Reasons for Damage

Although deterioration of pigmented structural glass itself is rare, or unheard of, failure of the mechanical support system which bonds the glass modules to the wall is almost always the cause of failure, cracking, slipping, or loss. Therefore, damage is usually attributable to one or a combination of the following:

• Deterioration of the Joint Cement
  
• Hardening and Failure of the Mastic Adhesive
  
• Impact Due to Accident/Vandalism

Deterioration of the Joint Cement

Historically, the cement joint between glass panels was intended to provide an integrated, watertight surface. Unfortunately, the traditional joint cement did not possess a long lifespan. Cracked or open joints have been the consequence, usually resulting from improper original application of the cement or from the normal thermal expansion and contraction cycle associated with weathering. Cracked or open cement joints then accelerated deterioration of the masonry substrate and/or the mastic adhesive bond by allowing water to penetrate the internal system. Water entering the system weakened the bond between the mastic and the masonry substrate or rusted the anchoring shelves. This caused the individual glass panels to gradually slip away from their original positions and fall.

Hardening and Failure of Mastic

Failure due to long-term hardening of the original mastic adhesive has accounted for a substantial loss of pigmented structural glass panels. The petroleum-based mastics normally possessed a 30 to 40 year lifespan. Once flexibility of the adhesive is lost, the glass panels become vulnerable to slippage and eventual destruction.

Impact Due to Accident/Vandalism

Glass breakage through impact is virtually impossible to prevent. The material is, by its nature, vulnerable to loss through vandalism or accident.

The pigmented structural glass panel has separated and fallen, exposing the substrate and hardened, brittle daubs of mastic. Photo: Douglas A. Yorke, AIA.

Maintenance and Repair of Pigmented Structural Glass

The maintenance of a dry masonry substrate, mastic, and metal anchors is essential to the longevity of a pigmented structural glass veneer. Thus, repointing cracked or open joints--particularly at ground level where glass abuts concrete--and caulking of slightly cracked glass panels is an ongoing concern. Where drainage to conduct water away from the wall is faulty or insufficient, the problem should be immediately corrected. For example, roof flashing, downspouts, and gutters should be repaired or new systems installed.

Repair of Cement Joints

Cracked or open cement joints, particularly in exterior applications, can present a serious preservation problem because they permit water to penetrate the internal system of a pigmented structural glass veneer. Rusting metal anchors or deteriorating mastic adhesive may be the result. Although the traditional joint cements are easily colored and may be neatly applied, they are no longer recommended for the repair of pigmented structural glass because their longevity is limited. Present-day silicone compounds, on the other hand, offer flexibility, relative impermeability to moisture, ease of installation, and a long lifespan. The proper color match can be obtained by mixing the compound with tinted polyester resins.

Patching Glass Cracks

Any glass panel that can be repaired should not be replaced. Thus, the decision to repair or replace damaged historic pigmented structural glass panels always needs to be made on a case-by-case basis. In many instances, the damage may be so minor or the likelihood of finding suitable replacement glass panels so small that repairing, reanchoring, and/or stabilizing the damaged glass is the only prudent choice.