Q&A Column

What is the difference between Type-I and Type-II concrete masonry?

Editor's note: ASTM C90 no longer defines Type I and Type II, but this column is included for historical reference.

ASTM C90 defines two types of concrete masonry units (CMU): Type-I Moisture-Controlled Units, and Type-II Non-Moisture-Controlled Units. Type-1 units must comply with moisture content requirements when delivered to the site. Proper use of this standard gives specifiers a way to limit residual drying shrinkage after installation.

Moisture content requirements are based on the following, as outlined in ASTM C90 Table 1. (Minnesota and surrounding areas have a mean relative humidity of 70%.)

  • Shrinkage properties of the units, which varies due to materials and curing methods used.

  • Humidity conditions at the site.

Areas with higher relative humidity permit a higher moisture content. In areas with lower relative humidity, lower moisture content must be maintained. Units that exhibit lower shrinkage are allowed higher moisture content at the time of delivery to the site.

Type-I units must be protected from rain or other moisture to ensure the block does not absorb additional moisture before installation. The tops of unfinished masonry must also be protected from the weather.

Concrete shrinks for two reasons:

  • Drying.

  • Hydration of the cement.

CMU that are almost completely hydrated and dried will shrink less. Two accelerated curing processes are used locally to produce Type-I units.

  • Low pressure curing with an extended controlled drying cycle. This system, which takes 24 hours, is called CO2 EnviroTrol. It is used locally by Amcon Block and Precast.

  • High pressure curing and drying. This system, known as autoclaving, takes about 12 hours. It is used locally by Anchor and Sheiley.

Both methods produce CMU that is Type I when delivered to the site.

Other manufacturers rely on air drying, which can take 30 days or more. This block will eventually meet Type-I standards, but only after a much longer drying period.

Properly spaced control joints are needed in masonry walls, regardless of the curing method. Use of Type-II units may require closer control joints or increases horizontal reinforcement.

Both of the two accelerated curing methods discussed produce CMU that shrinks less, shows minimal efflorescence, is more consistent in appearance, and is available with far less lead time than air-dried units.

Answer submitted by Jeanne Blonigen, CSI, Amcon Block & Precast