Analysis of Prestress Loss and Structural Performance of Box-Type Prestressed Concrete Girders on the Cakung Flyover

Recently, several prestressed concrete bridges and buildings have experienced severe cracking along the tendon path when prestress force has been transferred in the anchorage zone. The purpose of the present study is to explore characteristics of the local stress distribution and to study the failure mechanism on the anchorage zones of the precast prestressed concrete structures. To accomplish these objectives, a comprehensive experimental and analytical study has been conducted. Major variables include section geometry, tendon profile, and types of local reinforcements. From this study, it is found that the failure of the anchorage zones is initiated by the cracks along the tendon path, and eventually sudden explosive failure, with complete destruction on the anchorage zones. Section geometry such as tendon inclination, tendon eccentricity, and concrete cover thickness affect the cracking and ultimate loads. The cracking and ultimate load capacities for spirally reinforced anchorage zones are found to be larger than those for othogonal reinforced anchorage zones. A new concept of failure mechanism on the anchorage zone is suggested, and the proposed concept describes well the failure process of prestressed concrete members.

Concrete containment buildings (CCBs) are important safety structures in nuclear power plants; however, degradation may occur in CCBs as they age. For post-tensioned CCBs, prestressing losses could occur and may affect the CCBs’ performance under accident conditions. CANDU CCBs contain cement-grouted post-tensioning (P-T) cables. The grouting of P-T cables prevents direct monitoring of prestressing losses by traditional lift-off testing. Instrumented monitoring has been recommended as an indirect approach by some guidelines for integrity evaluation of CCBs with grouted prestressing systems. As part of the investigation on the relationship between instrumentation data and the integrity of CCBs, sensitivity analyses have been performed using finite element models to develop an understanding of the sensitivity of strain changes to degradation factors that contribute to prestressing losses, such as creep and shrinkage of the concrete, stress relaxation, and deterioration of prestressing systems. Strain measurements from a CANDU CCB were analysed to assess the measurement noise, which was compared with the predicted strain changes due to degradation to evaluate whether the degradation of concrete and prestressing systems can be captured by strain instrumentation. The analysis reveals that the strain changes due to degradation, except the creep and shrinkage during the early years of CCBs, were comparable with the level of noise observed in the measured strain data. Degradation mechanisms related to prestressing losses have conflicting effects on strain changes and are difficult to assess individually. Therefore, it could be difficult to detect the prestressing losses and the effect of individual degradation issues using strain instrumentation.

Several large thin-webbed box girders, with post-tensioned anchorage zones designed in accordance with AASHTO and ACI requirements, have experienced large cracks along the tendon path in the anchorage zones at the design stressing load. Cracking of this nature provides a path for penetration of moisture and salts and thus presents a potential corrosion and frost damage threat. In addition, such cracking negates a major reason for the use of prestressed concrete, the minimization of service load cracking. This report summarizes the major design-related observations and conclusions from an extensive analytical and experimental program which studied anchorage zone behavior of post-tensioned box girders. The experimental program investigated the primary variables affecting the formation of the tendon path crack: tendon inclination and eccentricity, section height and width, tensile splitting strength of the concrete, anchor width and geometry, and the effect of supplementary anchorage zone reinforcement, both active and passive. An extensive series of three-dimensional linear elastic finite element computer analyses was used to generalize these results and develop a failure theory to explain tendon path crack initiation based upon specified peak spalling strains at the edge of the anchorage. The theory agreed well with the experimental data over a wide spectrum of variables. (FHWA)

Prestressed concrete has been increasingly used in the construction of bridges due to its superiority as a building material. This has necessitated better assessment of its on-site performance. One of the most important indicators of structural integrity and performance of prestressed concrete structures is the spatial distribution of prestress forces over time, i.e. prestress losses along the structure. Time-dependent prestress losses occur due to dimensional changes in the concrete caused by creep and shrinkage, in addition to strand relaxation. Maintaining certain force levels in the strands, and thus the concrete cross-sections, is essential to ensuring stresses in the concrete do not exceed design stresses, which could cause malfunction or failure of the structure. This paper presents a novel method for monitoring prestress losses based on long-gauge fiber optic sensors embedded in the concrete during construction. The method includes the treatment of varying environmental factors such as temperature to ensure accuracy of results in on-site applications. The method is presented as applied to a segment of a post-tensioned pedestrian bridge on the Princeton University campus, Streicker Bridge. The segment is a three-span continuous girder supported on steel columns, with sensors embedded at key locations along the structure during construction in October 2009. Temperature and strain measurements have been recorded intermittently since construction. The prestress loss results are compared to estimates from design documents.

Experimental study of concrete beams prestressed with basalt fiber reinforced polymers. Part II: Stress relaxation phenomenon

Fiber reinforced polymers (FRPs) due to their specific high-strength properties become more and more popular and replace traditional structural materials like conventional steel in prestressed concrete structures. FRP reinforced structures are relatively new when compared to structures prestressed with steel tendons. For that reason only several studies and applications of pre-tensioned FRP reinforcement have been conducted until now. Moreover, researchers only considered short-term behavior of FRP reinforced concrete members. The precise information about long-term behavior of FRP reinforcement is necessary to evaluate the prestress losses, which should be taken into account in the design of prestressed RC structures. One of the most important factor influencing long term behavior of FRP reinforcement is stress relaxation. The overview of experimental tests results described in the available literature considering the prestress losses obtained in FRP prestressed concrete members is presented herein.

Monitoring and analysis of long-term prestress losses in post-tensioned concrete beams

Performance analysis of rolling multi-lug cable-strut joints with bearings under extreme loads

Prestress loss has a great influence on working performance of bridge structure, which is concerned highly by the engineers. In this paper, based on a large span continuous rigid frame bridge, the prestress change during the construction and prestress loss after closure is tested and analyzed by laying the magnetic flux sensor. The test and calculation results show that during segmental cantilever construction process, the measured values is very close to the theoretical value, indicating that the magnetic sensor is reliable. After the whole bridge is closed, the prestress value of T10, D6 and D2’ decreases with time, and the change trend is basically the same for 150 consecutive days for which have no any construction work on the bridge deck. This result has a high reference value for the design and construction of long-span prestressed continuous beam (rigid) bridge.

For prestressed concrete containment structure, prestress loss is a key factor that affects the performance of containment structure. Therefore, prestressed time-limited aging analysis (TLAA) is essential for containment structures. The main objective of prestressed TLAA is to assess the safety of containment structures after prestress loss occurred over time. This paper takes the in-service containment structure as an example to investigate the method of TLAA for grounted prestressed containment structure. Firstly, it introduces methods for prestressed TLAA. Secondly, a finite element model of containment structure is established to calculate the minimum required value (MRV) of prestress. The numerical model is verified by the pressure test results. Thirdly, prestress loss of tendons is calculated. Finally, the residual prestress of tendons are compared with the MRV of prestress to confirm whether the containment can service in a certain period. This study can provide guidance for goouted prestressed TLAA of containment structures.

The performance of prestressed concrete bridges (PCBs) is significantly affected by the prestress loss and overloading vehicles over their service life. Structural health monitoring is critical to prevent the structural collapse and life-cycle management and cost-effective maintenance for PCBs. This paper presents an innovative dynamic equilibrium-based internal force identification (IFI) approach for evaluating structural performance of PCBs, incorporating unknown prestress and moving forces. By integrating substructural modeling and Lobatto element-based Lagrangian interpolation, a new approach is proposed for accurate and efficient IFI. This technique is structured as a two-phase process: the first focuses on an enhanced methodology for identifying prestress and vehicle-induced excitation on a PCB, while the second introduces an improved approach for IFI from the identified prestress and excitation. To solve the inverse problem of identifying external forces resulting from prestress and moving forces, an improved regularization technique based on generalized singular value decomposition (GSVD), known as the compressed GSVD (CGSVD) technique, is introduced. This improved regularization method incorporates a compression factor to mitigate the ill-posedness of the inverse problem, enabling robust and precise IFI by minimizing the ill-posedness in identifying prestress forces and vehicle-induced excitation, using only displacement responses. By substructural modeling techniques, the number of sensors can be significantly reduced while enabling the IFI at arbitrary sections of PCBs. The proposed method is validated using experimental and numerical study on a simply supported prestressed concrete box girder bridge under general excitation and experimental study on a prestressed concrete [Formula: see text]-beam under moving loads. Results validate the method’s accuracy and effectiveness in IFI and in estimating prestress forces and vehicle-induced excitation, offering clear insights into PCBs’ structural performance.

To enhance the bearing capacity of cable–pylon anchorage zones in cable-stayed bridges, this paper proposes the integral steel wall plate (IWP) structure and investigates the structural performance of its application in anchorage zones with a steel anchor beam and with a steel anchor box. The proposed structure contains an end plate, a surface plate, and several perforated side plates, forming steel cabins that encase the concrete pylon wall, where the steel and concrete are connected by perfobond connectors on side plates. A half-scaled experiment and a finite element analysis were first conducted on the IWP with the steel anchor beam to study the deformation at the steel–concrete interface, as well as the stress distribution in steel plates and rebars. The results were compared with experimental data of a conventional type of anchorage zone. Then, finite element models of anchorages with steel anchor boxes were established based on the geometries of an as-built bridge, and the performance of the IWP structure was compared with conventional details. Finally, the effects of plate thickness and connector arrangement were investigated. Results show that the proposed IWP structure offers excellent performance when applied with an anchor beam or anchor box, and it can effectively reduce principal tensile stress on the concrete pylon wall compared with conventional anchorage details.

The post-tensioned concrete I-girder superstructure bridge made by high-performance concrete (HPC) typically exhibits deceptive time-dependent behavior issues with age of concrete. The most common changes are observed deformation, top and bottom fiber stress variation and loss of pre-stress in I-girder due to the secondary effects of shrinkage and creep in concrete. However, in design optimistic expectation of the time-dependent behavior of several post-tensioned concrete I-girder bridges are premature by the existing bridge code, design specification and available prediction models. Nevertheless, about 1–10 years the time-dependent behavior is observed inaccurate in prediction and introduced serviceability and sustainability problems. Consequently, more study is needed toward the time-dependent behavior prediction of the concrete bridges using HPC shrinkage and creep dataset and existing material prediction models. In the present study author’s own an experimentally measured HPC shrinkage and creep database is considered from the literature Gedam et al. 2015 and for same shrinkage and creep are predicted using existing material models such as the American Concrete Institute (ACI), the fib model code 2010 (fib), the Bazant and Bawaja (B3) and the Gardner and Lockman (GL). The results experimentally measured of the shrinkage and creep and predicted by existing material models are incorporated in incremental time-step analysis method to obtain time-dependent behavior results of the simply supported post-tensioned I-girder up to 800 days. Furthermore, outcome long-term behavior results from both an experimental and models are comparatively studied. It has been observed in comparative studies that the existing shrinkage and creep prediction models are not found sophisticated in prediction of the HPC shrinkage and creep properties. Also, it has a probability that in the conventional analysis, design and regional construction, use of any one out of the four existing material models may be produced serviceability and sustainability issues in long-term behavior prediction. In fact, these material models needed re-evaluation and modification, especially local environmental condition and indigenously sourced concrete materials properties.

In order to analyze the impacts of pre-stress loss on the long-term deflection for long-span PC continuous girder bridges, this paper presents a numerical analysis using the finite element analysis software MIDAS/Civil based on a long-span PC continuous box-section girder bridge in Shijiazhuang. Once the 3-D finite element model was established, the influences of different pre-stress loss levels and locations were analyzed in a numerical simulation. Pre-stress loss is often the key reason for long-term deflection in long-span PC continuous girder bridges, so we can estimate the development of deflection by considering these factors during the operation.

Composite beam theory for pretensioned concrete structures with solutions to transfer length and immediate prestress losses