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. © 2012 Springer Science+Business Media Dordrecht.Funding for this experimental research work has been provided by the Spanish Ministry of Education and Science and ERDF (Projects BIA2006-05521 and BIA2009-12722). Tests have been conducted at the Institute of Concrete Science and Technology (ICITECH), at the Universitat Politecnica de Valencia (Spain).Caro Forero, LA.; Martí Vargas, JR.; Serna Ros, P. (2013). Time-dependent evolution of strand transfer length in pretensioned prestressed concrete members. Mechanics of Time-Dependent Materials. 17(4):501-527. doi:10.1007/s11043-012-9200-2S50152717