Post-tensioned Concrete Members Research Articles

Transmission length of multiwire tendon in existing post-tensioned concrete structures

The paper presents unique research on real-scale post-tensioned concrete crane girders dismantled from an existing hall structure. The study aimed to determine the transmission length for bonded prestressing tendons in an anchorage failure situation, a critical factor for proper assessment of the safety of many building structures, mainly those built between the 1950s and 1980s. The results of the tests indicated the feasibility of prestressing force transmission from a 12Ø5 mm Freyssinet-type tendon to the element via bond stress, with implications extendable to the other 18Ø5 mm tendon type prevalent at the time. The key factor affecting transmission efficiency is the quality of tendon duct grouting, with good quality cement injection resulting in transmission length as short as 800 mm. Furthermore, a computational model for the transmission length of prestressing force in post-tensioned concrete members has been developed similarly to the Model Code 2020 for pretensioned concrete. The level I of approximation enables quick transmission length estimation, whereas refined analysis is possible with the level II approach. The developed model and the research findings facilitate a comprehensive assessment of the post-tensioned concrete members in anchorage or tendon failure scenarios. As a result of the analyses carried out, optimal strategies for the further use of the structures can be proposed, fostering a step towards sustainable management.

Analysis of beams prestressed with hybrid (Steel/Carbon Fiber-Reinforced Polymer) tendons using the conservation of energy approach

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.

Retrofitting of Distressed Post-tensioned Concrete Members by External Post-tensioning

Retrofitting of Distressed Post-tensioned Concrete Members by External Post-tensioning

Analytical investigation and experimental validation of anchorage zone to improve bursting force equation for post-tensioned concrete members

The anchorage zone is a critical component of post-tensioned concrete members, and its effect on transverse tensile stress and bursting force is of utmost importance. In the study undertaken, analytical investigation and experimental validation of the anchorage zone has been carried out in order to improve the bursting force equation presented in IS 1343 for post-tensioned concrete members. To determine the relative contribution of anchorage ratio (k), pre-stressing force, and duct opening in the modeling of the plate on bursting tensile stress and bursting force, 3D Finite Element Analysis (FEA) was conducted. The results indicate that the equation recommended by IS 1343 underestimates the bursting force. To address this, an updated design equation is proposed in the same form as presented by IS 1343.

New trends in assessing the prestress loss in post-tensioned concrete bridges

The evaluation of the current prestress force represents often a challenging issue during the assessment of existing post-tensioned concrete (PTC) members. In this contribution, two case studies are presented to show the efficiency of some experimental methods applied both to laboratory beams and to an in-service PTC bridge. First, the outcomes of an experimental campaign carried out on three PTC beams with a straight post-tensioned, non-grouted tendon are discussed. For each beam, three-point bending tests (3PBTs) were carried out applying increasing load steps until failure and varying the prestress force; at each load step, non-destructive tests (NDTs) were performed, namely, dynamic free vibration and ultrasonic tests. The variation of non-destructive parameters was evaluated as a function of the prestress force and of the structural damage. The second case study deals with a 50-year-old PTC bridge with grouted ducts. In this case, other than evaluating the existing geometry, structural details, and material properties, the following NDTs were performed: endoscopies, tests based on stress release, i.e., both saw-cut and wire-cut tests, and lastly, X-ray diffractometries (XRDs). The results indicate the high potential of this latter technique, which resulted more reliable and less invasive than the previous ones.

Artificial Neural Network Model for Predicting the Tendon Stress in Unbonded Posttensioned Concrete Members at the Ultimate Limit State

Artificial Neural Network Model for Predicting the Tendon Stress in Unbonded Posttensioned Concrete Members at the Ultimate Limit State

The Combined Strengthening Effect of CFRP Wrapping and NSM CFRP Laminates on the Flexural Behavior of Post-Tensioning Concrete Girders Subjected to Partially Strand Damage

The studies on unbonded post-tensioned concrete members strengthened with Carbon Fiber Reinforced Polymers (CFRPs) are limited and the effect of strengthening on the strain of unbonded pre-stressed steel is not well characterized. Estimating the flexural capacity of unbound post-tensioned members using the design methodology specified in the design guidelines for FRP strengthening techniques of bonded post-tensioned members does not provide a reliable evaluation. This study investigates the behavior of unbonded post-tensioned concrete members with partial strand damage (14.3% and 28.6% damage) and strengthened with CFRP laminates using a near-surface mounted technique with and without U-wrap anchorages. The experimental results showed that the use of CFRP laminates significantly affects strand strain, especially with the use of anchors. The CFRP reinforcement affected flexural strength, crack width, and midspan deflection. However, the flexural stiffness of strengthened members during the serviceability phases is critical as strand damage ratios increase. In comparison with the nondamaged girder, the NSM-CFRP laminates enhanced the flexural capacity by 11% and 7.7% corresponding to strand damage of 14.3% and 28.6% respectively. Additionally, semiempirical equations were proposed to predict the actual strain of unbonded strands whilst considering the effects of FRP laminates. The suggested equations are simple to apply and provide accurate predictions with little variance.

Determination of Johnson–Cook Material and Failure Model Constants for High-Tensile-Strength Tendon Steel in Post-Tensioned Concrete Members

The estimation of damage in steel tendons is important for evaluating the remaining capacity of existing tensioned members. This research focuses on calculating Johnson–Cook (LC) model and damage parameters of high-strength steel material through quasi-static and dynamic uniaxial tests. Finite element analysis is used to replicate the experimental procedure, and through dynamic image correlation analysis, the numerical results accuracy is verified. In this investigation, it is found that the JC model can accurately replicate deformation and stress concentration under different strain rates and triaxiality conditions and, thus, can be used for fracture analysis of prestressed concrete members.

Usage of EB-CFRP for Improved Flexural Capacity of Unbonded Post-Tensioned Concrete Members Exposed to Partially Damaged Strands

The study presents the performance of flexural strengthening of concrete members exposed to partially unbonded prestressing with a particular emphasis on the amount (0, 14.2, and 28.5%) of cut strands-symmetrical and asymmetrical damage. In addition to examining the influence of cut strands on the remaining capacity of post-tensioned unbonded members and the effectiveness of carbon fiber reinforced polymer laminates restoration, The investigated results on rectangular members subjected to a four-point static bending load based on the composition of the laminate affected the stress of the CFRP, the failure mode, and flexural strength and deflection are covered in this study. The experimental results revealed that the usage of CFRP laminates has a considerable impact on strand strain. In addition to that, the flexural stiffness of strengthened members becomes increasingly significant within the serviceability phases as the damaged strand ratios increase. The EB-CFRP laminates increased the flexural capacity by approximately 13%, which corresponds to strand damage of 14.28% and about 9.5% for 28.57% of strand damage, which represents one of the unique findings in this field. Additionally, semi-empirical equations for forecasting the actual strain of unbonded tendons were presented. The suggested equations are simple to solve and produce precise results. Doi: 10.28991/CEJ-2022-08-06-014 Full Text: PDF

Predictive model for stress at ultimate in internally unbonded steel tendons based on genetic expression programming

Predicting the stress increment (Δfps) and in turn, the total stress (fps) in internally unbonded steel tendons at ultimate flexural strength of post-tensioned concrete members is one of the major challenges due to the lack of bond between the steel tendon and surrounding concrete. This article presents the development of a new predictive model to evaluate the stress increment (Δfps) and in turn, the total stress (fps) based on a genetic expression programming (GEP). GEP has the ability to collect all the influential parameters, among all the cross-sectional and the member's geometrical properties, the mechanical properties of materials, and the applied load, in one single closed-form expression for predicting (Δfps). The proposed model was developed based on a comprehensive experimental database of 196 concrete specimens that were collected from literature and the experimental outcomes of 22 beams and one-way post-tensioned concrete slabs which were previously prepared, fabricated, and tested by authors of the present study. A parametric study was conducted to obtain an idea of the relationship between the independent variables and their effect on the stress increment in unbonded tendons at ultimate (i.e., the target parameter (Δfps)). To inspect the potentiality of the proposed (GEP) model against 22 analytical models available in the literature, an extensive comparative study was carried out. The performance of the proposed model was checked according to the values of the coefficient of determination (R2) and different achieved statistical errors. In comparison to the experimental values of (Δfps) and (fps), the proposed model showed the highest coefficient of determination (R2) of 0.78 and 0.93, respectively, compared to the other 22 analytical models. While the values of mean absolute error, relative absolute error, root mean square error, and root-relative square error to determine (Δfps) and (fps) were found to be (60.98, 76.96, 0.45, and 0.47) and (60.98, 76.96, 0.25, and 0.26), respectively.

Effect of duct type, size, and embedment on bond behavior of post-tensioned mono-strand concrete members

This research investigated the bond behaviour of mono-strand post tension systems with different duct types, sizes, and embedment length. Six specimens were considered in this study. Two different types of ducts were considered: plastic ducts and galvanic ducts. Two different duct sizes (9 cm × 4.2 cm and 6 cm × 2.4 cm), and embedment lengths (5 cm and 10 cm) for each duct type were considered in this study. Results showed that specimens with galvanic ducts had stronger bond than those with plastic ducts. In addition, it was found that extending the embedment length of the galvanic ducts form 5 cm to 10 cm improved the bond strength which was not the case for plastic ducts where the effect was null. Furthermore, specimens with galvanic ducts had greater bond strength when duct size was increased than those with plastic ducts.

An Impact-Echo Experimental Approach for Detecting Concrete Structural Faults

For the current problem of detection of grouting defects in posttensioned prestressed concrete members, the paper takes a single-layer arrangement of prestressed pipes as the object of study. The influence law of the main factors such as pipe material, defect size, defect critical surface location, and prestressing reinforcement location on the results of the impact-echo method for detecting concrete grouting defects was studied. Firstly, the ABAQUS finite element software was used to simulate these factors to obtain the influence law on the detection results, and a modal test was conducted to verify them. The results show that the impact-echo method can effectively test the location of defects and the degree of burial depth, and the pipe material influences the test results, and the impact of corrugated metal pipe is smaller and more accurate than the PVC pipe. In addition, the greater the plate thickness frequency drift rate, the larger the transverse size of the defect, so the plate thickness frequency drift rate and the measured defect depth are combined to quantitatively determine the depth of the defect.

Experimental Study on the Behaviours of Post-tensioned Concrete Members with Unbonded Tendons

This paper presents experimental results regarding the behaviours of eight simply supported partially prestressed concrete beams with internally unbonded tendons, focusing particularly on the effect of three different variables: concrete compressive strength, fc ′; jacking stress, fj ; and the prestressing index, ω. Increasing the concrete compressive strength from 35 MPa to 60 MPa was seen to lead to an increase in load-carrying capacity by about 10%. The load capacity was also affected, though to a lesser extent, by the jacking stress. The failure load of partially prestressed concrete beams with internally unbonded strands was increased by about 2.5%, to 10%, when the jacking stress increased from 0.5 to 0.7 of the strand’s ultimate strength, fpu . The test results showed that the prestressing index was the most influential parameter among the examined variables. The experimental results of the stress increase in unbonded strands at failure and the ultimate load resistance of members were also compared to the analytical results of various standards and research methodologies for verification.

Seismic behavior of unbonded post-tensioned precast concrete members with thin rubber layers at the jointed connection

Seismic behavior of unbonded post-tensioned precast concrete members with thin rubber layers at the jointed connection

Validity of Anchorage Zone Design for Post-Tensioned Concrete Members with High-Strength Strands

This paper evaluated the validity of a design method for anchorage zones in post-tensioned concrete members using Grade 2400 strands from American Concrete Institute (ACI) Building Code, ACI 318-19 and Association of State Highway and Transportation Officials (AASHTO) codes. For this purpose, the behaviors of anchorage zones in ten specimens were investigated and compared with ACI 318-19 and AASHTO codes, considering the jacking force and ultimate flexural load. The anchor shape, number of strands, and reinforcement details of the anchorage zone were considered as variables. The test results showed that ACI 318-19 and AASHTO codes could not predict where the maximum bursting force would occur. In addition, they calculated bursting force by summation of the pre-stressing forces from the individual strands, whereas the experimental results were not linearly proportional to the number of strands. The ACI 318-19 and AASHTO standards were inadequate to calculate bearing force of post-tensioned members using high-strength strands. Furthermore, additional stress in the anchorage zone occurred as the flexural load increased, which may have been caused by a failure in the anchorage zone. The experimental results indicated that the method used to predict stress and design of anchorage zone in current codes should be improved to consider stress distribution and additional stress in post-tensioned concrete members using high-strength strands.

Flexural Behavior of Posttensioned Concrete Beams with Unbonded High-Strength Strands

This paper assesses the applicability of high-strength strands to current design codes and various existing equations. To evaluate the flexural performance of posttensioned concrete members with Grade 2400 strands, a flexural experiment was conducted on eleven specimens. Test variables included the tensile strength of strands, the number of strands, the cross-section shape, and anchorage zone reinforcement details. The test results were compared with ACI 318-19, AASHTO, and equations of Du and Tao, Naaman and Alkhari, and Harajli to evaluate the applicability of flexural strength equations for posttensioned concrete members using unbonded high-strength strands. Results indicated that the provisions of ACI 318-19 and AASHTO design codes and the existing equations underestimated the increased stress of the high-strength strands. Additionally, results demonstrate that improved equations are needed to consider the strain-compatibility model, plastic hinge length, and relationship between bonded reinforcement, concrete, and prestressing steel in posttensioned members using high-strength strands.

Experimental investigation of simply-supported post-tensioned beam after anchorage system failure

In recent years, with the rapid development of China's transportation industry, some existing bridges have needed to be demolished and rebuilt. Bridge demolition is a complex process of structural system conversions. During demolition, the prestressed beam performance is not clear after anchorage system failure. Therefore, it is important to carry out research to better understand the behaviour of post-tensioned prestressed concrete members after anchorage failure. In this paper, the demolition project of the Qingkou interchange ramp bridge of the Fuzhou circumferential highway is taken as research background to carry out an experimental investigation on post-tensioned prestressed concrete beam specimens to study their mechanical performance before and after the failure of the anchorage system. The results show that the anchorage has a longitudinal extrusion effect on the concrete, and the contribution of the concrete and the rebar to the structure decreases from the end to the mid-span of the beams. The failure of the anchorage system affects the cracking load, the crack width and the ultimate bearing capacity, while it has no effect on the final crack distribution and the concrete failure mode. Beyond the limit of elasticity, the prestressing tendons and the concrete grout start to debond, and the bearing capacity of the beam decreases gradually. The crack resistance and the ductility of the prestressed concrete beams are also reduced. After concrete cracking and under the same load condition, the prestressing tendons bear a larger load in the case of anchorage system failure. It is suggested that after the failure of the anchorage system, the contribution of the residual stress in the prestressing tendons to the bearing capacity of the structure be taken into account until the limit of elasticity of the structure is reached.

Investigation and Improvement of Bursting Force Equations in Posttensioned Anchorage Zone

In posttensioned concrete members, the high local stress under the anchorage causes transverse tensile stress. Therefore, it is very important to predict the bursting force to determine appropriate reinforcement details. In the present work, the existing equations of the bursting force for the anchorage zone were evaluated and an equation for the bursting force based on finite element analysis was proposed to improve the model’s accuracy. Parametric analysis was performed considering the anchorage shape, tendon angle, and eccentric distance. The analytical results indicate that the existing equations underestimate or overestimate the bursting force. The proposed equation is able to predict the bursting force reasonably well for an anchorage zone with rectangular bearing plate, cavity, and eccentric distance.

Comparative Study on Performance of Corrosion Protective Systems for Post-Tensioned Concrete Members

Comparative Study on Performance of Corrosion Protective Systems for Post-Tensioned Concrete Members

Simplified finite element modeling of post-tensioned concrete members with mixed bonded and unbonded tendons

Tendon filler materials serve as a critical layer, protecting a post-tensioned bridge’s steel reinforcement against corrosion and subsequent strength loss. Recent adoption of flexible filler materials in post-tensioning tendons in bridge construction – an alternative to the more typical cementitious grout which is currently common in the U.S. – has implications on member flexural behavior. This paper describes the development and experimental validation of a simplified, computationally inexpensive finite element modeling (FEM) approach for unbonded tendons. Flexural members containing both unbonded post-tensioned steel and bonded pretensioned steel, or “mixed tendons”, were modeled. FEM results were then validated against experimentally obtained ultimate strength, tendon stress, and plastic hinge length data. The developed analytical approach described herein was found to predict the ultimate flexural strength, failure mode and tendon stress with good accuracy, within 5%. Prediction of the ultimate displacement was fairly accurately modeled (within 15%) for the mixed tendon beams.