Assessment of Jahn Permeable Mortar System in a Historic Bridge Abutment Application

Increasingly, the importance of historic preservation is competing with the requirements to enhance structural capacity of older unique structures. Preservation of the functional and aesthetic qualities limits potential change to meet the expanded needs of the traveling public. The Taftsville Covered Bridge in Woodstock, Vermont was deteriorating at a time the public needed increased capacity. Then in 2011, the tropical storm remnant of Hurricane Irene rendered the bridge impassable. Significant damage to the substructure and superstructure occurred. The bridge required a complete deconstruction and reconstruction to correct deficiencies and enhance capacity. Design and construction techniques focused on retention of the historic presence and value the bridge offered for over a century and a half. The Taftsville Covered Bridge has been repaired and improved many times over its life. This project continued that trend by restoring one abutment and pier with added strength by the use of Jahn mortar and grout. The existing laid-up stone was repointed with Mortar with minimal addition of new stone where large gaps were located. Once the shell formed by the existing stone and the new mortar set up, a flowable grout was placed within the stones to bond the stones together, thereby strengthening the substructures. The containment of the stones provided increased reliability in addition to strength. The feature the Vermont Agency of Transportation hoped to maintain was the high permeability of the abutment. The permeability of the Jahn system minimizes damage caused by internal hydrostatic pressures and the effects of chlorides locked within the stone substructures observed in other mortars and grout systems.