Post-tensioned Concrete Structures Research Articles

Effect of grouting quality on flexural behavior of corroded post-tensioned concrete T-beams

The durability of post-tensioned concrete (PTC) bridge structures has been greatly threatened by the corrosion of steel strands, especially in the case of insufficient grouting. In this paper, the effect of grouting quality on flexural behavior of corroded post-tensioned concrete (CPTC) beams is experimentally investigated with two groups of T-beams, i.e., five fully grouted beams in Group A and five partially grouted beams in Group B. Results indicate that when the mass loss of steel strands in Group A ranges from 0 % to 10.47 %, the cracking load decreases by 5.8821.57 %, and the ultimate bearing capacity decreases by 5.45–30.45 %. While for Group B, when the mass loss of steel strands ranges from 0 % to 14.26 %, the cracking load decreases by 3.75–25.00 %, and the ultimate bearing capacity decreases by 7.41–44.91 %. It is observed that as the degree of corrosion increases, both cracking load and ultimate bearing capacity of the beam gradually deteriorate, with the flexural capacity of partially grouted beams deteriorating faster. Built on this and with the model of bond reduction coefficient, an analytical method is proposed to predict the flexural capacity of CPTC beams under different grouting conditions. The developed method provides good predictions compared with experimental results of current study and from literature, paving the way for accurate assessment of the flexural bearing capacity of PTC beams with different grouting qualities in corrosive environments.

Durability of Modified Asphalt-Based Grouting Material for Post-Tensioned Concrete Structures

Durability of Modified Asphalt-Based Grouting Material for Post-Tensioned Concrete Structures

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.

SEISMIC PERFORMANCE AND DESIGN OF PRECAST CONCRETE SHEAR WALL STRUCTURAL SYSTEMS

Our understanding of the seismic behavior and constraints of reinforced concrete shear walls was to be improved by this work. The Alto Rio building's collapse as a consequence of an earthquake was evaluated in the first section. The second section made use of information from a three-dimensional seismic simulation test on a four-story, precast post-tensioned concrete shear wall structure that was built to scale. The aim was to create useful structural engineering models, perform analytical simulations to verify the structural models' capacity to mimic behaviors that are significant to structural engineers, and determine whether the analysis tools now available are enough to represent dynamic behavior.

Tension evaluation of prestressed strand based on combined features of nonlinear ultrasonic mixing

The estimation of prestressed strand stress levels is of great significance in ensuring the structurally reasonable performance of post-tensioned concrete structures, cable-stayed bridges, and arch bridges. This paper proposes a method to evaluate the remaining tensile stress on the strand through the ultrasonic mixing technique. The relationship between the strand tension and the nonlinear harmonics is established based on the theory of contact acoustic nonlinearity, and the acoustic nonlinear parameters at different tension levels are investigated by finite element analysis and step-loading experiments. The results show that the nonlinear parameters follow a power-law decrease with the increase in the tensile stress. To overcome the shortage of decreasing sensitivity of theoretical nonlinear parameters in monitoring early loosening, support vector regression is introduced to utilize complex high-order nonlinear generations in stress evaluation. With this approach, the accuracy of early-stage stress evaluation based on wave mixing has been remarkably improved.

Seismic behavior of earthquake-damaged hybrid connections reinforced with replaceable energy-dissipating elements

In recent years, self-centering post-tensioned concrete structures have received extensive research attention owing to their excellent seismic performance and post-earthquake functional recoverability. This study entailed the development of a novel repair method for earthquake-damaged unbonded post-tensioned hybrid connections (PTHCs) by using a novel energy-dissipating device to improve seismic resilience. To investigate the effectiveness of the repair method, four connections with different degrees of seismic damage were repaired using different repair methods, and quasi-static experiments were then conducted. The test variables included the position of the glass fiber-reinforced polymer (GFRP) arrangement, connection method of energy-dissipating devices, number of replaceable energy-dissipating elements (REDEs) of the damper, and initial prestressing force of post-tensioned (PT) strands. The experimental results indicate that the proposed repair methods can recover the lost capacities of PTHCs, and the mechanical properties of the reinforced self-centering precast connections (RSCPCs) can even be restored by replacing the REDEs under a second rare earthquake. The proposed energy-dissipating device facilitates increase or decrease of the REDEs to meet the energy dissipation and stiffness targets of the component for different levels of earthquake damage. The calculation results based on the recovery force model were in good agreement with the test results, showing that the proposed model could reasonably predict the hysteresis behavior of the RSCPCs.

Strain, crack, stress and shape diagnostics of new and existing post-tensioned structures through distributed fibre optic sensors

Prestressed concrete is commonly used in the construction of safety–critical structures such as large-span girders or slabs. The article focuses on post-tensioned elements, in which curvilinear multiwire strands are tensioned after concrete’s hardening to transfer the compressive force to the concrete. The overall idea is to confront external forces and achieve extended benefits, e.g. longer spans or reduced cracking. However, the design, construction and maintenance of such elements are particularly challenging because of their complexity, including spatial stress state in the anchorage zones and also the standard features of concrete structures like heterogeneity, material imperfections and, most importantly, local discontinuities (cracks). Reliable diagnostic solutions are therefore essential for both new and existing components. Distributed fibre optic sensing (DFOS) allows the measurements to be taken continuously over the entire structural length and its advantages have already been proven in civil engineering applications, especially those related to concrete structures. However, the novelty of the presented research lies in 1) simultaneous analysis of strains, stresses, cracks and displacements (shape changes); 2) direct comparison of results from monolithic sensors embedded in the concrete (for new structures) and those installed in pre-made near-to-surface grooves (for existing structures); 3)the application of the embedded composite strain sensor to the bi-directional analysis of the stress field within the anchorage zones; 4) possibility of strain transfer analysis between the cracked concrete, cable duct, injection and prestressing tendon during mechanical bending tests. The DFOS-based strain, crack and displacement results were also compared with reference measurements made with conventional spot techniques, achieving very good compliance. The research proved that a well-designed DFOS-based system, including appropriate measurement techniques, optical sensors, installation methods and post-processing algorithms, is the relevant tool for the diagnostics of post-tensioned concrete structures.

Structural behaviour of 2D post-tensioned precast beam-column sub-assemblages subjected to column loss scenario

Many previous studies have investigated alternative load path (ALP) mechanisms of 2D reinforced concrete (RC) beam-column sub-assemblages under column removal scenarios. However, how precast and prestressed RC structures respond under a column loss scenario receives far less attention, as evident from a lack of published test data. Herein, five 2D post-tensioned precast RC sub-structures subjected to a middle column loss scenario were tested by multi-point quasi-static loading. In all the specimens, individual precast members such as beams and columns were joined together through wet connection. The study focused on investigating the combined effects of post-tensioned tendon (PT) and wet connection on load-resisting mechanisms and the influence of basic parameters such as unbonded and bonded PT, T-beam effect, and boundary conditions. The test results indicated that both unbonded and bonded parabolic-shaped PT significantly enhanced compressive arch action (CAA) and catenary action (CA) beyond basic flexural capacity, and also changed the plastic hinge mechanism in the precast beams. The specimen with unbonded PT could provide greater residual capacity than bonded PT, but the former was severely damaged by fracture of normal reinforcement and crushing of concrete rather than fracture of the tendon. In addition, the effects of T-beam and boundary conditions are discussed in detail. Finally, new design criteria for precast and post-tensioned concrete structures under column removal scenarios are proposed for structural engineers to take advantage of high-strength tendons and avoid failure at critical locations by precast joints.

High-performance cementitious grouts for post-tensioned concrete systems – Performance specifications and prototype testing

Grouted, post-tensioned (PT) concrete structures are protected from tendon corrosion by filling the interstitial spaces with cementitious grouts. To achieve complete grouting, the cementitious grout must be sufficiently flowable and bleed resistant. Nowadays, many commercial pre-packaged grouts are available. However, simulated bleed measurements with prototype-scale tendon grouting tests have shown that pre-packaged grouts tend to form a highly porous layer of grout, which poses a severe threat to corrosion protection. This study focuses on evaluating the performance of a novel pre-blended grout produced on an industrial scale using the fluidity and fluidity retention tests, standard, wick-induced, pressure-induced, and inclined tube bleed tests. Also, the ability of the fresh grout to retain its properties against slight variations in the ambient temperature and water content was studied. The performance of the pre-blended grout on a real scale was evaluated and compared with a widely used site-batched grout composition using prototype tendon grouting tests. In addition, a set of stringent and comprehensive specifications were developed for applications in PT systems. It was also observed that the pre-blended grout considered in this study met all the proposed specifications, therefore, can be used for the corrosion protection of tendons in PT structures.

AE localization issues in heavy reinforced post-tensioned concrete beams

Damage localization is a relevant issue in structural health monitoring (SHM) of concrete structures. A greater level of insight into the structure can be so reached enabling more informed maintenance decisions and reducing operation and maintenance costs. The basic approach to acoustic emission localization, which is often referred to as the Time of Arrival (TOA) method, is based on detecting an AE source at a number of spatially-distributed sensors. However, in anisotropic media such as reinforced concrete, the basic assumptions for the TOA method become largely invalid. Different robust methods suitable to anisotropic media, incorporating an angular-dependent velocity term into the minimization process have been developed in the past. However, the wave-velocity profile is not always known. More recently a number of other authors have adopted various Bayesian approaches as part of AE localization strategies, including the use of a Markov chain Monte Carlo inference scheme as well as nonlinear Kalman filters. The use of a probabilistic approach in source location has become increasingly applied for AE source localization in complex large structures. In heavy reinforced post-tensioned concrete structures acoustic wave path can be significantly influenced by rebars as well as metallic post-tensioning ducts. Cracks opening can further influence acoustic propagation paths and greatly influence the reliability of AE source localization algorithms. In the present paper, different localization procedures have been developed and tested on post-tensioned concrete beams during loading and unloading cycles as well as on small homogeneous concrete blocks. In a highly emissive environment, significant difficulties have been reported also in event identification thus further reducing localization procedure reliability. EWGAE 35, Ljubljana, Slovenia, 13th – 16th Sep. www.ewg

Spiral Deployment of Optical Fiber Sensors for Distributed Strain Measurement in Seven-Wire Twisted Steel Cables, Post-Tensioned against Precast Concrete Bars.

On-time monitoring and condition assessments of steel cables provide mission-critical data for informed decision making, ensuring the structural safety of post-tensioned concrete structures. This study aimed to develop a spiral deployment scheme of distributed fiber optic sensors (DFOS) and to monitor/assess the post-tensioned force in seven-wire twisted steel cables, based on the pulse-pre-pump Brillouin optical time domain analysis. Each DFOS was placed in a spiral shape between two surface wires of a steel cable and glued to the steel cable by epoxy. Image observations were conducted to investigate the entireness and bonding condition between the optical fiber and the steel wires. Eight concrete bar specimens were cast, each with a pre-embedded plastic or metal duct at its center and each was post-tensioned by a steel strand through the duct once they were instrumented with two strain and two temperature sensors. The strand was loaded/unloaded and monitored by measuring the Brillouin frequency shifts and correlating them with the applied strains and the resulting cable force after temperature compensation. The maximum, minimum, and average cable forces integrated from the measured stain data were compared and validated with those from a load cell. The maximum (or average) cable force was linearly related to the ground truth data with a less than 10% error between them, after any initial slack had been removed from the test setup. The post-tensioned force loss was bounded by approximately 30%, using the test setup designed in this study.

Full-waveform inversion of ultrasonic echo signals to evaluate grouting quality of tendon ducts in post-tensioned concrete structures

Reliable techniques for non-destructive evaluation of reinforced and prestressed concrete structures are of utmost importance for the maintenance of aging infrastructure. A particular area of interest is the evaluation of grouting quality within tendon ducts in post-tensioned concrete. Detecting voids and water-filled cavities in plastic or metal ducts is challenging, especially at greater depths or in the vicinity of rebar. Conventional non-destructive inspection methods based on elastic waves, such as impact-echo or ultrasonic pulse echo method using synthetic aperture focusing and/or signal phase analysis, often lack sensitivity to those defects and/or rely on manual and subjective interpretation of the complex data. To overcome these problems, we present a synthetic feasibility study based on full-waveform inversion for reliable ultrasonic non-destructive testing. Full-waveform inversion is a powerful imaging technique that infers tomographic reconstructions of the material properties from ultrasound measurements. The method is widely used for geophysical applications based on seismic waves and has recently gained increasing attention for ultrasonic inspection applications. Using digital twins, we demonstrate the effect of voids and water inclusions on simulated ultrasonic echo signals. This information can be used in an iterative inversion algorithm to create a 3-D quantitative model of the specimen’s interior. We showcase in several numerical examples that even under uncertainties regarding aggregate size distribution and location of the rebar, full-waveform inversion can reveal defects related to the tendon ducts, which enables a robust and automated assessment of the grouting quality.

Robustness of Post-Tensioned Concrete Beam-Column Subassemblies under Various Column Removal Scenarios

To fully recognize the load-resisting mechanisms of post-tensioned concrete (PC) structures with realistic boundary conditions against disproportionate collapse, four beam-column subassemblies were extracted from a prototype building and the side columns and joints were reproduced to reflect the actual boundary condition. The parametric analysis was conducted, including location of the removed column (middle or penultimate) and strand profile (straight or parabolic). In addition, two RC counterparts were tested as a control group. Test results indicate that the unbonded post-tensioning strand (UPS) was able to enhance the structural robustness by increasing compressive arch action capacity of RC beams and developing catenary action. Compared with RC specimens, both PC specimens achieved much higher load resistance; herein, the PC specimen with straight strand profile obtained the highest load resistance due to two strands used, while the PC specimen with parabolic profile had higher deformation capacity. However, the existence of UPS increased the tensile force demand to the side column, leading to the flexural tension failure of the side column when the loss of a penultimate column was considered. Finally, an analytical study was carried out to quantify the load resistance from each dominant load-resisting mechanism.

Full-scale investigation of post-tensioned prestressed concrete bridge girders subjected to frost heaving in cold regions

Frost heaving is one of the major diseases of post-tensioned prestressed concrete structures in cold regions. This study investigates the effects and underlying mechanisms of frost heaving damages on the mechanical behaviors of full-scale post-tensioned prestressed concrete girders of a highway bridge. Forensic investigations were performed for three bridge girders through destructive tests, which included borehole, anatomy, and flexural tests, with an intact girder as the control. Three-dimensional nonlinear finite element models of the girders were established to understand the mechanisms of the effect of frost heaving. The results show that frost heaving reduced the effective prestress, mechanical properties of concrete, and bond between concrete and tendon, thus reducing the crack resistance and durability of the girders. This study provides new insights into post-tensioned prestressed concrete girders under frost heaving. The results are useful to improve the safety, serviceability, and durability of prestressed concrete bridges in cold regions.

High-performance cementitious grout with fly ash for corrosion protection of post-tensioned concrete structures

In order to provide corrosion protection of tendons in post-tensioned (PT) concrete systems, the congested interstitial spaces between the strands and duct are ideally to be filled with cementitious grouts. However, many currently used grouts do not have the desired fluidity and resistance to segregation – leading to formation of voids, frothy grout materials, and premature corrosion of strands. This paper presents a 3-phase study on the development of high-performance, pre-blended grouts. Several grouts were formulated with various proportions of ordinary portland cement (OPC), fly ash and chemical admixtures. The fluidity, bleed (i.e., standard, wick-induced, pressure-induced, and -inclined tube), softgrout, rheological, and shrinkage parameters of the formulated grouts were assessed; based on which a systematic selection of grouts was performed in three phases. Finally, a grout with 52 and 48% of OPC and fly ash, respectively (by volume) and optimal dosages of chemical admixtures was formulated and pre-blended in an industrial facility. This grout satisfies a set of performance specifications on fluidity, resistance against segregation (say, bleedwater and softgrout formations), and shrinkage. Therefore, the grout is capable of filling the ducts and providing long-term corrosion protection of PT strands.

Carbon value engineering: A framework for integrating embodied carbon and cost reduction strategies in building design

Value Engineering (VE) is a process where building materials, systems or design strategies are substituted to reduce capital costs without negatively impacting functionality. This research examines how Value Engineering can be adapted to also integrate the reduction of embodied carbon. A Carbon Value Engineering (CO2VE) framework is proposed to determine both capital cost and embodied carbon. The framework uses Pareto Principles to identify the primary contributors to these metrics, proposes alternative design strategies and uses Marginal Abatement Cost Curves (MACC) to visualise direct and indirect impacts of the changes. The framework is tested on an 18-storey building in Sydney. Results show that embodied carbon makes up 27–58% of the building's total lifecycle carbon emissions, depending on the future energy mix. The most significant contributor to embodied carbon and capital cost is the structural system. Alternative structural systems are evaluated with a post-tensioned concrete structure demonstrating an 8% reduction in embodied carbon and a 10% capital cost saving. A whole timber structure reduces embodied carbon by 13%–26% and cost by 5%. Embodied carbon savings are found to be comparable to conventional strategies to reduce operating carbon emissions such as the use of a high-performance building façade over the building's life.

Bond performance of grouted tendons at elevated temperatures

The bond between tendons and grout is crucial to the performance of bonded post-tensioned concrete structures. To study the bond performance of grouted tendons at elevated temperatures, pull-out tests were conducted on specimens comprising either a mono- or multi-strand tendon located centrally within a grouted duct inside a concrete cylinder, at test temperatures of 20–600°C. It was found that the bond strength deteriorated with a rise in temperature and the degradation of bond strength was more severe than that of other mechanical properties. Three types of failure mode were observed: pull-out, partial splitting and through splitting. The use of a smaller duct could improve the bond strength of specimens failing in pull-out mode. The influence of the strength of the outer concrete was found to be insignificant. Based on the distinct bond behaviour of different failure modes, a bond–slip model was developed by regression of the test results.

Internal Pressure Monitoring in a Post-tensioned Containment Building Using Operational Vibration

Nuclear material containment buildings are required to be periodically inspected to assure good quality conditions. To this end, there is increasing interest in developing efficient and economical testing techniques to monitor and inspect these structures. In this paper we study the dynamic behavior of a 1/3 mock-up containment structure and its internal pressure-dependence using testing techniques that may not interfere with the structure’s operation. The goal is to assess the possibility of monitoring internal pressure and potential leakage by measuring the structural frequency of vibration. The Vercors mock-up structure, a post-tensioned double-wall concrete structure of dimensions 15 m diameter and 30 m tall, was internally pressurized from 0 to 4 bar. We attached a pair of accelerometers at the outer surface of the inner structure and recorded signals periodically without imposing any controlled dynamic input; thus, we used an output-only dynamic analysis. We found that there is a clear positive correlation between internal pressure and the identified frequencies of vibration. We developed a multi-variable correlation, using several frequencies of vibration and the measured pressure, which could be used to monitor internal pressure. To study the underlying physical behavior, we built a finite element model. The observed frequency shift due to applied internal pressure could be partially explained by the effect of geometric nonlinearity, but it does not explain the entire range of frequency shift. Material nonlinearity should also be considered to assess this problem.

Delamination Cracking Behavior of Curved Post-Tensioned Concrete Structures

Delamination Cracking Behavior of Curved Post-Tensioned Concrete Structures

Stresses In Tendons Of Post-Tensioned Concrete Structures At Ultimate Limit State

Currently post-tensioned concrete structures, which are designed with bonded or unbonded tendons, are widely used in construction. The features of calculation of post-tensioned structures with unbonded tendons, which are contained in some other design codes, are considered. It is noted that the current design code for plain and reinforced concrete structures include the provisions for the calculation of post-tensioned concrete structures with bonded tendons. However, there are no recommendations for the calculation of post-tensioned concrete structures with unbonded tendons, taking into account the specific features of the stress-strain state of structures with such prestressing tendons. These features significantly affect both the ultimate moment resistance of structures and their deformations, usually in the unfavorable direction in comparison with structures in which the bonded tendons are accepted. Therefore, the main attention when developing a method for calculating of post-tensioned concrete structures with unbonded tendons was paid to determining the stresses in such reinforcement at the ultimate limit state capacity check. As a result, a method for determining the stresses and increase of the stress from the external load in the unbonded tendons was developed for calculating the ultimate moment resistance of structures. The proposed method presented in the article showed a close correspondence with the data obtained from other studies, in which the stresses in the unbonded tendons were determined by the results of experimental and theoretical studies.