Methods of Repair

Much historic cast stone is unnecessarily replaced when it could easily be repaired in situ, or left untreated. This is especially true of areas that exhibit isolated spalls from rusting reinforcement bars or anchorage, or installations where erosion of the matrix has left a rough surface of exposed aggregate.

The weathering of cast stone, while different from that of natural stone, produces a patina of age, and does not warrant large-scale replacement, unless severe cement matrix problems or rusting reinforcement bars have caused extensive scaling or spalling. Severe rusting of reinforcement bars on small decorative features, such as balusters, may signal carbonation (loss of alkalinity) of the matrix. Where carbonation of the matrix has occurred, untreated reinforcement will continue to rust. Replacement may be an acceptable approach for exposed and severely deteriorated features, such as hand railings, roof balustrades, or wall copings, where disassembly is unlikely to damage adjacent construction. Conversely, small areas of damage should generally be repaired with mortar "composites," or left alone.

A delaminated layer of ornamental cast stone on the Orpheum Theater, San Francisco, California (1925), was successfully re-attached using epoxy. The multiple delivery ports for the epoxy are removed after treatment and the holes patched. Photo: David P. Wessell.

Where the decorative facing of dry tamped cast stone has separated from core layers, injected grouts may be used to re-secure the facing. Re-attachment of a separated facing layer may be time consuming, and should be undertaken by a conservator, rather than a mason. This technique may be the best, most economical, approach for repair of figurative sculpture or unique elements that are not repeated elsewhere on a building. Theoretically, cementitious grouts are most appropriate for re-attaching separated facings, but hairline fissures may require the use of resin adhesives. Low-viscosity epoxies have been used for this purpose, and may be applied through small injection ports. Cracks that would allow adhesive to leak must be repaired prior to injection, of course. Holes made for adhesive injection will require patching after re-attachment is complete.