Masonry Magazine February 1989 Page. 27
TABLE 2-VALUES OF fm FOR MASONRY*
ACI STANDARD 531-79 (REVISED 1981)
Compressive test strength of masonry units, psi, on the net cross-sectional area
Compressive strength of masonry fm psi
| | Type M and S mortar | Type N mortar |
| --------- | ------------------- | ------------- |
| 6000 or more | 2400 | 1350 |
| 4000 | 2000 | 1250 |
| 2500 | 1550 | 1100 |
| 2000 | 1350 | 1000 |
| 1500 | 1150 | 875 |
| 1000 | 900 | 700 |
*Uniform Building Code (1982)
fm = 1350 psi for hollow or partially grouted unit construction (Grade N) with type M or S mortar.
fm = 1500 psi for fully grouted construction
FIGURE 4-Masonry Prisms for Compressive Testing Prisms
Duplicate the Project Construction and are Grouted
Solidly if the Structure is to be Similarly Grouted.
Face Shell
Bedding Only
7%"
15%"
Full Mortar Bedding
7%"
15%"
reinforcing, bolts, other embedded items, and cleanout closures must be properly located and securely in place before grouting is started and should be continuously inspected throughout the grouting operation. Horizontal reinforcing bars are properly positioned as the wall is erected. Vertical bars may be inserted from the top of the wall and are held in position at intervals not exceeding 192 bar diameter.
When design requirements result in a large amount of closely spaced steel reinforcement, a variation of the vertical steel placement may be employed. The vertical bars are tied into their proper position at the foundation of base of the wall prior to laying of concrete masonry units. Special open end "A" shaped or "H" shaped block are then placed around the vertical bars as the wall is erected. Figure 3 shows these open end units which are basically two core units with one or both ends cross webs removed.
High-lift grouting requires that the grout be placed in the cavities in lifts not exceeding four feet, and consolidated at the time of pouring by a flexible cable vibrator or 1"x2" wood pole to thoroughly fill all voids, spaces and interstices. A sufficient delay should be allowed between lifts to permit initial absorption of excess water into the masonry units, and the grout should then be reconsolidated prior to losing its plasticity. If the timing is suitable, reconsolidation of the previous pour and consolidation of the succeeding pour may be done in the same operation. The full height of any section of wall should be completed in one day.
Structural Design
# Masonry Strength
Allowable compressive, bearing, and shear stresses in reinforced hollow unit concrete masonry are based on the specified compressive strength of masonry (fm). The specified strength (fm) may be verified by either: (1) the compressive strength of the hollow units (net cross-sectional area), see Table 2; or (2) the compressive strength of prisms built with the same units, mortar, and workmanship proposed for the job (Figure 4). The prisms should be tested in accordance with the modified version of ASTM E447 (Standard Methods of Test for Compressive Strength of Masonry Prisms) contained in ACI 531.1 and which appears as follows:
Build and test a minimum of three prisms using the materials and workmanship to be used in the structure. In construcing the prisms, the mortar bedding, the grouting, the thickness and tooling of joints shall be the same as those to be used in the structure. Prisms shall be built in stack bond with a minimum of two units. The height to thickness ratio of a prism shall be 1.33 minimum and 3.0 maximum. The length of the prism shall equal or exceed the thickness. No reinforcement shall be placed in the prism. Grout only those wythes of the prism that are to be fully grouted in the construction. Place no grout in those wythes of the prism that are to be partially grouted or non-grouted in the construction. Prisms may be constructed of segments of units where necessary to accommodate the testing equipment, provided all the requirements for constructing the prisms are met and the reported strength is based on the net area of the unit segment plus grout area.
# Allowable Stresses
Masonry: Allowable stresses in inspected masonry are given in Table 3. Where there is no engineering or architectural inspection of construction as outlined in the applicable building code requirements, the allowable masonry stresses are reduced.
# Reinforcement
Stresses in steel reinforcement shall not exceed the following limits:
In Tension
Grade 40 Bars.
Grade 60 Bars.
Joint wire reinforcement
(50% of the minimum
ASTM specified yield
strength but not to
exceed
20,000 psi
24,000 psi
30,000 psi
All other reinforcement... 20,000 psi
In Compression
40% of the ASTM speci-
field yield strength, but
not to exceed
24,000 psi
For stresses due to wind or earthquake combined with dead and live loads, the allowable stresses (masonry and reinforcement) may be increased 33 percent provided the strength of the member is not less than that required for dead and live loads alone.