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