Detecting mild steel hairpin anchors in stone cladding
As-built conditions were determined by examining the stones that had already been removed as well as additional in-place pieces. This process revealed a host of problems, including cracked stones, poorly drilled stone anchor holes, honeycombed concrete at anchor locations, unstable existing repairs, and the apparently random use of mild steel stone anchors in lieu of the specified stainless steel units. These findings caused a concern both for the immediate stability of the cladding and the long-term durability of the facade.
The "hairpin" stone anchors on this building are the sole attachment between the stone cladding pieces and the precast concrete facade panels (see diagram). The long-term durability of the stone anchorage system was compromised by the corrision potential of the mild steel stone anchors. While the portion of the anchor within the precast concrete enjoys passive corrosion-protection from high pH of the concrete, the remaining portion in the stone is free to corrode when wet. Moisture can reach these anchors via absorption through the stone or through leaks in failed stone-to-stone sealant joints.