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Carbon fiber-reinforced polymer (CFRP) materials have been widely used in prestressed and non-prestressed systems for the flexural strengthening of concrete structures in many construction projects worldwide. Strengthening with non-prestressed CFRP materials, which is known as passive repair, could lead to an increase in the ultimate capacity of the concrete member. However, it has little effect on its serviceability performance (e.g., cracking, yielding, and deflection properties). Flexural strengthening with prestressed CFRPs provides an active repair technique that positively effects the strength and serviceability of structures. In addition, strengthening with prestressed fiber-reinforced composites efficiently uses the material's high tensile strength. Although different fiber-reinforced polymer (FRP) materials and forms have been used in prestressing applications, this paper focused on the use of CFRP laminates (e.g., sheets, strips, or plates). These laminates have been widely used due to their excellent mechanical properties, ease of application in various geometrical shapes, good conforming surface area of contact with structures, and low installation costs. A comprehensive literature review was presented on strengthening using prestressed CFRP laminates in conventionally and internally prestressed concrete structures (PCSs). The externally bonded and unbonded prestressed CFRP laminate applications that used different anchorage systems were presented. The flexural behavior, failure modes, and serviceability performance of the strengthened concrete members were discussed. The effects of the prestressing level on ductility and energy absorption and a summary of the recommended optimum prestressing level discussions were presented in this paper. Furthermore, a review of the theoretical studies that were conducted for prestressed CFRP applications was provided. The knowledge gaps in the research area and future research recommendations were presented. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Shanghai University / Shanghai Daxue Xuebao is the property of Journal of Shanghai University (Natural Sciences) Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Railway Construction Technology is the property of Railway Construction Technology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This is a proposal for a nonlinear analytical model that was developed to predict the structural response of post-tensioned beam with unbonded tendon. The structural response includes deflections under the service load before or after cracking, as well as the ultimate load capacity. A comprehensive procedure for the deflection calculation was implemented, considering the additional shear deformation and tension stiffening effect. The shear deformation calculation is based on the Timoshenko beam theory, considering the shear modulus reduction upon cracking. Furthermore, a modification of the tension stiffening model from CEB–FIP Model Code 1990 is proposed to account for the significant strain concentration in the post-yielding region. A fixed-point iterative procedure is introduced in the prediction model to compute the incremental strain in the unbonded tendon. Each iteration brings the estimated strain increase in the tendon closer to the actual value until it is reasonably accurate. Predictive accuracy of the proposed model in both load–deflection relationships and strain rates, as a function of deflections, is successfully validated against experimental beam tests and a nonlinear finite element analysis considering concrete plasticity, respectively.

Pre-stressing concrete is commonly used in construction. One of the challenges that pre-stressing concrete (PC) faces is that fire exposure reduces the strength and serviceability of PC elements, which requires unique design considerations. The pre-stressing strands are more sensitive to high-temperature, deform larger than mild steel, and break quicker. This research investigates the thermal and structural response of totally unbonded pre-stressing T-beams subjected to increased temperatures and continuous service loads. Thermal coating was proposed to avoid damage and load carrying capacity reduction under fire loading. Seven medium-sized post-tensioned concrete beams were tested in a fire exposure (600°C for 3 h) in order to examine the effect of the following variables: presence of thermal coating, the use of different pre-stressing ratios, increasing the concrete cover, and studying the fire effect at the end anchor, which represents the weakest point at the unbonded system. The proposed insulating technique improved pre-stressing beam behavior during fire exposure by improving crack distribution and lowering strength loss. The thermally insulated beam with a high pre-stressing ratio attained 99.3% of the control specimen load carrying capacity, compared to 86% for the unprotected beam. The deflection of the thermally isolated beam was 7.8 mm, compared to 22.7 mm for the unprotected specimen after 180 min of fire exposure. The use of low pre-stressing ratio caused the insulated pre-stressing strands to rapidly deform with low increases in temperature, which caused rapid crack formation. [ABSTRACT FROM AUTHOR]

The rapid progression of mass timber structures towards greater spans presents a challenge for the flexural behavior of glulam beams. In this study, this challenge was addressed by reinforcing the glulam beams with prestressing tendons which were bonded to the glulam beams by epoxy resin. The objective of this study is to investigate the influence of prestressing force levels and bonded types on the flexural behavior of prestressed glulam beams. Four-point bending tests were conducted on twelve glulam beams prepared by unreinforced, unbonded prestressed or bonded prestressed techniques. Experimental results revealed that the glulam beams exhibited increased ductility under prestressing force. Compared with unreinforced beams, the prestressed beams demonstrated a remarkable increase of up to 78.8 % in flexural capacity and 13.5 % in flexural stiffness. For the prestressed beams, the flexural capacity increased as prestressing force increases. Compared with the unbonded prestressed beams at the same prestressing force level, the bonded ones exhibited higher flexural capacity and flexural stiffness. Additionally, the bonded prestressed beams exhibited a slight increase in tendon stress at the end of the prestressing tendon during loading, indicating favorable bonding behavior of epoxy resin. Numerical models were developed to predict the non-linear flexural behavior of prestressed beams, and the numerical results showed good agreements with the experimental results. Parametric analysis based on the numerical models was conducted on the prestressed beams at various prestressing force levels. The positive effect of the bonded prestressed method on the flexual behavior of glulam beams was further substantiated. • Bending tests were conducted on unbonded and bonded prestressed glulam beams. • The influence of prestressing force levels and bond types was investigated. • Numerical models were developed to predict the nonlinear flexural behavior. • The enhancement mechanism was further elucidated through parametric analysis. [ABSTRACT FROM AUTHOR]

For tall buildings, wind demand under extreme wind loads is so large that design based on conventional elastic behavior can be difficult. A practical solution is to permit inelastic behavior to introduce hysteretic damping and reduce design wind force. Therefore, the inelastic behavior of structures subject to wind loads should be thoroughly investigated. The presumption is that applications currently applied in inelastic seismic design can be employed in inelastic wind design of concrete buildings. In this research, behaviors of elastic, bilinear, and self-centering single-degree-offreedom systems under along-wind load were studied for various design wind speeds using nonlinear time-history analysis. Selfcentering systems—in particular, unbonded post-tensioned concrete systems—given their wide use and high potential and flexibility for self-centering behavior, were found to have smaller maximum displacement than bilinear systems, with differences being considerable at higher wind speeds. Results found highlight the role of post-yield stiffness in reduction of maximum displacement. Self-centering systems were also found to be highly influenced by reverse-yielding values. Systems with reverse yield strength equal to the standard deviation of the elastic force (approximately 20% of [mean + background + resonant components]) demonstrated minimum displacement. In addition, design parameters such as ductility, overstrength, and displacement factor are reported for inelastic design based on the concept of a response modification factor. [ABSTRACT FROM AUTHOR]

This paper compares the flexural performance of two one-thirdscale models of a two-way unbonded post-tensioned flat plate. One of the slabs had a conventional uniform-banded tendon arrangement and the other had a banded-banded tendon arrangement. The models had three bays in each direction and each bay was 10 x 10 ft (3 x 3 m). The specimens were tested using a whiffle tree system to approximate a uniformly distributed load, and were loaded to service and factored load multiple times, then loaded to failure. The performance of the banded-banded specimen was very similar to the uniform-banded specimen at service and factored load. The failure loads for the banded-banded and uniform-banded specimens were 310 and 321 lb/ft2 (14,840 and 15,380 N/m²) of total load, respectively—considerably higher than the design factored load of 197 lb/ft² (9430 N/m²). [ABSTRACT FROM AUTHOR]

This paper investigates the feasibility and flexural behavior of reinforced concrete beams internally prestressed with straight unbonded aluminum alloy tendons by testing five partially prestressed beams and one reference beam. For each beam specimen, load-deflection curves, failure modes and cracking behavior, the relationship between load and strains in steel and prestressing aluminum alloy tendons were examined and analyzed. In particular, the effects of effective prestress, combined reinforcement index (CRI), and partial prestressing ratio (PPR) on flexure of concrete beams were discussed. The test results indicated that the spacing and width of concrete cracks of prestressed beams containing the same amount of bonded longitudinal steel reinforcement decreased with the increase of effective prestress, and the combined reinforcement index governs flexural behavior of the prestressed beams. The flexural crack width and displacement ductility exhibited a reduction with the increase of CRI. In addition, an analytical model was established to calculate the flexural strength and corresponding deflection at midspan of the concrete beams internally prestressed with unbonded aluminum alloy tendons by suggesting a new simplified curvature distribution, which is more accordant with the original curvature distribution. The proposed model provides a relatively good estimation of the flexural capacity and midspan deflection of the prestressed beams. [ABSTRACT FROM AUTHOR]

In this study, a 4-node lattice element was formulated based on the general lattice model. The experimental program was conducted to verify the 4-node lattice element analysis. In the experimental program, four kinds of RC beam-column specimens were constructed. Anchorage shapes of beam and bonding interfaces between the reinforcement and concrete in the beam part were used as parameters of the specimen. The material properties of unbonded reinforcement used in the 4-node lattice element analysis were derived from nonlinear fiber-section analysis. The load-displacement hysteresis behavior of the 4-node lattice element analysis results are compared with that of the experimental study. The improved lattice analysis method could be used to predict the hysteresis behavior quicker and more simply. It should now be possible not only to use the prediction of the unbonded reinforcement behavior more accurately, but also to apply nonlinear dynamic analysis to general reinforced concrete structures. [ABSTRACT FROM AUTHOR]

Abstract ACI 318-14 and 423.7-14 require the use of encapsulation system for an unbonded single-strand tendon for the purpose of corrosion protection and enhanced durability. In this study, load-carrying performance and hydrostatic tests were conducted for a newly developed, unbonded post-tensioning (PT) anchor with encapsulation, following the guidelines put forth in KCI-PS101 and ACI 423.7-14. The static load and fatigue tests as part of this study were used to evaluate the anchorage system behavior under static and cyclic loads, respectively. The hydrostatic test was used to demonstrate that the encapsulation system withstood at least 9.3 kPa of water pressure in a 24 h period according to ACI 423.7-14, and thus has been proven to promote the durability of unbonded PT single-strand tendons. Additionally, the push-in and jacking tests were used to assess the compressive and tensile strengths of the anchor as used in the field. Although these tests were not performed to failure for safety reasons, the anchor endured at least 1.18 and 1.02 times the nominal ultimate strength of a strand in each test, respectively. The authors’ previous load transfer tests were also reported in this paper, showing that test specimens with only two horizontal backup bars were capable of resisting at least 1.7 times the nominal ultimate strength of a strand. Based on the favorable results from the load-carrying performance and hydrostatic tests, the developed encapsulated anchor systems are being applied in actual practice as an economically feasible option in Korean PT industry and are expected to improve the corrosion resistance and durability of post-tensioned structures.

An experiment on detecting an unbonded defect under the surface of a glass model using Rayleigh waves was executed. A 1 MHz Rayleigh wave was propagated on the glass model, and the vibration distribution on the material surface was probed by a laser beam. No information of the defect was detected from the Rayleigh wave amplitude distribution. On the other hand, the phase information of Rayleigh waves propagating on the material surface was focused on and acquired. As a result, the measured phase change for the propagation distance x (difference in phase slope) was detected in the unbonded defect area. In addition, finite element method (FEM) analysis was performed under the same conditions as in the experiment to verify the experimental results. A phase change was also observed on the unbonded defect, as in the experiment. The possibility of this method as a new detection method for the effective evaluation of unbonded defects in a material is described. [ABSTRACT FROM AUTHOR]

• Energy Conservation Approach (CEA) is used to describe load versus deflection behavior. • Post-cracking curvature is idealized as parabolic shape to estimate beam deflection. • The stress in unbonded tendon is calculated using compatibility of deformation. • Simplified CEA shows that concrete non-linear behavior can be neglected after cracking. Durability of post-tensioned concrete members is greatly enhanced by using Carbon Fiber-Reinforced Polymer (CFRP) tendons in lieu of, or in combination with, unbonded and bonded steel tendons. However, current design and analysis methods are limiting the use of hybrid (i.e., a combination of unbonded and bonded steel and CRFP) tendons since they are only applicable to either bonded or to the rather complicated post-tensioned unbonded steel tendons. This paper presents an analytical approach developed to predict the overall response of concrete beams prestressed with hybrid tendons. The model is based on a structural idealization that uncouples the concrete beam and the unbonded tendon behaving as a trussed-beam system. The approach utilizes equilibrium equations, compatibility of deflection between beam and tendon, and the conservation of energy approach (CEA) applied at various limit states. Additionally, a simplified CEA is also presented at characteristic points for various cases including unbonded as well as hybrid tendons. Results are validated using experimental tests available in the literature indicating that CEA is a versatile and rational procedure that can accurately predict the overall flexural behavior of beams with hybrid tendons. A simplified version of the CEA using hand calculations is also developed for directly analyzing such beams with comparably accurate results. [ABSTRACT FROM AUTHOR]

Highlights • Flexural behavior of ungrouted and post-tensioned masonry beams is assessed. • Behavior of ungrouted and grouted post-tensioned masonry beams was similar. • US and Canadian codes predicted accurately the flexural capacity of the specimens. • Performance of specimens is mainly affected by masonry compressive strength. • Increase of masonry strength augmented load and displacement capacity at peak load. Abstract Precast and post-tensioned systems increase the efficiency of masonry beams in terms of labor and time of construction. This study is aimed at evaluating the flexural behavior of ungrouted post-tensioned concrete masonry beams, with eccentric bars, and the effect of the main design parameters on their structural performance when subjected to monotonic and unidirectional cyclic loading until failure. All beams failed in flexure by crushing of the masonry in the compression region. Masonry strength was the design parameter that had the most significant effect on the load-deflection behavior of post-tensioned masonry beams. The test results showed that the measured nonlinear load-deflection behavior and the correlation between bar stress increase and midspan deflection can be estimated using a multi-linear relationship. Beams tested under monotonic and unidirectional cyclic loading exhibited comparable load-deflection envelope response. [ABSTRACT FROM AUTHOR]

Previous research on unbonded post-tensioned precast concrete walls included the development of a nonlinear wall model based on fiber elements and of a design-oriented simple analytical model based on generalized closed-form expressions that define a trilinear idealized wall response under combined gravity and lateral loads. Each model considers several key limit states in the lateral-load response of unbonded post-tensioned precast concrete walls. The fiber model and simple model were previously validated using limited available experimental results on unbonded post-tensioned precast concrete walls. This paper presents the results of a parametric study conducted using the fiber model and the simple model to investigate the adequacy of the simple model for a broader range of parameters. Results show that the simple model accurately represents the lateral-load response obtained using the fiber model. Furthermore, the simple model and the fiber model show similar values of base shear and roof drift response, and they show similar trends for each parameter variation. Thus, the simple model is recommended for the seismic design of unbonded post-tensioned precast concrete walls. [ABSTRACT FROM AUTHOR]

It is necessary to use large-diameter (0.7 and 0.85 in. [17.8 and 21.6 mm]) strands in unbonded post-tensioned structures to address the question on flexural response, particularly regarding its strength. A series of experiments were conducted to investigate the ultimate stress of the large-diameter strands as unbonded tendons in simply supported members. Two crucial factors—prestressing reinforcement index and global reinforcement index—are obtained from the cross-sectional equilibrium equation and test data of the present and already-existing experiments. Both crucial factors are used to derive equations to predict the increase in stress and ultimate stress of large and normal diameter strands. Evidences support the behavior and demonstrate the feasibility of large-diameter strands for PT structure in terms of static loading. [ABSTRACT FROM AUTHOR]

Introduction of prestressed concrete slabs based on post-tensioned (PT) method aids in constructing larger spans, more useful floor height, and reduces the total weight of the building. In the present paper, for the first time, simulation of 32 two-way PT slab-edge column connections is performed and verified by some existing experimental results which show good consistency. Finite element method is used to assess the performance of bonded and unbonded slab-column connections and the impact of different parameters on these connections. Parameters such as strand bonding conditions, presence or absence of a shear cap in the area of slab-column connection and the changes of concrete compressive strength are implied in the modeling. The results indicate that the addition of a shear cap increases the flexural capacity, further increases the shear strength and converts the failure mode of connections from shear rigidity to flexural ductility. Besides, the reduction of concrete compressive strength decreases the flexural capacity, further reduces the shear strength of connections and converts the failure mode of connections from flexural ductility to shear rigidity. Comparing the effect of high concrete compressive strengths versus the addition of a shear cap, shows that the latter increases the shear capacity more significantly. [ABSTRACT FROM AUTHOR]

Precast concrete facilitates a construction method using durable and rapidly erectable prefabricated members to create costeffective and high-quality structures. In this method, the connections between the precast members as well as between the members and the foundation require special attention to ensure good seismic performance. Extensive research conducted since the 1980s has led to new precast concrete structural systems, designs, details, and techniques that are particularly suited for use in regions of high seismic hazard. This paper reviews the state of the art of these advances, including code developments and practical applications, related to four different systems: (1) moment frames; (2) structural walls; (3) floor diaphragms; and (4) bridges. It is concluded from this review that the widespread use of precast concrete in seismic regions is feasible today and that the jointed connection innovation introduced through precast research leads to improved seismic performance of building and bridge structures. [ABSTRACT FROM AUTHOR]

Two prestressed steel-reinforced concrete (PSRC) frames with unbonded tendons were tested under vertical monotonic and cyclic loading to study the static and seismic behaviors. In addition, the behaviors' differences due to the different loading form were compared to investigate the influence of cyclic loading on the mechanical properties. The result shows there is no obvious change after the frame cracks and it has a good capacity and ductility index, which is 3.36 under monotonic load. The hysteretic curve under cyclic load is relatively full and possesses a pinching effect, and the maximum energy dissipation coefficient is 1.36. Before the maximum load, stiffness, load and cracks of two frames are almost similar, during which stage the maximum crack width of the two specimens was 0.11 mm and 0.08 mm, and the degradation to frame resulted from the cyclic loading is not clear. After the maximum load, the capacity decrease of frame under monotonic load is slow, while the cyclic one is rapid. Moreover, the finite element analysis was made based on the OpenSEES and the calculation result has a good agreement with the test results. [ABSTRACT FROM AUTHOR]

This paper develops a simplified design approach to predict the flexural strength of unbonded post-tensioned masonry walls. The accuracy of different flexural expressions was investigated according to experimental and finite element modelling results. Using an analytical model and considering the stress-strain relationships for unconfined and confined masonry, force displacement curves were developed for eleven tested walls, with and without confinement plates. The developed force–displacement procedure was able to predict the lateral strength, stiffness and post-peak degradation of the behavior of the tested walls. Using a similar analytical procedure, a parametric study was performed to obtain the force-displacement response of walls with different features and to investigate the effect of different parameters including axial stress ratio, length, height and thickness of the wall, on the compression zone length. Multivariate regression analysis was performed to develop an empirical equation to estimate the compression zone length in unbonded post-tensioned walls. According to the results, the wall length and axial stress ratio were found to be the most significant factors affecting the compression zone length. Depending on the configuration of the wall, the compression zone length varied between 6.7% to 28% of the wall length. The proposed equation for compression zone length was then incorporated into the flexural analysis of post-tensioned masonry walls and validated against experimental results and finite element results. Comparing the prediction from Masonry Standards Joint Committee, MSJC (2013), and the proposed method reveals that ignoring the elongation of PT bars in strength prediction resulted in a considerable underestimation of the strength. Using the non-iterative proposed approach significantly improved the prediction. [ABSTRACT FROM AUTHOR]

Elongations have been used to assess the forces inside post-tensioned tendons immediately after the post-tensioning operation. However, the measured elongations are characterized by a high degree of uncertainty and variability due to site tolerances, and that the calculated elongations are based on the ideal equations using theoretical parameters. Building codes acknowledge such variability as a limitation and include tolerances for the elongation deviations between the calculated and measured values. Even so, the specified tolerances are quite stringent and unrealistic for relatively short tendons. In this study, to resolve this issue, a statistical analysis is carried out using a large database of tendon elongations measured from actual projects of bonded post-tensioned building structures. Based on the analysis of the data collected, a practical and rational tolerance between measured and calculated elongations is proposed. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The compression or tension-controlled failure mode of concrete beams prestressed with unbonded FRP tendons is governed by the relative amount of prestressing tendon to the balanced one. Explicit assessment to determine the balanced reinforcement ratio of a beam with unbonded tendons ( $$ \rho_{pfb}^{U} $$ ) is difficult because it requires a priori knowledge of the deformed beam geometry in order to evaluate the unbonded tendon strain. In this study, a theoretical evaluation of $$ \rho_{pfb}^{U} $$ is presented based on a concept of three equivalent rectangular curvature blocks for simply supported concrete beams internally prestressed with unbonded carbon-fiber-reinforced polymer (CFRP) tendons. The equivalent curvature blocks were iteratively refined to closely simulate beam rotations at the supports, mid-span beam deflection, and member-dependent strain of the unbonded tendon at the ultimate state. The model was verified by comparing its predictions with the test results. Parametric studies were performed to examine the effects of various parameters on $$ \rho_{pfb}^{U} $$ . [ABSTRACT FROM AUTHOR]

Most research on fatigue strengthening of steel has focused on carbon fiber-reinforced polymer (CFRP) strengthening of steel members with existing cracks. However, in many practical cases, aging steel members do not yet have existing cracks but rather are nearing the end of their designed fatigue life. Therefore, there is a need to develop a "proactive" retrofit solution that can prevent fatigue crack initiation in aging bridge members. Such a proactive retrofit approach can be applied to bridge members that have been identified to be deficient, based on structural standards, to enhance their safety margins by extending the design service life. This paper explains a proactive retrofit design approach based on constant life diagram (CLD) methodology. The CLD approach is a method that can take into account the combined effect of alternating and mean stress magnitudes to predict the high-cycle fatigue life of a material. To validate the retrofit model, a series of new fatigue tests on steel I-beams retrofitted by the non-prestressed un-bonded CFRP plates have been conducted. Furthermore, this paper attempts to provide a better understanding of the behavior of un-bonded retrofit (UR) and bonded retrofit (BR) systems. Retrofitting the steel beams using the UR system took less than half of the time that was needed for strengthening with the BR system. The results show that the non-prestressed un-bonded ultra-high modulus (UHM) CFRP plates can be effective in preventing fatigue crack initiation in steel members. [ABSTRACT FROM AUTHOR]

The behavior of rubber layers under pure compression has been investigated to considerable extent in the literature. The most widely used approach is the so-called pressure solution , which is based on several assumptions, most notably that the stress state is dominated by the hydrostatic pressure. Other approaches have also been considered, but for nearly incompressible material and thin layers their predictions are very similar to those of the pressure solution. Nearly all past studies on the subject have focused on rubber layers that are bonded to either rigid or flexible supports (or reinforcement). Unreinforced (i.e., single layer) rubber pads are often installed as unbonded, i.e., without steel end plates connecting them to their top and bottom supports. In an unbonded application, rubber pads rely solely on friction to develop shear resistance along the contact interfaces. This shear resistance is necessary to provide the pad with an adequately large vertical stiffness. The effect of the frictional restraint along the top and bottom contact surfaces and the influence of partial slip have received very little attention in the literature. In this paper, we present a theoretical analysis for the behavior of an unbonded rubber layer, including the effects of the elastomer’s bulk compressibility and the contact slip at the supports. Results of a finite element analysis are also presented and shown to be in good agreement with the results of the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper reports on an experimental study on four unbonded posttensioned masonry walls (PT-MWs). All walls had identical thickness, height, and length of 190, 2,000, and 1,400 mm, respectively, and were constructed using concrete masonry units (CMUs) and mortar type N, and were fully grouted. Different horizontal spacing values of 400, 600, and 1,200 mm were used between the posttensioning bars in the walls. Only Wall W4 had horizontal bonded reinforcement, located in the fourth and seventh courses. Two different levels of posttensioning force corresponding to an average posttensioning compressive axial stress on the masonry of 1.35 and 2.7 MPa were applied to the walls. Different initial posttension stresses in the bars ranging from 0.32 to 0.63 of the yield stress of each bar were applied to the walls. The walls were subjected to incrementally increasing in-plane lateral displacement cyclic load applied to the top of each wall. The experimental results including damage pattern, force displacement response, residual drift ratio, masonry compressive strain, wall rotation, damping, stiffness, stiffness degradation, and displacement ductility of the tested walls are presented and discussed in detail. The accuracy of ignoring the elongation of post-tensioning bars in predicting the strength of the tested walls is investigated based on the experimental results. The experimental tests showed that PT-MWs having the same total initial PT force but different stress levels in the PT bars displayed different lateral strengths. Doubling the total initial PT forces in the bars resulted in insignificant increases in the wall lateral strength. Finally, using bonded horizontal steel improved the lateral displacement capacity of the test specimen. Specimens that had horizontal reinforcement did not suffer postpeak shear cracking and hence were able to reach higher level of lateral displacement. [ABSTRACT FROM AUTHOR]

The anchorage of carbon fiber-reinforced polymer (CFRP) strips using CFRP anchors is gaining acceptance in strengthening applications of concrete members. CFRP anchors can fully develop the strength of CFRP strips when adequately detailed. However, parameters that influence the behavior and strength of CFRP strips and anchors are not well understood. In this study, 26 tests on concrete beams were conducted to study the influence of five key parameters on CFRP anchor effectiveness: 1) the width of the anchored CFRP strip; 2) the material ratio of CFRP anchor to CFRP strip; 3) the concrete strength; 4) the length/angle of anchor fan; and 5) the bond condition between a CFRP strip and concrete. Results indicate that narrow anchored CFRP strips developed higher stresses at fracture than wide strips and required smaller anchor material ratios to be fully developed. Test results provide valuable data for designing anchored CFRP strengthening systems. [ABSTRACT FROM AUTHOR]

Formulas for Hertzian frictionless elastic contact are well-known and have been used for a long time. However, they are not directly applicable for a crowned roller, a cylindrical roller with symmetrically rounded corners, deflecting a half-space. For this punch shape, the approximate analytical method of Barber and Billings (Int J Mech Sci 1990; 32(12): 991–997) was adapted to compute the load–deflection curve and the contact boundary avoiding calculation of pressure distribution. Comparisons were performed with finite element method (FEM) simulations, an adaptive Hertzian quadratic punch surface approximation (AHA) and other published results. Good agreement of analytical load–deflection curves with FEM results was noted but the AHA results deviated significantly from the former two. The FEM contact area resembled a dog-bone shape, the analytical a bow-tie shape and the AHA, by design, an ellipse. Though the AHA contact area was closer to the FEM result numerically, the contact boundary aspect ratio of the analytical model was closer to the FEM. The analytical results simulating conditions from other publications agreed quantitatively for the load–deflection curves and qualitatively for the contact boundary. The non-linear spring characteristic of the crowned roller was quantitatively evaluated for a diverse range of geometry. [ABSTRACT FROM AUTHOR]

Previous research using industry-representative standard posttensioning anchor components from the United States has shown that premature wire fractures of seven-wire, low-relaxation unbonded posttensioning strand can occur at strand strains of 0.01 or less. Building on this research, this paper describes an experimental investigation into whether significant improvements in the ultimate strand strains at wire fracture can be made by slightly increasing the taper angle of the anchor wedge outer surface with respect to the taper angle of the anchor housing cavity and slightly increasing the thickness of the anchor wedge as measured at the crown. The test results demonstrate that relatively small, controlled deviations from industry-representative standard wedge dimensions can significantly improve the ultimate performance of the strand at failure. Quality control measures are recommended related to the manufacturing of the wedges as well as the anchor housing for use in extreme design load conditions. [ABSTRACT FROM AUTHOR]

A full-scale experimental study was undertaken to observe and assess the behavior of continuous concrete beams with external prestressed bars. Three beams--a control beam and two comparable beams strengthened using external prestressed bars--were tested in two-point loading of the interior span of three continuous spans. The diameters of the external prestressed bars were 18 and 22 mm (0.71 and 0.87 in.), respectively. For the strengthened beams, V-shaped profiles were used with a deviator located at the middle bottom of the interior span, and the effective prestress was 45% of the yield strength of the external steel bars. Test results indicate that the external prestressing increased the load-carrying capacity by about 25% and the flexural stiffness by about l5%. An analytical model developed by the authors was applied that uses the beam collapse mechanism, plastic hinge length, and deformation of external bars depending on the beam deformation at the deviator point. Predictions of the nominal strength and the corresponding external bar stress compared closely to the test results. [ABSTRACT FROM AUTHOR]

Most research on fatigue strengthening of steel has focused on carbon fiber-reinforced polymer (CFRP) strengthening of steel members with existing cracks. However, in many practical cases, aging steel members do not yet have existing cracks but rather are nearing the end of their designed fatigue life. Therefore, there is a need to develop a “proactive” retrofit solution that can prevent fatigue crack initiation in aging bridge members. Such a proactive retrofit approach can be applied to bridge members that have been identified to be deficient, based on structural standards, to enhance their safety margins by extending the design service life. This paper explains a proactive retrofit design approach based on constant life diagram (CLD) methodology. The CLD approach is a method that can take into account the combined effect of alternating and mean stress magnitudes to predict the high-cycle fatigue life of a material. To validate the retrofit model, a series of new fatigue tests on steel I-beams retrofitted by the non-prestressed un-bonded CFRP plates have been conducted. Furthermore, this paper attempts to provide a better understanding of the behavior of un-bonded retrofit (UR) and bonded retrofit (BR) systems. Retrofitting the steel beams using the UR system took less than half of the time that was needed for strengthening with the BR system. The results show that the non-prestressed un-bonded ultra-high modulus (UHM) CFRP plates can be effective in preventing fatigue crack initiation in steel members.

The objective of this paper is to study the influence of thickness of individual elastomer layers (first shape factor) on the vertical and horizontal responses of fiber reinforced elastomeric bearings that are not bonded or mechanically fastened to their top and bottom supports. By definition, the first shape factor, S1, is the ratio of the plan area to the perimeter stress-free area of a single elastomer layer within the bearing. Results of a comprehensive 2D-finite element study on a large group of bearings with different shape factors of 10, 15, 20, 30, and 40 suggest that the shape factor S1 is a critical parameter in controlling both the vertical compression modulus of the bearing and the level of stress in the fiber-reinforcing layers. Furthermore, the secant horizontal stiffness and the stress demand in the bearing elastomer material are to a limited extent influenced by the parameter S1. [ABSTRACT FROM AUTHOR]

This paper reports on the experimentally observed response of five large-scale, isolated, multi-story, unbonded post-tensioned (UPT), precast concrete walls with horizontal joints under combined gravity and lateral loading. Test results show that the limit states that characterize the lateral load response of the walls occur as anticipated. The results demonstrate that the response of UPT precast concrete walls can be controlled by adjusting parameters such as the total area of post-tensioning (PT) steel and the level of prestressing. In addition, the results show that these walls can undergo significant nonlinear lateral drifts without significant damage while maintaining their self-centering characteristics, eliminating residual lateral drift. [ABSTRACT FROM AUTHOR]

The accurate prediction of tendon stresses is a critical step in calculating the nominal flexural strength of unbonded post-tensioned concrete walls. The lateral load behavior of these walls is largely characterized by a single crack developing at the wall base with the unbonded tendons providing the tension needed for flexural moment resistance. Unlike members with bonded reinforcement, stresses in unbonded tendons cannot be found from strain compatibility and instead must be related to the wall lateral deformation. Previously developed equations for predicting tendon stresses in unbonded post-tensioned flexural members, including those in ACI 318-08, have focused on concrete beams and are inappropriate for concrete walls. A finite element model validated with experimental data from the testing of concrete walls was used to demonstrate the inaccuracy of current design provisions and other available equations for predicting tendon stresses. To provide a more accurate prediction of the unbonded tendon stresses in post-tensioned concrete walls that are designed for use in regions with low seismicity, an equation is proposed that is suitable for inclusion in future versions of ACI 318. [ABSTRACT FROM AUTHOR]

Stable unbonded fiber-reinforced elastomeric isolators (SU-FREIs) are a viable device for seismic mitigation purposes in low-rise structures. SU-FREI bearings consist of alternating layers of elastomer and fiber fabric with shape factor and aspect ratio such that stable rollover occurs during lateral displacement. This study investigates the stability of SU-FREI bearings through experimental testing. Two experimental test procedures are employed. The first investigates stability during incrementally increasing axial loads under lateral cyclic excitation, and the second investigates the ultimate shear properties of SU-FREI bearings through monotonic lateral displacement under design axial load. Dynamic stability testing reveals that the tested SU-FREIs remain stable at axial loads significantly greater than the design load under a wide range of cyclic lateral displacement amplitudes. In addition, an increase in axial load results in lower effective stiffness and greater effective damping in the SU-FREI bearings. Although rollout instability was not encountered during ultimate shear property testing, shear tests provide insight into the stiffening effect encountered as the originally vertical faces of the SU-FREI bearings make contact with the top and bottom contact supports. [ABSTRACT FROM AUTHOR]

It is difficult to accurately predict the flexural strength of prestressed members with unbonded tendons, unlike that of prestressed members with bonded tendons, due to the unbonded behavior between concrete and tendon. While there have been many studies on this subject, the flexural strength of prestressed members with unbonded tendons is still not well understood, and different standards in various countries often result in different estimation results for identical members. Therefore, this paper aimed observe existing approaches and to propose an improved model for the ultimate strength of prestressed members with unbounded tendons. Additionally, a large number of tests results on flexural strength of prestressed members with unbonded tendons were collected from previous studies, which entered into a database to verify the accuracy of the proposed model. The proposed model, compared to existing approaches, well estimated the flexural strength of prestressed members with unbonded tendons adequately reflecting the effects of influencing factors such as the reinforced steel ratio, the loading patterns, and the concrete strength. The proposed model also provided a reasonably good estimation of the ultimate strength of over-reinforced members and high-strength concrete members. [ABSTRACT FROM AUTHOR]

Abstract: Unbonded precast reinforced concrete frame is a new assembly architecture structure. This thesis studies deeply the crack development characteristics, failure pattern, hysteresis curve and the displacement ductility of unbonded precast reinforced concrete frame, by analyzing one unbonded precast reinforced concrete frame under low reversed cyclic load test. We build a model using finite element analysis software to the test piece model analysis, the analysis result consistent with the experimental results. Experimental studies indicate that assembly of prestressed reinforced concrete frame structure has a good seismic performance. This unbonded precast reinforced concrete frame is a new kind of structural system complying with the development of architectural engineering, which is useful in application in the earthquake area. [Copyright &y& Elsevier]

Abstract: The model proposed in this paper calculates the structural response of post-tensioned prestressed beams with unbonded tendons, including both the deflections under service loading, before or after cracking, and the ultimate bearing capacity. Both monotonic and repeated load effects are considered. The originality of the model appears in the calculation of prestressed concrete serviceability after cracking, based on a non-linear macro finite element (M.F.E.). This is a beam finite element for bending problems characterized mainly by its homogeneous average inertia. A second innovation is the prediction of the overall lengthening of the unbonded tendons at all loading stages: cracking, serviceability, ultimate. Results show that the model gives very good predictions of the bending stiffness of the beam in the loading cycles, and the permanent residual deflection after unloading due to crack propagation. The calculations of the bearing capacity and the deflection at failure are also very accurate. [Copyright &y& Elsevier]

Shake table testing and an analytical study were conducted to investigate the seismic performance of unbonded prestressed hollow concrete columns constructed with precast segments. A prototype column was designed based on the current bridge specifications by AASHTO. A one-fourth-scale model column was constructed by match casting. Shake table testing that consisted of 15 runs based on the 1995 Hyogo-Ken Nanbu (Kobe) Earthquake was conducted. The specimen performed very well with essentially no residual displacement and only limited concrete spalling at the base. A parametric study was conducted using a nonlinear twodimensional finite element pushover analysis. An equation to estimate the strain in the prestressing strands was proposed. A simplified method to calculate the load-displacement relationship showed good correlation with the pushover analysis results. Unbonded prestress columns can be designed to provide excellent drift capacity with limited permanent displacements. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of post-tensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities of the concrete, prestressing tendon and anchorages have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the slab. The temperature distribution throughout the slab, time–deflection behaviour, time–longitudinal expansion, time–stress behaviour in the tendon, and the failure modes were predicted by the model and verified against test data. The study has shown that the coefficients of thermal expansion currently used in the European Code for calcareous and siliceous concrete can lead to inaccurate predictions of the structural behaviour. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the aggregate type, load ratio and boundary conditions. It was shown that by varying the boundary conditions the fire resistance was greatly affected. Although changing the aggregate type and load ratio affected the time-displacement response, the fire resistance defined by failure of the slab was not affected due to the splitting mode of failure above the tendon locations not being affected by these parameters. Comparison with the codes shows that the UK code BS8110 is generally unconservative, whereas the Eurocode EN1992-1-2 provides reasonable design rules. [Copyright &y& Elsevier]

Abstract: The potential use of mechanically-anchored unbonded fibre reinforced polymer (MA-UFRP) system to upgrade reinforced concrete (RC) slabs deficient in flexural strength is examined in this paper. A total of six RC slabs, each having 500mm width, 100mm thickness, and 1800mm length, were constructed and tested to failure under four-point bending. Each slab was reinforced with three No. 10 deformed steel bars at tension side that corresponded to 0.8% steel reinforcement ratio. One slab was used as a control while the other five slabs were strengthened with various fibre reinforced polymer (FRP) strengthening systems, each having 0.12% external FRP reinforcement ratio. Two slabs were strengthened with externally-bonded FRP (EB-FRP) system, one slab with end-anchorage and one slab without end-anchorage. The remaining three slabs were strengthened with MA-UFRP system having various anchors’ locations. Test results showed that MA-UFRP system resulted in up to 43% enhancement in the slab flexural strength. The strength of the slabs strengthened with MA-UFRP system was on average 18% lower than that of the slab strengthened with EB-FRP system with end-anchorage but only 8% lower than that of the slab strengthened with EB-FRP system without end-anchorage. The mid-span deflection at ultimate load of the slabs strengthened with MA-UFRP system was on average 56% higher than that of the slab strengthened with EB-FRP without end-anchorage, 5% higher than that of the slab strengthened with EB-FRP with end-anchorage, and only 15% lower than that of the control. [Copyright &y& Elsevier]

The behaviour of unbonded post-tensioned one-way concrete slabs is investigated experimentally and numerically in this paper. Two tests were conducted by the authors to measure the strains in the tendon during the post-tensioning stage and during the ultimate load test. The slabs were one-way simply-supported and reinforced with 15.7 mm nominal diameter seven-wire mono-strand tendons. Prestress losses were measured and a comparison with current design codes showed that the calculated design losses were higher than those measured. The load-deflection behaviour and modes of failure are presented for the two tests. A nonlinear finite element model, incorporating the correct load transfer from the tendon to the concrete, was developed and verified against the tests. The model also ensured that the profile of the tendon retained its correct shape during deformation. A parametric study was conducted to study the effects on the global structural behaviour due to the change in the slab's geometry, prestress load, concrete strength and boundary conditions. The experimental and numerical results were compared with values calculated using current design codes. It is shown that the ultimate loads calculated using current codes are conservative for the unbonded post-tensioned one-way concrete slabs investigated in this study. The results presented in this paper provide a valuable insight into the behaviour of unbonded post-tensioned concrete slabs during the construction phase and under ultimate load. [ABSTRACT FROM AUTHOR]

In an external prestressing system, there is no strain compatibility between the cable and the concrete at every cross-section, the increment of cable strain must be evaluated by taking into account the whole structure, rather than performing the calculation at each section, independently. In this study, a method for the calculation of cable strain, which is based on the deformation compatibility of beam and friction at the deviators, was proposed to predict entire response of externally prestressed concrete beams up to the ultimate state. Application of the developed method in numerical analysis of some examples was then performed. The predicted results showed that the structural behavior of externally prestressed concrete beams could be satisfactorily predicted from zero loading stage up to the ultimate loading stage. The proposed analysis reproduced experimental results such as deflection and increase of cable stress responses with remarkable accuracy.