CMU Basics  Strengths
Statement of cmu strength can be in a variety of expressions and must be understood to prevent confusion. The discussion following is offered to clarify those expressions, of which there are three: two for cmu strength, and one for the masonry assemblage.
CMU Strength
Compressive strength of an individual cmu is expressed in terms of gross or net psi.
Gross compressive strength is the psi calculated from the gross area of the unit, which is the entire crosssectional area including cells. A minimum average gross compressive strength of 1000 gross psi was specified in older UBC and ASTM specifications.


Net compressive strength is the psi calculated from the net area of the unit, which is the crosssectional area minus the average area of cells. The minimum average net compressive strength per UBC and ASTM is 1900 net psi. 

Masonary Assemblage Strength
FThe third expression of strength is notated as f'm, which UBC defines as the “specified compressive strength of masonry at the age of 28 days”. Notice it is the strength of masonry, not units. In other words, it is the compressive strength of the assemblage of masonry units, mortar, and, if required, grout. [See “Selecting f'm” in Technical Info]
f'm is the value used in design of the masonry wall. When the design utilizes prescribed minimum strengths of cmu, mortar, and grout per their respective standards, the given f'm is 1500 psi based on net area. Beyond that, UBC Section 2105.3 has three provisions for complying with the specified f'm value: 1) by prism testing, 2) based upon an approved prism test record, and 3) as selected from Table 21D.
Masonry prism testing. Compression testing of prisms affords the most accurate determination of f'm. A prism is a sample assemblage of masonry units, mortar joints, and if required, grout similar to the one shown. For concrete masonry, such testing has indicated “a rule of thumb” relationship between prism compressive strength and the individual strengths of cmu and grout from which it is constructed:
The f'm is approximately 80% of the cmu and grout strengths
or
Cmu and grout strengths are approximately 125% of the f'm value. 

For example, a HighStress unit of 3750 net psi should facilitate an f'm of 3000 net psi. (The compressive strength of grout must be equal to or greater than the compressive strength of the cmu.)
Masonry prism test record. The average of the test record must equal or exceed 1.33 f'm. Seldom, if ever, used.
Unit strength method. Alternatively, in Table 21D, UBC allows an “assumed” value of f'm to be selected based upon specifying the net compressive strength of the masonry unit. For example, specifying the same HighStress unit as above at 3750 net psi, an f'm of 2500 net psi would be allowed without substantiation by prism test. However, when full stresses are used in the design of concrete masonry, tests must be performed on both the cmu and grout. Only onehalf the allowable stresses may be used unless material strengths are substantiated by test.
We recommend specifying the f'm as required by design and verifying compliance by means of prism testing. In this way, testing more reliably portrays actual construction, and full values are realized in design and materials.
Compressive
Strength Comparisons 
CMU 
f'm 
f'm 
Unit Type 
Net Area 
80% of cmu and grout 
UBC Table 21D 
Std. cmu per
ASTM, UBC 
1,900 
1,520 
1,500 
High Stress 
3,750 
3,000 
2,500 
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