External Tendons Research Articles

Shear behavior of externally prestressed precast concrete segmental beams with both positive and negative moment regions

To investigate the shear behavior of externally prestressed precast concrete segmental beams with both positive and negative moment regions (EPCS P&N beams), eight specimens were subjected to failure under various shear span to depth ratios, moment ratios, and joint types. Throughout the testing process, critical crack initiation and development, joint slippage and opening, failure processes and modes, as well as strains in concrete, stirrups, and external prestressing tendons were meticulously recorded. At the point of failure, the concrete in the shear-compression zone near the top surface of the joint section (closest to the loading point in the positive moment region) or near the bottom surface of the joint section (closest to the interior support in the negative moment region) experienced crushing. Subsequently, the external tendons lost support and underwent elastic retraction, causing the two segments on either side of the critical web-shear crack to slide against each other. The elastic strain energy was released significantly as a consequence of the external tendons retracting, leading to the rupture of stirrups intersecting the failure cracks, followed by flange buckling. Based on the test results, a strut-and-tie model was developed to analyze the shear resistance mechanism, and a detailed flowchart for computing the shear resistance of EPCS P&N beams was established. The predictions from the proposed analytical method were compared with the test results, showing good agreement and confirming the rationality of the proposed shear resistance mechanism.

Damage Detection in External Tendons of Post-Tensioned Bridges

This study investigates damage detection in the tendons of post-tensioned segmental box girder bridges, focusing on the vibration-based technique and its application in conjunction with the Precursor Transformation Matrix (PTM). Due to the critical role of bridge tendons in structural integrity, efficient and timely detection methods are essential. The methodology combines theoretical modeling with Finite Element Method (FEM) simulations and empirical data collection to evaluate the PTM’s effectiveness in identifying tendon damage. Key results indicate that the PTM, when paired with vibration analysis, enhances the identification and localization of damage, proving to be a robust method in structural health monitoring. This approach not only speeds up damage detection but also potentially lowers maintenance costs by pinpointing specific damage sites, thereby preventing widespread structural failures. The findings offer a promising tool for ensuring the longevity and safety of post-tensioned bridges.

Some issues about the behaviour of external pre-stressed steel–concrete composite beams

A numerical experiment is performed to evaluate the behaviour of externally pre-stressed steel–concrete composite beams (EPSCCBs) up to their ultimate stages, considering second-order effects, tendon material and friction at tendon–deviator locations. The tendon force evolution with loading and slip at the steel–concrete interface is explored. A three-dimensional finite-element model is used to include all these aspects. The idea is to deconstruct the project of eight experimental EPSCCBs with different tendon profiles, considering the above-mentioned issues. The main findings indicate that the inclusion of second-order effects is essential for predicting the stress paths near the collapse loads and slip demands at the steel–concrete interface, while external tendons made of carbon fibre-reinforced polymer (CFRP) can be used as a substitute for classical steel ones, as they presented similar behaviours. The largest tendon force increments occurred for the CFRP tendons compared to other tendons made of glass, aramid and basalt fibre-reinforced polymers. Furthermore, it was found that a friction coefficient between 0.2 and 0.4 at the tendon–deviator interface better matches the available experimental tendon force evolution with loading. This finding seems to counteract the common assumption of using a null friction coefficient as suggested in other works.

Review and analysis of RC bridge half-joints strengthening techniques

In the past, the extensive use of the Gerber scheme in the construction of existing bridges has been justified by the advantages in terms of reduced stresses of the statically determined scheme and rapid execution. On the other hand, the presence of junction elements at the ends of the beams has over time led to a series of issues, especially related to their durability. In fact, the half-joints’ geometric configuration leads to the accumulation of degradation due to platform water seepage, resulting in the initiation of reinforcement corrosion, especially in the presence of chlorides derived from de-icing salts used in the winter season. Furthermore, these elements have been designed for lower loads than those specified in current regulations, and therefore, they may be in critical conditions where the strength reduction, due to existing degradation, can lead to failure. For these reasons, local repairs and/or strengthening interventions are necessary to ensure adequate levels of safety. Within this study, the topic of reinforcing half-joints is addressed, first by describing the currently available intervention techniques in the literature, and then by deepening an intervention technique based on the use of external post-tensioned bars, applied to a case study bridge through nonlinear numerical simulations. These analyses allow quantifying the performance improvements of this technique under both service conditions and ultimate loads, considering various parameters, including the pre-stress value applied to the external tendons and different scenarios of chloride-induced corrosion.

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.

Inspection for Voids in the Grout below the Protective Duct of an External Post-Tensioning Bridge Tendon Using a THz A-Scanner

Grout voids in the tendons of a post-tensioning bridge reduce their strength. Grout voids are also severe flaws causing corrosion of the steel strands in the tendons. Detecting voids during construction and operation of the tendons is essential to prevent tendon failure, which is critical to bridge safety. This study presents a method for inspecting external tendons for voids in the grout below the protective duct pipe using terahertz electromagnetic waves. Due to low attenuation in the high-density polyethylene duct and the large reflectivity difference between the duct/grout and the duct/void interfaces, terahertz waves are suitable for detecting voids in the grout inside tendons. For this study, we developed a mobile frequency-domain terahertz A-scanner that can be used to measure terahertz A-scan data in real time. It is shown that the mobile terahertz A-scanner can be used to assess the area of the grout void in external bridge tendons.

An Experimental Study on the Deterioration Behaviour of External Tendons Due to Corrosion

An external tendon failure of the PSC box girder was found due to corrosion in South Korea, and the maintenance authorities promoted a study on evaluation and maintenance methods. One of their concerns was to confirm that the tendons were constantly available through repair when voids or minor corrosion were found in ducts. This study, thus, conducted external tendon experiments with insufficient grout filling and corrosion, and the resulting decrease in deterioration behaviours was investigated. The corrosion was accelerated during the experiment, and the strains and prestressing force were measured. The test results indicated minimal reductions in prestressing force and strain as the corrosion was induced, followed by significant fluctuations when a corroded wire ruptured. Prior to the rupture, the strain development was approximately 130 µε. However, after the rupture, the strain increased significantly, reaching a maximum of 4,296 µε in compression and 2,776 µε in tension. It was also confirmed that enough grout ratio to strands resulted in meaningful stress redistribution among the strands and that the wire rupture caused various strain development due to impact load. Based on the test result, a method for corrosion monitoring after re-grout was also proposed using low-power sensing and strain gauges.

Hysteretic performance of prestressed concrete beams with AFRP tendons under cyclic loading

AbstractThis paper aims to evaluate the hysteretic performance of prestressed concrete beams with aramid fiber‐reinforced polymer (AFRP) tendons. Eight beams were constructed and tested under cyclic loading, with test variables covering the type of prestressing tendons (AFRP and steel), the ratio of internal tendons to external tendons (3:0 and 1:2) and the bond condition of internal tendons (fully bonded, partially bonded, and unbonded). Results showed that the failure of all the beams was dominated by concrete crushing and nonprestressed rebars rupturing at pure bending sections. AFRP prestressed beams exhibited commensurate bearing capacity, displacement ductility, energy dissipation and residual deformation with steel prestressed beams. The combination of internal and external tendons, and using unbonded or partially bonded tendons could improve the ductility of AFRP prestressed beams. Additionally, the finite element (FE) models of AFRP prestressed beams were developed using ABAQUS based on the coupling method under the local coordinate system. The simulated results closely matched the experimental results in terms of the failure mode and load–displacement curve. Parametric predictions, including the unbonded length of AFRP tendon, partial prestressing ratio, jacking stress and concrete strength were presented to extend the experimental works.

Damage Assessment of Segmental Concrete Beams with Different Types of Joints and Prestressed External Tendons

Precast segmental beams with prestressed external tendons have become increasingly common in bridge constructions owing to the merits of high-quality control and fast onsite construction with minimum environmental impact. Due to the nonlinear characteristics of the dynamic behavior of these segmental beams associated to the joint opening, the traditional condition monitoring and damage assessment methods based on vibration parameters either in time or frequency domain using the linear theory are unsuitable. To overcome this challenge, this paper proposes a damage assessment method through analyzing nonlinear structural dynamic responses for monitoring the conditions of segmental beams with different joint types and prestressed external tendons. Two damage indices are proposed to identify the occurrence of damage and to qualitatively indicate the damage severity. In the proposed approach, the measured dynamic responses of the segmental concrete beams under hammer impact are first adaptively decomposed into a finite number of mono-components by using variational mode decomposition (VMD) technique. Then, the instantaneous frequencies of the decomposed mono-components from the dynamic responses are obtained by conducting the Hilbert transform. Two damage indicators are defined for the damage assessment and for the identification of the damage location, respectively. Three four-span segmental beams with different types of joints (i.e. dry joints or epoxy joints) and prestressing tendon types (i.e. steel and carbon fiber reinforced polymer tendons) were constructed and tested under five loading levels with different damage severities. The feasibility and accuracy of the proposed approach are verified by comparing the identification results with the secant and initial stiffness extracted from the load–deflection curves at different loading levels. The effectiveness of these two damage indicators is also validated with the structural conditions observed in the tests. The results show that the proposed approach can successfully identify damage in segmental concrete beams.

Extending the life of bridges with external tendons

AbstractHistorically, expanding economies see a boom in the construction sector followed, some decades later, by issues associated with aging infrastructures. Sometimes, the bridges are performing as expected but a change in the codes or on the traffic loads trigger the need for wider or stronger structures. When this happens, the old paradigm of building bigger and more modern bridges is no longer valid, as we face one of the biggest challenges of our history: climate change. Sustainability is, or hopefully it will soon be, the main criteria when it comes to deciding how to intervene on an aging bridge. It is widely agreed that extending the life of an existing structure is more sustainable than building a new one.Amongst the many ways of retrofitting a bridge, external tendons made of prestressing steel are a versatile, well proven, and efficient way of strengthening existing structures, whether these were originally fitted with external tendons, or not. For bridges with damaged existing external tendons, specific replacement methods have been developed.In this paper, we will present a selected number of bridges where external tendons have been added or replaced, focusing on the lessons learnt whether for future strengthening projects or for the design of new structures.

Strengthening of Precast Segmental Bridge built in 1960s using External Tendons

AbstractRecently more attention has been paid to ageing infrastructure under increased traffic on the roads in Slovakia. As a result, selected prestressed concrete bridges have been inspected. Evidence of their poor technical condition has been discovered including serious faults such as large permanent deflection, wide cracks, extensive prestressing degradation, and others. The original conceptual design and poor quality of prestressing steel protection are often the decisive factor. This paper provides an example of bridge rehabilitation design of the concrete precast segmental bridge in an emergency condition. The bridge in question which is located over the water reservoir Ruzin was built in 1967 as one of the first‐generation precast segmental bridges in Slovakia. Its service life was successfully extended using structural system modification and strengthened using external prestressing. The initial assumptions for the rehabilitation have been based on the detailed analysis of the defects, and assessment of existing conditions including the analytical approach of residual prestressing determination. This concept represents the potential solution for achieving enhanced service life of similar bridges.

Experiences in assessment, replacement and repair of external tendons

AbstractThe paper presents experiences with damages that occurred when using external tendons. Reason for these damages could be both technical deficiencies of a tendon system (system = components, design, corrosion protection, installation, stressing etc.) or simply human mistakes. Damages often could occur at deviators and anchorages, since forces are transferred to the structure there and the tendon is loaded in transverse direction. Damages will be presented, explained (why it is a damage), which measure had been taken and of course also advice for the future (lessons learned). The damages occurred on site or during testing of external tendon systems. Hence, the paper shall help the reader/attendant to understand sensitivity of external tendons. Furthermore, to these damages experiences with replacement (mainly cutting of the old tendon) of external tendons with flexible filler mass are shared. “Experience” focusses mainly on the de‐installation, re‐installation, grouting and stressing process. Topics that will be covered are:‐ Improper design and/or installation of deviators and subsequent damages to the tendon sheath‐ Torsional strand failure due to improper deviation at stressing anchor‐ Limitations of the tendon path‐ Deficient encapsulation of the prestressing steel‐ Tendon force measurement options

Investigation of the unusual deformations of external tendons in concrete highway bridges

AbstractDuring a regular inspection of a multi‐span highway bridge, an unusual deformations of external post‐tensioned tendons were observed. These deformations were not noticed or recorded during the previous main inspections. A detailed visual inspection of the condition of external tendons was performed on all bridges with external tendons by mapping the location and extent of unusual deformation. Based on this, several non‐destructive tests were performed, where deformations were first noticed, from magnetic‐flux leakage to radiographic screening. Some local destructive investigations were subsequently carried out at several locations in two bridges, i.e. opening the tendons. The result of the tendons opening showed the presence of foreign objects and non‐tensioned strands, the latter being a possible cause for the deformations of that tendon. In both locations, the injection mass was compact with only hairy transverse cracks, both on the undeformed part as well as on the moderately deformed part of the tendon. In this paper, the latest results from this study are presented in detail.

A nonlinear finite element framework for static and dynamic analysis of structural cables with deviating supports

Structural cables are important load-bearing members in many light-weight and slender structures. The mechanical behaviour of tension cables is complex as it is dominated by geometric nonlinearities, complex support conditions and uncertainties arising from the composition of the cable cross section. Numerous models to describe static and dynamic cable behaviour were developed, differing in the model assumptions and the simplifications made. Most common are geometrical linearisation, dimensional reduction, and the disregard of localised support and stiffness effects. Some models take a combination of some geometric nonlinearities, cable bending stiffness and certain boundary effects into account. This paper first presents a literature review aimed at systematically categorising cable force identification models. Based on this, the paper proposes a novel numerical cable modelling framework that allows for accurate predictions of static and dynamic cable response, e.g. for the use in the inverse system identification techniques of Structural Health Monitoring. The saddled cable model is based on a nonlinear Finite Element formulation to account for the geometric and mechanical effects of deviating supports on structural cables even for very large sag cables. After validation with analytical solutions, the models show that the effect of self-weight distribution and cable bending stiffness considerably affects the identified internal force distribution, contact length on the saddle and cable form. Further, the model is used to predict the tension force based on the identified natural frequencies of an external tendon installed on a bridge. Compared with the actual tension force, the saddled cable model provided very high prediction accuracy, even when compared with other cable models. Therefore, it is shown to be very accurate and can be used to determine force-response relationships used in system identification techniques when increased accuracy is needed for such complex cable arrangements.

The Flexible Armor of Chinese Sturgeon: Potential Contribution of Fish Skin on Fracture Toughness and Flexural Response.

Fish skin is a biological material with high flexibility and compliance and can provide good mechanical protection against sharp punctures. This unusual structural function makes fish skin a potential biomimetic design model for flexible, protective, and locomotory systems. In this work, tensile fracture tests, bending tests, and calculation analyses were conducted to study the toughening mechanism of sturgeon fish skin, the bending response of the whole Chinese sturgeon, and the effect of bony plates on the flexural stiffness of the fish body. Morphological observations showed some placoid scales with drag-reduction functions on the skin surface of the Chinese sturgeon. The mechanical tests revealed that the sturgeon fish skin displayed good fracture toughness. Moreover, flexural stiffness decreased gradually from the anterior region to the posterior region of the fish body, which means that the posterior region (near the tail) had higher flexibility. Under large bending deformation, the bony plates had a specific inhibition effect on the bending deformation of the fish body, especially in the posterior region of the fish body. Furthermore, the test results of the dermis-cut samples showed that the sturgeon fish skin had a significant impact on flexural stiffness, and the fish skin could act as an external tendon to promote effective swimming motion.

Serviceability Performance of Externally Prestressed Steel-Concrete Composite Girders

The behavior of externally prestressed composite beams under short term loading has been studied. A computer program developed originally by Oukaili to evaluate curvature is modified to evaluate the deflection of prestressed composite beam under flexural load. The analysis model based on the deformation compatibility of entire structure that allows to determine the full history of strain and stress distribution along cross section depth, deflection and stress increment in the external tendons .The evaluation of curvatures for the composite beam involves iterations for computing the strains vectors at each node at any loading stage. The stress increment determined using equations depended on the member deflection at points of connection. The stress increment determined using equations depended on the member deflection at points of connection. The proposed model results for load –deflection response are compared with experimental data taken from Auyyb's beams. For beams with straight tendon profile the average discrepancy reached 5.77%, 8.48% and 5.23% corresponding to the 0.25, 0.5 and 0.75 of the maximum load, respectively. For beams with the draped tendon profile, the average discrepancy of the analytical deflections values reached 15.5%, 5.8% and 6.45% corresponding to the 0.25, 0.5 and 0.75 of maximum load, respectively.

Shear behavior of precast ultrahigh-performance concrete (UHPC) segmental beams with external tendons and dry joints

Shear behavior of precast ultrahigh-performance concrete (UHPC) segmental beams with external tendons and dry joints

Mechanistic understanding of precast UHPC segmental beams with external tendons and epoxy joints subject to combined bending and shear

Precast ultra-high-performance concrete (UHPC) segmental bridges (PUSBs) have become a competitive bridge type in modern civil engineering. To establish a thorough mechanistic understanding of PUSBs, seven specimens were fabricated and experimentally investigated. These specimens were designed to have different longitudinal reinforcements, construction methods, joint types, shear span-to-depth ratios (λ), stirrup ratios, and numbers of shear keys. Two failure modes were observed in the tests: flexural failure with crushing of the UHPC at the joints, and shear failure with shear tension sliding and concrete spalling of the web. Additionally, it was found that although the ultimate capacity, stiffness, and cracking load of the segmental beams were slightly lower than those of the monolithic beams, the segmental beams tended to exhibit better ductility and deformation capacity. As λ increased, the ultimate capacity and stiffness of the segmental beams decreased, whereas the growth rates of the deflection and tendon stress increased. Moreover, the stirrups showed a positive effect on the cracking control ability and shear strength of the beams. The three-keyed specimen exhibited better structural behavior than the single-keyed specimen. Finally, calculation methods were proposed to predict the cracking strength and ultimate capacity of PUSBs.

Nonlinear Analysis of External Post-Tensioning anchorage of Concrete Segment

Abstract: Precast concrete segmental bridges (PCSBs) have been the most common design technology used in the last decades. In these studies Non-linear structural performances of externally prestressed, precast concrete segmental bridges (PCSB). PCSBs with externally prestressed tendons have become very popular in construction because of economical and safety reasons, fast and practical construction, and outstanding serviceability. External tendons technique is widely used because it allows to inspect the cables and to replace them or to reinforce the tendons in case of damage while such kinds of actions are difficult to be taken in case of internal prestressing. It is widely recognized that segmental bridges have better durability, lower life-cycle costs and higher quality for maintenance than other types of bridges. However, there is lack of reliable computational model for analyzing behavior of post-tensioned PCSBs. Experimental analysis of bridge segment required more cost, time and effort for the analysis and also take cake of quality of material, cast process, curing, and testing. This research presents the results of stress and strain state of the local bearing area caused by a Prestressed cable anchor in reinforced concrete bridge segment. Nonlinear analysis of anchorage and rebar, concrete segment can be carried out by analytical solution calculated with the reference of ACI 318/19. ANSYS software is used for model and analyze the structure. The analysis process was carried out in 3 types : linear elastic analysis, nonlinear elastic analysis, and nonlinear analysis considering the destruction of concrete, and the value is compared with ANSYS software ( static structure).

Infrared thermography detection of grouting defects in external post-tensioned tendon ducts under construction hydration heat excitation

Most duct grouting defects are caused during the construction period, and its existence will seriously affect the safety performance of the structure. Timely detection and remediation during the construction period will reduce the high inspection and maintenance costs and the probability of structural failure later. This study proposed an approach for detecting grouting defects in external post-tensioned tendon ducts using infrared thermography (IRT) under the hydration heat of duct grouting material. IRT semi real-time detection of four different grouting defect specimens was performed indoors in the first 48 h after grouting completion to validate the proposed approach. Results showed that grouting defects with 0%, 40%, and 70% grouting compactness in high-density polyethylene ducts could be identified simultaneously from 12 h to 21.5 h after grouting completion, while defects in galvanized steel pipes (GSP) could also be identified after improving the emissivity and reflectivity of the pipe surface. Furthermore, grouting compactness could be quantified by the distance between the half maximum contrast, and PCA-based image preprocessing could reduce the quantitative error of grouting compactness detection. In addition, the detection in high temperature environment should be avoided because the rise in ambient temperature would reduce the identification ability of grouting defects. IRT technique could guide the grouting construction of external tendon ducts in actual engineering because of its ability to efficiently identify grouting defects during the hydration heat release stage of grouting material.