Evaluation of wrought iron for continued service in historic bridges

Doubts about the strength of wrought iron used in historic structures can be resolved with appropriate methods of evaluation. Since wrought iron is a composite material of metal and slag fibers, its performance in structures cannot be adequately evaluated by the techniques routinely used for testing structural steel. Both historic and modern test data show that wrought irons from different sources vary little in strength but greatly in ductility. Since adequate sampling for mechanical ductility tests is often difficult, alternatives based on metallurgical analysis can assist evaluation of the metal in historic structures. Test data show that more than 0.3% phosphorus in solid solution embrittles wrought iron. This level of phosphorus can be detected by optical microscope techniques. Excessive or poorly distributed slag fiber is the other principal source of inadequate ductility, and can be evaluated by metallographic examination either in place or on samples much smaller than those required for mechanical testing.