Building Codes stipulate material specifications that become manufacturing requirements. In particular, ASTM C 90 — Standard Specification for Loadbearing Concrete Masonry Units governs the manufacturing requirements for concrete block. It is the specification for loadbearing concrete masonry units. Contained in ASTM C 90 are requirements for physical dimensions, maximum absorption, strength, and linear shrinkage. Additionally, ASTM C 90 holds requirements for permissible variation in dimensions, or tolerances, and finish appearance requirements. Users of CMU specify that units meet ASTM C 90 because they know that such product provide the strength and integrity to construct safe buildings.

Testing procedures for the ASTM C 90 requirements are found in ASTM C 140 — Test Methods for Sampling and Testing Concrete Masonry Units and Related Units (compression, absorption, unit weight properties), plus ASTM C 426 — Test Method for Drying Shrinkage of Concrete Masonry Units. Drying shrinkage results are for a particular mix design and can be applied to different units manufactured with the same mix design. Recommendations are that CMU be tested every two years for ASTM C 90 compliance, unless there is a change in mix design, aggregate supplier, cement supplier, etc.

The National Concrete Masonry Association's Certified Laboratory Technician school provides the proper training so local testing labs correctly conduct the tests. Producers should use labs that employ NCMA certified technicians (a listing of certified laboratory technicians are found in the membership directory or at the NCMA web site, www.ncma.org). But producer should make sure to be smart clients and know the testing producers as well. Inaccurate testing that lead to faulty results can drive production processes in the wrong direction. A testing lab that reports unit failures may prompt a change in raw materials with no justification. On laboratory reports producers should demand to see received, immersed, saturated surface dry and oven dry weights for all the units tested. Ask the laboratory about their compression machine platen thickness and capping procedures.

If ever there is any doubt about testing results, the NCMA Laboratory is available to answer questions and at a producer's request, call a testing lab to advise then on testing procedures. Furthermore, the NCMA Laboratory can benchmark production and a local lab by conducting CMU testing and comparing the results. (A shipping agreement with Yellow Freight enables NCMA members to receive a 60 percent discount; note Scope, page 6).

By all means develop a relationship with your local lab to ensure proper testing of your products. As former President Reagan noted, “trust, but verify.”

Measuring CMU for physical dimensions

As noted, ASTM C 90 has requirements for physical dimensions, maximum absorption, strength, and linear shrinkage. Additionally, ASTM C 90 contains requirements for permissible variation in dimensions, or tolerances, and finish appearance requirements.

Physical dimensions of CMU contained in ASTM C 90 are measured by testing laboratories that verify your units' compliance. Very specific procedures for measuring physical dimensions are found in ASTM C 140. Overall dimensions must be measured with a steel scale with divisions not greater than 0.1 inch. Face shells and web thicknesses must be measured with a caliper having divisions not greater that 0.01 inch. Furthermore, overall dimensions, faceshells, and webs must be measured at specific locations. For example, webs must be measured at the thinnest point down from the top of the unit as manufactured.

The accuracy of the measurements is paramount. They will be used to see if the CMU complies with the dimensional requirements of ASTM C 90. These requirements required an average of three CMU and your testing laboratory should be reporting to you the measurements of all of these units. For each CMU in the test set, no overall dimension (width, length, and height) can differ by more that ±1/8 inch. Therefore, you should check your laboratory report making sure it reports all the dimensions and it shows how your CMU meets the ASTM C90 requirements.

Jeff Greenwald is Director of Research and Development for the National Concrete Masonry Association, Herndon, Va. In addition to ASTM, he is a member of The Masonry Society, American Concrete Institute, American Society of Civil Engineers, National Society of Professional Engineers, Building Officials & Code Administrators International, International Conference of Building (SBCCI), the (BOCA), and the (ICBO).