Effects of Redispersible Polymer Powder on Mechanical and Durability Properties of Preplaced Aggregate Concrete with Recycled Railway Ballast

The rapid-hardening method employing the injection of calcium sulfoaluminate (CSA) cement mortar into voids between preplaced ballast aggregates has recently emerged as a promising approach for the renovation of existing ballasted railway tracks to concrete tracks. This method typically involves the use of a redispersible polymer powder to enhance the durability of the resulting recycled aggregate concrete. However, the effects of the amount of polymer on the mechanical and durability properties of recycled ballast aggregate concrete were not clearly understood. In addition, the effects of the cleanness condition of ballast aggregates were never examined. This study aimed at investigating these two aspects through compression and flexure tests, shrinkage tests, freezing-thawing resistance tests, and optical microscopy. The results revealed that an increase in the amount of polymer generally decreased the compressive strength at the curing age of 28 days. However, the use of a higher polymer ratio enhanced the modulus of rupture, freezing-thawing resistance, and shrinkage resistance, likely because it improved the microstructure of the interfacial transition zones between recycled ballast aggregates and injected mortar. In addition, a higher cleanness level of ballast aggregates generally improved the mechanical and durability qualities of concrete

Impact Behavior of Unbonded Post-Tensioned Concrete Beams

This study experimentally investigates the effects of the unbonded post-tensioning on the response of reinforced concrete (RC) beams under impact loads. A total of fifteen specimens were cast and tested. All specimens had identical longitudinal reinforcement, but varying shear rein forrement ratios designed to be both flexural-and shear-deficient in static conditions according to the ACI code. In particular; thirteen unbonded post-tensioned (PT) beams were tested under drop weight impact applied at the midspan in simply supported conditions and compared with conventional type RC beams. The main variables investigated include increasing levels of PT force and the application of two different impact energies per each specimen type, shear- or flexural-deficient. The experimental results showed that the levels of PT force and shear reinforcements of the specimens played an important role in their overall behavior: With respect to beneficial design recommendations, relationships between the impact resistance and the static capacity are provided; a general increase in strength of around three times that of the static capacity is observed when impacted. Additionally, relationships between the ratio of the impact energy to static capacity are developed with regard to the maximum and residual member displacements

Vehicle Collision with Reinforced Concrete Columns Wrapped with Fiber-Reinforced Polymer Composites

The dominant failure mode for existing reinforced concrete (RC) columns exposed to vehicle collisions is generally shear. Fiber-reinforeed polymer (FRP) composites have been widely used to retrofit RC structures. This study numerically investigates FRP's effectiveness in reducing potential collision damage and preventing shear failure in full-scale RC pier columns retrofitted with FRP wraps under vehicle collision. Developed finite element (FE) models of plain and FRP-wrapped RC columns were well validated by comparing numerical results against laboratory impact test results. The validated FE modeling method was extended to build full-scale unstrengthened and FRP-wrapped RC pier column models. A parametric study was performed examining the effect of vehicle velocity and layer number and types of FRP wraps, as well as fiber orientation, on the structural responses. Numerical results showed that an increase in the number of layers of FRP wraps could significantly reduce column damage and displacements but has less influence on the dissipation of impact force, shear form, and moment of the columns. Two dimensionless damage indexes were proposed for the assessment of shear failure