# Track Modulus Assessment of Engineered Interspersed Concrete Sleepers in Ballasted Track
## 5 Conclusions  
This study assessed a section of track with interspersed timber sleepers and EIC sleepers that were designed to replicate timber-sleeper stiffness, and provided an analytical evaluation of the expected stiffness variation between traditional concrete sleepers, which can lead to an accelerated deterioration of the railway track and its components. Based on [PERSON]'s structural-analysis method, the conducted theoretical analysis demonstrated that the expected variation in the track-modulus value between sleeper types caused a difference in the load-support distribution. A field investigation was used to evaluate the performance of an interspersed sleeper structure composed of EIC and timber sleepers. Based on this background, the field experimentation results, and the evaluation of each rail-seat performance, the findings and conclusions were:  
* The theoretical analysis showed that interspersed standard concrete sleepers placed within a timber-sleeper track can experience a load up to five times greater than the load supported by the adjacent timber sleepers. * Based on measurements of sleeper displacement, both timber sleepers and EIC sleepers presented a similar track response. For timber sleepers, the median displacement value was 2.26 mm (0.089 in. ), and for EIC sleepers, it was 2.21 mm (0.087 in.). * The track-displacement results from the field instrumentation ranged from 1.22 mm (0.048 in.) to 4.32 mm (0.170 in.) for all timber sleepers. * The track-modulus results indicated a median modulus of 12.95 N/mm/mm (1878 lbs./in./in.) for the timber sleepers, and 12.79 N/mm/mm (1855 lbs./in./in.) for the EIC sleepers. * The variance in performance for the two sleeper types cannot be clearly distinguished with track-displacement or modulus results, which provides a favorable indication for future field performance. Based on the results, the desired performance of the EIC sleepers in maintaining consistent track modulus was demonstrated for this application and set of track conditions. This homogeneity in modulus is crucial to ensure a smooth ride quality and minimize track maintenance due to geometry deviations. The principal design characteristics that contributed to the EIC sleeper's performance were the reduced cross-sectional area and resulting reduction in flexural rigidity, and the resilient rail-seat plate system, with the latter providing most of the stiffness reduction. This study showed that properly engineered concrete sleepers can be interspersed in timber-sleeper track to achieve a similar track response. Nevertheless, the exact results for the track modulus obtained in this study are not entirely independent, and should not be extrapolated to other applications/locations without further study. Future work should include field experimentation in locations with a higher overall track modulus and different arrangements of the sleeper installation. Figure 13: The track modulus results for each individual sleeper instrumented in the field test zone. **Author Contributions:** Conceptualization, A.d.O.L. and J.R.E. ; methodology, A.d.O.L. and M.S.D. ; data collection, A.d.O.L., J.R.E. and M.S.D. ; data processing, A.d.O.L. and J.L. ; writing--original draft preparation, A.d.O.L., J.R.E., J.L. ; writing--review and editing, A.d.O.L., J.R.E. and M.S.D. ; supervision, J.R.E. ; project administration, A.d.O.L. ; funding acquisition, J.R.E. and A.d.O.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was primarily supported through funding from the National University Rail Center (NURail). The positions presented within this paper represent those of the authors and not necessarily those of the US Department of Transportation. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Restrictions apply to the availability of these data. Data are owned by a sponsoring agency but may be available from the authors with permission of the sponsoring agency.