Time-dependent evolution of strand transfer length in pretensioned prestressed concrete members
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Time-dependent evolution of strand transfer length in pretensioned prestressed concrete members L.A. Caro · J.R. Martí-Vargas · P. Serna
Received: 3 April 2012 / Accepted: 24 October 2012 © Springer Science+Business Media Dordrecht 2012
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. Keywords Bond · Concrete · Strand · Prestress · Transfer length · Time
1 Introduction During the manufacturing process of pretensioned prestressed concrete members, prestressing force is transferred from prestressing strands to concrete by bond at the ends of a member L.A. Caro · J.R. Martí-Vargas () · P. Serna Institute of Concrete Science and Technology (ICITECH), Universitat Politècnica de València, 4G, Camino de Vera s/n, 46022 Valencia, Spain e-mail: [email protected] L.A. Caro e-mail: [email protected] P. Serna e-mail: [email protected]
Mech Time-Depend Mater Fig. 1 Schematic prestressing stress diagram in a prestressed member after the prestress transfer
through the prestress transfer operation. According to ACI (2011), transfer length is the distance over which the strand must be bonded to concrete to develop the effective stress (fse ) in the prestressing strand. Figure 1 shows an idealization of the prestressing strand stress profile in a large-scale pretensioned prestressed member after the prestress transfer. Transfer length is a critical parameter in structural design for the calculation of both the flexural and shear capacities of the pretensioned prestressed concrete member, and also for the verification of transverse stresses near its ends (Russell and Burns 1996; Barnes et al. 2003). Several equations to predict transfer length have been proposed by codes and researches based on experimental results and theoretical studies. The majority of these equations take a pa
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