus have been newly developed by authors. In the method, layer of polyurea is inserted between concrete surface and FRP with the aim of improving bonding ability of the strand sheet. In this study, two kinds of test, bonding test and flexural test, were performed to clarify those effects of the polyurea soft layer on bonding ability of the strand sheet. To clarify the influence affected by temperature, bonding tests were carried out at two different temperatures, -20°C and 23°C. It was proved that inserting a polyurea soft layer was effective for improving bonding ability between the strand sheet and concrete surface at low temperatures and room temperatures, and flexural capacity of concrete beams strengthened with the strand sheet.

Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP-strengthened RC tensile members. A two-parameter fracture energy-based model was deployed to represent the bond-slip behavior of the FRP-to-concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond-slip behavior of the FRP-to-concrete interface. © 2011 Computer-Aided Civil and Infrastructure Engineering.

Loading tests were performed on RC beams with the depth and location of frost damage as variables. The results showed which side of the beams was damaged by frost (i.e., the compression side or the tension side) and indicated that the maximum bearing capacity, deformation properties and failure mode varied significantly with the extent of the deterioration. Specifically, with respect to deterioration on the compression side, deeper frost damage resulted in impaired performance in terms of rigidity and displacement under the maximum load, and the failure mode was more likely to shift to that of diagonal compression. For deterioration on the tension side, it was found that shallower frost damage corresponded to better deformation performance, with the failure mode shifting to a tied arch mechanism due to bond deterioration. However, deeper frost damage caused brittle diagonal tensile failure and may have resulted in reduced deformation performance.

We have developed advanced CFRP strand sheets which consist of bunch of hardened continuous fiber strands. It is assumed that the bonding ability of the strand sheet can be improved by putting a soft layer in between the strand sheet and concrete surface. In this study, the polyurethane resin was used to form a soft layer. In order to clarify the effectiveness of the polyurethane soft layer on the bonding ability of the strand sheet, bonding tests and beam bending tests in which the strand sheet was attached on concrete surface using epoxy resin with or without the polyurethane soft layer were carried out. As a result, it was found out that the polyurethane soft layer was effective for improving bonding ability between the strand sheet and concrete surface and flexural capacity of concrete beams strengthened with the strand sheet.

A two-dimensional (2D) nonlinear numerical analysis code by using the rigid body spring method (RBSM) was developed by the writers at Hokkaido University to simulate the behavior of reinforced concrete (RC) members strengthened with fiber-reinforced polymer (FRP) sheets. The code supports the nonlinear constitutive laws for the different materials and nonlinear bond stress-slip relationships for steel-concrete and FRP sheet-concrete interfaces. This study uses the aforementioned code to examine the uniaxial tension behavior of RC members strengthened with carbon fiber sheets (CFS). Experimental results are compared with relevant numerical outputs to validate the model and confirm its ability to simulate the experimental observations. This study also assesses the influence of the amount of CFS strengthening on the tension-stiffening behavior of strengthened members. Finally, this research also suggests new analytical expressions for the average stress-strain relationships of concrete and steel in tension in the presence of stiffening contributions from internal steel reinforcement bars and externally bonded CFS reinforcement. DOI: 10.1061/(ASCE)CC.1943-5614.0000175. (C) 2011 American Society of Civil Engineers.

The purpose of the present study was to determine the change in tensile behavior of a meso-scalc mortar specimen immersed in a solution of NaCl. In particular, mortar specimcns were immersed in a NaCl solution for 2 months, and then three-point bending tests and chemical analyses were conducted to determine the influence of the cement hydrate (CH and CSH) concentration on the tension-softening behavior of the material. To this end, tension- softening curves were obtained from load-displacement curves via a back analysis. Wc found that tensile strength and fracture energy were reduced as the concentration of CH decreased. Furthermore, the tensile-softening model for the deteriorated mortar was proposed. ©2010 by Hokkaido University Press All rights reserved.

Concrete would be deteriorated by various factors in severe environment. Chemical erosion of concrete in acidity environment is one of important deterioration mechanism. It is not clarified, however, how chemical characteristic change affects mechanical characteristics. Meanwhile, a meso-scale approach in which both aggregate and mortar can be modeled is on the verge of becoming possible to apply to such durability problems. In this paper, the relationships between chemical characteristic change (amount of CH and CSH) and mechanical characteristic change are examined experimentally and then the mechanical behavior is simulated by Rigid Body Spring Method which is a typical meso-scale analysis. In particular, meso-scale constitutive laws are obtained from the bending tests of deteriorated mortar specimens after immersion test. Finally the applicability and availability of the analysis with the developed constitutive laws are discussed based on comparison with experimental. © 2010 Taylor & Francis Group, London.

When attaching continuous fiber sheets on concrete members on site, bonding defection of the continuous fiber sheets likely happens. FRP strips could solve the problem associated with the continuous fiber sheets, though they also have demerit due to less adhesion property caused by small adhesive area. To solve these problems, we have developed advanced FRP strand sheets which consist of bunch of individually hardened continuous fiber strands. The FRP strand sheets can be easily attached on the surface of concrete members with a single adhesive. Thus, the advanced strand sheets assure better construction quality and enable to cut down on application time and/or costs. Series of tensile, lap splice, and bonding tests were conducted in order to evaluate the material characteristics of CFRP strand sheets. Bending test of RC (Reinforced Concrete) beams was also conducted to evaluate strengthening effect of the CFRP strand sheets. In each test, specimens strengthened with CFRP strand sheets, with ordinal carbon fiber sheets or with CFRP strips were prepared. In the bending test, it was observed that both bending stiffness and capacity of RC beams were increased by strengthening with CFRP strand sheets. It was also observed that ultimate load, which was determined by delamination of strengthening material; of specimen strengthened with CFRP strand sheets was the greatest among all the specimens. Furthermore, the influence of concrete strength and depth of cover concrete on flexural strengthening effect of the FRP strand sheets were investigated.

Mesoscale constitutive models of frost-damaged concrete are developed in this study through numerical simulation using a two-dimensional rigid body spring model (RBSM). The aim of the simulation is to predict the macrobehavior of frost-damaged concrete subjected to mechanical loading. The models also clarify the difference in failure behavior of concrete with and without frost damage. Zero strength elements and the concept of mesoscale plastic tensile strain are introduced into the normal RBSM springs to consider the experimentally observed cracking and plastic deformation caused by frost damage. The difference in the effect of frost damage on compression and tension behavior as found in the experiments is clearly predicted. Finally, analysis of a notched beam subjected to bending after different degrees of frost damage is carried out. The resulting load-deflection curves agree well with those obtained in the experiments. These good correlations confirm the applicability of the mesoscale model for predicting the macrobehavior of frost-damaged concrete. © ASCE 2009.

Analytical system for mortar failure under time-dependent loading was extended to high strain rate range by improving the time-dependent constitutive model of Rigid Body Spring Model. Using the model, experimental results of mortar specimens subjected to high strain rate loading and fatigue loadings were well simulated. Furthermore, damage accumulation of mortar under fatigue loading was explained in terms of the degradation in residual strength. It was confirmed analytically that damage accumulation of mortar is not proportional to number of loading cycles but suddenly increase near the failure point.© 2009 Taylor & Francis Group.

Mesoscopic analyses of mortar failure under high-stress creep and low-cycle fatigue loading are presented using a newly developed time-dependent constitutive model for Rigid Body Spring Model, which is a discrete analysis method. The failure process over time was successfully expressed by adopting a four-component combined mechanical model as the time-dependent model of connected springs, and by developing a new method for determining the failure state for load-controlled analysis. The numerical model provides reasonable results not only for the stress-strain characteristics under cyclic loading but also for the inapplicability of Miner's law under varying stress levels. The mechanism of the time-dependent failure of mortar was clarified by investigating the local stress-strain behaviors. © 2008 Japan Concrete Institute.

This study attempted to develop a model for the stress-strain relationship in compression of frost-damaged concrete subjected to fatigue loading. Concrete specimens were prepared and exposed to freeze-thaw cycles followed by application of static and fatigue loading. The strains induced during the freeze-thaw test were carefully measured as well as during a mechanical loading test. It was found that the static strength and the fatigue life of concrete decreases as increasing irreversible tensile strain was induced by frost action. A stress-strain model for frost-damaged concrete under application of static and fatigue loading based on the degradation of initial stiffness caused by frost damage was presented. The degradation of initial stiffness for damaged concrete was empirically formulated as a function of remaining expansion caused by freeze-thaw cycles. The plastic strain under the application of mechanical static and fatigue loading for frost-damaged concrete is higher than that for original concrete. Therefore, plastic strain for damaged concrete was formulated as not only the function of strain level under mechanical loading, but also the function of irreversible strain caused by frost action. The unloading and reloading stiffness factors were introduced to explain the change of stiffness as increasing the number of loading cycles by considering the effect of the degree of frost damage. © 2008 ASCE.

As is well known, in the current design code, the shear strength of beams can be calculated based on the modified truss theory, which cannot take into account the effects of the top flange area of T-beams. Reported experimental data show that the top flange has an effect on the shear capacity of T-beams with shear reinforcement. To predict the shear capacity of T-beams more precisely, the effect of the concrete top flange area on the shear resisting mechanism must be clarified. Comparison of test results for rectangular and T-beams yielded insights into the shear resisting mechanism of T-beams. Verification and clarification of the shear resisting mechanism of T-beams were performed based on the 3D nonlinear finite element code (CAMUI). Finally, a simplified method for determining the failure criteria for shear of RC T-beams is proposed. Copyright © 2007 Japan Concrete Institute.

High Performance Fiber Reinforced Concrete (HPFRC), which has a high compressive strength of about 130 N/mm2 and a high tensile ductility by the addition of short steel fibers, was used in this study. Axial tensile tests on reinforced HPFRC members were conducted in order to investigate the influence of fibers on tension stiffening. Therefore the fiber content and the dimensions of the fibers (aspect ratio) were varied in the HPFRC mixture . The average concrete stresses, which were derived from the test results after correction for the initial shrinkage strain, showed that an increase e of the fiber content and a larger aspect ratio lead to a higher tension stiffening. Moreover, a simplified equation to determine the tension stiffening in HPFRC was proposed on the basis of the axial tensile test results.

Based on a series of experimental tests on notched concrete beams externally bonded with unidirectional fiber-reinforced polymer (FRP) sheets, this paper investigates the bond characteristics of FRP sheet-concrete interfaces under dowel load, which acts vertically on the FRP sheet and leads to a mix-mode interface peeling. The peeling properties of FRP sheet-concrete interfaces under the dowel load are evaluated in terms of their interface dowel load-carrying capacity, critical interface peeling angle, and interface peeling fracture energy. Experimental parameters include strength of concrete substrate, tension stiffness of FRP sheets, properties of bonding adhesives, concrete surface treatment methods, and length of precrack set between the FRP sheet and concrete substrate. Analytical models clarifying the relationships among the interface dowel load-carrying capacity, the interface peeling angle, and the interface peeling fracture energy are built up and also verified by test results. Further, this paper shows how to use the interface peeling fracture energy calibrated from the present dowel tests for the practical design of spalling prevention, which is now becoming a popular application of FRP sheets for the maintenance and repair of existing concrete structures in Japan. © 2007 ASCE.

FEM in combination with an appropriate material model may serve as a suitable tool to analyze the structural performance of UHPFRC. Some reported constitutive models considering the effect of steel fibers are installed into the 3D FE program. The comparisons between the numerical and the experimental results are performed to confirm the applicability of the proposed models from the view point of structural performance. Moreover, a sensibility analysis is conducted to evaluate the effect of the material parameters on shear behavior of beams with UHPFRC. A new concept for a combination of the constitutive models in FEM is introduced in the present study. In the analysis, the constitutive laws are varied for different fiber directions in a numerical specimen following the phenomenal fact that orientation number differs for different considering directions in 3D space. Finally, the advantage of simulation by the proposed technique compared with current FE methods and test results are demonstrated on the structural level. © 2006 Taylor & Francis Group.

The pullout test is a conventional test method for calibrating interfacial shear bond characteristics of Fiber Reinforced Polymer (FRP)-concrete interfaces. However, due to the small bending stiffness of FRP sheets/strips and the highly non-linear interface fracturing mechanism, a well-recognized analytical approach to the accurate interpretation of the pullout test results remains to be achieved despite extensive studies particularly when the aim is to calibrate a local bond stress-slip model, which is necessary for developing bond strength and anchorage length models avoiding the use of empirical formulations. This paper introduces a newly developed non-linear bond stress-slip model for analyzing full-range strain distributions in FRP and shear bond stress distributions in the interface bond layer during pullout tests, along with a new anchorage length model and bond strength model that were developed accordingly. Compared with other existing bond models, the bond model described here has two advantages besides its simplicity: (1) it incorporates the most important interface parameter, the so-called interfacial fracture energy, in all analytical processes and links it successfully with all other important bond parameters; (2) it is a general and unified approach that allows for the first time consideration of the effects of the adhesive bond layer in non-linear analysis of FRP-concrete interfaces. Further, a unified bond stress versus slip expression is formulated to show the differences in local bond stress-slip relationships at the loaded and free ends in pullout tests, so that the effects of the bond length used in a pullout test on the calibration of the interfacial bond stress-slip model can be clarified. The reliability of all proposed models is verified through a comprehensive comparison of the experimental and analytical results. Copyright © 2006 Japan Concrete Institute.

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.

A new analytical method for defining the nonlinear bond stress-slip models of fiber reinforced plastics (FRP) sheet-concrete interfaces through pullout bond test is proposed. With this method, it is not necessary to attach many strain gauges on the FRP sheets for obtaining the strain distributions in FRP as well as the local bond stresses and slips. Instead, the local interfacial bond stress-slip models can be simply derived from the relationships between the pullout forces and loaded end slips. Based on a series of pullout tests, the bond stress-slip models of FRP sheet-concrete interfaces, in which different FRP stiffness, FRP materials (carbon FRP, aramid FRP, and glass FRP), and adhesives are used, have been derived. Only two parameters, the interfacial fracture energy and interfacial ductility index, which can take into account the effects of all interfacial components, are necessary in these models. Comparisons between analytical results and experimental ones show good accordance, indicating the reliability of the proposed method and the proposed bond stress-slip models.

Concrete is a heterogeneous material consisting of mortar and aggregate at the meso scale. Evaluation of the fracture process at this scale is useful to clarify the material characteristic of concrete. The authors have conducted meso scale analysis of concrete over a past few years by Rigid Body Spring Model (RBSM). In this study, three-dimensional analyses of mortar and concrete are carried out, which is necessary for the quantitative evaluation of concrete behavior especially in compression. Constitutive models at the meso scale are developed for the 3D RBSM analysis. Failure behaviors and strengths in compression and tension of mortar and concrete are predicted well by the analysis. In biaxial compression test of concrete, crack in normal direction to plane of specimen is simulated that cannot be presented by two-dimensional analysis. Copyright © 2005 Japan Concrete Institute.

In this study, a model for the interface between a concrete slab and a steel plate in composite slabs with studs is developed through a bending test for composite beams. A smeared reinforcing stud model is also implemented to consider the shear-reinforcing effect of taller studs. 3D nonlinear finite element analysis using the model for the interface and the reinforcing stud model developed in this study is found to predict punching shear behavior of composite slabs. Copyright © 2005 Japan Concrete Institute.

The mechanism of diagonal tensile failure of RC beams without shear reinforcement, which is difficult to solve by means of experimental and analytical study, is investigated. The close relationship between the fracture modes and the transfer stress at shear cracks is clarified, and the experimental results are verified by the finite element method taking into consideration the influence of a splitting tensile crack and dowel action. In RC members without shear reinforcement, the width of a shear crack increases owing to the occurrence of a splitting tensile crack along the main bars. As a result, the transfer stress of a shear crack cannot be generated and the shear crack grows and propagates rapidly. This paper also puts forth that FE analysis, in which not only shear crack but also splitting tensile crack are modeled discretely, can predict the size effect on diagonal tensile failure strength of RC beams without shear reinforcement.

Concrete is a heterogeneous material consisting of mortar and aggregate at the meso level. Evaluation of the fracture process at this level is useful to clarify the material characteristic of concrete. However, the analytical approach at this level has not yet been sufficiently investigated. In this study, two-dimensional analyses of mortar and concrete are carried out using the Rigid Body Spring Model (RBSM). For the simulation of concrete, constitutive model at the meso scale are developed. Analysis simulates well the failure behavior and the compressive and tensile strength relationship of mortar and concrete under uniaxial and biaxial stress conditions. Localized compressive failure of concrete is also simulated qualitatively.

This study investigates the dependence of the mechanical behavior of concrete, such as strength, stiffness, and deformation capacity on the damage caused by freezing and thawing cycles (FTC). A stress-strain model for concrete damaged by freezing and thawing prior to the application of mechanical loading was proposed based on plasticity and fracture of concrete elements. The FTC fracture parameter was introduced to explain the degradation in initial stiffness of concrete resulting from freezing and thawing damage. Based on experimental data, the FTC fracture parameter was empirically formulated as a function of plastic tensile strain caused by freezing and thawing with the assumption that the plastic strain was caused by the combined effects of FTC and mechanical loading damage. The stress-strain relationships obtained by the proposed model were compared with the experimental data.

The Mode II interlaminar fracture tests of hybrid composites with non-woven carbon tissue (NWCT) were carried out using the end-notched-flexure (ENF) specimen. The hybrid composites were made by interleaving NWCT prepreg between CFRP layers. CFRP specimen [0₂₄] as well as hybird specimen [0₁₂/T/0₁₂] were prepared to compare the Model II interlaminar fracture tough-ness. Here, the symbol "/T/" means that NWCT prepreg is located at a 0/0 mid-plane of the hybrid specimen. Three kinds of interlaminar cracks which have different positions in the NWCT layer were introduced to the hybrid specimen. It was found that the NWCT layer has the short fiber bridging effect due to in-plane random short carbon fibers and increases the interlaminar fracture toughness of laminated composites. The fracture surfaces and side-sections of the hybird specimens were observed with an optical microscope and a scanning elelctron microscope, and the failure mechanism was discussed.

This paper presents experimental results on the behavior in tension of reinforced concrete members strengthened with carbon fiber sheets (CFS). CFS reduced the crack spacing and brought about good crack width control as well as a significant change in the average bond stress and the average stress of steel reinforcing bars and concrete. The deterioration mechanism of the CFS bond properties near cracks was presented.

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.

Modeling of compression softening is essential to predict the ultimate behavior of RC members. In this study, constitutive laws of concrete under compression-tension state are extended according to the concept of the fracture energy and the influence of compression softening on ultimate behavior of RC members failing in shear compression mode was examined. It was found that the ultimate strength and deformation strongly depended on compression softening, namely compression fracture energy. It was also found that the analytical results agreed well with experimental ones, when compression fracture energy of 40 N/mm was used for normal concrete with compressive strength of 20 MPa.

The uniaxial tensile tests of Reinforced Concrete elements with Carbon fiber sheet (RCC) are conducted to clarify the basic mechanical characteristics which affect the tension stiffness of RCC. This paper mainly presents the difference between RCC and ordinary Reinforced Concrete member (RC) in the load carrying capacity, the average crack spacing, stress and/or strain distributions of steel, and the average stress - strain relationship of concrete. Crack spacing of RCC becomes smaller between 25% and 60% of that in RC as the amount of Carbon Fiber Sheet (CFS) increases. The strain distributions of steel in RCC before and after the yielding of steel differ from those in RC. The tension stiffness developed by the bond action of CFS increases as the amount of CFS increases but that of steel becomes less. With the steel and CFS contributions combined, the tension stiffness of concrete as a whole generally becomes greater than that in RC. There is, however, a case in which tension stiffness of concrete in RCC decreases more than that in RC, because the average bond stress of steel rapidly decreases after the yielding of steel.

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 100 mm. 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 confmement from the bolt. From the observed bond stress in CFS, the equation to predict the maximum local bond stress was proposed.

The simple-supported concrete beams reinforced with aramid FRP (AFRP) rods and carbon FRP (CFRP) sheets were tested to failure using a symmetric two-point concentrated static loading system. AFRP rods were used instead of steel reinforcing bars, and CFRP sheets were epoxy bonded to the tension face of the concrete beams to enhance their flexural strength. Moreover, a 5-cm wide strip of CFRP sheet in some places were wrapped around the web (hereafter, called "U· jacket") after the CFRP sheets were bonded. The strain distributions on AFRP rods and CFRP sheets, and flexural behavior of the beams with AFRP rods and CFRP sheets were examined experimentally. The results showed that; 1) peeling of CFRP sheets occurred near the maximum flexural moment region, 2) ultimate load and deflection of the beam with U-jackets were higher, and 3) the U-jacket was a significant factor affecting the ductile behavior of beams with CFRP sheets.

The authors have developed a continuous fiber flexible reinforcement (CFFR) which has good workability and can be used as a reinforcement in concrete. It consists of a plastic tube containing continuous fiber bundles and resin. The salient feature of this new material is that it can be easily formed at the construction site. The authors have conducted a study of fabrication methods of the material and an evaluation of its mechanical properties. This paper introduces the material and fabrication methods, and details the tensile strength of a member formed from this material. A member bent into a U shape was subjected to a load test to examine its basic properties. As a result, it was found that even when the material was bent, the applied tensile load was transmitted through it. Also, the load-deflection characteristic of the material and the stress concentration due to bending were influenced by the wall thickness of the plastic tube.

This study shows how reducing the thickness of the tube enclosing continuous fiber flexible reinforcement (CFFR) and providing mechanical anchorage at the end of CFFR improve the shear capacity of a beam specimen. It is also clarified in this study that from tensile tests on U shaped specimen of CFFR the shear capacity in a concrete beam reinforced with CFFR is greatly influenced by the deformational characteristics of CFFR, i.e., a slippage at bent portion of fibers and deformation of tube covering the fibers.

The aim of this study is to clarify the strength and the deformational ability of recycled concrete. It was clarified that the deformational behavior of recycled concrete filled steel tube (RCFT) was similar to that of normal concrete filled steel tube (CFT). It was shown that the stiffness of RCFT could be predicted approximately with consideration of the Young's modulus of recycled concrete which were lower than that of normal concrete. It was also shown that of the dilatancy in the elasto-plastic range, which is related to the Poisson's ratio, is important to predict the strength.

Simple-supported concrete beams reinforced with aramid FRP (AFRP) rods and carbon FRP (CFRP) sheets were tested up to failure under a symmetric two-point load application. AFRP rods were used instead of steel reinforcing bars, and CFRP sheets were bonded on the tension face. Moreover, in one specimen, three 5-cm wide strips of CFRP sheeting were wrapped around the web (U-jacket) after the CFRP sheets were bonded. The results showed that; 1) both the ultimate load and deflection of the beam with U-jackets were highly improved, and 2) the calculated deflection and strain correlated well with the measured values before the peeling failure.

The property of recycled concrete column encased by steel tube subjected to axial compression was studied. It was found that the column, where progress of fracture is faster than the normal concrete, has enough capacity to be utilized though its rigidity and ultimate strength are smaller than those of the normal concrete.

In this study, the shear resisting behavior of reinforced concrete (RC) beams with carbon fiber sheet (CFS) was examined experimentally. Shear strength of RC beams with CFS attached to the sides is defined as the summation of the shear force carried by concrete and the shear forces carried by stirrup and CFS in shear cracking zone. The concept for calculation of the shear forces carried by stirrup and CFS is also discussed.

In this study splitting crack of cover concrete due to corrosion of reinforcing bar and the critical volume increment of the reinforcing bar which causes full penetration of the splitting crack to the concrete surface are analyzed by a nonlinear finite element method. As a result, relationships between the dimension of the surrounding concrete, which is the concrete cover and the bar spacing, and the characteristics of the splitting crack, which is its direction and the critical volume increment are clarified.

Based on the finite element analyses, the equation for shear strength of deep beams without shear reinforcement is modified to predict the shear strength of deep beams with horizontal and vertical shear reinforcement. It is confirmed that the modified equation can estimate the experimental results.

Experiment on Fiber Reinforced Plastic (FRP) rods as shear reinforcement with the intersection of a shear crack in a beam is carried out in order to clarify the failure criterion of the FRP rod at the bent portion. The finite element analysis in which anisotropy of the FRP rod and bond slip between concrete and the FRP rod are considered, is also conducted. Strain and bond stress distributions of the FRP rod with bent portions are investigated in detail.

Non-linear finite element analysis is conducted in order to clarify the shear resisting behavior of prestressed concrete beams reinforced with FRP rods. Prestressing force, stiffness of tendon and web reinforcement are chosen as parameters. It is discussed how prestressing force and mechanical properties of reinforcement influence the shear strength as well as the shear force components in the shear resisting behavior.