Evaluating the early-age behaviour of full-scale prestressed concrete beams using distributed and discrete fibre optic sensors

This paper evaluates the results of a monitoring study that captures the early age behaviour of four 11.9 m prestressed concrete bridge beams utilising both distributed and discrete fibre optic sensor (FOS) arrays. The performance of the beams is evaluated before they are placed in-service as part of new concrete railway bridges in the Midlands in the UK. Two types of prestressed beams were monitored, two TY7 internal beams and two TYE7 edge beams. The beams incorporated high strength (up to 90.7 MPa) self-consolidating concrete. The entire manufacturing process which included early-age curing and the detensioning process was captured in great detail using the installed FOS system. An analysis of the curing strains within the beams revealed the significant effect that ambient temperature, curing duration, and formwork restraint has on the development of prestress losses prior to detensioning. Based on the distributed FOS readings, it was observed that the strain remained uniform along the length of the beams during the various beam monitoring stages. The measured strain data was then used to calculate prestress losses in the first six months after casting (prior to casting of the in-situ concrete bridge deck). The TY7 and TYE7 beams experienced losses that were 79% and 72% of the ultimate losses predicted using Eurocode 2 equations, respectively. Distributed strain measurements were used to provide estimates of the change in beam camber with time. The pre-camber values calculated using the recorded FOS strain data at the time of detensioning closely match the theoretically calculated values. However, camber values increased by up to 1.7 times in the first six months compared with the post-detensioning values and deviated significantly from the theoretically calculated values. The future aim of this research is to establish integrated FOS systems as viable tools for monitoring strain evolution in concrete bridges in order to establish comprehensive baselines to facilitate long term data-driven bridge monitoring programmes.The authors gratefully acknowledge the EPSRC and Innovate UK for funding this research through the CSIC Innovation and Knowledge Centre (EPSRC grant reference number EP/L010917/1)

Capacity optimization of a prestressed concrete railroad tie

Today the use of concrete ties is on the rise in North America as they become an economically competitive alternative to the historical industry standard wood ties, while providing performance which exceeds its competition in terms of durability and capacity. Similarly, in response to rising energy costs, there is increased demand for efficient and sustainable transportation of people and goods. One source of such transportation is the railroad. To accommodate the increased demand, railroads are constructing new track and upgrading existing track. This update to the track system will increase its capacity while making it a more reliable means of transportation compared to other alternatives. In addition to increasing the track system capacity, railroads are considering an increase in the size of the typical freight rail car to allow larger tonnage. An increase in rail car loads will in turn affect the performance requirements of the track. Due to the increased loads heavy haul railroads are considering applying to their tracks, current designs of prestressed concrete railroad ties for heavy haul applications may be undersized. In an effort to maximize tie capacity while maintaining tie geometry, fastening systems and installation equipment, a parametric study to optimize the existing designs was completed. The optimization focused on maximizing the capacity of an existing tie design through an investigation of prestressing quantity, configuration, stress levels and other material properties. The results of the parametric optimization indicate that the capacity of an existing tie can be increased most efficiently by increasing the diameter of the prestressing and concrete strength. However, researchers also found that current design specifications and procedures do not include consideration of tie behavior beyond the current tie capacity limit of cracking to the first layer of prestressing. In addition to limiting analysis to the cracking limit, failure mechanisms such as shear in deep beams at the rail seat or pullout failure of the prestressing due to lack of development length were absent from specified design procedures, but discussed in this project

INFLUENCE AND ON-SITE ASSESSMENT OF LONG-TERM PRESTRESSING LOSSES ON SHEAR STRENGTH OF BRIDGE GIRDERS

La valutazione della capacità portante residua di un impalcato da ponte in calcestruzzo armato precompresso (CAP), dopo oltre 50 anni di esercizio può risultare difficile, soprattutto se esso mostra difetti di degrado a causa della scarsa/assenza di manutenzione o addirittura esibisce un quadro fessurativo anomalo. A seguito di una campagna di ispezioni svolta su oltre 400 ponti della Provincia di Brescia, è emerso come il 6% dei ponti in CAP, che corrispondono a circa il 44% del patrimonio infrastrutturale del gestore, mostra fessure a taglio nelle anime, anomale per le condizioni di carico a cui il ponte solitamente è sottoposto. La natura di queste fessure può essere di varia origine, tra cui una sovrastima in fase progettuale del contributo a resistenza a taglio fornito dalla precompressione dell’elemento. Indagando in campo e in letteratura, si è scoperto che ditte di prefabbricazione negli anni 70-80, realizzavano elementi precompressi con scarsa armatura a taglio, se non addirittura assenza in alcuni casi. Notizia ritrovata anche nella normativa tedesca antecedente gli anni 70 per la realizzazione di elementi precompressi. Questo perché, essendo l’elemento precompresso, parte della resistenza a taglio veniva destinata al contributo della precompressione e quindi si poteva «risparmiare» in termini di armatura trasversale. Il presente lavoro di ricerca intende approfondire due aspetti: la valutazione della bontà di alcune tecniche diagnostiche proposte in letteratura per la valutazione della precompressione in situ e l’influenza delle perdite da precompressione sul quadro fessurativo e sulla resistenza a taglio di travi da ponte in scala reale. Per questi scopi è stata condotta una campagna sperimentale indirizzata alla realizzazione di 4 travi in CAP della lunghezza di 10 m, sezione a “I” alta 80 cm e con staffatura minima. Due travi sono state realizzate con la tecnologia di trefoli pre-tesi aderenti, che differiscono tra loro per un 30% di livello di precompressione assegnata. Le altre due travi, identiche, sono state progettate con un sistema di post-tensione dei cavi (non aderenti) che permette la variazione del livello di precompressione in modo controllato, coprendo così più scenari di perdite a lungo termine. Su questi elementi sono stati applicati 3 metodi semi-distruttivi per la valutazione della precompressione in situ basati sul rilascio tensionale, ovvero: carota strumentata, tagli paralleli all’intradosso e provino tronco piramidale. In aggiunta viene proposto e validato un nuovo metodo, simile ai tagli paralleli all’intradosso, ma eseguito sull’anima. Su questo, dimostratosi il più affidabile tra i metodi testati, sono stati realizzati modelli ad elementi finiti in 2D e 3D in campo elastico. Successivamente, è stata eseguita una prova di carico in 3 punti su ciascuna trave, valutando l’evoluzione del quadro fessurativo con la tecnica del Digital Image Correlation (DIC) e confrontando i risultati con quelli ottenuti dalla simulazione numerica eseguita con il software VecTor 2, basato sulla Modified Compression Field Theroy (MCFT). Durante le varie fasi è stato valutato se un’indagine non distruttiva come l’identificazione dinamica possa cogliere danneggiamenti dovuti a lesioni a taglio e una riduzione della perdita di precompressione. Infine, essendo le formule di verifica della resistenza a taglio proposte dai modelli delle varie norme (e.g. Eurocodice 2, fib Model Code, CSA, ACI) molto conservative per gli elementi precompressi con staffe, viene proposta una formulazione analitica basata sulla bozza del fib Model Code 2020 LoA IIb. La formulazione è stata validata grazie all’applicazione ad alcune prove sperimentali presenti in letteratura, ottenendo buoni risultati.The assessment of the residual load-bearing capacity of a precast prestressed reinforced concrete bridge deck (PRC) after more than 50 years of service can be challenging, especially if it exhibits degradation defects due to poor or missing maintenance or even if it is affected by any kind of cracking due to stress. Following an inspection program carried out on over 400 bridges in the Province of Brescia, it emerged that 6% of the PRC bridges, corresponding to approximately 44% of the infrastructure assets of the bridge manager, exhibit shear cracks in the webs that are unexpected for the load conditions to which the bridge is usually subjected. The nature of these cracks can have various causes, including an overestimation of the contribution to shear resistance provided by the prestressing of the element during the design phase. From a survey in the field and the literature, it was discovered that precast companies in the 1970s-1980s produced prestressed elements with low shear reinforcement, or even without reinforcement in some cases. This was also found in German regulations before the 1970s for the production of prestressed elements. This is because, being a prestressed element, part of the shear resistance was given to the contribution of prestressing, providing a significant saving in terms of transverse reinforcement. This research aims to investigate two aspects: the evaluation of the reliability of some diagnostic techniques proposed in the literature for the assessment of in-situ prestressing and the influence of prestress losses on the crack pattern and shear strength of full-scale bridge beams. For these purposes, an experimental program was conducted on 4 PRC beams with a length of 10 m, an 80 cm high I-section, and minimum web reinforcement. Two beams were constructed pre-stresses strands, which differ from each other by 30% of the assigned level of prestress. The other two identical beams were designed with a system of post-tensioning stands that allows the variation of the level of prestress in a controlled way, covering several long-term loss scenarios during the experiments. On these elements, three semi-destructive methods for in-situ prestressing evaluation based on tension release were applied: core trepanning, saw-cut at intrados, and blunt pyramidal specimen. In addition, a new method, similar to the parallel saw cuts at intrados, but performed on the web, is proposed and assessed. On this method, which proved to be the most reliable among the tested methods, 2D and 3D finite element models were also performed and discussed. Subsequently, a 3-point loading test was performed on each beam, evaluating the evolution of the crack pattern with the Digital Image Correlation (DIC) technique and comparing the results with those obtained from the numerical simulation performed with the VecTor 2 software based on Modified Compression Field theory (MCFT). During the various loading phases, it was evaluated whether a non-destructive investigation such as dynamic identification could detect damage due to shear cracking and a reduction in prestress loss. Finally, since the verification formulas for shear strength proposed by the models of various codes (e.g. Eurocode 2, fib Model Code, CSA, ACI) are very conservative for prestressed elements with stirrups, an analytical formulation based on the draft of the fib Model Code 2020 LoA IIb is proposed. The formulation was validated against to some experimental tests in the literature, obtaining good results

Influence of Concrete Compressive Strength on Transfer and Development Lengths of Prestressed Concrete

This research examines the relationship between concrete compressive strength and strand bond. The goal of this research was to develop an equation that relates strand bond to concrete compressive strength at strand release (approximately 1 day of age) and at 28 days of age, and those equations are presented in this investigation. Strand bond is assessed by measuring the transfer length and development length for prestressed beams cast in the laboratory. In the U.S., strand bond is predicted using transfer length and development length equations provided by the American Concrete Institute (ACI-318) Building Code and American Association of State and Highway Transportation Official (AASHTO) LRFD Bridge Design Specifications which were developed based on the 1950´s investigations. The equations provided by both ACI and AASHTO do not address concrete strength while equations, developed in this investigation, do account for the compressive strength of concrete at release and testing time. Although there has been much research conducted in this matter, this research provides a reliability data analysis relating to transfer and development lengths of prestressed concrete beams. Unlike many of the previous programs, this research includes strands of a known quality, the largest database of test specimens, and a variety of concrete mixtures and concrete strengths. This research concludes with the development of an analytical model to predict transfer length which includes concrete strength at release with fracture propagation around the strand

A study of the behaviour of post-tensioned brickwork beams.

SIGLELD:D48252/84 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

텐던공법에 따른 일방향 포스트텐션 콘크리트 부재의 내화거동

학위논문(석사) -- 서울대학교대학원 : 공과대학 건축학과, 2022.2. Thomas Kang.최근 콘크리트 구조물에서 포스트텐션 공법의 적용은 지하 구조물, 장경간 구조물, 격납 구조물에서 증가하고 있다. 이에, 다양한 재난 상황에서의 구조 안전성을 확보하기 위해 여러 건축법규가 제정 및 개정되었다. 내화성 측면에서, 국내 내화시험 방법을 제안한 KS F 2257과 화재안전 설계기준을 제안한 국가표준이 있으며, 해외의 IBC 2018과 ASTM E119 등이 있다. 그러나, 제안된 표준들은 세부적인 내용이 부족하며 모호한 부분이 있다. 또한, 규정에서 제시하고 있는 기준의 적절성에 대한 추가 연구가 필요하다. 따라서, 본 연구에서는 구조물의 화재성능 평가를 위해 일방향 부착식 포스트텐션 콘크리트 슬래브 2개와 비부착식 포스트텐션 콘크리트 슬래브 6개의 화재거동에 대한 실험을 4점 정적하중 가력 및 가열을 통해 수행하였다. 본 실험에서 고려된 변수는 텐던공법 종류, 피복두께, 그리고 목표 내화성능이다. 4개의 실험체는 2시간의 화재를 견딜 수 있게 설계되었으며, 나머지는 3시간의 화재를 견딜 수 있게 설계되었다. 모든 실험체는 실험체의 바닥, 상부, 텐던 위치에 열전대를 설치하여 온도변화를 계측하였다. 또한, 자체제작한 로드셀을 설치하여 비부착 포스트텐션 콘크리트 슬래브의 긴장력 변화를 긴장 및 실험 시 계측하였다. 변위는 슬래브 중심에 LVDT를 설치하여 계측하였다. 실험 결과, 부착식 포스트텐션 콘크리트 슬래브의 경우, IBC 2018에서 규정하고 있는 최소 피복두께를 확보하였을 경우, 모두 2시간 및 3시간 이상 지속하중에 견딜 수 있음을 입증하였다. 하지만, 비부착식 포스트텐션 콘크리트 슬래브는 예상 시간보다 조기에 파열되었다. 콘크리트 파열력이 실험결과에 중요한 영향을 끼쳤으며, 조기 종료에 대한 여러 요인들이 조사되었다. 비록 실험이 조기에 종료되었지만, 텐던공법 종류에 따른 온도 변화, 긴장력 변화, 변위 변화 등의 화재 거동의 차이를 비교 분석할 수 있었다. 부착식 포스트텐션 콘크리트 슬래브의 경우, 유한요소해석을 통해 목표 내화성능 대비 최소 피복두께에 대한 화재성능을 평가하였다. 본 연구에서는 텐던공법에 따른 포스트텐션 콘크리트 일방향 부재의 화재거동을 분석하였다. 수치해석 기법을 사용하여 최소 피복두께에 따른 내화성능을 분석하였고, 화재 실험결과를 또한 검증하고자 하였다. 이 연구의 결과는 기준 조항의 세부 사항을 개정하기 위한 기초 연구자료로서 활용될 수 있을 것으로 기대된다.Recently, the application of the post-tensioned (PT) method to concrete structures such as underground structures, long-span structures, and containment structures has increased. Structural safety under multiple hazards has been an issue for those structures, and therefore several building codes have been enacted and revised. In terms of fire resistance, there are domestic standards of Korean Standard (KS) F 2257 that specifies the fire resistance test method and National Legal that regulates fire safety design. There are also international standards such as IBC 2018 and ASTM E119 that specify fire safety design and fire test methods. However, existing standards include limited information and tend to be ambiguous in may clauses. Questions are continuously being raised as to whether or not the standards presented in the regulations are appropriate. In this study, fire tests of two one-way bonded PT concrete slabs and six one-way unbonded PT concrete slabs were conducted under four-point static loading and monotonically increasing heating to evaluate the fire performance of the structures. The variables considered in the test included tendon configuration, cover thickness, and fire resistance rating. Four specimens were manufactured to endure fire for two hours, others were designed to last three hours. All specimens installed thermocouples at the bottom, tendon, and top locations to measure temperature change. Self-made load cells were installed on unbonded cases to measure tendon stresses and their change at jacking and fire test stages. Deflection of slabs was measured at the center of specimens using an LVDT. Test results demonstrated that bonded cases sufficiently endured two hours and three hours of fire with the minimum cover thickness specified in IBC 2018. However, unbonded cases ruptured prematurely compared to the expected fire resistance rating. Concrete spalling and delamination significantly affected test results, and other factors were also investigated to address reasons for early termination. Although the test ended earlier, the difference in fire behavior according to the tendon configuration in terms of temperature change, tendon stress change, and deflection change could be analyzed. For bonded cases, finite element analysis has been performed to assess the fire performance with varied cover thicknesses under the design fire resistance rating. In conclusion, this study effectively analyzed the fire behavior of PT concrete one-way members with different tendon configuration. Numerical analysis was performed to check the fire performance with different cover thickness and verify the fire test results. Results from this study are expected to be used as basic research data to codify or revise fire design codes.Chapter 1. Introduction 1 1.1 Introduction 1 1.1.1 Bonded and unbonded post-tensioning systems 1 1.1.2 Fire safety engineering in structures 3 1.2 Scope and objectives 5 1.3 Organization 6 Chapter 2. Review of Codes, Specifications, and Previous Studies 8 2.1 Tendon stress at nominal flexural strength (fps) 8 2.2 Structural fire safety design 10 2.2.1 Variation of material properties according to temperature 10 2.2.2 Standards for fire tests 17 2.3 Previous studies 19 2.3.1 Bailey and Ellobody (2009) 19 2.3.2 Hou et al. (2013) 24 2.4 Discussion 28 Chapter 3. Specimen Design 30 3.1 Specimen outline 30 3.1.1 Specimen dimension 30 3.1.2 Material plan 31 3.2 Tendon design 31 3.2.1 Tendon configuration 31 3.2.2 Tendon layout 32 3.3 Specimen design 33 3.4 Discussion 37 Chapter 4. Specimen Construction and Test Setup 38 4.1 Specimen construction 38 4.2 Tendon jacking 41 4.3 Material property 44 4.3.1 Strand 44 4.3.2 Reinforcing steel 45 4.3.3 Concrete 45 4.4 Test setup and instrumentation plan 47 4.4.1 Test setup 47 4.4.2 Instrumentation plan 48 4.5 Discussion 50 Chapter 5. Fire Test Results 51 5.1 Input fire scenario 51 5.2 Temperature variation 52 5.2.1 Time-temperature curves at the bottom of slab 52 5.2.2 Time-temperature curves at tendon 53 5.2.3 Time-temperature curves for reinforcing bars 55 5.3 Deflection and deflection rate variation 55 5.4 Tendon tensile force variation 59 5.5 Concrete crack propagation, spalling and delamination 61 5.5.1 Concrete crack propagation 61 5.5.2 Concrete spalling and delamination 65 5.6 Discussion 68 Chapter 6. Numerical Analysis 70 6.1 Numerical modeling 70 6.1.1 Concrete modeling 71 6.1.2 Steel modeling 73 6.2 Heat transfer analysis 75 6.2.1 Heat transfer model 75 6.2.2 Heat transfer analysis 76 6.3 Thermo-mechanical analysis 79 6.3.1 Thermo-mechanical model 79 6.3.2 Thermo-mechanical analysis: prestressing 80 6.3.3 Thermo-mechanical analysis: loading 80 6.3.4 Thermo-mechanical analysis: heating 82 6.4 Parametric study 89 6.5 Discussion 92 Chapter 7. Conclusion 93 References 96 Appendix A : Final drawings 100 Appendix B : Time-temperature curves at the bottom of slab 105 국 문 초 록 109 감사의 글 111석

Modelling and parametric study of the re-anchorage of ruptured tendons in bonded post-tensioned concrete

The contribution of ruptured tendons to the residual strength of bonded post-tensioned concrete structures is currently assessed based on pre-tensioned concrete bond models. However, this approach is inaccurate due to the inherent differences between pre-tensioned and post-tensioned concrete. In this paper, a non-linear 3D finite element model is developed for the re-anchoring of a ruptured tendon in post-tensioned concrete. The model is validated using full-field displacement measurement from 33 post-tensioned concrete prisms and previous experimental data on beams from the literature. The influence of different parameters was investigated, including tendon properties (i.e. diameter, roughness), duct properties (i.e. diameter, thickness, material), initial prestress, concrete strength, grout strength, grout voids, stirrups, and strands, on the tendon re-anchorage. The most influential parameters are found to be tendon and duct properties