Masonry Magazine August 1994 Page. 23
NCMA TEK
An Information Series from National Concrete Masonry Association
SPECIFICATIONS FOR CONCRETE MASONRY UNITS
TEK 1-1
Codes & Specs (1994)
Keywords: arca (net vs. gross), ASTM specifications, compressive strength, concrete brick, dimensions, face shell and web thickness, specifications, testing, water absorption
INTRODUCTION
The most widely used standards for specifying concrete masonry units are published by the American Society for Testing and Materials (ASTM). ASTM standards for concrete masonry units contain minimum requirements that assure properties necessary for quality performance. Requirements include conformance to specified component materials, moisture content, compressive strength, water absorption, permissible variation in dimensions, face shell and web thicknesses, and finish and appearance criteria.
Currently, six ASTM standards apply to units intended primarily for construction of concrete masonry walls, beams, or columns and specialty applications:
Type of Unit
Concrete Building Brick
Calcium Silicate Face Brick
Load Bearing Concrete Masonry Units
Non-Load-Bearing Concrete Masonry Units
Catch Basins and Manhole Units
Prefaced Concrete Units
ASTM
Designation
C 55-93
C 73-85 (89)
C 90-93
C 129-85 (90)
C 139-73 (89)
C 744-73 (85)
The specifications listed above are developed under the jurisdiction of ASTM Committee C 15 on Manufactured Masonry Units and are the direct responsibility of Subcommittee C 15.03 on Concrete and Sand-Lime Units.
The first part of an ASTM standard number is the fixed designation for that standard. For example, ASTM C 55 is the fixed designation for concrete brick. The number immediately following the fixed designation indicates the year of the last revision. ASTM standards are required to be reviewed at least every 5 years. If no changes are made, the number in parenthesis after the date of last revision indicates the year of last reapproval.
LOAD-BEARING CONCRETE MASONRY UNITS, ASTM C 90
Prior to 1991, load-bearing concrete masonry units were covered by two separate standards; ASTM C 90 for hollow units and ASTM C 145 for solid units. Requirements for these two standards were identical in all respects except for compressive strengths and minimum cross-sectional area. Compressive strength requirements were based on the unit gross cross-sectional area. Although the net area strength of the concrete used to make the block is similar for hollow and solid units, gross area strengths of the units are quite different.
In 1990 the basis for compressive strength in ASTM C 90 was changed from gross area to average net area of the unit (see Figure 1). This change was mandated by the ever-increasing use of engineered masonry design, which uses net area strength as a basis for allowable stresses. Compressive stress based on gross area of units is used with masonry designed by empirical codes such as Chapter 9 of Building Code Requirements for Masonry Structures (ref. 1). Figure 1 illustrates gross area and net area for hollow concrete block.
Incorporation of net area strength permitted a change in the scope of ASTM C 90 to include both hollow and solid units and eliminated the need for a separate standard for solid units. Thus, ASTM C 145 has been discontinued.
Classification
Two types of concrete masonry units are covered by ASTM C 90-93: Type I, Moisture-Controlled Units, and Type II. Nonmoisture-Controlled Units.
Type I units are required to comply with the moisture content provisions listed in Table 1 at the time of delivery to the jobsite. The purpose of these requirements is to provide
Gross area = width (actual) x length (actual)
Net area = gross area x % solid
Figure 1 - Gross and Net Areas
MASONRY-JULY/AUGUST, 1994 23