Strengths
of CMU
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 2007 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 may require a bit more coordination in project management, compression testing of prisms does offer the most accurate verification of f'm. A prism is a sample assemblage of masonry units, mortar joints, and grout similar to the one shown.
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.
| CMU | f'm | ||
| Unit Type | Net Area psi |
Typical Prism Test |
Unit Strength Allowed* |
| Std. ASTM C90 | 1,900 | 1,520+ | 1,500 |
| High Stress | 3,750 | 3,000+ | 2,500 |
| * IBC Table 2105.2.2.1.2 | |||