Building Design and Construction

Determination of Masonry Compressive Strength

Allowable stresses are based, for the most part, on the ultimate compressive strength ƒ’m psi, of masonry used. This strength may be determined by prism testing, or may be based on the compressive strength of the bricks and the type of mortar used.
Compressive strength tests of brick should be conducted in accordance with ASTM ‘‘Standard Methods of Sampling and Testing Brick,’’ C67.
Prism Tests. When the compressive strength of the masonry is to be established by preliminary tests, the tests should be made in advance of construction with prisms built of similar materials, assembled under the same conditions and bonding arrangements as for the structure. In building the prisms, the moisture content of the units at the time of laying, consistency of the mortar, thickness of mortar joints, and workmanship should be the same as will be used in the structure.
Prisms. Prisms should be stored in an air temperature of not less than 65F and aged, before testing, for 28 days in accordance with the provisions of ASTM ‘‘Standard Method of Test for Compressive Strength of Masonry Assemblages,’’ E447.
Seven-day test results may be used if the relationship between 7- and 28-day strengths of the masonry has been established by tests. In the absence of such data, the 7-day compressive strength of the masonry may be assumed to be 90% of the 28-day compressive strength.
The value of ƒ’m used in the standard, ‘‘Building Code Requirements for Engineered  Brick Masonry,’’ is based on a height-thickness ratio h/ t of 5. If the h/ t of prisms tested is less than 5, which will cause higher test results, the compressive strength of the specimens obtained in the tests should be multiplied by the appropriate correction factor given in Table 11.2. Interpolation may be used to obtain intermediate values.
Brick Tests. When the compressive strength of masonry is not determined by prism tests, but the brick, mortar, and workmanship conform to all applicable requirements of the standard, allowable stresses may be based on an assumed value of the 28-day compressive strength ƒm computed from Eq. (11.1) or interpolated from the values in Table 11.3.

When there is no engineering or architectural inspection to ensure compliance with the workmanship requirements of the standard, the values in Table 11.3 under ‘‘Without inspection’’ should be used.

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