Prestressed concrete crossties represent one of the current strategies to upgrade the rail infrastructure to accommodate heavier loads and higher speeds. However, despite their strength, numerous members have exhibited durability issues that have led to their replacement before reaching their anticipated lifespan. Some of these issues are related to the development of cracks in critical regions. The source of the cracks is often associated with changes in the support conditions of the crossties, characteristics of the pre- or post-tensioning procedure, and the dynamic effects of the wheel-rail interaction. Unfortunately, overcoming these crack formations is challenging since the tensile stresses that generate them concentrate in specific parts of the members, such as the center or rail seat regions, while the common production practices (“long line” and “carousel”) prestress the entire length of the crossties, including zones where compressive forces are unnecessary or even counterproductive. To address this problem, this research project proposes an innovative prestressing approach designed to focus prestressing forces where they are most needed. In particular, the approach integrates traditional post-tensioning with localized forces exerted by shape memory alloys (SMAs) after activating their shape memory effect (SME). First, the concept was validated analytically through finite element models. Then, as an initial step in studying the technique in a practical context, four full-scale crosstie prototypes were cast, each featuring a specific arrangement of SMA reinforcement. These arrangements comprised plain NiTiNb and FeMnSi elements as well as two different FeMnSi-based adaptive prestressing system (APS) assemblies. The experimental program consisted of two phases. The first phase involved the activation of the SME through induction heating. The embedded SMA components of the specimens were heated above their activation temperature to trigger the development of their recovery stresses. After cooling down, the data collected by strain gauges and digital image correlation showed that the SMAs induced levels of local prestress in the crosstie specimens that ranged from approximately 5.2 MPa to 11.1 MPa. The second phase of the experimental program focused on evaluating the flexural performance of the specimens at their rail seat and center regions, following the testing protocol recommended by the American Railway Engineering and Maintenance-of-Way Association (AREMA). The results indicated satisfactory performance for specimens at the center region, exceeding the design limit by at least 19.90%. At the rail seats, the specimens displayed an acceptable performance under negative curvature bending but need further evaluation under positive curvature bending, as they currently meet the required flexural capacity but at reduced track speeds. Overall, the proposed approach shows promise as it demonstrates that a significant amount of end-to-end post-tensioning can be replaced with localized SMA prestressing and still meet important design criteria. Achieving a balance between both prestressing approaches could lead to mitigating the undesirable cracking patterns and reducing the reliance on traditional prestressing wires in regions where they are not beneficial.

The Final Report is available.