The importance of the indent geometry of prestressing wire used in the manufacture of prestressed concrete railroad ties is well known. In particular, it has been shown to have a major affect on the magnitude of the transfer length. Important parameters affecting transfer length include indent depth, indent volume and indent sidewall angle, with indent volume providing a major influence. Previous experimental evidence has revealed that the larger the indent volume, the shorter the transfer length. Furthermore, for full load bearing capacity, it is critical that the transfer length not exceed the distance to the rail seat. As a result, transfer length represents a key diagnostic parameter for evaluating the load bearing capability of prestressed concrete railroad ties, with potential for use as a quality control parameter.
Adequate transfer length alone, however, is not sufficient to guarantee tie performance. The geometry of the prestressing wire indents also plays a major role in the formation of cracking, which is of particular importance to the manufacture of concrete ties intended for high-speed rail applications. Insufficient consideration of cracking and de-bonding of prestressing wires associated with ties in service can result in severe splitting and complete tie failure. It should also be noted that while the wire specifications in standard ASTM A881 are intended to promote quality prestressed railroad tie behavior, they do not address either the detailed causes of cracking and splitting, or the specific indent features that are responsible.
Although indent geometry was known to influence both transfer length and splitting propensity, specific quantitative relationships between indent features and tie performance had not been identified. Accurate quantitative measurements of indent geometry on a statistically significant basis were simply not yet available. Recently, however, a high-resolution automated non-contact optical wire indent scanning system was developed for rapidly characterizing all relevant indent parameters including indent depth, indent width, indent sidewall angle, indent pitch, and indent volume. This paper presents, for the first time, development of reliable quantitative relationships between the relevant indent parameters and the tie performance indicators with a high degree of correlation. In addition, measurements using the new automated profiling system have revealed a new indent parameter that, on its own, shows direct correlation with transfer length. The physical concept behind this new parameter, referred to as the “Bite Per Length (BPL),” and its measured characteristics, will be presented along with its demonstrated experimental relationship to transfer length.