Masonry Magazine February 1992 Page. 11
NCMA-TEK
An Information series from National Concrete Masonry Association
ASTM Specifications for Concrete Masonry Units TEK 36B
Keywords: absorption, compressive strength, concrete brick, concrete masonry units, density, equivalent thickness, equivalent web thickness, face shell, grade, load bearing concrete masonry units, moisture content, net cross-sectional area, non-load bearing concrete masonry unit, sampling. solid units, split faced units, testing, web, weight classification.
Introduction
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.
The most widely used standards for specifying concrete masonry units are published by the American Society for Testing and Materials (ASTM). Currently, six ASTM standards which apply to units intended primarily for construction of masonry walls, beams, or columns and specialty applications are in use. These standards are identified as follows:
Type of Unit ASTM Designation
Concrete Building Brick C55-85
Calcium Silicate Face Brick C73-85 (89)
Load Bearing Concrete Masonry Units C90-90
Non-Load-Bearing Concrete Masonry Units C129-85
Catch Basins and Manhole Units C139-73 (89)
Prefaced Concrete Units C744-73 (85)
The standard specifications listed above are under the jurisdiction of ASTM Committee C-15 on Manufactured Masonry Units and are the direct responsibility of Subcommittee C15.03 on Concrete and Sand-Lime Units.
The first part of an ASTM standard number is the fixed designation for that standard. For example, ASTM C55 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 C90-90
A number of significant changes to specification requirements for load-bearing concrete masonry units have been implemented in the latest edition of ASTM C90.
Prior to 1991, load-bearing block were covered by two separate standards, ASTM C90 for hollow units and ASTM C145 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 upon gross cross-sectional area of block. Although the net strength of the concrete used to make the block is similar for hollow and solid units, gross area strength of the unit is quite different.
In 1990 the basis for compressive strength in ASTM C90 was changed from gross area to average net area of block. The necessity for 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 (ACI 530-88/ ASCE 5-88). 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 C90 to include both hollow and solid units and eliminated the need for a separate standard for solid units. Thus, ASTM C145 has been discontinued by ASTM.
Gross Area = Width (actual) x Length (actual)
Gross Area
Net Area = Gross Area x % Solid
Web
Thickness
Face Shell
Thickness
Figure 1 Difference Between Gross and Net Area
Classification
Two types of block are covered by ASTM C90-90: Type I, Moisture-Controlled Units, and Type II. Non-mois-