Q: Why are Type I (N-1) moisture control units no longer available?
-Moisture Unit N-1: Can you Help Investigate the Elusive?
Dear MUNCHIE:
Prior to 2000, ASTM C-90 included two designations: Type I moisture-controlled units, and Type II non-moisture controlled units. Type I (N-1) units were stipulated to have a maximum moisture content at the time of delivery. In theory, limiting the moisture content of the units reduces dry shrinkage, which would allow a maximum distance between control joints.
While true in theory, dry shrinkage is largely a function of the moisture content at the time of installation and not at the time of delivery. Because delivery rarely coincides with installation, units delivered as Type I had the potential to no longer meet the moisture requirements when installed. To avoid this, the designations were removed from C-90. This effectively made all concrete masonry units non-moisture controlled. While having no negative impact on the quality of CMU, industry control joint spacing recommendations were adjusted to accommodate the designation removal. For crack control, the following rule of thumb was established; horizontal joint reinforcement every 16 inches, with a maximum spacing between control joints 1 ½ times the height of the wall, not to exceed 25 feet.
Is this too many control joints MUNCHIE? Another way to figure control joint spacing would be to use the Alternative Engineered Method shown in NCMA TEK 10-3. Here, a ratio between horizontal reinforcement and control joint spacing can be figured. The more steel in the wall, the more space there can be between control joints. For both methods, special considerations need to be taken with large openings, corners, etc. To find NCMA TEK notes, go to our website and click on the Resources tab.
Heidi Jandris grew up immersed in all things concrete block. As a kid she helped her dad build block walls and as an adult worked by his side as a welder. She received a Bachelor of Architecture degree from Pratt Institute and a Masters of Sustainable Building Systems degree from Northeastern University’s College of Engineering. She is part of the 3rd generation of her family’s business. She provides technical services to the design community while researching and implementing ways to improve the efficiency and lower the environmental impacts of their products.
