Effect of Concrete Properties on Transfer Lengths in Concrete Rail-Road Ties

This paper presents findings from a current research project titled “Quantifying the Effect of Prestressing Steel and Concrete Variables on the Transfer Length in Pretensioned Concrete Crossties” that is funded by the Federal Railroad Administration (FRA). Specifically, the paper focuses on the effect of concrete properties on the resulting transfer lengths. These properties include concrete consistency (slump), compressive strength at the time of prestress transfer, the water-to-cementitious (W/C) ratio, the aggregate type, and the use of a viscosity-modifying admixture (VMA). Pre-tensioned concrete prisms were cast in the laboratory and transfer lengths were determined from surface strain measurements that were obtained prior-to and immediately after prestress transfer (de-tensioning). The concrete compressive strength at de-tensioning was determined using cylindrical concrete test specimens that were match-cured to the temperature of the pre-tensioned concrete members. The release strengths investigated were 3500 psi (24.13 Mpa), 4500 psi (31.03 Mpa), and 6000 psi (41.37 Mpa). The effect of concrete consistency on the transfer length was evaluated by varying the slump between 3″ (76.20 mm) and 9″ (228.60 mm) while maintaining release strength of 4500 psi (31.03 Mpa) and a W/C ratio of 0.32. The effect of W/C ratio on transfer length was evaluated by maintaining release strength of 4500 ±220 psi (31.03 ± 1.52 Mpa) and a slump of 6 ± 1/2″ (152.40 ± 12.7 mm) while varying the W/C between 0.27 and 0.42. These values represent the extreme values used in the North American concrete tie industry that were noted by the authors during research that was conducted in 2010–2011[15]. Results for each parameter type will be compared and discussed in this paper. Transfer length results obtained during earlier work [4] conducted by the authors at a W/C ratio of 0.32 will be compared. Finally, results will be presented from transfer length measurements that were obtained on identical-sized prisms that were manufactured with concrete mixtures that used different aggregate sources and also the use of a viscosity-modifying admixture.

Work presented in this paper was conducted on two wire samples with generic labels [WG] and [WH]. Laboratory prism specimens of size 3 ½″ (88.9 mm) × 3 ½″ (88.9 mm) each with 4 wires were cast to measure transfer lengths. These proportions were chosen to replicate the original concrete crosstie wire-to-concrete proportions. Potential usage of these prisms in estimating transfer lengths was validated in another phase [5] and is not discussed here.

Essential information obtained from results allowed researchers to discuss attributed influence of each concrete property on transfer length. The research knowledge acquired from this study will give proper insight about transfer length and will be helpful to manufacture a better product by adjusting concrete mix design.