Time-dependent evolution of strand transfer length in pretensioned prestressed concrete members

For design purposes, it is generally considered that prestressing strand transfer length does not change with time. However, some experimental studies on the effect of time on transfer lengths show contradictory results. In this paper, an experimental research to study transfer length changes over time is presented. A test procedure based on the ECADA testing technique to measure prestressing strand force variation over time in pretensioned prestressed concrete specimens has been set up. With this test method, an experimental program that varies concrete strength, specimen cross section, age of release, prestress transfer method, and embedment length has been carried out. Both the initial and long-term transfer lengths of 13-mm prestressing steel strands have been measured. The test results show that transfer length variation exists for some prestressing load conditions, resulting in increased transfer length over time. The applied test method based on prestressing strand force measurements has shown more reliable results than procedures based on measuring free end slips and longitudinal strains of concrete. An additional factor for transfer length models is proposed in order to include the time-dependent evolution of strand transfer length in pretensioned prestressed concrete members.     

Experimental Technique for Measuring the Long‐term Transfer Length in Prestressed Concrete

Abstract: This article presents a proposal of a test set‐up and methodology for testing the transfer length evolution through time of prestressing reinforcement in pretensioned prestressed concrete members, aimed at providing a basis for standardization. The proposed test method is based on the instantaneous and time‐related analysis of the prestressing reinforcement force profile at only one end of a pretensioned prestressed concrete member. The basis of the test method and the requirements of the prestressing frame and its components are presented, as well as the test procedure stages and the measurement devices. The interpretation of the test results and the criteria to determine both the initial and the long‐term transfer lengths are explained. A test method application and the equipment for testing seven‐wire prestressing strands have been designed. Some experimental results are provided to validate the test. A comparative analysis of test reliability with other existing experimental methods is also included.     

Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons

There is currently widespread use of high-strength steel tendons for prestressing and post-tensioning of concrete structures and as suspension cables for long-span bridges. Tendons normally consist of one or more seven-wire, helically wound steel strands or solid rods. There are more than 130,000 prestressed bridges in the United States that contain these tendons, and approximately 3,000 new bridges are constructed each year. The prestressing tendons are critical structural elements because the forces in the tendons counteract tensile stresses in the concrete that result from loads acting on a bridge. The tendons are frequently inaccessible for visual inspections and there is currently no accepted nondestructive evaluation technique to assess the condition of these tendons. The goal of this research is to examine ultrasonic stress measurement techniques for the condition assessment of prestressing tendons. This information could be used to compare the actual force in the tendon with its design values to determine if the tendon is performing below expectations. The focus of this paper is the characterization of the relationship between ultrasonic wave velocity and stress level in prestressing tendons. Measurements were made to determine constants that relate the change in ultrasonic velocity to the change in stress. The effects of dispersion in prestressing tendons, which act as circular waveguides for ultrasonic waves, are evaluated.     

Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons

Abstract

There is currently widespread use of high-strength steel tendons for prestressing and post-tensioning of concrete structures and as suspension cables for long-span bridges. Tendons normally consist of one or more seven-wire, helically wound steel strands or solid rods. There are more than 130,000 prestressed bridges in the United States that contain these tendons, and approximately 3,000 new bridges are constructed each year. The prestressing tendons are critical structural elements because the forces in the tendons counteract tensile stresses in the concrete that result from loads acting on a bridge. The tendons are frequently inaccessible for visual inspections and there is currently no accepted nondestructive evaluation technique to assess the condition of these tendons. The goal of this research is to examine ultrasonic stress measurement techniques for the condition assessment of prestressing tendons. This information could be used to compare the actual force in the tendon with its design values to determine if the tendon is performing below expectations. The focus of this paper is the characterization of the relationship between ultrasonic wave velocity and stress level in prestressing tendons. Measurements were made to determine constants that relate the change in ultrasonic velocity to the change in stress. The effects of dispersion in prestressing tendons, which act as circular waveguides for ultrasonic waves, are evaluated.     

Computer Aided Modeling of Magnetic Behavior of Embedded Prestressing Strand for Corrosion Estimation

This work presents a novel computer model which simulates the main magnetic flux (MMF) signal of prestressing strand subjected to a magnetic field. Magnetic inspection techniques have been shown to be effective in detecting hidden corrosion in embedded prestressing steel strand. However, such systems are not yet ready for field application. The development of computational models is a key step in making the technology practical. Magnetic behavior of prestressing steel strand is a feature that can be used to determine loss of mass or other defects in the strand. The magnitude of the MMF induced in the strand can be used to estimate the cross-sectional area of healthy steel in the strand. Thus, magnetic properties can be utilized to measure the amount of hidden corrosion in the strands embedded in prestressed bridges and, hence, be used to determine the capacity of the bridge. An MMF electromagnet-based sensor system was recently developed for corrosion detection using the same principle. 2D and 3D models of this sensor are developed using commercially available magnetic field simulation software. The models are validated using laboratory results from tests with prestressing strands using the MMF electromagnet-sensor system. A number of factors including diameter of the strand, thickness of the concrete cover, reluctance in the magnetic circuit and variations due to temperature rise that affect the magnitude of the induced magnetic field are considered. Accuracy of the model is compared with laboratory and theoretical values. The computational model can be used to estimate corrosion from field test results and advance the development of the MMF electromagnet-sensor system.     

An experimental investigation on flexural behavior of RC beams strengthened with prestressed CFRP strips using a durable anchorage system

This paper investigates the effectiveness and feasibility of a prestressed carbon fiber-reinforced polymer (CFRP) system for strengthening reinforced concrete (RC) beams. The proposed prestressing system with a novel anchorage allows the utilization of full capacity of the CFRP strips. Eight small-scale and two large-scale concrete beams strengthened different configuration of prestressed CFRP strips are tested under static loading conditions up to failure. The main parameters considered include the level of prestressing applied, ranging from 20% to 70% of the tensile strength of the CFRP strips, and the use of mechanical anchorages at both ends of the CFRP strips. Thanks to the durable anchorage, the full range of flexural behavior was investigated including post-debonding. The results indicate that the beams strengthened using prestressed CFRP strips exhibited a higher first-cracking, steel-yielding, and experimental nominal moments as the level of prestressing force increased up to a certain point. After analyzing prestress effects in small scale tests, an optimum prestress level for strengthening concrete beams using CFRP strips is proposed and verified in large scale tests.     

预应力钢-竹组合梁受弯挠度计算方法与试验研究

为分析预应力钢-竹组合梁的受弯挠度,以加载方式、张弦位置、预应力度为变量,对12根组合梁试件进行了设计与试验研究。在此基础上,假定梁变形分布符合正弦半波曲线,并考虑梁加载过程中几何关系变化与预应力反拱的影响,采用弹性理论建立了组合梁中预应力筋应力增量的计算方法,推导得出一点或两点加载、一点或两点张弦时,组合梁受弯挠度计算的统一公式。试验与理论计算结果的对比表明：该文提出的挠度计算方法可较好的预测组合梁在正常使用阶段的挠度;随着预应力度的增加,组合梁的等效抗弯刚度不断提高,且两点张弦时可获得更高的等效抗弯刚度。此外,对于初始预应力为零的试件,需采用可靠预紧措施,以保证体外预应力筋能够有效发挥作用。     

Shear creep of epoxy at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations of the long-term behavior of the epoxy used at the concrete–FRP interfaces. Double shear long-term test was performed on specimens composed of concrete blocks bonded to FRP sheets using epoxy. Three test replicates were examined under sustained shear stress for up to six month time period with two primary parameters: the shear stress level and the epoxy thickness. The investigation showed that shear stress level might have significant effect on the long-term behavior of epoxy at the concrete–FRP interfaces. Based on the experimental results, creep characteristics of epoxy in the concrete–FRP interfaces were evaluated. Finite element analysis was performed incorporating these creep characteristics to investigate the long-term deformations and stress redistributions in the test specimens. It was recognized that creep of epoxy might result in stress-redistribution at the concrete–FRP interfaces.     

Splitting failure of precast prestressed concrete during the release of the prestressing force

Bond between steel and concrete is fundamental for the transmission of stresses between both materials in precast prestressed concrete. Indented wires are used to improve the bond in these structural elements. The radial component of the prestressing force, increased by Poisson’s effect, may split the surrounding concrete, decreasing the wire confinement and diminishing the bonding. This work presents a testing procedure to obtain the bond–slip curves, between steel and concrete, during the releasing of the prestressing force. The experimental procedure allows study of the splitting failure of the concrete, induced by the action of the indented wire. The influence of the distance between wires, the thickness of concrete cover and the depth of the wire indentations on the bond and splitting are examined. Specimens with three depths of the wire indentations and three thicknesses of concrete cover were tested. Moreover, a numerical procedure is presented for modelling the bond–slip, taking into account the possible failure of concrete by splitting. The numerical procedure accurately reproduces the experimental records and improves knowledge of this complex process.     

Thickness effects on the fatigue strength of welded steel cruciforms

Over 100 fatigue tests were conducted on high strength welded steel (HSLA-80) cruciforms of different thickness. Tests were conducted under both constant and random amplitude axial loads to characterize thickness effects on fatigue strength. Specimens were similar in size, except for the thickness which was varied between four nominal values. Examination of both experimental and analytical results (obtained using linear cumulative damage and Rayleigh approximation) indicates thicker specimens exhibit lower fatigue lives under both constant and random amplitude loadings. These results, when compared with the commonly used ‘fourth root rule' thickness correction formula, indicate the latter to be generally conservative, particularly at low stress levels.     

Experimental studies of the bond response of three-wire strands and some influencing parameters

The bond behavior of prestressing strands is of great importance for the capacity of precast prestressed concrete structures. In the present study, the bond behavior of three-wire strands, and some influencing parameters, were examined by means of steel encased pull-through and push-in tests. The three mechanisms: adhesion, friction and other mechanical actions were found to be present at the strand-concrete interface at different slip values. The results from the experiments showed that the micro roughness of the strand surface strongly affected the initial bond response of the strand, that is the adhesion in the interface. The maximum bond capacity of indented three-wire strands was found to be directly connected to the geometric properties of the strand indents. The influence of the concrete strength on the bond capacity of the strand was hard to interpret. However, the density of the concrete matrix was found to be a better parameter for determine the influence of the concrete rather than the compressive strength.     

Experimental and analytical study of flat-plate floor confinement

Currently available analytical models were developed for homogeneous concrete and are therefore inapplicable to specimens cast with concretes of different strengths. The present study examines such composite structures, and more especially normal-strength floor concrete sandwiched between columns of high-strength concrete, as well as the aspect ratio (ratio of slab thickness to column dimension) and closely spaced slab reinforcement. The effect on column–slab joint strength of confinement by rectangular hoops and slab portions extending in all directions from the joint are investigated. Three series of experiments on column specimens were carried out and the experimental results compared with the analytical ones. The experimental results conform to the predictions made by the theoretical models. The same models were used to evaluate the effects on slab concrete behavior of confinement by lateral reinforcement and by a slab surrounding the column–slab joint. A surrounding-slab confinement factor was defined and developed for use in analysis. This study represents a first attempt to evaluate confinement effects in column–slab joints in the presence of surrounding slab. Application of the types of structure investigated here could yield improved strength and ductility, enabling smarter design.     

Prestress loss due to creep and shrinkage of high-strength calcium silicate element masonry with thin-layer mortar

Data on creep and shrinkage of high-strength calcium silicate element masonry with thin-layer mortar (CASIEL-TLM masonry) is not currently available in international literature. A novel application of this material in unbonded post-tensioned shear walls requires creep and shrinkage data to predict prestress loss. Therefore, creep and shrinkage were measured on 38 large-scale high-strength CASIEL-TLM specimens, including a TLM bed joint or a wall-floor connection, for a period of 300 days. Initial moisture content of the specimens and temperature and relative humidity of the environment were carefully controlled. Creep and shrinkage data were used to predict prestress loss by means of visco-elastic finite element simulations as well as a simple analytical expression. These predictions were validated by prestress loss experiments, conducted simultaneously with the creep and shrinkage experiments. Predictions for final prestress loss due to creep and shrinkage are below 16 or 24 % for CASIEL-TLM masonry with prestressing strands or prestressing bars respectively.     

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress–strain response of high strength concrete and the nonlinear stress–strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model, tension stiffening and bond slip between concrete and reinforcement (fibers, prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However, beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams.     

预应力度对预应力混凝土框架结构抗震性能影响研究

依据我国规范设计4榀不同预应力度、抗震等级为二级的预应力混凝土框架结构,基于OpenSees分析软件,采用纤维梁、柱单元对其进行数值建模,对一榀单层单跨预应力混凝土框架的低周反复加载试验进行数值模拟,滞回曲线和骨架曲线与试验结果吻合良好,验证了该数值建模方法的实用性;在此基础上,对4榀预应力混凝土框架进行了静力弹塑性分析;最后,对按我国规范设计的抗震等级为二级的PC框架结构,在裂缝控制等级为二级时,预应力梁中非预应力筋用量的确定方法及其适用性进行了理论分析。结果表明:按我国现规范设计的预应力混凝土梁中非预应力筋用量偏多,在大震下预应力混凝土框架结构难以实现“强柱弱梁”的破坏模式,底层柱底塑性铰出现过早;适当提高梁端截面的预应力度值,能够在一定程度上改善结构的屈服机制,增大结构的整体位移延性,提高结构的抗震能力。因此,建议在进行相关规范修订时,适当减少非预应力筋配筋、提高框架梁的预应力度限值,如按04规程设计的二级预应力混凝土框架结构,其预应力度限值可提高至0.8。     

Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A Study of Interfaces at Artificial Grouting Faults and Its Verification

This paper presents the progress of successful location of grouting faults in tendon ducts with ultrasonic imaging. The examples were obtained in the research group FOR 384 funded by DFG (German Research Foundation). The co-operation of experimental research and modeling allowed imaging and identification of grouted and ungrouted areas of tendon ducts (including strands) in a large test specimen (40 m²). In addition to the criteria for indicating grouting faults in post-tensioned ducts known until now the phase evaluation of reflected ultrasonic pulses is described. Experiments and modeling of wave propagation are presented for reflections at metal plates in concrete (thickness range 0.5 mm to 40 mm) and for tendon ducts including strands. The main part of the progress was achieved by automated measurements using dry contact transducers, 3D-SAFT reconstruction including phase evaluation and modeling considering wave propagation for typical elastic parameters and exact experimental site conditions. The results for shear waves as well as for pressure waves are compared in the frequency range from 50 kHz to 120 kHz.     

Durability of CFRP cables exposed to simulated concrete environments

One of the main causes of structural deficiency in concrete bridges is the deterioration of the constituent materials. Some transportation agencies have started exploring the use of carbon fiber reinforced polymers (CFRP) as a corrosion-resistant alternative to steel prestressing materials for longer lasting concrete bridge structures. To implement CFRP in prestressed concrete bridge structures with more confidence, an understanding of the challenges pertaining to durability under in-service environmental conditions is essential. This paper explores the effects of temperature with alkaline and alkaline/chloride solutions on material properties over time, in the context of reinforced and prestressed concrete structures for transportation applications. The durability testing yielded several key findings: (1) higher temperatures accelerate degradation, (2) moisture sorption was the primary process responsible for the observed degradation, with plasticization and microcracking as the controlling mechanisms leading to fiber–matrix interfacial debonding, and (3) a relaxation-based analytical model was implemented to predict residual properties after environmental exposure, showing promising agreement with experimental data.     

建筑用钢筋端头螺纹的高效精密滚压工艺及设备特性的研究现状

钢筋混凝土结构在大型建筑、道路桥梁、机场码头、核电等领域应用广泛,市场上高大工程建筑对大直径、超长钢筋的需求持续增加,超长钢筋由多根钢筋通过螺纹连接构成,实现每根钢筋连接端头螺纹的高效精密成形至关重要,是目前迫切需要解决的难题。分析了国内外钢筋两端螺纹的制造工艺,论述了钢筋端头螺纹滚压塑性成形的基本原理和工艺过程,对钢筋连接端头的剥肋尺寸、滚压模具的结构、工件与滚压模具之间的轴向运动以及滚压成形前模具的相位要求等多方面的问题进行了总结;对目前国内外钢筋端头螺纹滚压设备的基本原理、工作过程及其特点进行了总结,从而为建筑用钢筋两端螺纹的高效精密滚压成形工艺和设备技术的提升与推广奠定良好的基础。     

Experimental and analytical analysis of pretensioned inverted T-beam with circular web openings

This paper presents the results of a research project aimed at providing standard circular web openings to the popular precast pretensioned inverted T-beam. The main advantage of these openings is that mechanical equipment can pass through the webs of inverted T-beams. Another advantage is a slight reduction in inverted T-beams weight that would improve the demand on the supporting frame both under gravity loading and seismic excitation. Opening size and placement and required materials strengths were investigated. Also, the effects of using straight tendons, rather than two-point depression and one-point depression, of the prestressing strands were investigated. In this paper the nonlinear analysis and design of simply supported pre-tensioned inverted T-beam with circular web openings are presented. Two design parameters are varied such as: opening location and numbers of openings. The results from a nonlinear finite element analysis are substantiated by test results from five pretensioned inverted T-beam with web opening and one solid beam. Good agreement is shown between the theoretical and the experimental results. The effect of openings on the behavior of such beams at different stages of loading is presented. The test results obtained from this investigation show that the performance of the specimens with web openings was almost identical to that of the specimen without web openings. A simple design method for estimating the cracking load for the different crack patterns is proposed. Based on these tests, design recommendations are made that will allow the addition of web openings to inverted T-beam with minimal additional calculation.     

带栓钉波形钢板混凝土组合构件粘结滑移性能与承载力试验研究

通过11个带栓钉的波形钢板混凝土组合构件在单调荷载下的推出试验和1个自然粘结构件的对比试验,研究带栓钉波形钢板混凝土组合构件的破坏形态、裂缝模式、荷载-滑移特性、波形钢板应变分布和承载力等。结果表明:带栓钉波形钢板混凝土组合试件的破坏形态以混凝土劈裂为主。试件的荷载-滑移曲线由上升阶段、下降阶段、残余阶段三个部分组成。由于混凝土和栓钉的组合作用,波形钢板自由端存在受拉区,产生过零点现象。带栓钉波形钢板混凝土组合试件的抗剪承载力随栓钉直径、数量的增长呈线性增长,而在一定条件下,栓钉长度、钢板厚度对抗剪承载力影响不大,另外在200 mm范围内适当增大栓钉间距对抗剪承载力也有提高。基于试验结果和力的扩散原则,分别提出了考虑栓钉影响的波形钢板混凝土界面粘结滑移本构模型以及带栓钉的波形钢板混凝土推出试件的承载力计算公式,所提模型与试验结果吻合良好,承载力公式计算结果与试验结果总体相符且偏于安全。