Rationale

This ballot is proposing a series of changes to ASTM C1780 to align the requirements of the standard with the 2022 version of TMS 402/602 (Building Code Requirements and Specification for Masonry Structures). The 2022 version of those standards included a significant overhaul of requirements for all veneers (both anchored and adhered). As this version is now referenced in the 2024 International Building Code, it is appropriate to proceed with changes to align C1780. The 2022 TMS 402/602 contains two primary options for design and construction – the prescriptive option and the engineered option. For the prescriptive option, TMS 402/602 provides a detailed set of requirements for materials, substrates, fastening, and more. When those details are followed, additional design is not needed. For the engineered option, there are very few prescriptive requirements, instead requiring engineering of the system, but allowing a much larger range of materials and systems. The changes to ASTM C1780 proposed here are aligning the standard with the prescriptive options. The goal of these changes is that an installation that follows ASTM C1780 will be an installation compliant with the prescriptive option in TMS 402/602-22. Specific changes included in this ballot: • Added new Section 1.2 that states the standard is intended to cover prescriptive installations per TMS 402/602-22, and mentioning that the engineered design option is available for situations outside of the standard. • Added new Note 3 after 1.2 – this relocates existing Section 6.1 (before renumbering) regarding reference to the Appendix X1 for information on modifying the C482 shear bond test to evaluate non-standard installations. • Added new Section 4 on Significance and Use – This section is a required mandatory section per ASTM Form and Style Manual, but the standard never had it before. The language proposed identifies the limitations for the prescriptive design and construction option in TMS 402/602. If any installation does not meet these criteria, they would need to go to the engineered option. The addition of this new section also necessitates renumbering of the standard from that point on. • Updated new Section 5.5 on mortar – TMS 402/602 has particular requirements for mortars used in adhered masonry veneers. Most significantly, the setting bed must be a modified mortar meeting ANSI A118.4 or A118.15. Revisions to 5.5 are proposed to make this section clear and also align with the TMS requirements. • Updated New Section 7 on Substrate/Surface Preparation - Removed substrate options (i) Open Studs, Non-solid Sheathing and Metal Building Panel and (ii) Existing Cured Stucco from Section 7 because they are not allowed under the prescriptive design and construction option in TMS 402/602-22. Added Note 9 to Section 7 to let users know that these two options were historically included in this Practice and that the information has been moved to a new Appendix X2. • Updated new Section 7.3.2.2 on lath fastening – TMS 402/602 includes tables for fastener size and spacing based on the weight of the veneer, the type of structural backup, and the width of the cavity between the backup and the veneer. A reference to those requirements in addition to ASTM C1063 is added to 7.3.2.2. • Updated new Section 8.2.2 – Relocated Method A and Method B with Conventional Mortar because they are not allowed under the prescriptive design and construction option in TMS 402/602-22. These sections were moved to a new Annex A1 and a line added to Note 15 to point users to the Annex for installations using conventional mortar in the setting bed (such as installations using previous versions of TMS 402/602 or engineered applications that specify conventional mortar). • Additional cleanup – throughout, minor changes to clean up language and corrections to section references are changed as well. Note that a previous version of this ballot item was balloted under Main Committee Ballot C15 (25-02) Item #13. This ballot item is essentially the same as the previous version with some modifications made to address the negatives and comments. Significant modifications to the previous ballot item are highlighted in yellow in this ballot. Specific changes made in response to negatives and comments: • Section 4.2.1 – reduced maximum assembly thickness to 3.875 inches. While TMS 402/602 allows assemblies up to 4.625 inches, C1780 places additional limitations on cement-based adhered veneer assemblies. The maximum allowed unit thickness is 2.625 inches (per C1670 or C1877), and the maximum allowed thickness of the scratch coat and setting bed combined is 1.25 inches (Section 8.1.3). The proposed limit of 3.875 inches is consistent with those limitations. • New Note 4 was rewritten to clarify and improve language. • Section 5.5.3 – limit scratch coat to Type S mortar. TMS 402/602 allows either Type M or Type S mortar for scratch coat. Other recommendations (CMHA Installation Guide, manufacturer instructions) generally discourage Type M (concerns over shrinkage/cracking). The task group felt that limiting to Type S mortar is most appropriate. • Relocation of conventional mortar installation – addressed in rationale above. Because of the breadth of the changes throughout the standard, the entire standard is provided below for reference. Changes are limited to those identified in strikeout/underline.