Post-tensioned Concrete Bridge Research Articles

Machine learning-augmented multi-arrayed fiber bragg grating sensors for enhanced structural health monitoring by discriminating strain and temperature variations

AbstractFiber optic sensors (FOS) in long-term structural health monitoring (SHM) have drawn significant attention due to their pivotal role in detecting defects and measuring structural performance in diverse infrastructures. While using FOS, temperature variation due to environmental factors is still considered one of the major challenges to isolating sensing parameters. To address this issue, we reported a machine learning (ML)-augmented multi-parameter sensing system that enables simultaneous detection of strain and temperature effects based on one single fiber Bragg gratings (FBGs) sensor for SHM. The initial phase entailed designing, fabricating, and characterizing a novel FBG sensor in the laboratory, incorporating a set of four FBGs, each distinguished by distinct Bragg wavelengths. In the next phase, ML algorithms are employed to separate temperature effects from strain variations. As a proof of concept, mechanical loading tests are conducted on the sensor, exposing the FBG portion to various temperature conditions. In the final phase, data collected from a post-tensioned concrete bridge embedded with both strain and temperature FBG sensors are utilized, and the developed ML models are applied to observe real-environment outcomes. Despite the limited feature points of collected FBG spectrums, the developed ML models effectively address cross-sensitivity issues induced by temperature perturbations. The long-term benefit of using FOS is that it will enable a better understanding and utilization of aging infrastructure. This will potentially reduce embodied carbon of infrastructure in the future and assist in the global efforts to achieve Net-Zero.

Concrete and reliability of existing prestressed bridge structures

A large number of post-tensioned concrete bridges were built in the second half of the last century. They have often been insufficiently maintained during their lifetime (usually around 50 years). Nowadays, these structures exhibit significant deterioration, mainly due to leakages and also due to various other deficiencies such as a small concrete cover. Their load-bearing capacity needs to be verified. This paper focuses on estimating the load-bearing capacity calculation of existing post-tensioned concrete bridges. In the engineering practice, this is carried out using the partial factor method according to the currently valid standards (Czech standards ČSN and the Eurocodes), which often impose more stringent requirements than the original standards. The partial factor method then often leads to low load-bearing capacities. This study deals with the bridge for which a very low load-bearing capacity has been determined. For this reason, a comparative probabilistic analysis was performed, allowing for a better description of the uncertainties in the resistance and load effect variables. The probabilistic approach appears to be less conservative and yields a higher load-bearing capacity.

Redundancy and reliability levels of post-tensioned and grouted concrete cross-section in case of tendon failure

Since the 1960 s and 1970 s, the prestressing steel used on post-tensioned concrete (PTC) bridges has been known to be susceptible to stress corrosion cracking (SCC). This can result in brittle rupture of the prestressing steel. Objective of this study is to evaluate the reliability levels achieved with assessment method developed for verifying crack behaviour before failure in cross-section of PTC bridges with cross-section loss of prestressing steel. Additionally, the study aims to investigate of tendon loss on crack before failure behaviour. This paper applies a reliability analysis for methodology based on robustness analysis on crack before failure behaviour in PTC with deteriorating prestressing steel. The analysis involves examining cross-sections with various design criteria. The effects of choices made in designing the PTC structure are analysed and noted to have a significant impact on the safety margin of the structure, while the deterioration of prestressing steel does not significantly decrease the ductility of the cross-section if certain conditions are met. The use of stricter serviceability limit-state (SLS) criteria used in past design results the greater safety margin for the structure in the event of tendon loss, which therefore makes it beneficial for structural redundancy in the ultimate limit state (ULS).

Field investigation and numerical analysis of horizontal cracking in ledge girders of post-tension concrete bridges

This research addresses the pressing issue of horizontal crack initiation in ledge girders of post-tensioned concrete (PTC) bridges, which is a significant concern for the safety and longevity of long-span bridges. With field observations and advanced engineering computations, the study identifies the primary causes of these cracks and explores potential mitigation strategies. The research pinpoints the primary causes of horizontal crack initiation in PTC girders by analyzing 3D finite element modeling and comparing the results with the analytical solution suggested by a design code. The study also probes the potential of using external prestressing with a post-tensioning bar to mitigate horizontal cracking. The findings underscore the significance of adhering to the design code requirements for hanger reinforcement, which notably mitigates crack width in PTC girders. The research also highlights that higher prestress losses correspond to increased crack width, emphasizing the need to consider prestress losses for maintaining structural resilience. Furthermore, the study reveals that in all strengthening cases, the maximum crack width is notably reduced at the point of failure, albeit less effectively than when using rebar as per the design code. These findings underscore the potential of post-tensioning as a strengthening technique and the importance of adhering to design standards.

Experimental Investigation on Nonlinear Flexural Behavior of Post-Tensioned Concrete Bridge Girders with Different Grouting Conditions and Prestress Levels

Experimental Investigation on Nonlinear Flexural Behavior of Post-Tensioned Concrete Bridge Girders with Different Grouting Conditions and Prestress Levels

Damage of a post-tensioned concrete bridge – Unwanted cracks of the girders

The cracking of a post-tensioned T-beam superstructure, which was built using the incremental launching method, is analyzed in the paper. The problem is studied in detail, as specific damage was observed in the form of longitudinal cracks, especially in the mid-height zone of the girder at the interface of two assembly sections. The paper is a case study. A detailed inspection is done and non-destructive testing results of the girders are briefly discussed. The attention is especially focused on advanced and comprehensive numerical simulations of the bridge mechanical behavior. Linear and nonlinear static calculations are performed employing the Finite Element Method at global and local levels of precision, enabling deep insight into the bridge response during all the stages of bridge construction and after it is opened to traffic. The crack propagation process in local analyses is described by the application of the Concrete Damage Plasticity law, the parameters of which were carefully chosen. The predicted damage patterns closely resemble those observed at the site. The results reveal, that the girder damage process was initiated when centric prestressing was applied, because vertical reinforcement of the assembly section front-end surface was not designed. However, the damage did not compromise the safety of the bridge. Finally, the repair methods employed are described and also a discussion is presented on how to prevent the occurrence of such cracking.

Galvanic Corrosion of Strands in Re-Grouted, Post-Tensioned Concrete Bridges

Grouted, post-tensioned (PTD) concrete systems are widely used to construct bridges, typically with an anticipated corrosion-free service life of 100+ y. However, the usage of inadequate grout materials and grouting practices in PTD concrete systems have caused unwanted air voids in ducts, leading to strand/grout/air interface, carbonation of exposed grout layer, and localized corrosion of strands (say, within about 10 y to 20 y). Re-grouting of voids as a tendon repair strategy has led to accelerated galvanic corrosion of the portion of strands at the interface between the carbonated base grout and repair grout with different chemistry, raising concerns and reluctance in re-grouting of voids in tendons. This work focused on understanding and quantifying the galvanic corrosion at the interface of carbonated base grout and repair grout in a re-grouted tendon. The theoretical analysis based on mixed potential theory estimated a galvanic current density of approximately 2 µA/cm2 and showed that the galvanic coupling can increase the corrosion current density of the prestressing steel in the base grout by about two-fold. The study on prestressed steel in simulated solutions estimated a galvanic current density of approximately 20 µA/cm2. Then, the study on prestressing steel in grouts and the analytical simulation estimated galvanic current densities around 1.5 µA/cm2 to 2 µA/cm2 at 95% external relative humidity (ERH) and 25°C. A model relating the galvanic current density in grouted systems as a function of ERH was developed, which showed an exponential increase in the galvanic corrosion with an increase in ERH. Also, a case study showed that if the tendon anchorage region experiences 95% ERH for about 20 y, sufficient strand corrosion could happen, and structural behavior can change from ductile to brittle nature, which could be a serious concern for structures in the coastal zone.

Vision-Based Structural Monitoring: Application to a Medium-Span Post-Tensioned Concrete Bridge under Vehicular Traffic

Video processing for structural monitoring has attracted much attention in recent years thanks to the possibility of measuring displacement time histories in the absence of stationary points close to the structure, using hardware that is simple to operate and with accessible costs. Experimental studies show a unanimous consensus on the potentialities of vision-based monitoring to provide accurate results that can be equivalent to those obtained from accelerometers and displacement transducers. However, past studies mostly involved steel bridges and footbridges while very few applications can be found for concrete bridges, characterised by a stiffer response with lower displacement magnitudes and different frequency contents of their dynamic behaviour. Accordingly, the attention of this experimental study is focused on the application of a vision-based structural monitoring system to a medium-span, post-tensioned, simply supported concrete bridge, a very common typology in many road networks. The objective is to provide evidence on the quality of the results that could be obtained using vision-based monitoring, understanding the role and influence on the accuracy of the measurements of various parameters relevant to the hardware settings and target geometry, highlighting possible difficulties, and providing practical recommendations to achieve optimal results.

Assessment and strengthening of post-tensioned concrete box-girder bridges

Post-tensioned concrete box girders have been widely adopted for the construction of medium- to long-span bridges since the mid-20th century. Since the construction of the first such bridges, there has been rapid growth in the volume and weight of heavy vehicles, as well as an increase in exposure to adverse environmental conditions. At the same time, it is estimated that more than 50% of existing bridges in higher-income countries are over half-way through their design life and thus bridge owners are challenged with the management of aging structures. When deterioration in prestressed concrete bridges is identified during inspections, it leads to specific assessments and is usually followed by the need for bespoke strengthening design solutions. In many cases, the challenge in implementing the strengthening resides in maintaining structural capacity to minimise traffic disruption before the works are complete. This article covers the technical and asset management aspects related to the assessment and subsequent strengthening design for post-tensioned concrete box-girder bridges through the lens of case studies on two successful post-tensioned bridge rehabilitations. The strengthening solutions presented have been implemented with minimal disruption to traffic and have had a positive environmental impact by avoiding demolition and full deck reconstruction.

Sensory monitoring system of the post‐tensioned concrete bridge over the Odra River in Poland

AbstractThe paper presents the Structural Health Monitoring system installed on the posttensioned concrete road bridge over the Odra River in Poland. The bridge superstructure has a form of the 5‐span box girder with skewed supports, built using the cantilever method. During operation of the bridge (since 2010) many cracks arrived in the superstructure, mainly within the main bridge span with a length of 140 m. Therefore, as a part of a complex diagnostic process, the monitoring system were incorporated for assessment of the bridge performance and for observing further propagation of damage. The implemented monitoring system utilizes Fiber Bragg Grating (FBG) technique for measuring of strains, changes of cracks width and angular displacements of concrete box girder for various operating conditions of the structure including effects of regular road traffic, environmental influences and rheological phenomena. The paper presents applied testing and measuring procedures as well as results of the monitoring. Conclusions regarding effectiveness of the applied testing method in identification of the bridge condition are formulated and discussed.

Long-Term Health Monitoring Data Processing on Post-Tensioned Concrete Box-Girder Bridge by Wavelet-Based

The concrete box-girder bridge is designed to have a long service life of around 100 years. To ensure safety and performance degradation during long service life, a Structural Health Monitoring System (SHMS) has been implemented in the box-girder bridge. SHMS can reliably assess structural response due to real-time applied loads, detect anomaly activities and locate the structural damage in the structure. Several sensors have been implemented in the bridge to continuously record the behavior of the bridge in all environmental conditions. Due to real-time natural conditions, false alarms occur frequently in SHM due to the disruption of noises and lead to misunderstanding of who is evaluating. Nevertheless, numerous SHM data that have been collected make it complicated to determine the anomaly of the structures. Therefore, it required signal processing to maximize the potentialities of the massive SHM data, as well as the efficiency of the time work. In this study, wavelet transformation, a rapid and unsupervised signal processing approach, was used to analyze the huge signal data by removing noise, and separating different signal sources as well. Further, with time-frequency analysis and multi-resolution capabilities, the transformation of wavelet is a promising tool for analyzing long-term SHM data. The suggested approach is shown by using long-term strain data from a 40 m concrete box-girder bridge in 24h. The results showed that after the denoising process, the highest discrepancy between the reconstructed and original strain signal is 2.73 μƐ and lost their energy less than 1%. Hence, the strain gauge sensor was successfully able to eliminate the noise through wavelet technology.

An Analytical Model to Evaluate Short- and Long-Term Performances of Post-Tensioned Concrete Box-Girder Bridges Rehabilitated by an Ultrahigh-Performance Concrete Overlay

Potentially unfavorable stress redistributions could occur in post-tensioned concrete box-girder bridges upon deck rehabilitation by an ultrahigh-performance concrete (UHPC) overlay. The reason for such redistributions is that the deck forms an integral part of the load-resisting mechanism so that deck rehabilitation, namely, the process of removing the deteriorated deck and casting a fresh UHPC overlay, may cause large stress redistributions in the box girder. In addition, the significantly different shrinkage behavior between the freshly cast UHPC deck overlay and the old existing concrete deck below would result in restrained stress in the UHPC–old concrete interface. Therefore, how to evaluate the stress redistributions and the potentially unfavorable stresses in the box girder resulting from construction operations and the incompatible shrinkage is a crucial issue. Here, we propose an analytical model to assess the short- and long-term performances of the concrete box girder during and after deck rehabilitation. The analytical model was verified by using finite-element models incorporating the staged construction simulation and time-dependent behavior of materials. The model can be easily established and calculated with a great amount of time-saving compared with finite-element modeling, and it applies to different types of post-tensioned concrete box-girder bridges. The analytical model involves the balancing-load concept and the age-adjusted effective modulus method (AEMM). The balancing-load concept in the short-term analysis is to account for the prestressing force, and the AEMM in the time-dependent analysis is an effective tool to determine how stresses and strains vary with time due to the creep and shrinkage of a UHPC deck overlay. A parametric study was then conducted based on the analytical model, which involved the thickness of the deck layer to be rehabilitated and the addition of steel reinforcement in the new UHPC deck overlay. Analysis results indicated that pronounced tensile stresses developed in the UHPC deck overlay, which resulted from the restraint of the free shrinkage of the UHPC overlay by the subjacent concrete deck. An increase in the thickness of the newly cast UHPC deck overlay resulted in a decrease of the tensile stress in the UHPC deck overlay, and a rehabilitation thickness of 3.8–5.1 cm (1.5–2.0 in.) was deemed appropriate for the selected bridge according to the analysis.

Bridge deck analysis of transversely post-tensioned concrete box girder bridges

For rural bridges in Australia, a common design practice is pouring in-situ concrete on top of beams in order to tie all the beams together and distribute load. However, pouring concrete on-site creates more risk and contractors prefer to avoid it. Another method is using transverse post tensioning to tie beams. This article investigated the behaviour of transverse post-tensioning bars in providing load distribution between beams and ultimately comment on their effectiveness compared to in-situ poured decks. Currently, the industry has not completely investigated this matter in order to design post-tensioning accurately. Conservative estimates are currently used in industry today. Current practice is 50% of the design load on the beam where the load is applied in their design assumptions which is quite high. The team modelled concrete box girder bridges with transverse post-tensioning using grillage method. Several factors were investigated including bridge length and width, bridge skew and beam type. From the models, the team concluded that increasing the bridge span increases the load distribution, the load distribution difference is negligible for skew between 0 and 20 degrees and larger shear actions are observed with increased skew and width. It was determined that the worst-case total load on the beam where the load as applied was found to be 40.5%, 9.5% less than current practice. It is recommended that a similar investigation is conducted using a finite element method to gain a deeper understanding.

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.

Smartphone Application for Structural Health Monitoring of Bridges.

The broad availability and low cost of smartphones have justified their use for structural health monitoring (SHM) of bridges. This paper presents a smartphone application called App4SHM, as a customized SHM process for damage detection. App4SHM interrogates the phone's internal accelerometer to measure accelerations, estimates the natural frequencies, and compares them with a reference data set through a machine learning algorithm properly trained to detect damage in almost real time. The application is tested on data sets from a laboratory beam structure and two twin post-tensioned concrete bridges. The results show that App4SHM retrieves the natural frequencies with reliable precision and performs accurate damage detection, promising to be a low-cost solution for long-term SHM. It can also be used in the context of scheduled bridge inspections or to assess bridges' condition after catastrophic events.

Fusion of NDE Scanning and Photogrammetric Data for Concrete Bridge Decks and Girders Condition Assessment

The first case history would present 3-D results of Impact Echo Scanning and Ground Penetrating Radar (GPR) Scanning plus Spectral Analyses of Surface Waves tests of post-tensioned concrete bridge girders. The NDE results were overlaid on photogrammetric images of visible corrosion/cracking damage to the concrete bridge girders of an aging bridge due to de-icing salts and a marine environment. Cart-based scanning with GPR and Impact Echo and Spectral Analyses of Surface Waves NDE methods is commonly being conducted of both bare concrete decks and asphalt overlaid decks to map both top and bottom delaminations and depth of concrete cover. The data is being used in conjunction with chlorides and carbonation testing of concrete cores to recommend repairs and evaluate remaining service lives for bridges. Photogrammetric images of bare concrete in particular are being used to visibly compare areas of spalling, delamination and deck patching for improved interpretation, visualization and mapping of internal deck delaminations and other concrete damage with this data fusion approach to provide more accurate surface and full-depth images of bridge deck conditions.

Reliability analysis of deteriorated post-tensioned concrete bridges: The case study of Ynys-y-Gwas bridge in UK

Concrete structures are exposed to aggressive environmental conditions that lead to corrosion of the embedded reinforcement and pre-stressing steel. Consequently, the safety of concrete structures may be compromised, and this requires a significant budgets to repair and maintain critical infrastructure. Prediction of structural safety can lead to significant reductions in maintenance costs by maximising the impact of investments. The aim of this paper is to establish a framework to assess the reliability of existing post-tensioned concrete bridges. A time-dependent reliability analysis of an existing post-tensioned involving the assessment of Ynys-y-Gwas bridge has been presented in this study. The main cause of failure of this bridge was corrosion of tendons, making it a relevant case study to evaluate the effect of corrosion on bridge safety. Uncertainties associated with material properties, geometry, loads and corrosion parameters are taken into account. The probabilistic models of the uncertainties are combined with a non-linear finite element analysis to study the effect of tendons pitting corrosion on the post-tensioned concrete bridge. The limit state function considered is flexural strength. The Monte-Carlo simulation (MCS) method is used to compute the statistical parameters of the resisting bending moment through a MATLAB code running ABAQUS. It was found that the reliability index for the first year of bridge service life is below the minimum value acceptable for structures. The study confirmed that this bridge was a high risk structure due to its design and location. The proposed framework can be used by engineers and researchers as a tool to support decision for segmental post-tensioned (PT) bridges maintenance since they need for a regular inspection due to their risk to corrosion.

Probabilistic structural analysis of a real-life corroding concrete bridge girder incorporating stochastic material and damage variables in a finite element approach

A combined probabilistic approach based on response surface sampling (RSS) and importance sampling (IS) is developed to efficient assess the structural reliability of corroding concrete structures. In this approach, the failure probability and the corresponding reliability index can be calculated for any stochastic system, independent of the material and model complexity or the limit state function(s). The combined approach is applied to a real-life example where a corroding post-tensioned concrete bridge girder is considered. This bridge girder is modelled in a dedicated finite element analysis to assess the structural reliability under corrosion damage. Serviceability and ultimate limit state requirements are taken into account and the material and damage properties are the stochastic variables. A converged solution is found in 8 and 128 limit state function evaluations for the RSS and IS, respectively. The importance factors for the different variables are calculated. The full analysis highlights non-important parameters (strength of the traditional reinforcement) and indicates that the ultimate limit state of the girder is the limiting factor in all analyses. Further computational optimizations are proposed and implemented, highlighting the capabilities of the reduced model and the robustness of the proposed approach.

Time-Dependent Reliability Assessment and Updating of Post-tensioned Concrete Box Girder Bridges Considering Traffic Environment and Corrosion

AbstractIn the reliability assessment of a bridge in operation with regards to its ultimate limit state, it should be noted that the strengths of members and effects of traffic load vary continuous...

Comparison of the level of required prestressing of road bridges according to the historical development of technical standards in Slovakia

The paper deals with the level of required prestressing designed for box-girder bridges. Mainly focuses on the development of standards used in the design of prestressed concrete road bridges in the Slovak Republic. The standards for bridge design have been amended several times. A parametric study was performed on a model post-tensioned concrete bridge with a box-girder cross-section. The bridge has three-span with a main span of 50 m and end spans of 40 m. The study compares the normal stresses caused by traffic load along the length of the bridge according to various standards and technical regulations. The differences in design principles were investigated while the amount and geometry of the longitudinal prestressing of the bridge were changed as required by corresponding standards. The numerical model is considered as a beam element with neglecting of the torsional effects of the load. The parametric study points out the differences in the normal stresses along the bridge according to different design regulations and standards. The aim of the study is to compare level of prestressing according to the historical development of prestressed bridges