Evaluation of the Remaining Prestress Force and Center Negative Bending Moment in Crossties Removed From Track After 25 Years of Service

Abstract

Extensive research is currently being conducted by the team to understand the prestressing steel and concrete properties that cause high bonding stresses and lead to longitudinal splitting cracks, and how to mitigate this failure in future designs. One parameter of interest that affects the bonding stress is the amount of prestressing force in a crosstie. To help quantify the amount of prestress force necessary to provide a durable long-term crosstie, a study on existing crossties that have performed well in track for over 25 years was conducted to evaluate the center negative bending moment, and determine the remaining prestress force for each tie. The remaining prestress force in each tie was determined using a new proposed method in which ties are loaded in direct tension. The new test method was also conducted on new ties instrumented with vibrating wire strain gages to verify the method. The testing results reveal that a majority of the existing ties evaluated have a remaining prestress force in the range of 84–95 kips. These forces are significantly lower than the remaining prestress force after losses of newer tie designs. This can be seen when comparing the tension test results of the existing ties with the results of the new ties. Ties were loaded upside-down in four-point bending to determine their center negative cracking moments. The test setup and procedure used closely resembles the center negative bending moment test outlined in AREMA 30, with minor variations. For each different tie design in the study, the center negative design moment was calculated following the AREMA 30 procedure, and compared to the experimental cracking moments. Of the seven different existing tie designs investigated, four would meet the requirements of the current AREMA 30 center negative bending test.