Rationale

In 2023, ASTM C90 (loadbearing CMU) was revised to require that solid units actually be 100% solid, rather than having a cross-sectional area of 75% when compared to the gross cross-sectional area. This ballot is proposing the same change to ASTM C129 for non-loadbearing units. The rationale below from the original C90 ballot item is provided below, which provides the technical justification for this change. The same rationale holds true for non-loadbearing units as well. Until 2023, ASTM C90 defined ‘solid’ vs. ‘hollow’ masonry units based on the threshold of having a actual cross-sectional area of 75% of the total cross-sectional area. This definition has roots that extend back more than 80 years to when ASTM had two separate standards: C90 – Hollow Loadbearing CMU and C145 – Solid Loadbearing CMU. ASTM C145 was withdrawn in 1990 and C90 was rewritten to apply to all loadbearing CMU (hollow and solid). Distinguishing between solid and hollow CMU did have implications under the empirical design method, which based loads and stresses on the gross cross-sectional area (out-to-out dimensions) and did not account for the void area of the cells. Empirical design has been removed from current masonry codes and standards. Today, structural masonry analysis follows an engineering design approach that accounts for the open area of the cells and assembly. At this point, there is little value in having 75% solid units defined as ‘solid’ and few, if any, producers manufacture them. It also causes some confusion in architects, specifiers and users. This ballot item proposes to remove the non-intuitive definition such that ‘solid’ CMU will now be required to be 100% solid.