Specified Compressive Strength of Masonry

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The Specified Compressive Strength of Masonry (f'm), sometimes referred to as the "Laid-up Masonry Design Strength", is determined according to 2 methods outlined in IBC 2006 Section 2105.2.2. Those 2 methods are the unit strength method or the prism test method.[1]

The compressive strength of grout is typically specified to be equal to or greater than the compressive strength of the concrete masonry unit. For this reason the specified compressive strength tables found in the codes are based on the compressive strength of the masonry unit and the compressive strength of the mortar.[2]

"Cmu compressive strength is the psi calculated from the net area of the individual unit. The minimum average net compressive strength per ASTM C 90 is 1900 net psi. f'm, specified compressive strength of masonry is the value used in design of the masonry wall. This is the strength specification that really matters, as it is the strength upon which the engineer’s design is based. When the design utilizes prescribed minimum strengths of cmu and grout per their respective standards, the given f'm is 1500 psi based on net area.

Notice it is the strength of masonry, not units. It is the compressive strength of the assemblage of masonry units, mortar, and grout. IBC Section 2105.2 has two provisions for complying with the specified f'm value: 1) by unit strength method, and, 2) by prism test method.

Unit strength method. IBC allows an “assumed” value of f'm to be selected based upon specifying the net compressive strength of the masonry unit. (Table 2105.2.2.1.2) For example, specifying a High-Stress unit at 3750 net psi, an f'm of 2500 net psi would be allowed without substantiation by prism test.

Note: The California Building Code does not allow the unit strength method for design strengths over 1500 net psi. Prism testing must be used.

Prism testing. Although prism tests require more coordination in project management, compression testing of prisms does offer the most accurate determination of f'm. A prism is a sample assemblage of masonry units, mortar joints, and grout similar to the one shown. For concrete masonry, such testing has suggested “a rule of thumb” relationship between prism compressive strength and the individual strengths of cmu and grout from which it is constructed. where the f'm is approximately 80% of the cmu and grout strengths.

For example, a High-Stress unit of 3750 net psi should facilitate an f'm of 3000 net psi. (The IBC requires the compressive strength of grout must be equal to or greater than the f'm. However, the “80%” rule was based on UBC’s requirement of grout strength equal to or greater than the cmu compressive strength.)

When the f'm value exceeds 1500 net psi, 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.

Prism testing also allows the contractor to submit and supply a combination of cmu and grout most advantageous for the project schedule. The conservatively high cmu values utilized by the unit strength method may force materials to be made on special order with significant lead times. Many times, however, the f'm can be satisfied by available materials coupled with an appropriate grout strength."[3]

References

  1. International Code Council. International Building Code 2006
  2. Design of Reinforced Masonry Structures. Narendra Taly, PhD. McGraw Hill. 2001. Table A3.1 notes.
  3. Angelus Block Co, Inc. 14 Sept 2009. http://www.angelusblock.com/index.cfm?pcall=strength

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