During a minor earthquake of magnitude 4.4, part of a newly constructed stone veneer wall at a home in Northern California collapsed. Based on records of nearby instruments, shaking at the site of the house was at most approximately 0.10g. (The house was designed for lateral shaking of 0.33g.) According to the owner, more than a dozen stones fell from the top corner of one exterior wall during this earthquake (shown in Figures 1 and 2). The veneer at several other walls shifted slightly outward near the top, producing bulges in the wall face.

The investigation revealed that weak mortar bond strength contributed to the veneer failure. The anchorage system for the stone veneer contains ties within the mortar joints, rather than anchors within the stone units. This makes shear bond strength an important parameter in the performance of the veneer. The investigators needed to evaluate the mortar shear bond strength both in the field and in the laboratory. In-situ shear bond testing showed that some areas of the building had satisfactory mortar shear bond strength, while other areas were unsatisfactory. Laboratory testing revealed that the mortar mix used on the house varied widely, and did not comply with the building code. Before beginning reconstruction of the failed masonry, the investigating engineers developed and tested various repair mortar mix designs in the laboratory, and compared the results to the field tests.

Currently in the United States, there is no standardized means of comparing mortar bond shear strength results obtained in situ and in the laboratory. Although there are several laboratory testing standards for mortar bond strength, none is equivalent to the in-situ shear bond test; see Table 1.

This article discusses a project in which a laboratory test equivalent of the in-situ mortar shear bond test was needed. A laboratory test was designed as a modified version of the in-situ test. More research is needed on this or other laboratory shear bond test methods for eventual development into a formal standard.

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