Unbonded Prestressing Research Articles

Experimental characterization of indented dry connections for precast double‐curved concrete shells

AbstractThis paper presents the experimental validation of a connections system to connect segmented precast long‐span thin‐shell structures made with lightweight ultra‐high‐performance fiber‐reinforced concrete (UHPFRC). The work comprises the segmentation of the large‐span ultra‐thin shell to meet the requirements of prefabrication (construction and transport limitations), and the design and characterization of the connections. Once the assembling of the shell required the use of post‐tensioned unbonded prestressing, an indented dry connection was chosen. The connections are experimentally characterized with in‐plane and out‐of‐plane shear tests for different prestressing levels. The results are presented and discussed, demonstrating the balanced behavior of the indented dry connection under shear loading. The connections exhibited high ductility in shear, capitalizing on the use of a UHPFRC to fabricate the shell. The prestressing also demonstrated its efficacy, significantly enhancing both the stiffness and the capacity of the indented dry connection.

Shear strengthening of self-compacting lightweight concrete beams with steel fibers and unbonded prestressing bars

This study aims to investigate the shear behavior of self-compacting lightweight concrete (SCLC) beams strengthened with steel fibers and unbonded prestressing bars. For this, the experimental program included the use of hooked-end steel fibers with volume fractions of 0.5 %−0.75 %, and high-strength prestressing bars with varying prestressing forces and transverse reinforcement ratios. The addition of steel fibers in SCLC significantly increased the normalized shear capacity of beams, reaching up to 2.50 times higher values, whereas the substitution of normal coarse aggregates with lightweight aggregate resulted in a decrease of up to 10.1 %. The shear strengthening of SCLC beams with unbonded prestressing bars provided a substantial increase in the ultimate shear capacity up to 2.06 times higher than that of beam without reinforcement. Increasing the prestressing force of bars effectively restrained the occurrence of critical shear cracking, consequently enhancing the shear capacity. Utilizing hybrid reinforcement with fibers and prestressing bars resulted in a transition of failure mode from shear to flexural, attributed to the improved shear capacity. For the SCLC beams reinforced with steel fibers, the shear strengths were predicted by previous prediction models. When modifier for lightweight concrete was considered, the Imam’s model most precisely predicted the shear strengths with an average vu/vn ratio of 1.00 and SD of 0.07. The fib model Code, based on the modified compression field theory, provided accurate predictions for the shear capacities of SCLC beams with unbonded prestressing bars, exhibiting an average Vu/Vn ratio of 1.00 and SD of 0.09. The fiber-resisting component was reasonably considered in the model with an average Vu/Vn ratio of 1.02 and SD of 0.07.

The Cyclic Performance and Macro-Simplified Analytical Model of Internal Joints in RC-Assembled Frame Structures Connected by Unbonded Prestressed Strands and Mortise-Tenon Based on Numerical Studies

This paper introduces a novel type of connection that integrates unbonded prestressed strands (UPS) and mortise-tenon in an assembly frame structure (UPS-MTF). First, the damage process and failure modes of the joints under reciprocating horizontal loads were systematically analyzed using refined numerical models. The recommended values of the design parameters of the joints were derived from the parametric analysis results. Refined numerical modeling results reveal the diagonal compression strut mechanism within the core region of the joint. The diagonal compression struts model assists in establishing the theoretical calculation formula for the skeleton curve of shear stress–strain in the core region. Second, a genetic algorithm (GA) parameter was identified for the restoring force model of the core region to determine the parameters of the hysteresis rules. Finally, a macro-simplified analytical model of the joint was created based on the restoring force model of the core region, and parameter analysis was conducted to verify the applicability of this macro-simplified analytical model. The research results prove that the damaged form of the joint proposed in this paper originates from the shear and relative slip damage between the components in the core region. The axial compression ratio significantly affects the hysteretic performance of the joints, and the upper and lower limit values were identified for the axial compression ratio of the joints. The area and initial effective stress of the UPS exert a minimal effect on the hysteretic performance of the joint. Based on the method proposed in this paper for determining the restoring force model in the core region of the joints, the hysteresis curves obtained from the macro-simplified analytical model closely match the refined numerical analysis model results. This correspondence verifies the applicability of the macro-simplified analytical model.

Seismic Performance at Beam Ends of Unbonded Prestressed Precast Concrete Frames

Numerical modelling was conducted to investigate the seismic performance at beam ends of unbonded prestressed precast concrete frames. The established finite element model was verified by the test results. Through numerical simulation, the influence of such factors as the presence or absence of energy-dissipating reinforcement, the length of the unbonded section of energy-dissipating bars, the amount of prestressed or non-prestressed reinforcement on the seismic performance at beam ends of unbonded prestressed precast concrete frames were further analyzed. The results show that all prestressed precast beam-end models have similar initial stiffness. The bearing capacity and energy-dissipating performance of beam-end models without energy-dissipating bars are the lowest. The length of the unbonded section of energy-dissipating bars has no significant effect on the bearing capacity and energy-dissipating capacity of beam-end models, though the yield displacement increases obviously with the increase of the length of the unbonded section, as the local stress of energy-dissipating bars is reduced by setting the unbonded section. The bearing capacity of beam-end models can be improved by increasing the amount of prestressed or non-prestressed reinforcement. In the case of the same amount of prestressed reinforcement, increasing the amount of energy-dissipating bars can significantly improve the energy-dissipating capacity.

Modelling of fire behavior of RC girders strengthened by external prestressing

Since the 1950s, external post-tensioning has been a powerful tool for retrofitting existing bridges. In unbonded prestressing, the post-tensioning tendons are positioned outside the concrete cross-section, and the prestressing forces are transferred to the girder through end anchorages, deviators, or saddles. This implies that these could be critical elements for the bridge performance against fire. This paper examines reinforced concrete bridges strengthened by external prestressing and subjected to fire, using a mixed experimental and analytical approach. Initially, load tests are conducted on a model of a bridge beam strengthened by external prestressing to assess the effectiveness of the intervention until failure. The ultimate limit state (ULS) capacity of the beam is then evaluated both before and after rehabilitation. Subsequently, the fire performance is investigated through different scenarios defined by the fire load curves of Eurocode, evaluating the cross-sectional temperatures resulting from fire, estimating the loss of external prestressing efficiency, and assessing the beam’s overall fire resistance. This methodology is then applied to a real girder bridge with adjacent beams of the same type, extending the approach used on the experimental scaled beam to an actual bridge deck. Results are presented in terms of performance levels for different traffic load values and residual capacity over time from fire ignition, providing a safety assessment of girder bridges under fire exposure. The procedure is straightforward and immediately applicable to many existing reinforced bridges. The findings demonstrate how quickly external prestressing degrades, leaving the beam unprotected, and highlight the fundamental importance of the protective sheath for external tendons in limiting this degradation. Evaluating load levels based on exposure time also allows engineers to estimate the service time before rescue arrives, given the expected traffic level.

Flexural Behavior of Unbonded Prestressed Concrete Composite Beams Manufactured with Glass-Fiber-Reinforced Lightweight Aggregate Concrete and Conventional Concrete

Flexural Behavior of Unbonded Prestressed Concrete Composite Beams Manufactured with Glass-Fiber-Reinforced Lightweight Aggregate Concrete and Conventional Concrete

Effects of unbonded prestressing steel tendons and slit dampers on the seismic behavior of precast concrete beam-column joints

This article proposes a novel precast concrete (PC) beam–column joint that uses prestressing steel reinforcement tendons and external steel slit dampers (SSDs) to provide self-centering and energy dissipation capabilities. The SSDs are steel plates cut to a desired shape installed under the beam ends to control their yield through energy dissipation. Three exterior beam–column joint specimens were investigated through quasi-static reversed cyclic loading tests. The specimens included a PC beam–column joint with only an SSD and two PC beam–column joint specimens using unbonded prestressing steel tendons (two in one specimen and four in the other) and an SSD. A fiber-based model using the software Ruaumoko and a code-based design were developed and validated against the experimental results. The test results showed that without the tendon, the specimen was significantly damaged, whereas using unbonded prestressing steel tendons with an SSD endowed the beam–column joint with a higher lateral load capacity of up to 40%. In addition, the ductility improved significantly up to 1.88 times. The developed model and the code-based design were in excellent agreement with the experimental results. Overall, this innovative connection type offers an alternative for seismic-resistant PC structures, and its design enables easy repair after an earthquake.

Analytical Investigation on Catenary Action in Unbonded Prestressed Concrete Beam-Column Subassemblages

One reinforced concrete (RC) and four prestressed concrete (PC) beam-column subassemblages with various tendon profiles were designed and tested to investigate the effect of prestressed tendon profiles on the progressive collapse response of PC frames. An analytical model was developed to evaluate the PC frames’ catenary action (CA) response by identifying the fracture of longitudinal reinforcement and prestressed tendons in beams. The effect of the bond mechanism on the rupture strain of reinforcement was considered. Then, the proposed model was validated using experimental and simulation results. Finally, parametric studies were performed based on the calibrated model. Test results indicate that the prestressed tendon profile significantly impacts the CA response of PC frames against progressive collapse. The analytical model can effectively reflect the effect of the prestressed tendons on the CA resistance and displacement of PC frames.

Proposals for Flexural Capacity Prediction of Precast Segmental Concrete Beam Prestressed with Internal Un-Bonded CFRP Tendons

The application of CFRP tendons in precast segmental concrete beams (PSCB) as internal un-bonded prestressing reinforcement is a newly developed scheme to improve structural flexural performance. The stress increment of the un-bonded tendon, depending on the whole structural deformation, is a crucial value to be predicted for flexural capacity design. Due to the discontinuity of the opening joints, the deformation modes of segmental beams differ from the monolithic ones. The existing prediction methods built for monolithic beams can not be directly used for segmental beams. In this paper, the new prediction equations of the tendon stress increment and flexural capacity were put forward for PSCB with internal un-bonded CFRP tendons (PSCB-IUCFRP). Firstly, the differences between the deformation modes of monolithic and segmental beams were compared and clarified based on the numerical model analysis. Then, a parametric analysis was conducted on 162 numerical models, and the results were employed to evaluate the applicability of existing methods for PSCB-IUCFRP. The predictions of the ACI 318-14 model and the AASHTO LRFD model were both conservative and scattering compared with numerical results. The ACI 440.4R model underestimated the tendon stress increments of beams under one-point loading but overestimated it for those under two-point loading. According to the failure mode of PSCB-IUCFRP, a simplified curvature distribution mode was assumed, and the relation between tendon elongation and structural deflection was derived. The prediction equations for PSCB-IUCFRP were proposed using the back-calculated plastic hinge length. Compared with existing methods, the proposed equations considered the deformation characteristic of segmental beams and had clear physical significance. The predictions of the proposed method were in good agreement with the numerical and experimental results. Furthermore, a balanced prestressing reinforcement ratio equation is proposed for PSCB-IUCFRP to avoid tendon rupture-controlled failure. The proposed equations provide suggestions for the flexural design of PSCB-IUCFRP and will help to popularize this new structure.

Analysis of the flexural performance of Precast Box Girders Reinforced with a combination of Steel-Slab-Concrete Plus Unbonded Prestressing

Due to the serious development of cracks in precast box girders, there is no reliable method to detect the loss of prestress and carry out effective reinforcement of box girders. This paper proposes a reinforcement method of steel plate - concrete with unbonded prestressing, and applies this method to a 30m reinforced concrete prestressed box girder bridge. This paper uses FEA software to carry out a comparative analysis of the load-deflection curve of the reinforced mid-span section by comparing with the conventional bonded steel reinforcement method and the load test data, and through the analysis results, the calculation formula of the flexural load capacity of the steel plate-concrete with unbonded prestressing reinforcement method is derived. The results show that the flexural load carrying capacity of the strengthened mid-span section is significantly improved, and the proposed calculation formulae deviate from the finite element simulation results within 5%, which can be applied to actual projects.

Study on the flexural behaviours of precracked hollow core beams strengthened with core filling and unbonded prestressing steel strands

Extensive investigations show that the core-filling method can markedly improve the shear behaviour of hollow core beams (HCBs). However, few investigations were carried out to study the flexural behaviours of the HCBs strengthened with this method. In this study, the flexural behaviours were first studied, where the HCBs were strengthened with a hybrid strengthening method of the core-filling and external unbonded prestressing steel strands. Six types of HCB specimens were fabricated and investigated. The initial damage and prestress levels were selected as the main parameters. Before strengthening, the HCB specimens were loaded to 40.6 %, 66.3 % and 100 % of their ultimate load to pre-set some cracks in the concrete beams, which corresponded to the maximum crack widths of 0.2 mm, 0.4 mm and wider than 1.5 mm, respectively. Compared with the control beam, the strengthening with the core-filling improved the flexural load carrying capacity by 3.93 %, whereas the hybrid strengthening method improved by 33.67 %; the cracking loads for the two strengthening methods increased by 10.34 % and 128.28 %, respectively. Moreover, the experimental results showed that the initial flexural stiffness can be markedly improved with the hybrid strengthening method. However, the initial damage level had little influence on the final crack width. In addition, an analytical model was proposed to evaluate the flexural behaviors of the strengthened HCBs. The simulation results of the preload, ultimate load and failure modes agreed well with the experimental ones.

Experimental and numerical studies on the fire resistance of prestressed reactive powder concrete beams

The prestressed reactive powder concrete beam (PRPCB) is an advanced structural member that can take full advantage of the ultra-high compressive strength of reactive powder concrete (RPC) and the high tensile strength of tendons. The fire safety of PRPCBs has been mainly studied at the material level, with relatively few experimental studies being conducted on the structural member level with the principal reason of serious fire-induced spalling. This paper proposed an innovative curing method to prevent RPC spalling under high stress and extreme heating rates. A total of eight full-scale beams were tested in ISO 834 fire, considering the parameters of the cover thickness of tendon, load ratio, partial prepressing ratio (PPR), and bonded and unbonded prestress classifications. Cross-sectional temperatures, mid-span deflections, effective prestressing, crack patterns, failure modes, and fire resistances of the beams were obtained. Subsequently, fine thermal–mechanical coupling nonlinear finite element (FE) models were developed and verified, and an extensive parametric study was performed to simulate the fire responses of PRPCBs. The results indicated that the load ratio, cover thickness, and sectional dimensions notably affected the fire safety of PRPCBs, and PRPCBs fabricated as under-reinforced beams at ambient temperature may fail as lightly reinforced beams under fire exposure. According to these parametric studies, Eurocode 2: BSEN 1992-1-2 might overestimate the fire resistance of PRPCBs with small sectional dimensions by up to 3–15%. Therefore, fire resistance suggestions for PRPCBs are presented in this paper.

Experimental study on the shear behavior of precast concrete segmental simply supported beam with internal tendons and glue joints

Glue joints make precast concrete segmental beams with discontinuity, which is the weak link and an important part of the structure and needs to be given special attention in practical engineering. To test the shear behavior of five segmental glue joint beams and one monolithic beam, the test parameters included shear span ratios (1.2, 1.5), prestress levels (4 MPa, 6 MPa, 6.5 MPa), prestress types (unbonded prestress, bonded prestress), and beam types (segmental beam, monolithic beam). The results show that both segmental glue joint beams and monolithic beams exhibited typical brittle failure characteristics in the failure stage. The shear span ratio increased, the shear bearing capacity of the specimen decreased, and the deformation increased. The deformation of the specimen decreased when the prestress level increased. The bonded prestressed screw-thread bar improved the overall strength and stiffness of the segmental glue joint beam. The crack resistance of the monolithic beam was better than the crack resistance of the segmental glue joint beam, but the difference in shear bearing capacity between them was small. The test data were compared with the calculated values of shear bearing capacity of different formulas, and the results show that the calculated values of formulas of the American Association of State Highway and Transportation Officials (AASHTO (2003)) standard were in the best agreement with the test values. Based on the test results, the strain distribution and stress distribution of the specimens were further explored by combining the finite element method. The results show that cracks of different specimens would cross the glue joints. The yield strength of the prestressed screw-thread bar was not reached when the specimen was damaged.

FLEXURAL BEHAVIOR OF STEEL FIBER-REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE BEAMS PRESTRESSED WITH CARBON FIBER-REINFORCED POLYMER BARS

An unbonded prestressing anchorage system for carbon fiber-reinforced polymer (CFRP) tendons was introduced. Eight lightweight aggregate concrete (LWC) beams prestressed with unbonded CFRP tendons which used CFRP auxiliary reinforcements and one fabricated using normal weight concrete (NWC) were tested under a four-point bending condition. The influences of concrete type, of prestressed degree and, of clear span length on the cracking moment, on the moment-deflection response and, on the crack width were analyzed. The existing deflection and crack width models for unbonded FRP prestressed beams with steel auxiliary reinforcements were modified from the view of equivalent axial stiffness. The applicability of the modified models was evaluated based on the test results. It is shown that the serviceability limit state of the beams is mainly governed by the crack width limit. The ratios between the moment at crack width of 0.5 mm and the ultimate moment range from 41% to 52%. With the same total reinforcement amount, the increase in the prestressed degree helps delay the cracking of the specimens. Increasing the amount of the auxiliary reinforcement and of the unbonded prestressed tendons both improve the beam stiffness and restrain the propagation of the cracks. The modified Cheng’s and modified Meng’s deflection models provide accurate deflection predictions for LWC beams with unbonded prestressed and auxiliary CFRP reinforcements.

Effect of Axial Compression Ratio on Seismic and Self‐Centering Performance of Unbonded Prestressed Concrete Columns

To explore the influence of the applied axial compression ratio and preloaded axial compression ratio on the seismic performance of unbonded prestressed concrete columns, pseudo‐static tests were carried out on four prestressed columns and one ordinary column in this study. The seismic performance indexes of test columns were studied and analyzed, including failure modes, hysteresis curves, skeleton curves, stiffness, ductility, and energy dissipation. The test results show that compared with concrete columns with ordinary reinforcement, the hysteresis curve of reinforced concrete columns with prestressed tendons has a pinch phenomenon to a certain extent, and the energy dissipation performance becomes worse. For the prestressed columns, the greater the applied axial compression ratio, the worse the fullness of hysteresis curves and the energy dissipation performance, the greater the residual displacement, the faster the strength attenuation, and the worse the self‐centering performance. For the posttensioned unbonded prestressed concrete columns, the greater the preloaded axial compression ratio, the worse the energy dissipation performance of the test column, the slower the strength attenuation, and the better the self‐centering performance.

Design of prestressed, jointed columns for enhanced seismic performance

Jointed, prestressed columns have been shown to enable accelerated construction and to offer superior seismic performance, as compared with cast-in-place columns without prestressing. During an earthquake, such columns deform primarily through concentrated rotations at the joints, and special detailing at the ends of the columns prevents crushing and spalling. Unbonded prestressing steel, running vertically through the columns, provides a restoring moment that returns the columns to plumb after the ground motion stops. The use of prefabricated substructure elements speeds up the bridge assembly by eliminating building formwork, fixing steel, casting and curing concrete on-site. A variety of detailing strategies have been developed for jointed, prestressed columns, but their fundamental behavior is similar. More importantly, this behavior differs from that of conventional reinforced concrete columns for which current seismic code provisions were developed. This paper develops a displacement-based procedure for designing jointed, prestressed columns that is based on the framework provided by the AASHTO Guide Specifications for LRFD Seismic Bridge Design. To account for the unique characteristics of these systems, the design procedure introduces new performance criteria; it also provides recommendations for proportioning the prestressed and non-prestressed steel, calculating the effective column stiffness, and estimating the column displacement capacity. The focus of the paper is on columns with pretensioned strands and internal energy dissipaters, although the procedure would work for post-tensioned systems and those with external dissipaters as well. This design approach is critically evaluated through comparison to the results of both cyclic tests of cantilever columns and shaking table tests of a two-span bridge system.

Time-dependent analysis of simply supported and continuous unbonded prestressed concrete beams

An equation to estimate the prestress loss in continuous unbonded prestressed concrete beams is proposed in this paper. The equation accounts for the creep and shrinkage of the concrete and the relaxation of the prestressing steel. In addition, a numerical model is developed based on the finite element method for the time-dependent analysis of these types of concrete beams. The approach also accounts for the presence of non-prestressed steel reinforcement and the friction between the tendons and their ducts. Furthermore, a well-justified integral-form equation is proposed to compute the strain at each section of the unbonded prestressing steel accounting for the friction between the tendons and their ducts. This equation is based on the equation for frictional loss accepted by different standards. The strain at each section of the unbonded prestressing is calculated from the strain in the concrete at the level of the prestressed tendon along the whole beam. Numerical and experimental results are compared to validate the proposed equation and the finite element approach and the good performance of both in modelling the behaviour of this type of prestressed concrete member over time is shown.

Flexural Behavior of Unbonded Prestressed Concrete Bridge Girders

This paper presents an experimental and numerical investigation on the flexural behavior of unbonded prestressed concrete (PC) T bridge girders. Three unbonded PC T bridge girders with different prestress degrees spanning 3 m were selected to perform four-point bending tests and then determine the flexural performance. Flexural capacity, crack development and failure mode, load-deflection curves, strain in longitudinal rebars, and stress in prestressing strands of unbonded PC T bridge girders are experimentally analyzed. Subsequently, three-dimensional finite element (FE) models are built and validated by experiments to investigate the effect of different design parameters on flexural behavior of bridge girders. Results generated from experiment and numerical studies show that the flexural destruction behavior in unbonded PC T bridge girders experiences elastic, elastic-plastic, and ductility stages, similar to that of PC T bridge girders. The prestress degree and load location have significant influence on the destruction process in unbonded PC T bridge girders. A lower effective prestress degree can reduce the distribution range in cracks and also increase the width of cracks. Stress in prestressing strands under anchor increases rapidly after concrete presents obvious cracks, and the fracture area within prestressing strands increases with the elevation of prestress degree. The aim of this study is to provide a reference for the design and practical application of unbonded PC T bridge girders.

Comparative study of the flexural behavior of steel fiber-reinforced lightweight aggregate concrete beams reinforced and prestressed with CFRP tendons

A comparative experimental investigation of flexural behavior of lightweight aggregate concrete (LWC) beams reinforced and prestressed with carbon fiber reinforced polymer (CFRP) bars was presented and an unbonded prestressing anchorage system for CFRP tendons was introduced. A total of four beams prestressed with unbonded CFRP bars and four reinforced with CFRP bars were tested under four-point bending, taking into account the effect of steel fiber content and clear span length. Based on the experimental results, the application of prestressing force and the addition of steel fibers reduced the deflection and crack width of the beams. The larger clear span length resulted in wider crack width and lower tendon stress increment in the prestressed specimens. A geometry-based model was proposed to calculate the tendon stress increment in terms of deflection. Accordingly, decomposing analytical procedure into a bonded beam analysis and an unbonded member analysis, an iterative algorithm for the deflection of beams prestressed with unbonded FRP tendons was developed. The proposed models yielded reasonable stress increment and deflection predictions at service load level.

Dynamic Behavior of Externally Prestressed Continuous Beam considering Second-Order Effect

Considering precisely the second-order deformation of external tendon, the analytical solution of natural frequencies of 2-span externally prestressed continuous beam was obtained by the energy method. The effect of external prestress compression softening is between the zero effect of unbonded prestress compression and the effect of axial outside compression and is determined by the influence coefficient ranging within 0∼1. The influence coefficient is mainly related to the number of deviators and slightly related to tendon layout. Without deviator, the influence coefficient is 1, and the effect of external prestress compression softening is the same as the effect of axial outside compression. As the number of deviators increases, the influence coefficient gradually decreases from 1 to near 0, and the effect of external prestress compression softening is close to zero effect of unbonded prestress compression. With one or more deviators, the effect of external prestress compression softening is negligible. As the eccentricity and area of tendon increase, only the first symmetric frequency increases obviously, and other frequencies almost remain unchanged. The influence of tendon layout linear transformation on the frequency is negligible.