A comprehensive study of prestressing steel and concrete variables affecting transfer length in pre-tensioned concrete crossties

Abstract

A comprehensive study was conducted to determine the variation in transfer length of pre-tensioned prestressed concrete railroad ties with different parameters, including prestressing steel type and concrete variables. The in-depth evaluation included different prestressing reinforcement types that are employed in concrete railroad ties worldwide. The study consisted of two phases; Lab-Phase and Plant-Phase. Throughout the study, transfer lengths were determined from surface strain measurements of pre-tensioned concrete members. During the Lab-Phase, pre-tensioned concrete prisms were fabricated to replicate plant manufactured crossties. Different groups of prisms were fabricated during this phase, with each group used to determine the influence of selected prestressing steel or concrete variables on transfer length. A special jacking arrangement was employed to ensure that each of the reinforcements was tensioned to the same force. During the Lab-Phase, an 8-inch Whittemore gage was utilized to determine concrete surface displacements and thereby calculate surface strains. Later, during the Plant-Phase, pre-tensioned concrete railroad ties were fabricated at a concrete crosstie manufacturing plant with the same group of reinforcements. In-plant concrete surface strains were determined by utilizing both the Whittemore gage and two automated laser-speckle imaging (LSI) devices. Later, a long-term study was conducted on plant-manufactured crossties that were cast exclusively to utilize the mechanical (Whittemore) gage system. Various results from both the Lab-Phase and Plant-Phase are presented along with discussion. Potential benefits of laboratory prisms in estimating transfer lengths is also discussed. Results from both phases indicated that large variations in transfer lengths are due primarily to variations in the bond quality of the different prestressing tendons and the concrete strength at detensioning. Results pertaining to the variation in bond quality due to other concrete variables are also presented.