Hydraulic Mechanisms of Concrete-Tie Rail Seat Deterioration

摘要

Rail seat deterioration (RSD) is considered the most critical problem with concrete-tie performance on North American freight railroads. RSD is the deterioration of the concrete underneath the rail that results in problems such as wide gauge, insufficient rail cant, and loss of fastening toe load. Currently, the problem is not sufficiently understood to enable development of effective solutions. The primary causes of RSD appear to be high stresses at the rail seat, a loosened fastening system, the presence of moisture, and the presence of abrasive fines. RSD is considered to have up to six potential mechanisms, and this research investigates three of them: hydraulic pressure cracking, hydro-abrasive erosion, and cavitation erosion. A laboratory test apparatus and procedure were devised to measure the surface water pressure in a laboratory rail seat using tie pads of differing material composition and surface geometry. To evaluate hydraulic pressure cracking, a model of the effective stress in a concrete-tie rail seat was developed to estimate the water pressures on the rail seat surface and whether they could lead to damaging pore pressures in the concrete. Comparing the effective stress model and the measured surface water pressures, hydraulic pressure cracking appears to be a feasible mechanism for RSD given the correct combination of high rail seat loads, sufficient moisture, and a tie pad surface that develops high pressure. The measured surface water pressures were used to estimate the potential water velocity. By comparing these estimates with critical velocities for concrete erosion, it appears feasible that hydro-abrasive erosion contributes to RSD. The uplift action of the rail was simulated in the test apparatus, and the resulting suction was at or near the vapor pressure of water for most of the tie pads considered, suggesting that cavitation occurs in a concrete-tie rail seat. However, considering the size of the cavitation bubbles that could fit underneath the tie pad, and the observation that no high collapse pressures were measured, cavitation erosion is not a feasible mechanism for RSD. Mitigation options for preventing hydraulic pressure cracking are using a tie pad or pad assembly that does not seal water, reducing the occurrence of high impact loads, and using high-strength, air-entrained, low-permeability concrete. Care should be exercised when using pads that do not seal water, as this could contribute to hydro-abrasive erosion or abrasion.