Masonry Magazine January 1976 Page.18
Determine what equipment the lab has available as this has a direct bearing on the size of masonry specimens that can be tested. The equipment must be of a capacity that exceeds the load required to fail the specimen and should have a large enough opening so that a specimen with an h/t of at least 2 will fit into the machine.
DETERMINATION OF MASONRY STRENGTH
# Compressive Strength
The most desirable method for determination of masonry compressive strength remains the testing of masonry prisms. This should be done in accordance with Method B of ASTM E 447. The prisms should be constructed using the same materials, pattern bond and workmanship, and be of the same thickness as the actual walls being constructed. The length of the prism should not be less than its thickness. The height of the prism should be not less than twice its thickness. When prisms with height-to-thickness ratios of less than 5 are used, the ultimate strengths must be modified by the factors given in Table 1.
| Slenderness Ratiob | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 4.5 | 5.0 |
| :----------------- | :--- | :--- | :--- | :--- | :--- | :--- | :--- |
| Correction Factor | 0.82 | 0.85 | 0.88 | 0.91 | 0.94 | 0.97 | 1.00 |
ªThese values are different from those now given in the August 1969 BIA Building Code Requirements for Engineered Brick Masonry. They are based on subsequent research and more nearly reflect the masonry behavior in prisms with h/t less than 5. Height to thickness (h/t). CInterpolate to obtain intermediate values.
No structural reinforcing should be included; however, metal wall ties may be used. The prism shall not contain grout unless all cells of hollow units and/or the cavity of cavity walls are fully grouted in the actual wall construction.
Curing and Storage. After construction of the prisms, they should be stored in an area at the job site where they are protected from damage, but subject to the same curing conditions as the walls. They shall be allowed to stand for 48 hr. The prisms should then be protected top and bottom for transporting to the laboratory. The prisms then are stored in laboratory air at 70 F (21.1 C) 15 F (-9.4 C) and relative humidity between 30 percent and 70 percent. These prisms at maturity should be tested as described in Technical Notes 39A and ASTM E 447. See Fig. 1.
Early Testing. It may be desirable to establish strength relationships of prisms cured for less than 28 days to 28-day prisms. This can be done prior to start of construction. Build 2 or more sets of 3 prisms each, 5 preferred (1 set for each cure period). The designer may be interested in 3 or 7-day strengths as he may wish to know the masonry strength for placement of a floor system. The prisms should be cured at the site in air for 24 hr, then stored in an atmosphere with an ambient
temperature of not less than 65 F (18.33 C) for the remainder of the cure period. One set of prisms should be tested at 28 days and the other sets tested at the desired age level; i.e., 1, 3 or 7 days. From these data, relationships between shorter cure periods can be established with the 28-day strength obtained. If the relationship is not established by actual testing as described above, there is an approximation method which can be used to predict 28-day strengths. The 7-day strength can be assumed to be 90 percent of the projected 28-day strength. (See Technical Notes 35.)
# Diagonal Tensile (Shear) Strength
General. It is sometimes necessary when determining shear design stresses to establish masonry strengths to closer tolerances than the values established as a function of ultimate compressive strength, f'm. Methods used to establish these design values are given in Technical Notes 39A (ASTM E 72 and E 519). These test methods require large masonry specimens and are much too expensive to perform for normal quality control checks. Companion Specimens. Compressive prisms may be built as described in Technical Notes 39A and tested as companion specimens to the design shear tests. A minimum of 3 compressive prisms (preferably 5 prisms) should be built and tested. From the relationship betweeen these compressive prisms and design shear assemblage test results, judgment can be used to determine shear capacity. This relationship is based upon the units and mortar used on the project and "inspected workmanship".
# Flexural Bond Strength
General. ASTM Standard Methods of Test, E 518, are used primarily as a quality control check and to