Unbonded Posttensioning Research Articles

Moment redistribution and ductility of CFRP strengthened and non-strengthened unbonded post-tensioned indeterminate I-beams composed of UHSSCC

Strengthening might be necessary for unbonded post-tensioned (UPT) concrete members to account for mistakes at the design stage or to solve construction errors. Various strengthening techniques have been developed that use carbon-fiber-reinforced polymer (CFRP) to increase the load carrying capacity and fulfill serviceability requirements. Although many in situ post-tensioned beams are continuous, there has been little research done on such beams with unbonded tendons. The brittle nature of UPT members, particularly with high strength concrete and CFRP, raises concerns about the ability to redistribute the moments of a continuous member. Knowledge about the degree of moment redistribution (MR) and ductility response of ultra-high strength self-compacting concrete (UHSSCC) in strengthened and non-strengthened UPT indeterminate members is crucial. The current study presents the experimental results of research to determine the flexural response of six non-strengthened or strengthened full-scale continuous UHSSCC unbonded post-tensioned I-beams with emphasis on ductility and moment redistribution. The results showed that, despite a decrease in MR, displacement, and energy ductility, the confinement provided satisfied the high MR demand in presence of strengthening, unbonded prestressing and use of UHSSCC. The stress measured in unbonded strands approached the point of yielding at the onset of failure for non-strengthened and strengthened beams.

Experimental and Theoretical Serviceability of Strengthened and Nonstrengthened Unbonded Posttensioned Indeterminate I-Beams

Experimental and Theoretical Serviceability of Strengthened and Nonstrengthened Unbonded Posttensioned Indeterminate I-Beams

Shake-Table Test of a Full-Scale 4-Story Precast Concrete Building. II: Analytical Studies

In 2010, a full-scale, 4-story precast concrete building was tested on the E-Defense shake table. The lateral-force-resisting system of the building included unbonded posttensioned (UPT) concrete walls in one direction and bonded posttensioned frames in the orthogonal direction. The companion paper presents an overview of the experimental program and key test results for response in the wall direction; this paper describes the development and experimental verification of a nonlinear model in the wall direction of the test building using commercially available software. The good correlations between analytical and experimental results confirm the ability of the model to capture global and local responses, including story lateral displacements, story shear forces and moments, and gap opening due to rocking at the wall base. Test and model results provide valuable insight into dynamic responses and design aspects of a full-scale, three-dimensional UPT building, including the influence of component interactions and the role of the floor diaphragm.

Creep of prestressing steels in fire

SummaryWhen prestressing steel (PS) is applied as continuous unbonded tendons within concrete, as in the case of post‐tensioned (PT) construction, localized damage to the tendons may affect the integrity of the entire structure. When fire exposed, unbonded PT concrete structures are prone to tendon rupture because of a complex relaxation and strength reduction relationship influenced by high temperature PS creep. Explicit consideration of creep is necessary for the fire safe design of such buildings. High temperature PS creep has received limited research: few models can be used within the expected service stress range; none reliably model tertiary creep; and no studies are available investigating the influences of metallurgy on high temperature PS creep. A BS 5896 PS creep model is developed and validated. This model is used to provide PS failure insights at high temperatures. This understanding can be used to build validated models of structural fire experiments on PT concrete structural elements and to develop defensible, validated structural models that are essential towards performance based structural fire design of such systems. Finally, the high temperature creep performance of equivalent PS from different mills is compared, demonstrating strong influence of manufacturing and alloying techniques used. Future research is discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Hysteretic Modeling of Unbonded Posttensioned Precast Segmental Bridge Columns with Circular Section Based on Cyclic Loading Test

The seismic performance of unbonded posttensioned (PT) precast segmental bridge columns (PSBCs) with circular sections was investigated. Two precast segmental columns with/without energy dissipation (ED) bars were tested under quasi-static cyclic loading. Their damage patterns and hysteretic curves were compared. Three single-degree-of-freedom hysteretic models were formulated with the key parameters identified from the tests. Cyclic loading analyses of the two tested specimens were conducted using these hysteretic models. Time history analyses were also conducted utilizing the proposed three hysteretic models to observe their applicability in modeling the dynamic responses of PSBCs. Finally, parametric analyses of PSBCs were performed under the excitation of an ensemble of 20 history ground motions to study the influences of relative yield force, elastic period, and energy dissipation coefficient on the earthquake responses of displacement ductility demand, maximum absolute acceleration, and maximum absorbed energy.

Low-Damage Posttensioned Segmental Bridge Columns with Flexible End Joints for Seismic Accelerated Bridge Construction

A computational parametric study leading toward the development of a new type of low-damage precast concrete segmental column for accelerated bridge construction in seismic areas is outlined. In contrast to conventional monolithic concrete construction, accelerated bridge construction significantly reduces on-site construction time, total project delivery time, weather-related construction delays, and environmental impact, and it provides improved work zone safety for the traveling public and contractor personnel and high product quality and durability. Low-damage systems provide significantly reduced postearthquake downtimes, repair costs, and casualties. The proposed column design will incorporate internal unbonded posttensioning and flexible joints at the column ends, where large flexural demands occur. These joints will be (flexurally) deformable enough to accommodate large column lateral deformations (drift ratios of 10% or higher) without damage, and (axially) stiff enough to limit initial axial deformations due to initial posttensioning and gravity loads. To investigate this concept, a three-dimensional finite element model of a cantilever segmental concrete column incorporating a flexible joint segment at the bottom was generated, and a comprehensive parametric study was conducted. The parametric study investigated three types of polyurethane elastomers for the flexible joint and various geometric properties of the flexible joint in reference to the column geometry. The findings of this study, as supported by preliminary material testing, have shown that hyperelastic materials, such as stiff rubbers and polyurethanes, are good candidates for the proposed application.

Experimental Investigation of In-Plane Cyclic Response of Unbonded Posttensioned Masonry Walls

AbstractThis paper reports on an experimental study on four unbonded posttensioned masonry walls (PT-MWs). All walls had identical thickness, height, and length of 190, 2,000, and 1,400 mm, respectively, and were constructed using concrete masonry units (CMUs) and mortar type N, and were fully grouted. Different horizontal spacing values of 400, 600, and 1,200 mm were used between the posttensioning bars in the walls. Only Wall W4 had horizontal bonded reinforcement, located in the fourth and seventh courses. Two different levels of posttensioning force corresponding to an average posttensioning compressive axial stress on the masonry of 1.35 and 2.7 MPa were applied to the walls. Different initial posttension stresses in the bars ranging from 0.32 to 0.63 of the yield stress of each bar were applied to the walls. The walls were subjected to incrementally increasing in-plane lateral displacement cyclic load applied to the top of each wall. The experimental results including damage pattern, force displacem...

Experimental and Numerical Assessment of Bonded and Unbonded Post-Tensioned Concrete Members

A post-tensioned (PT) concrete member can be reinforced with either bonded or unbonded prestressing tendons. The bond condition of tendons may influence the flexural and shear performance of various types of PT concrete members. The main purpose of this study is to provide a direct comparison between members with the two different tendon systems and assess their structural performance in depth. To accomplish this, previously tested PT beams, one-way slabs, and slab-column connections are reexamined, and detailed nonlinear finite element analyses are conducted using the developed modeling techniques. A series of test results verifies that the developed model is reliable for both bonded and unbonded PT members. The results of the prior experimental and current numerical studies are used for better understanding the behavior of bonded and unbonded PT members.

Development and Testing of an Alternative Dissipative Posttensioned Rocking Timber Wall with Boundary Columns

AbstractThe unbonded posttensioned rocking and dissipative technology was first developed as the main outcome of the PREcast seismic structural systems (PRESSS) Program in the United States. After the first developments and significant refinement, the technology was extended to steel and, more recently, timber structures. The timber version, referred to as prestressed laminated (Pres-Lam) system can be either implemented for timber walls (single or coupled) or frames or a combination of the above, with unbonded posttensioning and supplemental dissipation devices. In unbonded posttensioned dissipative wall systems a combination of recentering capacity and energy dissipation leads to a controlled rocking mechanism, which develops a gap opening at the wall base. This generates an uplift displacement that is transferred to the floor diaphragm. This vertical displacement incompatibility can represent a potential issue if the connection detailing between floor and lateral resisting system is not designed proper...

Comparative Study of Unbonded Carbon Fiber and Steel Strands in Posttensioned Pier Caps

AbstractCarbon fiber strands offer a viable alternative to corrosion-prone steel strands in posttensioning applications. This study focused on a side-by-side comparison of two types of carbon fiber strands with different elastic moduli against steel strands in a 5½∶1 scaled model of a typical interior hammerhead pier of the San Antonio Downtown Y project with two identical cantilever overhangs. Two different strand arrangements were used for posttensioning, with eight and six strands, respectively representing an overdesign and a slight underdesign relative to the factored demand. The model was tested under service and factored flexure and shear loads. The study confirmed the feasibility of using carbon fiber strands in unbonded posttensioning of pier caps. Given the strength-based design of the pier cap, the elastic moduli of different carbon fiber strands did not seem to have affected the serviceability performance with respect to either cracking or deflection. Considering both serviceability and overlo...

Cyclic Loading Test of Unbonded and Bonded Posttensioned Precast Segmental Bridge Columns with Circular Section

Four 1/4-scale precast segmental bridge columns (PSBC) with different reinforcement types and arrangements and one monolithic reference column (MRC) were designed and tested under cyclic quasi-static loading. Both UPC (PSBC with unbonded tendons only) and UPCE (PSBC with unbonded tendons and bonded mild steel bars) use unbonded posttensioned (PT) strands to connect the segments. Bonded energy dissipation (ED) bars were added in UPCE to increase the ED ability of unbonded PSBC. Specimens BPC (PSBC with fewer bonded PT bars) and BPCII (PSBC with more bonded PT bars) use bonded PT bars to combine the segments together. The unbonded length was set in ED bars to delay the low-cycle fatigue damage. Test results showed PSBC only has minor concrete cracks or crushing at lower joints. The unbonded PT PSBC with ED bars had higher lateral strength, lower residual drift, and comparable ED with MRC. The bonded PT bar PSBC with appropriate bar arrangement also showed high lateral strength and medium energy-dissipation ability. However, bonded PT PSBC experienced large PT stress loss, which leads to a residual drift of 3% when unloading from 6% drift.

Experimental Seismic Performance of a Hybrid Sliding–Rocking Bridge for Various Specimen Configurations and Seismic Loading Conditions

AbstractThis paper presents the major findings of a shake table testing program on a large-scale (1:2.39) novel segmental concrete single-span bridge specimen. Emphasis is given on various specimen configurations and seismic loading conditions. The bridge specimen, termed hybrid sliding–rocking bridge, incorporated a box-girder superstructure with rocking joints and internal unbonded posttensioning (PT), and two single-column piers with internal unbonded PT. The pier columns included end rocking joints and intermediate sliding joints along the column height. Various configurations of the bridge specimen were considered with respect to the seismic mass and the superstructure-to-substructure connectivity. These configurations were subjected to far-field (F-F) and near-fault (N-F) ground motion ensembles scaled to various seismic hazard intensities. Asynchronous support excitation was also considered. The testing program included approximately 145 seismic tests. The dynamic response of the specimen was found...

Quasi-Static Cyclic Testing of Two-Thirds Scale Unbonded Posttensioned Rocking Dissipative Timber Walls

AbstractPosttensioning low-damage technologies were first developed in the late 1990s as the main outcome of the U.S. Precast Seismic Structural System (PRESSS) program coordinated by the University of California, San Diego, and culminated with the pseudo-dynamic test of a large-scale five-story test building. The extension of posttensioned techniques to timber elements led to the development of new structural systems, referred to as Pres-Lam (prestressed laminated timber). Pres-Lam systems consist of timber structural frames or walls made of laminated veneer lumber, glue laminated timber (Glulam), or cross-laminated timber (CLT). Pres-Lam walls consist of a rocking timber element with unbonded posttensioned tendons running through the length and attached to the foundation, which provides a centering force to the wall, while energy dissipation is supplied by either internal or external mild steel dissipaters. Previous tests carried out on posttensioned timber walls focused on small-scale (one-third) speci...

Large-Scale Seismic Testing of a Hybrid Sliding-Rocking Posttensioned Segmental Bridge System

AbstractIn this paper, a novel precast concrete segmental bridge system is introduced and experimentally investigated. The system consists of segmental members incorporating hybrid sliding-rocking (HSR) joints and internal unbonded posttensioning (PT). The HSR joints are plane interfaces that are oriented normal to the member axis and do not include shear keys or epoxy adhesives. The HSR joints are designed to exhibit sliding (slip-dominant, SD) or rocking (rocking-dominant, RD) to mitigate the applied seismic loading and reduce damage. The joint response is affected by the PT system, which can include straight or curved tendons. Two types of HSR members are developed and investigated: (1) members with SD joints and straight tendons, intended for substructure columns (HSR-SD columns), and (2) members with RD joints and curved tendons, intended for superstructure girders (HSR-RD girders). The SD joints are shown to offer energy dissipation with low damage through sliding in addition to moderate self-center...

Quasi-Static Cyclic Testing of a Large-Scale Hybrid Sliding-Rocking Segmental Column with Slip-Dominant Joints

This paper presents the findings of an experimental study that investigated the response properties of segmental cantilever columns incorporating internal unbonded posttensioning (PT) and slip-dominant (SD) joints. The SD joints exhibited controlled sliding that provided energy dissipation with low damage. All joints of these columns, except for the bottom one, were designed to be SD. The bottom joint was designed to be rocking dominant (RD) and exhibited rocking that offered self-centering to the system. Design objectives and equations are presented. These equations were used for the design of a large-scale cantilever column that was subjected to reverse lateral cyclic loading at its top end, reaching a maximum drift ratio of 14.9%. At small drift ratios (≤3%), the response was dominated by sliding of the SD joints that provided energy dissipation (damping). For medium drift ratios (between 3 and 10%), rocking at the bottom joint increased and provided self-centering properties to the system. For large drift ratios (>10%), the self-centering properties decreased, but the damping properties remained practically constant. Rocking at the SD joints remained small at all times. Minor spalling was observed in the SD joints, while concrete crushing was observed at the bottom joint.

Unbonded Transverse Posttensioning of a Railway Bridge in Haparanda, Sweden

The majority of railway lines in Sweden are designed to support axle loads of up to 250 kN. Because of increased transport needs on some lines, an axle load limit of at least 300 kN would be beneficial. To upgrade the Haparanda line in northern Sweden to 300 kN, the slabs in existing concrete trough bridges require a higher transverse shear resistance. Methods for in situ strengthening of bridge slabs in this way have not been fully developed, and this paper discusses the possibility of increasing the load capacity by horizontal prestressing. Internal, unbonded posttensioning was performed on one bridge on the Haparanda line, and the strengthening effects were investigated. The strengthening was designed according to the European Eurocode design regulations, and testing was conducted before and after the implementation. Strains in the main transverse reinforcement, caused by a train with an axle load of 215 kN, were completely counteracted by eight prestressing bars, stressed with 430 kN/bar. The results indicate that the actual strengthening effect is larger than what is predicted by the design equations. The Haparanda project showed that unbonded posttensioning can be implemented relatively fast and does not obstruct the ongoing railway traffic during installation.

Shake Table Studies of Energy-Dissipating Segmental Bridge Columns

Five one-third scale segmental bridge columns with plastic hinges incorporating different advanced materials were designed and tested on one of the shake tables at the University of Nevada, Reno. The columns were subjected to the Sylmar Earthquake record with increasing amplitudes until failure. All the models were cantilever with longitudinal steel dowels connecting the base segment to the footing. Unbonded posttensioning was used to connect the segments and to minimize the residual displacements. Energy dissipation took place mostly through the yielding of the longitudinal bars in the base segment. Conventional RC was used in the plastic hinge of a reference column. In one of the models, a built in elastomeric pad integrated with the footing and a concrete segment constituted the plastic hinge. The other two columns incorporated engineered cementitious composite (ECC) and unidirectional carbon fiber reinforced polymer (CFRP) fabrics at the lower two segments. The effectiveness of repair with CFRP wraps was also studied by repairing and retesting the reference column. The test results showed that the proposed models with advanced materials are suitable for accelerated bridge construction in high seismic zones because of their fast construction, high energy dissipation, minimal damage in the plastic hinge zone, and minimal residual displacement.

UPT rectangular and flanged shear walls of high-strength CASIEL-TLM masonry: Experimental and numerical push-over analysis

The mechanical behaviour of unbonded post-tensioned (UPT) shear walls of high-strength CAlcium SIlicate ELement masonry with Thin-Layer Mortar (CASIEL-TLM masonry) was investigated experimentally and numerically. Eight walls were tested with the following key variables: unit type, prestress level and cross-sectional shape (rectangular or T-shaped with interlocking of web and flange). An extensive measurement scheme was adopted that allowed derivation of average curvatures and strains in the bottom region of the wall in addition to wall displacements. Since UPT masonry is characterised by the absence of local compatibility between masonry and the UPT tendons, a numerical model for quasi-static, monotonic push-over analysis was developed that provides an iterative solution for the global interaction between masonry and UPT tendons. A common masonry stress–strain diagram was adopted in the numerical model. A peculiarity of CASIEL-TLM masonry is the kicker course, which reduces the stiffness of the bottom region of the shear wall. This layer was modelled with no-tension, linear-elastic behaviour and a reduced stiffness. Nevertheless, the model underestimates the experimental deformations of the rectangular shear walls, while the strength is in reasonable agreement. The walls with T-shaped cross-section failed prematurely by shear of the web-flange interface, resulting in diagonal splitting cracks in the interlocking units. This paper deals with the experimental results of UPT CASIEL-TLM masonry shear walls with rectangular and T-shaped cross-section and with the numerical modelling of the overturning behaviour of UPT shear walls with rectangular cross-section.

Behavior of Precast Concrete Shear Walls for Seismic Regions: Comparison of Hybrid and Emulative Specimens

AbstractThis paper discusses the lateral load behavior of two, 0.40-scale, hybrid, precast concrete shear wall test specimens and the behavior of a third precast specimen designed to emulate monolithic cast-in-place RC shear walls. The walls had identical overall geometry and were constructed by placing rectangular precast panels across horizontal joints. The hybrid walls used mild steel bars [Grade 400 (U.S. Grade 60)] and high-strength unbonded posttensioning (PT) strands for lateral resistance, whereas the emulative wall used only mild steel bars. The mild steel bars crossing the base joint were designed to yield and provide energy dissipation, with the PT steel in the hybrid walls reducing the residual displacements of the structure. The mild steel bars at the base of the emulative wall and one of the hybrid walls used Type II mechanical splices, while the other hybrid wall used continuous bars grouted into the foundation. Because of the lack of PT steel, the emulative wall developed a large residual ...

Two-plastic-hinge and two dimensional finite element models for post-tensioned precast concrete segmental bridge columns

Recent studies have confirmed that unbonded post-tensioned (PT) precast concrete segmental bridge columns are capable of undergoing large lateral deformation with negligible residual drift. To provide a clear guideline for the modeling of the columns for practicing engineers as well as researchers, this paper presents two types of numerical models: (i) a two-plastic-hinge model using the sectional moment–curvature analysis procedure at two segment interfaces and (ii) a two-dimensional (2D) finite element model using truss and beam-column elements in the computer program PISA. Three unbonded PT precast concrete-filled tube segmental bridge column specimens are cyclically tested. Two specimens have mild steel bars crossing to different column heights for studying the effects of anchorage position on the hysteretic energy dissipation (ED) capacity. The test results show that (1) the mild steel bars (“ED bars”) can increase hysteretic energy dissipation, and Specimens 1–3 have equivalent viscous damping of 6.5–8.8%, (2) a plastic hinge length in the first or second segment varies with anchorage position of ED bars and lateral displacement, and (3) an equivalent unbonded length along which the strain in the ED bar is assumed uniformly distributed on each of the two sides is 5–6bar diameter. A 2D finite-element model is utilized to predict the cyclic behavior of the specimens. Parametric studies using finite-element models are also conducted to investigate the effects of ED bar area, initial strand force, and aspect ratio on the cyclic behavior.