The conservation of historic roads and bridges

Public Roads, Summer, 1994

Highways, themselves, can be historic architecture in need of conservation. To qualify for listing on the National Register of Historic Places, a structure needs to be at least 50 years old. The average age of highway bridges in the New York City area is 67 years. Construction of the interstate system began in the 1950s; in the next decade, stretches of it could be determined to meet the criteria of the National Register of Historic Places.

Besides being old, historic property must be significant to the cultural heritage of the United States. A prime example of a historically significant highway structure is the Brooklyn Bridge, which was built over 100 years ago and still serves as a critical part of the highway system. The construction of this bridge united Manhattan to Long Island, with enormous consequences for the urban development of New York City. John Roebling established the suspension method as the preferred way of spanning long distances and greatly enhanced the worldwide reputation of American civil engineering. For all these reasons, Roebling's creation, the Brooklyn Bridge, has been placed on the National Register of Historic Places and is considered a national historic landmark.

Some historic bridges have more in common, in terms of their construction materials, with medieval cathedrals than with modern steel or reinforced concrete structures. Although the Brooklyn Bridge pioneered the use of steel in large engineering structures, its steel suspension cables are linked to the anchorages by wrought iron chains. (15) Wrought iron has been used since antiquity to strengthen and tie together large stone structures. Moreover, the towers of the Brooklyn Bridge are massive load-bearing granite masonry, built by masons stone by stone--just as in the medieval cathedrals. John Roebling, the engineer of the Brooklyn Bridge, emphasized this resemblance by including Gothic arches in the architectural design of the towers. Consequently, lessons learned in the conservation of Gothic cathedrals can be directly relevant to renewal of these highway bridges.

Many historic bridges have sculptural features that must be conserved. For example, the Taft Bridge in Washington, D.C. is now undergoing restoration. The precast concrete lion sculptures at either end of the bridge require protection from the weather, and they are being replaced with bronze replicas.

Even stretches of highways may be preserved. Sections of U.S. Route 66 are being placed on the National Register as significant elements of the American culture.

Unfortunate experiences in architectural conservation have demonstrated the importance of understanding the interactions between repair materials and the original structural materials. Misguided restoration efforts have caused more damage than they have cured. On occasion, thin cementitious grout has been injected into the walls of historic buildings in order to consolidate them. One result has been the formation of soluble salts such as magnesium carbonates and sulfates that form damaging surface efflorescences. In cases where the original material contained significant amounts of gypsum, chemical reactions can lead to disastrous expansive forces. This process caused such severe tensile stresses in the brick walls of the Marienkirche in Lubeck, Germany, the iron reinforcements snapped, and the walls had to be extensively rebuilt. (14)

Another recurring problem in architectural conservation is the application of various kinds of plastic coatings to stone and other porous materials to consolidate or waterproof them. Aside from aesthetic problems, these coatings typically produce surface layers with different physical properties than the underlying material. These differences can, under thermal cycling, lead to significant interfacial stresses, which can, in turn, lead to spalling of the surface. The creation of an impermeable barrier can trap moisture and salts at the interface, again leading to failure. This latter problem is illustrated by the photograph of a Venetian stone sculpture.

Conclusion

Conservation science has much to offer to the field of highway research and development. This type of technology transfer becomes especially important with the completion of the Interstate Highway System. In the future, the emphasis will no longer be on new construction but rather on the conservation of the transportation infrastructure already in place.

References

(1) Strategic Issues in Civil Infrastructure Systems Research: Executive Summary, National Science Foundation, Engineering Directorate, Strategic Planning Committee, Arlington, Va. 1993.

(2) R.A. Livingston. "Architectural Conservation and Applied Mineralogy," Canadian Mineralogist, Vol. 24, 1986, pp. 307-22.

(3) R.A. Livingston. "Influence of the Environment on the Patina of the Statue of Liberty," Environmental Science and Technology, Vol. 25, No. 8, 1991, pp. 1400-08.

(4) R.G. Newton. "The Weathering of Medieval Stained Glass," Journal of Glass Studies, Vol. 7, 1975, pp. 161-68.