Masonry Magazine August 1994 Page. 25
Table 3-Strength, Absorption, and Weight Classification Requirements for Concrete Masonry Units, ASTM C 90-93
| Minimum Compressive Strength", psi (MPa) | Maximum Water Absorption, lb/ft³ (kg/m³) (Average of 3 units) |
|---|---|
| Average Net Area | Weight Classification Oven Dry Weight of Concrete, lb/ft³ (kg/m³) |
| Average of 3 Units | Individual Unit | Light weight, less Medium weight, 105 (1682) Normal weight, 125 |
| | | than 105 (1682) to less than 125 (2002) (2002) or greater |
| 1900 (13.1) | 1700 (11.7) | 18 (288) 15 (240) 13 (208) |
"To prevent water penetration, proper detailing, construction, flashing, and drainage should be provided. "Higher compressive strengths may be specified where required by design. Consult with local suppliers to determine availability of units of higher compressive strength.
There is now one grade, which is not given a grade classification. Compressive strength based on net cross-sectional area is required to be at least 1700 psi (11.7 MPa) for individual units and 1900 psi (13.1 MPa) for the average of three units (Table 3). Net area used for compressive strength determination is the "average" net area of the block, calculated by determining unit net volume from water displacement tests described in ASTM Test Methods C 140 (ref. 3). For cored units having tapered face shells and cross webs, average net area is typically the net cross- sectional area at approximately mid-height of the block.
Maximum permissible water absorption is shown in Table 3. Absorption is a measure of the total water-fillable void content of the concrete. It is determined from the weight difference between a saturated and an oven-dry condition and is expressed in terms of pounds of water per cubic foot of concrete. The maximum permissible absorption ranges from 13 pcf (208 kg/m³) to 18 pcf (288 kg/m³), depending on the density of the concrete. Since absorption is a measure of the total water-fillable void content, aggregates with relatively large pores, such as some light weight aggregate, would have a greater absorption than dense, non-porous aggregates, given the same compaction. As a result, light weight units are permitted higher absorption values than medium or normal weight units.
Finish and Appearance
Provisions relating to finish and appearance prohibit defects that would impair the strength or permanence of the construction, but permit minor cracks incidental to the usual manufacturing methods.
For units which will be used in exposed wall construction, the presence of objectionable imperfections is based on viewing the face or faces from a distance of 20 feet (6.1 m) under diffused lighting. Similarly, the specification requires that color and texture shall be specified by the purchaser, and that an approved sample consisting of not less than four units, representing the range of color and texture permitted, be used to determine conformance.
CONCRETE BUILDING BRICK ASTM C 55
ASTM C 55 applies to solid brick size units, solid concrete veneer, and facing units larger than brick size. Two grades of concrete brick are included in ASTM C 55:
-Grade N-For use as architectural veneer and facing units in exterior walls and for use where high strength and resistance to moisture penetration and severe frost action are desired.
-Grade S For general use where moderate strength and resistance to frost action and moisture penetration are required.
Compressive strength and absorption requirements for the two grades of concrete brick are listed in Table 4. ASTM C 55 includes requirements for Type I and II units, as does C 90. Moisture content requirements for Type I concrete brick are the same as in ASTM C 90-93 (Table 1).
NON-LOAD-BEARING CONCRETE MASONRY UNITS, ASTM C 129
ASTM C 129 covers hollow and solid non-load- bearing units, intended for use in non-load-bearing partitions. These units are not suitable for exterior walls subjected to freezing cycles unless effectively protected from the weather. ASTM C 129 requires that these units be clearly marked to preclude their use as load-bearing units. Minimum compressive strength requirements are 500 psi (3.5 MPa) for individual units and 600 psi (4.1 MPa) for an average of three units, based on net cross-sectional area.
ASTM C 129 covers Type I and Type II blocks, which must meet identical moisture content and linear drying shrinkage requirements to those in ASTM C 90 (Table 1).
CALCIUM SILICATE FACE BRICK, ASTM C 73
ASTM C 73 covers brick made from sand and lime. Two grades are included:
-Grade SW Brick intended for use where exposed to temperatures below freezing in the presence of moisture.
-Grade MW - Brick intended for exposure to temperatures below freezing, but unlikely to be saturated with water. Physical requirements are listed in Table 5.
PREFACED CONCRETE AND CALCIUM SILICATE MASONRY UNITS, ASTM C 744
ASTM C 744 for prefaced units establishes requirements for the facing materials applied to the surface of masonry units. For the concrete masonry units onto which the prefaced surface is molded, C 744 references the requirements contained in specifications C 55, C 90, and C 129. Facing requirements in C 744 include: resistance to crazing, surfaceburning characteristics, adhesion, color permanence, chemical resistance, cleansability, abrasion, and dimensional tolerances.