RESEARCH SCHOLAR, INDIAN INSTITUTE OF TECHNOLOGY
Investigating use of compiste strands in place of Steel strands in Prestressed Concrete application
The transfer of prestress force from prestressing strands to the surrounding concrete is dependent on the bond between the two materials. It is essential to understand the actual bond stress distribution along the transfer length to determine the transfer zone in pretensioned concrete. The first part of this study examine the effects of excessive slip on steel strand transfer length and moment envelop of pretensioned bridge girders. For this, a 3-D nonlinear Finite Element Model has been developed to simulate the transfer of prestress force from steel strands to concrete in pretensioned bridge girders. Overall slips of the steel strands over the transfer length were calculated from the Finite Element Model using a theoretical relationship proposed by previous researchers. Results obtained from the Finite Element analysis showed that the transfer length in the girders increases substantially due to the slippage of the strands. Investigation of the flexural behaviour of the bridge girder with varying strand surface conditions showed that the moment capacity of the girder with significant strand slips is reduced in the development region. It can thus be concluded that, strand slipping measurement should be added to the quality control procedures for pretensioned members. Steel strands are susceptible to corrosion and show very high prestress loss. A potentially alternative to prestressed Steel strands, would be Carbon Fiber Reinforced Polymer (CFRP) Strands and Aramid Fiber Reinforced Polymer (AFRP) Strands. Composite rods manufactured from artificial polymer materials such as Glass, Aramid and Carbon fibres can be adopted as strands for prestressing of concrete members. The use of composite materials like Fibre Reinforced Polymer (FRP) strands is evolving and is currently shifting from theoretical investigations into prototype installation. In the second part of this study, a 3-D linear Finite Element Model has been developed to simulate the transfer of prestress force from Steel, Carbon and Aramid fibre strands to concrete in a pretension member. The Finite Element Model has been validated using experimental studies. Overall slip of steel and FRP strands were calculated from the Finite Element Model using a theoretical relationship proposed by previous researchers and the end slips were directly measured from the Finite Element Model.
Abstract: In pretensioned concrete members, the bond between prestressing strands and concrete in the transfer zone is necessary to ensure the two materials can work as a composite material. This study develops a computer program based on the Thick-Walled Cylinder theory to predict the bond behavior within the transfer zone. The bond was modeled as the shearing stress acting at the strand-concrete interface, and this generated a normal stress to the surrounding concrete. The stresses developed in the concrete often exceeded its tensile strength, which resulted in radial cracks at the strand-concrete interface. These cracks reduced the concrete stiffness and redistributed the bond strength along the transfer zone. The developed program was able to determine the bond stress distribution, degree of cracking, and transfer length of the prestressing strands. The program was validated using a data set of transfer lengths measured at the University of Arkansas and a data set collected from the literature.
Pub.: 15 Nov '16, Pinned: 27 Jul '17
Abstract: The transfer of prestress force from prestressing strands to the surrounding concrete is dependent on the bond between the two materials. Understanding the actual bond stress distribution along the transfer length results in optimized design of the transfer zone of prestressed concrete members. Equations of estimating the transfer length in ACI 318 code and AASHTO LRFD bridge design specifications simply take into account the effect of the strand diameter only. The objective of this study is to provide a generalized procedure for determining the bond stress–slip relationship accurately by incorporating the effects of additional parameters, such as concrete compressive strength at prestress release, center-to-center strand spacing, and concrete bottom cover. First, the bond stress distribution along the transfer length of a prestressed concrete member is formulated based on longitudinal slip–strain compatibility, force equilibrium and invariable bond stress–slip relationship along the transfer length. Second, a generalized Inverse Problem-Solving approach is introduced to determine best parameter coefficients through minimizing the discrepancy between the calculated and measured results. Two types of measurements (i.e., transfer length and end slip) reported in the literature are utilized to demonstrate the proposed approach. Predicted transfer length and end slip values using the calibrated bond stress–slip relationship show better agreement with the test data compared to those predicted by ACI 318 code and AASHTO LRFD bridge design specifications. Third, a computational procedure is developed and an example is presented to assist engineers using the developed formulae for determining the bond stress distribution along the transfer length of prestressed concrete members.
Pub.: 30 Jan '15, Pinned: 27 Jul '17
Abstract: Accurate prediction of strand transfer length is important for prestressed concrete members when calculating anchorage zone stresses and shear capacity. Currently, most design specifications oversimplify strand transfer length due to the large number of factors affecting it. This research investigated two of these factors to better understand strand-concrete bond and to improve transfer length predictions: 1) concrete nonlinearity due to concrete plastic strains surrounding prestressing strands, and 2) strand confinement due to groups of strands, concrete, or reinforcement. Nonlinear finite element analyses (FEA), which were calibrated using test data, were used to study material nonlinearity and confinement. Transfer length was investigated using models of both a simple prismatic beam with 25 strands commonly used in laboratory research and a bulb-tee girder with 40 strands commonly used in bridge construction. The results show that strand-concrete bond and transfer length are strongly influenced by concrete nonlinearity. Confinement affects transfer length because of its influence on plastic strains in concrete.
Pub.: 08 Apr '17, Pinned: 27 Jul '17
Abstract: 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.
Pub.: 16 Nov '12, Pinned: 27 Jul '17