Masonry Magazine February 1992 Page. 12
Moisture-Controlled Units
Type I units are required to comply with the moisture content provisions listed in Table 1. The purpose of these requirements is to provide the specifier with a method of limiting residual drying shrinkage of units in the wall regardless of the shrinkage properties of the units (up to the maximum unit shrinkage of 0.065%).
These provisions prescribe maximum allowable moisture contents depending upon: (1), the shrinkage properties of the units, and (2), the atmospheric humidity conditions at point of use. Table 1 recognizes that inherent linear drying shrinkage properties of concrete will vary depending on materials and manufacturing methods used. Also, the moisture content of concrete responds to average relative humidity conditions of the atmosphere in which it is located. Thus, a lower moisture content is required when units are to be used in an arid environment, (less than 50% average annual humidity); and a higher moisture content is permitted if units are used in a humid atmosphere (above 75% humidity). (The appendix of ASTM C90 includes a map of the U.S. showing mean annual relative humidity, %, covering the U.S.) Likewise, units having lower drying shrinkage properties are permitted to have higher moisture content at time of delivery than higher shrinkage units. Type I units must be protected from rain or other moisture at the job site before they are placed in the wall.
Type II units, which are manufactured without special consideration given to controlling moisture content, are extensively used. With exposed Type II units, closer control joint spacing, or increased horizontal reinforcement may be advisable depending on the potential shrinkage properties of the units.
TABLE 1
Moisture-Content Requirements for Type I Units
| | Moisture content, max, % of Total Absorption (Average of 3 Units) |
| :---------------------------- | :------------------------------------------------------------------ |
| | Humidity conditions at Job Site or Point of Use |
| Lincar Shrinkage, % | Humid |
| 0.03 or less | 45 |
| From 0.03 to 0.045 | 40 |
| 0.045 to 0.065, max | 35 |
| "Mean annual relative humidity above 75%. | |
| "Mean annual relative humidity 50 to 75%. | |
| Mean annual relative humidity less than 50%. | |
Physical Requirements
Physical requirements prescribed in ASTM C90-90 cover the following applicable requirements:
* Moisture content requirements for Type I units.
* Face shell and web thickness requirements for hollow units.
* Strength and absorption requirements.
Minimum face shell and web thickness requirements for hollow load-bearing units are listed in Table 2. The values listed are deemed to be minimums necessary to obtain satisfatory structural performance. In addition to minimum permissible web thicknesses for individual cross-webs, the specification requires a minimum total thickness of cross-webs per foot of block length. Face shell thicknesses for 10" and 12" hollow units are permitted to be reduced to 1-1/4" if allowable design loads are reduced in proportion to the reduction in thickness from the basic face shell thicknesses listed.
Provisions for face shell thickness have been extended to include face-shell thickness for split face units. As shown in footnote A of Table 2, a maximum of 10% of a split face shell area may be less in thickness than the minimum prescribed thickness, but not less than 3/4 inches. Footnote B clarifies the intent of the minimum equivalent web thickness requirements to be not applicable to the portion of a unit to be filled with grout. This provision was included to avoid exclusion of special units such as bond beam block because they may not meet web thickness requirements. Footnote C, which permits reduction in face shell thickness if allowable design load is reduced accordingly, clarifies that such units, when fully grouted, do not require reduction in allowable load.
TABLE 2
Minimum Thickness of Face Shells and Webs
| Nominal Width (W) of Units, in. | Face-Shell Thickness (PST) min, in. | Webs min, in. | Equivalent Web Thickness, min, in/linear ft |
| :------------------------------- | :----------------------------------- | :------------- | :----------------------------------------- |
| 3 (76.2) & 4 (102) | 3/4 (19) | 3/4 (19) | 1-5/8 (136) |
| 6 (152) | 1 (25) | 1 (25) | 2-1/4 (188) |
| 8 (203) | 1-1/4 (32) | 1 (25) | 2-1/4 (188) |
| 10 (254) | 1-3/8 (35) | 1-1/8 (29) | 2-1/2 (209) |
| 12 (305) | 1-1/4 (32) | | |
| | 1-1/2 (38) | 1-1/8 (29) | 2-1/2 (209) |
| | 1-1/4 (32) | | |
*Average of measurements on 3 units taken at the thinnest point, when measured as described in Methods C140. When this standard is used for split face units, a maximum of 10% of a split face shell area may have thickness less than those shown, but not less than 3/4 inch. When the units are solid grouted the 10% limit does not apply.
"Sum of the measured thickness of all webs in the unit, multiplied by 12 and divided by the length of the unit. Equivalent web thickness does not apply to the portion of the unit to be filled with grout. The length of that portion shall be deducted from the overall length of the unit for the calculation.
"This face shell thickness (FST) is applicable where allowable design load is reduced in proportion to the reduction in thickness from basic face shell thicknesses shown, except that allowable design loads on solid grouted units shall not be reduced.
Minimum face shell and web thicknesses are not prescribed for "solid" units. The section properties are controlled by requiring that the net cross-sectional area in every plane parallel to the bearing surface to be not less than 75% of the gross cross-sectional area measured in the same plane.
In the previous editions of ASTM specifications covering hollow units and solid units, two grades of block (N