Acknowledgements

This report has been prepared with funds provided by the United States Department of Transportation to the Mountain-Plains Consortium (MPC). The authors would like to acknowledge the support of the Structural Engineering Laboratory and the Composite Materials, Manufacture and Structures Laboratory at Colorado State University for the use of test and measurement equipment critical to the collection and evaluation of the data presented in this report. Finally, the authors would like to thank Shell Resins and PPG Fiberglass for material donations used in the preparation of test specimens.

Disclaimer

The contents of this paper reflect the views of the authors who are responsible for the facts and the accuracy of the information presented. This document is disseminated under the sponsorship of the Department of Transportation, University Transportation Centers Program, in the interest of information exchange. The United States Government assumes no liability for the contents or use thereof.

Abstract

Investigations into the effects Z-spiking with fiber reinforced polymer (FRP) rods have on the flexural stiffness and repair of deteriorated railroad crossties were conducted. These tests were conducted to provide a pilot study for an ultimate goal of repairing full-scale bridges. This report presents the results found from flexural load testing conducted on deteriorated Douglas fir railroad crossties reinforced with FRP shear spikes; epoxy-resin shear strength testing; and finite element modeling of repaired crossties. Reinforcement was provided by installing 12.7mm (0.5in.) diameter polyglass polyester resin fiberglass rods bonded in place using epoxy-resin. The rods were inserted into the deteriorated railroad crossties perpendicular to the primary bending axis to provide horizontal shear reinforcement and improve flexural stiffness. Shear spikes were installed in pairs starting at each end of the crosstie specimen and moving inward. Load tests were conducted to determine the gain in effective stiffness as the number of spikes increased. A total of ten rows were sequentially inserted into each crosstie specimen with load tests performed after each successive row installation. Results from load tests show an average increase in flexural stiffness of 58 percent with all ten rows of shear spikes inserted into the crossties. The results show the use of FRP shear spikes as a reinforcement method dramatically increased the flexural stiffness of railroad crossties and helped to offset the deterioration present from decay. In addition, the shear spikes and epoxy-resin show a strong possibility for repairing decay voids present on the surface of the deteriorated crossties as well as their interior portions.

Executive Summary

In many installations timber railroad bridges that are 50-100 years old are necessary for daily operation. Numerous timber-based highway bridges exist too, primarily for secondary roads. The latter are often in jurisdictions where new construction funds are very limited. Hence, economical repair of bridges is vital to the nation's infrastructure. Fiber-reinforced composites are extremely popular for infrastructure and in situ infrastructure repair. Common approaches are fiberglass wrap (bandages) or adding reinforced plates (patches) to the sides of the members. These require the members be removed from the bridge for the repair to be made. They also degrade with time because of exposure. Alternatives to these techniques that do not require member removal and are embedded in the member are invaluable to low-cost, long-lasting repair.

A prior research project explored an innovative alternative to fiberglass wrap and patch repair techniques, termed z-spiking. Adapting this method, fiberglass rods (shear spikes) are inserted from the bottom to the top of a wood member passing through the areas of damage. Pre-drilled holes and an injected adhesive are used to bond the spikes to the wood. They serve to tighten the member to restore overall stiffness and add horizontal shear resistance, among other benefits. This procedure proved very successful in rejuvenating small wood specimens (based on nominal 2 x 2 and 2 x 4 dimension lumber). Repairs to layered and split members resulting in strong recovery of the flexural strength and stiffness comparable to undamaged control specimens. The work described is the outcome of a pilot study in which the technique was applied to larger wood members. A set of 35 deteriorated wood railroad crossties were incrementally reinforced with shear spikes. Load tests were conducted to asses the gain in stiffness (related to the non-reinforced condition) as the number of shear spikes increased. On average the gain after all spikes were inserted was 58 percent, with the range being 32 percent to 98 percent. An empirically-based analytical model was developed to determine the benefit to be gained by reinforcing a given crosstie if its non-reinforced stiffness is known.

Z-spiking provides a fundamentally new, more structurally effective, low-cost alternative to presently limited repair methods based on fiber composites. This project shows promise of leading to an invaluable technology for repairing aged timber bridges on short- and main-line railroads and on secondary roads. Continuation of the research to full-size bridge members is warranted.