Post-tensioning System Research Articles

Investigation of Passive Controlled Post-Tensioning System on the Structural Behaviour of Precast Reinforced Concrete Beam–Column Connections

Precast structures are widely used in many parts of the world. This construction technique is more commonly preferred for low-rise industrial buildings than multi-story structures. The most commonly used column–beam connection in precast buildings is the dowel connection (DC). Past earthquakes in various parts of the world have shown that these connections do not provide sufficient resistance. The main deficiencies of such connections are that they are sheared or stripped due to the shear force demand from the in-plane effects of large earthquakes, and that they do not provide sufficient resistance to the overturning moments from the out-of-plane effects of the earthquakes. Correspondingly, many prefabricated buildings have collapsed during earthquakes, causing loss of life and property. This study proposes using post-tensioning tendon (PT) systems and systems created by adding steel springs (PTS) to eliminate the weaknesses in column–beam connections in precast structures. To this end, real-sized column and beam specimens used in precast buildings were produced, and experiments were conducted under the cyclic loads defined by the American Concrete Institute (ACI) Committee, Report 374, simulating earthquake effects for three different connection types (DC, PT, and PTS). It was observed that the proposed PTS connection type dissipated approximately one-third of the energy transferred to the joint through elastic deformation in the springs, compared to the DC and PT connection types. This indicates that the PTS specimens transferred significantly less energy to the column–beam connection region. Consequently, the PTS system exhibited much less damage in the column foundation and especially the column–beam connection areas than other test specimens. In conclusion, it can be stated that the use of the PTS connection type in prefabricated structures has high potential to reduce damages due to dynamic loads such as earthquakes.

Experimental study and numerical modelling of post-tensioning systems on the transverse direction of composite steel deck-concrete structures

This study introduces the first investigation into the impact of post-tensioning layouts on the specific transverse direction of steel-deck concrete structures due to the significant limitations of current practice. The experimental work investigates the flexural capacity, structural failures, and crack characteristics of post-tensioned composite (PTC) systems using different tendon profiles and anchorage positions. Three specimens were tested under the four-point bending tests, in which one specimen employed a straight tendon and the remaining two used a profiled tendon (parabolic) with different anchorage positions. The specimens failed due to flexural failure with heavy crushing of concrete and without any damage to the steel sheeting. As compared to conventional concrete structures, the testing revealed a unique crack pattern in the composite system, with cracks initiating 90 degrees at the top edge of the ribs instead of the bottom face of the concrete. The load capacity of specimens using profiled tendon was higher by 15.2 % in the service stage and 17.3 % in the ultimate stage than that of straight tendon. The anchorage positioned at the mid-depth of the section brings the most effective placement to the PTC specimens, where the stress losses can be reduced by 2 % to 4 % and the bending stiffness increased by 4 %. The proposed finite element (FE) models of the PTC specimens were developed and validated against experimental results, ensuring the use of modelling techniques available in the software package ABAQUS.

Chemical and electrochemical repair methods for the anchorage regions of grouted, post-tensioned concrete systems

Grouted, post-tensioned concrete systems are commonly used for constructing bridges with an intended corrosion-free service life of 100+ years. However, the use of inadequate grout materials and improper grouting techniques have caused voids at the anchorage regions – resulting in premature corrosion (say, within one or two decades) of strands. In addition, re-grouting the voids with repair grout has aggravated the corrosion of strands at the interface between the base grout (usually carbonated) and repair grout due to the differences in their chemical properties – raising concerns and leading to reluctance in re-grouting the voids in tendons. This work focused on developing non-invasive chemical (re-alkalization) and electrochemical (galvanic cathodic protection) methods to repair the anchorages of grouted, post-tensioned concrete systems. Immersion experiments were conducted to assess the efficiency of alkaline solutions in re-alkalizing the carbonated grouts. It was found that 1 M Ca(OH)2 solution can restore the pH of the carbonated grouts in about a day. Moreover, the electrochemical impedance spectroscopy studies indicated that 1 M Ca(OH)2 solution can re-passivate the embedded strands within a week. It can be sometimes challenging to achieve complete re-alkalization and re-grouting of voids; hence, a redundant electrochemical system that could work from outside the anchorage was found necessary. Therefore, experiments were conducted to evaluate the viability of galvanic anodes in protecting the anchorages. It was found that a thin layer of grout surrounding the strand would be sufficient to provide ionic conductivity for the galvanic anode (connected to the strand-end outside the anchorage) to protect the strand portions inside the anchorage. Based on the study, recommendations for the repair of the anchorage regions using chemical and electrochemical methods are presented.

Load transfer behavior of 3D printed concrete formwork for ribbed slabs under eccentric axial loads

3D printed concrete (3DPC) has primarily been used for non-structural applications, with limited exploration into its potential for structural load-bearing applications. This is mainly due to the layered structure of 3DPC and the lack of compatibility of conventional reinforcement strategies to be integrated within the 3D concrete printing process. The post-tensioning of 3DPC structures presents an effective solution to improve the load-carrying capacity and cracking behavior of 3DPC. However, understanding the load transfer behavior and failure modes of 3DPC structures under a particular post-tensioning configuration are crucial to determine the permissible post-tensioning load for a structure without premature failure and to understand the distribution of stresses within the concrete structure under post-tensioning loads. The current study aims to investigate the load transfer behavior and failure mode of a 30 mm thick 3DPC formwork designed for one-way ribbed slabs when subjected to end-anchorage post-tensioning. For this purpose, a series of mechanical characterization and load transfer experiments were conducted on ribbed 3DPC formwork. The mechanical characterization investigations involved examining the compressive and flexural strength, as well as the elastic modulus of 3DPC, under various loading orientations relative to the print path. In the load transfer experiments, the end anchorage post-tensioning system was simulated by applying the axial load to the specimen via end plates bonded to the specimen. The variable parameters of the load transfer experiments were the boundary conditions, the eccentricity of the axial load from the neutral axis, and the topology of the ribbed 3DPC formwork. A digital image correlation system was used to study the axial and transverse strain evolution during the load transfer experiments. The experimental results showed that, regardless of the eccentricity of the applied load, the ribbed 3DPC formwork specimens exhibited higher axial load capacity when the load was applied near the bottom flange rather than the top flange. This was because of the reduced effective cross-sectional area for compression when the load was positioned near the top flanges, a consequence of the selected formwork topology, where top flanges were discontinuous to allow for pouring of the cast concrete within the formwork.

Seismic performance assessment and fragility analysis of concrete structures equipped with self-centered hybrid wall systems

ABSTRACT In this study, comprehensive analytical and parametric studies were conducted on seismic performance, collapse and fragility curves of concrete structures equipped with self-centered unbonded post-tensioned hybrid wall system in comparison with conventional structural wall system. For this purpose, analytical models of prototype buildings were developed and to further study the behaviour of hybrid wall, parametric analysis was performed considering four parameters: wall aspect ratio, initial stress of post-tensioned tendons, the total area of PT tendons, the total area of mild bars. The performance of buildings was evaluated under non-linear quasi cyclic and time history method considering two seismic hazard levels, the design-based earthquake level and the risk-targeted maximum credible earthquake level. Hysteresis response, energy dissipation ability and damping ratio, relative self-centering efficiency, residual drift, peak roof drift, column curvature ductility, incremental dynamic analysis data, seismic collapse assessment and fragility curves for collapse and repairable limit states were studied.

Enhancing the Flexural Capacity of Deteriorated Low-Strength Prestressed Concrete Beam Using Near-Surface Mounted Post-Tensioned Carbon Fiber-Reinforced Polymer Bar

This study aimed to address the critical issue of age deterioration in prestressed concrete (PSC) structures by investigating the strengthening of aged PSC structures using a near-surface mounted (NSM) post-tensioned carbon fiber-reinforced polymer (CFRP). A total of nine PSC beams, each with a length of 6.5 m, were fabricated for a four-point bending test. Various experimental parameters were taken into account, including the strengthening method, compressive strength of concrete in the PSC beam, and the prestressing force of the PSC beam. The results indicated that the NSM post-tensioned CFRP strengthening system proved more efficient when compared to the NSM non-post-tensioned CFRP strengthening system. The flexural capacity of the NSM post-tensioned CFRP strengthening system, under the deteriorated low-strength PSC beam, increased by up to 30.9% compared to the PSC reference beam. Additionally, the experimental results were compared to a finite-element analysis, and a parametric study was conducted to examine the material properties of the PSC beam. Consequently, the NSM post-tensioned CFRP strengthening system is expected to be an effective solution for addressing the issue of deteriorated low-strength PSC structures.

Experimental and analytical investigation of T-beams retrofitted through the external post-tensioning using steel and various FRP strands

This study aimed to scrutinize the potential benefits of using fiber-reinforced polymer (FRP) materials as unbonded strands in post-tensioning systems for reinforced concrete (RC) flexural members. The research meticulously evaluated the influence of these FRP materials on global and local response levels. To this end, four-point loading tests were performed on T-beam specimens, and the experimental results were compared with numerical predictions. The study also incorporated insights from previous research on external post-tensioning variables. The results of this study, with their practical implications, demonstrate a strengthening technique that significantly enhances the load-carrying capacity at ultimate and yielding points, as well as the crack initiation load of non-strengthened specimens. The strengthening technique also significantly influences deflection during yielding and maximum loads and plays a crucial role in restoring deflection after unloading. However, it is essential to note that strengthening with external post-tensioning does reduce the member’s ductility. The choice of materials also emerges as a significant factor that leads to noticeable differences in crack widths and distribution among the specimens. These practical findings hold considerable importance and offer valuable insights into the potential use of FRP materials in post-tensioning systems for RC flexural members.

Flexural behaviour of the segmental precast concrete decks post-tensioned by GFRP rods

This paper introduces an innovative post-tensioned segmental concrete deck system internally reinforced and tied with GFRP reinforcements for application in pontoon decks and other deck structures in aggressive marine environments. Utilisation of the GFRP reinforcements in floating concrete structures is essential because of their non-corrosive characteristics. Six large-scale segmental decks following the specifications of Queensland maritime infrastructure were designed, manufactured, and tested to assess the reliability of the new construction system under static flexural loading in the flatwise and edgewise orientations. One segmental deck served as a reference with hand-tight post-tensioning, while the remaining specimens were connected by the GFRP rods with varying levels of post-tensioning. All decks were tested up to failure, allowing for an investigation of their flexural strength, load-strain behaviour, joint opening, and failure mechanism. The results showed that post-tensioning the GFRP rods improves the flexural performance of the segmental decks. The higher the level of post-tensioning, the higher the contact area between the segments at the joint is achieved. A numerical model was developed to understand the detailed mechanism of both flatwise and edgewise specimens. A strain reduction coefficient in the segmental concrete deck under flexure is introduced accounting for the joint presence to reliably calculate the stress in the post-tensioned internal GFRP rod when the concrete in the joint crushes. The system investigated can increase maritime and recreational infrastructure's construction efficiency and provide creative solutions in the GFRP-reinforced concrete structures in the building and construction industry.

Sustainable concrete structures by optimising structural and concrete mix design

ABSTRACT Sustainable concrete construction can be achieved by combining optimised structural designs with low-carbon concrete (LCC). This paper demonstrates the importance of sustainable construction by analysing various design options for a typical suspended slab system in high-rise buildings. Pearson (2020) noted that concrete slabs make up approximately 47% of the embodied carbon (EC) in commercial and residential structures, with slabs alone comprising around 70% of the total concrete volume in residential buildings. This presents an opportunity to reduce EC through structural design choices. The analysis in this paper involved designing a post-tensioned (PT) flat slab system for a residential development in Sydney and then re-designing it as a reinforced concrete (RC) slab by adjusting slab thickness and reinforcement system. A key finding was that transitioning from an RC to a PT slab resulted in a 44% total EC reduction when using a 100% General Purpose Cement (GPC) blend. Further EC reductions can be achieved by implementing LCC. The chosen LCC blend for this study is 23% Fly Ash, 23% Ground Granulated Blast Furnace Slag, and 54% Cement (FA/GGBFS/GPC), demonstrating a 36% reduction in EC compared to a 100% GPC blend while maintaining acceptable workability, constructability, mechanical, and durability properties.

Experimental and numerical investigation of punching and post-punching shear capacities of post-tensioned flat plate slab-column joints influenced by prestressing levels

Post-tensioned slab systems serve as an effective method to enhance the progressive collapse resistance of concrete flat plate structures by significantly improving the punching shear and post-punching capacities of their slab-column joints. While the existing research mainly focused on the punching shear behaviour of post-tensioned slab-column joints in small deformation stages, the post-punching behaviour in large deformations has been overlooked. This study aims to fill this research gap thorough experimental and numerical studies. Experimentally, four slab-column joints, including three post-tensioned specimens (with varied prestressing levels) and one specimen without tendons, were designed and tested. Detailed failure process and load-resisting mechanisms of the specimens at different deformation stages were discussed. The test results confirmed the efficiency of post-tensioning in improving the punching shear performance. Varying prestressing levels have influenced the development of punching shear capacity but did not affect the growth rate of load resistance in the initial post-punching stage. Numerically, validated finite element models were used to predict the post-punching capacities of the specimens. The numerical results indicated that post-tensioning helped increase post-punching capacities, but such an enhancement was independent of the prestressing levels. Additionally, the influence of the rupture strains of both tendons and reinforcement on the post-punching capacity was investigated through a parametric study.

Degradation of structural safety levels in prototype balanced beam systems

Post-tensioned balanced beam structures are very sensitive to natural deterioration and excessive environmental attacks, which can lead to relatively rapid drops in safety levels. Since partial failure or corrosion of pre-stressing tendons can be difficult to detect, the strategy of last resort to ensure that safety levels are acceptable remains to perform a mechanical load test, with the risks that this entails. In the case of some iconic balanced beam systems, destructive investigations are further limited by the need to comply with the deontological standards valid for cultural heritage structures. The capacity and robustness of balanced beam systems can therefore be estimated by relying primarily on accurate mechanical models and sensitivity analyses against scenarios that may degrade safety. To this end, reference models can be corroborated by information from various sources, including experimental campaigns and archives. This study, in particular, focuses on the evaluation of the residual capacity offered by Morandi's prototype balanced beam scheme, to aid prognosis, and possibly support “decision-making” for possible reuse. Finally, an experimental and numerical application to the most complex post-tensioned concrete balanced beam system ever designed by Riccardo Morandi is presented, i.e. the roofing system of Pavilion V of Torino Esposizioni dating back to the late 1950 s.

Analytical and experimental response of timber joists strengthened with an external underslung post-tensioned system

Floors made of timber load-bearing joists often do not meet modern load demands and strict code requirements. In pursuit of retrofitting such floors, this paper investigates the feasibility of strengthening timber joists with an external underslung post-tensioned system composed of a V-shaped tension chord and a midspan strut. An analytical approach is employed to assess the system’s response under live loads. Based on these findings, a short parametric study is used to identify the key parameters influencing the structural behaviour of the strengthened joists. To validate the theoretical results, an experimental programme is conducted on 9 full-scale specimens obtained from an existing XI-century building. The specimens were divided into three series based on a variable span-to-rise ratio of the underslung post-tensioned system. The experimental tests confirm the correct functioning of the system when applied to timber joists. Furthermore, the comparison between the analytical and experimental outcomes demonstrates consistent agreement across the various cases investigated, confirming the efficiency of the proposed system in increasing the load-carrying capacity of the timber joists. Specifically, the underslung post-tensioned system significantly enhances the load-carrying capacity of timber joists, increasing the flexural strength and stiffness up to 2.0-2.5 times and contributing to support up to 35% of the live loads.

Quantitative monitoring of tensile force in post-tensioning systems using piezoelectric wave method

ABSTRACT The tensile force of a post-tensioning system is essential in prestressed concrete structures. Since the tensile force is related to the contact pressure at the interface between the anchor head and the bearing plate, this study utilizes the piezoelectric wave method to quantitatively monitor the reflected wave energy at the interface. A 3D solid element model was developed to simulate the propagation of stress waves in a simplified anchorage system across different contact areas. Three independent tests were conducted on a simplified 2-strand anchorage system, using the reception coefficient as a monitoring index. The empirical fitting formula between the reception coefficient and the load is derived from the loading data and verified using the unloading data. The experimental results demonstrate good monotonicity and stability within the test range. The study also conducted two temperature-based tests to investigate the effect of small temperature variations on the proposed method. The results indicate that temperature fluctuations of less than 20°C have little effect on the test method. Then, the proposed method is applied to a real 2-strand anchorage system in a concrete beam. The proposed method serves as a reference for quantitatively monitoring the prestressing force of a post-tensioning system.

Post-tensioning system as a strengthening solution for masonry arch bridges: numerical and experimental results

One of the most common bridge typologies in roadway and railway networks in Italy is the masonry arch bridge, typically built more than a hundred years ago. To keep these structures operatives with increasing traffic load, strengthening interventions are required in some cases. The solution investigated in this work is the external post-tensioning system, which consists of applying external loads to the arch by means of tensioned cables, resulting in a better configuration of internal forces. An experimental campaign was conducted on pre-damaged single span masonry arches by anchoring the cables to the intrados of the arch. The results of cyclic eccentric vertical load tests show an increase in displacement and load-carrying capacity. Numerical tests were performed to validate the experimental results using the rigid-block analysis method. The comparison between the experimental and numerical results is reported in terms of limit load capacity.

The Italian Guidelines for special inspections of post-tensioned concrete bridges: review and case study

Post-tensioned (PT) concrete bridges are structural systems characterized by intrinsic fragility. The assessment of their health condition is very difficult due to their nature, which prevents the detection of degradation in PT elements through conventional investigation methods and/or visual inspections. For this reason, the Italian Guidelines for the maintenance of existing bridges envisage special inspections to assess the condition of the post-tensioning system. According to the dedicated Guideline, the special inspection procedure consists of four steps. First there is Step O, which identifies the number and location of PT samples to be investigated based on the historical•critical review of available documentation and the visible defects in the post-tensioned members. Then, in Step 1 a preliminary analysis of the identified PT elements is performed, identifying the presence and location of active defects and grout voids, as well as the probability of corrosion of the reinforcement through non-destructive tests. In Step 2, the analysis of defects becomes more accurate, determining the probability of corrosion and estimating the level of stress in the concrete and in post-tensioned cables through locally destructive tests. Step 3 is applied if the outcome of Step 2 is uncertain and involves the experimental analysis of the overall structural response of the bridge. Eventually, in case the presence of damaged PT elements is confirmed, a load assessment of the structure is performed. The objective of this study is to evaluate the effectiveness of the investigation method proposed by the Italian Guidelines, highlighting its strengths and potential weaknesses. The procedure is applied to the Dora Riparia River Bridge, located in Avigliana (Turin) on provincial road SP197. The bridge consists of seven spans, 21.4 m long, and is characterized by a simply supported static layout; each span has four post-tensioned I-shape beams and four post-tensioned diaphragms.

Comparison of types of grouting to guarantee zero tendon slip with post-tension system prestressed concrete bridge anchors

Prestressed concrete is internal stresses of appropriate magnitude and distribution are given in such a way that the stresses caused by external loads are resisted to a desired level. The use of prestressed concrete is mostly applied in bridge construction, especially in the bridge girders. During the stressing girder process, grouting is carried out to prevent the anchor from slipping. The strength of the adhesion of the grouting material used determines the homogeneous nature and whether the prestressed concrete section next to it can meet the anchor load. If the bond strength does not meet the requirements, it will cause slippage in the anchors which will ultimately affect the strength of the prestressed concrete structure due to quite large prestress losses due to slippage. The method used is to test the adhesive strength of the strands that have been grouted with f.a.s. 1:3 using each grouting material such as PCC (Portland Composite Cement), PPC (Portland Pozzoland Cement), OPC (Ordinary Portland Cement) and Sika Grout 215 which is then compared with the grouting material that adheres best. The results obtained were Sika Grout 215 bond strength of 3.6629 MPa, PCC bond strength of 3.2986 MPa, PPC bond strength of 2.5743 MPa and OPC bond strength of 2.307MPa.

ДОСЛІДЖЕННЯ НАПРУЖЕНО-ДЕФОРМОВАНОГО СТАНУ ЗАЛІЗОБЕТОННОЇ МОНОЛІТНОЇ ПОПЕРЕДНЬО НАПРУЖЕНОЇ ПРОГОНОВОЇ БУДОВИ ШЛЯХОПРОВОДУ

Introduction. The experience of operation of reinforced concrete span structures of bridges from the time of their use shows the need for their constant improvement. Due to the use of pre-tensioning of reinforcement in reinforced concrete span structures of bridges, it was possible to increase the crack resistance of such structures and operational reliability. This is especially relevant when it is necessary to increase the length of spans of reinforced concrete span structures. Problems Statement. The use of monolithic prestressed span structures with tension on concrete (post-tensioned systems) is the newest for Ukraine. They have many advantages compared to prefabricated prestressed reinforced concrete. But it is necessary to solve a number of problems in the design and construction of such structures in order to achieve the efficiency of structures that would serve the designed term, using all their advantages. Therefore, the study of such structures, assessment of their stress-strain state will allow in the future to find solutions for their improvement, development of appropriate calculation recommendations, construction of such structures. Purpose. Study of the stress-strain state of the structure of the monolithic non-separated prestressed span structure of the overpass with tension on concrete (post-tensioned system) in the city of Kherson.

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.

PixelFrame: A reconfigurable, precast, post-tensioned concrete structural system for a circular building economy

The demand to build for a growing urban population conflicts with the need to reduce the AEC (Architecture, Engineering and Construction) sector’s contributions to global CO2 emissions. Structural systems are a key factor in this challenge, as they contribute substantially to a building’s mass and are typically overdesigned. In this paper, we present PixelFrame, a new precast concrete structural system that is algorithmically designed to use less carbon upfront and be reused across multiple lifecycles. We achieve a high reuse potential for structural elements through building load demand analysis, a segmental externally post-tensioned design, and an integrated optimal assignment strategy. In addition to improving reuse potential, a segmental optimization strategy also yields higher efficiency in comparison to traditional prismatic concrete structural elements. The detailing of concrete mixes is informed by a statistical analysis of anticipated building load demands and a custom clustering asymmetric distance function and clustering algorithm. Compared to previous work in this area, our contribution is original in its bidirectional algorithmic approach; both the target building structure and the circular material inventory are computationally generated to achieve versatility while minimizing emissions.

Short-Term Efficiency of Using Sustainable BFRP Bars in Post-Tensioning Systems for One-Way RC Slab

Short-Term Efficiency of Using Sustainable BFRP Bars in Post-Tensioning Systems for One-Way RC Slab