On the shear strength of polyurethane coated railroad ballast

Unbound nature of railroad ballast aggregates causes abrasion and degrada- tion predicaments due to heavy train axle loads. Polyurethane reinforcement of ballast layer is a candidate solution to these problems. However, not many polyurethane reinforcement applications have been studied until the begin- ning of last decade. Therefore, the technology requires considerable research e orts to fully develop. As part of this thesis study research, direct shear testing was undertaken for two di erent railroad ballast aggregates, granite and limestone. Both aggregate materials satis ed the AREMA No. 24 railroad ballast gradation requirements. Although the unbound granite samples showed higher shear strength properties than the unbound limestone samples, polyurethane rein- forcement increased shear strengths of the limestone aggregate samples more than the granite aggregate samples in direct shear (shear box) tests. The granite samples had somewhat of a coarser gradation when compared to the size distribution of the limestone samples. An image aided discrete element modeling approach was used to simulate the shear box tests and calculate number of particle contacts and sample porosities for both the granite and limestone gradations. The simulation results showed that even though the di erences in porosity were only in the order of 2% to 3%, the number of particle contacts for the limestone samples was 50% more than that for the granite samples. In conclusion, number of particle contacts in an assembly of aggregates coated with polyurethane was found to be an important factor contributing to the shear strength properties of chemically bonded ballast aggregate layers